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clipka <nomail@nomail> wrote:
> Well, from an outsider's perspective - given that everything falling into the
> black hole comes to a standstill at the EH...
Slowing down asymptotically is not the same thing as being stopped.
Also, as I have been saying, since it will emit constantly less and less
photons, which are more and more red-shifted, at some point it will be
practically impossible to observe the object. It will just fade to black.
--
- Warp
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Darren New <dne### [at] san rrcom> wrote:
> You give one particle that is constantly emitting photons in all directions
> a push due south towards the singularity. You have an observer due north of
> the singularity looking south. The observer due north will continue to see
> photons coming off the particle indefinitely?
You can distribute a finite amount of photons over an infinite amount
of time with a simple 1/x formula.
--
- Warp
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nemesis <nam### [at] gmailcom> wrote:
> If we assume blackholes exist, without much evidence so far
The GR equations seem to work with everything we can measure, at
macroscopic scales. At no point it has been measured that these equations
begin to diverge from reality.
In other words, we have *no evidence* that the GR equations are not
valid in all possible situations, such as with extremely dense objects.
Thus, until we get evidence of the contrary, the only logical conclusion
is to assume that the equations hold also for extreme situations, for
example when an object collapses to be smaller than its Schwarzschild
radius.
GR equations might not fully reflect what happens at atomic scales,
but neither do these measures provide any evidence that GR equations
don't work at macroscopic scales.
--
- Warp
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On Thu, 22 Jan 2009 09:18:19 +0100, "scott" <sco### [at] scottcom> wrote:
>> Depends on definition if speed, but as it is usually specified based on
>> the
>> observer's timeframe: Yes. Gravitation near a black hole is so strong
>> that -
>> from an observer's POV - through time delation and the warping of space it
>> causes "that poor old sod over there" to slow to a halt.
>
>I always thought that it could be possible to go forward in time by some
>arbitrary amount just by making a close orbit around a black hole. The
>closer you go, the more time you can jump forward. So just type in the year
>3050 to your ship, it fires you off towards the nearest black hole, and a
>few months later you return back towards Earth in the year 3050 +/- a few
>months ;-)
>
>Or doesn't it work like that?
>
No you run widdershins round the Earth very fast and you arrive in tomorrow
before the rest of the world does.
Or doesn't it work like that?
--
Regards
Stephen
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> No you run widdershins round the Earth very fast
Ah but why use expensive rockets and stuff to go fast round the Earth when
you can just aim towards a black hole and get a huge speed-up for free (plus
it returns you roughly back towards Earth if you aim correctly).
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scott <sco### [at] scottcom> wrote:
> > No you run widdershins round the Earth very fast
> Ah but why use expensive rockets and stuff to go fast round the Earth when
> you can just aim towards a black hole and get a huge speed-up for free (plus
> it returns you roughly back towards Earth if you aim correctly).
I think the slingshot effect is not dependent on the density of the larger
object, only on its mass and velocity (or, in other words, momentum). So
a black hole wouldn't make any difference compared to a regular star of
the same mass and velocity.
--
- Warp
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> I think the slingshot effect is not dependent on the density of the
> larger
> object, only on its mass and velocity (or, in other words, momentum). So
> a black hole wouldn't make any difference compared to a regular star of
> the same mass and velocity.
But presumably an object with the same mass but higher density would allow
you to get a faster slingshot without crashing into it (because it's
smaller)?
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scott <sco### [at] scottcom> wrote:
> But presumably an object with the same mass but higher density would allow
> you to get a faster slingshot without crashing into it (because it's
> smaller)?
In theory yes. OTOH it still depends on the momentum of the larger object.
You could get a stronger slingshot from a star of the same mass if it's
moving faster than the black hole.
Yet, this is not as simple as it sounds in any case. The slingshot effect
is always relative to something. For example interplanetary gravitational
assist is relative to the Sun. The Sun itself cannot be used for a slingshot
effect inside the solar system (it could be used for a slingshot relative
to the galaxy, but not relative to the solar system).
With this taken into account, can you just go from Earth to the nearest
black hole, get an enormous speedup and come back at 100x the speed and
slam onto Earth at that speed? From a gravity assist only, I don't think so.
I think it would be against conservation of energy. If you were travelling
from Earth to another star system, then maybe, but I don't think it works
in the closed case.
There are also some practical things to take into accout:
- If you go so close to a black hole that it will give you a stronger
slingshot effect than a regular star would, the tidal forces would
probably rip you apart. Not very practical.
- Black holes usually have an accretion disc around them, usually much
larger than the size of a normal star of the same mass. The accretion
disc could slow you down or be dangerous.
- The humongous amounts of radiation around a black hole would probably
be enough to fry you to ashes in a fraction of a second, no matter what
kind of shielding you use, especially if you go so close that you would
get a larger speed boost than from a regular star.
--
- Warp
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nemesis <nam### [at] gmailcom> wrote:
> Does the singularity or blackholes even exist or are merely the point
> where GR equations break?
I honestly suspect exactly that... see Achilleus and the turtle: With the wrong
approach, you get the result that the fastest runner can't catch up with the
turtle... while reality proves he can.
In that old greek paradoxon it is rather obvious that it's a problem of a wrong
approach, because the reference system used is far from common sense. However,
with black holes, the reference system picking ever smaller intervals of
distance and time - that of an outside observer - seems more natural at first
glance, so it's not *that* obvious that we might just be using a wrong
approach.
