User:Hemantt1/sandbox/The Research

Time Begins
We all travel in time. During the last year, I've moved forward one year and so have you. Another way to say that is that we travel in time at the rate of 1 hour per hour. “We like the idea of time travelling because if we go back in time, many of us would like to change things to make the future better. And if you travel to the future, you’d want to come back with some wonderful discovery and share it to improve human life. So the desire to time travel is quite a positive fantasy to have,” But the question is, can we travel in time faster or slower than "1 hour per hour"? Or can we actually travel backward in time, going back, say 2 hours per hour, or 10 or 100 years per hour? It is mind-boggling to think about time travel. What if you went back in time and prevented your father and mother from meeting? You would prevent yourself from ever having been born! But then if you hadn't been born, you could not have gone back in time to prevent them from meeting. The great 20th century scientist Albert Einstein developed a theory called Special Relativity. The ideas of Special Relativity are very hard to imagine because they aren't about what we experience in everyday life, but scientists have confirmed them. This theory says that space and time are really aspects of the same thing—space-time. There's a speed limit of 300,000 kilometers per second (or 186,000 miles per second) for anything that travels through space-time, and light always travels the speed limit through empty space. Special Relativity also says that a surprising thing happens when you move through space-time, especially when your speed relative to other objects is close to the speed of light. Time goes slower for you than for the people you left behind. You won't notice this effect until you return to those stationary people. Say you were 15 years old when you left Earth in a spacecraft traveling at about 99.5% of the speed of light (which is much faster than we can achieve now), and celebrated only five birthdays during your space voyage. When you get home at the age of 20, you would find that all your classmates were 65 years old, retired, and enjoying their grandchildren! Because time passed more slowly for you, you will have experienced only five years of life, while your classmates will have experienced a full 50 years. So, if your journey began in 2003, it would have taken you only 5 years to travel to the year 2053, whereas it would have taken all of your friends 50 years. In a sense, this means you have been time traveling. This is a way of going to the future at a rate faster than 1 hour per hour. Time travel of a sort also occurs for objects in gravitational fields. Einstein had another remarkable theory called General Relativity, which predicts that time passes more slowly for objects in gravitational fields (like here on Earth) than for objects far from such fields. So there are all kinds of space and time distortions near black holes, where the gravity can be very intense. In the past few years, some scientists have used those distortions in space-time to think of possible ways time machines could work. Some like the idea of "worm holes," which may be shortcuts through space-time. This and other ideas are wonderfully interesting, but we don't know at this point whether they are possible for real objects. Still the ideas are based on good, solid science. In all time travel theories allowed by real science, there is no way a traveler can go back in time to before the time machine was built. Is time travel possible? To look at whether time travel is possible we need to distinguish between travelling back and travelling forwards. “From a philosophical point of view, we are constantly time travelling! As we age in our ordinary everyday lives, we are forever travelling into the future'. “The more interesting idea is being able to access a future everyone else is travelling slowly towards. So you move forward without ageing. And that, in theory, is actually perfectly possible,” he says. I am confident time travel into the future is possible, but we would need to develop some very advanced technology to do it. We could travel 10,000 years into the future and age only 1 year during that journey. However, such a trip would consume an extraordinary amount of energy. Time travel to the past is more difficult. We do not understand the science as well. Actually, scientists and engineers who plan and operate some space missions must account for the time distortions that occur because of both General and Special Relativity. These effects are far too small to matter in most human terms or even over a human lifetime. However, very tiny fractions of a second do matter for the precise work necessary to fly spacecraft throughout the solar system.

Quantum Time
Max Planck is sometimes considered the father of quantum theory In the first half of the 20th Century, a whole new theory of physics was developed, which has superseded everything we know about classical physics, and even the Theory of Relativity, which is still a classical model at heart. Quantum theory or quantum mechanics is now recognized as the most correct and accurate model of the universe, particularly at sub-atomic scales, although for large objects classical Newtonian and relativistic physics work adequately. If the concepts and predictions of relativity (see the section on Relativistic Time) are often considered difficult and counter-intuitive, many of the basic tenets and implications of quantum mechanics may appear absolutely bizarre and inconceivable, but they have been repeatedly proven to be true, and it is now one of the most rigorously tested physical models of all time. Quanta One of the implications of quantum mechanics is that certain aspects and properties of the universe are quantized, i.e. they are composed of discrete, indivisible packets or quanta. For instance, the electrons orbiting an atom are found in specific fixed orbits and do not slide nearer or further from the nucleus as their energy levels change, but jump from one discrete quantum state to another. Even light, which we know to be a type of electromagnetic radiation which moves in waves, is also composed of quanta or particles of light called photons, so that light has aspects of both waves AND particles, and sometimes it behaves like a wave and sometimes it behaved like a particle (wave-particle duality). An obvious question, then, would be: is time divided up into discrete quanta? According to quantum mechanics, the answer appears to be “no”, and time appears to be in fact smooth and continuous (contrary to common belief, not everything in quantum theory is quantized). Tests have been carried out using sophisticated timing equipment and pulsating laser beams to observe chemical changes taking place at very small fractions of a second (down to a femtosecond, or 10−15 seconds) and at that level time certainly appears to be smooth and continuous. However, if time actually is quantized, it is likely to be at the level of Planck time (about 10-43 seconds), the smallest possible length of time according to theoretical physics, and probably forever beyond our practical measurement abilities. It should be noted that our current knowledge of physics remains incomplete, and, according to some theories that look to combine quantum mechanics and gravity into a single “theory of everything” (often referred to as quantum gravity – see below), there is a possibility that time could in fact be quantized. A hypothetical chronon unit for a proposed discrete quantum of time has been proposed, although it is not clear just how long a chronon should be.

Copenhagen Interpretation
One of the main tenets of quantum theory is that the position of a particle is described by a wave function, which provides the probabilities of finding the particle at any number of different places, or superpositions. It is only when the particle is observed, and the wave function collapses, that the particle is definitively located in one particular place or another. So, in quantum theory, unlike in classical physics, there is a difference between what we see and what actually exists. In fact, the very act of observation affects the observed particle. Another aspect of quantum theory is the uncertainty principle, which says that the values of certain pairs of variables (such as a particle’s location and its speed or momentum) cannot BOTH be known exactly, so that the more precisely one variable is known, the less precisely the other can be known. This is reflected in the probabilistic approach of quantum mechanics, something very foreign to the deterministic and certain nature of classical physics. This view of quantum mechanics (developed by two of the originators of quantum theory, Niels Bohr and Werner Heisenberg), is sometimes referred to the Copenhagen interpretation of quantum mechanics. Because the collapse of the wave function cannot be undone, and because all the information associated with the initial possible positions of the particle contained in the wave function is essentially lost as soon as it is observed and collapsed, the process is considered to be time-irreversible, which has implications for the so-called “arrow of time”, the one way direction of time that we observe in daily life (see the section on The Arrow of Time). Some quantum physicists (e.g. Don Page and William Wootters) have developed a theory that time is actually an emergent phenomenon resulting from a strange quantum concept known as entanglement, in which different quantum particles effectively share an existence, even though physically separated, so that the quantum state of each particle can only be described relative to the other entangled particles. The theory even claims to have experimental proof recently, from experiments by Ekaterina Moreva which show that observers do not detect any change in quantum particles (i.e. time foes not “emerge”) until becoming entangled with another particle. Many Worlds Interpretation The Copenhagen interpretation of quantum mechanics, mentioned above, is not however the only way of looking at it. Frustrated by the apparent failure of the Copenhagen interpretation to deal with questions like what counts as an observation, and what is the dividing line between the microscopic quantum world and the macroscopic classical world, other alternative viewpoints have been suggested. One of the leading alternatives is the many worlds interpretation, first put forward by Hugh Everett III back in the late 1950s. According to the many worlds view, there is no difference between a particle or system before and after it has been observed, and no separate way of evolving. In fact, the observer himself is a quantum system, which interacts with other quantum systems, with different possible versions seeing the particle or object in different positions, for example. These different versions exist concurrently in different alternative or parallel universes. Thus, each time quantum systems interact with each other, the wave function does not collapse but actually splits into alternative versions of reality, all of which are equally real. This view has the advantage of conserving all the information from wave functions so that each individual universe is completely deterministic, and the wave function can be evolved forwards and backwards. Under this interpretation, quantum mechanics is therefore NOT the underlying reason for the arrow of time.

Quantum Gravity
Quantum gravity, or the quantum theory of gravity, refers to various attempts to combine our two best models of the physics of the universe, quantum mechanics and general relativity, into a workable whole. It looks to describe the force of gravity according to the principles of quantum mechanics, and represents an essential step towards the holy grail of physics, a so-called “theory of everything”. Quantum theory and relativity, while coexisting happily in most respects, appear to be fundamentally incompatible at unapproachable events like the singularities in black holes and the Big Bang itself, and it is believed by many that some synthesis of the two theories is essential in acquiring a real handle on the fundamental nature of time itself. Many different approaches to the riddle of quantum gravity have been proposed over the years, ranging from string theory and superstring theory to M-theory and brane theory, supergravity, loop quantum gravity, etc. This is the cutting edge of modern physics, and if a breakthrough were to occur it would likely be as revolutionary and paradigm-breaking as relativity was in 1905, and could completely change our understanding of time. Any theory of quantum gravity has to deal with the inherent incompatibilities of quantum theory and relativity, not the least of which is the so-called “problem of time” – that time is taken to have a different meaning in quantum mechanics and general relativity. This is perhaps best exemplified by the Wheeler-DeWitt equation, devised by John Wheeler and Bruce DeWitt back in the 1970s. Their attempt to unify relativity and quantum mechanics resulted in time essentially disappearing completely from their equations, suggesting that time does not exist at all and that, at its most fundamental level, the universe is timeless. In response to the Wheeler-DeWitt equation, some have concluded that time is a kind of fictitious variable in physics, and that we are perhaps confusing the measurement of different physical variables with the actual existence of something we call time. Imaginary Time While looking to connect quantum field theory with statistical mechanics, theoretical physicist Stephen Hawking introduced a concept he called imaginary time. Although rather difficult to visualize, imaginary time is not imaginary in the sense of being unreal or made-up. Rather, it bears a similar relationship to normal physical time as the imaginary number scale does to the real numbers in the complex plane, and can perhaps best be portrayed as an axis running perpendicular to that of regular time. It provides a way of looking at the time dimension as if it were a dimension of space, so that it is possible to move forwards and backwards along it, just as one can move right and left or up and down in space. Despite its rather abstract and counter-intuitive nature, the usefulness of imaginary time arises in its ability to help mathematically to smooth out gravitational singularities in models of the universe. Normally, singularities (like those at the centre of black holes, or the Big Bang itself) pose a problem for physicists, because they are areas where the known physical laws just do not apply. When visualized in imaginary time, however, the singularity is removed and the Big Bang functions like any other point in space-time. Exactly what such a concept might represent in the real world, though, is unknown, and currently it remains little more than a potentially useful theoretical construct.

