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I thought Warp might like seeing this, since he has talked about relativity
and warped space time and (I think) dark matter here before.
http://www.newscientist.com/article/mg18925423.600-three-cosmic-enigmas-one-audacious-answer.html
--
Darren New, San Diego CA, USA (PST)
Why is there a chainsaw in DOOM?
There aren't any trees on Mars.
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Darren New <dne### [at] san rrcom> wrote:
> I thought Warp might like seeing this, since he has talked about relativity
> and warped space time and (I think) dark matter here before.
>
http://www.newscientist.com/article/mg18925423.600-three-cosmic-enigmas-one-audacious-answer.html
"One such problem arises from the idea that once matter crosses a
black hole's event horizon - the point beyond which not even light can
escape - it will be destroyed by the space-time "singularity" at the
centre of the black hole. Because information about the matter is lost
forever, this conflicts with the laws of quantum mechanics, which
state that information can never disappear from the universe."
General relativity predicts one thing, quantum mechanics another. Why
must GR be wrong and QM right? Why couldn't it be the other way around?
"Another problem is that light from an object falling into a black
hole is stretched so dramatically by the immense gravity there that
observers outside will see time freeze: the object will appear to sit
at the event horizon for ever."
I'm not sure that's exactly correct. From the point of view of an external
observer, when an object approaches the event horizon of a black hole the
frequency of the light coming from that object decreases. If the frequency
of the light decreases to zero, it's basically not emitting any light at
all and thus cannot be observed.
While from the outside point of view the object never actually reaches
the event horizon, and consequently the frequency of the emitted light
never reaches true zero, there's probably a limit to how low the frequency
can be for it to be observed (as there are such limits in almost everything
related to quantum particles).
After all, the object cannot (and does not) emit photons forever. It stops
at some point (ie. when it crosses the event horizon), so no infinite amount
of photons can reach the external observer. The external observer sees the
photons coming less and less frequently, until the frequency becomes so low
that it's practically nonexistent.
"The team's calculations show that the vacuum energy inside the shell
has a powerful anti-gravity effect, just like the dark energy that
appears to be causing the expansion of the universe to accelerate. [...]
All observations used as evidence for black holes - their
gravitational pull on objects and the formation of accretion discs of
matter around them - could also work as evidence for dark energy
stars."
I don't really understand how the same object can both repel (with
antigravity) and attract (with gravity) at the same time. Isn't that
a bit contradictory? Which is it?
"Dark energy stars and black holes would have identical external
geometries, so it will be very difficult to tell them apart,"
If that is so, then wouldn't it behave in the same "problematic" way
with regard to an external observer watching an object fall towards this
star? What would be the difference?
The article doesn't claim that time dilation near massive objects doesn't
happen. It just says that time dilation near the event horizon of a black
hole causes problems with respect to quantum mechanics. If these dark stars
behave externally so much like black holes that they would be very hard to
distinguish from them, how does it solve any such "problem"? The time
dilation would still be there due to the mass of the former star being
accumulated close to the Schwarzschild radius.
--
- Warp
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Warp escreveu:
> Darren New <dne### [at] sanrrcom> wrote:
>> I thought Warp might like seeing this, since he has talked about relativity
>> and warped space time and (I think) dark matter here before.
>
>>
http://www.newscientist.com/article/mg18925423.600-three-cosmic-enigmas-one-audacious-answer.html
>
> "One such problem arises from the idea that once matter crosses a
> black hole's event horizon - the point beyond which not even light can
> escape - it will be destroyed by the space-time "singularity" at the
> centre of the black hole. Because information about the matter is lost
> forever, this conflicts with the laws of quantum mechanics, which
> state that information can never disappear from the universe."
>
> General relativity predicts one thing, quantum mechanics another. Why
> must GR be wrong and QM right? Why couldn't it be the other way around?
Besides, I thought Hawking's radiation coming from blackholes explained
that no information is lost or am I lost here? :/
Well, I'm still munching the possibility of being nothing more than a
hologram in a credit card... :P
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Warp wrote:
> General relativity predicts one thing, quantum mechanics another. Why
> must GR be wrong and QM right? Why couldn't it be the other way around?
Did it say GR is wrong? That quote just says it conflicts.
(FWIW, I'm seeing more articles that imply GR is going to loose to QM than
vice versa, but that might just be because it's easier to experiment on QM
than on GR.)
> I'm not sure that's exactly correct.
It's definitely a weird situation. I think the progress of time for someone
falling in seems to be asymptotically zero, but I don't know that means
anything for anyone outside or inside the doomed spacecraft.
> While from the outside point of view the object never actually reaches
> the event horizon,
Why not? It doesn't slow down - it just moves slower *inside* the space
ship, yes?
> and consequently the frequency of the emitted light
> never reaches true zero, there's probably a limit to how low the frequency
> can be for it to be observed (as there are such limits in almost everything
> related to quantum particles).
I think that's quite the problem there. That's where GR and QM disagree: QM
says there are lower limits on size, frequency, etc, while GR says space is
smooth and continuous.
> I don't really understand how the same object can both repel (with
> antigravity) and attract (with gravity) at the same time. Isn't that
> a bit contradictory? Which is it?
That does indeed sound strange. Perhaps if either one of us were deeply
schooled in this... :-)
> The article doesn't claim that time dilation near massive objects doesn't
> happen. It just says that time dilation near the event horizon of a black
> hole causes problems with respect to quantum mechanics.
I think it's saying the singularity causes the problem with QM, not the
event horizon. Specifically, when the black hole evaporates due to Hawking
radiation, you've lost the information (namely, the spin and charge and
such) of the particles that fell in.
With a big enough black hole, you'll never know when you cross the event
horizon. If your event horizon is galaxy-sized, the gravitational gradient
is very mild.
> If these dark stars
> behave externally so much like black holes that they would be very hard to
> distinguish from them, how does it solve any such "problem"?
There wouldn't need to be a singularity in the middle of the black hole.
If/when it evaporates, all the stuff that fell in can come back out the same
way.
I think. I am not a cutting-edge theoretical physicist.
--
Darren New, San Diego CA, USA (PST)
Why is there a chainsaw in DOOM?
There aren't any trees on Mars.
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nemesis wrote:
> Besides, I thought Hawking's radiation coming from blackholes explained
> that no information is lost or am I lost here? :/
AIUI, that's exactly where the information is lost. The Hawking radiation is
from quantum uncertainty *outside* the black hole, unrelated to what's
inside. That's *why* the information has to be on the *surface* (so to
speak) of the black hole.
--
Darren New, San Diego CA, USA (PST)
Why is there a chainsaw in DOOM?
There aren't any trees on Mars.
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nemesis <nam### [at] gmailcom> wrote:
> Besides, I thought Hawking's radiation coming from blackholes explained
> that no information is lost or am I lost here? :/
OTOH Hawking radiation is also an attempt to unify a bit quantum
mechanics with GR. There's practically no concrete proof that it's
actually happening. The physics involved are also rather exotic
(although perhaps not for a quantum mechanic).
