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Faster than light travel

Umbran

Mod Squad
Staff member
I thinks that’s survivors bias.

Dude, that's how science works. As a theory passes more tests, you gain more confidence that it is correct.

After all originally it was a steady state theory of the universe.

That's a misleading statement.

I can give an analogy. Do you know any calculus? Take the indefinite integral of x dx, you end up with the answer x2/2 + c. Without knowing the boundary conditions of the integral, you don't know what that constant c is.

There's a similar thing in GR . It has a term (called the Cosmological Constant) for which there's nothing in theory to determine what it is, and it is equivalent to a boundary condition - the overall energy density of the vacuum. It can only be set by observation. As a working choice, he took a value that made sense to him. It is NOT a prediction of GR - it was Einstein imposing his desire for a steady-state universe on his solution. But eventually observation revealed that the universe wasn't steady state, and we accepted new estimates for the constant.

Note that it is a cosmological constant. Not a local constant. It isn't relevant for short length scales (like, a couple hundred yards across an Alcubierre drive spaceship).

Also I believe there was supporting work showing why black holes were impossible. That too fell to observation. Most of the counter things that “checked out” are forgotten.

Not really. Not in the way you implied here.

Einstein published general relativity (in 1915). Within a matter of days1 of receiving a copy, Karl Schwarzschild, colleague and friend of Einstein, wrote back detailing a solution2 to the equations. Schwarzschild had worked out the shape of spacetime around a massive spherical object, like a planet or a star. And, in that solution is it bleedingly obvious that for a massive enough object, you get a singularity.

Einstein liked Schwartzschild's work. As I recall, he didn't have any objection to the idea of "frozen stars" as they were called at the time. However, others were not so sanguine. Arthur Eddington famously said, “There should be a law of nature to prevent a star from behaving in this absurd way.”

And, they set out trying to find reasons why a frozen star couldn't form, or a flaw in Einstein's General Relativity that would keep this from happening. It was rather like people trying to poke holes in the math of the Alcubierre drive, honestly. Thing is, they were all incorrect. No changes to GR were required.

This is how science works. Attempts to disprove a thing, or poke holes in it, are tests. The forumulation of Einstein's general Relativity has been tested many times over - from detection of precession of Mercury's orbit, to discovery of gravitational lensing, to the radiation coming off Cygnus-X1 (the observatin of a black hole), to detection of gravitational waves. No changes to the overall formulation have been required. There are some things predicted by GR that can't be tested yet, but everything it has predicted that we can test, turns out to be correct. It is stunning in this regard, but true.

The outlying issue is quantum mechanics, which is incredibly difficult to match up with Relativity. But, we have yet to figure out what changes, exactly, this may require in GR, if any. It may be QM that needs adjustment.






1. Which was remarkable, because in December 1915, Schwartzschild was literally in a trench fighting WWI.

2. In this context, "solution," means, "application to a particular scenario". Einstein's equations are general, like saying y = mx +b. You need to apply it to a particular situation to say what m and b are, and then you can give a Y for any X you like. Einstein's original paper was the construction of the general form, and did not include applications to particular scenarios.
 

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darjr

I crit!
Dude, that's how science works. As a theory passes more tests, you gain more confidence that it is correct.

Dude that's super frustrating, assuming I meant science in general.

I didn't. I meant picking theories that 'worked out' as support for this idea. Lots of ideas, like this one, don't, yes that's science, but that's my point. This conjecture is fun but a long way from testable.

Anyway I'd be thrilled if it worked out.
 
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Umbran

Mod Squad
Staff member
Dude that's super frustrating, assuming I meant science in general.

I was pointing out that this is no different from science in general.

I meant you cherry picking theories that 'worked out' as support for this idea.

The one thing you mentioned is not actually a prediction of GR. GR will give you different forms of universes, depending on what you give as a cosmological constant. The constant is an input, not a prediction.

It is like, if you use Newton's Laws, and look at a falling object - the distance covered under acceleration is x = at2. If you put a=5 m/s rather than the actual 9.8 m/s, you get an answer hat doesn't match reality. That's not the model being incorrect, that's giving the model bad input.

