Science fictions have given rise to many ideas be it making contact with the alien life forms, AI ruling over humanity or telekinesis, and what not. But, the one that has been stuck in the minds of people is the possibility of time travel. This theme comes from the fact that the equations of Special and General relativity permit time travel. Movies like interstellar, predestination, looper, etc. have indeed shown how everything would change if one travels in time in either direction.
Let’s explore the argument rationally using science. The first and foremost approach to the concept of time travel goes hand in hand with the Special Theory of Relativity. Fast moving objects experience a slower rate of time or to say in a more clinical fashion, their clocks tick slowly compared to a stationary observer. But, it’s only a one way ticket in time and is equivalent to travelling in the same temporal direction at different rates. At speeds faster than the speed of light, time flows backward and one could steer to a point in spacetime before one departs.
A Basis for Unexplored Particle Physics
To do this with regular mass (or inertial mass), an infinite amount of energy is required. But, hold on to your hats, we can modify the equations of SR by allowing mass to take values in the complex domain. Now, the object in this realm would only travel faster than the speed of light, no less, i.e. only backwards in time. We have a name for a particle that can exhibit this kind of peculiar behaviour, you guessed it right, Tachyon. Sounds cool. Well, don’t get your hopes too high right now because no tachyons have been discovered so far and might never will.
Is that it? No time travel for us? Well, not so fast. We still have another arrow in our quiver to continue our quest for travelling in time. This arrow is none other than the big boy of modern physics, the General Theory of Relativity (GR). Theory incorporates all the formalisms of SR and explains the force of gravity as the curvature of fabric of spacetime due to the presence of mass and energy. There is a famous quote by John Wheeler, “Spacetime tells matter how to move, matter tells spacetime how to curve”. Solutions of equations of GR give rise to a hypothetical concept of Einstein-Rosen bridge, or to say in layman terms, “Wormholes”.
Concept of Wormholes and Warp Drive
Let’s warp and stretch the spacetime in such a way as to create a tunnel between two points such that the internal distance is short even if the mouths of the tunnel are light years apart. Voila, we have a wormhole. One of the gladdening things about it is that one can teleport between distant points in space as well as in time. Let’s take you back in time. Here’s how to do it: Take a wormhole that should be large enough to be traversed. Rotate one end to close to the speed of light- its rate of time flow would slow down relative to the other end. An offset is created in time in such a way that one portal is permanently stuck in the past with a fixed time difference. Now, you can travel through the future into the past. Sounds easy, right? But, here’s a catch. Wormholes are not at all stable. In order for them to be stable, we need some kind of matter that can counteract the effect of gravity. This special type of matter is called exotic matter. It has started to get a bit out of hand. So this idea might be a non-starter one. Here’s one more thought: Wormholes can be opened with anything that has negative energy density. Some scientists argue that this has already been achieved through the Casimir Effect, in which the energy of the vacuum is lowered between two nearby conducting plates. But, there is no clear way of translating this to a large scale negative energy distribution that would keep wormholes open.
Some other time travelling options open up using Alcubierre Warp Drive and this too requires exotic matter. In short- if one has access to exotic matter, time travel seems plausible.
Forget about exotic matter for a moment. Let’s think out of the box for a moment. Shall we? Focus on the energy conditions in GR. These are basically a set of requirements that do things like prevent negative energy and enforce energy conservation. But these conditions have sometimes been violated like in the case of the Casimir Effect. Thus, throwing the ideas of wormholes and Warp Drive out of the window is not justified.
Tipler Cylinder in Brief
Time machines can also be built without the use of negative or imaginary mass. This can be done using a Tipler Cylinder as proposed by Frank Tipler. According to him, build an infinitely long cylinder of extreme density and set it in rotation extremely quickly about its main axis. It produces a frame dragging effect and forms closed timelike curves that can be used to travel back in time.
All in all, it looks like we have abundant methods to take things back in time, but they all seem useless for actually making time machines for any arbitrary reason. We can make a reasonable argument that fundamental laws of physics don’t prohibit us from travelling back in time. However, the chain of cause and effect (in simple words, Causality) breaks down and we are left with paradoxes like grandfather paradox, predestination paradox, bootstrap paradox etc. which play with our common sense in a way in which we perceive reality. But there are no true paradoxes- only seeming paradoxes that point to a gap in our understanding. S.W. Hawking explained it beautifully with his Chronology Protection Conjecture, saying that “The laws of physics will prevent time travel or allow it only when it doesn’t create paradoxes”.
The ideology behind arguments that may or may not be conceivable could go on. But here’s a good takeaway:
- Time travel is possible only if the closed timelike curves are self consistent. In simple words, as long as the backwards time travelling configuration of matter leads exactly to the same forward travelling configuration, i.e. loop creates itself. For instance, you want to kill your grandfather, only to become your own grandfather…and on and on. Don’t think too hard about it.
- If Many Worlds interpretation of Quantum Mechanics is found to be true (in the future), you going back in time and killing your grandfather doesn’t make your grandfather vanish right away as he is still alive in other timelines (for this, watch a movie named Coherence). Again, don’t think too hard.
- Until and unless we have a theory of Quantum Gravity, we cannot fully rule out the idea of time travel.
Some scientists say that time travel is now only an engineering problem. But deep down we all know, the statement is much further from the truth. That’s why I wrote: Time Travel is Possibly Impossible.
PS: I haven’t explained everything in detail. If you want to learn more, checkout the Wikipedia page or go through a humongous book named Gravitation by Misner, Thorne and Wheeler.