> If we assume blackholes exist, without much evidence so far, how about
> going even further in the imagination? What if our 4D space-time isn't
> but a section of higher dimensional spaces, like a julia fractal section
> rendered in povray? Perhaps the blackhole is then just a curve along
> this surface and the poor fellow ends up in another region of
> space-time. The original outside observer will never see him again,
> unless he goes all the way back through the worm hole...
That description matches quite well what I think is actually happening -
although with a different punchline. That wormhole will take the traveller to
another region of spacetime(!) indeed: Far-away future! The only problem with
it is that it's a one-way ticket: If he enters the wormhole again, he will end
up yet again "someplace later", not back where he started.
> Those greeks philosophers were just full of marijuana. And pasta. They
> were first possibly the first thinkers of mankind to think too much on
> far too abstract subjects rather than more mundane and practical
> matters... :P
:) I guess the same goes for Buddhism and the like... but they seem to have
gotten some things astonishingly right. Talk about atoms - the ancient greeks
"invented" them. Talk about the quantum theory question of whether anything
actually exists that is not observed - Zen has been asking that question long
time ago.
But let's get back to more practical matters: I get the munchies now... where's
my pizza? :)
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nemesis <nam### [at] gmailcom> wrote:
> Since we're speculating, if space is really expanding, do you think in
> absolute terms we're now much larger than dinosaurs back in their day?
> Of course, fossils won't tell, cause they expanded ever since too. ;)
Hum... well, I guess the cosmological constant isn't *that* big...
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nemesis <nam### [at] gmailcom> wrote:
> So that's why light takes a lot more time to reach us and thus the after
> image just before the crossing is visible. OTOH, time and space are two
> sides of the same coin, isn't it?
That's what GR says - yes.
Or, well, maybe they're rather two hemispheres on the same sphere... or
something along that line...
Did you realize that if you're accelerating, you're just turning around in
space-time? (Well, basically, that is - it's a kind of weird type of
rotation...)
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> In theory yes. OTOH it still depends on the momentum of the larger
> object.
> You could get a stronger slingshot from a star of the same mass if it's
> moving faster than the black hole.
Of course, are black holes known for not really having much momentum then
compared to stars?
> Yet, this is not as simple as it sounds in any case. The slingshot effect
> is always relative to something. For example interplanetary gravitational
> assist is relative to the Sun. The Sun itself cannot be used for a
> slingshot
> effect inside the solar system (it could be used for a slingshot relative
> to the galaxy, but not relative to the solar system).
I thought the idea of the slingshot was that you wait until you are at
maximum velocity near the planet/star and then fire your rockets, firing
them at this point gives you way more speed than firing them at much lower
speeds, in effect meaning you need less fuel overall to get somewhere. I
didn't think about using movement of planets and stars as a slingshot.
> With this taken into account, can you just go from Earth to the nearest
> black hole, get an enormous speedup and come back at 100x the speed and
> slam onto Earth at that speed? From a gravity assist only, I don't think
> so.
No, I think the path would be pretty symetrical, so you would speed up to
100x as you passed the black hole, then slow back down to the same speed you
set off with as you approach Earth. Just like you were orbiting the black
hole in a non-circular orbit, like a comet orbits the sun.
> There are also some practical things to take into accout:
Hehe yes I imagined!
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clipka <nomail@nomail> wrote:
> Did you realize that if you're accelerating, you're just turning around in
> space-time? (Well, basically, that is - it's a kind of weird type of
> rotation...)
Aren't you confusing gravity with acceleration?
--
- Warp
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scott <sco### [at] scottcom> wrote:
> I thought the idea of the slingshot was that you wait until you are at
> maximum velocity near the planet/star and then fire your rockets
No. That's a so-called powered slingshot and it can boost the effect
of the slingshot. However, the slingshot effect can be achieved with
pure inertia, without firing any rockets.
> I didn't think about using movement of planets and stars as a slingshot.
Well, that's the whole idea. (The powered version is just a boost, but
not really the basic effect.)
--
- Warp
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> Well, that's the whole idea. (The powered version is just a boost, but
> not really the basic effect.)
Hehe I guess it's easy to forget the planets are actually moving pretty damn
quick :-)
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clipka <nomail@nomail> wrote:
> nemesis <nam### [at] gmailcom> wrote:
> > Does the singularity or blackholes even exist or are merely the point
> > where GR equations break?
> I honestly suspect exactly that... see Achilleus and the turtle: With the wrong
> approach, you get the result that the fastest runner can't catch up with the
> turtle... while reality proves he can.
The major objections against singularities are more or less philosophical,
rather than based on hard science. It's "hard to believe" that mass could
be compressed into a point of zero volume.
However, if reading about quantum mechanics and general relativity has
taught me something, it's that the Universe doesn't always work intuitively,
and there are lots of things which are "hard to believe", yet demonstrably
happen.
If someone objects to the notion of a singularity, he would have to show
some evidence that GR doesn't work as predicted in this case. There must be
some property of the Universe which makes GR not work in this situation,
something which actually stops the singularity from forming. What could this
phenomenon be? Has anyone ever measured such phenomenon to exist?
If not, then any objection against singularities is more or less
philosophical. "It doesn't sound right" is not hard science.
The thing is, if GR equations are right, and there is a lot of evidence
suggesting that they are, then there simply is no way for a singularity
to not to form when a mass collapses to be smaller than its Schwarzschild
radius. All geodesics inside the event horizon, including time geodesics,
point towards the center. There's no known way for matter/energy to stop
going towards this center. Just advancing in time makes it advance towards
this center. Even if you tried to apply some force to a particle to stop
it from getting closer to the center, you couldn't, not even if you applied
an infinite force.