--
- Warp
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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> > General relativity predicts one thing, quantum mechanics another. Why
> > must GR be wrong and QM right? Why couldn't it be the other way around?
> Did it say GR is wrong? That quote just says it conflicts.
> (FWIW, I'm seeing more articles that imply GR is going to loose to QM than
> vice versa, but that might just be because it's easier to experiment on QM
> than on GR.)
OTOH GR has resisted the test of time rather well. Time and again new
tests go just like GR predicts, and no way else. For example some time ago
they published data about the exact measurement of the distance between
the Earth and the Moon for the past 40 years (which can be done with
millimeters of accuracy using lasers and those corner-box mirros there),
and this distance has changed exactly as GR predicts, to umpteeth decimals
of accuracy.
In fact, the entire theory of dark matter and dark energy exists
*because* of GR, not regardless of it. Galaxies don't behave like they
should, according to GR, if the visible matter would be everything there
is in the galaxy. Neither does the Universe, expanding against GR, unless
there's some umeasured force making it do so.
It's not like there wouldn't be alternative theories. For example,
similarly to how Newtonian mechanics are a good approximation at small
scales and velocities but start failing at larger ones, it has been
proposed that GR also works well for stellar sizes but not for galactic
ones. That, in a similar way, it starts deviating as we go larger and
larger, and this can be seen from the behavior of entire galaxies and
galaxy groups. This would explain the galaxy rotation abnormality without
having to resort to dark matter which cannot be measured.
OTOH such theories are not considered viable because they contradict
other measurements, which in turn confirm GR even at galactic scales.
(I think measuring eg. galactic lensing has been used for this purpose.)
That's why the dark matter and dark energy theories are currently considered
the most believable.
> > I'm not sure that's exactly correct.
> It's definitely a weird situation. I think the progress of time for someone
> falling in seems to be asymptotically zero, but I don't know that means
> anything for anyone outside or inside the doomed spacecraft.
You mean besides being ripped apart by infinite tidal forces? ;)
From a timescale point of view, an observer falling to a black hole
would not see any change in timescales for himself. How he sees the
rest of the universe, however, is another story.
> > While from the outside point of view the object never actually reaches
> > the event horizon,
> Why not? It doesn't slow down - it just moves slower *inside* the space
> ship, yes?
To the external observer it looks like the falling object slows down in
every aspect. For example if the falling object had a clock (and let's
forget those tidal forces ripping it apart) and the external observer
would look at this clock with a telescope, the clock would slow down
indefinitely. Well, until no photons arrive anymore from the object and
it could not be observed anymore.
> > and consequently the frequency of the emitted light
> > never reaches true zero, there's probably a limit to how low the frequency
> > can be for it to be observed (as there are such limits in almost everything
> > related to quantum particles).
> I think that's quite the problem there. That's where GR and QM disagree: QM
> says there are lower limits on size, frequency, etc, while GR says space is
> smooth and continuous.
Does QM say that space is not continuous?
> > The article doesn't claim that time dilation near massive objects doesn't
> > happen. It just says that time dilation near the event horizon of a black
> > hole causes problems with respect to quantum mechanics.
> I think it's saying the singularity causes the problem with QM, not the
> event horizon.
I think the article is talking about the event horizon in this case
because it talks about the time dilation which happens as an object falls
towards it. This is a direct prediction of GR.
What happens to the object *after* it has crossed the event horizon
is a rather different issue.
> Specifically, when the black hole evaporates due to Hawking
> radiation, you've lost the information (namely, the spin and charge and
> such) of the particles that fell in.
Assuming Hawking radiation indeed exists...
> With a big enough black hole, you'll never know when you cross the event
> horizon.
I have heard this, I have a very hard time understanding how it would be
possible.
Space is *really* warped near the event horizon. The closer you get to
it, the more warped it is. If you were to look out of your spacecraft as
it's falling, the universe would look really weird. The closer you are
to the event horizon, the weirder.
And after you cross the event horizon... Who knows. But you certainly
would *not* see the universe in any normal way, if at all. You probably
wouldn't be able to measure anything of the universe at all (because,
if the GR equations are right, *all* geodesics inside the event horizon
point directly towards the center).
Some people seem to think that there could be objects "floating" around
inside the event horizon, and that someone could be there and see nothing
unusual. However, if all geodesics are pointing directly towards the
center, I have hard time believing that you could perceive the space
around you as anything "normal". No matter what you do, you go inevitably
towards the singularity. Even trying to stay still is impossible because
time geodesics also point towards the singularity, and advancing in time
moves you towards it.
Thus also light is moving directly towards the singularity. This would
make it impossible to make any observations about anything else inside
the event horizon because any light hitting you is coming from "above"
and going "down".
Of course this assuming tidal forces haven't obliterated you into
subatomic particles.
> If your event horizon is galaxy-sized, the gravitational gradient
> is very mild.
Maybe, but trying to make any observation about your surroundings
would be completely impossible, if I have understood correctly. Inside
the event horizon everything goes towards the singularity and there are
no other possible paths.
I bet this would make eg. a human body keeping its shape a bit difficult.
--
- Warp
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Warp wrote:
> OTOH GR has resisted the test of time rather well.
On the big scale it has. But then, so has QED at the small scale. :-)
> OTOH such theories are not considered viable because they contradict
> other measurements, which in turn confirm GR even at galactic scales.
Yes. I was pretty impressed when they found colliding galaxies where the
visible matter stopped (due to interactions) but the dark matter could be
seen "splashing" without stopping if you look at lensing effects.
>> It's definitely a weird situation. I think the progress of time for someone
>> falling in seems to be asymptotically zero, but I don't know that means
>> anything for anyone outside or inside the doomed spacecraft.
>
> You mean besides being ripped apart by infinite tidal forces? ;)
Yes.
> From a timescale point of view, an observer falling to a black hole
> would not see any change in timescales for himself. How he sees the
> rest of the universe, however, is another story.
I think that's what I was saying. I don't understand why anyone inside *or*
outside would think it takes infinite time to fall in.
> To the external observer it looks like the falling object slows down in
> every aspect.
So it stop, hovering against gravity, milimeters above the event horizon? I
find that difficult to understand.
> For example if the falling object had a clock (and let's
> forget those tidal forces ripping it apart) and the external observer
> would look at this clock with a telescope, the clock would slow down
> indefinitely. Well, until no photons arrive anymore from the object and
> it could not be observed anymore.
But inside, he'd still be falling. And outside, he'd still be falling, yes?
He'd just look stopped yet still falling to those outside.
>> I think that's quite the problem there. That's where GR and QM disagree: QM
>> says there are lower limits on size, frequency, etc, while GR says space is
>> smooth and continuous.
>
> Does QM say that space is not continuous?
I believe that's correct. Or that gravity at least is not continuous
(because nothing is continuous). I don't really understand it, but my
layman understanding is that GR's math only works if "forces" are
continuous, and QM says that "forces" are not.
>> I think it's saying the singularity causes the problem with QM, not the
>> event horizon.
>
> I think the article is talking about the event horizon in this case
> because it talks about the time dilation which happens as an object falls
> towards it. This is a direct prediction of GR.
