Faster-than-light and 'hard' scifi

Well, it's just that pretty much all theories build upon that fact that the speed of light is a constant. Except that it's not, as you just said.

And then there's black holes. They screw physics up.

edited 5th Sep '12 7:58:37 PM by AnotherDuck

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Actually, the answer is not to travel in a straight line moving as fast as possible. Someone already mentioned an interdimensional drive. What if this dimension was infact a super compressed version of ours. In that case meters there would be kilometers or more here.

Read the site i linked above. The nature of FTL doesn't matter. Accelerating beyond c is impossible according to relativity, and thus won't happen anyway, but even travel methods that get around this have to deal with the "pick two" problem. Although of course positing that accelerating beyond c IS possible is one of the solutions that pick FTL and causality, ditching relativity. Science does not deal in why. Why is in the realm of theology, science only deals in what and how. Science can tell you how we found that c is 300 Mm/s and how that relates to the way the universe works, and what changes there would be to the world if you changed that (drastic ones, see "atoms can't exist"). By "why" does not enter into it. That is like asking why gravity is configured so apples fall down, not up. Can tell you how it works that they fall down (actually, gravity is probably the least understood of everyday phenomena) but not why.

Go to a physics course at your local university. I'm serious. Relativity is complicated and you will want someone to answer your question if you're trying to understand it.

edited 6th Sep '12 1:27:49 AM by McKitten

Because the photon is massless.

Yeah, I know that doesn't help. I'm not entirely sure which of the rules say that massless things always move at c, but they do; the photon is massless, hence always moves at c. (Special Relativity is basically a description of the conspiracies physics pulls to make sure they always travel at c.)

Although, c's value is what it is because that's how the meter's defined: the distance light travels in 1/299,792,458th of a second.

edited 6th Sep '12 3:23:08 AM by Yej

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A little note re:

The fact that most Wikipedia citations on the topic are far more nuanced makes me think of filing this into "most likely not true/confusing general and special relativity"

Plus, all objections with wormholes, Alcubierre drives etc. also in said citations rarely even mention causality.

"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard Feynman

There is a wiki page on the "pick two" problem? That seems surprising. Even leaving aside however that wiki is not a good source when it comes to highly specialized knowledge, why would any wiki page on possible FTL methods mention causality? It is a problem in fiction, not in reality because all those theoretical FTL methods physics has discovered so far have been practically impossible (requiring more energy than the universe contains and the like)

If you doubt the problem exists, you are very welcome to demonstrate that it is impossible to find a Lorentz Transformation where the order of two events outside of the observers lightcone is reversed. Probably not enough to get you a nobel price, but it'll at least be frontpage article in Sci Am, and make a whole lot of mathematicians look really stupid.

^a) I am talking about wikipedia's citations and b) most serious FTL proposals deal with General Relativity and the rules there aren't quite as simple as in Special one and c) the citations are mostly theoretical studies. Some of them deal with causality. None says that FTL through a wormhole is by default a causality violation.

"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard Feynman

Causality is not a physical law; it's a philosophical concept.

...if you don’t love you’re dead, and if you do, they’ll kill you for it.

@Mc Kitten: Saying that science doesn't deal in "why" is essentially a Mathematician's Answer. Similarly, asking "why gravity is configured so apples fall down, not up" is an erroneous question by the same logic, because gravity doesn't make things fall down, it makes them be drawn towards the centre mass, provided the object of said mass is large enough to affect the apple to be drawn towards it by its gravitational force. Or something.

Also, if you ask, "Why does an apple fall down (from our perspective)?" I'm sure you'd find a perfectly reasonable answer science deals with.^Answer Because gravity.

Try "What proves that c is actually a constant?" if you want a technically correct question.

edited 6th Sep '12 10:34:54 AM by AnotherDuck

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I think i'd rather call it logical construct, but that's just nitpicking, you're right that it isn't a a physical law. And yet, it is so ingrained to our way of thinking that we have to be very careful when throwing it out the window. No it isn't. It's pointing out that why questions (grammatical loopings aside) are generally question of an intentional nature, whereas science only deals in ontology. I.e. one can keep the asking- "why?" game up forever, but in science at a certain point, the answer is simply "why not?". As in, why is pi 3.14.. and so on. There is no scientific answer to such questions, because sooner or later all science boils down to empirical observations which are the way they are simply because the world is the way it is.