The only possible conclusion is that all the matter and energy compresses
into a point of zero volume. If the singularity does not form in reality,
then something must explain why. Something more than philosophical objections.
--
- Warp
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scott <sco### [at] scottcom> wrote:
> > Well, that's the whole idea. (The powered version is just a boost, but
> > not really the basic effect.)
> Hehe I guess it's easy to forget the planets are actually moving pretty damn
> quick :-)
It helps understanding the gravity slingshot if you think of the effect
as a 100% elastic collision. If you do the math, you will see how the
larger object transfers momentum to the smaller object in such a collision.
And elastic collision is, AFAIK, exactly what the effects of a gravity
assist are, from an overall point of view.
--
- Warp
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"nemesis" <nam### [at] gmailcom> wrote:
> Yes, but according to the article you linked, QM physicists are giving another
> explanation for the same effects without the paradoxes blackholes carry, the
> same ones we were avidly discussing about previously: the "lost information"
> and the "after-image" effect of something crossing the EH as seen from an
> external observer. Of course, they are trying to explain it by relying on a
> supposed dark matter star that has never been detected before either. :P
Interestingly though, the real-existing phenomenon of "black holes" was
originally a purely hypothetical prediction of GR - not some already known
effect that needed to be explained. And it did predict their properties quite
well as it seems. So this is strong evidence that the GR is a very good tool to
make predictions.
So in order to show that *their* theory is at least an equally good tool, they
would actually have to demonstrate how their own theory explains not only the
same *phenomenons* as GR, but *postulates GR itself* - or demonstrate how their
theory is just a different way of looking at GR.
> OTOH, will we ever witness such object, blackhole or dark matter star? Such
> humongous gravitational force surely has many objects orbiting it,
> "overcrowded" as clipka mentioned, and most of it being gas and stars, so what
> is the chance of seeing what isn't supposed to be seen except against a clear
> starry background?
It would also probably an unhealthy place to stay even without being ripped
apart by tidal forces - with gamma ray jets and all that around.
BTW, speaking of it: When falling into a black hole I guess it may be best not
to try to fight the inevitable... suppose you manage to stay at a fixed
distance from the center - with all the blueshift of the photons falling in,
you're gonna get your daily maximum dose of gamma rays in a few split
seconds... so better fall in to experience the same blue shift (effectively
leaving the shift unchanged from your perspective) and hope that the
singularity is just a transition to a better life somewhere - um, I mean
someWHEN - else...
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"triple_r" <nomail@nomail> wrote:
> Convincing evidence, but I'm a sucker for the visually spectacular ones:
>
> http://en.wikipedia.org/wiki/Elliptical_Galaxy_M87
Yeah, definitely a sight!
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"scott" <sco### [at] scottcom> wrote:
> I always thought that it could be possible to go forward in time by some
> arbitrary amount just by making a close orbit around a black hole. The
> closer you go, the more time you can jump forward. So just type in the year
> 3050 to your ship, it fires you off towards the nearest black hole, and a
> few months later you return back towards Earth in the year 3050 +/- a few
> months ;-)
>
> Or doesn't it work like that?
Exactly that way. Although maybe you'd need quite some acceleration to get there
- and back again - soon enough for 3050 ;)
And then again, speeding up, travelling a while at high speed, then putting the
gear shift to reverse and head back may do the job just as well. No need for a
black hole. You wouldn't want your Windows CE based guidance computer to crash
*there*...
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Warp <war### [at] tagpovrayorg> wrote:
> clipka <nomail@nomail> wrote:
> > Well, from an outsider's perspective - given that everything falling into the
> > black hole comes to a standstill at the EH...
>
> Slowing down asymptotically is not the same thing as being stopped.
Why do geeks always have to be nitpickers? ;)
But then again, notice that I wrote "at the EH" - so what I wrote was actually
that lim[d->r] v(d) = 0...
(phew... last second save ;))
> Also, as I have been saying, since it will emit constantly less and less
> photons, which are more and more red-shifted, at some point it will be
> practically impossible to observe the object. It will just fade to black.
"infra-brown", to be precise ;)
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Warp <war### [at] tagpovrayorg> wrote:
> > Ah but why use expensive rockets and stuff to go fast round the Earth when
> > you can just aim towards a black hole and get a huge speed-up for free (plus
> > it returns you roughly back towards Earth if you aim correctly).
>
> I think the slingshot effect is not dependent on the density of the larger
> object, only on its mass and velocity (or, in other words, momentum). So
> a black hole wouldn't make any difference compared to a regular star of
> the same mass and velocity.
Allows for a much more "slender" parabola though... and, besides, a smaller
radius to the star's center, for more gravity and therefore more of the
time-dilating effect ;)
(someone sing "let's do the time-warp again"...)
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Warp <war### [at] tagpovrayorg> wrote:
> With this taken into account, can you just go from Earth to the nearest
> black hole, get an enormous speedup and come back at 100x the speed and
> slam onto Earth at that speed? From a gravity assist only, I don't think so.
> I think it would be against conservation of energy. If you were travelling
> from Earth to another star system, then maybe, but I don't think it works
> in the closed case.
It does.
The trick is that the slingshot affects both bodies: That Huge Planet Over There
and yout Teeny Weeny Space Machiney.
