Claiming somethings impossible just because you don't like its implications isn't skepticism, either. And I doubt anyone on this board is seriously advocating that time travel is "real". It's speculation. Lighten up, Francis!

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

I understand your point about not rejecting time travel outright, and perhaps my rhetoric is too harsh on that possibility, but the point I'm making is we allow equally harsh rhetoric, and say "hear hear", all the time on this forum-- directed at nonscientific possibilities with no less support than time travel. That's really my point, not that it's silly to discuss time travel (which it is, but if it's fun to do so, fine), but rather that it is no different than discussing any other "fun" topic like miracles, ghosts, ESP, etc. I probably shouldn't include alternative medicine, because it is important for people's actual well being to know what works medically and what doesn't, but even there the "lighten up" argument can be used-- if people want to spend money on something they think helps them even if it doesn't, why not lighten up on that too? The basic reason is that if we think the truth matters more than what's fun, we have to apply that rule across the board, not cherry pick the magic.

Indeed, if, as we've heard suggested above, that magic is just technology, and time travel is magic, then any future society with access to time travel and other magical technologies can exhibit any magic they want in our current realm, so we cannot rule out the possibility that any magic that doesn't violate a well established law is completely real even in the here and now-- it is "allowed by physics". There's no evidence for it, true, but there's no evidence for time travel either.

So if time travel is possible, anything is possible? Does not compute.

The possiblity of time travel is (somewhat) supported by certain mathematics. I've never heard of an equation for miracles, ghosts or the boogeyman. ESP may or may not be consistent with known physics. But it doesn't matter to this discussion, because if time travel is possible or not, it'll stand or fall on its own merits, not because you choose to lump it together with a bunch of other questionables.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Why not? If we accept the argument that "anything not prohibited by physics is just a matter of sufficient technology", then if time travel is possible, we may assume that in some future time all possible technology will likely be developed, ergo everything not explicitly prohibited by physics becomes possible right here and now. That means any magic, or any miracle, reported by anyone, might actually be just that-- a miracle of technology from a future time acting via a time machine on the present. It is "not prohibited by the physics".
That's because all those phenomena are more complex than can be described by a single equation. What is the equation that tells us a human being can live? Time travel (in the form of CTCs that don't close on an event, which is not what anyone means by time travel anyway) can be related to mathematical solutions to current physics, but nothing that is physical or real. Walking through walls is also a mathematical solution to Newton's laws, it is impossible not because it is not a solution, but because it doesn't include all the relevant physics.
You act as if my criticism of contemplating time travel stems from association with those other things, which has never been my position. I'm perfectly content to let time travel stand "on its own merits"-- which is, not at all. The issue is, the application of a different standard of evidence when something comes from science fiction or imaginary technology rather than some other form of magic. I'm not even saying that it is inappropriate to speculate, I'm saying it's inappropriate to do so in an inconsistent manner where some absurd notions are tolerated and others are rejected, completely arbitrarily. Don't you find it significant that so far everyone on this thread has already stated categorically that they don't personally believe time travel is possible? What can they say about ghosts or ESP that is stronger than that?

Agreed.

Another point I think you more-or-less made: We have the likes of Hawking coming up with a chronology protection conjecture, which sounds to me as if they are thinking, "Well this piece of physics seems to predict time travel, but I don't want it to be true so I shall struggle to come up with something to counter it."

Whereas we do not get physicists or biologists struggling to come up with ESP Negation Hypothoses or Ghost Exclusion Theories or Memory of Water Suppressant Conjectures.

That in itself suggests to me that time travel is now in a different category to those things.

Not really.

For one thing, although time travel is not explicitly forbidden, it is not exactly allowed either. Only through some freaky physics could one might possibly maybe perform time travel and even if so, that would require energies beyond comprehension.

Add to this the paradoxes, displacement ( everything is in motion- moving only through time would not necessarily land you on Earth in the distant past) and a great many other problems that strongly suggest that just because some physics and mathematics do not absolutely forbid it, doesn't make it possible either. Looking at more factors than just those two seems to forbid it quite well.

You are suggesting that some folks are coming up with "Cosmological Constant"s in order to account for the lack of time travel. This too is a "not necessarily." Many folks are looking for working models to explain the observations.

I am on this thread and I have not said that. I said I accept that it might be impossible - entirely different terms to those I would use to describe my attitude towards ghosts and ESP.

What I would find impressive and convincing would be an easy-to-understand explanation of why the accelerated wormhole idea would not work. Perhaps followed by an explanation of why the rotating cylinder idea would not work.

