Absolutely. But how are you going to test a wormhole solution in GR, to know if that's b.s. or not?
My endeavor has never been to dictate what is possible and what isn't, it has been to point out that the very expression "what is possible' is not very scientifically meaningful until you give it meaning in some experimental context. Time travel has not done that contextual work, not at all. All I have argued is that it should be treated in the same light as other magical ideas that have zero experimental backing, and not given a "free pass" because it can be vaguely expressed in the language of physics (CTC solutions in GR are vaguely connected to the usual concept of time travel, the latter generally meaning a loop that closes on some event. I argue that it is very unclear what CTC solutions actually mean, and they offer no kind of concrete evidence that time travel is even remotely "possible" in any meaningful context for that word).
Now you have it.

All of which seems very interesting and worthwhile from the point of view of understanding the fundamental physics there, but note that achieving a bizarre mangling of time for a single particle is not necessarily a step toward time travel. There are many things that single particles can do that are still manifestly impossible for macro systems. Granted, GR is essentially a classical theory, so one might imagine that what a particle can do in GR can be extended to a macro system at a vastly higher energy scale, but that may be confusing the different meaning of time at the particle level and at the macro level. For example, we can already make an electron go in a loop that closes on the event of its creation-- that's one interpretation of electron-positron pair creation. But that logic does not extend to macro systems, which use an entropic meaning for the arrow of time and are subject to irreversible dynamics because of all the noise modes they couple to.
IMHO, Alcubierre drive is a perfect example of physics taken to an extreme that is indistinguishable from magic. We have no evidence the concept has any bearing on reality. It is like a mathematical proof with a step that is isomorphic to "then a miracle happens".

Are you objecting to the fact that I can argue my position? And as for definitions, don't you know that dictionaries are useless for science? Dictionaries use inclusive definitions, science uses exclusive ones. You see, you do have a lot to learn about science-- the appropriate definitions are a huge part of scientific progress, and using common dictionaries (force, time, energy, particle, golly the list goes on and on) will never cut it.

So my bringing up the need for contextual information to make that meaningful was lost on you? Then let me give you more concrete examples of the uselessness of that definition for scientific thought. First of all, applied literally, that definition is meaningless, because using it with modrrn formulations of physics would imply that anything is possible. Secondly, it gives us no guidance whatsoever, because without including context in the meaning, you can't use that definition to even distinguish levels of difficulty. The point is, there simply is no absolute meaning for that word, but let's say we are using the operational definition that if humanity continues to advance its technology for another million years (and is that possible in itself?), then will it develop time travel as a mode of conveyance? Will it develop ESP, or astral projection? Will it create a magic wand that can turn a person into a newt? How about combining that with time travel, so it could turn us into newts tomorrow morning, or any other miracle? All pretty much the same meaning for possible, sorry.Again, it is not my goal to arbitrarily dictate what is and what is not possible, but rather to point out that time travel is no different from the other magical concepts that we have no evidence are possible. General relativity solutions may give us guidance into a good place to look for time travel, perhaps at the level of a particle in some special circumstances, but that is not evidence it is possible, it merely allows it to be phrased as part of a physical model expressly because it is so much more simply describable than those other miracles. And when we set aside all our "gee whiz" inclinations to want all those things to be possible, in the cold light of rational thought, we recognize that they are probably not, to the point of foolishness.

Actually, there's a pretty big difference-- those threads aren't trying to tell you something about science, aren't challenging you to question the assumptions that people often make that the equations of science are the way to determine the ultimate constraints on reality instead of seeing them as a way that the practitioner interacts mathematically with the art of making predictions. What has always been true in science, always, is that determining the nature of what is possible in reality in any given context can only be done with experiment. Since there is zero experimental support for time travel, ESP, and ghosts, they are all in exactly the same boat. Theory is a guide to what can be made possible only when the theory represents a means to extend experimental results, like the action of gravity if one does not leave a realm for which it has already been demonstrated. That is precisely what is lacking for time travel, it is an extrapolation way beyond anything with any experimental support. This is just the truth, I'm sorry if you don't like it, but it really just proves my point of how people's skeptical blinders come on as soon as science fiction is involved-- a phenomenon I see over and over.

It is true that GR predicts something funky should happen when stellar densities exceed neutron stars, and sure enough we see something funky happening there. It also predicts light should be bent by gravity, and sure enough we see that. So it has great predictive potential-- now why do people take that and extrapolate it to absurd extremes like macroscopic time travel? That's the real question. That's the same fallacy as taking Newton's laws and inferring that reality is deterministic, or taking the Big Bang model and inferring that the universe was once a single point. How many times do we need to learn this lesson about what our own science really is? A conceptual guide for how to provide a context that makes our questions meaningful, but the hands of the practitioner are always all over the result. But you don't want to know this, it's too challenging, so you decide that it is angry ranting without any evidence to support that view.

