# Faster than light travel



## Umbran (Mar 10, 2021)

So, there are solutions to Einstein's equations for General Relativity that allow for effective faster than light travel - you may have seen them referred to as real life "warp bubbles".  The problem with them is that they need a lot of what physicists call "exotic matter" - matter with really weird properties like negative energy density, stuff that we have not observed in nature, and do not know how to make.

Someone's found some new solutions that don't require exotic matter.  However, to move a ship of 100 meter radius, it'd take energies equivalent to the mass of a few hundred Jupiters.

But, that's progress, right?










						Breaking the warp barrier for faster-than-light travel
					

If travel to distant stars within an individual's lifetime is going to be possible, a means of faster-than-light propulsion will have to be found. To date, even recent research about superluminal (faster-than-light) transport based on Einstein's theory of general relativity would require vast...




					phys.org


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## darjr (Mar 10, 2021)

Halo Drives still seem the most realistic possibility. Just got to get our selves to the nearest black hole pair.
Maybe the Halo mechanism could be used to provide the energy needed here?


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## Morrus (Mar 10, 2021)

The Gallifreyans just captured a black hole and use it as a power source. Simple, eh?


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## Ryujin (Mar 10, 2021)

Morrus said:


> The Gallifreyans just captured a black hole and use it as a power source. Simple, eh?



Same with the Romulans. They must have been on to something.


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## MarkB (Mar 10, 2021)

Ryujin said:


> Same with the Romulans. They must have been on to something.



Yeah, I never could decide who was the more crazy in Star Trek - the Federation for flying around with fuel tanks full of antimatter, or the Romulans for riding around on black holes.


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## aco175 (Mar 10, 2021)

I'm still waiting on my jet pack they promised me back in the day.


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## Umbran (Mar 10, 2021)

darjr said:


> Halo Drives still seem the most realistic possibility. Just got to get our selves to the nearest black hole pair.




The nearest known black hole is some 1100 light years away.  When you want to get to something 4 ly from Earth (like Proxima Centauri), having to first go 1100 ly to QV Telescopii isn't exactly a win.



darjr said:


> Maybe the Halo mechanism could be used to provide the energy needed here?




I would question the ability of any normal matter to manage that amount of energy all at once. Using the drive system in the OP requires bringng the energy requirements down by about 30 orders of magnitude.


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## darjr (Mar 10, 2021)

Well yea, but a 30 magnitude reduction is asking a lot. So meet somewhere higher. And yea it’s a hard nut to crack anyway it’s sliced.


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## Umbran (Mar 10, 2021)

darjr said:


> Well yea, but a 30 magnitude reduction is asking a lot. So meet somewhere higher.




I don't think you have the option to meet somewhere higher.  The laws of physics are a harsh mistress. 

We are talking about how much energy can be in a ship a few hundred yards across. 

Consider, if you managed to put all the mass (and so, all the energy) of a single Jupiter in a space about 6 meters across, it would collapse into a black hole.  If you put it all in a space between one and five kilometers across, it collapses into a ball of neutronium.

Basicially, in order to power the ship, you need a ball of neutronium ten times larger than the ship!  The matter/energy you need cannot fit within the ship, much less be managed by the ship's systems.  Not going to the stars in that.

If you bring it down by 30 orders of magnitude, we are talking about completely converting maybe a kilogram material into energy. For scale, all the energy released by the Hiroshima bomb was less than a gram of mass.  So, we are then still talking about containing a thousand Hiroshimas within a space of, say, a modern sports stadium.  Which may be possible, but still pretty fantastic. 

But, above that... you're not going to the stars if you vaporize the ship when you turn the engine on.


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## Ryujin (Mar 10, 2021)

Umbran said:


> I don't think you have the option to meet somewhere higher.  The laws of physics are a harsh mistress.
> 
> We are talking about how much energy can be in a ship a few hundred yards across.
> 
> ...



I recall a book series (was it by Vernor Vinge?) in which an ancient race has set up compact cylinders of stellar mass, rotating at near light speed, to act as anchor points for folding space. My memory of it is quite vague, as I read it decades ago, but this was based at least loosely on the understanding of physics at the time. Simply put you would approach at a given vector in order to "link up" two of these points, so that you could enter a space fold that would transit you to your destination. This removes the necessity for having super massive objects housed within a relatively tiny ship but, then again, there is the issue of getting these anchors in the locations you wish to visit.

So, on that theory, we could have interstellar travel in a few mere millennia


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## Umbran (Mar 10, 2021)

Ryujin said:


> I recall a book series (was it by Vernor Vinge?) in which an ancient race has set up compact cylinders of stellar mass, rotating at near light speed, to act as anchor points for folding space. My memory of it is quite vague, as I read it decades ago, but this was based at least loosely on the understanding of physics at the time. Simply put you would approach at a given vector in order to "link up" two of these points, so that you could enter a space fold that would transit you to your destination.




