Discussing Time Travel

Last post I did a short guide on Special Relativity and I briefly mentioned time dilation which touches on the concept of Time Travel. Time Travel, like Parallel Universes or Teleportation is something that at a glance seems improbable, “beyond the boundary of physics” and just a complete work of science fiction, but interestingly enough all three of these areas are studied in-depth within the field of physics.

Einstein thought about space as a piece of fabric and essentially called it the fabric of space-time. We can think about time as a dimension, like length and width and height, there is a temporal dimension. Time travel is simply our progression through this dimension and we are all time travelling at this very moment because we are experiencing the passing of time. Time travelling to the future is a crucial everyday task, and it happens more naturally than people may acknowledge.

Fine I guess this type of future time travel is a little boring. Let’s go back to special relativity and the twin paradox. Two weeks ago I introduced the twin paradox which is when one twin moves off into space at a very high-speed, like close to the speed of light, and the other twin stays stationary on Earth, when the twin in the spaceship comes back from the journey they are essentially years younger than the other twin and everyone else on Earth. Time runs slower for objects travelling close to the speed of light so the twin on the rocket actually experiences less time than its other twin. This also leads me onto another thought experiment I saw described in a documentary by cosmologist Stephen Hawking where he used a perhaps more imaginable scenario. So, in the far future we somehow managed to build an extremely high speed train. This train is so fast that it circles the Earth just over seven times per second, at a speed of somewhat 0.95-0.99c. All passengers on board this train is told that it is a point of no return as it is a time travelling machine though an irreversible one. The train begins to travel at a constant speed at a super high rate. The passengers stay on for a week and time dilation is occurring however because of its nature they don’t experience things in slow motion but rather everything outside their window appears in fast motion. As the train stops after the week journey they step out into the unknown, a strange new advanced technological civilisation – 100 years into the future.

Mass warping space-time

Einstein’s theory tells us that mass distorts the fabric of space-time therefore clocks run slower around these regions of mass in the Universe: called Gravitational Time Dilation. For example, we would experience slower progression of time on Earth than someone in space (ignoring effects of velocity). The bigger the mass, the more it distorts space-time thus this is another exploitation opportunity for time travel into the future: by orbiting a black hole. Note that black holes aren’t as evil as they seem, they are perfectly orbitable with enough velocity if you don’t accidentally cross its event horizon, but if you do..good luck on surviving (warning: don’t try this at home). Anyway, orbiting a black hole has similar effects as the hypothetical high-speed train due to its great gravitational field strength. After a comparable short period of black hole orbiting you will return to a very different Earth of the future.

Travelling to the future is easy, much easier than travelling to the past. But the idea of time travelling to the past is much more popular amongst people. In a recent would you rather game with friends, a question I asked them was: Would you rather time travel to the past or the future? Frankly most of them said to the past. Why? Answers ranged from “I want to erase bad memories and undo mistakes” to “I want to change the historical timeline and prevent the world wars from happening”. Now, most people think time travel itself is impossible though we have clearly showed that it isn’t. However time travel to the past is still a tricky concept to grasp. Special Relativity states that time runs slower and slower approaching light speed, and for particles like photons that travel at the speed of light time is non-existent; it stops. So theoretically if we travel faster than the speed of light, time runs backwards thus if an object is able to achieve that speed then it is time travelling to the past. Previously I talked frequently about high-speed figures like 0.9c but I never mentioned anything like 2c or 5×10^8 m/s because velocities like these simply don’t apply. In fact it is impossible for any object with mass to ever reach the speed of light, we can get close to it e.g. 0.995c but never get to the value of c itself. When approaching the speed of light, the object would start to increase in mass rather than speed, so according to E=mc^2 an infinite amount of energy would be required to push the object to such speed…and infinite energy seems very easy to obtain indeed.

Connecting two points in space-time

Is your past time travel dream pretty much entirely shattered? Don’t worry we still have portals. Time travel to the past is the subject of quite a number of novel and film plots. I try not to be too critical with the uses of cupboard portals/time machines in these stories since they’re simply for entertainment purposes and I suppose it doesn’t matter that much but yes Science does have its own theoretical version: Wormholes. Wormholes are a pretty fascinating topic because the whole thing has always been a mystery. Wormholes are shortcuts through the fabric of space-time, and passing through one could take you to different region of space and time, potentially the past. They connect two points in the Universe like a bridge and are mathematically predicted using calculations within the frameworks of Einstein’s General Relativity. A few physicists have also speculated wormholes to exist inside of a black hole. Though many are hesitant with the practicalities of this since we have yet to spot the existence of a single wormhole. Moreover there are a mountain of problems associated with time travelling through a wormhole in the first place, including size and stability.

To conclude, time travelling to the future is doable though to the past is a little complicated with the rise with several paradoxes. Let’s end with a quote from Stephen Hawking, “Time travel may be possible, but it is not practical.”

Author – Susan Chen

Susan is a 5th year high school student currently studying three STEM subjects at Scottish Higher level-Mathematics, Physics and Chemistry (Crash Course). She particularly loves ideas in cosmology and hopes to embark on an academic journey in the area of theoretical physics.