And what part of that paragraph could not be applied to ESP or any other magical concept? How does that in any way distinguish time travel, and why would you think I've said your thirst for knowledge about it is somehow invalid? What I've said is that it is magic, like those others, and many people may have a thirst for knowledge about applying scientific methodology to magical ideas. The only difference with time travel is that theoretical physics gives us a better guide for where to look for that particular magic. It's still just as much magic as those others, and if something experimental happens for any of them, they will instantly be elevated to the level of real science-- but not before.
Where do you get any of that, because I actually support my positions? You think I'm angry, or shouting? I say I am none of those, but you read it in because you feel challenged. Yes, I am trying to challenge you to think, I'm sorry if that sounds like "ranting" to you.

Thank you Neverfly. I will admit to making my case forcefully, and in the impersonal venue of an electronic forum that can come off as attacking, but my purpose is to challenge people to look critically at their assumptions, and that must be a bit inflammatory to be successful. It's nothing personal, the inflammatory rhetoric is just to stir things up a bit-- the argument stands entirely on the merits of logic and evidence.

Well don't give up the day job, then!

I've seen how people successfully challenge others to think. First they acknowledge the person's curiosity. They say things like, "Yes, I can see where you're coming from. But have you considered X?" They direct them to useful books or websites. They ask guiding questions. They try to explain in terms that the naive but curious person might understand.

Suppose one of my students came up to me and said, "I am trying to come up with a scenario where someone can travel to Alpha Centauri and back in a day. Now I've heard that you can't go faster than light, but surely it's just a speed like any other? I mean, can't you just accelerate up to the speed of light, and then add a bit more thrust?"

My approach would be something like, "Okay. Do you know why the speed of light is special? No? Right, you'll need to know a bit about relativity. There's a very good book in the library which will introduce you to the idea, but here's a few thoughts to get you started..." And so on. Maybe concluding with, "So unless you can come up with some really radical new physics, I'm afraid you're stuck with a minimum of a nine year round trip."

At the end of this the student might have learned something, might even have become fascinated by the actual science.

What I would not do is say, "Faster than light? That's science fiction rubbish! You might as well talk about learning to levitate or turn lead to gold. How are you going to overcome the infinite mass? You can't! End of story. Hey, are you feeling challenged now?"

But that is precisely what I have done. Except it's not that you need to know more about relativity, it is that you need to know more about where relativity came from: you need to know more about what physics itself actually is, and has always been. I can't refer you to a book because this is just what I have figured out about physics by observing it, though I'm sure such books exist. My argument stands on the evidence I bring to it: to wit, the history of science, and careful inspection of what scientific theories actually are. They are guidelines for making experimental discoveries, and they have the hands of the practitioner all over them. There are no fundamental theories that can actually be applied according to a rote prescription, unless you are talking about ultra-simple or idealized systems. Indeed, the whole purpose of science is to create a language that can be used by the practitioner, where the words are the simple idealized systems to which I refer. The attitude that "the fundamental laws of physics provide everything you need to describe reality" is exactly like asserting that "a dictionary provides everything you need to write poetry". No it does not, the act of describing reality, and what is possible in any given context, requires a lot of work for the practitioner, it's not there in the equations. This is just the truth, look at how physics is done.

Yes, and I have equated "radically new physics" with "magic", because that's the same thing.

From where I sit, this is just your opinion. Your objections sound very much like Eddington's to Chandrasekhar's work on White Dwarfs. After all, Eddington objected, Chandrasekhar was trying to combine QM and GR, and that just couldn't work. In addition, his calculations led to such an outlandish conclusion such as what's now known as a black hole.

Then there is this quote by you:

Which isn't much different from Eddington's "I think there should be a law of nature to prevent a star from behaving in this absurd way.”

Both of which are saying the are some new physics to prevent either from happening.
(you really should find copy of Eddington's speach at the January 1935 meeting to realize how similar your objections sound)

It also appears to me that you are just cherry picking which equations of GR you are willing to accept. You kept demanding experimetal evidence for CTCs and when I asked for direct evidence for Gravity Waves, you gave me indirect evidence (the spindown). This seems a bit inconsistent on your part, especially after this comment by you earlier in this thread:

We have a hint for Gravity Waves, based on the equations, but we still don't have direct experimental evidence.

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

I mentioned that example above, because the theory of gravity had to either be wrong in ways that interfere with successful predictions it made, or something had to give when stellar densities reached Pauli exclusion levels. Chandresekhar's work did combine those two theories, but it pushed neither into a domain that was particularly far from where the experimental results already existed on lesser scales. So the prediction that something must give when you put them together was not such a stretch-- but note that exactly what does give is still a matter of exploration even to this day.
A star collapsing under it's own gravity does not seem at all to parallel the possibility of time travel. I must note that I am indeed talking about time travel as it is normally discussed in science fiction, not time travel of a single particle (which as I said happens all the time) nor even CTCs that close on some causally unconnected event and are actually just challenges to standard intuition about the meaning of simultaneity of two distant events. What I'm saying is absurd is the concept of using technology to create a time machine in the normal sense, where people go back and visit their own history, whether one thinks they were somehow already there before or not. It's pure magic, and no over-extrapolated esoteric solution of general relativity makes that science.

