Time travel

Time is the progression from past, to present to future that will inevitably occur and is irreversible. Time serves several functions:

Sequencing events
Quantifying duration of events
Quantifying the intervals between events
Quantifying rates of change

Essentially, time is what a clock reads.

Our idea of time has evolved since the time of Isaac Newton. For Newton, time is like an arrow, that would travel unimpeded toward it's target. However, for Albert Einstein, time is more like a river, that could gently speed up or slow down in curved space.

Time travel is a concept, in fiction and philosophy, where, one would travel to a different point in time, in a way analogous to how one can move to different points in space. Typically, it is based on the use of a time machine, which could either be some sort of vehicle, spacecraft or portal that would act to connect points in spacetime, that would otherwise not be connected.

Time travel would diverge from the conventional flow of time from past to future. It has also never been verified, in any way, either observational, theoretical or experimental. That being said, along with the fact that time travel presents theoretical problems, it is a potential reality, that, it is not possible.

A problem that arises with time travel is the problem of causality. This is the notion that: cause precedes effect. If an effect precedes a cause, we have a time paradox or a temporal paradox. A temporal paradox is a kind of logical contradiction, pertinent to the flow of time, and to the idea of time travel. Here are some examples of a temporal paradox:

Grandfather paradox: This is a change to the past, that generates an inconsistency in time. It is a change to the past that makes the present, somehow, impossible. The name of the paradox, refers to the best example of the paradox. If a man were to travel backward in time and kill his grandfather, he would be preventing his own existence. If you destroy your ancestor, you cannot logically exist.
Causal loop or Information paradox: This paradox refers to a sequence of events where: the first event, will cause another event, and that second event, is the cause of the first event. They would appear to come from nowhere. This is when information comes from the future and appears to have no origin.
Bilker's paradox: This is when one, with a prior knowledge of the future, does something to make the future impossible.
Sexual paradox: In this scenario, you have traveled back in time and acted in a way that results in you being your own parent. This is a biological impossibility, not allowed by the mechanics of DNA.

Whether it is physically possible or not, however, really, is not one-hundred percent certain. Relativity does allow time travel to the future, however, this is not possible within our current technology (obviously). Time travel into the past is theoretically possible by general relativity. However, the required conditions, also, may not be physically possible. The support for time travel in physics is very limited at the present. The closest real ideas we have are connected to wormholes and quantum mechanics.

Albert Einstein

General relativity

Albert Einstein's theory links space and time into a single continuum. That being said, any wormhole that connects two distance points in space would also connect two different points in time. Does this mean that the theory of Albert Einstein allows time travel?

Einstein was bothered by the possibility of time travel when he first formulated general relativity. Perhaps, since space and time could warp, they could warp to such a degree that allows for time travel.

W. J. Van Stockum

W. J. Van Stockum, was an important early contributor to the development of general relativity. Stockum showed in 1937, a possible solution to Einstein's equations that would allow for time travel. This situation would make use of a concept in general relativity known as "frame-dragging." This is when a rotating mass can actually drag and distort surrounding spacetime. Stockum's idea involved a cylinder of infinite size (although not physically possible), spinning around at the speed of light, it would drag the spacetime around with it. The traveler, who moves across the cylinder will move at fantastic velocities. They would appear, to an outside observer, to be moving faster than the speed of light. If one made a complete trip around the cylinder, they would have returned before they left, hence, went back in time!

The faster the cylinder spun, the further back in time you would go. However, you could not go back farther than the creation of the cylinder itself. However, there are two phenomenon that make such a time machine impossible:

Cylinders cannot be infinitely long.
The cylinder, would move near the speed of light. Hence, the centrifugal forces would be so large, that the material that the cylinder is composed of would fly apart.

Kurt Godel

Godel, in 1949, proposed a solution to general relativity where, the universe, is rotating. That being said, if one could travel in a rocket ship around this hypothetical Godel universe, you would return to your starting point and shift back in time. In this scenario, you could, in principle:

Travel between any two points in spacetime in the universe.
Any event, from any period of the universe's history could be visited, despite how far in the past.