Guess which one *seems* to be affected most...
So if you use a black hole to slingshot you around back home, at the same time
you sort of slingshot the black hole around your spacecraft into the opposite
direction... it's just that the black hole won't bother *much*, being the fat
lazy sucker it is.
> - If you go so close to a black hole that it will give you a stronger
> slingshot effect than a regular star would, the tidal forces would
> probably rip you apart. Not very practical.
Nah, I don't think so. Would tidal forces rip you apart at the surface of the
sun? I doubt. At half the star's radius you already get 4 times the
gravitational pull, but only about 2 times the tidal forces (gravity being
proportional to the inverse radius' square, so its gradient being only linearly
proportional to the inverse radius, if I get the math right).
> - Black holes usually have an accretion disc around them, usually much
> larger than the size of a normal star of the same mass. The accretion
> disc could slow you down or be dangerous.
Only if they are supermassive (those you'd expect to find at galaxy centers) or
happen to have a companion star.
> - The humongous amounts of radiation around a black hole would probably
> be enough to fry you to ashes in a fraction of a second, no matter what
> kind of shielding you use, especially if you go so close that you would
> get a larger speed boost than from a regular star.
Again, only if there's an accretion disk. Remember: Black holes don't emit
anything - only their surroundings may ;)
However, maybe you also want to check the magnetic field of the black hole - I'm
not sure about the orders of magnitude, but I wouldn't want all my electrons go
spinning off to the left while the protons go off to the right - not to speak
of the resulting synchroton radiation ;)
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scott wrote:
> I always thought that it could be possible to go forward in time by some
> arbitrary amount just by making a close orbit around a black hole.
You can go forward in time by some arbitrary amount just by sitting still.
:-) It's just a question of whether you're still alive when you finish.
--
Darren New, San Diego CA, USA (PST)
"Ouch ouch ouch!"
"What's wrong? Noodles too hot?"
"No, I have Chopstick Tunnel Syndrome."
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Warp <war### [at] tagpovrayorg> wrote:
> clipka <nomail@nomail> wrote:
> > Did you realize that if you're accelerating, you're just turning around in
> > space-time? (Well, basically, that is - it's a kind of weird type of
> > rotation...)
>
> Aren't you confusing gravity with acceleration?
Aren't you forgetting that Einstein postulated that you can't tell the
difference of the one from the other? ;)
After all, that's the *basic foundation* of GR...
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Warp wrote:
> With this taken into account, can you just go from Earth to the nearest
> black hole, get an enormous speedup and come back at 100x the speed and
> slam onto Earth at that speed? From a gravity assist only, I don't think so.
> I think it would be against conservation of energy.
I don't believe that's right.
Consider rolling a small ball at 10 kph on a level surface. It runs into a
large heavy ball (say, 1000x the mass) rolling at 1 kph the other way and
bounces off. Now the large heavy ball is rolling at 0.999 kph, and the small
ball is rolling at 1000 kph (or 10,000 kph or something) back the way it came.
Clearly energy and momentum are conserved there.
You'd wind up changing the course of the black hole much less than you'd
change the course of your spaceship, is all.
I also think the black hole would be better, because you can get closer and
get a bigger time dilation from the gravity also.
> There are also some practical things to take into accout:
We're talking about using black holes for time travel and you're considering
practical things? :-)
> - The humongous amounts of radiation around a black hole would probably
> be enough to fry you to ashes in a fraction of a second,
FWIW, it's the accretion disk that generates the radiation, so you could
kill two birds with one stone by finding one without.
--
Darren New, San Diego CA, USA (PST)
"Ouch ouch ouch!"
"What's wrong? Noodles too hot?"
"No, I have Chopstick Tunnel Syndrome."
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clipka wrote:
> (someone sing "let's do the time-warp again"...)
With time dilation even:
http://www.youtube.com/watch?v=4GoTnd99rBo
--
Darren New, San Diego CA, USA (PST)
"Ouch ouch ouch!"
"What's wrong? Noodles too hot?"
"No, I have Chopstick Tunnel Syndrome."
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scott escreveu:
>> Well, that's the whole idea. (The powered version is just a boost, but
>> not really the basic effect.)
>
> Hehe I guess it's easy to forget the planets are actually moving pretty
> damn quick :-)
Yeah, think about it: Earth is rotating about the sun, which is
rotating about the Milky Way nucleus, which is speeding in a cluster
with many closer galaxies through ever accelerating expansive space.
We're actually cruising unknown frontiers at speeds vastly superior to
200km/sec (which is about the speed of the sun as it rotates about the
galaxy nucleus according to wikipedia). Who needs a spaceship, huh?
and all this while you sit in front of your desk staring at the monitor,
holding your cup of coffee with a steady hand... :P
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Warp <war### [at] tagpovrayorg> wrote:
> GR itself doesn't forbid the distance between two points in space growing
> faster than c.
Btw, another situation where GR predicts relative motion larger than c
is inside the ergosphere of a rotating black hole.
--
- Warp
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clipka <nomail@nomail> wrote:
> > Aren't you confusing gravity with acceleration?
> Aren't you forgetting that Einstein postulated that you can't tell the
> difference of the one from the other? ;)
Just because you can't tell appart gravity from acceleration doesn't
mean that all accelerations are caused by the same phenomenon as gravity.
> After all, that's the *basic foundation* of GR...
The foundation of GR is that gravity does not, in fact, cause
acceleration at all. An object in "free wall" is all about inertia
and nothing about acceleration (even though it may not look like it
to us).
