Manual How to Time Travel: Explore the Science, Paradoxes, and Evidence

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No experiment or observation has ever indicated that time travel is occurring in our universe. Much work has been done by theoretical physicists in the past decade to try to determine whether, in a universe that is initially without time travel, one can build a time machine--in other words, if it is possible to manipulate matter and the geometry of space-time in such a way as to create new paths that circle back in time.

The simplest way currently being discussed is to take a wormhole a tunnel connecting spatially separated regions of space-time and give one mouth of the wormhole a substantial velocity with respect to the other. Passage through the wormhole would then allow travel to the past.

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Although the theoretical properties of wormholes have been extensively studied over the past decade, little is known about how to form a macroscopic wormhole, large enough for a human or a spaceship to pass through. Some physicists believe it may be possible to grab one of these truly microscopic wormholes and enlarge it to usable size, but at present these ideas are all very hypothetical.

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Stephen Hawking has formulated a "Chronology Protection Conjecture," which states that the laws of nature prevent the creation of a time machine. At the moment, however, this is just a conjecture, not proven. In all the searching, however, only one bit of physics has been found that might prohibit using a wormhole to travel through time. In , Deborah A. Konkowski of the U. Naval Academy and I showed that the energy in the vacuum state of a massless quantized field such as the photon would grow without bound as a time machine is being turned on, effectively preventing it from being used.

Later studies by Hawking and Kip S. Thorne of Caltech have shown that it is unclear whether the growing energy would change the geometry of space-time rapidly enough to stop the operation of the time machine. Recent work by Tsunefumi Tanaka of Montana State University and myself, along with independent research by David Boulware of the University of Washington, has shown that the energy in the vacuum state of a field having mass such as the electron does not grow to unbounded levels; this finding indicates there may be a way to engineer the particle physics to allow a time machine to work.

It is increasingly clear that the question may not be settled until scientists develop an adequate theory of quantum gravity. Friedman of the physics department at the University of Wisconsin at Milwaukee has also given this subject a great deal of consideration: "Special relativity implies that people or clocks at rest or not accelerating age more quickly than partners traveling on round-trips in which one changes direction to return to one's partner. In the world's particle accelerators, this prediction is tested daily: Particles traveling in circles at nearly the speed of light decay more slowly than those at rest, and the decay time agrees with theory to the high precision of the measurements.

Once gravity is included, however, spacetime is curved, so there are solutions to the equations of General Relativity in which particles can travel in paths that take them back to earlier times. Other features of the geometries that solve the equations of General Relativity include gravitational lenses, gravitational waves and black holes; the dramatic explosion of discoveries in radio and X-ray astronomy during the past two decades has led to the observation of gravitational lenses and gravitational waves, as well as to compelling evidence for giant black holes in the centers of galaxies and stellar-sized black holes that arise from the collapse of dying stars.

But there do not appear to be regions of spacetime that allow time travel, raising the fundamental question of what forbids them--or if they really are forbidden. For several simple physical systems, solutions to the equations of physics exist for any starting condition. In these model systems, something always intervenes to prevent inconsistency analogous to murdering one's grandfather.

Two different answers are consistent with our knowledge. The first is simply that the classical theory has a much broader set of solutions than the correct theory of quantum gravity. It is not implausible that causal structure enters in a fundamental way in quantum gravity and that classical spacetimes with time loops are spurious--in other words, that they do not approximate any states of the complete theory.

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A second possible answer is provided by recent results that go by the name chronology protection: One supposes that quantum gravity allows microscopic structures that violate causality, and one shows that the character of macroscopic matter forbids the existence of regions with macroscopically large time loops. To create a time machine would require negative energy, and quantum mechanics appears to allow only extremely small regions of negative energy.

And the forces needed to create an ordinary-sized region with time loops appear to be extremely large.

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Newsletter Get smart. Such travel raises neat questions, such as whether you can go back in time and kill your own grandparent a puzzle sometimes referred to as " the grandfather paradox ". Sutter pointed out that the physics of our universe appear to forbid this situation, at least as far as we can see. But surprisingly, some of Einstein's equations from the theory of general relativity may allow time travel into the past.

That theory basically discusses how huge objects distort space-time , which we feel as gravity. So how could Einstein's theory make time travel possible? Well, one way would be to break the cosmic speed limit and go faster than the speed of light — but that likely wouldn't work, because an object going at that speed would have infinite mass. Another possibility would be to form "wormholes" between points in space-time, although this would likely work for only small particles.

There are even more exotic possibilities out there, such as using black holes, huge cylinders or cosmic strings to play with the fabric of space-time.

How to time travel : explore the science, paradoxes, and evidence (Book, ) []

But all of these scenarios end up violating other known physics, like requiring negative mass or infinitely long rotating cylinders. Why does general relativity allow past time travel, but other physics always jump in to spoil the fun? We honestly don't know. But that doesn't mean that scientists are giving up.

How to Time Travel: Explore the Science, Paradoxes, and Evidence

This is a theoretical form of matter that cannot be seen or otherwise sensed with telescopes, but does show itself in its gravitational effects on other bodies. Even the world-renowned physicist Stephen Hawking was entranced by the idea of time travel before his death this year, when he discussed in the Daily Mail how a black hole could make it possible.

The ship and its crew would be traveling through time," he wrote in However, physicist Amos Iron at the Technion-Israel Institute of Technology in Haifa, Israel, said a machine circling a black hole would probably disintegrate before moving that quickly.

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