What's almost impossible Let's start with the bad news. We probably can't travel back in time and watch the Egyptians build the pyramids. In the last century scientists came up with a number of theories that suggested it is indeed plausible to take a leap into the future; going back in time, unfortunately, is much more complicated. But it's not necessarily impossible. Setup Timeout Error: Setup took longer than 30 seconds to Albert Einstein laid the groundwork for much of the theoretical science that governs most time travel research today. Of course, scientists like Galileo and Poincaré that came before him helped, but Einstein's theories of special and general relativity dramatically changed our understanding of time and space. And it's because of these well-tested theories that we believe time travel is possible. One option for would be a wormhole, also known as an Einstein-Rosen bridge. Along with physicist Nathan Rosen, Einstein suggested the existence of wormholes in 1935, and although we've yet to discover one, many scientists have contributed their own theories about how wormholes might work. Stephen Hawking and Kip Thorne are probably the most well known. Thorne, a theoretical physicist at CalTech, even helped Christopher Nolan with the science behind Interstellar. So let's just assume that wormholes do exist. In the late 1980s, Thorne said that a wormhole could be made into a time machine. According to Einstein's theory of general relativity, a wormhole could act like a bridge though space-time by connecting two distant points with a shortcut. Certain types of wormholes, it's theorized, could allow for time travel in either direction, if we could accelerate one mouth of the wormhole to near-light speed and then reverse it back to its original position. Meanwhile, the other mouth would remain stationary. The result would be that the moving mouth would age less slowly than the stationary mouth thanks to the effect of time dilation—more on this in a second. But there are several major caveats of traveling back in time with this method. Chief among them is the simple fact that we'd need a method for creating wormholes, and once created, the wormhole would only allow us to travel as far back as the point in time when it was created. So we'll definitely never be spectators to Great Pyramids' construction. The other really serious caveat is that we'd need a way to move one of the mouths of the wormhole nearly the speed of light. In their seminal 1988 paper on wormholes, Thorne and his colleagues assumed that "advanced beings [would] produce this motion by pulling on the right mouth gravitationally or electronically." We can't do that right now, however. What we can do is travel into the future—but only by a little bit. Going forward in time There are two ways we may, one day, be able to time travel forwards. You may have heard of Cryogenics. This is when someone who’s died is frozen instead of being buried or cremated. The theory is they can be “woken up” in the future when we have the technology to bring them back to life. Or a machine or device could be developed so that some people age more slowly than others around them. This way they’d live longer and see a future beyond the average person’s life span. Another very different way of travelling into the future is more like what you’d see in science fiction. This is might involve travelling in a rocket or spaceship at a very high speed, close to the speed of light. “We can’t establish equality with the speed of light but it is possible, in theory, to travel nearly as fast as the speed of light,” So imagine you’re in a spaceship travelling very fast away from the Earth and you stay in orbit for a year. You would age at the same rate as if you were still on the Earth, by a year, but when you returned, the earth may have aged hundreds of years. “This is way beyond the technology we have at the moment,” he says. “But... in theory, it is possible.” This idea is explored further in what's known as the 'Twin Paradox'. Take a look at the video below: Gotta get back in time! So we may one day be able to time travel forwards but what about going back? “Here we hit the philosophical problem that always comes up when you discuss travelling back in time,” "Suppose you could go back in time and you accidentally killed your grandparents. Wouldn’t that mean you couldn’t exist either?" Something similar happens in the film ‘Back To The Future’. This whole idea is known as the ‘grandfather paradox’ (this is explained in more detail later in this big question). So how could you go back in time and see people who’ve died - some of them hundreds of years ago? “What happens with the Tardis is controlling space and time around it. And of course, the Tardis got its name from what it does - Time and Relative Dimension in Space. So imagine a graph where space is laid out on a two-dimensional coordinate but time moves up and down, backwards and forwards. That’s how we think of going back in time - time is relative. But space is fixed.” What's almost certainly possible In recent years, we've seen some aspects of Einstein's fanciful theories proven true. The latest and perhaps most exciting theory is the aforementioned effect called time dilation. Though we've based technology on the theory for decades, an experiment finally proved this year that time dilation is absolutely a real phenomenon. It's also a phenomenon that could allow us to travel into the future. Time dilation basically refers to the idea that time passes more slowly for a moving clock than it does for a stationary clock. The force of gravity also affects the difference in elapsed time. The greater the gravity and the greater the velocity, the greater the difference in time. Black holes, like the one depicted in Interstellar, for instance,would produce a massive amount of time dilation, due to their extreme gravitational pull. Thanks to the space program, we've actually been dealing with this effect for many years. This is why the clocks on the International Space Station tick just a little bit more slowly than clocks on Earth do. Since the space station is moving so fast and is affected by less gravity, time moves more quickly. It's also why no clock on Earth is perfectly accurate, since the effect of time dilation means that time moves more slowly closer to the planet's surface. Okay, maybe one is almost perfect. A better example of time dilation at work involves GPS satellites. The GPS chip in your smartphone works because there are 24 satellites circling the globe at all times that triangulate your location based on how long it takes time-stamped information to travel to and from the device. Go back and stop an avalanche! However, going back in time brings us up against the laws of physics where some processes are reversible and some irreversible. Some processes such as, say, an avalanche can’t unhappen. Likewise, on a much smaller scale, if you drop an egg, you can’t unbreak it. A spinning top, however, or a pendulum, is reversible because it keeps moving, repeating itself. Nothing really happens. “Within physics, we’re very aware of these issues. The world is irreversible. And so are our lives. We grow, we age, we die,” You can’t reverse the effects of an avalanche or similar catastrophic event. And that’s why going back in time is not possible nor ever likely to be. However, scientists learned when building the system that the atomic clocks on GPS satellites do indeed run a little bit fast, since they're moving 9,000 miles per hour in orbit. To be specific, they lose 8 microseconds a day. That's hardly perceivable, but it's enough to throw off the location data. And so GPS technology makes adjustments to the clocks on board to account for the relativistic effects. The equation used is kind of complicated.

Ancient Philosophy
Kalachakra, the Wheel of Time, is a representation of the cyclic view of time in some ancient philosophies Since the earliest days of philosophy in ancient India and Greece, the true nature of timehas exercised some the greatest minds in history. Mythology In ancient times, mythology and other traditional narratives were used to try and make sense of the universe we find ourselves in. In Greek mythology, Khronos (or Chronus to the Romans) was the personification of time, not to be confused with Cronus, the Titan and father of Zeus. The Greeks had two different words for time: chronos refers to numeric or chronological time, while another word kairos refers to the more qualitative concept of the right or opportune moment. The figure of Khronos was typically portrayed as a wise old man with a long grey beard, similar to the later European folklore image of Old Father Time, although he was originally described in very early Greek mythological tales as serpentine in form, with three heads, of a man, a bull and a lion. A separate figure, Geras, was the Greek god of old age, usually depicted as a tiny shrivelled-up old man. The Horae or Hours were the goddesses of the seasons and the natural flow of time, generally portrayed as personifications of nature in its different seasonal aspects, and with the cycle of the seasons themselves symbolically described as the dance of the Horae. Other mythologies had their own time-related gods, such as Heh the Egyptian deification of eternity or infinity, Zurvan the Zoroastrian god of infinite time (and the father of the twin spirits of good and evil), Elli the Norse god of old age, etc.

Wheel of Time
In ancient Indian philosophy, as expounded in early texts such as the Vedas of the late 2nd millennium BCE, the universe goes through repeated cycles of creation, destruction and rebirth (with each cycle lasting 4,320 million years according to some sources). This led to a cyclic view of time, the so-called “wheel of time” or Kalachakra, in which there are repeating ages over the infinite life of the universe. This was coupled with a belief in an endlessly repeated cycle of rebirths and reincarnations for individuals. The wheel of time concept is found in Hinduism and Buddhism, as well as in the beliefs of the ancient Greek Orphics and Pythagoreans, but also in other disparate religions and beliefs such those of the Maya, the Q’ero Indians of Peru and the Hopi Indians of Arizona. The idea of time as consisting of endlessly repeated cycles is perhaps an unsurprising one given the observed repetitiveness of other natural phenomena, such as the day-and-night cycle, the motion of the tides, the monthly cycle of the Moon, the annual cycle of the seasons, etc. It does, however, seems to presuppose an overall linear ordering in some sort of “hypertime” of all the cycles, so that each cycle can be distinguished from its predecessors and successors because it occurs at a different point in hypertime.

Ancient Greece
The early Greek philosophers generally believed that the universe (and therefore time itself) was infinite with no beginning and no end. In the 5th Century BCE, the Sophist philosopher Antiphon asserted that time is not a reality (hypostasis), but a concept (noêma) or a measure (metron). Also in the 5th Century BCE, Parmenides saw time (as well as motion and many other everyday things that we take for granted) as nothing more than an illusion because, he argued, all change is impossible and illusory (time as an illusion is also a common theme in Buddhist thought). Parmenides, then, believed that reality was limited to what exists in the here and now, and the past and future are unreal and imaginary. His near-contemporary Heraclitus, on the other hand, firmly believed that the flow of time is real and the very essence of reality. Zeno’s Paradoxes were devised at least partially to support Parmenides’ doctrine that change and plurality and the passage of time are merely illusory and lead to paradoxes and absurdity. In the best known of these, Achilles and the Tortoise, Achilles allows the tortoise a head start of, say, 100 metres in a footrace. After some finite time, Achilles will have run 100 metres, bringing him to the tortoise’s starting point, but during this time the slower tortoise has run a much shorter distance, say, 10 metres. It will then take Achilles some further time to run that distance, by which time the tortoise will have advanced yet further, etc, etc, so that whenever Achilles reaches somewhere the tortoise has been, he still has further to go. Because there are an infinite number of points Achilles must reach where the tortoise has already been, Zeno argues that he can never overtake the tortoise, and the tortoise must win the race. Indeed, another corollary of this paradox is that neither Achilles nor the tortoise can ever actually finish the race, as they are constantly having to cover an ever smaller distance, ad infinitum, and, as Zeno averred, “it is impossible to traverse an infinite number of things in a finite time”. Plato, in the 4th Century BCE, believed that time was created by the Creator at the same instant as the heavens. But in an attempt to slightly be more scientific, Plato identified time with the period of motion of the heavenly bodies. Plata was also aware of the so-called “Great Year”, a complete cycle of the equinoxes around the ecliptic (effectively the return of the planets and the “fixed stars” to their original relative positions, a process that takes about 25,800 years). The Pythagoreans and some Stoic philosophers like Chrysippus saw the end of this cycle as the end of time iself, after which history would start to repeat itself all over again in an endless repetition. Plato’s student Aristotle saw time as an attribute of movement, as something that does not exist on its own but is relative to the motions of things. He called time “the numeration of continuous movement” or “the number of change in respect of before and after”. Aristotle argued that time is essentially a measurement of change, and therefore cannot exist without some kind of succession or change, and also that it requires the presence of a soul capable of “numbering” the movement. Although he saw time as the measure of change, he stressed that it was not the same thing as change, because a change may occur faster or slower. Aristotle also believed that, although space was finite (with only some undefined void existing beyond the outermost sphere of the heavens), time was infinite, and that the universe has always existed and will always exist. Furthermore, he believed that time was continuous, not discrete or atomistic, in the same way as a line can be divided and sub-divided ad infinitum. Aristotle was also the first to frame a commonly-mentioned paradox about the existence of time, recapitulated by St. Augustine several centuries later: if time essentially consists of two different kinds of non-existence (the future or the “no longer”, and the past or the “not yet”) separated by a nothing (the instantaneous and vanishing present or “now”), how then can we talk of time as actually existing at all?

The Dark Ages
Mithraism, a mystery religion influenced by ancient Zoroastrianism, and a strong competitor to Christianity in the early years CE, believed in a finite “Time of the Long Dimension” which repeated itself in cycles of 12,000 years, within the overall container of infinite time. In general terms, Zoroastrianism saw the world around us as a kind of battlefield between a bad god and a good one, and saw time as the duration of this battle. The early Christian theologian St. Augustine (4th – 5th Century CE) probably thought more deeply about the nature of time than any philosopher since the ancient Greeks, but his deep thoughts remained inconclusive. Echoing the earlier comments of the Neo-Platonist Plotinus, St. Augustine famously encapsulated the experience of so many of us, when he observed: “What then is time? If no one asks me, I know; if I wish to explain it to one that asks, I know not”. He was only able to conclude that time was some kind of a “distention” of the mind which allows us to simultaneously grasp the past in memory, the present by attention, and the future by expectation. St. Augustine also adopted a subjective view of time, that time is nothing in reality but exists only in the mind’s apprehension of reality.