Just because gravity doesn't cause acceleration doesn't mean that
*nothing* can cause (true) acceleration.
--
- Warp
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nemesis <nam### [at] gmailcom> wrote:
> We're actually cruising unknown frontiers at speeds vastly superior to
> 200km/sec
With respect to what?
Motion is always relative.
--
- Warp
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clipka wrote:
> Talk about atoms - the ancient greeks "invented" them.
Yes, and they also didn't believe in irrational numbers even after they were
proved to exist. So I don't think you can pick one success out of history
and call it a "prediction".
I mean, there's really only two or three possibilities: Stuff is atoms,
stuff is continuous, stuff is a mixture. :-)
The greeks had no evidence of atoms, as far as I know. It was just a guess.
It wasn't until Mendeleev (or the work that lead up to the periodic table)
or so that people actually had evidence that atoms were the right answer.
--
Darren New, San Diego CA, USA (PST)
"Ouch ouch ouch!"
"What's wrong? Noodles too hot?"
"No, I have Chopstick Tunnel Syndrome."
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clipka <nomail@nomail> wrote:
> Warp <war### [at] tagpovrayorg> wrote:
> > With this taken into account, can you just go from Earth to the nearest
> > black hole, get an enormous speedup and come back at 100x the speed and
> > slam onto Earth at that speed? From a gravity assist only, I don't think so.
> > I think it would be against conservation of energy. If you were travelling
> > from Earth to another star system, then maybe, but I don't think it works
> > in the closed case.
> It does.
> The trick is that the slingshot affects both bodies: That Huge Planet Over There
> and yout Teeny Weeny Space Machiney.
> Guess which one *seems* to be affected most...
> So if you use a black hole to slingshot you around back home, at the same time
> you sort of slingshot the black hole around your spacecraft into the opposite
> direction... it's just that the black hole won't bother *much*, being the fat
> lazy sucker it is.
Actually it would work if the black hole was moving towards us. Then
the spacecraft could go there and rob part of this momentum and get a
speed boost back towards Earth.
If the black hole was moving away from us then it would certainly not
work.
Think about it as an elastic collision. It's a pretty accurate way of
thinking about it.
> > - If you go so close to a black hole that it will give you a stronger
> > slingshot effect than a regular star would, the tidal forces would
> > probably rip you apart. Not very practical.
> Nah, I don't think so. Would tidal forces rip you apart at the surface of the
> sun? I doubt.
Remember: When the Shoemaker-Levy was on collision course against Jupiter,
tidal forces broke it in 9 parts well before it reached the surface. And
Jupiter is a lot less massive than the Sun.
Ok, maybe if your spacecraft is sturdy enough, it might survive a
slingshot from the Sun. However, we are talking about black holes here,
and black holes have several times the mass of the Sun (because stars
as small as the Sun cannot collapse to black holes), and getting very
close to the event horizon (for it to make a difference compared to a
regular star).
> > - The humongous amounts of radiation around a black hole would probably
> > be enough to fry you to ashes in a fraction of a second, no matter what
> > kind of shielding you use, especially if you go so close that you would
> > get a larger speed boost than from a regular star.
> Again, only if there's an accretion disk. Remember: Black holes don't emit
> anything - only their surroundings may ;)
They don't emit anything, but by their nature as being really massive
objects they surely tend to aggregate lots of nasty stuff orbiting around
them, and this stuff often emits large amounts of radiation by several
means.
For example one concern about sending probes to orbit Jupiter is the
large amount of radiation there (which could break electronics if not
shielded properly). Jupiter, as a very massive object, gathers tons of it.
And Jupiter is a lot less massive than a normal black hole.
> However, maybe you also want to check the magnetic field of the black hole - I'm
> not sure about the orders of magnitude, but I wouldn't want all my electrons go
> spinning off to the left while the protons go off to the right - not to speak
> of the resulting synchroton radiation ;)
Yeah, that too.
--
- Warp
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Warp wrote:
> The major objections against singularities are more or less philosophical,
> rather than based on hard science. It's "hard to believe" that mass could
> be compressed into a point of zero volume.
I think that maybe used to be the case, but I think the objections are also
that it's in conflict with QM right now.
> If someone objects to the notion of a singularity, he would have to show
> some evidence that GR doesn't work as predicted in this case.
I thought that's why it's called a singularity? :-)
> There must be
> some property of the Universe which makes GR not work in this situation,
> something which actually stops the singularity from forming. What could this
> phenomenon be? Has anyone ever measured such phenomenon to exist?
QM uncertainty, and symmetry rules that indicate you can't lose information
about individual particles. You *can't* compress something down to zero size
and not know both where it is and how fast it's moving, for example. QM also
says you can't have two fermions in the same quantum state in the same
place. (More precisely, the probability that it happens is zero, once you
add up all the possible virtual paths.) Hence, you can't collapse an entire
star into a single point.
I don't think the objects are just "it's hard to believe". It's just that
nobody knows how to resolve the problem, since the incompatible theories
both satisfy all the experimental evidence gathered so far.
> The thing is, if GR equations are right, and there is a lot of evidence
> suggesting that they are,
... at the macroscopic level ...
But a singularity is, by definition, microscopic. :-)
> There's no known way for matter/energy to stop going towards this center.
You wouldn't think a single electron could be larger than a house, either,
but they've done that in a laboratory.
