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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