If, for whatever reason, someday we discover that FTL is somehow possible despite the laws of relativity and causality, and said laws of relativity and causality are, hypothetically, somehow still apparently intact, what's the best guess we could come up with as to why this is apparently the case?
The Many Worlds Hypothesis, which also goes for other kinds of time travel besides FTL. If sending information or traveling to to the past results in the creation of a new timeline or travel to one, then there's no temporal paradox because it's not your past.
That does have the interesting implication that after every FTL trip you arrive in a different universe than the one you started from.
The real problems come in with things like the Fermi Paradox. Without FTL colonizing a whoile galaxy can be done on the order of a million or so years, and without FTL intergalactic travel is possible. Then it becomes really really strange why we exist and why earth was not demolished or settled by a civilisation whose homeworld is outside the observable universe from our point of view and who has had a billion year head-start. Settled hundreds of millions of years ago at that.
Also FTL kind of implies time travel, and if there is time travel why is there no civilization from billions of years in the future colonizing our past?
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If you look at the equations we know, you're dealing with imaginary mass-energy. That happens with 3-phase electrical circuits when the loads don't balance. Something much more sophisticated will need to be figured out.
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Teleportation, time travel (back in time) and FTL should each allow each other.
Law of physics already proved, that they tend to change (or rather expand on approximations) near extremes. It is entirely possible that different equations govern what happens when you attain FTL and time stops behaving as you would expect it to. Reality already proved to be non-euclidean, why can't it be non-non-euclidean? It's also entirely wild guess.
Do not forget the most plausible explanation when reality breaks down - a new patch to our simulation came out.
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But if a different copy of you and your ship from a very similar universe that just lost their versions of you and your ship by having them transferred to our universe show up in their place, what's the big problem?
What if two or more versions of you arrive at the same destination, and none at all in others? What if the divergences start getting noticeable, so when you arrive back where you started the world is different than you remembered?
It's actually really not that hard to make time travel gel with causality. Honestly, quantum mechanics already does it -- it's called "interference", where unobserved probability amplitudes can cancel each other out so that only some states are ever possible. If you throw Schrodinger's equation at a box that's periodic in time (which is equivalent to a time machine), you just... get the same old limited set of quantized particle-in-a-box solutions that you do anywhere else.
-- Of course, what they mean is a little bit more interesting: only states that are consistent can exist. This basically just means that only timelines that contain no time travel, or timelines that contain consistent timetravel -- ontological loops, "you already changed the past" -- have nonzero amplitude (= "actually exist.")
Which also answers this nicely:
It doesn't answer the Fermi Paradox, however -- why haven't we been conquered yet? To explain that, you need some more exotic assumptions, like a simulation hypothesis ("all the more advanced civilizations just Sublimed and left us lame-os behind in the old universe.) There's even a decent argument that our universe is a natural simulation anyway (look up holographic theory, for example), so this is at least a little less ridiculous than it looks on the surface.
-- Of course, what they mean is a little bit more interesting: only states that are consistent can exist. This basically just means that only timelines that contain no time travel, or timelines that contain consistent timetravel -- ontological loops, "you already changed the past" -- have nonzero amplitude (= "actually exist.")
Which also answers this nicely:
'Cause they didn't, so they didn't.
It doesn't answer the Fermi Paradox, however -- why haven't we been conquered yet? To explain that, you need some more exotic assumptions, like a simulation hypothesis ("all the more advanced civilizations just Sublimed and left us lame-os behind in the old universe.) There's even a decent argument that our universe is a natural simulation anyway (look up holographic theory, for example), so this is at least a little less ridiculous than it looks on the surface.
I'm gonna need math to back that up. I't definitely does not gel with the class I took.
It's basically just a couple of lines, really. We start with the wave equation for a free particle:
LaTeX:
\[
\begin{align}i \hbar \partial_t \psi &= \frac{p^2}{2m} \psi\\ &= -\frac{\hbar^2}{2m} \partial_x^2 \psi\\ \implies\psi&\propto e^{-i\left(kx+\frac{\hbar k^2}{2m}t\right)}\end{align}
\]
And then you impose a periodic boundary condition on t, so that you permit only wavefunctions s.t. psi(t+T) = psi(t) -- but since the spatial and temporal components of every solution are coupled (you'll note that I can write the above entirely in terms of 'k'), this basically just eliminates every wavefunction whose period isn't an integer divisor of the the length of the temporal 'box'. Whiiiiich is basically exactly the same result as in the spatial 1D box case, except that the restriction is on k^2 instead of k.
Which, yeah, changes the result in some important ways (for starters, the energy levels are evenly spaced now), but ultimately, the point stands -- it's not really fundamentally different.
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Physics is the "we're always right" science that changes every day.
Physics has theories. Thermodynamics has laws.
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One possible answer to that: we might be the first species in the universe actually smart enough to ask that question.
Not sure if you are being facetious here?
Thermo is physics, and the only difference between theory and law is that we decided to stop calling things laws because it gives science illiterate people the wrong idea.