Yeah.  I think that solution came from Tipler.  He thought of it as a time machine, but in those solutions, time travel implies FTL space travel, so you can use it for that too.

Mind you, it isn't enough for it to be of stellar mass.  It has to be of high density.  IIRC, the original solution requires an infinite cylinder of neutronium, spinning so its surface is moving at relativistic speeds.  It has been conjectured that havingthe cylinder of finite length is also workable, but if the edge effects don't make you into spaghetti, they limit where/when you can go to further.


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## Mustrum_Ridcully (Mar 10, 2021)

Does this particular method actually still even allow  FTL travel, or does it just allow you to travel non-relativistically close to the speed of light? I seem to remember someone commenting on that detail, but I am not sure if it was correct. I don't expect to be able to understand the actual paper, unfortunately. 
But it would be something I read before about the Alcubierre Drive - aside from needing exotic matter with negative energy, you would still need something moving superluminally to create the desired configuration of masses for FTL travel. Which is kinda begging the question.But there was an alternate approach were you basically first build a space high way placing the masses where they need to be by "foot" at non-FTL speed, and from that point on, you can travel on that route at FTL speed. So no Star Trek warp drive and going anywhere interesting outside our star system would still take humanity decades, centuries or millennia, but once they build their warp highways, people could move quickly between planets. Or we're lucky and someone built one for us already and we just need to find the next drive up ramp.


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## Ryujin (Mar 10, 2021)

Umbran said:


> Yeah.  I think that solution came from Tipler.  He thought of it as a time machine, but you can use it for space travel too.
> 
> Mind you, it isn't enough for it to be of stellar mass.  It has to be of high density.  IIRC, the original solution requires an infinite cylinder of neutronium, spinning so its surface is moving at relativistic speeds.  It has been conjectured that havingthe cylinder of finite length is also workable, but if the edge effects don't make you into spaghetti, they limits the where/when you can go to further.



The book that I was thinking of was "The Avatar" by Poul Anderson, though Vinge did use it by a different name and configuration.






						Tipler cylinder - Wikipedia
					






					en.wikipedia.org


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## darjr (Mar 10, 2021)

It assumes that a local area of space time can travel faster than the speed of light. We already know that waves in space time cannot. It, in itself, is kind of educated speculation.

I think.


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## Umbran (Mar 10, 2021)

Mustrum_Ridcully said:


> Does this particular method actually still even allow  FTL travel, or does it just allow you to travel non-relativistically close to the speed of light?




The claim on this one is outright FTL.  I haven't been over the paper myself yet.  My tensor calculus is rusty enough that it'll be a chore.



Mustrum_Ridcully said:


> I seem to remember someone commenting on that detail, but I am not sure if it was correct. I don't expect to be able to understand the actual paper, unfortunately.
> But it would be something I read before about the Alcubierre Drive - aside from needing exotic matter with negative energy, you would still need something moving superluminally to create the desired configuration of masses for FTL travel.




Maybe yes, maybe no.  It basically depends on one basic issue.

In the original formulation, the Alcubierre Drive has a flaw (actually several, but I'm talking about one)- the people inside the bubble are causally disconnected from the rest of the universe while it is in operation.  You configure it, launch, and it does its thing until you arrive at your destination.  You cannot do things like, say, steer an Alcubierre Drive, because the passengers cannot communicate with the bubble.

A workaround for that _may_ exist.  It amounts to firing little Alcubierre Drive packets from within the bubble to the main bubble.  The same solution can get around the problem you mention, if it works.

On a personal note - I get wistful with this stuff.  I learned General Relativity from Larry Ford at Tufts, who has done a significant amount of work on the Alcubierre Drive solutions.  If he'd had funding, I'd likely have been his grad student, and I'd have been working on this really cool stuff, and probably still be in academia as a result.  _sigh_.


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## Umbran (Mar 10, 2021)

darjr said:


> It assumes that a local area of space time can travel faster than the speed of light. We already know that waves in space time cannot. It, in itself, is kind of educated speculation.
> 
> I think.




I'm not sure what "It" is assuming.  But in general, no.  The solutions work out - the basic questions are over whether other things prevent them from working.  For example... stopping an Alcubierre drive may sterilize any system you are entering with a massive burst of radiation.


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## Ryujin (Mar 10, 2021)

Umbran said:


> I'm not sure what "It" is assuming.  But in general, no.  The solutiosn work out - the basic questions are over whether other things prevent them from working.  For example... stopping an Alcubierre drive may sterilize any system you are entering with a massive burst of radiation.



