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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