Time Travel
By M.K Ibn Hadj Kacem
CONTENTS
I - Introduction
1 - Concept
2 - Back to the past
3 - Forward to the future
II - Risks
1 - Disrupting the space-time continuum
2 - The butterfly Effect
3 - Positional Errors
III - Paradoxes
1 - Backward travel - The grandfather paradox
2 - Forward travel - The twin paradox
IV - Not-so paradoxical ?
1 - Novikov self-consistency principle
2 - Parallel universes - Worldlines
V - Experiments conducted
2 - The experiment of Lijun Wang
3 - Quantum tunneling
VI - John Titor - the “self-proclaimed” time traveler
I - Introduction
1 - Concept
The concept of time travel is the movement between two points in time as if it was movement in space, most of the time, time travel is always considered with its narrow meaning from science fiction movies, that’s why it seems to get a lot of support from philosophy and pop culture whilst little to none from physics. To understand the broader more interesting meaning of time travel, we need to split the concept into two categories…
2 - Back to the past
The first category is the most controversial, since time is known to never move backwards, so that makes it the most interesting too, it’s been first started (of course) by popular culture such as fiction novels and continued on until reaching the ears of scientists, so they considered its study, although reluctantly…
Many theories exist about the possibility of time travel back in time, it is theoretically possible by one of these methods :
- Faster-than-light travel (FTL) : According to relativity, special relativity to be precise, if a signal or object A travels towards B faster than light then B will receive it before it was sent. Although this doesn’t imply human travel, since it would take an infinite amount of energy to accelerate a slower-than-light object to ONLY the speed of light. But it would be interesting to send data to the past (see the tachyon quantum field theory).
- Cosmic strings and black holes : A simplistic version of this theory is the following : To fire two infinitely long cosmic strings past each other (or just one string past a black hole) at very high speeds, then move around them in a carefully calculated figure eight. In theory, you would be able to emerge anywhere, anytime! [These are purely theoretical objects that might possibly be left over from the creation of the universe in the Big Bang(which is also a theory). A black hole contains a one-dimensional singularity – an infinitely small point in the space-time continuum. A cosmic string would be a two-dimensional infinitely thin line that has even stranger effects on the fabric of space and time. Although no one has actually found a cosmic string, astronomers have suggested that they may explain strange effects seen in distant galaxies]
- Wormholes : Wormholes are a hypothetical warped spacetime stated by the field equations of general relativity, (another) simplistic explanation is : It’s just a bending of space-time, so you can think of it as a shortcut to move yourself to a different space at the same time or, what we need here, a different time in the same space.
Those theories have been heavily discussed, the first one for example can only be used for data transmission and for very short periods of time, but can be useful for example in resolving NP problems in polynomial time (see David Deutsch work). The second one is purely theoretical, cosmic strings and black holes have to be observed first. As of the third one, there is the condition that it must be a Lorentzian traversable wormhole, and you cannot travel to a time prior to the one in which the wormhole has been created.
3 - Forward to the future
This kind of time travel is also theoretically but is has actually been demonstrated at very small time scales. But in traveling to the future, we have to distinguish two processes, one is the instant transfer from one point in time to another future point, and the other is to simply accelerate time to experience future sooner than it should happen, since the second one is only a matter of perspective (or referential as far as relativity is concerned) it is considered the most probable and real of all time travel concepts.
- Wormhole future travel : It’s the same principle of wormholes explained in backward time travel, and since travel to the past can be made only back to the time of the creation of the wormhole at maximum, then it would be more interesting to travel the other way around, with the supposition that the wormhole is still maintained until that time.
- Velocity-based time dilation : Time dilation is very famous, from Albert Einstein's special and general theories of relativity, it states (another simplistic explanation) that the more velocity an object gains, the more time passes slowly from his perspective, and by consequence, the world moves faster and he’s fast forwarding to the future. Since results are only perceivable nearing the speed of light (with actual time stopping if the object speed is C) the current record being about 20 milliseconds for the cosmonaut Sergei Avdeyev.
- Gravitational time dilation : Under the theory of general relativity, for instance : Residing inside of a hollow, high-mass object or residing just outside of the event horizon of a black hole, or sufficiently near an object whose mass or density causes the gravitational time dilation near it to be larger than the time dilation factor on Earth. These conditions can generate slower time experience for the object in question. It has the same effect as velocity-based time dilation although it hasn’t been practically observed.
II - Risks
1 - Disrupting the space-time continuum
<< [...]Space-time does not evolve, it simply exists. When we examine a particular object from the standpoint of its space-time representation, every particle is located along its world-line. This is a spaghetti-like line that stretches from the past to the future showing the spatial location of the particle at every instant in time. This world-line exists as a complete object which may be sliced here and there so that you can see where the particle is located in space at a particular instant. Once you determine the complete world line of a particle from the forces acting upon it, you have 'solved' for its complete history[...] >> The previous assumption is called the space-time continuum, and according to relativity it is the basis of ‘existence’, since an event already happened, we are sure that it is part of its world-line, if we go back in time and change it, we shift it from its own world-line, and thus shifting the whole space-time continuum, although the consequences are unknown, a lot of theorists think that ‘existence’ itself could be compromised.