Whoops! Shades of the Voyager Probe, from "Space: 1999." Suddenly alien societies are looking for the race that committed genocide, while transmitting messages of friendship.


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## darjr (Mar 10, 2021)

There are a lot of solutions that work out. Being real and verifiable with experiment or observations is a whole other thing


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## Umbran (Mar 10, 2021)

darjr said:


> There are a lot of solutions that work out. Being real and verifiable with experiment or observations is a whole other thing




Of course.  However, when so many things about General Relativity _are_ verified by experiment and observations, the first place to look for a failure point isn't in the very basic math.  The more likely culprits are around the requirements (like exotic matter), or the fact that it may vaporize anything inside it with Hawking radiation.


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## darjr (Mar 10, 2021)

I thinks that’s survivors bias. After all originally it was a steady state theory of the universe. The counter was shown through observation. 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” and later shown to be wrong through experiment and observation are forgotten.

it is WHY Enstiens theory is so remarkable.


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## Umbran (Mar 10, 2021)

darjr said:


> 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. 



darjr said:


> 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).



darjr said:


> 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 (Mar 10, 2021)

Umbran said:


> 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 (Mar 10, 2021)

darjr said:


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




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



darjr said:


> 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.




darjr said:


> 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.


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## freyar (Mar 11, 2021)

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).


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## Umbran (Mar 11, 2021)

freyar said:


> 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.



freyar said:


> 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 (Mar 14, 2021)

Umbran said:


> 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).


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## Flamestrike (Mar 20, 2021)

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.


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## Mustrum_Ridcully (Mar 20, 2021)

Flamestrike said:


> 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.


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## Umbran (Mar 20, 2021)

Flamestrike said:


> 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.


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## shawnhcorey (Mar 22, 2021)

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:

Use FLT travel to create 2 events: one is you starting FTL travel, the other ending it.
Change your velocity so that the event of you starting FTL travel is far in your future.
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).


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## Umbran (Mar 22, 2021)

shawnhcorey said:


> 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.


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## Morrus (Mar 22, 2021)

shawnhcorey said:


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



What makes it better?


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## Umbran (Mar 22, 2021)

shawnhcorey said:


> 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 (Mar 22, 2021)

Morrus said:


> What makes it better?



Gravity travels at the speed of light. Yes, it's been measured. So if light cannot escape a black hole, how can gravity? With a MECO, there is no event horizon and so, it still has gravity.


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## Morrus (Mar 22, 2021)

shawnhcorey said:


> So if light cannot escape a black hole, how can gravity?



You’re saying gravity is slowed by gravity?


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## shawnhcorey (Mar 22, 2021)

Umbran said:


> 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.


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## Umbran (Mar 22, 2021)

Morrus said:


> 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.


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## shawnhcorey (Mar 22, 2021)

Morrus said:


> 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.


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## Morrus (Mar 22, 2021)

shawnhcorey said:


> 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.


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## Umbran (Mar 22, 2021)

shawnhcorey said:


> 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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## shawnhcorey (Mar 22, 2021)

Umbran said:


> 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.




But eventually the bubble must be popped. So there are still 2 events outside each other's light cone: creating the bubble and popping it.


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## Umbran (Mar 22, 2021)

Morrus said:


> 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.




If anyone who doesn't do tensor calculus tells you they understand General Relativity, they are simply incorrect.  So, don't sweat it.  Grokking in fullness requires heavy math that not everyone gets exposed to.

The point shawn is making is an issue if you only look at gravity as a particle or a wave - the typical quantum mechanical view.  But, as we know, QM and Relativity don't get on so well.  And, back in classical mechanics, there's a _third_ thing gravity is - spacetime curvature.  

General Relativity is all about calculating that spacetime curvature.  And, it _does not care_ about the whole particle/wave thing.  From this viewpoint, there is nothing traveling from within the hole to the outside world, so the speed with which it does so is irrelevant.


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## shawnhcorey (Mar 22, 2021)

Morrus said:


> 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.




And that is why there is a search for alternatives.

"Black holes do not exist—at least, not as we know them, says renowned physicist Stephen Hawking, potentially provoking a rethink of one of space's most mysterious objects." No Black Holes Exist


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## Umbran (Mar 22, 2021)

shawnhcorey said:


> But eventually the bubble must be popped. So there are still 2 events outside each other's light cone: creating the bubble and popping it.




Sure.  But you get no paradox, no closing of the curve, unless a round-trip is allowed, which may not be the case for the Alcubierre Drive.


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## shawnhcorey (Mar 22, 2021)

Umbran said:


> The point shawn is making is an issue if you only look at gravity as a particle or a wave - the typical quantum mechanical view.  But, as we know, QM and Relativity don't get on so well.  And, back in classical mechanics, there's a _third_ thing gravity is - spacetime curvature.