Middle Ages
In the Middle Ages, Christian philosophers had to reconcile the concept of time with the creation of the universe by God Christian and Muslim philosophers tried their best to incorporate the ideas of Aristotle into their theology during the early Middle Ages, but they struggled mightily with his belief that time was infinite. Perhaps the first Christian writer to put forward a solid argument against the ancient Greek notion of an infinite past was the Alexandrian philosopher John Philoponus in the 6th Century. The doctrine was further developed and institutionalized by the Christian scholastics of the 11th-13th Century, including Thomas Aquinas and St. Bonaventure, as well as Muslim philosophers such as Al-Kindi and Al-Ghazali, and the Jewish philosophers Maimonides and Saadia Gaon. Christianity and the other Abrahamic faiths, Islam and Judaism, believed in an all-powerful and infinite God (in contradistinction to everything else, which was therefore finite), and so medieval Christian, Muslim and Jewish philosophers and theologians developed the concept of the universe having a finite past with a definite beginning (the moment of its creation by God). Time, therefore, was necessarily finite in nature, a doctrine known as temporal finitism. The general Christian view is that time will come to a definite end with the end of the world, in the so-called “end-times” and the cataclysm of the Apocalypse. The 13th Century Italian theologian Thomas Aquinas objected to Aristotle’s assumption of infinite time on the grounds that, although the universe could in theory have existed infinitely into the past, in fact it did not (it began with God’s creation of the Earth a finite time ago), warning that our imagination cannot always be trusted to tell us how things really are. The 13th Century philosophers Henry of Ghent and Giles of Rome made the rather fine distinction that the continuum of time does actually exist in reality and not just as a mind-dependent concept, but that it can only be distinguished into earlier and later parts by the mind. Various versions of Christian creationism persist to this day, although not all are quite as literal as that of the medieval philosophers, or of the 17th Century bishop James Ussher, who famously concluded in the that the Earth was created by God on Sunday, 23rd October 4004BCE, at precisely 6pm! Young-Earth creationists still believe that God created the Earth, sometime within the last ten thousand years or so, over a period of 6 days, literally as described in the Genesis creation narrative. Others have even specified that He deliberately created it with the appearance of age, complete with fossils, rock strata, etc. Old-Earth creationists, on then other hand, have attempted to update their beliefs to take account of the scientifically proven age of the Earth (around 4.6 billion years) by claiming that the six days of creation in Genesis were not ordinary 24-hour days, but “God-days”, which may be the equivalent of millions or billions of years of human time. Still others claim that life was created relatively recently by God, but on a pre-existing old Earth. In the 14th Century, the French mathematician Nicole Oresme was perhaps the first to try and put the study of time on a mathematical and scientific basis. He asked the question as to whether the celestial motions of the Sun, Moon and planets are commensurable, and so whether there is a “basic” time interval of which the day, month and year are all exact integer multiples. Oresme suggested that a creator of the universe might well have arranged things so, but his conclusion was that that no two celestial motions are actually commensurable, and so there is no such basic time interval. Every individual perceives the flow of time differently Time perception refers to a person’s subjective experience of the passage of time, or the perceived duration of events, which can differ significantly between different individuals and/or in different circumstances. Although physical time appears to be more or less objective, psychological time is subjective and potentially malleable, exemplified by common phrases like “time flies when you are having fun” and “a watched pot never boils”. This malleability is made particularly apparent by the various temporal illusionswe experience. As a field of study within psychology and neuroscience, time perception came of age in the late 19th Century with the studies of the relationship between perceived and measured time by one of the founders of modern experimental psychology, Gustav Theodor Fechner. We do not so much perceive time itself, but changes in or the passage of time, or what might be described as “events in time”. In particular, we are aware of the temporal relations between events, and we perceive events as being either simultaneous or successive. We also have a perception of the sequence or order of these events. Our sense of time seems to have originated as a product of human evolution, and it is not a purely automatic or innate process, but a complex activity that we develop and actively learn as we grow. Humans are, as far as we know, the only animals to be consciously aware of the passage of time and our own impermanence and mortality, and to have a consciousness of the past that is anything more than pure instinct and behavioural conditioning. How We Perceive Time Although psychologists believe that there is a neurological system governing the perception of time, it appears not to be associated with specific sensory pathways, but rather uses a highly distributed system in the brain (see the section on Biopsychology). Time perception therefore differs from our other senses – sight, hearing, taste, smell, touch, even proprioception – since time cannot be directly perceived, and so must be “reconstructed” in some way by the brain. Neurotransmitters such as dopamine and norepinephrine (adrenaline) are integrally involved in our perception of time, although the exact mechanism is still not well understood. The human brain appears to possess some kind of “internal clock” (distinct from the biological or circadian clock) which is linked to specific dopamine levels, or possibly even several different clocks working together but independently, each of which may dictate our time perception depending on the particular context (see the section on Biopsychology for more detail). When the brain receives new information from the outside world, the raw data does not necessarily arrive in the order needed to process it properly. The brain therefore reorganizes the information and presents it in a more easily understandable form. In the case of familiar information, very little time is needed for this process, but new information requires more processing and this extra processing tends to makes time feel elongated. This is part of the reason why a child’s summer seems to last forever, while an old person’s well-practiced routine seems to slip away faster and faster. The more familiar the task, the less new information the brain needs to process, and the more quickly time seems to pass. To some extent also, the perception of time is associated with other cognitive processes such as attention. Measuring the duration of an event – whether it be the length of time to leave a sauce to simmer, estimating how fast to run to catch a ball, or calculating whether there is enough time to drive through a yellow light – requires a certain amount of attention, and new events appear to take longer than familiar events because more attention is paid to them. For instance, in psychological tests, if the same picture is shown again and again, interspersed every so often with a different picture, the different picture is perceived by the observer as staying on-screen for longer, even if all the pictures actually appear for the same length of time. The difference arises from the degree of attention paid to the pictures. The perception of time durations is also crucially bound up with memory. It is essentially our memory of an event (and perhaps, even more specifically, our memory of the beginning and end of the event) that allows us to form a perception of, or a belief in, its duration. We infer, albeit subconsciously, the duration of an event from our memory of how far in the past something occurred, of how long ago the beginning and end of the event occurred. It is not clear whether this is done by some measure of the strength of a memory trace that persists over time (the strength model of time memory), or by an inference based on associations between the event and other events whose date or time is known (the inference model). There is increasing evidence that an animal’s metabolic rate affects the way it perceives time. In general, larger animals have a slower metabolic rate, and time passes relatively rapidly for them. Smaller animals, conversely, tend to have faster metabolisms, and experience time as passing relatively slowly, so that they can perceive more events in the same period. Studies have shown that small animals can in fact distinguish very short and very quick-changing events, which is one reason why a fly can avoid a swatter with such apparent ease. In evolutionary terms, the ability to perceive time on very small scales may be the difference between life and death for small, vulnerable animals. Sequence and Duration We perceive time as series of events in a sequence, separate by durations of various lengths. Our experience is not limited to a single series of events, though, but we experience a plurality of overlapping events, sequences and durations. A metronome ticking at a rate of two or three times a second is perceived as an integral sequence, as a rhythm. When the ticks are less frequent, though, say at intervals of three seconds, the sounds appears to be no longer perceived as a sequence in the same way, and each sound impulse remains an isolated perceptual event. Similar results occur with slowed down speech or music: music or spoken sentences are only recognizable as such when their rhythmic patterns and phrases are presented at an optimal speed that allow them to be recognized as a perceptual unity. The perception of a duration requires a minimum of about 0.1 seconds in the case of visual stimuli such as a flash, or much less (0.01 to 0.02 seconds) in the case of auditory stimuli. Stimuli of any shorter time than these are therefore perceived as instantaneous, and as not representing any duration at all.

Physics of Time
In the sciences generally, time is simply what a clock reads, but this hides a whole host of different conceptions of time used in physics Physics is the only science that explicitly studies time, but even physicists agree that time is one of the most difficult properties of our universe to understand. Even in the most modern and complex physical models, though, time is usually considered to be an ontologically “basic” or primary concept, and not made up of, or dependent on, anything else. In the sciences generally, time is usually defined by its measurement: it is simply what a clock reads. Physics in particular often requires extreme levels of precision in time measurement, which has led to the requirement that time be considered an infinitely divisible linear continuum, and not quantized (i.e. composed of discrete and indivisible units). With modern atomic time standards like TAI and UTC (see the section on Time Standards) and ultra-precise atomic clocks (see the section on Clocks), time can now be measured accurate to about 10−15 seconds, which corresponds to about 1 second error in approximately 30 million years. But several different conceptions and applications of time have been explored over the centuries in different areas of physics, and we will look at some of these in this section. In non-relativistic or classical physics, the concept of time generally used is that of absolute time (also called Newtonian time after its most famous proponent), time which is independent of any perceiver, progresses at a consistent pace for everyone everywhere throughout the universe, and is essentially imperceptible and mathematical in nature. This accords with most people’s everyday experience of how time flows. However, since the advent of relativity in the early 20th Century, relativistic time has become the norm within physics. This takes into account phenomena such as time dilation for fast-moving objects, gravitational time dilation for objects caught in extreme gravitational fields, and the important idea that time is really just one element of four-dimensional space-time. Relativity also allows for, at least in theory, the prospect of time travel, and there are several scenarios which allow for the theoretical basis of travel in time. There are even theoretical faster-than-light time-travelling particles like tachyons and neutrinos. However, the concept of time travel also brings with it a number of paradoxes, and its likelihood and physical practicality is questioned by many physicists. Quantum mechanics revolutionized physics in the first half of the 20th Century and it still represents the most complete and accurate model of the universe we have. Time is perhaps not as central a concept in quantum theory as it is in classical physics, and there is really no such thing as “quantum time” as such. For example, time does not appear to be divided up into discrete quanta as are most other aspects of reality. However, the different interpretations of quantum theory (e.g. the Copenhagen interpretation, the many worlds interpretation, etc) do have some potentially important implications for our understanding of time. Most physicists agree that time had a beginning, and that it is measured from, and indeed came into being with, The Big Bang some 13.8 billion years ago. Whether, how and when time might end in the future is a more open question, depending on different notions of the ultimate fate of the universe and other mind-bending concepts like the multiverse. The so-called arrow of time refers to the one-way direction or asymmetry of time, which leads to the way we instinctively perceive time as moving forwards from the fixed and immutable past, though the present, towards the unknown and unfixed future. This idea has it roots in physics, particularly in the Second Law of Thermodynamics, although other, often related, arrows of time have also been identified. Time travel — moving between different points in time — has been a popular topic for science fiction for decades. Franchises ranging from "Doctor Who" to "Star Trek" to "Back to the Future" have seen humans get in a vehicle of some sort and arrive in the past or future, ready to take on new adventures. The reality, however, is more muddled. Not all scientists believe that time travel is possible. Some even say that an attempt would be fatal to any human who chooses to undertake it. Understanding time What is time? While most people think of time as a constant, physicist Albert Einstein showed that time is an illusion; it is relative — it can vary for different observers depending on your speed through space. To Einstein, time is the "fourth dimension." Space is described as a three-dimensional arena, which provides a traveler with coordinates — such as length, width and height —showing location. Time provides another coordinate — direction — although conventionally, it only moves forward. (Conversely, a new theory asserts that time is "real.") Most physicists think time is a subjective illusion, but what if time is real? Einstein's theory of special relativity says that time slows down or speeds up depending on how fast you move relative to something else. Approaching the speed of light, a person inside a spaceship would age much slower than his twin at home. Also, under Einstein's theory of general relativity, gravity can bend time. Picture a four-dimensional fabric called space-time. When anything that has mass sits on that piece of fabric, it causes a dimple or a bending of space-time. The bending of space-time causes objects to move on a curved path and that curvature of space is what we know as gravity. Both the general and special relativity theories have been proven with GPS satellite technology that has very accurate timepieces on board. The effects of gravity, as well as the satellites' increased speed above the Earth relative to observers on the ground, make the unadjusted clocks gain 38 microseconds a day. (Engineers make calibrations to account for the difference.) In a sense, this effect, called time dilation, means astronauts are time travelers, as they return to Earth very, very slightly younger than their identical twins that remain on the planet. Through the wormhole General relativity also provides scenarios that could allow travelers to go back in time, according to NASA. The equations, however, might be difficult to physically achieve. One possibility could be to go faster than light, which travels at 186,282 miles per second (299,792 kilometers per second) in a vacuum. Einstein's equations, though, show that an object at the speed of light would have both infinite mass and a length of 0. This appears to be physically impossible, although some scientists have extended his equations and said it might be done. A linked possibility, NASA stated, would be to create "wormholes" between points in space-time. While Einstein's equations provide for them, they would collapse very quickly and would only be suitable for very small particles. Also, scientists haven't actually observed these wormholes yet. Also, the technology needed to create a wormhole is far beyond anything we have today. While Einstein's theories appear to make time travel difficult, some groups have proposed alternate solutions to jump back and forth in time. Infinite cylinder Astronomer Frank Tipler proposed a mechanism (sometimes known as a Tipler Cylinder) where one would take matter that is 10 times the sun's mass, then roll it into very long but very dense cylinder. After spinning this up a few billion revolutions per minute, a spaceship nearby — following a very precise spiral around this cylinder — could get itself on a "closed, time-like curve", according to the Anderson Institute. There are limitations with this method, however, including the fact that the cylinder needs to be infinitely long for this to work. An artist's impression of a black hole like the one weighed in this work, sitting in the core of a disk galaxy. The black-hole in NGC4526 weighs 450,000,000 times more than our own Sun. Another possibility would be to move a ship rapidly around a black hole, or to artificially create that condition with a huge, rotating structure. "Around and around they'd go, experiencing just half the time of everyone far away from the black hole. The ship and its crew would be traveling through time," physicist Stephen Hawking wrote in the Daily Mail in 2010. "Imagine they circled the black hole for five of their years. Ten years would pass elsewhere. When they got home, everyone on Earth would have aged five years more than they had." However, he added, the crew would need to travel around the speed of light for this to work. Physicist Amos Iron at the Technion-Israel Institute of Technology in Haifa, Israel pointed out another limitation if one used a machine: it might fall apart before being able to rotate that quickly. Cosmic strings Another theory for potential time travelers involves something called cosmic strings — narrow tubes of energy stretched across the entire length of the ever-expanding universe. These thin regions, left over from the early cosmos, are predicted to contain huge amounts of mass and therefore could warp the space-time around them. Cosmic strings are either infinite or they’re in loops, with no ends, scientists say. The approach of two such strings parallel to each other would bend space-time so vigorously and in such a particular configuration that might make time travel possible, in theory.