Yeah. Really, I didn't think QM had even started on predicting what gravity
would do and how it would work, so I'm quite over my head here. I don't know
where they're getting "quantum effects cause time to slow down" or whatever.
I was saying that the GR singularity in the black hole is what is in
conflict with the information-is-retained part of QM.
>> Specifically, when the black hole evaporates due to Hawking
>> radiation, you've lost the information (namely, the spin and charge and
>> such) of the particles that fell in.
>
> Assuming Hawking radiation indeed exists...
If it doesn't, you've still lost the information. :-)
>> With a big enough black hole, you'll never know when you cross the event
>> horizon.
>
> I have heard this, I have a very hard time understanding how it would be
> possible.
>
> Space is *really* warped near the event horizon.
Not necessarily. It's almost really warped just outside, and it's just a
little bit more warped inside. So while the slope is steep, the second
derivative isn't. At least, that's how I understand it. :-)
> Some people seem to think that there could be objects "floating" around
> inside the event horizon, and that someone could be there and see nothing
> unusual. However, if all geodesics are pointing directly towards the
> center,
I'm not sure how you can have a geodesic outside in an "orbit", and a
geodesic a milimeter away a "straight line" into the middle. They all point
directly towards the center in their own frame of reference, perhaps? I
think it's over my head.
> I have hard time believing that you could perceive the space
> around you as anything "normal".
I would think if you look inwards, you'd see nothing. If you look outwards,
everything would be very (infinitely?) blue-shifted. I've heard it, but I
can't think how it could be true.
> Of course this assuming tidal forces haven't obliterated you into
> subatomic particles.
I think the bits we've heard is that the tidal forces aren't necessarily
that high. The tidal force is the difference between your head and your
feet, not the absolute "slope" of the spacetime.
> Maybe, but trying to make any observation about your surroundings
> would be completely impossible, if I have understood correctly.
It sounds right to me.
> I bet this would make eg. a human body keeping its shape a bit difficult.
That's a good point. Hrm. It would certainly seem to interfere with
circulation, for example.
Actually, I've read a short fiction story about just such a situation.
Something was randomly bobbling cities, nobody knew what it was, but you
could only move inwards, could only see what's behind you, except the
quantum uncertainty let you see a few feet ahead, move back enough that your
circulation still works, etc. There were people trained to go in at the
border when it showed up, helping people get to the center, staying safe
until it unbobbled itself.
And then there's "Redshift Rendezvous", which I read decades ago and which
has apparently just now resurfaces. A novel about bad guys taking over a
hyperdrive spaceship. "Hyperdrive" as in "the speed of light is different
here", as in "a dozen meters a second", and how the captain takes advantage
of the odd physics to rescue the situation. A very fun story.
--
Darren New, San Diego CA, USA (PST)
Why is there a chainsaw in DOOM?
There aren't any trees on Mars.
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Darren New escreveu:
> Warp wrote:
>> Does QM say that space is not continuous?
>
> I believe that's correct. Or that gravity at least is not continuous
> (because nothing is continuous). I don't really understand it, but my
> layman understanding is that GR's math only works if "forces" are
> continuous, and QM says that "forces" are not.
Now you tell me the universe is discrete! That's unreal! :P
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Warp wrote:
> OTOH GR has resisted the test of time rather well.
BTW, the article I read recently that talked about this was discussing a
gravity-wave detector where the amount of random noise they were getting was
pretty close to the amount of noise predicted by this whole
holographic-universe kind of thing.
It was a popular-science article, but google turns up
http://arxiv.org/abs/gr-qc/9906003
that pretty much sums it up, given what we're talking about. :-)
Basically, since the "hologram" of the universe is at 37 billion lightyears,
or whatever it is, but space is much bigger than that surface area, you get
some "blurring" at the quantum level that makes the "pixels" big enough to
actually measure, closer to 10^-15 than 10^-35 or some such. Way over my
head, but interesting.
--
Darren New, San Diego CA, USA (PST)
Why is there a chainsaw in DOOM?
There aren't any trees on Mars.
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Warp <war### [at] tagpovrayorg> wrote:
> I have heard this, I have a very hard time understanding how it would be
> possible.
>
> And after you cross the event horizon... Who knows. But you certainly
> would *not* see the universe in any normal way, if at all. You probably
> wouldn't be able to measure anything of the universe at all (because,
> if the GR equations are right, *all* geodesics inside the event horizon
> point directly towards the center).
I saw a simulation of this a while back. Your field of view increases as you
pass through the horizon, until 360+ degrees shrink to a circle directly above
you. As you go further and further into the black hole, the whole universe
above you shrinks to a point. Strictly speaking, though, you can observe
anything in the universe that you could before since you are still traveling
slower than the speed of light. The event horizon is only the point at which
light cones tip past vertical so that even light moving directly outward can no
longer make it. There really isn't anything special to the person passing
through it; it's just the point of no return.
> Some people seem to think that there could be objects "floating" around
> inside the event horizon, and that someone could be there and see nothing
> unusual. However, if all geodesics are pointing directly towards the
> center.
They don't really have to point directly toward the center, do they?. It's only
that NONE of them point outward. If you're inside the event horizon, you still
have an infinite number of choices, but 'out' is not one of them.
- Ricky
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Darren New <dne### [at] sanrrcom> wrote:
> > From a timescale point of view, an observer falling to a black hole
> > would not see any change in timescales for himself. How he sees the
> > rest of the universe, however, is another story.
> I think that's what I was saying. I don't understand why anyone inside *or*
> outside would think it takes infinite time to fall in.
Things get weird on the extremes with relativity.
For example a photon might travel billions of years from a star and
collide on Earth. However, from the point of view of the photon itself,
what is the distance it has travelled and how long it took for it to
travel it?
> > For example if the falling object had a clock (and let's
> > forget those tidal forces ripping it apart) and the external observer
> > would look at this clock with a telescope, the clock would slow down
> > indefinitely. Well, until no photons arrive anymore from the object and
> > it could not be observed anymore.
> But inside, he'd still be falling. And outside, he'd still be falling, yes?
> He'd just look stopped yet still falling to those outside.
Not stopped. Asymptotically slowing down.
(But as I said in my original post, I don't think it would be possible to
observe the object indefinitely from the outside. It just stops sending
photons at some point.)
> > Does QM say that space is not continuous?
> I believe that's correct. Or that gravity at least is not continuous
> (because nothing is continuous).
Unless I'm mistaken, the "graviton" is one of those particles invented
by quantum mechanics for the simple reason that "because everything seems
to consist of particles, then there must exist a particle for every
phenomenon we can see, including gravity".
In other words, it feels more a question of principle than a result of
measurements or predictions of some theory. Gravitons must exist. Why?
Because.
I really can't understand why quantum mechanists have such a problem
in thinking about gravity as timespace geometry rather than as the exchange
of some exotic particles which cannot be detected. I don't understand
what would be the problem. Why do we *need* a gravity particle? What
would it explain that gravity-as-timespace-geometry doesn't?
