Einstein's Twin Paradox Explained

The idea of time machine was first imagined in 1895. Since then time travel is a widely recognized concept in science, philosophy and fiction. This post is written jointly by Kaushik Kashyap and Vedang Sati.

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Time travel: we have all heard about it and surely it takes our mind on a wild imaginary ride when we attempt to understand it. It is a kind of travel not in space (which we know has three dimensions) but it is the travel in the time dimension, the fourth one which we do not quite understand. So our question stands: is it theoretically possible to travel in time?

At first, we have to understand what time really is so that we know what we have to travel through. In non-relativistic or classical physics, the concept of time generally used was that of absolute time.. which is independent of any observer and same throughout the universe. This was thought of first by English scientist Sir Isaac Newton back in the day who proposed that time progressed at consistent pace for everyone everywhere and is essentially imperceptible and mathematical in nature.

But in relativity, time is not absolute meaning that it is interchangeable with space and all together known as the space-time. Time is therefore considered as a valid dimension or "pathway" for someone to walk on. From the relativistic theory of Albert Einstein, we now know that the rates of time actually run differently depending on relative motion meaning that time effectively passes at different pace for different observers travelling at different speeds.

We can accordingly see that there could be two types of time travel: into the future and into the past. And of course we already are moving into the future all the time but we're doing so at the regular rate. Could we make it so that this pace of time going forwards is increased? There is a way: sending elementary particles on round trips in a particle accelerator at 99% of light speed is routine. The result is that the inner clock of such a travelling particle runs much slower than that of a particle of the same species that remains at rest.

Let us have a look at the thought experiment "twin paradox" which was first conceived by Einstein while he was working on the theory. In this, a hypothetical astronaut returns from a near-light speed voyage in space only to find his stay-at-home twin many years older than him because travelling at relativistic high speeds has allowed the astronaut to experience only, let's say, one year of time, while ten years have gone by on the Earth.

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The clock in the spaceship literally registers a shorter duration for the journey than the clock in mission control on Earth. This would then lead one to think that we can control the time of the object with the speed of the object but it is wrong. Because Einstein has explained it, the real paradox, which happens from the fact that there's "no favourable reference frame" in relativity. Why can’t the twin in the spaceship define himself as being at rest? From that point of view, it would be the Earth that moves away before returning to the spaceship.

And if that is the case, couldn’t the travelling twin apply time-dilation to everyone who stay on the Earth? By that argument, shouldn’t it be the humans of earth that remain younger than expected once the twins are reunited? We all must eventually agree though that only one of the twins' perspective has to be correct. Which one is it then? So this is the actual "twin" or "dual" paradox of time dilation as put forth by Einstein in the 20th century.

Moving forward, general relativity does raise the prospect, well at least theoretically, of travel through time, that is, 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.

From general relativity, we can say that time passes more slowly for objects in strong gravitational fields than for the objects which stay far from such fields. There are all kinds of space and time distortions near black holes where the gravity can become very intense. Thus if one of the twins is orbiting around a black hole and the other's orbiting around the earth the question of the paradox, "which twin is older" is answerable.

How about travelling into the past? Haven't we all seen "Back to the Future" and wondered how messed up it could get if we too did actually move backwards in time? And many scientists say the very premise of pushing a button and going back to yesterday violates the law of causality. However there are also some who think otherwise. Professor Michio Kaku has said, "Time is a river. It speeds up, meanders, and slows down. It can also have whirlpools and even fork into two rivers."

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That last bit, "fork into two rivers," is important because then moving backwards in time could become at least thinkable. Why because as soon as we push the button we go back into an alternate world or reality. We do not cause harm to our previous reality as in the case of "grandfather paradox". The idea was first proposed by British physicist David Deutsch who used the terminology of multiple universes to solve the grandfather paradox. Deutschian time travel involves the time traveler emerging in a different universe other than his own but very similar to his own.

Time travel will remain only conceptual and debatable except if we are able to develop enough advanced technology for it to become achievable. Until then we will use our earth bound telescopes as time machines. Because when you look into one you'd actually be looking into the past stages of the universe.. meaning that the star you observe today might not even exist in the first place. Turns out that if aliens knew exactly where to point their telescopes they could see dinosaurs at least in principle.