I think you left out a step there. How do you “build a wormhole”?
Why not do the research on both, if they both are important? BTW, it is a very bad idea to jerk around research budgets. When you lose someone, it is hard to get them back.
I think we’re arguing at cross-purposes at this point.
The constant “c” itself had to be measured in order to be known, etc.
Sure, FTL causes time travel. That’s not exactly news.
As to the results of that time travel, who knows? My money is on not being able to change the past, as whatever you do in the past has already happened, in which case there’s no paradox in the above. This is what GR seems to suggest, but it’s far from a complete theory.
Probably a better example of causality weirdness is spontaneous creation. I have a time machine. I send plans for [magic device that does whatever I want] back through the time machine. Thus we have a perfectly good origin for the magic device - we recieved the design from our future self - but it was never really ‘created’.
Of course such a thing probably wouldn’t arise naturally, and is unlikely to be exploitable, but the example holds on a smaller scale.
Anyway, none of these really provide a convincing argument for causality. They pretty much amount to “Well, if we didn’t have causality then things wouldn’t be causal… and that would be weird.”
Nitpick: The example I cited involves no portion of the system to go faster than light. Also, it’s possible to create scenerios combining warp drives and wormholes that create (however convoluted) causality-violations. You’re right, the take-home message is if we can get around the light-speed barrier somehow, causality can be violated, and that’s way weird.
But is it really that easy to shrug off causality? I mean, since the wormhole examples do not (so far as we know) violate GR, and yet lead easily to causality violaitons (unless I’m mistaken), this is the biggest reason many folks conjecture these modes of transport are impossible, and the rigorous explanation for this will become clear once we figure out quantum gravity.
Hey, cause precedes effect, effect precedes cause, whatever, is that the approach to this we should take?
FWIW, I’ve read the only possible way to rescue traversible wormholes from the causality paradox (the strongest indicator we have they can’t exist), is the many-worlds interpretation of quantum mechanics. In this model, the future the wormhole in the above scenerio leads to is just on of many existing futures, and just so happens to be one that follows a past where I never went back through the wormhole and killed that poor guy’s great-grandfather.
Thing is, how did the wormhole know which timeline I’m on, so as to avoid causality violations? Granted, with a nearly infinite number of possible timelins, the chance I wind up in a causality-violating situation may be small, but it’s not zero, unless the system “knows” what I’m going to do. If causality is absolutely inviolable (I can’t imagine how things could be any other way), then even with many-worlds, I have no free will. Things are already arranged such that, even with wormholes, I’ll never create a paradox, because I simply can’t. The past, the future, all are determined.
Loopydude, it"s been suggested (sorry I can give a cite/site) that one way around the wormhole/ timetravel paradox is to introduce some sort of restrictions on when and where FTL can occur. In the wormhole example, let’s say that one end of the wormhold is near Earth and the other is 100 light-years away. Does that mean you can know what’s happening 100 years from now? Yes and no. You won’t know what’s happening on Earth 100 years from now: at the other end of the wormhole you’ll be just behind the light that left Earth (in normal space) at the time of your departure. In this sense, the wormhole doesn’t travel through time; it simply establishes a new space with it’s own local “now”.
Now if you built a second wormhole, going the other way, then you would run into problems. You could in theory journey to the Earth of 200 years in the future, and then return to our present. So to avoid this, we have to impose a cosmic censorship principle. Say, “no two space-time bridges can be close enough to form closed timelike loops”.
It’s beyond my education to know whether this censorship principle is arbitrary, or violates the idea of no privileged reference frames, or is consistant. But that is the theory as I understand it.
But what physical law or principle could you appeal to that says “you just can’t have two wormholes in said configuration, and that’s that”?
The point is, even without quantum gravity or some as-of-yet unimagined theory of everything, some people already have already come up with some potentiall viable, physics-based mechanisms by which the time-travel interdiction conjecture can be supported. One example I have alluded to is the fact that creating temporal loops allows even vacuum fluctuations in the vicinity of the wormhole to feed back and destroy the wormhole almost instantaneously. Hence, even if you can prop the mouth of the wormhole open and thread its entire length with exotic matter so as to avoid collapse into a singularity, regular positive energy is magnified in the vicinity of the hole with destructive consequences.
All of these arguments are very likely totally moot: Nobody knows how to make matter with negative mass, much less concentrate a gas-giant’s-worth portion of such weird stuff into a hula-hoop-shaped configuration (or any other configuration) so as to keep a natural (or even artificial) wormhole open. Exotic matter may well prove to be a complete physical impossibility.