And I would say the only difference is that the Memory of Water Suppressent Conjecture is just more obvious, that's all. We know that atoms lose their memory as they encounter random fluctuations, so we don't need such a conjecture. Hawking is just looking for the equivalent that makes time travel impossible. As for the others, we have no idea how ghosts or ESP could be possible, and no evidence they are, so we don't feel we need to understand why these things don't exist. With time travel, we do have an idealized solution that sounds like time travel (though as I've said the condition for CTCs is quite a bit less restrictive than what people mean by time travel), so it is relevant to ask ourselves why that doesn't exist. Fine, ask away-- that's quite different for imagining that it could be possible. The issue on the table is, what is the criterion for a legitimate openmindedness about the possible existence of any non-phenomenon? And, are we really applying a consistent standard, or are there different standards for pet theories of science fiction?

Oh come on! Physics either forbids it or it allows it. There is no fuzzy region between possible and impossible.

And of course it's going to be freaky physics! That doesn't stop it being physics.

What paradoxes? (Please don't say "grandfather".)

Why would that be an objection?

Such as?

Such as?

Again, show me where the wormhole models break down and maybe you'll have a convert. All you've done here is say it's difficult - and I don't think anybody will disagree with that!

Then tell me-- on what basis can you support a contention that time travel is more plausible than the existence of ghosts and ESP? The latter have countless proponents and anecdotal evidence (none scientifically reliable, of course, we're not in disagreement there), while time travel has none of that-- it just has an obscure and almost certainly spurious solution to a highly idealized equation that I wager does not include remotely complete physics, like the intentional and time-dependent generation of a CTC.
Where does the wormhole come from? How much energy does that cylinder need, and is the introduction of that energy included in the mathematical time dependence of the solution? No, the wormhole would need to already be there (few people even think that wormholes themselves are physically realistic owing to stability problems), and the spinning up of the cylinder is not even included in the physics of that solution. It's me walking through a wall-- allowed by some of the physics, but only if the physics is incomplete. The Emperor has no clothes.

Of course there is such a fuzzy region. Does physics allow me to walk through a wall? It conserves energy and momentum, how much physics does one need to put in to make it impossible? Can even more physics make it possible again? How do you know that no technology could ever make it possible for me to walk through a wall, and how do you know that any technology could? It's totally fuzzy.
Like the freaky physics of walking through walls?
Stability and energy considerations are both completely sufficient to blast the wormhole idea to smithereens, not to mention the complete lack of experimental evidence in their favor. To me it's no different than people trying to spawn new mini universes in their laboratories, long on "gee whiz" appeal but still an embarassment to science when the Emperor has no clothes, no matter how brilliant the Emperor is. It's the same feeling I get when I see people taking scientific equipment into haunted houses, or tests of ESP. It might be viewed as science at some level, but you just know it's baloney, if you have not turned your baloney sensor off because of that gee whiz appeal.

Paul, Ken G just answered it better than I could have- So I'll let his argument stand. Just so you don't think I was ignoring you

Thank you for that.

All you seem to be saying here is that we don't know everything. At the end of the day there either is a (hitherto unknown) way of passing through bricks, or there isn't.

And now I'm going to have to go to work.

Actually, Paul, that was my point. And it seems to me, that is what Ken is arguing for.


Not something I came up with, but I think it's a good point.

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Some try to tell me, thoughts they cannot defend,... - Moody Blues.

What I'm saying is that any miracle or magic you can name might be possible with the appropriate technology/physics that is hitherto unknown, so classifying time travel differently than any other magic/miracle is unwarranted-- yet in practice it is viewed as a plausible possibility despite having no more experimental confirmation than all the rest. The OP stated some reasons for discrediting time travel, but also included a remark that if we substitute "ESP" for "time travel" reads like this:

I have heard that there have been some seriously considered theories about ESP but I know nothing about them. I was wondering if anyone had any input on the topic.

Now, my point is, what kind of response do you think that would have received?

I don't know. Have there been any peer-reviewed articles on ESP published in Physical Review Letters?

OK, Ken, lets take a look at the walking-through-walls example that you've equated to time travel.

My understanding is that individual particles can effectively "walk through walls" at the quantum level - quantum tunnelling. [Stop me right here if I'm wrong]. This is a useful effect and can be harnessed in various ways.

So... we build something based on this phenomenon that maybe doesn't itself walk through walls but is still useful. This device is based on sound scientific principles and clearly works (reliably). That immediately differentiates it from ESP.

Now, let's consider a more far-fetched scenario where we use the quantum tunnelling to move two particles from one side of the barrier to the other without going over it. Then two particles have "walked through a wall". Is that allowed by quantum physics? I think it is.

OK, let's move a million particles through the wall. Still nothing wrong with that, just a scaling up of the effect. Might not be useful; might take more energy than just opening the door and walking through but it's not impossible.

I think the same argument can be followed for time travel. If we can measure it at the quantum level, we can possibly use the effects in some way. If there is a proof that a) we can never use the effects, or b) it can not be measured or demonstrated, then the concept of time travel does return to fantasy and wishful thinking but until then, it's a valid concept and worthy of further study.

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Spike
:)