For example, let's look at the Chandra situation. He made a prediction that certain stars would do certain things. That's science. He might have been right, and he might have been wrong, but he was not imagining magical technologies, he was simply making a testable prediction. When CTCs are framed in that kind of language, they are science. When framed as precursors to time machines, they are make believe, just as much as an ESP machine based on electromagnetic "auras" or an astral projection machine based on quantum mechanical tunneling. I just find it curious how some of these make believe technologies acquire an air of authenticity, while others are derided as pandering to the lunatic fringe.

It is certainly important to recognize that a very thin line is being tread whenever one tries to figure out what real science is and what it is capable of. The reason I feel new physics will appear to block time travel is because it is an example of taking the concept of time too seriously to think we could really manipulate it that way. But note that my core argument does not claim new physics is needed to block time travel, it's that new physics would be needed to allow it to become a technological possibility. That's why I call it magic, just like all those others-- not because I know it to be impossible, I'm just ultra-skeptical that it will be possible, just as I am with those others.

Have I claimed that gravity waves are real? I just don't think they are magic, because they arise from similar kinds of solutions to GR that have already worked in predicting things like neutron star spindown, nor is anyone talking about building a gravity wave generator and using it to go surfing. GR certainly has great predictive power, as does all of physics, and just like all of physics, it is easy to over-extrapolate that into all kinds of pretty ridiculous predictions about what is technologically possible (like Adams' infinite improbability drive based on quantum mechanics, though at least that's meant as a joke).

I'll let you know as soon as you start. So far all you've been doing is asserting your position.


Ah, there we go. You don't like the idea, so it's "magical". Gotta love that logical, rational arguing you're doing. Very scientific of you.

It may not be your goal, but it's certainly your result.

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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

Can anyone explain to me how time travel is not magic?

We have yet to be inundated with time travelers from the future...

Maybe because in multiple worlds time travel, time travel means leaving your family and friends behind,They will never see you again forever. Not a fun prospect for the loud shirt, khaki shorts, camera brigade. And in single universe, they are here, they just can't say anything. As well, maybe our era is lost to history, our feats taking on an Atlantian gleam of fools gold. We are but myths of legands, at best. We may simply have been forgotten. We are arrogant if we think we are so important, that people would travel through TIME just to visit us. Besides, the 21'st centaury is DANGEROUS, they still KILLED people back then. And the food, oh mercifal marty, is made out of MEAT, taken from ANIMALS, they KILL the animals to get the MEAT. It's DISGUSTING. And the SMELLS, ugh.. they burned oil FROM UNDERGROUND, to make their cars go. It is all just so, SO GROSS!

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How do you know? They've got excellent motivation to stay hidden, don't want to have to memorize a bunch of new kings when they get home.

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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

That is so patently false it is amazing. Read the thread again.

Had my argument actually been "I don't like it", then your point would have been logically relevant. As that is not my point, yours isn't. What I like or don't like has no bearing on this issue at all-- what does have bearing is at what point is science actually informing technology, versus just being a form of mental recreation that uses the language of science without any of the backbone of it.

Yes, I've heard of this form of time travel, which is invented to avoid the paradoxes to which you speak-- but certainly does not avoid the "magic" problem. Multiple worlds is also a perfect example of the over-extrapolation of scientific philosophy to the point of being pure make believe. There is still not a single shred of evidence that multiple worlds exist to the extent that we could visit them, they are purely a pedagogical tool for helping people reduce cognitive dissonance around certain aspects of quantum mechanics. I claim that that dissonance actually originates from not really understanding what quantum mechanics is usable for, but that's another thread. The bottom line is, over-interpretation and over-extrapolation of scientific theories leads to magical thinking. Worse, it's a flavor of magical thinking that gets a free pass from being recognized as such. That has always been true, over all the history of science (consider, for example, Plato's perfect solids, or Kepler's music of the spheres or Newton's mysticism or Wigner's role of consciousness in measurement), and is still true today. That is my main point here (note, Noclevername, that this has nothing at all to do with "what I like").

This presupposes that the technology for time travel will not be available for eons, or else this time would indeed be interesting to visit. If the technology is not available for eons, then it would have to be so far advanced from anything we recognize that who knows what other capabilities we will have-- capabilities that we now label as magical. So indeed, that would not separate time travel from being considered magic today, and would not put it in a separate boat from any other magical idea that is out there.