Curved space-time means the radial distance is increased to infinite. Since everything must travel the speed of light or slower, nothing can escape a black hole, not even gravity.

Some think that the curved space-time detaches itself from the mass and remains in its inertial frame but this has been shown to be incorrect.


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## shawnhcorey (Mar 22, 2021)

Umbran said:


> Sure.  But you get no paradox, no closing of the curve, unless a round-trip is allowed, which may not be the case for the Alcubierre Drive.




If you can do FTL once, you can do it many times.


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## Umbran (Mar 22, 2021)

shawnhcorey said:


> And that is why there is a search for alternatives.
> 
> "Black holes do not exist—at least, not as we know them, says renowned physicist Stephen Hawking, potentially provoking a rethink of one of space's most mysterious objects." No Black Holes Exist




From that same article: _"I would caution against any belief that Hawking has come up with a dramatic new solution answering all questions regarding black holes," said theoretical physicist Sean Carroll at the California Institute of Technology, who did not participate in this study. "These problems are very far from being resolved."_


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## Umbran (Mar 22, 2021)

shawnhcorey said:


> If you can do FTL once, you can do it many times.




Moving slower than light, there are places in spacetime, places outside your light cone, that you cannot reach.  If you can move at sub-light speed, you can move many times, but there are places you can't get to.

There's no reason, _a priori_, that moving FTL removes _ALL_ restrictions on travel. It may be that, though you are moving FTL, you could do so many times, but there would still places you cannot get to.


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## shawnhcorey (Mar 22, 2021)

Umbran said:


> There's no reason, _a priori_, that moving FTL removes _ALL_ restrictions on travel. It may be that, though you are moving FTL, you could do so many times, but there would still places you cannot get to.




Except that special relativity allows you to change things by merely changing your velocity. If there are 2 events outside each other's light cone, the order in which they happen can be changed by changing your velocity. The past is not fixed; it depends on your point of view.


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## tomBitonti (Mar 22, 2021)

Umbran said:


> 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.



That's true for light originating at the event horizon.  Light originating within the event horizon follows a path that reaches the singularity at the center of the black hole.  Any direction in which light can be emitted within a black hole points towards the singularity.
Thanks!
TomB


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## Morrus (Mar 22, 2021)

It's very cool that this thread involves people talking about stuff that the rest of us don't understand -- and I don't mean that sarcastically, I heartily approve! I love that this place can host that sort of conversation, even if it's beyond me personally!


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## Umbran (Mar 22, 2021)

tomBitonti said:


> That's true for light originating at the event horizon.  Light originating within the event horizon follows a path that reaches the singularity at the center of the black hole.  Any direction in which light can be emitted within a black hole points towards the singularity.




I'm aware.  

Sorry, I do more editing of my comments for a broader audience than some of the rest of you, and that was a level of complexity I didn't think relevant for this discussion.


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## Umbran (Mar 22, 2021)

shawnhcorey said:


> Except that special relativity allows you to change things by merely changing your velocity. If there are 2 events outside each other's light cone, the order in which they happen can be changed by changing your velocity. The past is not fixed; it depends on your point of view.




But, _again_ - sub-light, there are places you _CANNOT_ get to.  If they are outside your light cone when you start, there is _NO WAY_ to ever get there.

Some of the reading I've done on the Alcubierre drive suggests that it may have similar, but far more complicated, aspects.  There may be regions of spacetime that it cannot reach, such that the closed curve is impossible.  

Whether it is true?  Moot, until they bring the energy requirements down by 30 orders of magnitude or so.  That's unlikely to happen in any of our lifetimes, so I am happy to toy with the possibility.  If you aren't happy to do so... that's your choice.


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## Ryujin (Mar 22, 2021)

Really, REALLY hoping that this is all moot and that "The Road Not Taken" is accurate, with respect to faster than light travel.









						The Road Not Taken (short story) - Wikipedia
					






					en.wikipedia.org


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## shawnhcorey (Mar 22, 2021)

Umbran said:


> But, _again_ - sub-light, there are places you _CANNOT_ get to.  If they are outside your light cone when you start, there is _NO WAY_ to ever get there.




There is no way to get there without FTL travel. But with FTL travel, even the far past is accessible.


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## tomBitonti (Mar 23, 2021)

Umbran said:


> I'm aware.
> 
> Sorry, I do more editing of my comments for a broader audience than some of the rest of you, and that was a level of complexity I didn't think relevant for this discussion.