Light speed is such a case. It is easy to answer how we derived that c = ~300 Mm/s, and how we can observe that it is constant, but a question of why is it 300 and not 400 is not answerable by science. Science can answer why in this universe it would not be congruent with our observation of reality for it to be 400, I.e. science can answer why the observation of 300 is not an error, but that is a different question.

If we're talking about technically correct questions, that's still not quite it. Science also doesn't deal in proofs. There is a lot of supporting evidence for c being constant from rather direct to very indirect observations, but proof is for maths and philosophy only.

Again, why would that be mentioned? Violating causality is a big problem for common sense (and writing stories) but not for physics. Although it throws up very interesting question (mostly about how causality paradoxa would actually resolve in reality) but there is nothing that says a theory that implies a violation of causality has to be wrong. (as a theory violating, for example, conservation of energy would be extremely likely to be wrong)

edited 6th Sep '12 1:40:25 PM by McKitten

I can accept 'logical construct'.

But the fact that it's an ingrained way of thinking means that sometimes, throwing it out the window is the best way to make the story work. Science fiction is often about, or at least includes a healthy dose of "humans are forced into a situation that requires that they figure out how to deal with/cope with/understand something utterly foreign to them". Tossing causality out the window is a great way of doing that.

...if you don’t love you’re dead, and if you do, they’ll kill you for it.

...something that has happened quite regularly in the thousands of years of human history. Just look at the philosophies such as Flat Earth or variable gravity*

.

Yep. The fact is that Causality isn't necessarily inviolable. We don't even know if it objectively exists. It's just a comfortable way of expecting things to work. That's why I recommended Diane Duane's The Wounded Sky. She writes some lovely parts in there where Scott is trying to understand how causality can work when you've stepped outside of normal time. It's a really good exploration of an intelligent, thoughtful person having to readjust the way they see the world operating.

edited 6th Sep '12 3:21:17 PM by Madrugada

...if you don’t love you’re dead, and if you do, they’ll kill you for it.

I think that many writers will try to conform to causality to avoid Mind Screw scenarios.

"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard Feynman

That too. The point I'm trying to make is that it isn't really on a par with FTL and Relativity in the Pick-Any-Two triad. It's not vital, just handy and comfortable.

...if you don’t love you’re dead, and if you do, they’ll kill you for it.

I think it's fair to say that the theoretical framework of Newtonian mechanics does describe reality "not at all", yes. The framework suggests that time is absolute, that the speed of light has no special status, that velocities can be linearly added, and so on. Those are not inaccurate, those are flat-out wrong, because they are contrary to experimental results in the relativistic domain. Obviously, that does not mean that the mathematical models Newtonian mechanics derives from that framework are useless in the non-relativistic domain. Which is why I phrased it as I did in the first place, as you seem to have overlooked. *scowls*

There seems to be a widespread misconception that Relativity addresses the behaviour of things moving as fast as and faster than light. In my understanding, it does no such thing. Its theoretical framework simply posits that the speed of light represents a certain kind of discontinuity, and it then develops mathematical techniques to describe the mechanics of classical (i.e. non-quantum) bodies in the subluminal domain. Two of those results are that massive bodies can't smoothly accelerate to reach or exceed the speed of light and that massless particles can't move any slower than the speed of light.

That's the part that hard Sci-Fi isn't allowed to mess with, because diverging from it means diverging from observation.

Now, these mathematical models are of course extensible to the superluminal domain, but the theory itself never suggests that such an extension would be valid. Relativity doesn't, for example, predict that tachyons (FTL particles) exist. Nor does it predict that tachyons don't exist. The only link between the two is that scientists who, in the absence of any empirical data, want to speculate about tachyons, find it convenient to employ those extensions of the relativistic model. This approach is taken because, in spite of its dubious validity, it's the best candidate there is.