Please, feel free to find an actual prediction of GR that has turned out to be false. As I said, this is a truly amazing thing about GR. There's a stack of things that we cannot test... yet. But something that actually failed? Good luck finding that.


This conjecture is fun but a long way from testable.

Yes... and no. To deal with a large object, yes, it cannot be tested. However, the principle may be testable. Very small amounts of negative energy density can be created today using the Casimir effect. It may be possible, then, to make a teenie-tiny bubble, and measure it using laser interferometry. There are teams out there trying to do it now. Unfortunately, at the moment, getting a pattern of negative energy density that should make a bubble is extremely difficult, and I believe all tests to date have been inconclusive.
 

freyar

Extradimensional Explorer
This is interesting for sure, and I wish I had some spare time to read the paper. I do wonder if there's some subtle type problem that could crop up, like instability when there's matter inside the bubble or something. Warp drives are a little worrisome because of the time travel issues that come up.

One thing to keep in mind is that the matter/energy required to form the bubble likely needs to be much larger than the mass of the ship inside, or else you'd expect the gravity of the ship to deform the bubble seriously, possibly enough to ruin the effect (unless the author already looked at this).
 

Umbran

Mod Squad
Staff member
This is interesting for sure, and I wish I had some spare time to read the paper. I do wonder if there's some subtle type problem that could crop up, like instability when there's matter inside the bubble or something. Warp drives are a little worrisome because of the time travel issues that come up.

Yeah. But, well... Einstein allows for closed timelike curves. We may find time travel distasteful, but the Universe is under no obligation to have its laws be aesthetically pleasing. Or make sense.

One thing to keep in mind is that the matter/energy required to form the bubble likely needs to be much larger than the mass of the ship inside, or else you'd expect the gravity of the ship to deform the bubble seriously, possibly enough to ruin the effect (unless the author already looked at this).

I haven't gone through the math myself, but the article calls for a 100 yard radius ship a mass/energy equivalent to hundreds of times the mass/energy of Jupiter. Upthread, my back-of-the-envelop calculation puts that as being unworkable on several levels. It may be just short of collapsing into a black hole, but not by all that much.
 
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freyar

Extradimensional Explorer
Yeah. But, well... Einstein allows for closed timelike curves. We may find time travel distasteful, but the Universe is under no obligation to have its laws be aesthetically pleasing. Or make sense.
For sure. But self-consistency, which causality is important for, seems like a pretty important principle.

I haven't gone through the math myself, but the article calls for a 100 yard radius ship a mass/energy equivalent to hundreds of times the mass/energy of Jupiter. Upthread, my back-of-the-envelop calculation puts that as being unworkable on several levels. It may be just short of collapsing into a black hole, but not by all that much.
Also agreed. I was referring to attempts to reduce the energy required to make the bubble. If it gets too small, then you have to worry about whether the ship itself destabilizes it (but maybe the author looked at that already).
 

Higgs field manipulation.

If we can manipulate/ trick the Higgs field into reading the mass of an object at zero (or manipulate the matter to register on on the Higgs field as a mass of zero) we're good to go.
 

Mustrum_Ridcully

Adventurer
Higgs field manipulation.

If we can manipulate/ trick the Higgs field into reading the mass of an object at zero (or manipulate the matter to register on on the Higgs field as a mass of zero) we're good to go.
That would only get us to the speed of light, though. Still no FTL.
And if every atom in your body suddenly has no (inertial) mass, it might not really be your body anymore. Just - stuff flying off into all directions at the speed of light.
 

Umbran

Mod Squad
Staff member
Higgs field manipulation.

If we can manipulate/ trick the Higgs field into reading the mass of an object at zero (or manipulate the matter to register on on the Higgs field as a mass of zero) we're good to go.

So, that's an incomplete understanding of the Higgs field.

The Higgs field does not "read" anything. It is through interaction with the Higgs field that fundamental particles (electrons, quarks, and others that have no smaller parts - what we'd call "fundamantal particles") get their mass. But this is only a small fraction of the total mass of your day-to-day physical matter.

Most of the mass of protons and neutrons comes not from quark interaction with the Higgs field, but from Strong force interactions between the constituent quarks. That energy binding the quarks together is mass (E=mc2). Then, there's mass in an atom that comes from the energy binding the protons and neutrons to each other.

