Time travel is the phenomenon of going forward or back in our timeline, or just moving to a different time than someone else. Time travel can be any change in time, even seconds, it’s just the idea of being in a different time than you originally were.
The idea of time travel is popular among the sci-fi genre of media and literature. The main contraption that they use is usually a time machine—a high tech machine capable of transporting you across spacetime. Obviously we don’t have that in our current time, otherwise that world and spacetime would probably be massively changed or destroyed. However, what most people don’t realize is that we already do have a form of time travel.
First introduced by Albert Einstein and used by thousands of physicists, the special and general theory of relativity has parts that allow for time travel. There were also the famous Newton that Einstein used to create the general theory of relativity. Even modern-day scientists such as Stephen Hawking are making advances in spacetime advances, such as his publications on the black hole.
The theory of special relativity was originally developed because physics prior to that time could not explain why the speed of light was always constant regardless of how the observer was moving. Einstein, however, realized there it was because previous scientist only referred to a single reference frame, when in reality, in conjunction to Galileo’s past work, said that there were in fact a lot of reference frames. So how do these frames affect the speed of light source?
In reality then don’t. Regardless of where you are observing something from, the speed of a constant source won’t change. For example, if you were moving toward a car that was approaching you, the speed of a car would appear to be going faster than if you were standing still. Conversely, if you were walking away from the approaching car, then the car would seem to be going a lot slower. So the basics of special relativity walk about the relative speed between an observer and source, or the speed of the object relative to your position at a certain point in time.
Before Einstein’s discoveries, scientists were trying to determine the relative speed of light. Their idea was to measure the speed of light from the sun to the earth, and that when the earth moved away from the sun, the relative speed of light would be faster, and when the earth moved towards the sun, the light’s speed would be slower. However regardless of who measured it, and at different angles, their calculations always came to be the same.
In response to this mystery, Maxwell said that the speed of light was a constant, c; a natural law. Einstein said that as the Earth moves, all calculations will be slightly off, and thus clocks will always be wrong by a certain amount just to maintain the idea that the speed of light is constant.
So how does the idea that the speed of light is constant relate to time travel? To explain this we can use our earlier example of a car and the observer. Assume that the observer remains still and the car passes by at a constant speed; their reference frames will be different. At one point in time during this situation, the frame of the person won’t be the same as the car’s. The car would be moving at frames faster than the eye could trace, thus the car would appear to pass by slower so your eye could still see everything despite the car going by at high speeds. While you are viewing the frames of the car slower, the car’s frames are still normal, and so because he is caught up in frames, while you are still behind, in theory he is a head in time as he is a couple frames ahead.
Because of the idea of different reference frames, by theory we can move forward in time, even though it’s not a significant jump. However, this example of the theory of relativity can’t be used for reverse travel, because even if you are going in reverse at a high speed, you’ll just be ahead in frames in the opposite direction.
In the movie, Back to the Future, the main characters use a car to move between time. This was based off of special relativity as the characters needed to reach a high speed before they were moved in time. Basically, they worked on the idea that because the car is going faster than what the observer perceives, they are ahead in time. While their idea on special relativity was right, the exaggeration of it was a lot. Firstly 88mph is clearly not enough to travel in time to a completely different period. Most people drive that speed on the highway. Secondly, the theory of special relativity is mostly suited for forward travel, and doesn’t make sense for backwards travel.
While special relativity is a branch of relativity, it only uses time, length, and mass. On the other hand, general relativity talks about time and space together, otherwise known as spacetime. First published in 1915, Einstein said that spacetime curved in response to gravity, energy, and momentum. Gravity causes time to speed up (the more the gravity the faster) and having a lot of energy and momentum causes something that is travelling to move and curve spacetime.
One of the main ideas behind the general relativity is the idea of the “principle of equivalence”. Basically, if a person was sitting still on the surface earth, and another person was accelerating at a certain speed, and each dropped some object at the same time, each observer would not be able to tell whether the speed of each object was different. They would appear to be at the same speed, and they would not be able to tell whether gravity was different in the testing environment.
Back to the previous idea of, as gravity increases, then time speeds up. So if you were next to a person and for some reason you had a bit more gravity, then you will always be a bit more ahead in reference frames as opposed to the other person. Similarly, the farther you are away from the source, the slower time ticks for you.
