"How could one build a time machine? 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.
"Easily said--but where does one obtain a wormhole? 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 speculative theories of quantum gravity tell us that space-time has a complicated, foamlike structure of wormholes on the smallest scales--10^-33 centimeter, or a billion billion times smaller than an electron. 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.
"Even if we had a wormhole, would nature allow us to convert it into a time machine? 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.
"Theoretical physicists have studied various aspects of physics to determine whether this law or that might protect chronology and forbid the building of a time machine. In all the searching, however, only one bit of physics has been found that might prohibit using a wormhole to travel through time. In 1982, Deborah A. Konkowski of the U.S. 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.
"Perhaps the biggest surprise of the work of the past decade is that it is not obvious that the laws of physics forbid time travel. It is increasingly clear that the question may not be settled until scientists develop an adequate theory of quantum gravity."
John L. 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.
"Within the framework of Special Relativity, the fact that particles cannot move faster than light prevents one from returning after a high-speed trip to a time earlier than the time of departure. 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.
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Add CommentBefore the blogs were canceled here on Sci Am I had a blog under the username "PHAYEZ" which dealt with, among other things, time and measurement. I put forward that time only exists as a feature of three dimensional existence. Time is the "velocity/distance of 3D matter relative to the velocity/distance of other 3D matter". Outside of three dimensions "TIME" as such does not exist and infinity is equal to zero "time". Also outside of three dimensions measurements cannot be made of anything which makes mathematics, with all due respect, irrelevant since the language of mathematics has, as a syntax, products of measurement. The existence and non-existence of time is a spatial relationship oxymoron which is difficult to grasp when you are an entity who is wholly dependent on being made up of atoms which are moving through space.
Reply | Report Abuse | Link to thisOne final comment, if I may, on the speed limit of light and the fact that even information cannot exceed the speed of light. At the speed of light time is zero, in other words for the light there is no time that passes so that regardless of where it arrives, it arrives instantaneously. Nothing can move faster than instantaneously, even information...
Pierre
username: PHAYEZ (Edmonton,Alberta,Canada)
Before considering this question we must ponder whether we understand time properly or whether we know enough about Big Bang. It seems highly likely that an antimatter universe and anti-time was created alongside the world we observe. Antimatter seems the best candidate for Dark Matter, and Anti-time could well explain the bizarre nature of quantum mechanics, given that the Quantum World that we observe can only be described in probabalistic, uncertain terms, compared to the larger scale Relativistic world. It would seem that at Big Bang there was a flip over in the time dimension of the Quantum world, with that of the anti-matter universe. Thus when Quantum observations are performed, we are actually witnessing time-travel in a sense.
Reply | Report Abuse | Link to thisAlso the reason that Gravity propagation has never been detected is most likely because it too propagates in anti-time. This explanation would also explain the phenomenon of Entanglement. Entanglement would seem to be a microcosm of our connection to the Antimatter universe, which has been in place since Big Bang. Gravity thus appears to operate by virtue of this matter/antimatter connection and not due to a mediatory particle such as the Higgs Boson. The best way of understanding the Status Quo of the Matter/Antimatter universe is to consider Entropy, denoted by S. I would propose that S + ~S = K, where ~s stands for anti-entropy. Thus time-travel as such can only be possible if it does not violate this law. if it were theoretically possible to get a body to travel backwards in time, then it would appear that this law would be violated, unless it's equivalent in the antimatter universe were to travel forward in time by an equivalent amount. It would therefore seem that as the two universes are connected, that time travel in one of these universes would be negated by the affect on the other and therefore it would not be possible.
I was watching Into the Universe with Stephen Hawking which was talking about time travel. I was truly fascinated by the show itself, but I did find one flaw in Stephen Hawkings beliefs. I respect the man more than words can describe, but while watching the show he said, I do not believe that we can travel into the past, but I do believe we can travel into the future. He explained that time around objects of great mass is slower than farther away from those objects. He later explained how we could fly around a black hole, which decreases time in half compared to time here on earth. For example, 16 minutes would pass by on earth while only 8 minutes pass on the black hole. I understand what he is talking about, but this same concept can be applied to travel into the past. This means that if someone were to travel around an object (planet, black hole, etc.) that was smaller than earth for an extended period of time, they could travel back in time when arriving back on earth. I remember Stephen talking about paradoxes, one example being if a person were to travel into the past and shoot themselves. I understand how this makes no sense in the laws of physics, but Stephens belief that travel into the past cannot happen makes no sense either. I would appreciate some intelligent feedback on this subject and I mean no disrespect to Stephen or anyone else.
Reply | Report Abuse | Link to thisI was watching “Into the Universe with Stephen Hawking” which was talking about time travel. I was truly fascinated by the show itself, but I did find one flaw in Stephen Hawking’s beliefs. I respect the man more than words can describe, but while watching the show he said, “I do not believe that we can travel into the past, but I do believe we can travel into the future.” He explained that time around objects of great mass is slower than farther away from those objects. He later explained how we could fly around a black hole, which decreases time in half compared to time here on earth. For example, 16 minutes would pass by on earth while only 8 minutes pass on the black hole. I understand what he is talking about, but this same concept can be applied to travel into the past. This means that if someone were to travel around an object (planet, black hole, etc.) that was smaller than earth for an extended period of time, they could travel back in time when arriving back on earth. I remember Stephen talking about paradoxes, one example being if a person were to travel into the past and shoot themselves. I understand how this makes no sense in the laws of physics, but Stephen’s belief that travel into the past cannot happen makes no sense either. I would appreciate some intelligent feedback on this subject and I mean no disrespect to Stephen or anyone else.
Reply | Report Abuse | Link to thisEssentialy your traveling back to the past if you hang out with pluto or an astroid if you look at it that way, but it can also be thought of as traveling to the future slower then earth meaning that earth just gets ahead of you, or you get ahead of it, what ever wich way. So it depends on how you look at it.
Reply | Report Abuse | Link to thisTime travel seems reasonable
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They are offering $50 for the first person to find a verifiable contradiction between their model and observable physics.