I think the real answer is that people just don't know yet. GR says one
thing, and every experimental investigation we can do matches GR. QM says
another thing, and every experimental investigation we can do matches QM.
However, we don't have the capability (yet) of doing any experiment that
would encompass both GR and QM in the same experiment that would give
different answers in the area where they conflict.
Now, what would be fascinating would be for someone to come up with an
experiment that proves you can't measure things in a way that would make QM
and GR conflict. :-)
--
Darren New, San Diego CA, USA (PST)
"Ouch ouch ouch!"
"What's wrong? Noodles too hot?"
"No, I have Chopstick Tunnel Syndrome."
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Darren New <dne### [at] sanrrcom> wrote:
> You'd wind up changing the course of the black hole much less than you'd
> change the course of your spaceship, is all.
The black hole would have to be moving towards the general direction
of Earth, in that case.
> I also think the black hole would be better, because you can get closer and
> get a bigger time dilation from the gravity also.
But you get tons of other problems, as I described in my post. :P
> > - The humongous amounts of radiation around a black hole would probably
> > be enough to fry you to ashes in a fraction of a second,
> FWIW, it's the accretion disk that generates the radiation, so you could
> kill two birds with one stone by finding one without.
As very massive objects, and ones usually formed from collapsing stars
(the collapse leaving behind tons of stuff), I think it would be rather
difficult to find "naked" black holes, without the accompanying stuff.
--
- Warp
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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> You give one particle that is constantly emitting photons in all directions
>> a push due south towards the singularity. You have an observer due north of
>> the singularity looking south. The observer due north will continue to see
>> photons coming off the particle indefinitely?
>
> You can distribute a finite amount of photons over an infinite amount
> of time with a simple 1/x formula.
And therefore, yes, you would see photons coming off the particle
indefinitely. :-) That's kind of what I was asking. If you see photons
coming off forever, then you know that shortly before the most recently
observed photon, the object still hasn't crossed the EH.
So since the person falling in will never see himself cross the EH (because
his time slows), and the person outside will never see him cross, then stuff
falling into a black hole does *not* fall into the singularity. Nothing
crosses the EH, so nothing gets compressed into the singularity, yes?
(Discounting the black holes that have paths to the singularity that don't
cross an EH, of course.)
--
Darren New, San Diego CA, USA (PST)
"Ouch ouch ouch!"
"What's wrong? Noodles too hot?"
"No, I have Chopstick Tunnel Syndrome."
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Darren New <dne### [at] sanrrcom> wrote:
> QM also
> says you can't have two fermions in the same quantum state in the same
> place.
At least not in normal, almost-cartesian space, with extremely weak
gravity.
How about inside space which is so curved that it forms an event
horizon because of a practically infinite gravity?-)
Gravity is such a weak force in normal situations that you usually don't
have to even take it into account when making QM calculations. However,
inside a black hole gravity, rather obviously, overwhelms all other
forces and becomes absolutely dominant. What happens to eg. fermions
when gravity is immensely stronger than eg. the strong nuclear force?
Under such pressures matter degenerates. Do fermions even stay as
fermions, or do they degenerate to something more elemental? Something
which might not be bound to QM laws?
> > The thing is, if GR equations are right, and there is a lot of evidence
> > suggesting that they are,
> ... at the macroscopic level ...
> But a singularity is, by definition, microscopic. :-)
Gravity is in effect also at subatomic levels, even though it's often
ignored because it's so weak in normal circumstances.
> > There's no known way for matter/energy to stop going towards this center.
> You wouldn't think a single electron could be larger than a house, either,
> but they've done that in a laboratory.
"You wouldn't think" is different from "there's no known way". The
former is something related to my intuition. The latter is related to
current scientifical global knowledge.
(Well, always with a big "AFAIK". I'm *not* a scientist.)
--
- Warp
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clipka escreveu:
> "nemesis" <nam### [at] gmailcom> wrote:
> Interestingly though, the real-existing phenomenon of "black holes" was
> originally a purely hypothetical prediction of GR - not some already known
> effect that needed to be explained. And it did predict their properties quite
> well as it seems. So this is strong evidence that the GR is a very good tool to
> make predictions.
It is, no doubt. A very solid model, which QM enthusiasts are still
plotting to supplant. :)
Evidence was found of objects with imense gravitational fields occupying
relatively tiny areas in space and this matches with GR's predictions.
But there's as of yet no evidence that singularities physically exist or
that matter entering a blackhole is forever lost, except as a somewhat
vague Hawking radiation...
So, the QM guys thought it would be a good idea to imagine an alternate
model to explain such objects without relying on singularities and lost
information. And it's testable in lab, right here on Earth. Of course,
they are trying to somehow fit the lab effect to the observable
phenomena, but without explaining why such effect would occur in the
surface of stars...
> So in order to show that *their* theory is at least an equally good tool, they
> would actually have to demonstrate how their own theory explains not only the
> same *phenomenons* as GR, but *postulates GR itself* - or demonstrate how their
> theory is just a different way of looking at GR.
QM so far has made good predictions is the small, very small. :)
> BTW, speaking of it: When falling into a black hole I guess it may be best not
> to try to fight the inevitable... suppose you manage to stay at a fixed
> distance from the center - with all the blueshift of the photons falling in,
> you're gonna get your daily maximum dose of gamma rays in a few split
> seconds... so better fall in to experience the same blue shift (effectively
> leaving the shift unchanged from your perspective) and hope that the
> singularity is just a transition to a better life somewhere - um, I mean
> someWHEN - else...
a very good suggestion too. :)
Perhaps it's like passing through a corridor in flames: you should run
towards the end of it... :P
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Darren New <dne### [at] sanrrcom> wrote:
> So since the person falling in will never see himself cross the EH
I'm not sure where you are getting this one. I have not heard of it.