Presentism and four-dimensionalism
Do non-present things exist? Four-dimensionalists say that they do, and presentists say that they don't1. Four-dimensionalists believe that time is a fourth dimension, orthogonal to the three spatial dimensions. In the same way that things exist at other points in space, says the four-dimensionalist, we should accept that things exist at other points in time. Just as Shania Twain exists, but not here, John Denver exists, but not now. On the four-dimensionalist view, the universe is an existing space-time manifold, containing everything that has happened, everything that is happening and everything that will happen. It follows from the four-dimensionalist picture that there is no single time that can be nonindexically regarded as the present. Presentists deny the analogy between time and the spatial dimensions, and insist that the only things that exist are the things that exist now. Shania Twain doesn't exist here, but she does exist. John Denver doesn’t exist now, so he doesn’t exist at all. The best that can be said about John Denver, according to the presentist, is that he did exist. There are those who say that presentism is vacuously true, those who say that it is obviously false, and those who say that presentists and fourdimensionalists are talking past each other2. We think that the disagreement between presentism and four-dimensionalism is both real and interesting, but we won't try to defend that view here. In fact, our purpose is to deny what has sometimes been taken to be a defining difference between the two views: their disagreement over the possibility of time-travel.

A common view about time-travel
A time traveler is one who takes a trip, starting at some time and arriving at some earlier or later time, where the duration of the trip is unequal to the difference between the arrival and departure times. So, if I leave here now, travel for an hour, and arrive somewhere a thousand years in the future, then I am a time traveler. Someone who goes to sleep and wakes up a thousand years later is not a time traveler,although he may feel like one. Wormholes and how scientists have created them on Earth! Wormholes are theoretical tunnels in space-time believed to connect different parts of our Universe. But they may not only exist in space. Scientists have created one in the lab… with magnets! Time travelers have personal experiences much like ours, except that the times that follow one after the other in their personal times are not all the times that follow one after the other in external time. (More on this later.) Is time travel possible? Philosophers disagree. But the common view is that time travel is possible if the four-dimensional view of time is correct, but is not possible if presentism is true. We call this the common view because we know many people who subscribe to it, and because the four-dimensional framework tends to be assumed by those who defend the possibility of time-travel. The common view has been defended in print by William Godfrey-Smith, who says that ‘the metaphysical picture which underlies time travel talk is that of the block universe, in which the world is conceived as extended in time as it is in space’3.The common view also seems to be behind John Bigelow’s claim that the absence of time-travel stories prior to the nineteenth century counts as evidence that everyone was a presentist until then (although Bigelow might just be saying that we needed to think in non-presentist terms before we could realise that time-travel is possible)4. We shall argue against the common view. We will focus upon a class of fairly straight-forward stories – stories which are commonly (and with good reason) regarded as time-travel stories, and which are commonly (and with good reason) regarded as compatible with four-dimensionalism. Our goal will be to show that if these stories really are compatible with four-dimensionalism, and presentism is at all plausible, then they are compatible with presentism too. It may be that there are other sorts of stories which can be correctly regarded as time-travel stories and which are not compatible with presentism, but we will not concern ourselves with those5. We’ll be happy if we can show that the presentist should accept the possibility of at least some time-travel stories.

The Nowhere Argument for the common view
Why think that time-travel is incompatible with presentism? One might argue as follows. According to the four-dimensionalist, the past, present and future all exist. If you want to travel to the past or to the future, you have a destination. There is somewhere for you go. The past and future are there waiting, as it were, for the time traveler to pay them a visit. But this is not the case on the presentist model. On the presentist model, the past and future do not exist, so there is nowhere for the timetraveler to go. Time travels features extensively in fiction, but there is some theoretical basis to the idea Arguably, we are always travelling though time, as we move from the past into the future. But time travel usually refers to the possibility of changing the rate at which we travel into the future, or completely reversing it so that we travel into the past. Although a plot device in fiction since the 19thCentury (see the section on Time in Literature), time travel has never been practically demonstrated or verified, and may still be impossible. Time travel is not possible in Newtonian absolute time (we move deterministically and linearly forward into the future). Neither is it possible according to special relativity (we are constrained by our light cones). But general relativity does raises the prospect (at least theoretically) of travel through time, i.e. the possibility of movement backwards and/or forwards in time, independently of the normal flow of time we observe on Earth, in much the same way as we can move between different points in space. Time travel is usually taken to mean that a person’s mind and body remain unchanged, with their memories intact, while their location in time is changed. If the traveller’s body and mind reverted its condition at the destination time, then no time travel would be perceptible. Time Travel Scenarios Although, in the main, differing fundamentally from the H.G. Wells concept of a physical machine with levers and dials, many different speculative time travel solutions and scenarios have been put forward over the years. However, the actual physical plausibility of these solutions in the real world remains uncertain. At its simplest, as we have seen in the section on Relativistic Time, if one were to travel from the Earth at relativistic speeds and then return, then more time would have passed on Earth than for the traveller, so the traveller would, from his perspective, effectively have “travelled into the future”. This is not to say that the traveller suddenly jumped into the Earth’s future, in the way that time travel is often envisioned, but that, as judged by the Earth’s external time, the traveller has experienced less passage of time than his twin who remained on Earth. This is not real time travel, though, but more in the nature of “fast-forwarding” through time: it is a one-way journey forwards with no way back. There does, however, appear to be some scientific basis within the Theory of Relativity for the possibility of real time travel in certain scenarios that there are some solutions to the field equations of general relativity that describe space-times so warped that they contain “closed time-like curves”, where an individual time-cone twists and closes in on itself, allowing a path from the present to the distant future or the past. Gödel’s solution was the first challenge in centuries to the dominant idea of linear time on which most of physics rests. Although a special case solution, based on an infinite, rotating universe (not the finite, non-rotating universe we actually find ourselves in), other time travel solution have been identified since then that do not require an infinite, rotating universe, but they remain contentious. In the 1970s, controversial physicist Frank Tipler published his ideas for a “time machine”, using an infinitely long cylinder which spins along its longitudinal axis, which he claimed would allow time travel both forwards and backwards in time without violating the laws of physics, although Stephen Hawking later disproved Tipler’s ideas. In 1994, Miguel Alcubierre proposed a hypothetical system whereby a spacecraft would contract space in front of it and expand space behind it, resulting in effective faster-than-light travel and therefore (potentially) time travel, but again the practicalities of constructing this kind of a “warp drive” remain prohibitive. Other theoretical physicists like Kip Thorne and Paul Davies have shown how a wormhole in space-time could theoretically provide an instantaneous gateway to different time periods, in much the same way as general relativity allows the theoretical possibility of instantaneous spatial travel through wormholes. Wormholes are tubes or conduits or short-cuts through space-time, where space-time is so warped that it bends back on itself, another science fiction concept made potential reality by the Theory of Relativity. The drawback is that unimaginable amounts of energy would be required to bring about such a wormhole, although experiments looking into the possibility of creating mini-wormholes and mini-black holes are being carried out at the particle accelerator at CERN in Switzerland. It also seems likely that such a wormhole would collapse instantly into a black hole unless some method of holding it open were devised (possibly so-called “negative energy”, which is known to be theoretically possible, but which is not yet practically feasible). Stephen Hawking has suggested that radiation feedback, analogous to feedback in sound, would destroy the wormhole, which would therefore not last long enough to be used as a time machine. Actually controlling where (and when) a wormhole exits is another pitfall. Another potential time travel possibility, although admittedly something of a long shot, relates to cosmic strings (or quantum strings), long shreds of energy left over after the Big Bang, thinner than an atom but incredibly dense, that weave through the entire universe. Richard Gott has suggested that if two such cosmic strings were to pass close to each other, or even close to a black hole, the resulting warpage of space-time could well be so severe as to create a closed-time-like curve. However, cosmic strings remain speculative and the chances of finding such a phenomenon are vanishingly small (and, even if it were possible, such a loop may well find itself trapped inside a rotating black hole). Physicist Ron Mallet has been looking into the possibility of using lasers to control extreme levels of gravity, which could then potentially be used to control time. According to Mallet, circulating beams of laser-controlled light could create similar conditions to a rotating black hole, with its frame-dragging and potential time travel properties. Others are looking to quantum mechanics for a solution to time travel. In quantum physics, proven concepts such as superposition and entanglement effectively mean that a particle can be in two (or more) places at once. One interpretation of this (see the section on Quantum Time) is the “many worlds” view in which all the different quantum states exist simultaneously in multiple parallel universeswithin an overall multiverse. If we could gain access to these alternative parallel universes, a form of time travel might then be possible. At the sub-sub-microscopic level – at the level of so-called quantum foam, tiny bubbles of matter a billion-trillion-trillionths of a centimetre in length, perpetually popping into and out of existence – it is speculated that tiny tunnels or short-cuts through space-time are constantly forming, disappearing and reforming. Some scientists believe that it may be possible to capture such a quantum tunnel and enlarge it many trillions of times to the human scale. However, the idea is still at a very speculative stage, It should be noted that, with all of these schemes and ideas, it does not look to be possible to travel any further back in time that the time at which the travel technology was devised. Faster-Than-Light Particles The equations of relativity imply that faster-than-light (superluminal) particles, if they existed, would theoretically travel backwards in time. Therefore, they could, again theoretically, be used to build a kind of “antitelephone” to send signals faster than light, and thus communicate backwards in time. Although the Theory of Relativity disallows particles from accelerating from sub-light speed to the speed of light (among other effects, time would slow right down and effectively stop for such a particle, and its mass would increase to infinity), it does not preclude the possibility of particles that ALWAYS travel faster than light. Therefore, the possibility does still exist in theory for faster-than-light travel in the case of a particle with such properties. There is a rather strange theoretical particle in physics called the tachyon that routinely travels faster than light, with the corollary that such a particle would naturally travel backwards in time as we know it. So, in theory, one could never see such a particle approaching, only leaving, and the particle could even violate the normal order of cause and effect. For a tachyon, the speed of light is the lower speed limit, while the upper speed limit is infinity, and its speed increases as its energy decreases. Even stranger, the mass of a tachyon would technically be an imaginary number (i.e. the number squared is negative), whatever that might actually mean in practice. It should be stressed that there is no experimental evidence to suggests that tachyons actually exist, and many physicists deny even the possibility. A tachyon has never been observed or recorded (although the search continues, particularly through analysis of cosmic rays and in particle accelerators), and neither has one ever been created, so they remain hypothetical, although theory strongly supports their existence. Research using MINOS and OPERA detectors has suggested that tiny particles called neutrinos may travel faster than light. Other more recent research from CERN, however, has put the findings into dispute, and the matter remains inconclusive. Neutrinos are not merely hypothetical particles like tachyons, but a well-known part of modern particle physics. But they are tiny, almost-massless, invisible, electrically neutral, weakly-interacting particles that pass right through normal matter, and consequently are very difficult to measure and deal with (even their mass has never been measured accurately).

