Physics Concerning Time Travel

## Physics of Time Travel

The physics of time travel follow the same principle, to reach another point of space time faster than light can (x speed of light equals x amount of time). The way this is accomplished is through multiple

ways.

       The most well-known theory is the Einstein-Rosen Bridge (wormhole), which consists of a black hole (immense gravity from a collapsed star) and a theoretical "white hole" (hypothetical region of spacetime which cannot be entered from the outside, although matter and light can escape from it). The supposed wormhole has negative energy density which allows for the vacuum of gravity to pull an object through it and transport it to another point of space through the way that space is bent by the gravity. Then these two points link together by the curved bend of space allowing for a "shortcut" through space. A diagram that can explain this is having two points on a piece of paper, the folding the paper so the two points touch.

The reason that the black hole wouldn't collapse it due to the centrifugal force as it's rotating so fast

making a sort spinning ring of neutrons. The way time is affected is by RPM (revolutions per minute i.e. speed of rotation). If the rotational speed of one side of wormhole is faster than the other, then the aging of both sides would be different allowing for future/past travel. If the side closest to the observer is rotating faster, then the aging process of the opposite side would be slower meaning the time that would be traveled would be theoretically the past. If the opposite side is rotating faster than the observer's, then the travel would be to the future due to the fact that the fact the time is moving faster on the opposing side.

       Another theory has to do with cosmic lines (thin strands of energy that bend space-time into a wedge, originated from the Big Bang). The way time travel is achieved through these strands is when two lines are approaching each other at high speeds. The lines bend space-time downward into a wedge allowing for the observer to travel in between the lines while light would follow the wedge created through the lines allowing the observer to cross the gap faster than light can, meaning that theoretically the observer has reached the past but this method is un-reversible.

Possibly one of the simplest methods of time travel can be achieved through Einstein’s theory of special relativity. His theory states “the laws of physics are the same for all non-accelerating observers, and that the speed of light in a vacuum was independent of the motion of all observers”. This theory also means that if an object were to travel near the speed of light, time would go slower.

In the diagram above, there is a laser (represented by arrows) that starts from the bottom of the ship, goes to the ceiling of the ship, then is reflected back to the bottom of the ship. The purple and red arrows are both the same laser, but they are viewed differently by the two astronauts. The astronaut on the moving ship (astronaut 1) views the laser as going up and down, but the astronaut on the stationary ship (astronaut 2) views it differently. Astronaut 2 sees the laser move in a diagonal motion because astronaut 2 is the stationary while astronaut 1 is moving. Not only does astronaut 2 see the laser travel diagonally, astronaut 2 sees the laser travel further, and the speed of the laser never changes, so the only way for the laser to start and finish at the same place, same time, is for the time to change. Time is slower on the moving ship and faster on the stationary ship. This phenomenon, the difference between the times, is known as time dilation, however, this phenomenon is only noticeable when the speed of the ship is near the speed of light.

        Time dilation can also be explained through the equation of time (Time = Distance / Speed). The speed is constant for both perspective but not the distance. On the ship with astronaut 1, the distance, say ten meters, is divided by a constant speed, and the time is given for ship 1. On ship 2, the distance, say 40 meters, is greater and when divided by the constant speed , the time is greater or faster than the speed on ship 1.

         The loop theory is another theory in which light is used to distort gravity, therefore distorting time. This is achieved by circulating light fast enough which starts the circulation of gravity creating a loop. The circulation of gravity creating this loop has an effect on space by also circulating it to create the effect of slowing down time.

## Problems That Occur

       The problems that hinder these uses of time travel can be the resources that are needed to achieve these time travel properties. For example, the wormhole theory would require a  type of "exotic" matter. This matter requires a negative mass which doesn't exist by normal means, although a type of matter called quark-gluon plasma has negative density but it was created from the Big Bang. It can only be created through heating matter up to 2 x 10^12 K (kelvin).

       The problem with the theory of the cosmic lines is that the source originated from the Big Bang and it is difficult to find two strings coming towards each other fast  enough, and the distance away from Earth in light years can also detrimental as the distance can outlive an average human lifetime.

       The problem with the loop theory is a particle accelerator is needed to push a photon fast enough to create this loop and then keep its speed, although the faster the photon moves the more mass it gains meaning double the energy is required (2y x 2...) so a large/infinite energy source would needed to satisfy the energy requirement

       The problem with time dilation is that it only allows travel into the future and not the past.

## Time Travel Paradoxes

### Grandfather Paradox

You are a time-travelling assassin who travels back into the past to kill your grandfather. You are about to kill your grandfather when he was eighteen years old, and right when you are about to, what will happen after it. It is an impasse, for if you kill your grandfather, your dad nor you would exist, so you would not be able to travel back into time to kill your grandfather. This type of paradox is also known as an inconsistent causal loop.

### Consistent Causal Loop

This paradox is one of the main contributions to the “Back to the Future 2”. Biff Tannen, a wealthy, corrupt villain who is the antagonist in “Back to the Future 2”, gets rich by going back into the past to give his past-self a sports almanac containing the results of all sports in the future, so the Biff in the past can make money in betting on sports events. The Biff in the past does so and becomes wealthy and corrupt. After become wealthy and corrupt, the past Biff has to go back into the past to give another Biff the same sports almanac; this cycle will repeat infinetly.