Therefore, I believe that the notion that "any fundamental change [to Relativity] that would allow FTL without causality violations [...] would mean major changes to reality" is fundamentally flawed, in a way. Reality as we know it is on one side of the light barrier, the supposed causality violation is on the other, and Relativity itself tells us that there is a discontinuity between the two domains. The expectation that Relativity could meaningfully describe FTL phenomena may well be just as naive as the historical expectation that Newtonian mechanics could meaningfully describe relativistic phenomena - except that the older model claimed to be able to do so, while the younger quite expressly doesn't. Which, to repeat myself, is why the theoretical framework of the older model is simply wrong, whereas the one of the younger seems to apply beautifully to the limited domain of its applicability.

So, reconciling relativistic phenomena and FTL is, at the conceptual level, a completely straightforward proposition. One just posits that the superluminal extensions of Relativity are indeed invalid and that FTL is instead governed by TBD (TachyoBaroDynamics?), or whatever. Neither of the two needs to reduce to the other, they simply describe distinct classes of phenomena, just as, say, gases and plasmas are treated distinctly in statistical mechanics. There would have to be some underlying theory which ties Relativity and TBD together, just as particle physics ties the various branches of StatMech together, but that doesn't even have to be addressed, unless the author really really wants to.

The reason that it's not quite that straightforward in practice is that FTL isn't much use as a plot-device unless one also has some mechanism in mind to get classical bodies across the light barrier and back again. That's where it's awfully easy to misstep and make up stuff that does directly disagree with observationally supported predictions of Relativity, and where IMO nobody short of a physicist has much hope of keeping things plausible as far as scientifically literate readers are concerned.

Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.

To do this, you basically have to majorly redefine how spacetime works. Good luck.

edited 6th Sep '12 5:34:37 PM by Yej

Da Rules excuse all the inaccuracy in the world. Listen to them, not me.

@Mc Kitten: And apparently, you don't deal in actual answers. Whether science deals with "why" questions of intentional values or not is irrelevant when that's not what the actual question was about. Also, what science deals in is irrelevant when it comes to me asking a question. Lastly, it was an answer with no practical use, hence a Mathematician's Answer.

And I still think you're wrong.

edited 6th Sep '12 5:59:55 PM by AnotherDuck

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From the point of view of classical electromagnetism, the answer is simple: Light is an electromagnetic wave, which, like any wave, will propagate at a speed depending on the relevant characteristic properties of the medium it traverses. To illustrate, for a sound wave in air, those properties are the density and pressure of that air, because those tell you how the medium reacts if something, like the membrane of a loudspeaker, "pushes" against it. Analogously, for light in vacuo, those properties are the electrical permittivity and magnetic permeability of the vacuum, because those also tell you how the medium reacts if something "pushes" electric and magentic fields into it - which is why those two properties also appear as the proportionality constants in the electric and magnetic force laws, just as G appears as in the more familiar gravitational force law. Calculating the speed of light from those two constants and the fundamentals of wave mechanics is entirely straightforward.

Thus, the answer to your question, from this point of view, is also straightforward: The speed of light is constant because there is, as far as we can tell, no difference between one region of vacuum and another.

Presumably, what you really want to know is why that should have anything to do with massive bodies. After all, we're certainly able to move things through air at speeds higher than that of sound. I think the answer to this lies in wave-particle duality - a supersonic plane is essentially different from a sound wave, but every matter particle has a quantum-mechanical wave nature, which does make it essentially similar to an electromagnetic wave.

As far as Relativity is concerned, this isn't the issue, though. Rather, the special status of the speed of light as the watershed between two domains within which particles behave completely differently is simply one of its axioms, and by their very natures scientific and mathematical theories do not have the capacity to investigate their own axioms. To do that, one has to step outside the theory.

If what you really want to know is why Relativity prohibits FTL, then my answer would be that it doesn't really do any such thing, cf my preceding post.

And if none of these answers satisfy you, then I'm going to have to agree with McKitten. All of this is outside the realm of common sense, and so the best that popular science can do for us is provide analogies, all of which will quickly fall apart once one starts to question them in earnest. Real understanding unfortunately requires looking at the predictive models themselves, and those perforce involve advanced concepts and mathematics which one just has to work one's way through in a systematic manner, such as that provided by a formal course of study.

Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.

Thanks. That's at least a partial answer to the full question (of which I only asked a part), but enough concerning what can be achieved in this thread, I think.

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Are you sure?