Which brings me back to the ultimate reason for even discussing any of this wildly-speculative physics mumbo-jumbo: There is virtually no reason at all to think we will ever be able to traverse stellar distances by any means other than to build very fast sub-light-speed spacecraft and fly them around essentially the way we do now. Galactic colonization is possible, even likely, but it will take millions of years before completion, and by the time intelligent life has traversed the Milky Way, it will have become distinct and utterly isolated from its origins, except as an ancient memory. Racial survival is trumped as a concept by the overwhelming likelihood that a different race will reach the far destination than the one that set out on the journey.
That maybe? 
Any, as I posted earlier, I don’t think we need exotic physics to colonize the galaxy (altough FTL would be awfully nice!)
Except, of course, by travelling to the past, you are changing it, because you are appearing in a time and space in which you originally weren’t, unless time travel is limited to things like what Billy Pilgrim could do.
I have no idea who Billy Pilgrim is beyond what his Wiki entry says, which isn’t terribly enlightening.
That being said, I see no reason to believe that you weren’t originally there. It seems perfectly consistent to me that you had been there all along because you travelled there from the future.
You could view time travel as a ‘loop’ along the time line. Nothing is actually changing, we’re just ‘moving’ along that line, and some things split off and move along the loop. The loop exists independently of the particular point on the diagram you’re at, it merely seems to be created and people to be travelling from the perspective of an observer.
See previous disclaimers about this being purely hypothetical. I don’t really believe time travel is likely, I just don’t think it entirely beyond the realms of plausibility.
Incidentally, I’m not sure what I was thinking when I said that E = m c^2 follows from (E/c, p) transforming as a 4-vector. You need some sort of additional assumption in there. As it stands it’s simply not true.
Slaughterhoue Five opens with the lines, “Billy Pilgrim has become unstuck in time.” Pilgrim spends the rest of the novel bouncing back and forth in time, but he can only go to points of his life.
Except, of course, that when the event originally occurs, the future hasn’t happened yet.
Yeah, then I quit doing acid and stopped having those kind of thoughts.
Well, there’s only one way to find out.
So what you’re saying is that because cause has to follow effect, causality must be true? Hmm. Something about the argument strikes me as a tad circular…
Or physics apparently. From the GR perspective, viewing time as a fourth dimension, that’s exactly the right way to look at it.
And yours wasn’t? Your statement about being there because you were always there. Sounds sort of like the whole warm and fuzzy schtick New Agers spout about everything in the universe being one.
I’m sorry, but I don’t recall reading anything which says one can bend time, but not space, or vice versa. Everything I’ve read has stated that you can’t bend one without bending the other. Admittedly, I’ve not read any articles from The Journal of Theoretical Physics on it, but I’ve read enough to know that the whole area of temporal physics is still pretty much the crapshoot which Einstein said God didn’t like. Everyone has a different theory about what time travel is, how it might be achieved, and what might be possible with it. Until someone comes up with hard scientific data supporting one theory or another, it’s all just WAGging.
Err… umm… yes, well of course it does. Except for the part where it’s completely, totally, and utterly different. I haven’t the faintest idea what you’re talking about here, so I can’t even begin to address this point.
Sure. I never suggested space didn’t bend, it’s just rather difficult to describe 4D diagrams so I simplified. Space bending wasn’t relevant to the argument, thus I ignored it. The point was that you don’t really have a system that is ‘evolving in time’. You have a fixed 4D system, and a perception of time is just a trajectory on it. Hence it seems to make sense that if time travel is just a loop in the system, you can’t change the past - if you travelled back in time, then you were there in the ‘past’.
Sure it is. I’m not claiming it isn’t. There is however such a thing as an educated guess; I’m merely pointing out that it’s not something that is completely ruled out by current theories. It’s just improbable given our current understanding.
IOW, you were always there. Which gets us back to the weird New Agey warm and fuzziness that you don’t understand. If time travel is possible, in the manner in which you suggest, then I see nothing to rule out me taking a fully loaded Apache helicopter back with me and using it to shorten the American Revolution by a few years.
Ok. I see where the confusion comes from. What I consider to be a perfectly sensible and internally consistent viewpoint based on scientific principles, you consider to be fuzzy hippy talk. Gotcha. Given that you’re dismissing something out of hand without an actual argument to back it up, I amend the previous statement to not considering the point worth responding to.
Your objection is something that really can’t be answered expect in the case of a specific propose method of time travel (and a better theory of temporal physics than we have now). In the specific case we’ve considered in this thread (the wormhole), there are two things preventing this. Firstly, as a nitpick, you can’t go back in time past the creation of the original wormhole. That rules out the American revolution. This doesn’t matter especially, as we can insert just about any other proposed event you want to change.
