"A recent surprise is that one can circumvent the 'grandfather paradox,' the idea that it is logically inconsistent for particle paths to loop back to earlier times, because, for example, a granddaughter could go back in time to do away with her grandfather. 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.
"Then why do there seem to be no time machines? 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. 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.
"To summarize: It is very likely that the laws of physics rule out macroscopic time machines, but possible that spacetime is filled with microscopic time loops.
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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.