The Concept of Time Travel

Time travel is a notion that is not restricted to only science fiction but it’s something you would inherently think about when you delve deeper into the concepts that make up modern physics. For starters, Imagine being trapped in a rocket ship that travels at the speed of light. At that speed, it doesn’t matter for time to be affected by your surroundings, you would still travel through time at the same rate - 1 second per second.

Now this may not seem like much of a significant observation, but when Einstein put forward the theory of special relativity in the year 1905, this was one of the profound results of that theory. Before we delve deeper into the concept of time travel, we first need to understand some of Einstein’s popular theories that made him one of the greatest physicists of all time.

Einstein’s theory of special relativity

The theory of special relativity explains how speed affects space, time, and mass. According to the theory, tiny amounts of mass (m) can be interchangeable with large amounts of energy (E) as defined by the classic equation E = mc2,  which offers a means for the speed of light to define the link between energy and matter.

Another implication of this theory, apart from that mentioned in the previous section, is that photons - or any massless particle, for that matter, cannot experience time in their own frame of reference. If you hypothetically had the photon’s POV, you would observe the universe to contract down to a single point and absorption and emission would all happen at the same time.

So why does all of this matter for time-travel?

Unfortunately unlike photons, we have mass. And for any object that has mass, it is always going to travel at a speed less than the speed of light. And it doesn’t matter in which frame of reference you are, you will always observe light travel at the speed of light in vacuum (c). 

Now this is a significant and remarkable observation. What it means is that if you observe someone in motion relative to you, their clock would appear to run slower than yours.

The Light Clock and Einstein’s Twin Paradox

To help better understand this, imagine a “light-clock”, one where light bounces off mirrors in up-down and transverse directions. A simple light clock consists of two mirrors separated apart at a known distance wherein a pulse of light is made to bounce between the two mirrors. In principle, we can make a light clock if the mirrors are separated at a distance of approx. 150,000 km, then each individual mirror will be struck by the pulse of light once a second i.e. the round trip from one mirror to the other would take the light pulse one second. In the case of our experiment, the faster the other person travels relative to you, the more the light’s velocity will seem to move in the transverse direction, rather than the up-down direction. Light moves in such a way that, to the external observer, it appears to trace out a series of triangles (see the animation attached). Hence, their clock will appear to run slower. 

Similarly your clock would appear to move slower with respect to them and they’ll see time passing more slowly for you! Clearly, this can’t be the case for both of you, one of you will be older than the other the next time you meet. But who among you two would it be?

This gives rise to Einstein’s “twin paradox” problem. Both the “twin paradox” problem and the light clock experiment, which is theoretical proof for Einstein’s theory of special relativity, are vast topics in themselves and it would be difficult to incorporate them in the scope of this blog itself and would be dealt with extensively in future content. 

"Doc, You’re My Only Hope."

You’d think that your dream of time-traveling is all but shattered. But hold on to the steering wheel of your beloved DeLorean as we crack the code. 

To summarize everything that has been discussed so far, assuming you started off in the same frame of reference (at rest on Earth, for example), and you wind up in that same frame of reference at a later time, the person who did the traveling will have aged less, having had time pass at the “slow” rate, while the person who remained at home will have had time pass at the “normal” rate.

So if you want to travel ahead in time quickly, just accelerate to a near light speed, move at that speed for some time, and then come back to your initial state of motion. Do that and you can, depending upon the fuel left in your spaceship, witness the evolution and destruction of humanity, the death of our star, galactic collisions, and all the other cool "sci-fi" stuff you see in movies.

 So there you go! Now you can travel into the future!

But… What about going back in time? What if you wanted to alter your memories, correct those wrong answers in your test or tell your ancestors about what the future beholds for them?  Going backward in time is a whole another story which we will explore in the future blog posts.

Bibliography

Lewis, Huey. “Michael J. Fox in Back to the Future (1985).” IMDb, https://www.imdb.com/title/tt0088763/?ref_=nm_knf_t_1. 

Stein, Vicky. “Einstein's Theory of Special Relativity.” Space.com, 1 February 2022, https://www.space.com/36273-theory-special-relativity.html. 

https://www.wikipedia.org/

Author: Divyanjay Sinha