It is my impression that for spacetime topologies which aren't what is called "simply connected", meaning, among others, all topologies that include wormholes (as a matter of definition, really), the "default" assumption would indeed have to be that of failing the stronger causality conditions. It would then be up to each "theoretical study" to show that this is either not the case at all, such as by partitioning the spacetime by means of suitable event horizons, or instead to show that the weaker conditions that are still being met are sufficient to at least prevent causality paradoxes and the like.

Simply ignoring the issue doesn't sound like a very sound approach to me, by that light.

What do you mean by it not being "a big problem"? The concepts of cause and effect are deeply engrained in most physical theories - "stress-energy causes space-time to curve, and curvature of spacetime causes stress-energy to flow accordingly", that sort of thing. Even quantum mechanics works within that basic paradigm, for the most part. So, violations of causality at a fundamental level would pretty much abolish all of physics.

Violations of the familiar patterns of causality at a shallower level, such as the temporal order of cause and effect, "merely" produce phenomena that the theories can't handle, such as when a waveform travels along a CTC and ends up amplifying itself ad infinitum, which I guess isn't quite as major a problem as the previous one but still pretty major in my book.

Violations of the kind one encounters in conventional time travel fare, like killing one's ancestors, aren't really covered by physics at all, so if that was what you meant, then I'm agreeing with you.

edited 6th Sep '12 6:52:34 PM by kassyopeia

Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.

Indeed, but what I'm saying is that the author has absolutely no need to spell out how the new "workings" of spacetime, and so doesn't even have to come up with them in the first place. You just posit that the effects are as described, allowing Relativity to hold sway in the one domain and TBD in the other. As long as TBD doesn't make any predictions regarding the relativistic domain, just as Relativity already doesn't really make any predictions regarding the FTL domain, there is no possible way for that supposition to disagree with observations, is there?

Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.

The thing is, Relativity does make a prediction about FTL speeds; it says FTL signals look like time travel. (Just like Newtonian mechanics makes predictions about near-c velocities: nothing special happens.) To have a different result, you have to redefine everything, probably in a way that vastly complicates the structure of space itself.

(And even then, preserving causality in the face of FTL is probably not possible.)

Da Rules excuse all the inaccuracy in the world. Listen to them, not me.

According to my understanding, saying that Relativity applies to FTL is like saying that Ye Olde Mappes apply to uncharted areas, and saying that to get rid of causality-violating FTL you have to "not pick" all of Relativity is like saying that you have to ignore the charted coastlines to get rid of the dragons which supposedly There Be in said uncharted areas.

I realize that your understanding is different, but I know of nothing that would prevent one from simply refusing to extend the relativistic model beyond the various discontinuities and divergences that occur at the speed of light. This is what one usually does when models exhibit such behaviours, you know.

I may be wrong, and relativistic predictions regarding superluminal phenomena may be integral to modelling subluminal phenomena properly. That would make it a much less valuable tool, though, and I do not believe that to be the case.

Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.

Your understanding is mistaken. It is not about using relativity to try and predict what happens when something moves at FTL speeds. You are correct that that obviously wouldn't work because it is something relativity can't model (like black holes). It is about the very fundamentals of relativity, and FTL breaks them, specifically the relativity of simultaneity and the absolute past and future of an event. That's why it doesn't matter how you manage FTL. I recommend reading the sharp blue post again, it has the best diagrams on the matter that i'm aware of. Think of it not in terms of FTL movement but of lightcones. As long as you only move around in the lightcone of an event, simultaneity is not preserved, but the past and future stay absolute, i.e. movement backwards in time can not be observed, no matter what the vector in the lightcone. But if something moves outside of a lightcone (i.e. goes FTL) then it can move backwards in time. (and because all frames of reference are equal, there is always a point of view from which it actually does) If that were not the case, the whole way relativity works would be different. If moving outside of the lightcone did not lead to time travel, then the relativistic effects inside the lightcone would also have to be different. Which means major changes to relativity. (like c not being constant, or frames of reference not being equally valid) Theories in physics are not maps where you can have blank spots, or just erase parts and redraw them. Everything is connected to everything else, change one thing and you change everything.

edited 9th Sep '12 9:05:45 AM by McKitten