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A question about ftl that recently came to my mind.

Ciber said:
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"
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I mean, you say that, but like, the concept of distance is currently under fire, because any attempt to quantize gravity as a field theory does weeeeeiiird things to spacetime and there's a decent contingent of physicists who are seriously considering the idea that spacetime is just an emergent result of quantum correlations.

Also, FTL doesn't really do anything to causality? Local causality, anyway, remains just fine. It's true that you get some, again, weird nonclassical correlations when you zoom out, but ... we already have to live with that, it's called quantum mechanics :V.

(Now. It's true that, if a CTC exists, then you do have to give up on locality; not everything is predictable just from things in your environment. But again, locality is also under fire, as part of the same giant question mark called 'quantum gravity.')
 
Quadhelix said:
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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What I was told is that, in science, a Law is a description of observations--for instance a (rather simplified) Law of Gravity could be "Stuff falls down"
While a Theory is an explanation for those observations, so a Theory of Gravity would be an explanation of why "Stuff falls down".
So a theory can never "graduate" to a law, because one is an observation, and the other is an explanation. Also, while it is rather unlikely (after sufficient observations) for a law to be proven wrong, it is a lot more common for a theory to be proven wrong (phlogiston, aether, miasma disease theory, etc).

tramp_stamp said:
Physics is the "we're always right" science that changes every day.

Physics has theories. Thermodynamics has laws.
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So while perhaps facetious, there is a certain amount of truth in it.
 
I'm with linkhyrule on this one. Assuming that FTL is possible, and relativity is basically correct, the only sensible expectation is that the universe will remain consistent. This isn't some extra rule added to fix time travel -- it's how the universe always works, in every situation. Closed timelike curves may be unknown, but there's no reason to expect them to be an exception.

That said, we've got the chronology protection conjecture -- and virtual particle loops as a possible cause -- suggesting that such curves may still end up nonexistent, even with FTL as a possibility. The implication is basically that FTL will work up until it turns into time travel, at which point your spaceship will explode. I'm not kidding.

This has some interesting consequences, however.
 
Ciber said:
I'm gonna need math to back that up. I't definitely does not gel with the class I took.
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You were paying attention in the class, congratulations.

It's trivially true ( I am going to vastly simplify here) that "quantum entanglement" theoretically does produce time travel: in essence, a photon will flip back and forth between two possible states – but the flips will exactly mirror the flipping of another photon somewhere else, if the two are entangled.

Two scientists each studying their own photon will therefore get the same results at the same time, faster than the speed of light.

As for the two observers with their photons, while they might achieve the same result simultaneously, they could not confirm the fact with each other any more quickly than light could travel between them.So it's white-board possibility assuming that we have "god eye" view of the whole process, which we don't have in reality. With that "goe eye view" we can construct paradoxes, under which information can go backwards in time.

In grubby and often disappointing reality, usual wave collapse and Born' rule take it off the table.

Now, to the question in OP: it seems that the Alcubierre drive, or theortical process that describes a situation in which space-time is squashed in front of a spacecraft, pulling it forward, while space-time behind the craft is expanded, creating a pushing effect would be way to go with "plausible" FTL.

(plausible as far as ignoring problem of accelerating mass - any mass- to relativistic speed can be called plausible).
 
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I would hazard to use "plausible". It would require a matter with a mass that can only be described using complex numbers. That's usually a sign of unphysicality.
 
Korto said:
What I was told is that, in science, a Law is a description of observations--for instance a (rather simplified) Law of Gravity could be "Stuff falls down"
While a Theory is an explanation for those observations, so a Theory of Gravity would be an explanation of why "Stuff falls down".
So a theory can never "graduate" to a law, because one is an observation, and the other is an explanation. Also, while it is rather unlikely (after sufficient observations) for a law to be proven wrong, it is a lot more common for a theory to be proven wrong (phlogiston, aether, miasma disease theory, etc).
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A scientific law is a prediction based based on a set of observations. A scientific theory is a prediction based on a set of observations and includes even more observations and measurements that provide an explanation as to why things happen the way they do.

A theory can't "graduate" to a law, because for one, it would be a "demotion". And it would be a "demotion" because it would mean removing all the evidence making up the 'why'.