Sure, but this is important to understanding how things work inside a black hole.  There are a lot of misconceptions that are cleared up knowing that all movement within a black hole at less than or equal to the speed of light points to the center of the black hole.
Thanks,
TomB


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## Umbran (Mar 23, 2021)

tomBitonti said:


> Sure, but this is important to understanding how things work inside a black hole.




Except, of course, it isn't.  For the majority of readers here, it is a curiosity, and nothing more.  It doesn't even fit into the science literacy needed to make reasonable decisions for day-to-day life in a technological culture.  

There is a strong argument that since by definition these things cannot be observed, they fall into the pile of "non-falsifiable" items that aren't even science, _per se._


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## tomBitonti (Mar 23, 2021)

Umbran said:


> Except, of course, it isn't.  For the majority of readers here, it is a curiosity, and nothing more.  It doesn't even fit into the science literacy needed to make reasonable decisions for day-to-day life in a technological culture.
> There is a strong argument that since by definition these things cannot be observed, they fall into the pile of "non-falsifiable" items that aren't even science, _per se._



Well sure.  What can be said about the interior of a black hole is arguably philosophy, not physics.  One simple alternative view of black holes is that they have no interior.  No currently conceivable experiment can measure anything about the interior.  The point is important for anyone wanting to deepen their understanding of things.  Special relativity is a subject that extends beyond black holes.  But the real problem is that the statement is quite misleading.  I don't think it should be provided even casually, as it leads folks to incorrect thinking.  For example, it seems to allow a chain of in-falling devices, each with an emitter and a receiver, relaying signals outwards, while removing the incremental red-shift of the prior device.
Thanks!
TomB


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## GreyLord (Mar 23, 2021)

tomBitonti said:


> Well sure.  What can be said about the interior of a black hole is arguably philosophy, not physics.  One simple alternative view of black holes is that they have no interior.  No currently conceivable experiment can measure anything about the interior.  The point is important for anyone wanting to deepen their understanding of things.  Special relativity is a subject that extends beyond black holes.  But the real problem is that the statement is quite misleading.  I don't think it should be provided even casually, as it leads folks to incorrect thinking.  For example, it seems to allow a chain of in-falling devices, each with an emitter and a receiver, relaying signals outwards, while removing the incremental red-shift of the prior device.
> Thanks!
> TomB




Isn't Physics...pure physics...mostly a math driven thing?  Thus, there are things that they can't even observe but which they can see the math predict which are theories.

In such a way, couldn't the math be part of the evidence regarding the interior of a black hole or special relativity?


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## shawnhcorey (Mar 23, 2021)

GreyLord said:


> In such a way, couldn't the math be part of the evidence regarding the interior of a black hole or special relativity?




You can prove anything with math; it all depends on your assumptions. Nobody knows what the interior of a black hole is like, so the assumptions run wild. I read one article where gravity was reversed inside the event horizon. Everything was pushed out to the event horizon. The math was good but the assumptions were debatable.


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## tomBitonti (Mar 24, 2021)

GreyLord said:


> Isn't Physics...pure physics...mostly a math driven thing?  Thus, there are things that they can't even observe but which they can see the math predict which are theories.
> 
> In such a way, couldn't the math be part of the evidence regarding the interior of a black hole or special relativity?



well, math isn’t evidence.  Math is how we organize evidence and create a tractable model for how things work.  For black holes, my understanding is that it’s our past experience that guides us.  Past experience says that physics doesn’t stop applying in places we can’t see.  Then we don’t consider matter that has fallen outside of the visible universe because of cosmological expansion to have left the universe.  Likewise, we expect physics to work more or less the same inside a black hole as outside.  “Less” is mostly about what happens at the exact center, which has conditions that our understanding of physics can’t describe.
I was agreeing with Umbran’s second statement.  (While setting it aside.  I think it is irrelevant to my original point.)  We can’t measure what happens inside a black hole.  Strictly speaking, as an unmeasurable thing, it is not proper material for science.
Thanks!
TomB


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## briggart (Mar 24, 2021)

tomBitonti said:


> well, math isn’t evidence.  Math is how we organize evidence and create a tractable model for how things work.  For black holes, my understanding is that it’s our past experience that guides us.  Past experience says that physics doesn’t stop applying in places we can’t see.  Then we don’t consider matter that has fallen outside of the visible universe because of cosmological expansion to have left the universe.  Likewise, we expect physics to work more or less the same inside a black hole as outside.  “Less” is mostly about what happens at the exact center, which has conditions that our understanding of physics can’t describe.
> I was agreeing with Umbran’s second statement.  (While setting it aside.  I think it is irrelevant to my original point.)  We can’t measure what happens inside a black hole.  Strictly speaking, as an unmeasurable thing, it is not proper material for science.
> Thanks!
> TomB



There is no fundamental reason why matter falling outside the cosmological horizon would be lost to us forever, it depends on the nature of the current accelerated expansion phase. For all we know this could be temporary, similar to the primordial inflationary expansion, and if that is the case at some time in the remote future "we" will get back in touch with that matter. On the other hand, I don't think we have any reasonable idea of how we could get back in touch with matter inside a black hole horizon at all.