Timespace curvature *is* something which can be (and has been) measured.
A graviton isn't.
> I don't really understand it, but my
> layman understanding is that GR's math only works if "forces" are
> continuous, and QM says that "forces" are not.
How is, for example, momentum quantized? What is the particle which is
the smallest unit of momentum? Or kinetic energy? Or inertia?
Must everything be quantized?
> > Assuming Hawking radiation indeed exists...
> If it doesn't, you've still lost the information. :-)
If you add two pieces of information together, has some information been
lost?
> >> With a big enough black hole, you'll never know when you cross the event
> >> horizon.
> >
> > I have heard this, I have a very hard time understanding how it would be
> > possible.
> >
> > Space is *really* warped near the event horizon.
> Not necessarily. It's almost really warped just outside, and it's just a
> little bit more warped inside. So while the slope is steep, the second
> derivative isn't. At least, that's how I understand it. :-)
Well, given that the event horizon is, by definition, the area of space
were the curvature of space is so large that even photons have no way out,
and that this curvature of space increases as you approach the event
horizon, I have hard time believing you wouldn't notice anything when you
approach and cross the event horizon.
When you start approaching the event horizon light starts visibly bending
because of the curvature of space. Any light which traverses closer to the
event horizon gets "sqeezed", and thus everything seems to flatten.
Wikipedia has a good image about this: Assuming GR is right, what a
"bare" black hole (ie. one with no accretion disc or other stuff orbiting
around) would look like against a starry background, from a relatively
close distance:
http://en.wikipedia.org/wiki/File:Black_Hole_Milkyway.jpg
I suppose the size of the black hole only affects the rate at which
the change happens as you get closer to the event horizon, rather than
the strength of the visual effect.
(Btw, that image might not be totally correct, as it seems to lack
red-shifting of the light closer to the event horizon...)
--
- Warp
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triple_r <nomail@nomail> wrote:
> I saw a simulation of this a while back. Your field of view increases as you
> pass through the horizon, until 360+ degrees shrink to a circle directly above
> you. As you go further and further into the black hole, the whole universe
> above you shrinks to a point.
I believe you are talking about this:
http://www.spacetimetravel.org/expeditionsl/expeditionsl.html
However, if you read the explanation carefully, none of the images
simulates what happens *inside* the event horizon. All of them are from
the outside, closer and closer to the event horizon. The last image is
"taken" from a distance of 1.005rs, where 'rs' is the Schwarzschild radius,
ie. the radius at which the event horizon is located.
(OTOH, I still think those images don't take red/blue-shifting into
account, so they might not be completely accurate in their coloration,
even if they are in their geometry.)
> > Some people seem to think that there could be objects "floating" around
> > inside the event horizon, and that someone could be there and see nothing
> > unusual. However, if all geodesics are pointing directly towards the
> > center.
> They don't really have to point directly toward the center, do they?.
I'm a complete layman on the subject and the GR equations go *well* over
my head. The Schwarzschild solution to those equations goes even more
beyond my comprehension, so everything I know about the subject is from
reading things put into layman terms.
I have understood that all geodesics point directly towards the center
(at least in the Schwarzschild solution, ie. a non-rotating non-charged
black hole), but I may well have understood wrongly.
I'm sure, though, that the equations predict that it's physically
impossible to avoid advancing towards the singularity. Even trying to
stay still, without moving, is impossible. (I suppose the only possibility
to avoid this would be some weird quantum mechanical effect.)
--
- Warp
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Warp wrote:
> (Btw, that image might not be totally correct, as it seems to lack
> red-shifting of the light closer to the event horizon...)
There's clearly redshift a ways out from the horizon. I believe what you see
between the red ring and the horizon is light that has been "slingshotted"
around the hole - that is, you're seeing stars from behind your back. That
light would blue-shift going in, and red-shift coming out again, to balance
out.
--
Darren New, San Diego CA, USA (PST)
Why is there a chainsaw in DOOM?
There aren't any trees on Mars.
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Warp wrote:
> Things get weird on the extremes with relativity.
Sure.
>> But inside, he'd still be falling. And outside, he'd still be falling, yes?
>> He'd just look stopped yet still falling to those outside.
>
> Not stopped. Asymptotically slowing down.
So, the steeper the gravity gradient, the slower he falls? That doesn't make
sense? He actually *slows down* as he falls? Nothing ever goes thru the
event horizon?
His *clock* slows down. His actual progress thru space doesn't. Yes?
> Unless I'm mistaken, the "graviton" is one of those particles invented
> by quantum mechanics for the simple reason that "because everything seems
> to consist of particles, then there must exist a particle for every
> phenomenon we can see, including gravity".
I wouldn't be surprised if that's the case. I never heard of anyone even
proposing properties of a graviton, let alone finding evidence.
> I really can't understand why quantum mechanists have such a problem
> in thinking about gravity as timespace geometry rather than as the exchange
> of some exotic particles which cannot be detected.
I don't know they have a problem with it. That was my comment.
>> I don't really understand it, but my
>> layman understanding is that GR's math only works if "forces" are
>> continuous, and QM says that "forces" are not.
>
> How is, for example, momentum quantized?
The uncertainty principle comes into play. I'm not sure I understand all of
it. Possibly something to do with renormalization in the QM equations: if
you don't assume there's a lower limit on scale, you get infinities in all
the QM equations (as divergent summation series). It doesn't seem to matter
much *what* the lower limit is, which is what Feynman got the Nobel for
proving. But Plank length has always been bandied around in the stuff I've
read. (Plank length is something like ten orders of magnitude smaller than a
proton or some such, so ...)
> Must everything be quantized?
I don't understand it well enough. I'm just saying what I've heard said - GR
assumes space that's continuously differentiable everywhere outside an event
horizon, and QM requires it not to be that way.
>>> Assuming Hawking radiation indeed exists...
>
>> If it doesn't, you've still lost the information. :-)
>
> If you add two pieces of information together, has some information been
> lost?
If hawking radiation doesn't exist, then you have no way of seeing what's
inside the event horizon, and hence the information on the spin of the
particles in there is lost?
> I suppose the size of the black hole only affects the rate at which
> the change happens as you get closer to the event horizon, rather than
> the strength of the visual effect.
I think that's what I was trying to express. With a sufficiently large black
hole, you might not know you crossed the event horizon because the rate of
change of curvature is so slow. Of course you know you're in trouble.
There's just no "bump" as you cross over. And if you're in free-fall
anyway, what might you notice?
--
Darren New, San Diego CA, USA (PST)
Why is there a chainsaw in DOOM?
There aren't any trees on Mars.
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Darren New wrote:
> Possibly something to do with renormalization in the QM
To be a little more clear:
The way you calculate if particle P goes from place X to place Y is to
calculate first the probability of oging there directly, then the
probability of going there by going X->A->Y, then X->A->B->Y, then
X->A->B->C->Y, for all possible A, B, C, etc.
In QED (quantum mechanics of photons and electrons), the -> bit multiplies
by a rather small number, so the infinite summation comes to a total pretty
fast, so you don't have to do too many before you get numbers that match
experiements.