The possibility of travel backwards in time is generally considered by scientists to be much more unlikely than travel into the future. The idea of time travel to the past is rife with problems, not least the possibility of temporal paradoxes resulting from the violation of causality (i.e. the possibility that an effect could somehow precede its cause). This is most famously exemplified by the grandfather paradox: if a hypothetical time traveller goes back in time and kills his grandfather, the time traveller himself would never be born when he was meant to be; if he is never born, though, he is unable to travel through time and kill his grandfather, which means that he WOULD be born; etc, etc. Some have sought to justify the possibility of time travel to the past by the very fact that such paradoxes never actually arise in practice. For example, the simple fact that the time traveller DOES exist at the start of his journey is itself proof that he could not kill his grandfather or change the past in any way, either because free will ceases to exist in the past, or because the outcomes of such decisions are predetermined. Or, alternatively, it is argued, any changes made by a hypothetical future time traveller must already have happened in the traveller’s past, resulting in the same reality that the traveller moves from. Theoretical physicist Stephen Hawking has suggested that the fundamental laws of nature themselves – particularly the idea that causes always precede effects – may prevent time travel in some way. The apparent absence of “tourists from the future” here in our present is another argument, albeit not a rigorous one, that has been put forward against the possibility of time travel, even in a technologically advanced future (the assumption here is that future civilizations, millions of years more technologically advanced than us, should be capable of travel). Some interpretations of time travel, though, have tried to resolve such potential paradoxes by accepting the possibility of travel between “branch points”, parallel realities or parallel universes, so that any new events caused by a time traveller’s visit to the past take place in a different reality and so do not impact on the original time stream. The idea of parallel universes, first put forward by Hugh Everett III in his “many worlds” interpretation of quantum theory in the 1950s, is now quite mainstream and accepted by many (although by no means all) physicists. Traveling to Portland is possible, because Portland is right there waiting for you. But traveling to the Land of Oz is impossible, because there is no such place. Traveling to the past or future is more like traveling to the Land of Oz, if presentism is true. You can't travel to somewhere that doesn't exist, so, if presentism is true, you can’t travel to other points in time. So presentism implies the impossibility of time-travel. Call this the Nowhere Argument6. The Nowhere Argument is not a good argument. If it were a good argument, then it would rule out not just the possibility of time-travel, but the ordinary passage of time as well. Consider what is about to happen to all of us. We are about to take a journey into the immediate future. Now it is over. If presentism is true, however, then the time that we just traveled to did not exist when we started the journey. We just traveled to a time that did not exist, and we are about to travel to a time that does not exist. If it were impossible to travel to times that do not exist, then we would not be able to make these journeys in time, and the ordinary passage of time from one moment to the next would be impossible. This would be a disastrous result for presentism. One way or another, the presentist has to account for the passage of time from one moment to the next. One way or another, the presentist has to make room for travel to non-existent times. As the Nowhere Argument rules this out, it proves too much. Assuming that presentism can get to the starting line, the Nowhere Argument must be a bad argument. We think that the Nowhere Argument is probably at the heart of the common view that time travel and presentism are incompatible. (Although there is another relevant argument against the possibility of time travel – an argument from endurantism, rather than from presentism. This argument is the topic of section 9 below.) It may be, however, that defenders of the common view have a subtler version of the argument up their sleeves. Perhaps there is something in the presentist’s story that allows her to say that the travel to non-existent times that occurs in ordinary passage is possible, but travel to times in the past or distant future is not. To show that this is not the case, we will now try to give a positive account of the possibility of presentist time travel. Our broad strategy will be to present David Lewis's defense of the possibility of four-dimensionalist time travel, and show how it can be translated into presentist terms7. We will tell a coherent time-travel story and explain its coherence in four dimensional, and then in presentist, terms. If our presentist explanation works, then presentism and time-travel are compatible.

Time and the Big Bang
We can model quite accurately the evolution of the universe since the Big Bang 13.8 billion years ago The general view of physicists is that time started at a specific point about 13.8 billion years ago with the Big Bang, when the entire universe suddenly expanded out of an infinitely hot, infinitely dense singularity, a point where the laws of physics as we understand them simply break down. This can be considered the “birth” of the universe, and the beginning of time as we know it. Before the Big Bang, there just was no space or time, and you cannot go further back in time than the Big Bang, in much the same way as you cannot go any further north than the North Pole. As theoretical physicist Stephen Hawking notes in his 1988 book A Brief History of Time, even if time did not begin with the Big Bang, and there was another time frame before it, no information is available to us from that earlier time-frame, and any events that occurred then would have no effect on our present time-frame. Any putative events from before the Big Bang can therefore be considered effectively meaningless (or at least the province of philosophical speculation, not physics). Events after the Big Bang The universe is expanding, and all the galaxies are moving further and further away from each other. In fact, we now know that this expansion is accelerating faster and faster (largely as a result of the mysterious dark energy that pervades the universe). If we were to play the movie of this expansion in reverse, we would see the universe become smaller and small as we go back in time, until ultimately the matter and energy of the whole universe is concentrated into a microscopic point some 13.8 billion years ago. We can model this process remarkably closely (at least until the very early nanoseconds or less), and physicists have been able to piece together the major events in the evolution of universe, beginning with the tiniest fractions of a second after the Big Bang: •	Planck Epoch (the first 5.39 x 10-44 seconds after the Big Bang) – events (if any) occurring within this time must necessarily remain pure speculation. •	Grand Unification Epoch (10-43 to 10-36 seconds) – the force of gravity separates from the other fundamental forces, and the first elementary particles are created. •	Inflationary Epoch (10-36 to 10-32 seconds) – the universe undergoes an extremely rapid exponential expansion, known as cosmic inflation, and any existing particles become very thinly distributed. •	Electroweak Epoch (10-36 to 10-12 seconds) – the strong nuclear force separates from the other two forces (electromagnetism and gravity), and particle interactions create large numbers of exotic particles, including W and Z bosons and Higgs bosons. •	Quark Epoch (10-12 to 10-6 seconds) – the four fundamental forces assume their present forms, and quarks, electrons and neutrinos form in large numbers as the universe cools off to below 10 quadrillion degrees (although most quarks and antiquarks annihilate each other upon contact, a surplus of quarks survives, which will ultimately combine to form matter). •	Hadron Epoch (10-6 seconds to 1 second) – the universe cools to about a trillion degrees, allowing quarks to combine to form hadrons like protons and neutrons, and electrons colliding with protons fuse to form neutrons and give off massless neutrinos. •	Lepton Epoch (1 to 10 seconds) – most (but not all) hadrons and antihadrons annihilate each other, and leptons such as electrons and positrons dominate the mass of the universe. •	Nucleosynthesis (3 minutes to 20 minutes) – the temperature of the universe falls to about a billion degrees, so that atomic nuclei can begin to form as protons and neutrons fuse to form the nuclei of the simple elements of hydrogen, helium and lithium. •	Photon Epoch (10 seconds to about 240,000 years) – the universe is filled with plasma, a hot opaque soup of atomic nuclei and electrons, and the energy of the universe is dominated by photons, which continue to interact frequently with the charged protons, electrons and nuclei. •	Recombination/Decoupling (about 240,000 to 300,000 years) – the temperature of the universe falls to around 3,000 degrees, and ionized hydrogen and helium atoms capture electrons, neutralizing their electric charge and binding them within atoms; the universe finally becomes transparent to light, making this the earliest epoch potentially observable today. •	Dark Age or Era (about 300,000 to 150 million years) – the universe is literally dark, with no stars having formed to give off light; only very diffuse matter remains, and all activity tails off dramatically, with the universe dominated by mysterious “dark matter”. •	Reionization Epoch (about 150 million to about 1 billion years) – the first quasars form from gravitational collapse, and their intense radiation reionizes the surrounding universe, which goes from being neutral back to being composed of ionized plasma •	Star and Galaxy Formation (300 – 500 million years onwards) – small, dense clouds of cosmic gas start to collapse under their own gravity, until they trigger nuclear fusion reactions between hydrogen atoms and create the very first stars, which gradually cluster into galaxies. •	Solar System Formation (8.5 – 9 billion years after the Big Bang) – our Sun, a late-generation star incorporating the debris from generations of earlier stars, and the Solar System around it, form roughly 4.5 to 5 billion years ago. The Ultimate Fate of the Universe We can also model, with reasonable confidence, the ultimate fate of the Universe Our Sun is gradually getting larger, hotter and brighter, and the Earth will probably become uninhabitable within about a billion years from now. In about 5 billion years, our Sun is expected to turn into a red giant star, after which it will gradually shrink and cool into a small, dense white dwarf star, and ultimately into a dark, dead black dwarf star(in about 10 billion years from now). The rest of the universe, though, will continue its expansion and evolution. There are several possible scenarios in physics for the ultimate fate of the universe, depending on the universe’s overall shape or geometry (i.e. whether it is flat, open or closed), on how much dark energy it contains (dark energy is an invisible, hypothetical form of energy with repulsive anti-gravity that permeates all of space, and that may explain recent observations that the universe appears to be expanding at an accelerating rate), and on the so-called “equation of state” (which essentially determines how the density of the dark energy responds to the expansion of the universe). Further advances in fundamental physics may be required before we can make predictions about the future of the universe with any level of certainty, but we can still look at the possibilities. Without the repulsive effect of dark energy, the effects of gravity will eventually stop the expansion of the universe and it will start to contract until all the matter in the universe collapses to a final singularity, a mirror image of the Big Bang known as the “Big Crunch”. This also offers intriguing possibilities of an oscillating or cyclic universe, or “Big Bounce”, where the Big Crunch is succeeded by the Big Bang of a new universe, and so on, potentially ad infinitum, corresponding to a cyclic view of time. If the acceleration of the expansion of the universe caused by dark energy increases without limit, one hypothesis is that the dark energy could eventually becoming so strong that it completely overwhelms the effects of the gravitational, electromagnetic and weak nuclear forces. This would result in galaxies, stars and eventually even atoms themselves being literally torn apart, sometimes referred to as the “Big Rip”, with the universe as we know it ending dramatically in an unusual kind of gravitational singularity within the relatively short time horizon of just 35 – 50 billion years. Time under this model would therefore be finite, rather than cyclic or infinite, in nature. However, the most likely scenario, given our current knowledge of the constantly increasing effects of dark energy, is that the universe will continue expanding forever at an exponentially accelerating rate, ultimately turning space into an almost perfect vacuum as the remaining matter-energy becomes more and more diluted, a scenario sometimes referred to as “Heat Death” or the “Big Freeze“ or the “Big Chill”. Over a time scale of 1014 (a hundred trillion) years or more, the universe would reach a state of maximum entropy and thermal equilibrium at a temperature of very close to absolute zero, where it simply becomes too cold to sustain life or motion of any kind, and all that would remain are burned-out stars, cold dead planets and black holes. Eventually, after an almost unimaginable 10100 (a googol) years, even the black holes will have evaporated away, leaving nothing but random isolated particles floating in emptiness, with little or no prospect of ever interacting with other particles. The implication of this model is that, although time was finite in the past, it will be potentially infinite in the future, although in a scenario like this, where change is practically impossible, the very concept of time becomes effectively meaningless. The problem with an infinite, eternal universe is that even the most unlikely events will eventually occur (and not only occur, but occur an infinite number of times). In such a scenario, every event would theoretically be equally likely to happen, which effectively undermines the basis for all probabilistic predictions of local experiments. A solution to this problem, according to physicist Raphael Bousso and his collaborators, is to conclude that time WILL eventually end, and he has set about calculating the probability of how and when time will end given five different cut-off measures. Two of these scenarios resulted in time having a 50% chance of ending within 3.7 billion years; in two other scenarios, time has a 50% chance of ending within 3.3 billion years; in the fifth (much less likely) scenario, the time scale is very short and time is overwhelmingly likely to end within the next second. In this hypothetical situation, the end of time is envisioned as similar to an outside observer’s description of a matter system falling into a black hole: everything would gradually slow down and eventually just stop. Multiverse An alternative model of the universe sees it as just one of a potentially infinite number of other parallel universes in an overall multiverse (a word actually coined as long ago as 1895 by the American philosopher and psychologist William James). Such a scenario is actually thrown up by many different physical theories, including quantum mechanics, string theory, brane theory, etc, and is increasingly being seen as a real possibility and as a solution to many of the inconsistencies and inexplicabilities in our current theories. It has also been proposed as an explanation for how our universe appears to be so fine-tuned for life as we know it, by calling on the anthropic principle (the idea that the universe is only as it is because we are here to observe this particular version of it).