This doesn't really affect the overall discussion about FTL, black holes, etc., but there is some ground for looking at cosmological and black hole horizons in different ways.


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## shawnhcorey (Mar 24, 2021)

briggart said:


> There is no fundamental reason why matter falling outside the cosmological horizon would be lost to us forever, it depends on the nature of the current accelerated expansion phase. For all we know this could be temporary, similar to the primordial inflationary expansion, and if that is the case at some time in the remote future "we" will get back in touch with that matter. On the other hand, I don't think we have any reasonable idea of how we could get back in touch with matter inside a black hole horizon at all.
> 
> This doesn't really affect the overall discussion about FTL, black holes, etc., but there is some ground for looking at cosmological and black hole horizons in different ways.




Einstein's gravity states that the radial distance is increased to infinity. Since nothing can travel faster than light, anything that falls into a black hole will never reach the event horizon. It will fall forever.

This is one of the unresolved anomalies of black holes. Which is why some scientists are looking at alternatives.


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## Umbran (Mar 24, 2021)

GreyLord said:


> Isn't Physics...pure physics...mostly a math driven thing?




There is no such thing as "pure physics".  No science is of value if it doesn't mix in, get its hands dirty, and actually demonstrate itself through experiment or observation of predictions.



GreyLord said:


> Thus, there are things that they can't even observe but which they can see the math predict which are theories.




Beware of what you call "theory", for the common and technical uses vary.  For people actually in the field, a "theory" is a model well established and tested by many high quality experiments.  A model that has not been tested is an _hypothesis_.

Not that the world is consistent about this, but it is important.  Einstein's General Relativity is referred to a a theory, because it has withstood a whole lot of testing.  Meanwhile, so called "String Theories" have not, and are more properly considered hypotheses.



GreyLord said:


> In such a way, couldn't the math be part of the evidence regarding the interior of a black hole or special relativity?




Mathematics can be an inspiration, sure.  But a mathematical model is not "evidence".


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## Umbran (Mar 24, 2021)

shawnhcorey said:


> This is one of the unresolved anomalies of black holes. Which is why some scientists are looking at alternatives.




To quote Tyson - "The Universe is under no obligation to make sense to you."

It is less an anomaly, and more a thing that makes some physicists uncomfortable.  Like mathematics, what makes you uncomfortable can be inspiration, but it isn't evidence.


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## shawnhcorey (Mar 24, 2021)

Umbran said:


> To quote Tyson - "The Universe is under no obligation to make sense to you."
> 
> It is less an anomaly, and more a thing that makes some physicists uncomfortable.  Like mathematics, what makes you uncomfortable can be inspiration, but it isn't evidence.




If an object is accelerated forever, it will gain infinite kinetic energy. This would violate the 1st law of thermodynamics. Black holes make me more than comfortable.

The universe does not have to make sense but science is making sense of the parts that we can.


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## Umbran (Mar 24, 2021)

shawnhcorey said:


> If an object is accelerated forever, it will gain infinite kinetic energy.




Sure, but that's a nonsensical statement, because "forever" is not an attainable moment.  At any given time, it has finite KE.

Moreover, there's going to be a point in that descent in which the object fails to be a classical, or even relativistic object.  Long before forever, it will spaghettify into bits that are better described as quantum objects, and that simple classical description will no longer hold.  And, eventually, we'd expect that it would cease to be a separate object from the star that collapsed - the quantum objects having gained enough energy to do something like tunneling to the singularity, come into the region where spacetime is more like a quantum foam, or the like - and there's nothing to accelerate any more.



shawnhcorey said:


> This would violate the 1st law of thermodynamics.




The Laws of Thermodynamics only hold for closed systems.  The interior of a black hole is certainly open at the event horizon, and may well be open at the singularity.


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## tomBitonti (Mar 24, 2021)

shawnhcorey said:


> Einstein's gravity states that the radial distance is increased to infinity. Since nothing can travel faster than light, anything that falls into a black hole will never reach the event horizon. It will fall forever.
> 
> This is one of the unresolved anomalies of black holes. Which is why some scientists are looking at alternatives.