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A little bit facetious, but not by much.
Energy is constant, Entropy always increases, and Heat always flows towards the lower temperature.
It's possible to calculate the entropy of a black hole just by knowing its mass. I don't know how to do it off the top of my head, but it's the maximum amount of entropy a singularity can possibly have.
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Not entirely correct, theory becomes law when you got border testing it and getting predicted results.
I'm sure that's how you think it should work. But that's not how it's actually used in practice.
Perhaps more important, the OP is essentially asking "If FTL is real, what then?" Not "Is FTL real?"
And FTL isn't actually against the known laws of physics; it's probably not possible, but as it is now various physicists regularly come up with this or that theoretical technique to pull it off as a thought experiment. Equally regularly someone else then comes up with reasons why it either won't work or isn't practically achievable, but it's always on a case by case basis since there's no actual flat "FTL is impossible" in physics.
It actually reminds me of how before the Laws of Thermodynamics were formulated scientists would regularly design perpetual motion devices as thought experiments to be shot down; I suspect that we are in a similar situation and eventually some fundamental law will be discovered outright forbidding FTL.
And FTL isn't actually against the known laws of physics; it's probably not possible, but as it is now various physicists regularly come up with this or that theoretical technique to pull it off as a thought experiment. Equally regularly someone else then comes up with reasons why it either won't work or isn't practically achievable, but it's always on a case by case basis since there's no actual flat "FTL is impossible" in physics.
It actually reminds me of how before the Laws of Thermodynamics were formulated scientists would regularly design perpetual motion devices as thought experiments to be shot down; I suspect that we are in a similar situation and eventually some fundamental law will be discovered outright forbidding FTL.
I'm not sure how much more fundamental you can get than "You cannot formulate a consistent set of physical laws which allow FTL while also having both cause and effect and the concept of distance"
Except that isn't so, various ways to do so have already been mentioned. And as said, physicists do regularly come up with hypothetical FTL methods.
Physicists just normally treat FTL as impossible because Occam's Razor; there's no evidence it actually is possible, so there's no reason to postulate any mechanism for how it works outside the occasional hypothetical conjecture.
Sending something through a FTL jump causes it to change.
For example, say someone with brown eyes was aboard a FTL spacecraft. A short jump could mean for example, that when they came out into the normal universe again, their eyes were now blue, and had always been blue*.
Changes are seemingly**, but the scale of the changes is directly correlated to how far the FTL ship traveled. Longer jumps = greater changes. All FTL jumps take the same amount of time between leaving the normal universe and reentering it, no matter the distance traveled. The closest to a "speed limit" is the scale of the changes which the mission planners and crew are willing to accept.
* From their perspective and memories. Furthermore, any evidence (pictures, medical records, etc) aboard the ship would change to indicate that things had always been that way. Outside records and memories would remain the same.
** Actually, the ship is just getting switched with an alternate version of itself and its crew from a parallel world. With small jumps, the differences between the ship sent out and the ship received are minor, but large jumps can lead to stuff like getting the Terran Empire's version of the ship or one crewed with highly evolved sapient troodon descendants from a world where the KT asteroid strike missed earth, etc.
I recall a novel where FTL worked like that, but not the title. There's a scene where the characters are passing through the poorly understood natural gateways that allow FTL while speculating that FTL works like that, while without their knowledge their described appearance changes with each FTL transition. I especially recall the woman who is described as resembling the Mona Lisa, except that after each transition the Mona Lisa is also different to match.
Quadhelix
Professional Windmill Hunter
The best answer I can come up with on that would be if FTL, for whatever reason, worked relative to only one preferred frame of reference.
That would mean that, although any particular FTL trip would be time travel from the perspective certain frames of reference, there would be no way to turn your ship around and arrive at your launch pad before your had taken off.
Just as an aside, that sounds suspiciously close to the aphorism, "Relativity, Causality, FTL. Pick 2."
When I run a web search for your aphorism, I get results on Relativity that don't mention locality, but I also get a bunch of results on quantum entanglement -- which makes me wonder if you're maybe inadvertently pulling Bell's inequality into this discussion -- after all, the upshot of Bell's inequality could be rephrased as "Quantum Mechancs, Determinism, Locality: pick 2."
So far as I understood things, "laws" tend to be models that were originally built to match the empirical data without a particular motivating rationale for the model, whereas "theories" tend to be models that were originally built around some motivating rationale, with the empirical details being developed later.
So, for example, Kepler's laws of planetary motion were developed from observing how the planets moved without any understanding of why the moved that particular way. Newton's law of gravitation, then, was developed (IIRC) by deriving that a force proportional to 1/r^2 would reproduce Kepler's laws. The theories of Relativity, on the other hand, were originally extrapolated from relatively simple premises -- Special Relativity from the proposed invariance of the speed of light and General Relativity from the concept that an observer in freefall sees the same physics as an observer not under the influence of gravity. Although both theories of Relativity are incredibly well-tested, both are still theories, not laws.
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