Parallel universes may physically exist within the same dimensional space as our own universe, but beyond our cosmological horizon; they may exist within black holes; they may exist in other inaccessible dimensions; they may exist very close to our own, or even locked inside or superimposed on it in other dimensions. Some of these parallel universes may even have completely different physical constants and physical laws to ours. By definition, though, we can only ever experience our own universe, and just do not have – and never will have – the ability to see or interact with (or, for that matter, prove the existence of) the rest of the multiverse, and so it remains necessarily hypothetical. This kind of universe of course also has implications for time, and it may be that what see perceive as time and the arrow of time is only a localized part of an overall concept of time.

Paradoxes of time travel
Time travel, I maintain, is possible. The paradoxes of time travel are oddities, not impossibilities. They prove only this much, which few would have doubted: that a possible world where time travel took place would be a most strange world, different in fundamental ways from the world we think is ours. I shall be concerned here with the sort of time travel that is recounted in science fiction. Not all science fiction writers are clear-headed, to be sure, and inconsistent time travel stories have often been written. But some writers have thought the problems through with great care, and their stories are perfectly consistent.1 If I can defend the consistency of some science fiction stories of time travel, then I suppose parallel defenses might be given of some controversial physical hypotheses, such as the hypothesis that time is circular or the hypothesis that there are particles that travel faster than light. But I shall not explore these parallels here. What is time travel? Inevitably, it involves a discrepancy between time and time. Any traveler departs and then arrives at his destination; the time elapsed from departure to arrival (positive, or perhaps zero) is the duration of the journey. But if he is a time traveler, the separation in time between departure and arrival does not equal the duration of his journey. He departs; he travels for an hour, let us say; then he arrives. The time he reaches is not the time one hour after his departure. It is later, if he has traveled toward the future; earlier, if he has traveled toward the past. If he has traveled far toward the past, it is earlier even than his departure. How can it be that the same two events, his departure and his arrival, are separated by two unequal amounts of time? It is tempting to reply that there must be two independent time dimensions; that for time travel to be possible, time must be not a line but a plane.2 Then a pair of events may have two unequal separations if they are separated more in one of the time dimensions than in the other. The lives of common people occupy straight diagonal lines across the plane of time, sloping at a rate of exactly one hour of time1 per hour of time 2. The life of the time traveler occupies a bent path, of varying slope. On closer inspection, however, this account seems not to give us time travel as we know it from the stories. When the traveler revisits the days of his childhood, will his playmates be there to meet him? No; he has not reached the part of the plane of time where they are. He is no longer separated from them along one of the two dimensions of time, but he is still separated from them along the other. I do not say that two-dimensional time is impossible, or that there is no way to square it with the usual conception of what time travel would be like. Nevertheless I shall say no more about two-dimensional time. Let us set it aside, and see how time travel is possible even in one-dimensional time. The world—the time traveler’s world, or ours—is a four-dimensional manifold of events. Time is one dimension of the four, like the spatial dimensions except that the prevailing laws of nature discriminate between time and the others—or rather, perhaps, between various timelike dimensions and various spacelike dimensions. (Time remains one-dimensional, since no two timelike dimensions are orthogonal.) Enduring things are timelike streaks: wholes composed of temporal parts, or stages, located at various times and places. Change is qualitative difference between different stages—different temporal parts—of some enduring thing, just as a “change” in scenery from east to west is a qualitative difference between the eastern and western spatial parts of the landscape. If this paper should change your mind about the possibility of time travel, there will be a difference of opinion between two different temporal parts of you, the stage that started reading and the subsequent stage that finishes. If change is qualitative difference between temporal parts of something, then what doesn’t have temporal parts can’t change. For instance, numbers can’t change; nor can the events of any moment of time, since they cannot be subdivided into dissimilar temporal parts. (We have set aside the case of two-dimensional time, and hence the possibility that an event might be momentary along one time dimension but divisible along the other.) It is essential to distinguish change from “Cambridge change,” which can befall anything. Even a number can “change” from being to not being the rate of exchange between pounds and dollars. Even a momentary event can “change” from being a year ago to being a year and a day ago, or from being forgotten to being remembered. But these are not genuine changes. Not just any old reversal in truth value of a time-sensitive sentence about something makes a change in the thing itself. A time traveler, like anyone else, is a streak through the manifold of space-time, a whole composed of stages located at various times and places. But he is not a streak like other streaks. If he travels toward the past he is a zig-zag streak, doubling back on himself. If he travels toward the future, he is a stretched-out streak. And if he travels either way instantaneously, so that there are no intermediate stages between the stage that departs and the stage that arrives and his journey has zero duration, then he is a broken streak. I asked how it could be that the same two events were separated by two unequal amounts of time, and I set aside the reply that time might have two independent dimensions. Instead I reply by distinguishing time itself, external time as I shall also call it, from the personal time of a particular time traveler: roughly, that which is measured by his wristwatch. His journey takes an hour of his personal time, let us say; his wristwatch reads an hour later at arrival than at departure. But the arrival is more than an hour after the departure in external time, if he travels toward the future; or the arrival is before the departure in external time (or less than an hour after), if he travels toward the past. That is only rough. I do not wish to define personal time operationally, making wristwatches infallible by definition. That which is measured by my own wristwatch often disagrees with external time, yet I am no time traveler; what my misregulated wristwatch measures is neither time itself nor my personal time. Instead of an operational definition, we need a functional definition of personal time; it is that which occupies a certain role in the pattern of events that comprise the time traveler’s life. If you take the stages of a common person, they manifest certain regularities with respect to external time. Properties change continuously as you go along, for the most part, and in familiar ways. First come in fantile stages. Last come senile ones. Memories accumulate. Food digests. Hair grows. Wristwatch hands move. If you take the stages of a time traveler instead, they do not manifest the common regularities with respect to external time. But there is one way to assign coordinates to the time traveler’s stages, and one way only (apart from the arbitrary choice of a zero point), so that the regularities that hold with respect to this assignment match those that commonly hold with respect to external time. With respect to the correct assignment properties change continuously as you go along, for the most part, and in familiar ways. First come infantile stages. Last come senile ones. Memories accumulate. Food digests. Hair grows. Wristwatch hands move. The assignment of coordinates that yields this match is the time traveler’s personal time. It isn’t really time, but it plays the role in his life that time plays in the life of a common person. It’s enough like time so that we can—with due caution— transplant our temporal vocabulary to it in discussing his affairs. We can say without contradiction, as the time traveler prepares to set out, “Soon he will be in the past.” We mean that a stage of him is slightly later in his personal time, but much earlier in external time, than the stage of him that is present as we say the sentence. We may assign locations in the time traveler’s personal time not only to his stages themselves but also to the events that go on around him. Soon Caesar will die, long ago; that is, a stage slightly later in the time traveler’s personal time than his present stage, but long ago in external time, is simultaneous with Caesar’s death. We could even extend the assignment of personal time to events that are not part of the time traveler’s life, and not simultaneous with any of his stages. If his funeral in ancient Egypt is separated from his death by three days of external time and his death is separated from his birth by three score years and ten of his personal time, then we may add the two intervals and say that his funeral follows his birth by three score years and ten and three days of extended personal time. Likewise a bystander might truly say, three years after the last departure of another famous time traveler, that “he may even now—if I may use the phrase—be wandering on some plesiosaurus-haunted oolitic coral reef, or beside the lonely saline seas of the Triassic Age.”3 If the time traveler does wander on an oolitic coral reef three years after his departure in his personal time, then it is no mistake to say with respect to his extended personal time that the wandering is taking place “even now”. We may liken intervals of external time to distances as the crow flies, and intervals of personal time to distances along a winding path. The time traveler’s life is like a mountain railway. The place two miles due east of here may also be nine miles down the line, in the westbound direction. Clearly we are not dealing here with two independent dimensions. Just as distance along the railway is not a fourth spatial dimension, so a time traveler’s personal time is not a second dimension of time. How far down the line some place is depends on its location in three-dimensional space, and likewise the locations of events in personal time depend on their locations in one-dimensional external time. Five miles down the line from here is a place where the line goes under a trestle; two miles further is a place where the line goes over a trestle; these places are one and the same. The trestle by which the line crosses over itself has two different locations along the line, five miles down from here and also seven. In the same way, an event in a time traveler’s life may have more than one location in his personal time. If he doubles back toward the past, but not too far, he may be able to talk to himself. The conversation involves two of his stages, separated in his personal time but simultaneous in external time. The location of the conversation in personal time should be the location of the stage involved in it. But there are two such stages; to share the locations of both, the conversation must be assigned two different locations in personal time. The more we extend the assignment of personal time outwards from the time traveler’s stages to the surrounding events, the more will such events acquire multiple locations. It may happen also, as we have already seen, that events that are not simultaneous in external time will be assigned the same location in personal time—or rather, that at least one of the locations of one will be the same as at least one of the locations of the other. So extension must not be carried too far, lest the location of events in extended personal time lose its utility as a means of keeping track of their roles in the time traveler’s history. A time traveler who talks to himself, on the telephone perhaps, looks for all the world like two different people talking to each other. It isn’t quite right to say that the whole of him is in two places at once, since neither of the two stages involved in the conversation is the whole of him, or even the whole of the part of him that is located at the (external) time of the conversation. What’s true is that he, unlike the rest of us, has two different complete stages located at the same time at different places. What reason have I, then, to regard him as one person and not two? What unites his stages, including the simultaneous ones, into a single person? The problem of personal identity is especially acute if he is the sort of time traveler whose journeys are instantaneous, a broken streak consisting of several unconnected segments. Then the natural way to regard him as more than one person is to take each segment as a different person. No one of them is a time traveler, and the peculiarity of the situation comes to this: all but one of these several people vanish into thin air, all but another one appear out of thin air, and there are remarkable resemblances between one at his appearance and another at his vanishing. Why isn’t that at least as good a description as the one I gave, on which the several segments are all parts of one time traveler? I answer that what unites the stages (or segments) of a time traveler is the same sort of mental, or mostly mental, continuity and connectedness that unites anyone else. The only difference is that whereas a common person is connected and continuous with respect to external time, the time traveler is connected and continuous only with respect to his own personal time. Taking the stages in order, mental (and bodily) change is mostly gradual rather than sudden, and at no point is there sudden change in too many different respects all at once. (We can include position in external time among the respects we keep track of, if we like. It may change discontinuously with respect to personal time if not too much else changes discontinuously along with it.) Moreover, there is not too much change altogether. Plenty of traits and traces last a lifetime. Finally, the connectedness and the continuity are not accidental. They are explicable; and further, they are explained by the fact that the properties of each stage depend causally on those of the stages just before in personal time, the dependence being such as tends to keep things the same.4 To see the purpose of my final requirement of causal continuity, let us see how it excludes a case of counterfeit time travel. Fred was created out of thin air, as if in the midst of life; he lived a while, then died. He was created by a demon, and the demon had chosen at random what Fred was to be like at the moment of his creation. Much later someone else, Sam, came to resemble Fred as he was when first created. At the very moment when the resemblance became perfect, the demon destroyed Sam. Fred and Sam together are very much like a single person: a time traveler whose personal time starts at Sam’s birth, goes on to Sam’s destruction and Fred’s creation, and goes on from there to Fred’s death. Taken in this order, the stages of Fred-cum-Sam have the proper connectedness and continuity. But they lack causal continuity, so Fred-cum-Sam is not one person and not a time traveler. Perhaps it was pure coincidence that Fred at his creation and Sam at his destruction were exactly alike; then the connectedness and continuity of Fred-cum-Sam across the crucial point are accidental. Perhaps instead the demon remembered what Fred was like, guided Sam toward perfect resemblance, watched his progress, and destroyed him at the right moment. Then the connectedness and continuity of Fred-cum-Sam has a causal explanation, but of the wrong sort. Either way, Fred’s first stages do not depend causally for their properties on Sam’s last stages. So the case of Fred and Sam is rightly disqualified as a case of personal identity and as a case of time travel. We might expect that when a time traveler visits the past there will be reversals of causation. You may punch his face before he leaves, causing his eye to blacken centuries ago. Indeed, travel into the past necessarily