Well, what is the state of the universe at a particular time?  Is it based on our awareness of an event based on when a signal from that event reaches us?  Or do we consider that time has elapsed since the event created the signal that we perceived?  The sense that there is a universal “now” doesn’t work in special relativity.
That said, I struggle with this same question.
It is true that we never see a crossing of the event horizon.  The signal is stretched out over longer and longer time and red shifted away.  But see the preceding paragraph.
Also, the infinite distance problem was, I thought, a problem of the coordinate system being used.  A more careful choice of coordinate systems avoids infinities at the event horizon.  (That is at the edge of my understanding.  I’m thinking others could explain this last point better.)
Thanks!
TomB


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## Umbran (Mar 24, 2021)

tomBitonti said:


> Also, the infinite distance problem was, I thought, a problem of the coordinate system being used.  A more careful choice of coordinate systems avoids infinities at the event horizon.




So, spacetime at and across the event horizon is smooth.  There is no physical discontinuity.  Signals trying to escape to the distant stars get further and further red-shifted as you approach the event horizon, but that's not a physical infinity.  It only seems like one to a distant observer.  

The only real "infinity" in a black hole is at the singularity itself, which we don't get to look at.


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## shawnhcorey (Mar 24, 2021)

Umbran said:


> Sure, but that's a nonsensical statement, because "forever" is not an attainable moment.  At any given time, it has finite KE.
> 
> Moreover, there's going to be a point in that descent in which the object fails to be a classical, or even relativistic object.  Long before forever, it will spaghettify into bits that are better described as quantum objects, and that simple classical description will no longer hold.  And, eventually, we'd expect that it would cease to be a separate object from the star that collapsed - the quantum objects having gained enough energy to do something like tunneling to the singularity, come into the region where spacetime is more like a quantum foam, or the like - and there's nothing to accelerate any more.
> 
> ...




The pint is that the KE is forever increase, with no known source. That violates the 1st law.

The quanta cannot tunnel into the singularity. They are too far away. The probability of them doing so is almost zero.

The universe is a closed system because we can see a fixed distance into the past. All the energy we see back to the big bang is finite. Or would be if there weren't any black holes.



tomBitonti said:


> Well, what is the state of the universe at a particular time?  Is it based on our awareness of an event based on when a signal from that event reaches us?  Or do we consider that time has elapsed since the event created the signal that we perceived?  The sense that there is a universal “now” doesn’t work in special relativity.
> That said, I struggle with this same question.
> It is true that we never see a crossing of the event horizon.  The signal is stretched out over longer and longer time and red shifted away.  But see the preceding paragraph.
> Also, the infinite distance problem was, I thought, a problem of the coordinate system being used.  A more careful choice of coordinate systems avoids infinities at the event horizon.  (That is at the edge of my understanding.  I’m thinking others could explain this last point better.)
> ...




The problem with the coordinate system is that a flat-space system is used. But space is not flat. It is impossible to get close to the event horizon. It is infinitely far away.



Umbran said:


> So, spacetime at and across the event horizon is smooth.  There is no physical discontinuity.  Signals trying to escape to the distant stars get further and further red-shifted as you approach the event horizon, but that's not a physical infinity.  It only seems like one to a distant observer.
> 
> The only real "infinity" in a black hole is at the singularity itself, which we don't get to look at.




Signals from above the event horizon get red-shifted. There are no signals from the event horizon or below it. Nothing can escape the event horizon.


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## Umbran (Mar 24, 2021)

shawnhcorey said:


> The pint is that the KE is forever increase, with no known source. That violates the 1st law.
> 
> The quanta cannot tunnel into the singularity. They are too far away. The probability of them doing so is almost zero.




The probability of tunneling is _almost_ zero, at any particular moment.  But there's two things to remember:

1) the probability increases as energy increases. So, as you like to keep noting that the energy goes to infinity, so, then, does the probabilty approach certainty, because the barrier height is fixed.

2) Long before that certainty, though, we note that _almost_ zero probability at any particular moment means a certainty over "forever".  So, eventually, it happens. 



shawnhcorey said:


> The universe is a closed system because we can see a fixed distance into the past. All the energy we see back to the big bang is finite.




We don't see all the way back to the big bang.  As you look back in time, the universe eventually becomes opaque before then.  We also don't even see the entire universe.  We only see a _visible_ universe, limited by lightspeed. There's no indication I'm aware of that the physical universe is not infinite in extent, meaning that the energy overall may well be infinite. There's not a lot of win to be had in discussing the thermodynamics of a system with infinite energy...

We also don't see into black holes.  Beyond the event horizon is, for many intents and purposes, outside the visible universe, and possibly outside the physical universe.



shawnhcorey said:


> Or would be if there weren't any black holes.




Interesting point - I said the interior of the black hole is thermodynamically an open system.  That's true.  However, that open state is one-way.  The only things that can come out of a black hole are the mass/energy, charge, and information that went in, and that only by Hawking radiation.  Other than that, black holes are Las Vegas - what happens inside them stays inside them.  That'll be important in a minute.