The only thing I have heard is that an *external* observer never sees
anything "touch" the event horizon.
Time for an observer itself is always the same, regardless of where
he is.
--
- Warp
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Warp escreveu:
> point towards the center. There's no known way for matter/energy to stop
> going towards this center. Just advancing in time makes it advance towards
> this center.
I like how Hawking puts it: "The singularity is the future: as you
advance towards it, you also moves towards the future" or something to
that effect...
> The only possible conclusion is that all the matter and energy compresses
> into a point of zero volume. If the singularity does not form in reality,
> then something must explain why. Something more than philosophical objections.
Well, if it was like that in the beginning, why couldn't it be possible
now, huh? Which also begs the philosophical question of whether passing
through the EH leads to a whole new universe beginning from yet another
big bang...
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nemesis <nam### [at] gmailcom> wrote:
> Which also begs the philosophical question of whether passing
> through the EH leads to a whole new universe beginning from yet another
> big bang...
I don't think the spacetime geometry inside the event horizon of a
black hole would allow for a universe to exist in there.
--
- Warp
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Warp escreveu:
> nemesis <nam### [at] gmailcom> wrote:
>> We're actually cruising unknown frontiers at speeds vastly superior to
>> 200km/sec
>
> With respect to what?
>
> Motion is always relative.
>
to the center of the galaxy?
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Warp escreveu:
> nemesis <nam### [at] gmailcom> wrote:
>> Which also begs the philosophical question of whether passing
>> through the EH leads to a whole new universe beginning from yet another
>> big bang...
>
> I don't think the spacetime geometry inside the event horizon of a
> black hole would allow for a universe to exist in there.
In there or out there? ;)
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nemesis escreveu:
> Warp escreveu:
>> nemesis <nam### [at] gmailcom> wrote:
>>> Which also begs the philosophical question of whether passing through
>>> the EH leads to a whole new universe beginning from yet another big
>>> bang...
>>
>> I don't think the spacetime geometry inside the event horizon of a
>> black hole would allow for a universe to exist in there.
>
> In there or out there? ;)
I believe the singularity could only get a proper artistic rendering
from Escher. :)
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nemesis <nam### [at] gmailcom> wrote:
> Warp escreveu:
> > nemesis <nam### [at] gmailcom> wrote:
> >> We're actually cruising unknown frontiers at speeds vastly superior to
> >> 200km/sec
> >
> > With respect to what?
> >
> > Motion is always relative.
> >
> to the center of the galaxy?
Well, I suppose that since we are in a rotating frame of reference, and
given that it's physically possible (at least in theory) to determine an
inertial frame of reference centered at the center of our galaxy, we could
be moving at that speed with respect to this inertial frame of reference.
--
- Warp
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Darren New escreveu:
> clipka wrote:
>> Talk about atoms - the ancient greeks "invented" them.
>
> Yes, and they also didn't believe in irrational numbers even after they
> were proved to exist. So I don't think you can pick one success out of
> history and call it a "prediction".
>
> I mean, there's really only two or three possibilities: Stuff is atoms,
> stuff is continuous, stuff is a mixture. :-)
>
> The greeks had no evidence of atoms, as far as I know. It was just a
> guess. It wasn't until Mendeleev (or the work that lead up to the
> periodic table) or so that people actually had evidence that atoms were
> the right answer.
Indeed. And atoms are not even atomic in the original sense of the
greek word... ;)
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nemesis wrote:
> holding your cup of coffee with a steady hand... :P
... which only means you need more coffee.
--
Darren New, San Diego CA, USA (PST)
"Ouch ouch ouch!"
"What's wrong? Noodles too hot?"
"No, I have Chopstick Tunnel Syndrome."
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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> You'd wind up changing the course of the black hole much less than you'd
>> change the course of your spaceship, is all.
>
> The black hole would have to be moving towards the general direction
> of Earth, in that case.
I don't have the math to be sure that's true. Wouldn't it be possible to
make it moving away less quickly?
I mean, certainly if there's a solid object moving away from you, you can
land on it and jump off and go up. Stand on a trampoline going 1 MPH
downwards - can you jump up?
You might be right, tho, if there's no physical contact, but as I say I
don't want to learn the math to figure it out. :-)
> But you get tons of other problems, as I described in my post. :P
Yah. I posted before I read that, methinks.
> As very massive objects, and ones usually formed from collapsing stars
> (the collapse leaving behind tons of stuff), I think it would be rather
> difficult to find "naked" black holes, without the accompanying stuff.
Unless you manufactured it yourself, yes. :-)
--
Darren New, San Diego CA, USA (PST)
"Ouch ouch ouch!"
"What's wrong? Noodles too hot?"
"No, I have Chopstick Tunnel Syndrome."
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Warp <war### [at] tagpovrayorg> wrote:
> If someone objects to the notion of a singularity, he would have to show
> some evidence that GR doesn't work as predicted in this case. There must be
> some property of the Universe which makes GR not work in this situation,
> something which actually stops the singularity from forming. What could this
> phenomenon be? Has anyone ever measured such phenomenon to exist?
>
> If not, then any objection against singularities is more or less
> philosophical. "It doesn't sound right" is not hard science.