In QCD (quantum mechanics of nuclear reactions), the -> bit multiplies by a
much larger number, so you have to trace out the possibilities eight or ten
deep to get decent numbers.
In either case, you really have to add up an infinite number of
possibilities (including where B is a spacelike distance from both X and Y)
to get the 100% correct answer, but the formula diverges. If you stop at any
given distance, then divide the result by that length (for some meaning of
the word "divide"), you get the answer that agrees with experiment. (That
step is "renormalization".)
So it *seems* like there's a lower limit to the number of times you can
divide a space up and come up with the right numbers in QM. And the
mathematics of GR are (I'm led to believe) don't work without continuous
differentiability. So ... there ya go.
--
Darren New, San Diego CA, USA (PST)
Why is there a chainsaw in DOOM?
There aren't any trees on Mars.
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Warp <war### [at] tagpovrayorg> wrote:
> I'm a complete layman on the subject and the GR equations go *well* over
> my head. The Schwarzschild solution to those equations goes even more
> beyond my comprehension, so everything I know about the subject is from
> reading things put into layman terms.
I think we all have to agree on this point. I pretty much stopped reading these
layman's explanations on topics like relativity and quantum mechanics, figuring
that all the effort would be better spent learning the real thing. I picked up
a textbook on the subject a couple weeks ago, but haven't gotten far enough to
say anything more intelligent. Here's an interesting set of notes, though:
http://preposterousuniverse.com/grnotes/ It has a pretty neat explanation of
the Lorentz transformations, but it takes a bit more effort to dig into GR.
- Ricky
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Darren New <dne### [at] sanrrcom> wrote:
> > Not stopped. Asymptotically slowing down.
> So, the steeper the gravity gradient, the slower he falls? That doesn't make
> sense? He actually *slows down* as he falls? Nothing ever goes thru the
> event horizon?
No, it looks *from the outside*, from the point of view of an external
observer, like his time is slowing down.
Remember that speed and time is always relative to who is observing.
> > Unless I'm mistaken, the "graviton" is one of those particles invented
> > by quantum mechanics for the simple reason that "because everything seems
> > to consist of particles, then there must exist a particle for every
> > phenomenon we can see, including gravity".
> I wouldn't be surprised if that's the case. I never heard of anyone even
> proposing properties of a graviton, let alone finding evidence.
Actually there are a few things which can be said of gravitons. In other
words, "if gravitons existed, they must have these properties". For example,
they would have spin 2.
(And no, I haven't the faintest idea what "spin" means. I just read
wikipedia. :P )
> >>> Assuming Hawking radiation indeed exists...
> >
> >> If it doesn't, you've still lost the information. :-)
> >
> > If you add two pieces of information together, has some information been
> > lost?
> If hawking radiation doesn't exist, then you have no way of seeing what's
> inside the event horizon, and hence the information on the spin of the
> particles in there is lost?
Why is it lost? Just because you can't see it doesn't mean it doesn't
exist.
If something moves to the other side of the cosmological horizon (from
our perspective), is the information "lost"?
> > I suppose the size of the black hole only affects the rate at which
> > the change happens as you get closer to the event horizon, rather than
> > the strength of the visual effect.
> I think that's what I was trying to express. With a sufficiently large black
> hole, you might not know you crossed the event horizon because the rate of
> change of curvature is so slow. Of course you know you're in trouble.
> There's just no "bump" as you cross over. And if you're in free-fall
> anyway, what might you notice?
I think that you would notice everything being warped.
--
- Warp
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Warp <war### [at] tagpovrayorg> wrote:
> However, if you read the explanation carefully, none of the images
> simulates what happens *inside* the event horizon. All of them are from
> the outside, closer and closer to the event horizon. The last image is
> "taken" from a distance of 1.005rs, where 'rs' is the Schwarzschild radius,
> ie. the radius at which the event horizon is located.
Btw, if it's not clear why the universe seems to shrink into a small
circle above as you get closer to the event horizon, I drew a picture.
It demonstrates the reverse light paths which arrive at the observer
(ie. like reverse raytracing), which shows why most of the field of
view is covered by the event horizon when you are close enough to it:
http://warp.povusers.org/snaps/EventHorizon.png
--
- Warp
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From: Phil Cook v2
Subject: Re: Physics, relativity, quantum, etc.
Date: 20 Jan 2009 11:24:23
Message: <op.un2c6hs4mn4jds@phils>
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And lo On Tue, 20 Jan 2009 01:05:02 -0000, Darren New <dne### [at] sanrrcom>
did spake thusly:
>> Unless I'm mistaken, the "graviton" is one of those particles invented
>> by quantum mechanics for the simple reason that "because everything
>> seems
>> to consist of particles, then there must exist a particle for every
>> phenomenon we can see, including gravity".
In one sense, if you arrange the known particles you get gaps similiar to
what occured with the first periodic tables...
> I wouldn't be surprised if that's the case. I never heard of anyone even
> proposing properties of a graviton, let alone finding evidence.
... it has been a while but I recall someone running the theory that if
gravity was a particle (as postulated by the Standard Model) then the
properties it would have to have to function in the way we see it would
match certain criteria (as Warp mentioned spin 2 being one of them) that
so happened to match one of the gaps. Working from another direction
string theory 'created' as a necessary outcome a 'particle' then had spin
2 etc.
--
Phil Cook
--
I once tried to be apathetic, but I just couldn't be bothered
http://flipc.blogspot.com
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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>>> Not stopped. Asymptotically slowing down.
>
>> So, the steeper the gravity gradient, the slower he falls? That doesn't make
>> sense? He actually *slows down* as he falls? Nothing ever goes thru the
>> event horizon?
>
> No, it looks *from the outside*, from the point of view of an external
> observer, like his time is slowing down.
Right. Understand that my confusion is between these two sentences:
"He is slowing down."
"His time is slowing down."
> Remember that speed and time is always relative to who is observing.
I would think that from the outside observer, his time is slowing down even
while his velocity is increasing.
I find it hard to believe that in falling into a black hole, your velocity
decreases.
> Actually there are a few things which can be said of gravitons. In other
> words, "if gravitons existed, they must have these properties". For example,
> they would have spin 2.
Yep. Stuff like that. But I think that's because of where it would fall in
the tables.
> (And no, I haven't the faintest idea what "spin" means. I just read
> wikipedia. :P )
"Spin" is a fascinating concept related to symmetry. A "spin 1/2" particle
means if you turn it 180 degrees, it looks the same. So a "two of diamonds"
playing card would be "spin 1/2" - it looks the same right side up as upside
down. (Discounting, of course, that it's 3D, so has a back that's different
from the front.)
An "ace of spades" has "spin 1" - if you turn it 180 degrees, the spade in
the middle will be pointing down instead of up, so you have to turn it 360
degrees.
And yes, a "spin 2" particle really is as strange as it sounds. You have to
turn it around 720 degrees before it's symmetric again. Don't ask me how
that works out.
That's about all I know about spin, tho, so ...