2 - The butterfly Effect
<< [...]one flap of a butterfly's wings would be enough to alter the course of the weather forever, generating cyclones and typhoons as it progresses[...] >> The principle of the butterfly effect has originated from Chaos Theory, and its equivalent time travel is the fact that a simple tiny change in a far away past, could generate disastrous differences in the present, for example, considering someone just crashed a corn bean, that one, after a hundred years could have given birth to millions of corn cubs but didn’t, causing famine or shortages.
3 - Positional Errors
An error almost all science fiction movies commit, which is not taking into consideration of the motions in the universe, considering the time machine travels in time but stays stationary in space, this looks convenient, however the earth's rotation around itself and around the sun hasn’t been taken into consideration, thus there’s a big possibility the time traveler will find himself in empty silent space. Aside from that, supposing the machine travels both spatially and temporally with some advanced calculations, this still doesn’t guarantee that a person, animal or even a tree wasn’t at that place at that time… Thus making time travel for humans a seriously dangerous process.
III - Paradoxes
Along with time travel enthusiasts we find time travel haters, and they’re doing a great job, the aim of announcing a paradox is to use the absurd reasoning process : If a time travel situation leads to a paradox, then time travel is impossible in itself.
1 - Backward travel - The grandfather paradox
This is a simple idea, consider an individual named John, who travels back in time when his grandfather Adolf was 7 years old, and then kills him, this will lead to John’s father not being born, meaning John himself will not exist, and by consequence there will exist no one who will travel in time and kill Adolf the grandfather in the first place. And this, my friends, is one hell of paradox ! Making backward time travel impossible.
2 - Forward travel - The twin paradox
The twin paradox is a direct application of relativity, not a simplistic explanation this time, since we all know Einstein’s twin travel story for time dilation, so we consider again two identical twins, on traveling at high speed into space and the other is stationary at earth. Each twin from his referential sees the other twin as in motion, thus speeding and experiencing slower time. Making both of them forward time travelers which is paradoxical.
This paradox however is flawed, because its an incorrect and naive application of time dilation, in order for two perceptions to be symmetrical, both space-time paths need to be symmetrical too.
IV - Not-so paradoxical ?
1 - Novikov self-consistency principle
The principle asserts that if an event exists that would give rise to a paradox, or to any "change" to the past whatsoever, then the probability of that event is zero. It would thus be impossible to create time paradoxes. And the universe would favorize the most improbable event just to avoid that situation.
If we apply this principle to the grandfather paradox, all the events would be possible for John aside from the killing, he would either mistake his grandfather with another person or just not deliver a fatal blow...
2 - Parallel universes - Worldlines
The Multiverse is a theory in which our universe is not the only one, but many universes exist parallel to each other. These distinct universes within the multiverse theory are called parallel universes. This theory is expanded even more to the Superverse, which is a collection of parallel universes in which every outcome of every decision made has its own universe where it exists. So every object have his own worldline and thus changing the past won’t affect the future, it will just move perceived reality into the universe where that worldline exists. So in the case of John, he will just disappear (in the time machine) from his initial worldline in which he’s born to the worldline in which he won’t be born, preserving the equilibrium of “one worldline with John and another without” he can also go back without disrupting anything but noticing that nothing has changed in his “universe”.
V - Experiments conducted
This experiment has been aggressively refuted, but no one can firmly conclude due to the experimental impossibility for the time being, to put this experiment in a simple way, a photon has both a wave and a particle behaviour, and this depends on the observation, in other words when we want to observe the photon as wave, it “chooses” to behave like a wave, and similarly as particle. The aim of this experiment is to use two entangled photons and observe them as particles, and then switch to observe them as waves at the last minute, and knowing that a photon can never be a particle and a wave at the same time, the experiment proved that switching time doesn’t matter, raising the theory that the photon is “capable of knowing the future”.
2 - The experiment of Lijun Wang
The procedure of this experiment was to send packages of waves through a bulb of caesium gas in such a way that the package appeared to exit the bulb 62 nanoseconds before its entry. So the wave package seemed to have moved faster than light, or even backward in time. Of course this has been refuted too, by the fact that the wave package warps, but none of the waves in the sum do so.
3 - Quantum tunneling
This is more an experiment to try faster-than-light movement rather than time travel itself, scientists have conducted an experiment in which microwave photons traveled "instantaneously" between a pair of prisms that had been moved up to 0.91 m apart, using a phenomenon known as quantum tunneling. And I already stated that faster than light movement allows going backward in time.
VI - John Titor - the “self-proclaimed” time traveler
John Titor is a name used on bulletin boards from 2000 to 2001 by a poster claiming to be a time traveler from 2036. John Titor stopped posting in 2001. For five months he had patiently answered questions about time travel and what the world is like in 2036. With his final post, he announced that he was to travel in time again. A cryptic warning remains his last message. << [...]Bring a gas can with you when the car dies on the side of the road.Farewell.John[...]>> . Every day on the internet, people create identities for themselves. But John Titor seemed different. For hours every day he would respond to complex questions about time travel, about the future, and about his own life. When asked to share details of what was to happen in the immediate future, Titor refused, saying that it was either dangerous or he did not know. His responses angered some of the skeptics. Instead of leaving the site, Titor began to post more frequently, and with proof such as pictures and diagrams.
M.K Ibn Hadj Kacem
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