involves reversed causation. For time travel requires personal identity—he who arrives must be the same person who departed. That requires causal continuity, in which causation runs from earlier to later stages in the order of personal time. But the orders of personal and external time disagree at some point, and there we have causation that runs from later to earlier stages in the order of external time. Elsewhere I have given an analysis of causation in terms of chains of counterfactual dependence, and I took care that my analysis would not rule out casual reversal a priori.5 I think I can argue (but not here) that under my analysis the direction of counterfactual dependence and causation is governed by the direction of other de facto asymmetries of time. If so, then reversed causation and time travel are not excluded altogether, but can occur only where there are local exceptions to these asymmetries. As I said at the outset, the time traveler’s world would be a most strange one. Stranger still, if there are local—but only local—causal reversals, then there may also be causal loops: closed causal chains in which some of the causal links are normal in direction and others are reversed. (Perhaps there must be loops if there is reversal: I am not sure.) Each event on the loop has a causal explanation, being caused by events elsewhere on the loop. That is not to say that the loop as a whole is caused or explicable. It may not be. Its inexplicability is especially remarkable if it is made up of the sort of causal processes that transmit information. Recall the time traveler who talked to himself. He talked to himself about time travel, and in the course of the conversation his older self told his younger self how to build a time machine. That information was available in no other way. His older self knew how because his younger self had been told and the information had been preserved by the causal processes that constitute recording, storage, and retrieval of memory traces. His younger self knew, after the conversation, because his older self had known and the information had been preserved by the causal processes that constitute telling. But where did the information come from in the first place? Why did the whole affair happen? There is simply no answer. The parts of the loop are explicable, the whole of it is not. Strange! But not impossible, and not too different from inexplicabilities we are already inured to. Almost everyone agrees that God, or the Big Bang, or the entire infinite past of the universe, or the decay of a tritium atom, is uncaused and inexplicable. Then if these are possible, why not also the inexplicable causal loops that arise in the time travel? I have committed a circularity in order not to talk about too much at once, and this is a good place to set it right. In explaining personal time, I presupposed that we were entitled to regard certain stages as comprising a single person. Then in explaining what united the stages into a single person, I presupposed that we were given a personal time order for them. The proper way to proceed is to define personhood and personal time simultaneously, as follows. Suppose given a pair of an aggregate of persona-stages, regarded as a candidate for personhood, and an assignment of coordinates to those stages, regarded as a candidate for his personal time. If the stages satisfy the conditions given in my circular explanation with respect to the assignment of coordinates, then both candidates succeed: the stages do comprise a person and the assignment is his personal time. I have argued so far that what goes on in a time travel story may be a possible pattern of events in four-dimensional space-time with no extra time dimension; that it may be correct to regard the scattered stages of the alleged time traveler as comprising a single person; and that we may legitimately assign to those stages and their surroundings a personal time order that disagrees sometimes with their order in external time. Some might concede all this, but protest that the impossibility of time travel is revealed after all when we ask not what the time traveler does, but what he could do. Could a time traveler change the past? It seems not: the events of a past moment could no more change than numbers could. Yet it seems that he would be as able as anyone to do things that would change the past if he did them. If a time traveler visiting the past both could and couldn’t do something that would change it, then there cannot possibly be such a time traveler. Consider Tim. He detests his grandfather, whose success in the munitions trade built the family fortune that paid for Tim’s time machine. Tim would like nothing so much as to kill Grandfather, but alas he is too late. Grandfather died in his bed in 1957, while Tim was a young boy. But when Tim has built his time machine and traveled to 1920, suddenly he realizes that he is not too late after all. He buys a rifle; he spends long hours in target practice; he shadows Grandfather to learn the route of his daily walk to the munitions works; he rents a room along the route; and there he lurks, one winter day in 1921, rifle loaded, hate in his heart, as Grandfather walks closer, closer,. . . . Tim can kill Grandfather. He has what it takes. Conditions are perfect in every way: the best rifle money could buy, Grandfather an easy target only twenty yards away, not a breeze, door securely locked against intruders. Tim a good shot to begin with and now at the peak of training, and so on. What’s to stop him? The forces of logic will not stay his hand! No powerful chaperone stands by to defend the past from interference. (To imagine such a chaperone, as some authors do, is a boring evasion, not needed to make Tim’s story consistent.) In short, Tim is as much able to kill Grandfather as anyone ever is to kill anyone. Suppose that down the street another sniper, Tom, lurks waiting for another victim, Grandfather’s partner. Tom is not a time traveler, but otherwise he is just like Tim: same make of rifle, same murderous intent, same everything. We can even suppose that Tom, like Tim, believes himself to be a time traveler. Someone has gone to a lot of trouble to deceive Tom into thinking so. There’s no doubt that Tom can kill his victim; and Tim has everything going for him that Tom does. By any ordinary standards of ability, Tim can kill Grandfather. Tim cannot kill Grandfather. Grandfather lived, so to kill him would be to change the past. But the events of a past moment are not subdivisible into temporal parts and therefore cannot change. Either the events of 1921 timelessly do include Tim’s killing of Grandfather, or else they timelessly don’t. We may be tempted to speak of the “original” 1921 that lies in Tim’s personal past, many years before his birth, in which Grandfather lived; and of the “new” 1921 in which Tim now finds himself waiting in ambush to kill Grandfather. But if we do speak so, we merely confer two names on one thing. The events of 1921 are doubly located in Tim’s (extended) personal time, like the trestle on the railway, but the “original” 1921 and the “new” 1921 are one and the same. If Tim did not kill Grandfather in the “original” 1921, then if he does kill Grandfather in the “new” 1921, he must both kill and not kill Grandfather in 1921—in the one and only 1921, which is both the “new” and the “original” 1921. It is logically impossible that Tim should change the past by killing Grandfather in 1921. So Tim cannot kill Grandfather. Not that past moments are special; no more can anyone change the present or the future. Present and future momentary events no more have temporal parts than past ones do. You cannot change a present or future event from what it was originally to what it is after you change it. What you can do is to change the present or the future from the unactualized way they would have been without some action of yours to the way they actually are. But that is not an actual change: not a difference between two successive actualities. And Tim can certainly do as much; he changes the past from the unactualized way it would have been without him to the one and only way it actually is. To “change” the past in this way, Tim need not do anything momentous; it is enough just to be there, however unobtrusively. You know, of course, roughly how the story of Tim must go on if it is to be consistent: he somehow fails. Since Tim didn’t kill Grandfather in the “original” 1921, consistency demands that neither does he kill Grandfather in the “new” 1921. Why not? For some commonplace reason. Perhaps some noise distracts him at the last moment, perhaps he misses despite all his target practice, perhaps his nerve fails, perhaps he even feels a pang of unaccustomed mercy. His failure by no means proves that he was not really able to kill Grandfather. We often try and fail to do what we are able to do. Success at some tasks requires not only ability but also luck, and lack of luck is not a temporary lack of ability. Suppose our other sniper, Tom, fails to kill Grandfather’s partner for the same reason, whatever it is, that Tim fails to kill Grandfather. It does not follow that Tom was unable to. No more does it follow in Tim’s case that he was unable to do what he did not succeed in doing. We have this seeming contradiction: “Tim doesn’t, but can, because he has what it takes” versus “Tim doesn’t, and can’t, because it’s logically impossible to change the past.” I reply that there is no contradiction. Both conclusions are true, and for the reasons given. They are compatible because “can” is equivocal. To say that something can happen means that its happening is compossible with certain facts. Which facts? That is determined, but sometimes not determined well enough, by context. An ape can’t speak a human language—say, Finnish—but I can. Facts about the anatomy and operation of the ape’s larynx and nervous system are not compossible with his speaking Finnish. The corresponding facts about my larynx and nervous system are compossible with my speaking Finnish. But don’t take me along to Helsinki as your interpreter: I can’t speak Finnish. My speaking Finnish is compossible with the facts considered so far, but not with further facts about my lack of training. What I can do, relative to one set of facts, I cannot do, relative to another, more inclusive, set. Whenever the context leaves it open which facts are to count as relevant, it is possible to equivocate about whether I can speak Finnish. It is likewise possible to equivocate about whether it is possible for me to speak Finnish, or whether I am able to, or whether I have the ability or capacity or power or potentiality to. Our many words for much the same thing are little help since they do not seem to correspond to different fixed delineations of the relevant facts. Tim’s killing Grandfather that day in 1921 is compossible with a fairly rich set of facts: the facts about his rifle, his skill and training, the unobstructed line of fire, the locked door and the absence of any chaperone to defend the past, and so on. Indeed it is compossible with all the facts of the sorts we would ordinarily count as relevant is saying what someone can do. It is compossible with all the facts corresponding to those we deem relevant in Tom’s case. Relative to these facts, Tim can kill Grandfather. But his killing Grandfather is not compossible with another, more inclusive set of facts. There is the simple fact that Grandfather was not killed. Also there are various other facts about Grandfather’s doings after 1921 and their effects: Grandfather begat Father in 1922 and Father begat Tim in 1949. Relative to these facts, Tim cannot kill Grandfather. He can and he can’t, but under different delineations of the relevant facts. You can reasonably choose the narrower delineation, and say that he can; or the wider delineation, and say that he can’t. But choose. What you mustn’t do is waver, say in the same breath that he both can and can’t, and then claim that this contradiction proves that time travel is impossible. Exactly the same goes for Tom’s parallel failure. For Tom to kill Grandfather’s partner also is compossible with all facts of the sorts we ordinarily count as relevant, but not compossible with a larger set including, for instance, the fact that the intended victim lived until 1934. In Tom’s case we are not puzzled. We say without hesitation that he can do it, because we see at once that the facts that are not compossible with his success are facts about the future of the time in question and therefore not the sort of facts we count as relevant in saying what Tom can do. In Tim’s case it is harder to keep track of which facts are relevant. We are accustomed to exclude facts about the future of the time in question, but to include some facts about its past. Our standards do not apply unequivocally to the crucial facts in this special case: Tim’s failure, Grandfather’s survival, and his subsequent doings. If we have foremost in mind that they lie in the external future of that moment in 1921 when Tim is almost ready to shoot, then we exclude them just as we exclude the parallel facts in Tom’s case. But if we have foremost in mind that they precede that moment in Tim’s extended personal time, then we tend to include them. To make the latter be foremost in your mind, I chose to tell Tim’s story in the order of his personal time, rather than in the order of external time. The fact of Grandfather’s survival until 1957 had already been told before I got to the part of the story about Tim lurking in ambush to kill him in 1921. We must decide, if we can, whether to treat these personally past and externally future facts as if they were straightforwardly past or as if they were straightforwardly future. Fatalists—the best of them—are philosophers who take facts we count as irrelevant in saying what someone can do, disguise them somehow as facts of a different sort that we count as relevant, and thereby argue that we can do less than we think—indeed, that there is nothing at all that we don’t do but can. I am not going to vote Republican next fall. The fatalist argues that, strange to say, I not only won’t but can’t; for my voting Republican is not compossible with the fact that it was true already in the year 1548 that I was not going to vote Republican 428 years later. My rejoinder is that this is a fact, sure enough; however, it is an irrelevant fact about the future masquerading as a relevant fact about the past, and so should be left out of account in saying what, in any ordinary sense, I can do. We are unlikely to be fooled by the fatalist’s methods of disguise in this case, or other ordinary cases. But in cases of time travel, precognition, or the like, we’re on less familiar ground, so it may take less of a disguise to fool us. Also, new methods of disguise are available, thanks to the device of personal time. Ever since Albert Einstein published in1905 his Theory of Relativity, the fact than nothing can travel faster than light has become an essential part of our view of the Universe. Light travels at 300,000 kilometers per second, that is, about a billion kilometers per hour. In everyday life this speed is so huge that it seems almost infinite. But distances in space are HUGE! ... Light takes 8 minutes to reach us from the Sun. ... 4 years from the closest star. ... 2 million years from the Andromeda Galaxy (in our cosmic backyard) ... And over 10 billion years from the most distant galaxies known. Even within our solar system, the limit of the speed of light implies serious restrictions to communications: the instructions sent to interplanetary probes  must be send sometimes hours in advance. If we ever want to reach the stars, the speed limit imposed by light represents a big problem! The simple answer is that the theory of relativity forbids it. There are two specific reasons why this is so: I)  The “mass” of an object (or rather its “inertia”) increases with speed and in fact becomes infinite at precisely the speed of light. II)  The second reason is more complex, but much more fundamental, and is related to concept of “causality”. It turns out that travelling faster than light violates causality, that is, effects can come before causes, with is of course absurd. The concept of “relativity” goes back to Galileo, and comes from the fact that movement is always “relative to something”. In other words, there is no “absolute motion”, jus as there is no “absolute rest”. (Aristotle would not have agreed) An observer inside a closed room with no windows, somewhere inside a ship in a calm sea, can not tell if the ship is moving with respect to distant land or if it isn’t. No physical experiment can measure “absolute speed”. The results of all experiments are independent of the speed of the observer.