>> If hawking radiation doesn't exist, then you have no way of seeing what's
>> inside the event horizon, and hence the information on the spin of the
>> particles in there is lost?
>
> Why is it lost? Just because you can't see it doesn't mean it doesn't
> exist.
It's lost because it cannot influence other particles in the local area. Of
course, that's because the particles outside the horizon are a spacelike
distance from the particles inside the horizon, so maybe you're right. Maybe
it's the equivalent of being outside the cosmological horizon.
> If something moves to the other side of the cosmological horizon (from
> our perspective), is the information "lost"?
Maybe. I don't know. It can't affect us, but it can affect other things in
the local area. Technically, the particles *could* affect things a spacelike
distance away, which is what all the yadda about "quantum teleportation" is
about.
>> I think that's what I was trying to express. With a sufficiently large black
>> hole, you might not know you crossed the event horizon because the rate of
>> change of curvature is so slow. Of course you know you're in trouble.
>> There's just no "bump" as you cross over. And if you're in free-fall
>> anyway, what might you notice?
>
> I think that you would notice everything being warped.
Sure. But would you notice it significantly more one inch inside a
galaxy-size event horizon more than you'd notice it one inch outside a
galaxy-size event horizon? I don't know, but I don't think that's how it
works - the event horizon, as I understand it, isn't a discontinuity.
--
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:
> I would think that from the outside observer, his time is slowing down even
> while his velocity is increasing.
>
> I find it hard to believe that in falling into a black hole, your velocity
> decreases.
Cheating a little and looking ahead in my textbook, I find the following
sentence regarding the path of a particle falling radially into a black hole:
2/(3*sqrt(2m)) * ( (r_0)^(2/3) - r^(2/3) ) = tau-tau_0
"Looking at this equation, the mysterious surface r=2m makes no appearance. The
body falls continuously to r=0 in finite proper (inertial) time, in contrast to
the result seen by an outside observer. In fact, one can say that the entire
evolution of the physical universe has occurred by the time the body passes the
surface r=2m."
In other words, as seen by an outside observer, you reach the event horizon and
sit. Forever. As seen by you, on the other hand, you go speeding right on
through like it's not even there. The surface only has significance for an
observer far away.
- Ricky
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Darren New escreveu:
> Warp wrote:
>> (And no, I haven't the faintest idea what "spin" means. I just read
>> wikipedia. :P )
>
> "Spin" is a fascinating concept related to symmetry. A "spin 1/2"
> particle means if you turn it 180 degrees, it looks the same. So a "two
> of diamonds" playing card would be "spin 1/2" - it looks the same right
> side up as upside down. (Discounting, of course, that it's 3D, so has a
> back that's different from the front.)
>
> An "ace of spades" has "spin 1" - if you turn it 180 degrees, the spade
> in the middle will be pointing down instead of up, so you have to turn
> it 360 degrees.
>
> And yes, a "spin 2" particle really is as strange as it sounds. You have
> to turn it around 720 degrees before it's symmetric again. Don't ask me
> how that works out.
Wonderful explanation. So far, from my days of chemestry in college,
all I (thought I) knew was that spin 1 was an electron spinning in one
direction and spin -1 in the other direction, whatever it be. :P
>> If something moves to the other side of the cosmological horizon (from
>> our perspective), is the information "lost"?
>
> Maybe. I don't know. It can't affect us, but it can affect other things
> in the local area. Technically, the particles *could* affect things a
> spacelike distance away, which is what all the yadda about "quantum
> teleportation" is about.
quantum entanglement! Beam me into a blackhole! :D
>> I think that you would notice everything being warped.
Including your body! :P
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triple_r wrote:
> In other words, as seen by an outside observer, you reach the event horizon and
> sit. Forever.
So at what point does the black hole increase in mass? Why would a black
hole ever grow if nothing falls thru the event horizon?
And at what distance do you start slowing down?
If you take the Earth and condense it down to a black hole, then stop a
particle in orbit 100 miles above where the surface used to be, the particle
will accelerate towards the black hole, right? But to avoid crossing the
event horizon, it must logically decelerate, as seen from the POV of someone
in orbit (say). You wouldn't stop at the original altitude of the surface,
but you would stop before you get to the event horizon, so in this case,
gravity must be causing you to decelerate. 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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Darren New escreveu:
> triple_r wrote:
>> In other words, as seen by an outside observer, you reach the event
>> horizon and
>> sit. Forever.
>
> So at what point does the black hole increase in mass? Why would a black
> hole ever grow if nothing falls thru the event horizon?
He's talking about what is seen from an external observer, which is the
same explanation Hawking gave in one of his books. The observer sees a
kind of after image that seems to ever approach the event horizon
without ever touching it, but in reality the crossing of the EH already
took place and nothing of what happens in the inside is seen...
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nemesis <nam### [at] gmailcom> wrote:
> Darren New escreveu:
> > triple_r wrote:
> >> In other words, as seen by an outside observer, you reach the event
> >> horizon and
> >> sit. Forever.
> >
> > So at what point does the black hole increase in mass? Why would a black
> > hole ever grow if nothing falls thru the event horizon?
> He's talking about what is seen from an external observer, which is the
> same explanation Hawking gave in one of his books. The observer sees a
> kind of after image that seems to ever approach the event horizon
> without ever touching it, but in reality the crossing of the EH already
> took place and nothing of what happens in the inside is seen...
Besides, the mass of a spherical object, as measured from outside the
sphere, is independent of how the mass is distributed. It could be all
in the center or it could be all evenly distributed. From the outside
it would measure the same.
That's the reason why you can eg. calculate orbits assuming the Earth
is a point, rather than a sphere. (Well, as long as the orbit doesn't
collide with the surface of the Earth, of course.)
--
- Warp
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nemesis <nam### [at] gmailcom> wrote:
> > Maybe. I don't know. It can't affect us, but it can affect other things
> > in the local area. Technically, the particles *could* affect things a
> > spacelike distance away, which is what all the yadda about "quantum
> > teleportation" is about.
> quantum entanglement! Beam me into a blackhole! :D
If quantum teleportation exists, I wonder if it would be theoretically
possible to teleport a particle out of a black hole.
--
- Warp
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nemesis wrote:
> kind of after image that seems to ever approach the event horizon
> without ever touching it, but in reality the crossing of the EH already
> took place and nothing of what happens in the inside is seen...
Hmmmm.... I'll have to think on it.
--
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:
> If quantum teleportation exists, I wonder if it would be theoretically
> possible to teleport a particle out of a black hole.
And therein lies the clash of QED and GR. :-)
--
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 escreveu:
> nemesis wrote:
>> kind of after image that seems to ever approach the event horizon
>> without ever touching it, but in reality the crossing of the EH
>> already took place and nothing of what happens in the inside is seen...
>
> Hmmmm.... I'll have to think on it.
I think it has something to do with the last photons coming out of the
object entering the EH are severely slowed down by the massive gravity
and only reach you after much more time than normal has passed.
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Warp escreveu:
> If quantum teleportation exists, I wonder if it would be theoretically
> possible to teleport a particle out of a black hole.
