User:ManishGuptaK/sandbox

Why time travel is not possible

Einstein’s special theory of relativity explained that space and time are not separate entities but interwoven into a single spacetime continuum and further explained that faster you travel through the space, the time dilates and moves slowly for you. This ultimately explained why the speed of light is constant regardless of the frame of reference of the observer. Einstein's general theory of relativity explained that mass creates distortion in spacetime and its effect is observed as gravity. It further explained how mass effect the space around it which in turn affects the movement of the mass. Einstein did not perform any scientific experiments to come up with these theories. He came up with these theories purely based on thought experiments and this is where his genius lied.

Over time, Einstein’s theory of relativity has been proven correct with experiments and observed phenomenons. Some of the predictions made by the theory of relativity have also been proven correct such as gravitational lensing and gravitational redshift. Most recently scientists detected gravitational waves caused by events such as the collision of two black holes as predicted by Einstein.

Before we talk about whether time travel is possible or not, let's first understand the concept of ‘now’ so that we understand what past and future means relative to 'now'. The ‘now’ is defined by a point in spacetime where you are at present. Remember that space and time are part of the same continuum and cannot be viewed separately. Traveling into your past means to be at the exact point in spacetime where you were from where you are now. On the same note, travel into your future means to be at the exact same point in spacetime where you will be from where you are now. We also need to understand that the concept of ‘now’ is very local and personal since what is ‘now’ for me is not the same as what is ‘now’ for you even if we are standing next to each other. If you look at the stars and galaxies, you are not seeing how they look now but how they looked when the photon that is reaching your eye just now started its journey from the star possibly hundreds of thousands of years ago. On the same token, even when we look at somebody who is sitting in the same room as us, we are not sharing the same ‘now’ moment. We are looking at each other in their past however minuscule that is because light will take some time to travel that distance between the two.

Let's also try to understand conceptually what time travel means. In order to be able to go back and forth in space and time, every point in spacetime that defines the past, present and future must co-exist. In other words the future state must already be determined and occurred and frozen so that we can get to it when we want to and on the same token all past states must exist simultaneously so we can get to it when we want to. And that does create the dilemma since I can go back in time at will and change the event in the past that will change the future. Which means there must be an infinite number of combinations of events in past and future that must co-exists in parallel and if does then the question arises which future or past event am I going to go if I decide to travel in the future.

Now let’s try to understand if all of this chaos, that needs to exist for spacetime travel, is possible at all. We travel through spacetime continuum at the fixed speed and that is the speed of light. This explains the concept of time dilation. The faster you move through the space, the slower the time will flow for you. If we move at the speed of light, the time will cease to exist. On the other hand if you stop moving at all in space, the time will move at the speed of light. In other words, your speed through time can be from zero to the speed of light, nothing less that zero and nothing more than the speed of light. To understand this better, imagine in a two dimensional graph, x-axis represents your movement through space and y-axis represents your movement through time. Now if your speed on this graph is fixed that is the speed of light then faster you travel along the x-axis (space), the slower you will move along the y-axis (time) and vice versa. If you travel along the x-axis (space) at the speed of light which is the maximum speed you can travel with then your speed along the y-axis will be zero and similarly if stop moving along  x-axis (space), i.e., you have no motion in space then your speed across y-axis (time) will  be the speed of light. This can be further demonstrated with the example below

In the x-y coordinates of space and time, you can only move in any direction along the x-axis (space), however you can only move in upward direction along the y-axis (time) with the speed anywhere between 0 and speed of light. In the example above, during 1, since you are traveling through spacetime continuum at the speed of light and you have a movement along the x-axis (space), you will experience time dilation. During 2, you are entire motion is along space and you are moving through the speed of light. As a result, you do not have any movement through time and time has ceased to exist for you (this is an example of a photon. Any object with mass cannot travel at the speed of light as the mass becomes infinite at the speed of light). In 3, you have zero movement along the space and hence your entire movement is now along the time. During this time you are moving through time at the speed of light and you are not experiencing any time dilation. Now you decide to come back to the same point in space where you were at the end of 1. This is represented by 4. Notice that even though you are back to the same point in space, in spacetime continuum you are not since the time has elapsed for you.

In order to time travel, you need to be able to come back to a specific x,y coordinate or go to a specific future x,y coordinate at will. In order to go to an x,y coordinate that you will be in the future, you will have to move through the spacetime continuum at a speed faster than light to be able to overtake the normal course of nature and this is now allowed. similarly to move back to any x,y coordinate that is in the past, you will have to move through spacetime continuum at a negative speed that again is not possible. And hence time travel is not possible.

Twin Paradox explained: Scientists have argued that travel into the future is already possible. For example take two twin Mary and Johnny on earth. Johnny goes on a spaceship at extremely high speed and comes back to earth after years and notices that Mary who stayed back on earth has aged a lot more than him. This is because Johnny’s spaceship was moving at a very high speed and experienced time dilation and hence will aged slowly relative to Mary. And since Johnny is younger than Mary now, it seems he has arrived into Mary’s future. In other words, seemingly time travel into the future took place for Johnny. But did it?

If you think carefully, even though Mary stayed back on earth and Johnny traveled through the spaceship, both were travelling through the spacetime continuum at the constant speed of light and none of them time traveled. Both experienced time dilation but at different rates since their speed through the space was different. That is all. If Mary is now 24 years old and Johnny is only 23 years old, this is there individual “Now”. Mary still cannot change any events in Johnny’s past and similarly Johnny cannot change any events for Mary that have  not taken place at all since all the events in Mary’s life up to that point have already taken place and events taking place right now are in her current present. Therefore except for time dilation nothing else has happened here, definitely not time travel, as neither of the twins can alter each other’s events in the past or future.