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## The Theories of Time Travel

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### The Theories of Time Travel

Why it is theoretically possible?

Who is responsible for these conclusions?

Why it is believed to be impossible?

Einstein’s Equations of Space-time

- Curvature of space and time is determined by the matter-energy content of the universe
- Possible to find configurations of matter-energy that can force the bending of time and allow time travel
- When we find these configurations that allow time to bend backwards Einstein’s general relativity breaks down and quantum theory becomes dominant

The Breakdown of Einstein's General Relativity and Theory of Gravity

- Breakdown in large gravitational fields, which are necessary for the backward bending of time.
- However both quantum theory and the theory of gravity are united in the Hyper-space theory which exists in ten-dimensional space.

Kurt Gödel

- Kurt Gödel was the first person to find solutions to Einstein’s equation that allowed for time travel.
- This was the first solid mathematical foundation for time travel.

Kurt Gödel’s Solutions

- Assumed that the universe was filled with gas or dust and that its was rotating.
- Concluded “By making a round trip on a rocket in a sufficiently wide curve , it is possible in these worlds to travel into any region of the past, present, and future and back again.”
- He found the first closed timelike curves, CTCs, in general relativity.

Einstein's view of time vs.Gödel’s

- Einstien: a river flowing through space, which could be moving at different speeds dependent on it surroundings.
- Gödel: this river could be bent smoothly backwards into a circle, much like an eddy or whirlpool in real river.

What was wrong with Gödel’s theory?

- Einstein was very perplexed by Gödel’s solutions but could not find anything theoretically wrong because Gödel used Einstein’s own equations.
- Weak spot was the in the assumption that the universe was rotating.
- Experimentally we have found no evidence that this is true, so for now Gödel’s results can be ruled out , but still remains a possibility if the universe did rotate then CTCs and time travel would be physically possible.

Kip Thorne

- “ If Einstein is the architect of the space-time shortcut (wormholes) then Kip Thorne is its structural engineer.”-Micheal Moyer
- Along with colleagues, developed first serious proposal for a time machine.

How the idea of Wormholes came about?

- 1935: Einstein and colleague Nathan Rosen examined the scenario where an tiny rip in a black hole connected to another tiny rip in a different black hole.
- Know as the Einstien-Rosen bridge.
- Today referred to as a wormhole and creates a shortcut through space-time.
- Thorne’s time machine centered around wormholes.

The problem with wormholes are:

- One: the gravitation forces at the center of the black hole are astronomically large.
- Two: wormholes might be unstable, and that small disturbances could cause the Einstein-Rosen bridge to collapse.
- Three: in order to penetrate a wormhole to the other side, one would have to go faster than the speed of light.
- Four: the intense radiation given off by the black hole might serve to close the wormhole by itself.
- Five: because time slows down in a wormhole and comes to a complete stop at the center (point of singularity), it would take an infinite amount of time to travel through the wormhole.

How Thorne discovered his time machine?

- He started by working the problem backwards.
- He wanted a solution to Einstein’s equations where the traveler would survive a comfortable, round trip through a stable wormhole, that would be measured in days not millions of years.

At the beginning of these calculations and theories, Thorne and colleagues stated that this was a engineering problem for some future civilization and had no realistic applications for today or sometime in the near future.

- Their solution for these black holes turned out not to be the typical black hole solution.
- It was referred to as a “transversible wormhole.”

Thorne’s Wormhole

- One way of creating this wormhole involves two chambers each containing two parallel metal plates and to induce a huge electrical field, vastly greater than anything possible today.
- This electric field rips the fabric of space-time and creates a wormhole between the two chambers.

Thorne’s Wormhole

- A second way to create a transversible wormhole is to utilize already existing black holes.
- The problem occurs at the center, because no matter would be able to pass through it.
- Throne concluded that if an exotic matter with anti-gravity properties was introduced to the center it would in a sense widen the center and allow matter to flow through it.

Thorne’s Wormhole

- Now we would have a stable wormhole than moves objects geographically in space-time, when Thorne really wanted a wormhole that moved objects in time.
- To do this he applied Einstein's Special Theory of Relativity.

Brief description of the Special Theory of Relativity

- For Thorne’s purpose, he used the idea that time slows down for objects that move quickly,relative to the speed of light.
- This principle, time dilation, is shown in Lorentz’s transformations:

Δt=γΔt’ and

where γ=1/(1-(v²/c²))^½

The Birth of a Time Machine

- By surrounding the first chamber, the first wormhole, in negative energy and towing it around the universe at close to the speed of light, it would age more slowly than the non-moving chamber.
- Overtime the two cambers would become desynchronized in space-time, i.e. the two chambers would exist in different times.
- To travel back in time, an traveler would enter the stationary end of the wormhole, the second chamber, and exit at the moving wormhole, the first chamber, many years before he actually enter the second chamber.

Why we don’t have time machines today?The Problems:

- The biggest and most obvious problem is nature of the matter and energy needed to create these wormholes was know as “exotic.”
- This exotic matter does not violate any laws of physics, a key idea that hints to its possible existence.

Problem 2:

- The amount of energy needed to create and maintain a wormhole with exotic matter is like nothing we have produced with today’s technology.
- There is theory that negative energy can be used to create areas of space-time in which time is bent into a circle with transversible wormholes.

Negative Energy

- Was discover by Henrik Casimir, through the use of quantum theory.
- Experimentally showed existence of negative energy, by using two large neutral metal plates separated by a very small distance, inside of a vacuum.
- He used the Heisenberg Uncertainty Principle, to say that in between the plate there exists trillions of particles and anti-particles constantly appearing and disappearing.
- Know as “virtual particles” and they create a minute net attractive force between the plates.

Time Travel and String TheoryRichard Gott III

- Proposed that Cosmic Strings could be used to allow backward time travel.
- These strings are infinitely long and not wider than an atom.

Time Travel and String Theory

- These strings have very strange gravitational properties: a loop of string acts like a regular body of mass, while a straight piece has no direct gravitational force even though it has a large amount of mass per length.
- Gott’s theory was that two parallel strings traveling in opposite directions and at close to speed of light would cause such a distortion in space-time that a CTC would form and allow matter to enter this riff then exit before it ever entered.

The Problem with Time Travel in String Theory

- We don’t know enough about Cosmic Strings to find the correct configurations, motions, and the physical properties of their infinite extension.
- The downside is that we don’t have the proper information to use Gott’s scenario, but the upside is if we did, there are no laws of physics that state it can’t theoretically happen.

The Limitation of Time Travel

- So far to today, every plausible time machine would not allow a traveler to go back in time before the time machine was created.
- This is the one consistent limitation of time travel, but still does not explain the paradoxes that could arise by traveling to the past.

Paradoxes of Changing the Past:

- Aside from the actual construction of a time machine the most challenging part would be the result of changing the past.
- Stephen Hawking proposed a simple solution to this problem called the Chronology Protection Hypothesis.
- It states that nature would always find a way of preventing wormholes and other ways of changing the past.

Paradoxes of Changing the Past:

- Some physicists believe that most paradoxes can be explained with Quantum Mechanics and the Theory of Relativity, using matrices and a series of mixed states which are the possible outcome of changing the past.
- This leads us to the theory of multiple or parallel universes.

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