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] 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?

"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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