However, it is possible that a Godel universe could collapse on itself due to it's own gravity. The idea is that the centrifugal force from the Godel universe's rotation would balance out this force of gravitation. The Godel universe must be rotating at a certain velocity. Larger universes would have a greater tendency to collapse, hence, would have to rotate faster.

Einstein will also point out: the universe does not rotate, it expands.

Ezra T. Newman

In 1963, Ezra T. Newman, Theodore W. J. Unti and Louis A. Tamburino, in the mid-1960s discovered a bizarre solution to Einstein's equations. The solution they found was so weird that they dubbed it the "NUT solution". This solution allows for closed timelike curves and time travel.

NUT solutions allow a strange kind of time travel and allow for a strange distortion of space. On the surface, the solution resembles a typical black hole. However, if you took a trip around this black hole, 360 degrees, you would not return to where you started. You would wind up on another sheet of the universe. The topology of a NUT universe is like a spiral staircase.

Kip Thorne

A wormhole

In 1988, Kip Thorne, Michael Morris and Ulvi Yurtsever showed that it could be possible to build a time machine if someone could obtain some exotic negative matter (which has never been seen to exist) or energy (which only exists in small quantities). The idea behind negative matter and energy is that they can make a wormhole traversable. This means you can make a mild two-way trip through it. However, the problem with this kind of matter is that it is repelled by ordinary matter. It possesses antigravity. This makes it extremely difficult to find (if it even exists at all). That is not to say that there may not be some negative matter floating in deep space, that is possible. Negative energy, on the other hand, is possible, however, it is extremely rare.

Hendrik Casimir

Casimir effect...

Casimir effect... another visual...

Hendrik Casimir, in 1933, showed:

Two uncharged parallel metal plates, placed only a few nanometers apart, can create negative energy.
There was a very small attractive force between these two plates.

This effect was measured in 1948, and it was shown that negative energy is a physical possibility. This is known as the Casimir effect and it exposes a bizarre feature of the vacuum of space. In quantum mechanics, the vacuum is filled with virtual particles that come in and out of nothingness. The Heisenberg Uncertainty principle allows for this brief violation of the conservation of energy. There is a probability that an electron and antielectron pair can come into existence, and annihilate with one another.

Since there is very little distance in between the two plate (several nanometers), then, there will be more virtual particles around the plates than there are in between them. This would create a force that slightly pushes together the plates. The force would grow stronger as the distance of separation decreases.

Thus, the time machine would look something like this: there would be two parallel plates separated by some short distance. The plates would be re-shaped into a sphere with an inner and outer shell. A wormhole would (somehow) connect these two spheres, creating a tunnel in space. One of these spheres, in sent into some sort of advanced high-speed spacecraft, soaring at some velocity near the speed of light, causing time to slow down. This is a consequence of relativity. Thus, if one could traverse through this wormhole, from the Earth, into this accelerated sphere, where time has slowed down: they would wind up sometime in the past! However, this kind of time machine could not take you back farther than the creation of the time machine itself.

However, there are problems with assembling this kind of time machine:

One must obtain large quantities of negative energy, which is rare indeed. Stephen Hawking has showed that, for all wormhole solutions, negative energy is required.To keep the mouth of the wormhole open and stable, large amounts of negative energy are necessary. Negative energy created using the Casimir effect would result in a wormhole too small to travel through. There are other places, besides using the Casimir effect, that we may find negative energy:

A rapidly moving mirror would accumulate negative energy in front of it as it travels near the speed of light. However, this would only produce a small amount of negative energy.
High powered laser beams can extract negative energy. There exists both positive and negative energy in some pulses of laser energy. However, they will last for only 10^-15 seconds. It could be possible, however difficult, to extract the negative energy from these laser energy states.
Black holes have some negative energy near their event horizon. Due to the Heisenberg uncertainty principle, it is possible for black holes to emit radiation and slowly evaporate energy. This radiation can escape the enormous gravity of a black hole. If positive matter is thrown into a black hole, the event horizon expands. However, if negative matter is thrown into the black hole, it’s event horizon will contract. That is what black hole evaporation is: the production of negative energy near the event horizon.