Secondly, at least one censorship principle has been proposed for wormholes - I freely admit to not understanding the details, but if your wormholes are close enough together (and long enough lasting) that a photon can follow a closed loop between then, you get feedback from (virtual?) photons which overload the thing. This would, at the very least, make time travel back to somewhere on earth extremely difficult (probably not impossible mind you - I haven’t worked through the numbers. Just Very Difficult). There may be other censorship principles at work.
Finally, and this is a bit of a cop out, there’s nothing to stop you in principle. However, if you haven’t already done it, obviously either it never ocurred to you to do so, or events conspired against it. Maybe the time cops arrested you before you could get around to doing it.
Anyway, this is really quite a hijack. If you want to continue we should probably move it to another thread.
Yes, it’s a hijack, or at least a sidetrack . . . Whether FTL travel can produce time-travel paradoxes or not is tangential to the OP. The point I was making is, the long-term (defining “long-term” in the millions or even billions of years) survival of the human race is assured if and only if we can establish a stable, self-sustaining human presence elsewhere than the surface of our homeworld. Even if that presence is only within our own solar system, if you live long enough to see that, then I think you can die with the sure and certain knowledge that the human race will find a way to colonize the nearer stars, or establish space habitats that can survive independently of the fate of any particular star system. Therefore, it is worthwhile and important to do everything we can, in the short term, to develop space-travel technology, even if such technology does not immediately provide us with any spinoff benefits (which it usually does), and even if it does not lead immediately to a new space-based industrial revolution (as Pournelle, Heinlein, Niven, Dyson, etc., have promised us). Am I wrong?
my 2 cent- opinions that we are alone, or the opposite- what validity have they without research? exploration? we dont even know if we are alone in this galaxy- so many times in humanitys history… “scietific evidence” has supported “conclusions” which are really assumpitons. i know this much- in earths past the ecological systems of the past have been devestated by various things- super volcanoes, earthquakes and asteroids and who knows what else… at the least we should have an inner and outer space defense perimeter- just to protect the homeworld from any natural incoming disasters… beyond that we need the ability to detect any incoming object larger than half a mile in mass within our solar system… this is stuff akin to putting a fence up to keep the wolves out cutting the grass to avoid snakes… basic simple stuff… i will never understand, with all the evidence we have, how the general population can ignore the total lack of safety earth exists in… the billions of years old earth with the millions of year old human race… i mean its like explaining 2+2 and as to the persons math equation that there is nothing- that is irrelevant until we can actually SEE-
The coelacanth might be around until the oceans freeze or boil – but with space travel, our descendants might be around even after that.
Uzi: It is only after a country becomes wealthy that population growth starts to fall and not over extend the already limited resources.
Has India “become wealthy”? Because their population growth rate has apparently already started to fall, and the same may be true for China, but I don’t think of either of them as a “wealthy country”.
silenus: As a member of Western Civilization you are, by definition, rich. To bring everyone up to our level will require massive amounts of cheap resources.
There are quite a few people here in Western Civilization who live on the streets of dirty crime-ridden slums without enough to eat. There is no guarantee that just because a society as a whole possesses massive amounts of resources, all the members of that society will be “brought up to our level” (“our level” being that of literate people with sufficient money and leisure to hang around on the Internet). It depends on how the society chooses to use those resources.
Look, I like dreaming of the final frontier as much as anybody (well, almost anybody), but I just don’t think it’s valid to try to sell space exploration as providing the answer to the problem of poverty by glutting us with cheap materials and technology. In the first place, as we can already see in Terran societies, wealth and high quality of life don’t automatically “trickle down” to everybody just because the elites attain them. In the second place, cheap resources and widespread technology often bring new problems of their own (as we see, for example, in some of the side effects of massive fossil fuel use).
I say sure, keep a limited budget for developing space exploration (though I agree that right now the best investment is in unmanned vehicles). But let’s not try to push it as the best way, much less the only way, of achieving a more prosperous global society and a healthier planet.
BG: * Even if that presence is only within our own solar system, if you live long enough to see that, then I think you can die with the sure and certain knowledge that the human race will find a way to colonize the nearer stars, or establish space habitats that can survive independently of the fate of any particular star system. Therefore, it is worthwhile and important to do everything we can, in the short term, to develop space-travel technology, even if such technology does not immediately provide us with any spinoff benefits (which it usually does), and even if it does not lead immediately to a new space-based industrial revolution (as Pournelle, Heinlein, Niven, Dyson, etc., have promised us). Am I wrong?*
Well, yeah, you are, if the “sure and certain knowledge” that you “think” will materialize about extra-solar-system travel turns out to be bullshit. No guarantee that it will, of course; but if I’m going to devote budgets on the scale of “doing everything we can” to such an enterprise (hee), I’d like to have a much more solid basis for it than your “thinking” that “the human race will find a way” to do it.