For another, they can't "graduate" or "demote" (which is why, for example, we have a law describing gravity and a theory describing gravity):

Article:
Both scientific laws and scientific theories are produced from the scientific method through the formation and testing of hypotheses, and can predict the behavior of the natural world. Both are typically well-supported by observations and/or experimental evidence.[29] However, scientific laws are descriptive accounts of how nature will behave under certain conditions.[30] Scientific theories are broader in scope, and give overarching explanations of how nature works and why it exhibits certain characteristics. Theories are supported by evidence from many different sources, and may contain one or several laws.[31]

A common misconception is that scientific theories are rudimentary ideas that will eventually graduate into scientific laws when enough data and evidence have been accumulated. A theory does not change into a scientific law with the accumulation of new or better evidence. A theory will always remain a theory; a law will always remain a law.[29][32][33] Both theories and laws could potentially be falsified by countervailing evidence.[34]
 
Ciber said:
I would hazard to use "plausible". It would require a matter with a mass that can only be described using complex numbers. That's usually a sign of unphysicality.
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And I would disagree with you.

While I don't remember all the details, sqrt(-1) does show up in 3-phase electrical circuits.

Voltage fluctuations can do it.

I'm not an Electrical Engineer or Physicist an I know this.
 
tramp_stamp said:
And I would disagree with you.

While I don't remember all the details, sqrt(-1) does show up in 3-phase electrical circuits.

Voltage fluctuations can do it.

I'm not an Electrical Engineer or Physicist an I know this.
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Complex numbers show up a lot in electrical math, because it's convenient. You can map voltage to the amplitude, map the phase angle to the angle of the number in polar notation, and the resulting equations all come out fairly simple. It's even intuitive -- voltages add normally if they're in phase, and the more they're out of phase the more... interesting things get. I've got a DC electrical simulation which I've considered extending to AC, and complex numbers are how you'd do it -- it means you can use large time-steps, yet still get the correct results without attempt to model a sine curve, even though the actual voltage is in fact a sine curve.

(Assuming there's no reactive or inductive load, anyhow.)

But that convenience doesn't mean that any of the fundamental, physical particles in the system have complex-valued mass or energy. It's just summing up vast statistical assemblies of them in a useful way.
 
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Baughn said:
Complex numbers show up a lot in electrical math, because it's convenient. You can map voltage to the amplitude, map the phase angle to the angle of the number in polar notation, and the resulting equations all come out fairly simple. It's even intuitive -- voltages add normally if they're in phase, and the more they're out of phase the more... interesting things get. I've got a DC electrical simulation which I've considered extending to AC, and complex numbers are how you'd do it -- it means you can use large time-steps, yet still get the correct results without attempt to model a sine curve, even though the actual voltage is in fact a sine curve.

(Assuming there's no reactive or inductive load, anyhow.)

But that convenience doesn't mean that any of the fundamental, physical particles in the system have complex-valued mass or energy. It's just summing up vast statistical assemblies of them in a useful way.
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I'm not an Electrical Engineer, nor do I have a College degree. The last time I was asked to design part of an Electrical System it was for a stadium, I had four days to do it, and I somehow got it mostly right.
 
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If my boss asks me to design anything electrical I will outright refuse. A new machine one of the factory engineers built just caught fire due to electrical fault over the weekend and burned down our inspection department.
We employ a full time electrician. I say hook up these motor & sensors & buttons and he gets it done way less flammable than I would.
 
Ciber said:
If my boss asks me to design anything electrical I will outright refuse. A new machine one of the factory engineers built just caught fire due to electrical fault over the weekend and burned down our inspection department.
We employ a full time electrician. I say hook up these motor & sensors & buttons and he gets it done way less flammable than I would.
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Someone fucked up.

The 12V lead-acid battery my car has is protected by a 100Amp fuse. It can provide a jump start to another car by way of positive-to-positive and negative-to-negative (negative-to-ground is recommended when using jumper cables).
 
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Ciber said:
I would hazard to use "plausible". It would require a matter with a mass that can only be described using complex numbers. That's usually a sign of unphysicality.
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Indeed.

Even though I also like it as a fun thought experiment nerds put waaaay too much stock into the Alcubierre drive.

The way it was invented was basically the guy asking himself "hey, can I describe a metric (so a shape of spacetime) that is globally flat but two points that have a space-like separation are connected by some weirdness warping that creates a path that is locally time-like everywhere" - so basically an FTL connection as far as faraway observers are concerned. He did that and then as an afterthought he ran the Einstein equations backwards to see what mass and momentum distribution corresponds to that metric.

The result was the well known issue of "lots of mass, some of it negative magically distributed along the path and moving in the exact right way. Not actually complex mass but negative for sure but still - this does not in any way describe anything that could be plausibly described as a "mechanism" for FTL travel because it's entirely unclear if such a mass distribution is physical.
 
Second Mover said:
Indeed.

Even though I also like it as a fun thought experiment nerds put waaaay too much stock into the Alcubierre drive.