The next problem is the question: where do we find these wormholes? Do wormholes occur naturally in the spacetime foam? What is the spacetime foam? John Wheeler, was trying to answer a question first posed by Zeno, the Greek philosopher: what is the smallest distance? What Wheeler stumbled upon was that, at the Planck length (10^-35 meters), the curvature of spacetime could be much bigger. Space was not smooth at all at the Planck length. There was large curvature and resembled a foam. This lumpy space is filled with bubbles that pop in and out of the vacuum very quickly. These bubbles are actually, tiny wormholes and tiny universes. These tiny wormholes and baby universes can pop in and out of the vacuum with ease. So, if there are wormholes in the quantum foam, then, perhaps, some advanced civilization could somehow stabilize one with negative energy and expand it. This is possible, however, would be extremely difficult.

What should also be noted is Stephen Hawking's 1992 chronology protection conjecture." This proposal was that time travel is prevented by the laws of physics, on all, except submicroscopic scales.

Charles Misner

Charles Misner discovered a simplified universe that could be applicable to understanding time travel. This is called Misner space and a good way to understand it is by picturing the room you are sitting in, as the entire universe. Opposite walls would be identified with each other. If you enter one wall, you will immediately emerge from the other wall. The ceiling is also identified with the floor. You would also see a carbon copy of your backside, looking through the transparent walls. However, you would never see your face, only a carbon copy of the back of your head. The reason Misner space is useful for our conversations right now about time travel, is that, it has the same topological properties as a wormhole. However, Misner space is much easier to work with mathematically. If we could make the walls move in a Misner space, time travel could be theoretically possible. You could travel so fast that you actually travel back in time. According to Hawking, the left and right wall would be representative of the two mouths of the wormhole. However, there is an instability: if a beam of light goes from the right wall to the left wall, it would gain energy every time it emerges from the right wall, until has reached infinite energy, which is impossible.

"Opposite walls would be identified with each other. If you enter one wall, you will immediately emerge from the other wall. The ceiling is also identified with the floor. You would also see a carbon copy of your backside, looking through the transparent walls. However, you would never see your face, only a carbon copy of the back of your head. "

This is an image of what Misner space would look like.

J. Richard Gott III (Princeton)

J. Richard Gott III from Princeton, in 1991, proposed another solution to the Einstein equations that would allow for time travel. This solution allowed for a phenomenon known as cosmic strings. Cosmic strings are a prediction by some cosmologists and are relics of the Big Bang. The width of the cosmic string is thinner than an atomic nucleus. However, they may extend for millions of light-years in length. Gotts idea was that, two cosmic strings, heading toward each other, can be used as a time machine, right before they collide. Space could contract if you were to make a trip around a pair of colliding cosmic strings. Space would shrink. You would only appear to be exceeding the speed of light, to a distant observer, thus, special relativity would not be violated. You could take a trip to the past. However, enormous amounts of energy would be required to do this. You would need strings with a weight of about 10 million billion tons per centimeter. These strings would also have to be moving in opposite directions at 99.999999996% the speed of light. However, this is possible. High energy protons have been observed reaching these velocities. That being said, cosmic strings, if they exist are rare. Colliding cosmic strings may be even rarer.

A simulation of what a network of cosmic strings may look like...

There are certain spacetime geometries in general relativity (Einstein, 1915) that would allow one to time travel to the past. These are the solutions that allow for travel faster than the speed of light. In relativity, if you can move faster than light, you can go backward in time. However, this could require an infinite amount of energy. Kurt Godel, showed a solution to the Einstein field equations that can allow a CTC (closed timelike curve) in 1949. This is a worldline (a history or path traced out by a particle in spacetime), that returns to its place of origin, thus allowing a loop in time. However, the conditions required are too extreme to probably ever even be possible. Nothing, however, is certain until we have a full theory of quantum gravity, such as string theory.

