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Quantum physics is the science of the very small- for example, it is used to describe the behaviour of electrons and photons. This is opposed to classical physics which deals with sizes more familiar with human experience, including astronomical bodies (planets, galaxies etc).

The word 'quantum' in this sense means the 'minimum amount of any physical entity involved in an interaction' and is fundamental to the field- it means that certain characteristics of matter can take only discrete values. For example, the energy of an electron is limited to certain specific values- electron energy is said to be 'quantised'.

Some aspects of quantum mechanics can seem counter-intuitive or even paradoxical, because they describe behaviour quite different than that seen at larger length scales, where classical physics is an excellent approximation. In the words of Richard Feynman, quantum mechanics deals with "nature as She is – absurd".

Wave particle duality: the two slit experiment


At the heart of quantum mechanics is the idea that many types of energy behave in some respects like waves and in other respects like particles. This is best illustrated by the two-slit experiment.

Imagine a laser beam firing a continuous stream of photons at a metal sheet with a small slit. We would expect a single slit of light to appear on a screen, and indeed that is what we see.

Likewise, if we add a second slit right next to the first, we would expect two slits of light. However that is not what is observed. What is observed is a series of high and low intensity areas- also known as an interference pattern. This is due to the wave like nature of light. A wave of light exits from each slit, yet they interfere with each other- sometimes two peaks or troughs combine resulting in a high intensity region on the screen; sometimes a peak and trough combine and cancel each other out, causing the observed low intensity areas.

Now, what happens if we slow down the rate of fire from the laser such that only one photon leaves at a time? We may naively expect that this time, if we left the experiment running long enough, we would observe two slits on the screen. However, we still see an interference pattern.

What happens is therefore that each photon travels as a wave, and thus interferes with itself. Yet when it hits the screen at the end, we only see a single point of light- it has thus 'collapsed' into a particle. Thus, light has the properties of both particles and waves.

Probability waves and wave function collapse
Unlike with water or sound waves, the waves of quantum physics do not correspond to the actual presence of matter. Rather, they correspond to the probability that a particle will be found at a certain location if we look for it at that location. In the two slit experiment, a peak would correspond to a high probability that a photon would be found at that location.

It is important to note that these waves do not simply represent science's understanding of where the photon (for example) is, and that in actuality the photon is only ever at one place at a time. The photon truly is, until we measure it, everywhere at once. This is known as quantum superposition. Schrödinger's cat is a well known thought experiment used to illustrate this phenomenon.

Theoretically, these probability waves exist throughout the entire universe. That is, if the two slit experiment above is carried out in London, there is a finite probability that the photon would be detected in the Andromeda galaxy only a moment later. This applies to 'classical' objects too. A car is made up of nothing but atoms, each of which obeys the laws of quantum mechanics. There is a finite chance that a car in London could disappear and re-appear on Mars a second later (of course we would expect to wait many lifetimes of the universe to observe this, which is why we don't see it). Note that the 'movement' of objects in this way is not constrained by the speed of light- for more on this see quantum entanglement.

As noted in the two slit experiment, when we observe such waveforms (for example, place a screen to detect the location of the photon), the energy 'collapses' into a particle. If we observe an electron at a certain point, we can say that it is there with 100% certainty, thus it takes on the properties of a particle and ceases to be a wave. This is known as wave function collapse. All attempts to measure the energy result in wave function collapse.

This leads to some strange results. In the two-slit experiment, if we place a measuring device next to each slit to observe which one each electron goes through, the interference pattern is no longer observed, we instead see two strips of light. This is due to the waveforms 'collapsing' at the slits, thus they are no longer able to interfere with each other.

The process by which waveforms collapse is still not fully understood and remains an active area of research.

Quantum entanglement
Quantum entanglement describes the phenomenon of two separate particles becoming 'entangled', and which can then influence each other instantly regardless of the distance between them. Einstein called this "spooky action at a distance".

Experiments have been performed where two electrons have become entangled (the details of which are beyond the scope of this article) and then fired off in opposite directions. We then choose a property of these electrons to measure, for example its spin. The spin of an electron can only be either clockwise or anti-clockwise. Until we measure the spin of each electron, there is precisely a 50% chance of it being found spinning in one direction or the other.

However, much like with the two-slit experiment, the electrons are said to be in a 'superposition' of both states until we measure them. It is not the case that the electrons are spinning one way or the other prior to measurement.

When we do measure the spin of each particle, we always find that it is spinning one way or the other. What is interesting about this phenomenon however is that as soon as we measure one of the electrons, the wavefunction of the other electron will instantly collapse and, if measured, will always be found spinning in the same direction as its entangled twin (or always spinning in the opposite direction- the spins are said to be 100% correlated such that one can be predicted with 100% certainty if the other is known).

For example, if electron A is measured and found to be spinning clockwise, electron B will be found to be spinning in the same direction when it's measured. Experiments have proved mathematically that electron B was not always spinning clockwise, it took on this property only when electron A was measured.

The above is true even if the two electrons are light years apart. Measure one, and you will instantly cause the other to collapse into one spin or the other, even though it would take light many years to travel between them. This is known as the non locality of quantum physics and is one of the fundamental differences between quantum and classical physics.

Most scientists do not, however, consider this experiment to breach special relativity which (broadly) states that information can never travel faster than the speed of light. This is because it is impossible to use quantum entanglement to send information from one place to another instantly. The first electron's spin, when measured, is always randomly clockwise or anti-clockwise. It is this randomness that prevents information being sent at faster than the speed of light.

The Theory of Everything
Quantum physics describes the behaviour of the very small, whereas classical physics describes the behaviour of everything else. However, large bodies such as planets are made of nothing but lots of very small particles- therefore, in theory, we should be able to use both quantum and classical physics (specifically, general relativity) to describe its behaviour- that is, the two laws should be compatible with each other. A combined theory of both quantum and classical physics would be able to describe the behaviour of anything in the universe, and has been termed The Theory of Everything.

Unfortunately, such a theory does not yet exist, because the laws of quantum physics break down and simply make no sense when applied to larger bodies. Attempts to find a Theory of Everything are thus an active area of physics research (string theory), with many physicists considering it to be the single largest unsolved problem in physics today.