So, no, manipulating the Higgs field will not solve that problem.
 

shawnhcorey

Explorer
First of all, all FLT travel is time travel. In special relativity, if there are 2 events not in each other's light cone, you can change the order the happen in by changing your velocity. To go back to your past:
  1. Use FLT travel to create 2 events: one is you starting FTL travel, the other ending it.
  2. Change your velocity so that the event of you starting FTL travel is far in your future.
  3. Use FLT travel to go back where you started.
You are now in your past.

Also, black holes are not the final word on super-massive objects. An better theory is magnetospheric eternally collapsing object (MECO).
 

Umbran

Mod Squad
Staff member
Also, black holes are not the final word on super-massive objects. An better theory is magnetospheric eternally collapsing object (MECO).

Lots of people have issues with the idea of singularities, and for nearly a century now, there's been folks looking for arguments that they are not possible. MECOs are another in the long series.

In the end, the only clear measure of "better" is fitting the observational data more cleanly, which the MECO model does not, as yet, do. Beyond that all distinctions of "better" or "worse" are matters of human aesthetics, not science.
 


Umbran

Mod Squad
Staff member
First of all, all FLT travel is time travel.

This may not be exactly true. Einsteinian closed timelike curves are, yes. However, the Alcubierre drive may not be. The wall of the "warp bubble" is a barrier between the interior and the rest of spacetime that prevents those inside from having causal connection with the rest of the universe. This may prevent the people inside from following a closed timelike curve - it rather depends on what slowing down and opening the bubble looks like.
 
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shawnhcorey

Explorer
This may not be exactly true. Einsteinian closed timelike curves are, yes. However, the Alcubierre drive may not be. The wall of the "warp bubble" is a barrier between the interior and the rest of spacetime that prevents those inside from having causal connection with the rest of the universe. This may prevent the people inside from following a closed timelike curve - it rather depends on what slowing down and opening the bubble looks like.

This is a common misconception. It does not matter how FTL travel is done. All that matters is that there are 2 events outside of each other's light cone. And the events are starting FTL travel and ending it.

The only way to do FTL travel without time travel is to have a preferred frame of reference. But since both special relativity and general relativity assume there is no preferred frame of reference, having one would invalidate them.
 

Umbran

Mod Squad
Staff member
You’re saying gravity is slowed by gravity?

Not any more than gravity slows down light - which is to say not at all. Things with zero mass (photons and gravitons) move at a fixed speed in vacuum. Rather than slowing down, they lose energy - meaning the their wavelengths get red-shifted. Saying "light cannot escape a black hole" is saying that light originating within the black hole gets red-shifted to nothing.
 

shawnhcorey

Explorer
You’re saying gravity is slowed by gravity?

Is light slowed by gravity? No. The supposition that is given is that gravity expands the radial distance. For a black hole, it is expanded to infinite. Since everything travels at the speed of light or slower, nothing can escape a black hole.
 

Morrus

Well, that was fun
Staff member
Is light slowed by gravity? No. The supposition that is given is that gravity expands the radial distance. For a black hole, it is expanded to infinite. Since everything travels at the speed of light or slower, nothing can escape a black hole.
My brain is just kinda hurting by the concept that something which causes gravity is the very thing that prevents gravity escaping it. I get stuck in this circular self-referential loop, which basically means I don't understand it.
 

Umbran

Mod Squad
Staff member
This is a common misconception. It does not matter how FTL travel is done. All that matters is that there are 2 events outside of each other's light cone. And the events are starting FTL travel and ending it.

Again, the warp bubble complicates this. It takes the interior of the bubble outside everybody's light cone - things inside are causally disconnected from the rest of the universe while it exists. If the event that is "staring" effectively happens inside, there's no violation.

There are some other aspects of the Alcubierre drive that may limit where one goes at what speeds, which may prevent a closed timelike curve from forming. Basically, you may be able to take such a drive from point A to point B faster than light, but it may be that you could not return to your starting point (at least, not any sooner than light would make the round trip) - so the curve cannot be closed.
 

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