Wormhole

Hypothetically, a traversable wormhole, since it is a connection between separate points in space and time, could be used as a time machine. There are two ways one would go about constructing such a device:

One end of the wormhole would have to be accelerated to some fraction of the speed of light. Then, it would have to return to it’s point of origin. In this scenario, what is needed for time travel, is very fast speeds. This is a consequence of time dilation: a difference measured in elapsed time by two observers, as a consequence of their velocities being different relative to each other. In other words: a moving clock, ticks slower than a stationary clock. That being said: the entrance to the wormhole that is accelerated to this relativistic velocity, will, be younger. One moving from the younger, accelerated end of the wormhole, to the other end, would thus, be moving back in time, from the point of view of an outside observer.
One of the entrances to the wormhole would have to be moved into another gravitational field with a more intense gravitational pull than the original entrance to the wormhole. Then, it would be moved near the original entrance. In this scenario, what is needed for time travel, is very strong gravity. This is a direct consequence of gravitational time dilation: a clock near a source of strong gravitation will tick slower. Hence, the entrance to the wormhole that is placed in an area of stronger gravitational pull, will have aged less and will be younger. Thus, if one were to go in the younger end, he would come out of the older end, when the older end, was the same age as the younger end (when it was initially entered). From the point of view of an outside observer, you have gone back in time!

Image simulation of what a traversable wormhole may look like.

The idea, for both of these methods, has to do with time dilation, a difference in elapsed time as measured by two different observers. Time dilation is either caused by the objects moving at different velocities relative to each other, or, the two objects being at different positions, relative to the location of a source of gravitation. In either case: the end of the wormhole that moved, aged less. The moved side of the wormhole is younger. Thus, an observer, going through the wormhole, starting at the younger end, would come out of the older end, at a point in time when the older end’s age was the same as the younger end’s age when it was entered: thus, going back in time from the point of view of an outside observer!

A closed timelike curve

Closed timelike curve

These are worldlines (the path that particles trace out in spacetime), that form closed loops, in other words, they return to where they started. There are solutions to general relativity that allow for such a construction.

Alcubierre drive

This is a speculative idea based on general relativity named after Miguel Alcubierre, who first proposed it in 1994. In this hypothetical scenario: a spacecraft would achieve an apparent: faster than light travel, if a field with negative mass could be created. The way the spacecraft would do this, is by contracting the space in front of it and expanding the space behind it. Since objects cannot accelerate faster than light in normal spacetime, the Alcubierre drive, would shift the surrounding spacetime, allowing an object to arrive at its destination, faster than light.

This warp drive alters the topology of space by shrinking the space in front of you and expanding the space behind you. Remember, space itself, can expand faster than the speed of light. Thus, if space can be shrunk faster than the speed of light, than perhaps, it is possible to travel faster than the speed of light. According to Alcubierre, this is a real solution to Einstein's equations. It is within the laws of physics, however, you would need large amounts of positive energy (to compress the space in front of you) and negative energy (to lengthen the distance behind you) to power your ship. Also, there is a problem with using the Casimir effect to produce this negative energy. The plates would have to be a Planck length (10^-35 meters) away from each other. This is impossible by conventional means.

Alcubierre drive

Tipler cylinder

This is an approach to time travel that requires a dense spinning cylinder. The cylinder would have to be infinitely long, and be spinning about its axis at a sufficient speed. A spacecraft flying around the cylinder in a circular path could move backwards or forwards in time. However, ordinary matter, sadly is not strong enough to construct such a device.

A Minkowski diagram of what a tachyonic antitelephone would probably look like

Tachyonic antitelephone

This is a hypothetical device that can send signals to the past in theoretical physics. The idea is that, if a signal travels faster than light, it can be received, before it is sent. Thus, the signal has moved backward in time. However, tachyons, (the hypothetical particles that move faster than light) have not been shown to exist. This scenario remains hypothetical.

Sadly, there is very little support from theoretical physics for time travel at the present.

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