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Philosophy and Physics of Time Travel

Written by Yuvraj Dhillon

“No one knows what the future holds. That’s why its potential is infinite."—Okabe Rintarou, “Steins; Gate”

Imagine waking up one day with your mad scientist uncle bustling into your room, screaming at the top of his lungs, “I did it! I created a time machine! Now get inside the DeLorean so we can stop World War 3 from happening!” No one would miss the opportunity, as going to the future or revisiting the past will always be an awesome experience. Also yeah, you could totally save the world too. Although we must ask ourselves first, is time travel actually possible, or is it forever going to stay in the realm of science fiction, like in franchises like “Back to the Future”, “Star Trek” and “Steins; Gate”?


(The DeLorean time machine featured in “Back to the future”)


Time travel is the movement between different points of time, which can be analogous to moving between different points in space.

In order to understand how time travel can be like, I shall try to explain the physics behind time travel, the scientific theories established up to date, and the various paradoxes.


What is time?


Time, as described by Albert Einstein, is nothing but an illusion. According to his theory on Special Relativity, time is relative for every observer in the universe. Time is not a constant quantity, as most would believe. This is because of time dilation, which is a consequence of the Theory of Relativity, as described by Albert Einstein.


Einstein's Theory of Special Relativity says that time slows down or speeds up depending on how fast you move relative to something else. Approaching the speed of light, a person inside a spaceship would age much slower than his twin at home.


Picture a four-dimensional fabric called space-time. When anything that has mass sits on that piece of fabric, it causes a dimple or a bending of space-time. The bending of space-time causes objects to move on a curved path and that curvature of space is what we know as gravity.


(Gravity can be described as the curvature of spacetime due to mass)


This can be understood by the following analogy. Imagine placing a massive metal ball on a trampoline. The trampoline bends under the weight of the ball and gets curved. Now gather some small marbles and gradually drop them at a little distance from the ball. It can be seen that the marbles swirl around the massive ball. If the ball is our sun, then the marbles are the planets in our solar system, and the trampoline is space-time. The curvature of the trampoline is equivalent to the curvature of space-time. The more the space-time fabric is curved, the stronger gravity is.



Einstein’s General Theory of Relativity provides for possibilities for time travel, though none of them have been achieved practically yet.


Time travel possibilities


1) Through the wormhole: Wormholes are tunnel-like structures that link two points in space-time. They can connect any two points, even if they are billions of light-years apart. Thus, we can take a shortcut to reach a point in space-time not ordinarily traversable. A wormhole is formed when spacetime is curved so much between the two points that it forms a pathway between them.


(A wormhole is like a tunnel between any two points)


2) Black holes: Another possibility would be to move a ship rapidly around a black hole, or to artificially create that condition with a huge rotating structure.

"Around and around they'd go, experiencing just half the time of everyone far away from the black hole. The ship and its crew would be traveling through time," physicist Stephen Hawking wrote in the Daily Mail in 2010.


(The first-ever image of a black hole in 2019)


3) Using quantum mechanics: Quantum mechanics could provide certain mechanisms for time travel. Quantum-mechanical phenomena such as quantum teleportation, the EPR paradox, or quantum entanglement might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact, some interpretations of quantum mechanics such as the Bohm interpretation presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles. This effect was referred to as "spooky action at a distance" by Einstein since he theorized that there was nothing faster than the speed of light.


(Quantum Entanglement has been demonstrated to be faster than light)


Time travel paradoxes


A time paradox is an apparent logical contradiction in the idea of time and time travel.some of them are:


1) The Grandfather Paradox: If you go back in time and kill your grandfather, then it would mean your dad was never born, implying you weren’t either. But that means you never existed to go back in time in order to kill your grandfather, therefore your grandfather didn’t die, ergo you are alive. This is one of the biggest paradoxes of time travel.


(The paradox in a nutshell)


A possible resolution is that a time traveler can do anything that did happen in the past, but can’t do anything that didn’t happen in the past. Doing anything that did happen in the past results in no contradiction, so the paradox can be avoided.


2) The Fermi Paradox: It states, “If time travel is possible, why aren’t their time travelers from the future in the present?” Answers vary, from time travel not being possible, to the possibility that visitors from the future cannot reach any arbitrary point in the past, or that they disguise themselves to avoid detection.


Time Machines


It is generally understood that traveling forward or back in time would require a device — a time machine — to take you there. Time machine research often involves bending space-time so far that time lines turn back on themselves to form a loop, technically known as a "closed time-like curve."

(Candice: Mom, They made a Time Machine!)


To accomplish this, time machines often are thought to need an exotic form of matter with so-called "negative energy density." Such exotic matter has bizarre properties, including moving in the opposite direction of normal matter when pushed. Such matter could theoretically exist, but if it did, it might be present only in quantities too small for the construction of a time machine.


(Exotic matter hasn’t really been found yet)


However, time-travel research suggests time machines are possible without exotic matter. The work begins with a doughnut-shaped hole enveloped within a sphere of normal matter. Inside this doughnut-shaped vacuum, space-time could get bent upon itself using focused gravitational fields to form a closed time-like curve. To go back in time, a traveler would race around inside the doughnut, going further back into the past with each lap. This theory has a number of obstacles, however. The gravitational fields required to make such a closed time-like curve would have to be very strong, and manipulating them would have to be very precise.


(Closed time-like curves could allow time machines to exist)


Summary


With numerous theories about time travel floating around, many experimental results failing, it makes you think a time machine is never going to exist practically. However, we shouldn’t give up yet, as we haven’t really achieved the holy grail of all physics - a quantum theory of gravity. If we can find a Quantum description of gravity, it might help us know if time travel is really possible or not, because Einstein’s theory of General Relativity doesn’t describe how gravity works at the subatomic level. I mean, quantum physics allows particles to travel through walls, so why not?


Even if actually changing the past is possible, there’s the philosophical question of, “Should it be allowed?”. Manipulating the past to your advantage is definitely something that can happen, should a time machine fall into the wrong hands. Countries can use it for their own benefit, to win a world war before it even happens, or assert world dominance by re-writing history to their favor, “playing god” where they shouldn’t. A nihilist could wipe out the whole human race just by getting the right idea from the future to do it. Very few people would actually use time travel for the progress of science, technology, and mankind.


So yes, contrary to popular belief, time travel could actually be more dangerous than it could be fruitful.

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