I'm not saying "it doesn't sound right" - I'm saying I *believe* something else
;)
It wasn't meant to be a scientific statement or any other claim of truth in the
first place. Just a purely philosophical statement amidst an otherwise
dominated by science and wild thoughts...
I believe science will find that GR is wrong close to its singularities,
because... well, because I love the idea that there might be some
mind-bogglingly simple way of looking at both GR and QM, which science just
hasn't been imaginative enough to discover ;)
> The thing is, if GR equations are right, and there is a lot of evidence
> suggesting that they are, then there simply is no way for a singularity
> to not to form when a mass collapses to be smaller than its Schwarzschild
> radius. All geodesics inside the event horizon, including time geodesics,
> point towards the center. There's no known way for matter/energy to stop
> going towards this center. Just advancing in time makes it advance towards
> this center. Even if you tried to apply some force to a particle to stop
> it from getting closer to the center, you couldn't, not even if you applied
> an infinite force.
Hm - let's take this a step further, and salt it with a bit of quantum thinking
(which has successes of its own):
- If the equations say that inside the EH you can't do anyhing to a particle to
prevent it from going straight towards the center, then this might be a hint
that particles there can't *interact* in the first palce.
- If particles can't *interact*, there is no way to measure time; so the time a
particle would take from entering the EH until it reaches the center would be
"mu" (in the Zen sense, not some greek-lettered constant).
- If the equations say that as soon as a particle crosses the EH it will head
straight towards the center within a time of "mu", then it *might* be a hint
that the EH is *identical* with the center after all...
> The only possible conclusion is that all the matter and energy compresses
> into a point of zero volume.
No - this is the most *straightforward* conclusion. But being straightforward,
it may hinder thinking along other lines, so I choose to discard it and let my
imagination run wild.
Occam's razor is a good tool, but applying it to some "brainstorming" will just
kill inspiration.
It's also a good thing to occasionally check whether the concept once regarded
as "most simple" actually still *is* the most simple.
> If the singularity does not form in reality,
> then something must explain why. Something more than philosophical
> objections.
Fine. So? I *love* some good crazy mix of science, philosophy and wild
speculations, nothing wrong there for me ;)
Then again, I could turn my speculation indeed into an objection: As of now, no
singularity (i.e. a condition where a formula gives "infinity" as an answer)
predicted by any scientific theory has ever been observed in true life. In
fact, various theories that predicted singularities have been *proven* to be
inconsistent with reality exactly at or close to these conditions. Therefore,
it is reasonable to assume that singularities predicted by scientific theories
actually indicate that the theory *fails* at these conditions.
This resoning is actually not philosophical in nature, but rather very
scientific, and may qualify for a theory in its own right. The statement is
clear: "If the formulae derived from a theory in the domain of natural science
result in "infinity" values for certain conditions, then it is invalid for
these conditions." Predictions can be easily derived from it, e.g. "The GR does
not accurately describe the conditions within the EH of a black hole". In this
case, putting the predictions to the test might be problematic, but I guess
other theories can be found.
Of course one must be careful with this reasoning. Finding that a theory
predicts singularities doesn't mean the theory as a whole msut be at fault - it
just should be taken as a warning to be careful about predictions made by the
theory close to these conditions. In fact, a theory may be perfectly fine
except *exactly* at these conditions. And, after all, this reasoning is just
another theory, so it *may* be wrong after all. But from evidence seen so far,
I guess it can be considered quite robust.
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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> QM also
>> says you can't have two fermions in the same quantum state in the same
>> place.
>
> At least not in normal, almost-cartesian space, with extremely weak
> gravity.
>
> How about inside space which is so curved that it forms an event
> horizon because of a practically infinite gravity?-)
AFIAK, gravity's effect is so weak in QED at least that nobody knows how it
affects things.
Note that I'm not saying "you can't have two fermions in the same state."
I'm saying that's what QM says. And since it conflicts with GR's
predictions, one of the other needs to be adjusted, unless you can prove
that you can't prove either one is right.
Sure, it's possible that QM breaks down in extremes of GR, just like it's
possible GR breaks down at extremes of QM. I'm not saying one is right or
the other. I'm saying it's not just "philosophical objections" to
singularities, but actual mathematics that matches experimental measurements
to 15 decimal places.
(I think gravity is something like 40 decimal places out, tho, so it's
certainly possible QM isn't taking account of it properly.)
> Under such pressures matter degenerates. Do fermions even stay as
> fermions, or do they degenerate to something more elemental? Something
> which might not be bound to QM laws?
It's entirely possible that's what happens.
> Gravity is in effect also at subatomic levels, even though it's often
> ignored because it's so weak in normal circumstances.
Yep. And QM is in effect at levels the size of a star, similarly. :-)
>>> There's no known way for matter/energy to stop going towards this center.
>
>> You wouldn't think a single electron could be larger than a house, either,
>> but they've done that in a laboratory.
>
> "You wouldn't think" is different from "there's no known way". The
> former is something related to my intuition. The latter is related to
> current scientifical global knowledge.
Sure. I was simply addressing your "philosophical objection" bit. :)
And there *is* a known way for it to stop going towards the center: QM.
If either of us knew whether GR or QM was more right in this situation, we'd
already been in Stockholm picking up our prize. :-)
--
Darren New, San Diego CA, USA (PST)
"Ouch ouch ouch!"
"What's wrong? Noodles too hot?"
"No, I have Chopstick Tunnel Syndrome."
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