Bah, it all becomes easy when you know we live in a hologram: just look
at it from a different angle... ;)
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nemesis <nam### [at] gmailcom> wrote:
> I think it has something to do with the last photons coming out of the
> object entering the EH are severely slowed down by the massive gravity
> and only reach you after much more time than normal has passed.
Photons don't slow down. The speed of light in vacuum is a universal
constant and doesn't change.
What photons do is that they redshift.
--
- Warp
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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> > If quantum teleportation exists, I wonder if it would be theoretically
> > possible to teleport a particle out of a black hole.
> And therein lies the clash of QED and GR. :-)
Thinking about it, unless I'm completely mistaken, quantum mechanics
doesn't disagree with the notion that *information* cannot be transported
at superluminal speeds (even if quantum effects can).
Transporting a particle out of a black hole, which in practice would be
getting information out of the black hole, would require superluminal speed,
so it would violate this basic principle.
--
- Warp
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nemesis wrote:
> object entering the EH are severely slowed down by the massive gravity
As Warp said. My "have to think about it" would be the "if the occupant of
the spaceship never experiences the crossing, and people outside can never
see/measure it actually happen, does it ever actually happen?"
Kind of a "tree falls in the woods" question if anything.
--
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:
> Thinking about it, unless I'm completely mistaken, quantum mechanics
> doesn't disagree with the notion that *information* cannot be transported
> at superluminal speeds (even if quantum effects can).
QM doesn't show that particles cannot be transported at superluminal speeds.
Indeed, you have to take superluminal speeds into account when you're doing
the infinite sums in order to make them come out.
GR says information cannot be transported at superluminal speeds, which is
why Bell's Inequality is of interest.
--
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:
> Warp wrote:
> > Thinking about it, unless I'm completely mistaken, quantum mechanics
> > doesn't disagree with the notion that *information* cannot be transported
> > at superluminal speeds (even if quantum effects can).
> QM doesn't show that particles cannot be transported at superluminal speeds.
> Indeed, you have to take superluminal speeds into account when you're doing
> the infinite sums in order to make them come out.
Unless I'm mistaken (hmm, I'm using that expression a lot), it's a bit
more complicated than that.
Just because, according to quantum mechanics, a particle can interact
over great distances with no delay doesn't mean that *information* can
be transferred over that distance without the delay (it's one of those QM
weirdnesses that I have no hope of fully understanding). I have the
impression that quantum mechanics doesn't disagree with GR about this
subject.
For example it was specifically noted in the relatively recent test
result of quantum teleportation that although the particles were (at least
apparently) teleported at superluminal speeds, the technique could still
not be used to transfer information from one place to another at superluminal
speeds. Thus there was no contradiction with GR.
GR itself doesn't forbid the distance between two points in space growing
faster than c. This is actually what is currently mostly accepted as
happening in the universe, and the cause for the so-called cosmological
horizon (the farthest parts of the Universe are receding from us faster
than c, which means we can *never* get any information about those parts
by any possible means, which in turn means that there's a "horizon" which
effectively completely conceals the rest of the Universe from us). This is
accepted because it actually doesn't break GR equations.
However, even in this circumstance *information* cannot be transferred at
superluminal speeds between two points in space. In other words, even if
we invented a machine which would create new space between two points so
fast that the two points recede from each other faster than c, this could
not be used to transport anything between the points faster than c.
> GR says information cannot be transported at superluminal speeds, which is
> why Bell's Inequality is of interest.
I think that the first sentence in the related wikipedia page is the
best summarization of QM I have seen:
"Bell's theorem is a theorem that shows that the predictions of
quantum mechanics (QM) are not intuitive, and touches upon several
fundamental philosophical issues related to modern physics."
--
- Warp
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Warp wrote:
> Just because, according to quantum mechanics, a particle can interact
> over great distances with no delay doesn't mean that *information* can
> be transferred over that distance without the delay
I think you're right. Indeed, most recent tests on Bell's Inequality seem to
indicate that the non-local variables indeed do not exist. I didn't at first
understand what you were saying.
But we weren't talking about information. My comment was in response to you
saying that particles can't be transported out of a black hole. That is
essentially what Hawking radiation is. However, since Hawking radiation is
random, information is not leaving the black hole.
In other words, I think we're agreeing with each other.
--
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:
> And at what distance do you start slowing down?
>
> If you take the Earth and condense it down to a black hole, then stop a
> particle in orbit 100 miles above where the surface used to be, the particle
> will accelerate towards the black hole, right? But to avoid crossing the
> event horizon, it must logically decelerate, as seen from the POV of someone
> in orbit (say). You wouldn't stop at the original altitude of the surface,
> but you would stop before you get to the event horizon, so in this case,
> gravity must be causing you to decelerate. Yes?
Here's a couple plots to show the scales involved in your question. For the
earth, the Schwarzschild radius is about 9mm. If you start from 1m, I've
attached plots showing what an external observer sees and what the dead guy
sees. (The units of time are meters, so you need to divide that by c to get
time.) I'm not sure it's correct to think of it as deceleration since that's
really a notion we have from flat space-time. For this case, space-time is
infinitely warped, so even though it may appear that you decelerate to an
outside observer, in fact you accelerate all the way to the center. You can
also see here that the event horizon means nothing to the guy passing through
it.
- Ricky
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Attachments:
Download 'rel1.png' (15 KB)
Preview of image 'rel1.png'
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Sorry. Web viewer. Here's the second plot.
- Ricky
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Attachments:
Download 'rel2.png' (15 KB)
Preview of image 'rel2.png'
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Warp <war### [at] tagpovrayorg> wrote:
> > With a big enough black hole, you'll never know when you cross the event
> > horizon.
>
> I have heard this, I have a very hard time understanding how it would be
> possible.
>
> Space is *really* warped near the event horizon. The closer you get to
> it, the more warped it is. If you were to look out of your spacecraft as
> it's falling, the universe would look really weird. The closer you are
> to the event horizon, the weirder.
>
> And after you cross the event horizon... Who knows. But you certainly
> would *not* see the universe in any normal way, if at all. You probably
> wouldn't be able to measure anything of the universe at all (because,
> if the GR equations are right, *all* geodesics inside the event horizon
> point directly towards the center).
Hm... I never thought about this in detail. I always imagined the event horizon
as nothing more than a theoretical construct, still a good deal away from the
singularity at its center - the distance beyond which a "fixed" outside
observer cannot see anything anymore. I also imagined that a non-fixed observer
in free fall towards the black hole would percieve the event horizon to be a bit
closer to the singularity, because after all light would only have to reach him,
not be able to zoom off to infinity...
.... but then again, if the light from inside the event horizon could still reach
a free-falling observer outside it, then what would stop the light from zooming
off from *there* to infinity?
So from this, and hearing that it would actually take an eternity (as seen from
an outside observer) to *reach* the event horizon, it sounds logical to me that
in fact the event horizon *is* the singularity...
.... whoa! so we have infinitely small spots the size of planets or larger... now
*that* is what I call weird...
