Where are we with time travel?

Sci Tech Dec 25, 2017

The Dilettante’s guide to manipulating time

By Thirumalaesh A (B.E Robotics and Automation, 2017-2021)
Flash, Captain Rip Hunter and his crew, Joseph Cooper, Marty McFly, Doctor Who and many more characters made our ‘time’ worthwhile, with time itself being the greatest power that man can harness. Travelling through time has been one of the greatest human fantasies of all time. Oh, what wouldn’t the Americans give to find the truth about JFK’s death and what wouldn’t I do to witness my own future!
Scientifically, the theories about time travel and its implications have been evolving, with the difficulty in comprehending them remaining constant. The contemporary understanding of time and time travel is based on the special theory of relativity developed by Albert Einstein. Setting aside the complex equations and derivations, understanding time travel in strictly fictional terms ,is a doable task.

The fabric of space-time:

Scientific breakthroughs occasionally shatter the lens of the conceived reality through which we perceive the world around us. The relativity theory took concepts that are familiar to us – light and time, things which are beyond our manipulation and provided equations for us to be able to manipulate these godly entities. The biggest take away was that time and space make up this intricately interwoven web and that modifying one would affect the other. The effect of relativity in space alone is something we are familiar with. Let us take a glass of water resting on a table in front of you during a train journey. It would rather seem moving with the train if you see the same glass from the station after you get down. The speed of an object (glass of water) as we perceive, actually changes depending on the speed of the observer (us).
The effect of relativity in space-time has been illustrated in movies like “Interstellar” (The accurate visualizations of the giant blank hole in the movie actually led to three scientific papers by the physicist Dr. Kip Thorne). To explain Einstein’s theory, let us take the same train with a distance of  ‘d’ between its ceiling and floor (See picture below).

Source: Reddit

Suppose, we throw a ball inside the train in a vertically straight fashion. If we are on the train, this ball just goes up and down, again and again, assuming no loss of energy in a straight line. For someone else who traces its path, from outside the train, it also moves horizontally. (As the glass of water moves horizontally, so does the ball. It just happens to have a vertical motion too) So, the triangular, zig-zag path in the above figure actually represents the path seen by someone outside the train. Note that the distance travelled by the ball, in this case, is more than in the first case (Between the floor and ceiling). We assumed that there is no loss of energy – that means that the speed of the ball is constant. If the speed is constant and the distance covered is more, the time interval also has to increase (as v = s/t). Another word for putting this is that the time slows down.
Intuitively, it is obvious that faster the train goes, more is this time dilation. So, if your time goes slower than mine, you can go to my future, in a way. The upper limit of such speed is said to be the speed of light by the theory of special relativity. If you were to travel through this train at 99.94% the speed of light for 5 years, corresponding to your watch, I would’ve aged 10 years. This means that you would have travelled 5 years into my future. It’s similar to the ending of “Interstellar”, when Murph was in her deathbed as her father returned to hold his promise good. So, theoretically, the time travel to future can be a train in which you can hop on to travel few years ahead of others’ time.  

The deal-breaker?

The obvious restriction starts with the fuel to keep the train running, let alone accelerating it to 99.94% of the light speed. It is more complicated that the mass of the object also changes with its speed, and the mass of such train would increase to infinity, as it reaches light speed. So, all the fuel in the world could not supply that. That is why that 99.94% of light speed was used in the last paragraph. Such energy sources don’t exist in our hands, right now to make space for bigger discoveries in the future, but physicists like Stephen Hawkings already have figured out other forms of manipulating time.
From our school geometry, straight lines exist and the angles of a triangle add up to 180 degrees; in a perfect world, envisioned by Euclid, this is. We know that a very large straight line on the surface of the earth is actually a part of a circle when viewed from space. Einstein made a peculiar statement that we live in a universe where space and time are intertwined. To understand this effect, we can think of a polyurethane rectangular sheet. This sheet (two-dimensions) would represent the spacetime in the universe. If we were about to place a small spherical ball (say a football) on that, the sheet would get elongated near the sphere. Einstein predicted (which was proved later) that such effect happens in three-dimensional space too. So, a heavier object like Earth will bend the space around it. (As shown in figure below)

