Anyone have any good reads they want to recommend?
On the popular level, "Black holes and time warps - Einstein's outrageous legacy" has some material about time travel. Also, there are a few articles by Robert Forward (I don't recall the titles) and John Cramer's "Alternate views" column (which you can find online).
If you want serious papers, you can try the following, if you can find them. (Most of them are quite advanced)
At what level?
pervect gave a number of good technical references.
For an excellent, non-technical reference, have a look at the second edition of Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction by Paul Nahin. This is a wonderful book that is written for the educated layperson.
Physicist (and relativist) Kip Thorne wrote a foreword for the second edition of this book, and here's a quote from this foreword: "It now is not only the most complete documentation of time travel in science fiction; it is also the most thorough review of serious scientific literature on the subject - a review that, remarkably, is scientifically accurate and at the same time largely accessible to a broad audience of nonspecialists."
In a few posts in this sci.physics thread, I attempted to explain a wormhole time machine like one in pervect's fourth technical referenece.
The spacetime diagram in my first post looks better when Show original under show options is used.
I should probably add the "billiard ball" paper, which addresses a billard ball version of the Grandfather paradox. The "Grandfather paradox" is a causal loop where someone goes back in time and kills his own grandfather). The billiard ball version involves a billiard ball whose future version travels back in time and obstructs the original from entering a wormhole.
H. G. Wells "The Time Machine" should convince you not to try.
If you get the Science Channel, they have a couple episodes on time travel. One is called TIME TRAVEL as fate would have it. It's pretty decent. Get's into parallel universes and the Grand Father paradox, amongst many other things. It's only 30 min.
Hi. it's probably too late for me to post this but I'm bored so might as well. Before you go searching for the secret of time travel you should know that every modern book on theoretical physics contains at least one chapter on time travel and every one says the same thing. It's blah, blah, blah, more energy than is availllable iin the entire universe...blah, blah, blah, find a way to reposition a black hole, blah, blah, blah. Some physicists even reject (it can be rejected) the idea that time travel is possible all together. So are there any good time travel references? Not really, unless you're bored.:grumpy:
Just have your deflector emit chromoton particles and enter the tear in the space time continuum that forms in front of the ship --
Sorry, just saw First Contact on TV the other day.
Here is my opinion on time-travel paradoxes:
For the sake of the discussion, time travel is possible.
Could a person go back in time with the intention, for example, of killing Hitler, hoping to prevent the war? Obviously he can, but could he succeede?
The grandfather paradox comes to mind, but there is more to it than that.
Well assuming time travel IS possible, it is fair to speculate upon what restrictions or requirements are imposed on such travel.
I would assume what ever the tool or mechanism (Worm hole etc.) the following limitation would apply:
Based on the time as measured in local vacuum; that is not on a “slow time caused be a large Mass. And the MBR as a local fixed “preferred reference frame”.
That any movement forward or backwards in time by an amount “T”.
Would result in an associated one way displacement of spatial location by a minimum of twice T times the speed of light (2cT).
With this limitation there would be no possibility of undoing Hitler’s work or invoking the Grandfather Paradox.
However others would remain that would say even this sould not be possible.
Correct, he couldn't do anything to stop the war. Could he do something that would change it?
There's no need to invent totally fictitious laws of physics (introducing a local preferred reference frame would be seen as very implausible, for one thing), which we aren't supposed to do in this forum anyway. Better to stick to known laws of physics like general relativity, which already suggest at least a possibility of time travel. Most physicists who analyze time travel or "closed timelike curves" using GR (like Kip Thorne, for example) assume that there is only a single unchangeable spacetime, which would mean you could never change anything in the past, although you might play a role in causing events that were already part of your history all along. There have also been some attempts to apply QM to these situations to reach the same conclusion that only self-consistent histories are possible (see the Novikov self-consistency principle, for example), although I've also read that the physicist David Deutsch argues that the many-worlds interpretation of QM would imply that time travel would take you to a different alternate "world", meaning you could change the past (but Stephen Hawking argued in a lecture that even if the MWI is true, time travels would stay in a single self-consistent 'world'--see his comments in this lecture).
There is no difference between stopping and changing.
Defining a common “local” preferred frame based on the MBR for any speed object can be done using SR, not implausible at all based on current science. What cannot be done is defining “A Single” preferred frame that would be common for any location in the universe – the fact of the Hubble Constant scientifically demonstrates that the local frame at some distant location cannot be the same frame as our nearby location.