.... and it makes me imagine that it is actually impossible to fall *into* a
black hole: If you'd take that trip, you'd experience it as being torn to
pieces by tidal forces, and in the next moment evaporating as hawking radiation
- seeing that the rest of the universe has aged some few gazillions of years...
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clipka wrote:
> So from this, and hearing that it would actually take an eternity (as seen from
> an outside observer) to *reach* the event horizon,
This still doesn't make sense to me, I fear.
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?
> it sounds logical to me that
> in fact the event horizon *is* the singularity...
But it's not.
> If you'd take that trip, you'd experience it as being torn to
> pieces by tidal forces,
Tidal forces are a difference in gravity gradient between head and feet.
It's not an absolute number. A sufficiently small pebble would experience
very little tidal effect regardless of how close to the primary it orbits. :-)
--
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:
> (But as I said in my original post, I don't think it would be possible to
> observe the object indefinitely from the outside. It just stops sending
> photons at some point.)
Strictly speaking I guess it doesn't - the probability of it sending photons
just decreases asymptotically to zero...
> (Btw, that image might not be totally correct, as it seems to lack
> red-shifting of the light closer to the event horizon...)
No red-shifting necessary, because before it gets out of the gravitational well
- experiencing redshift - it gets into it first, experiencing a corresponding
ammount of blueshift.
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Darren New wrote:
>> it sounds logical to me that
>> in fact the event horizon *is* the singularity...
>
> But it's not.
By which I mean:
"singularity" has a specific meaning in mathematics, and the event horizon
doesn't meet that definition, while the "center" of the black hole does.
Take the graph of tan(x). At pi/2, there's a singularity. At 0, it just goes
thru a 45-degree slope (and a point of inflection). That's kind of the
difference.
--
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 escreveu:
>> If you'd take that trip, you'd experience it as being torn to
>> pieces by tidal forces,
>
> Tidal forces are a difference in gravity gradient between head and feet.
> It's not an absolute number. A sufficiently small pebble would
> experience very little tidal effect regardless of how close to the
> primary it orbits. :-)
hah! Thanks for the tip! I'll roll up into fetal position when nearing
a black hole... :)
Anyway, trying to understand something that until now has not been
observed and is only but a prediction from a math model that may be
incomplete -- which is how many scientists see the singularities of GR
equations -- seems a bit off. Some scientists are suggesting that
blackholes are in fact stars made out of dark matter and energy.
http://www.newscientist.com/article/mg18925423.600-three-cosmic-enigmas-one-audacious-answer.html
whoa, head spins...
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Darren New <dne### [at] sanrrcom> wrote:
> "Spin" is a fascinating concept related to symmetry. A "spin 1/2" particle
> means if you turn it 180 degrees, it looks the same. So a "two of diamonds"
> playing card would be "spin 1/2" - it looks the same right side up as upside
> down. (Discounting, of course, that it's 3D, so has a back that's different
> from the front.)
IIRC it's just the other way round - so you have to turn quite a lot of
subatomic particles around twice to get them look the same again...?
> It's lost because it cannot influence other particles in the local area. Of
> course, that's because the particles outside the horizon are a spacelike
> distance from the particles inside the horizon, so maybe you're right. Maybe
> it's the equivalent of being outside the cosmological horizon.
Does there exist such a thing? It doesn't have this "one way ticket" thing to it
like black holes do.
AIUI the cosmological horizon is just the distance we can see *now*. Everything
beyond will become visible to us over time. Theoretically speaking.
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clipka wrote:
> IIRC it's just the other way round - so you have to turn quite a lot of
> subatomic particles around twice to get them look the same again...?
It's possible I got that wrong. Indeed, it's possible I don't know anything
at all about the subject that I think I do. :-) Were this an actual
scientific forum, I'd have shut up a long time ago.
>> It's lost because it cannot influence other particles in the local area. Of
>> course, that's because the particles outside the horizon are a spacelike
>> distance from the particles inside the horizon, so maybe you're right. Maybe
>> it's the equivalent of being outside the cosmological horizon.
>
> Does there exist such a thing? It doesn't have this "one way ticket" thing to it
> like black holes do.
I'm not sure what you speak of.
> AIUI the cosmological horizon is just the distance we can see *now*. Everything
> beyond will become visible to us over time. Theoretically speaking.
Oh, the event horizon. No, I think it's caused by space itself expanding,
due to the big bang sort of thing. If it all collapses again, that's a
different question.
--
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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nemesis wrote:
> hah! Thanks for the tip! I'll roll up into fetal position when nearing
> a black hole... :)
You're not kidding. Larry Niven actually did a sci-fi story about that.
> equations -- seems a bit off. Some scientists are suggesting that
> blackholes are in fact stars made out of dark matter and energy.
Dude. Scroll up to the first post in this conversation. :-)
--
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 escreveu:
> Dude. Scroll up to the first post in this conversation. :-)
oh, crap! So that's where this tab got open... :P
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Darren New <dne### [at] sanrrcom> wrote:
> If you take the Earth and condense it down to a black hole, then stop a
> particle in orbit 100 miles above where the surface used to be, the particle
> will accelerate towards the black hole, right? But to avoid crossing the
> event horizon, it must logically decelerate, as seen from the POV of someone
> in orbit (say). You wouldn't stop at the original altitude of the surface,
> but you would stop before you get to the event horizon, so in this case,
> gravity must be causing you to decelerate. Yes?
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.
From that poor old sod's perspective, gravitation near a black hole is so strong
that it keeps speeding him up - his feet much stronger than his head (if he
plunges in feet first) - until he is ripped to isolated subatomic particles.
Paradoxically, while this happens, I think that he will at the same time see his
feet slow down to a standstill, just as an outside observer would. So maybe he's
not torn apart after all..!?!
(In a similarly paradoxical fashion, the particles at his head can still easily
influence those at his feet - but the influence of the feet particles on the
head particles will drastically diminish (remember them sending out less
photons?)
This actually shows that gravitation is a quite strange beast in the zoo of
forces: The influence of gravitation does *not* diminish while closing in on a
black hole.
So either gravitation cannot be a "normal" QM-style force (else the object would
send out fewer and fewer gravitons as its own timeframe slows down with respect
to an outside observer) - or the object's e=mc^2 increases towards infinity as
it comes close to the event horizon, but why should it? After all, we have seen
that from an outsider's point of view the object actually slows to a halt.
Hum... sounds to me like bad news for the people at the LHC - and speaks for the
"gravitation = spacetime geometry" view of GR.
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nemesis <nam### [at] gmailcom> wrote:
> Darren New escreveu:
> > nemesis wrote:
> >> kind of after image that seems to ever approach the event horizon
> >> without ever touching it, but in reality the crossing of the EH
> >> already took place and nothing of what happens in the inside is seen...
> >
> > Hmmmm.... I'll have to think on it.
>
> I think it has something to do with the last photons coming out of the
> object entering the EH are severely slowed down by the massive gravity
> and only reach you after much more time than normal has passed.
Yeah - interestingly, the distance between a point near the EH and a point far
away is shorter when moving towards the EH than when moving away from it on the
very same route...
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