Upthread, I mentioned that I sometimes edit and use simplified language to keep things simple an comprehensible.  Well, now we have to be more accurate as we invoke the First Law of Thermodynamics.  It does _NOT_ say, "you cannot have infinite kinetic energy".  It says that, in a closed system, energy cannot be created nor destroyed.  However, we note that it _can_ be transformed.

So, let us consider a closed system that contains one black hole, and one rock.  We drop the rock into the black hole.  And you say, well, that rock now accelerates infinitely toward the singularity, develops infinite kinetic energy, and breaks the first law! 

Then I ask - what was the rock's potential energy of position at the start of the experiment? I answer for you - the rock starts infinitely far from the singularity, so its potential energy of position was _infinite_!  No energy is being created!

Moreover, we can be even more strict, and note that the First Law really says that you cannot get more energy (as work) out of a closed system than you put into it.  And now Black Hole Las Vegas comes up.  I cannot _extract_ any of that kinetic energy from inside the black hole.  Inside the event horizon, all world lines point inexorably inward.  There is no path out for that energy.  Thus, the First Law still holds. 




shawnhcorey said:


> Signals from above the event horizon get red-shifted. There are no signals from the event horizon or below it. Nothing can escape the event horizon.




Yep.  Exactly.  For that reason, that which goes on inside the event horizon cannot violate the First Law of Thermodynamics.


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## tomBitonti (Mar 24, 2021)

shawnhcorey said:


> If an object is accelerated forever, it will gain infinite kinetic energy. This would violate the 1st law of thermodynamics. Black holes make me more than comfortable.



Note that a thing _can_ (theoretically) accelerate forever tending to a finite velocity.  As long as the amount of acceleration decreases over time.  A point falling towards the event horizon of a black hole, once it gets close enough, to a distant observer, actually appears to _decelerate_ forever, slowing due to special relativity and the relative position of the observer as the horizon is approached.
Thanks!
TomB
PostScript: Just found this, which has some interesting insights:




__





						Waterfall
					






					jila.colorado.edu
				



"A former student once called the ergosphere the place where little children come from, because nothing can remain at rest there. "


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## shawnhcorey (Mar 24, 2021)

tomBitonti said:


> Note that a thing _can_ (theoretically) accelerate forever tending to a finite velocity.  As long as the amount of acceleration decreases over time.  A point falling towards the event horizon of a black hole, once it gets close enough, to a distant observer, actually appears to _decelerate_ forever, slowing due to special relativity and the relative position of the observer as the horizon is approached.
> Thanks!
> TomB
> PostScript: Just found this, which has some interesting insights:
> ...




Note the word "appears". Yes, if you assume space is flat, it appears to decelerate. But space is not flat. In reality, the object is constantly accelerating and the acceleration keeps increasing.


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## shawnhcorey (Mar 24, 2021)

Yes Umbran, the 1st law cannot be violated. But black holes appear to do so. So something is wrong.


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## Umbran (Mar 25, 2021)

shawnhcorey said:


> Yes Umbran, the 1st law cannot be violated. But black holes appear to do so.




I have already outlined how they _don't_ do so.  There is no violation.


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## shawnhcorey (Mar 25, 2021)

Umbran said:


> I have already outlined how they _don't_ do so.  There is no violation.



No, you have no stated where all that energy is coming from.


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## Umbran (Mar 25, 2021)

shawnhcorey said:


> No, you have no stated where all that energy is coming from.




In a very real sense, it is a figment of your imagination, and doesn't exist, so it does not "come from" anywhere.  There is no finite time at which this energy can be observed in the universe, which makes it a non-physical fiction, a bedtime story to scare children.  Where does the energy for the body of Peter Pan come from?

The above is technically correct, but I expect it won't satisfy you.  So, I have another, equivalent phrasing.

In an imaginary sense (and, I mean this in terms of the actual math - it is quantum mechanical, so imaginary numbers are involved) the energy is borrowed from the vacuum, and returns to the vacuum when the falling item joins with the singularity (which, despite your protestations, it does do), leaving the overall system of rock + hole with the same mass-energy it started with.  This is the usual source for internal, non-physical energies that crop up in math from time to time.


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## shawnhcorey (Mar 25, 2021)

An object falling into a black hole will be accelerated forever. That means its KE is always increasing. That energy must have a source.

A black hole starts as a very large star. Anything falling into the star has a maximum speed when it hits the surface. This means it has a maximum KE.

Then the star goes supernova and leaves a black hole. There is no limit to the KE an object falling into a black hole can acquire. So where did this energy come from?


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