What is implausible is this threads premise being used for the “sake of the discussion” that “time travel is possible”.
Just as the speculations[\b] of time travel within the Theories of GR QM MWI, etc. even by notables like Thorne, Novikov, Deutsch, or Hawking are just that, speculations and not a part of demonstrated science.
Which is why those theories must still be considered theories and NOT “known laws of physics” not yet at the level of say the Conservation Laws of momentum, mass & energy.
Even the best of them GR & QM have unresolved questions and doubts still being tested, including the idea that time travel is not ruled out by those theories, that alone to me means they could likely be incomplete theories, and thus not ready to be describe as part of the definitive “laws of physics”.
I can see this discussion is going downhill since I last visited it....
True statements: nobody except Ron Mallet is seriously working on an actual time machine.
Important notes: Papers challenging Mallet's approach have been published. They apparently have not convinced Mallet of the error in his approach, but they've convinced (for example) me. (See the Wikipedia article on Mallet for more details or look for his name in old PF posts).
False statements: "Blah blah blah, more energy than is available in the entire universe" - this is total technobabble, with no informational content, and it does not represent the level of serious work that has been done on the topic. For instance, you'll not find the above remarks or anything resembling them in anything I quoted, even the popular works.
If you are imagining the local laws of physics would work differently in that frame than they would in any other local frame in that region, that would be a major violation of known physics, akin to re-establishing the existence of the aether. And if the local laws of physics don't work differently in that frame than in any other, then the frame isn't "preferred" as physicists use the term.
My point was that this forum certainly allows discussion of the theoretical predictions of well-confirmed theories such as general relativity, but it doesn't allow discussion of phenomena which aren't predicted by any mainstream theory. And what is your basis for judging time travel any more "implausible" than, say, the idea that black holes have event horizons, which is also a theoretical prediction of GR for which there is no direct experimental evidence?
He could not have stoped it, because if he did, why would he go there. If he had, by accident killed a general, he could change the war, but not to any extent that would have taken away his reason for trying to stop it.
Plausibility of event horizons vs. time travel
I wish to avoid "debunking" in this forum, but I will attempt to cautiously inject what I hope everyone here will agree is a reasonable "reality check".
Few astronomers would agree that there is no (more or less) direct observational evidence for event horizons. Although you could probably quibble endlessly over what is "direct" versus "indirect" observational evidence (I assume "experimental" was a slip), most astronomers have felt for quite some time that the evidence that black holes exist (indeed are ubiquitous) in nature is overwheliming; see for example http://arxiv.org/find/astro-ph/1/au:+Rees_M/0/1/0/all/0/1, particularly surveys like http://www.arxiv.org/abs/astro-ph/0401365
This does not imply at all that further observations which tend to confirm or challenge the existence of event horizons in nature are without interest; far from it, as the papers of Rees demonstrate! I might also mention that various groups are trying to lay the foundations for the theory of optical and acoustic analogs of black holes, which might one day allow experimental (as in, "laboratory experiments") studies of effects such as Hawking radiation which we expect to be associated with event horizons.
Most physicists do seem to regard "time travel" (in the sense in which this term has recently been employed in various speculative papers in gravitation physics) as implausible, but this remains an area of active research. See http://arxiv.org/find/gr-qc/1/ti:+AND+curves+AND+Closed+timelike/0/1/0/all/0/1 and http://arxiv.org/find/gr-qc/1/ti:+AND+time+machine/0/1/0/all/0/1.
Yes, I should have said "empirical".
There is certainly plenty of evidence that very dense objects that fit the profile of black holes exist, but the existence of an event horizon around such a dense object is itself a theoretical prediction of GR, not something there is currently any clear observational evidence for. The second link you provide says:
Of course there is plenty of observational evidence that general relativity is an accurate theory in other circumstances, so I suppose you could count that as indirect evidence for any given prediction it makes, but then time travel is also a theoretical prediction of GR.
They do regard it as implausible, but not because they doubt that general relavity allows it (the technical term would be 'closed timelike curves'), it's more a matter of hunches about what a future theory of quantum gravity will look like, based partly on semiclassical attempts to incorporate QM into GR.
Separate names with a comma.