Space-time warping. (Source: Stanford University)

So when anything that passes through this curvature (Gravity!) will have to slow down as it has larger distance to cover. But, another point from the special relativity is that the speed of light is constant (So, the ball in the train illustration can be light instead and the assumption of constant speed is redundant). This means that light when travelling through such dents in the universe takes a longer time to travel. This is similar to the train argument, and the same form of time travel can be achieved. This led Prof. Hawkings’ to speculate that by orbiting a black hole, which are the largest masses known in the universe, our time would slow down relative to some others’ time.(This was again illustrated in “Interstellar” as the different time zones of the spacecraft and the planets orbiting the giant black hole). But, again, the possibility and energy to orbit a black hole is far from reality.

Why light?

The constant speed of light (3 x 10^8 m/s), from the special relativity, sets the upper limit for these arguments and is also one that we cannot obtain without increasing our mass out of bounds. But, something travels at that speed and it is light itself. How can it pull off the stunt without the mass problem? The overt answer tends toward the ‘relatively’ massless property of photons.
Based on our current understanding, the property of mass is instilled in a physical body based on its interaction with a theoretical field called Higgs field.  There was a hype, a few years back, around something dubbed the “God” particle, which is a misnomer actually. The particle was named Higgs-Boson to balance the equation, like a side product, of how objects acquire mass. It is also part of the Standard Model of quantum physics (Like the periodic table in chemistry), which describes the smallest particles that make up our world. While the object gets its mass as a result, the Higgs-Boson particles are also produced.
The importance is that these particles are observable, which actually took more than 50 years to do so. Large Hadron Collider (LHC), the tunnel showed below, which belongs to the European agency CERN detected such particles in 2012. Seemingly, photons (which make up light) do not interact with this particle and thus remain massless. Our current understanding, no way complete, provides evidence for all the earlier discussions to exist. A more layman understanding of this idea is presented, here in TED-Ed and in CERN website.

An aerial view of CERN, Switzerland

Back to the Past?

For the sake of consideration, if we were to assume time travel to the past was possible, it would open a Pandora’s box! The most significant one would be the formation of Paradoxes. (The time when flash was grieving and went ahead and saved his mom creating Savitar. Ring a bell?). The best example of paradoxes would be the ‘Grandfather paradox’. If person A had to travel back in time and kill his cruel grandpa in his teens, that would mean his father would never be born and in turn he wouldn’t be born meaning he wouldn’t be able to travel to the past to kill his grandpa and thus his grandpa would be alive. This is an infinite loop – a paradox.

The Grandfather Paradox (Source: occultusregnum.com )

There have been few resolutions that were obtained for this paradox. These are even more abstract concepts like simultaneous reality, where more than one path in time exists. We won’t get into that mess though. Travelling through time back would be impossible if the relativity theory is to be considered, although the usage of wormholes could be substituted (“Interstellar” again). Now we go into the realm of complete fiction. A wormhole is basically a shortcut through this space-time. If I could create a wormhole at a certain point in space-time and accelerate one entrance to approximately the speed of light, jump in and come out through the second entrance, I could technically travel back in time. The limitation is that you can only travel back in time until the creation of the wormhole beyond which is impossible. That is, if I managed to create a wormhole today, I cannot use it to visit the dinosaurs but the men of the future could use it to visit us.
A theorized depiction of a wormhole (Source: express.co.uk)

Thus we can understand that the journey to the future is definitely possible while the past is best left alone. After all, history never fails to repeat itself!
If you are interested in the cool physics of time and universe, you should check out the books (Link to the buyer is provided) about time by Stephen Hawking, about Black Holes by Jean-Pierre Luminet, the theory of special relativity by Albert Einstein and about particle physics by Dick Teresi.
THIRUMALAESH A (B.E.ROBOTICS AND AUTOMATION, 2017-2021) is a fervid reader, writer and follower of current affairs who hopes on developing a better tomorrow through innovation in technology.
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