Darren New <dne### [at] sanrrcom> wrote:
> You give one particle that is constantly emitting photons in all directions 
> a push due south towards the singularity. You have an observer due north of 
> the singularity looking south.  The observer due north will continue to see 
> photons coming off the particle indefinitely?

  You can distribute a finite amount of photons over an infinite amount
of time with a simple 1/x formula.

-- 
                                                          - Warp

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nemesis <nam### [at] gmailcom> wrote:
> If we assume blackholes exist, without much evidence so far

  The GR equations seem to work with everything we can measure, at
macroscopic scales. At no point it has been measured that these equations
begin to diverge from reality.

  In other words, we have *no evidence* that the GR equations are not
valid in all possible situations, such as with extremely dense objects.
Thus, until we get evidence of the contrary, the only logical conclusion
is to assume that the equations hold also for extreme situations, for
example when an object collapses to be smaller than its Schwarzschild
radius.

  GR equations might not fully reflect what happens at atomic scales,
but neither do these measures provide any evidence that GR equations
don't work at macroscopic scales.

-- 
                                                          - Warp

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On Thu, 22 Jan 2009 09:18:19 +0100, "scott" <sco### [at] scottcom> wrote:

>> Depends on definition if speed, but as it is usually specified based on 
>> the
>> observer's timeframe: Yes. Gravitation near a black hole is so strong 
>> that -
>> from an observer's POV - through time delation and the warping of space it
>> causes "that poor old sod over there" to slow to a halt.
>
>I always thought that it could be possible to go forward in time by some 
>arbitrary amount just by making a close orbit around a black hole.  The 
>closer you go, the more time you can jump forward.  So just type in the year 
>3050 to your ship, it fires you off towards the nearest black hole, and a 
>few months later you return back towards Earth in the year 3050 +/- a few 
>months ;-)
>
>Or doesn't it work like that?
> 

No you run widdershins round the Earth very fast and you arrive in tomorrow
before the rest of the world does.

Or doesn't it work like that?
-- 

Regards
     Stephen

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> No you run widdershins round the Earth very fast

Ah but why use expensive rockets and stuff to go fast round the Earth when 
you can just aim towards a black hole and get a huge speed-up for free (plus 
it returns you roughly back towards Earth if you aim correctly).

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scott <sco### [at] scottcom> wrote:
> > No you run widdershins round the Earth very fast

> Ah but why use expensive rockets and stuff to go fast round the Earth when 
> you can just aim towards a black hole and get a huge speed-up for free (plus 
> it returns you roughly back towards Earth if you aim correctly).

  I think the slingshot effect is not dependent on the density of the larger
object, only on its mass and velocity (or, in other words, momentum). So
a black hole wouldn't make any difference compared to a regular star of
the same mass and velocity.

-- 
                                                          - Warp

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>  I think the slingshot effect is not dependent on the density of the 
> larger
> object, only on its mass and velocity (or, in other words, momentum). So
> a black hole wouldn't make any difference compared to a regular star of
> the same mass and velocity.

But presumably an object with the same mass but higher density would allow 
you to get a faster slingshot without crashing into it (because it's 
smaller)?

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scott <sco### [at] scottcom> wrote:
> But presumably an object with the same mass but higher density would allow 
> you to get a faster slingshot without crashing into it (because it's 
> smaller)?

  In theory yes. OTOH it still depends on the momentum of the larger object.
You could get a stronger slingshot from a star of the same mass if it's
moving faster than the black hole.

  Yet, this is not as simple as it sounds in any case. The slingshot effect
is always relative to something. For example interplanetary gravitational
assist is relative to the Sun. The Sun itself cannot be used for a slingshot
effect inside the solar system (it could be used for a slingshot relative
to the galaxy, but not relative to the solar system).

  With this taken into account, can you just go from Earth to the nearest
black hole, get an enormous speedup and come back at 100x the speed and
slam onto Earth at that speed? From a gravity assist only, I don't think so.
I think it would be against conservation of energy. If you were travelling
from Earth to another star system, then maybe, but I don't think it works
in the closed case.

  There are also some practical things to take into accout:

- If you go so close to a black hole that it will give you a stronger
slingshot effect than a regular star would, the tidal forces would
probably rip you apart. Not very practical.

- Black holes usually have an accretion disc around them, usually much
larger than the size of a normal star of the same mass. The accretion
disc could slow you down or be dangerous.

- The humongous amounts of radiation around a black hole would probably
be enough to fry you to ashes in a fraction of a second, no matter what
kind of shielding you use, especially if you go so close that you would
get a larger speed boost than from a regular star.

-- 
                                                          - Warp

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nemesis <nam### [at] gmailcom> wrote:
>   Does the singularity or blackholes even exist or are merely the point
> where GR equations break?

I honestly suspect exactly that... see Achilleus and the turtle: With the wrong
approach, you get the result that the fastest runner can't catch up with the
turtle... while reality proves he can.

In that old greek paradoxon it is rather obvious that it's a problem of a wrong
approach, because the reference system used is far from common sense. However,
with black holes, the reference system picking ever smaller intervals of
distance and time - that of an outside observer - seems more natural at first
glance, so it's not *that* obvious that we might just be using a wrong
approach.

> If we assume blackholes exist, without much evidence so far, how about
> going even further in the imagination?  What if our 4D space-time isn't
> but a section of higher dimensional spaces, like a julia fractal section
> rendered in povray?  Perhaps the blackhole is then just a curve along
> this surface and the poor fellow ends up in another region of
> space-time.  The original outside observer will never see him again,
> unless he goes all the way back through the worm hole...

That description matches quite well what I think is actually happening -
although with a different punchline. That wormhole will take the traveller to
another region of spacetime(!) indeed: Far-away future! The only problem with
it is that it's a one-way ticket: If he enters the wormhole again, he will end
up yet again "someplace later", not back where he started.

> Those greeks philosophers were just full of marijuana.  And pasta.  They
> were first possibly the first thinkers of mankind to think too much on
> far too abstract subjects rather than more mundane and practical
> matters... :P

:) I guess the same goes for Buddhism and the like... but they seem to have
gotten some things astonishingly right. Talk about atoms - the ancient greeks
"invented" them. Talk about the quantum theory question of whether anything
actually exists that is not observed - Zen has been asking that question long
time ago.

But let's get back to more practical matters: I get the munchies now... where's
my pizza? :)

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nemesis <nam### [at] gmailcom> wrote:
> Since we're speculating, if space is really expanding, do you think in
> absolute terms we're now much larger than dinosaurs back in their day?
> Of course, fossils won't tell, cause they expanded ever since too. ;)

Hum... well, I guess the cosmological constant isn't *that* big...

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nemesis <nam### [at] gmailcom> wrote:
> So that's why light takes a lot more time to reach us and thus the after
> image just before the crossing is visible.  OTOH, time and space are two
> sides of the same coin, isn't it?

That's what GR says - yes.

Or, well, maybe they're rather two hemispheres on the same sphere... or
something along that line...

Did you realize that if you're accelerating, you're just turning around in
space-time? (Well, basically, that is - it's a kind of weird type of
rotation...)

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