The way it was invented was basically the guy asking himself "hey, can I describe a metric (so a shape of spacetime) that is globally flat but two points that have a space-like separation are connected by some weirdness warping that creates a path that is locally time-like everywhere" - so basically an FTL connection as far as faraway observers are concerned. He did that and then as an afterthought he ran the Einstein equations backwards to see what mass and momentum distribution corresponds to that metric.

The result was the well known issue of "lots of mass, some of it negative magically distributed along the path and moving in the exact right way. Not actually complex mass but negative for sure but still - this does not in any way describe anything that could be plausibly described as a "mechanism" for FTL travel because it's entirely unclear if such a mass distribution is physical.
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Basically this. If you want to go FTL the square root of -1 is going to show up somewhere and you're dealing with imaginary mass-energy in ways we do not yet understand.
 
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Everytime someone discusses ftl i think to myself, have they considered how to keep humans alive going at that speed or would it have to be slowly ramped up to?

I imagine the first test done with monkeys ends with goo on the back wall due to insane acceleration

Maybe gravitic control to counter the force of whatever acceleration occurs will be necessary because even without gravity a massive change in inertia chances blackout or liquification. . . Liquidation. . .turning into human soup, get what i mean?

Its why space fighters in movies are pure fiction because the turns and manoeuvres used would cause blackout and death at the speeds portrayed to anything with flesh

The weight of your body moving one direction once changed by your ship sharply in any direction going at speed would rip all the fleshy bits from your skeleton wich might still be in the seat because of a harness

Space would be filled with stuff like this because you won't feel acceleration without gravity (or very little), then splat
 
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FTL is by definition space magic. If you don't want inertia to affect your space magic, then don't.
 
@MightyMutt
It's literally impossible to accelerate to FTL speeds. You can't get there from here -- it doesn't require infinite acceleration, infinite acceleration would only get you to the speed of light and not past it.

This is really a statement about geometry. Our universe is Lorentzian (roughly), which means the geometry of (time + 1 space dimension) looks hyperbolic, which means... there's a rupture, of sorts, between time and space. Acceleration corresponds to smooth rotation on a space-time map, and there is no smooth rotation that will get you from STL to FTL. Hence there's no need to worry about the needed acceleration; no amount of acceleration will do you any good regardless.

The Alcubierre drive, while certainly impossible for other reasons, sidesteps that problem by sort of... sliding sideways? It doesn't involve the ship accelerating, it involves creating conditions such that the section of spacetime the ship is in moves "sideways" on the space-time diagram, moving the ship at FTL without the ship actually accelerating. Other, more sensible FTL approaches also have to sidestep the problem. For Krasnikov tubes, or wormholes, they do that by changing the overall geometry from Lorentzian to something more convenient... still no acceleration required.
 
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That brings up the question of if it is possible to formulate a consistent universe that is globally Lorentzian but not locally
 
Ciber said:
That brings up the question of if it is possible to formulate a consistent universe that is globally Lorentzian but not locally
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We live in one, so yes. :lol:

Lorentzian space-time is just the special case of Einsteinian space-time where there's no gravity, and the global geometry is exactly flat. Which as far as we can tell is the case.
 
@Vorpal's thoughts on this would be interesting but I haven't seen them around in ages.

tramp_stamp said:
And I would disagree with you.

While I don't remember all the details, sqrt(-1) does show up in 3-phase electrical circuits.

Voltage fluctuations can do it.

I'm not an Electrical Engineer or Physicist an I know this.
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The two aren't analogous. The reason sqrt(-1) shows up frequently in electrical engineering is because a complex number is a two-axis coordinate system, which allows for the representation of geometric concepts. Electricity in the form of alternating current has 'phases', which are sine curves with certain offsets. Those come from trigonometry -- which is geometry.
 
Similarly, while imaginary energies do in fact show up in perturbative quantum theory, they're an 'abuse of notation' used to cram time dependence into a time-independent theory. If you solve the equation exactly, the complex energies go away.

(In fact, you can argue that the non-observability of complex anythings is a keystone of modern physics -- because gauge invariance can be derived from the requirement that only the magnitude of probability magnitudes are observable, and so you must be able to introduce any arbitrary complex phase anywhere you like. And the forces that exist are precisely the maximal set of renormalizable gauge-invariant fields.

+Gravity. Have I mentioned that gravity is weird yet? :V)
 
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Ciber said:
FTL is by definition space magic. If you don't want inertia to affect your space magic, then don't.
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As is freshman high school algebra. The square root of -1 often shows up when the solution to ax^2+bx+c = 0 isn't a real number.

I was taught how to solve those equations two years before I was old enough to even apply for a licence to drive a car.
 
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