In the movie Interstellar (spoilers ahead if you haven’t watched it sorry), there is a scene where the main character and his crew circle around a black hole in order to propel themselves back home. So they used newton’s laws to do this; while they circled around the strong gravity of the blackhole, they used a counter momentum (one of the other parts of the ship) to move themselves out of the gravity’s range, acting like a slingshot to propel the ship to safety. However because the blackhole was so dense and heavy, the main character ends up skipping 60 years on Earth. This is what General Relativity calls bending time using gravity. Basically, because there was a lot of gravity, it bent the space time continuum, which made it so that time was a lot faster because time moved closer to the observer. Because time was now pushed towards the observer, they would see and experience time more quickly, and in turn move through time.
Time dilation is the concept of how times moves relative to an observer. For example, when describing it in terms of special relativity, a clocking moving faster than another clock would experience time slower than the stationary clock. In response to general relativity, time dilation says that when a clock approaches a body with a large gravitational field, the clock will again experience slower time.
One of the few famous conjectures that arose in response to the discoveries of time travel was the grandfather paradox. Suppose you managed to travel back in time and kill your grandfather, what would happen to the spacetime continuum? If you did kill your grandfather, then you wouldn’t cease to exist because that line wouldn’t exist for the next time loop. However if you didn’t exist, you wouldn’t be able to kill your grandfather in another loop. In theory, you couldn’t ever remove someone in your line like that, because it would just create a giant paradox with you trying to kill your grandfather, but you wouldn’t exist, so in reality you couldn’t.
Another theory is the Novikov self-consistency principle. This is one of the few theories that suggest ways that reverse time travel is possible. The principle also suggests how to avoid paradoxes, which would be caused by altering events in the past. By looking at special relativity, if you wanted to go back in time you would have to move notably slower than everyone else, thus in technicality you would be behind in time, or you could make everyone go exponentially faster than you.
There is also the idea of parallel dimensions. Assume you have a grudge on someone, and you steal their sandwich at lunch. This action won’t affect that person and you in another dimension. Your actions in are completely independent of one another in another dimension (tongue twister).
On top of this, there is a variation of these dimensions which uses the ideas of timelines. So the timeline you are currently in is your main one. Now you failed a test, and you go to change it, thus creating a different timeline branched off of your main one. While this theory is similar to the parallel dimensions one, the difference is that every timeline of you is connected through some means. So you’ve gone and changed your test and you’ve aced it. When you return to your originally timeline, you find the world in ruins because you altered the curve of the test and someone got a “b” on their test. To fix it you would have to the second timeline you created, fix it there, thus causing a third one, but at this point the third and originally are linked as well, leaving it back to how it was and it fixes the second timeline as well.
Actually time travelling is virtually impossible with our level of technology. All current ideas on how to build a time machine are all theoretical. After watching movies, most people attempt to simplify the process of building a time machine. Movies just try to sound realistic and throw in a bunch of fictional science and fictional technology to make it more believable. Throwing a “flux capacitor” into some giant box will not make it a time machine. In reality, building a time machine does not seem possible for a long time, and with our current level of technology we can’t even plan on what we would need for it, and how the machine would work.
Of course with this logic, all movies that talk about time travelling with futuristic technology aren’t true. With no knowledge on time machines, there is no way any movie could possibly be accurate. However, for movies that try to be realistic, such as Interstellar, their physics are mostly accurate. Again, with the scene of the black hole, the characters use General Relativity to describe effects of gravity on time. Even then, most movies still try to be entertaining, thus exaggerating physics in some way, such as when one of the characters ACTUALLY enters a black hole, and lives, coming out some 60+ years later.
So the phenomenon of changing a person’s time compared to others is called time travel. Starting in the late 1800’s and the early 1900, scientists began researching early stages of time travel to determine the accuracy of their physics labs. However it then began to become more spacetime oriented, and we started our ideas of time travel.
Our main ideas about spacetime physics are found in the theory of relativity through the ideas of special relativity and general relativity. Of course we have a ton of other time travel theories, such as parallel dimensions and the idea of timelines, and a bunch of paradoxical cases such as the Grandfather paradox.
Time travel has even found its way to the media and the show business. Popular movies such as Interstellar and Back to the Future were heavily based on einstein’s work. Project Almanac just used a fictional time machine, but sort of demonstrated the dangerous part of time travelling, paradoxes.
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