Einstein’s postulates
In 1905, Einstein tries to solve some puzzles related to the propagation of electromagnetic waves (light) and takes two basic postulates: 1. The principle of relativity (Galileo): The laws of physics are independent of the speed of the speed of the observer. 2. The speed of light is ABSOLUTE, independent of the speed of the source, or of the observer. It is a universal constant. All the theory of “special relativity” can be derived from these two postulates. The second postulate is crucial. It implies that the Galilean law of addition of velocities is wrong! This is because is one of the two velocities is the speed of light, then the sum must also be equal to the speed of light, regardless of the value of the other one. Einstein’s two postulates have a series of consequences that turn upside down our concepts of space, time, mass and energy: 1. The flow of time is relative! 2.  Distances are relative! 3.  Simultaneity is relative! 4.  Mass and energy are equivalent! 5. The mass (inertia) of an object increases with its speed! The mass of an object increases with speed. This is because kinetic energy is equivalent to mass, and so it adds to the objects inertia. At the speed of light the mass becomes infinite! It is clearly impossible to push an object with infinite mass any faster, which implies that we can not go faster than light.

M = m01−v2/c2
Light signales define a “cone” in space-time centered on a given event: the “light cone”. Observers that move relative to one another (O and O‘) do not agree on the simultaneity of events, nor on the time ordering of some events. ... But the always agree on where is the light cone of a given event. A: I sent a letter to my friend yesterday. B: My friend received the letter today. A: I woke up in Mexico at 8:00 (local time) B: My friend had a late lunch at 3:00 pm (local time) A and B happend at the same time No, A came before B! No, no, no, no, B came before A! In relativity, causality is defined by the light cones: •  Different observers DO NOT AGREE on the time order of events that are outside each others light-cones. •  But the ALWAYS AGREE on the time order of events inside each others light-cones. This means that, if an object moves faster than light (outside the lightcone), then they will be perfectly reasonable observers that will see such an object arrive at its destination before it left! Travelling faster than light violates causality! For centuries we have known that it is possible to move faster than sound (cannon balls do it all the time). Flying faster than sound was a technological problem, never a physical problem: it is very hard to make an airplane than can fly faster than sound in a stable way. The light barrier is completely different, it is not a technological problem but a physical problem. It is forbidden by the laws of physics. As it turns out, one can use the second great contribution of Einstein to eliminate part of the problems with faster than light travel. This second great contribution is the theory of “General Relativity”. General relativity is in fact our modern theory of gravity. It replaces Newton’s old law of “Universal Gravitation”.

Principle of equivalence (Einstein, 1907):
•  Galileo: All objects fall with the same acceleration in a gravitational field. •  Newton: The “inertial” and “gravitational” mass of an object are the same. •  Einstein: The laws of physics in free fall are identical to those of special relativity. In other words, in free fall the “force of gravity” vanishes. “I had the happiest thought of my life …”             Albert Einstein,  The principle of equivalence implies that the trajectory of an object in a gravitational field is a property of “space”, sine it is the same for any object (this is very different to what happens with electric and magnetic fields). If objects follow curved paths it must be because space itself is “curved”. According to General Relativity (GR), space and time are not fixed: The geometry of space and the floe of time can be altered by the presence of large concentrations of mass-energy (gravity). In other words, gravity is equivalent to the “curvature” of space-time. In flat space, parallel lines remain parallel, lines that cross keep always the same angle and the angles inside a triangle always add to 180 degrees. In a curved space none of these statements are true any more. Also, in a flat space, if we transport a vector around a closed path it comes back the same. In a curved space it does not. If we accept that the geometry of space can be altered, then there are several ways one can think off that will allow us to cheat and move faster than light. One idea is to simply make a short “tunnel” that connects two distant regions of space. Such tunnels are known an “Einstein-Rosen” bridge, or less technically as “wormholes”. When we go through a wormhole we move a very short distance but find ourselves suddenly very far from where we started. But, are wormholes allowed in GR? Actually, yes!, The first solution ever found (the Schwarzschild solution of 1916), that describes a spherical black-hole in fact also contains a wormhole. This was discovered by Einstein and Rosen 1935 (but beware, a black-hole and a wormhole are different things). But the wormhole inside a Schwarzschild black-hole is quite useless because: 1. It is dynamical, and closes in on itself so fast that not even light can get through. 2. It actually leads to a separate “parallel” Universe, and not to another region of our Universe. We have no idea how to make one. In the 1980´s, Carl Sagan was writing a science fiction novel (“Contact”). He needed his characters to travel to a distance star faster than light. But as a scientist he couldn’t say “they just did”, he needed something better. He decided to ask his friend Kip Thorne. Kip, together with his student Michael Morris, studied under what conditions one could have “traversable” wormholes in GR. Wormholes have been used for decades in science-fiction, sometimes under different names: “portals”, “stargates”. Some authors imagine doorways that take us to the stars in a single step (effectively a door-sized, and extremely, short, wormhole). Another idea for travelling faster than light using GR is known as the “warp drive” spacetime (or warp bubble). This was suggested by Alcubierre (i.e. myself) in 1994. With this method, one does not move “through space”, but rather “with space” (similar to a travelator in an airport). For a “warp drive” we would need to create a violent localized expansion of space behind us, and an opposite contraction of space in front of The spaceship would sit inside a “bubble” of undisturbed flat space, and would fill no acceleration whatsoever! The idea behind a warp drive comes from the expansion of the Universe: distant galaxies move away from each other. The further they are, the faster they move. In fact, very distant galaxies move away from each other faster than light. This DOES NOT contradict relativity, since in reality it is not the galaxies moving, but rather the space in between expanding! Both wormholes and warp drives have a serious drawback: in order to curve space in the desired way they require “negative energy”, that is a form of mass-energy that produces a gravitational repulsion instead of an attraction, or in other words, antigravity. And the require huge, star-sized quantities of it. We have never found antigravity in nature. But as far as why know it is not forbidden! Better find it by next week, or the next episode will be rather boring ... Until recently, any form of antigravity was pure speculation. But today this has changed! The study of supernovae at cosmological distances has shown without a doubt that the expansion of the Universe is in fact accelerating. This can only be understood if we have some energy field that produces some form of antigravity: the so-called “dark energy”. And what is dark energy? We don’t know yet, it is one of the big mysteries of modern physics. 1. Logical inconsistencies or the grandfather paradox: I travel 70 years to the past and kill my grand father before he has any children. But in that case I was never born, so how did I travel to the past? 2. Strange loops or, where is the watchmaker?: An old man gives me a nice gold watch. I travel to the past and meet a kid and like him so much that I give the watch to him. The child grow to be be the old man that gave me the watch. Who made the watch? But beware: in relativity, any method to travel faster than light can in principle be used to travel back in time (a time machine). ¡Horror! Physicist hate time travel to the past because it brings with it a series of nasty paradoxes. 3. Information from nothing or, who invented that?: I travel to the past and meet and old Beethoven, almost deaf, tired and unwilling to write music. I hum to him the “Ode to Joy”. He loves it and composes his 9th symphony around. Who composed the Ode? Conjectures	   Time travel paradoxes are so nasty that they make most physisists believe that time travel is impossible. Stephen Hawking has even postulated his “chronology protection conjecture” that essentially states that the laws of physics prohibit time travel to the past. The conjecture has not been proven (it wouldn’t be a conjecture it it had), but there are good arguments in its favor based on quantum field theory. Notice that the conjecture does not prohibit faster than light travel. It just states that is a method to travel faster than light exists, and one tries to use it to build a time machine, something will go wrong: the energy accumulated will explode, or it will create a black hole.

Conclusions
So, can we travel faster than light? • It is forbidden in special relativity. •  General relativity seems to allow it by creating some particular distortions of the geometry of space. But they all seem to require antigravity, which has not been discovered (but cosmology hints to some form of antigravity in the dark energy). • And if it is possible, we must confront the problem of time travel to the past and the causality problems is causes. We do not have the final answer ... But we have discovered that the question can indeed be asked in a meaningful way. And also than not all the doors are closed. Time will tell ...