## Friday, August 24, 2007

### Time travel unravelled

Time travel has always intrigued me. Be it Einstein's Theory of Relativity, which states that the faster you travel, the slower time moves, or HG Wells' Time Machine, the possibility of time travel itself is exciting!

Let me share the result of my recent thought experiments:
We know from our physics textbooks that the faster you travel, the slower time moves. i.e., if you travel faster than the speed of light, you can travel back in time. Find it difficult to believe? Here's how:
Imagine that you are taking off from the earth in a super speed rocket. For simplicity's sake, imagine that your body itself is the rocket and you are moving away from the earth, into space. After moving several million miles into space away from earth, you reach the speed of light, and move forward at that constant speed. Now, at the precise moment (lets call this z)when you achieved the speed of light, certain light beams emitted by the earth were right next to you i.e., you were neck to neck with those light particles, which portrayed a scene of the earth at time n. Lets call this scene s. In the next one second, you have travelled 300,000 kms which is the speed of light. The scene s has also travelled the same distance with you! So it you look at the earth at z + 1 seconds, you see the same scene that you would have seen at z! This is because the scene has moved with you! If you continue to move at the same speed, you continue to see the same scene! Its as if time has stopped!
Let me extend this thought experiment further. Imagine you are travelling at twice the speed of light. Now, as you move ahead, you are overtaking the light particles! So, at z+1, you will watch a scene that is equivalent to z - 1. If you continue at this speed, you will progressively overtake light particles and will progressively watch the scenes in reverse! Which means, you will see the earth rotate from west to east! IT'S AS IF YOU ARE TRAVELLING BACK IN TIME!

Are you with me so far? Good! Now let me introduce a twist in the tale. All this while, I told you that you were moving away from the earth. Imagine now that you spontaneously turn around and move toward the earth. As you move toward the earth (say faster than the speed of light), the light particles overtake you. This means you again see the earth moving from east to west, but at a faster rate than what the physics text books say. i.e., if a is the moment of time when you turned around spontaneously, at a+1, you will see the scene from a+2! The faster you move toward the earth, the faster these scenes will hit you. The earth will now appear to whir rapidly from east to west. Its as if you are catching up with the backlog of time (remember - you were moving back in time till now). Its as if you are moving forward in time! By the time you are back on earth, you are at that precise moment that you would have been, had you not undertaken your space odyssey! (This is an important result, as it resolves the grandfather paradox - we will get back to this later).

Let me now extend the second part of the above experiment further. We have noticed that when you moved toward the earth faster than light, you moved forward in time. So, instead of moving toward the earth, had you moved toward the Andromeda galaxy, you would have been moving forward in time with respect to the Andromeda Galaxy! Which means behind your back you were moving back in time and on the front, you were moving forward in time! This sounds absurd, how is it that the same person can move both forward and backward in time at the same time? Actually, its not that absurd. Imagine a line connecting points E and A. Now, imagine you are an ant walking along that line, from E towards A. As you walk from E to A, you move away from E and towards A. Wow! This means behind your back you are moving away from E and on the front, you are moving toward A! See the connect between moving from E to A and Earth to Andromeda Galaxy??

You still with me? Good. Now for some complications.

We have described what happens when we move in a straight line. What if, instead of moving away from the earth, you moved around the earth, circling it? With one complete circumnavigation, you are back at the same point where you started. So, you moved away from point X and moved toward point X, at once! So when you land back at X, are you ahead in time? Or have you moved back in time? The answer is neither! You will actually be in the present i.e., where you would have been had you not traveled at all! That’s for another post!

Confusing? Its actually quite simple. Remember that in our earlier example, we moved away from the earth in a straight line and turned around to move back in a straight line. The result, you were back where you were supposed to be. Effectively, you moved, turned, returned, completing a loop. A circumnavigation of the earth is also a loop! Its just that instead of moving in a thin oblong loop, you have now moved in a wider, elliptical loop! So, you started from Mumbai, India, moved west until you hit say Mexico, and returned. Just that instead of retracing the path, you instead continued around the earth! So, how is that different from our oblong loop? Not convinced? Let me explain further. We have established that as we move away from a given spot, we move back in time and as we move toward a given spot, we move ahead in time. If you are starting from point X on the equator in your world tour, as you move away from it, you are moving back in time vis-à-vis point X. After you are halfway around the earth, if you continue moving along the equator, you are in effect moving toward X. Which means, you are moving ahead in time vis-à-vis X! With every step that you take towards X, you cancel out the time lag and catch up with time, such that when you reach X, you are exactly at the moment that you would have been, had you not moved at all!

Now back to the grandfather paradox of time travel that I said is resolved. This is the grandfather paradox: imagine I travel back in time and kill my grandfather. Since my grandfather is dead, there is no way that I could have been born. If I was not born at all, then how did I go back in time and kill my grandfather in the first place? So, if I did not kill my grandfather, then I am born and hence can go back in time to kill the grandfather! The loop continues. From our explanation above, we have demonstrated that you can only “view” the past, but not affect it! This means, you can go back in time and see your grandfather, but can’t shake hands with him. The same holds for your grandfather – he can see you, but can’t hug you! Can you hear what he is saying? My guess is you can’t. Neither can you smell the aroma of the rich coffee he is brewing on the stove.

I do not know if my theories above stand up to the scrutiny of the equations and theories of higher physics. This thought experiment has however, given an incomparable joy…. the kind of joy that I experienced when I tallied my first balance sheet. Is this close to ananda???