"clipka" <nomail@nomail> wrote:
> 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...

Again, from the book since I probably can't state it any better:

"In this case, we will find that while the surface r=2m has some unusual
properties, the singularity is due to the choice of coordinates, and so is a
*coordinate singularity*.  Simply put, by using a different set of coordinates
we can write the metric in such a way that the singularity at r=2m is removed.
However, we will see that the point r=0 is due to infinite curvature and cannot
be removed by a change in the coordinates."

I may be guilty of having implied that the curvature is infinite at the event
horizon, but it looks like only the redshift to an outside observer is.  Kinda
learning this as I go, so you'll have to forgive me.

I know most of this is focused on general relativity, but here's a nifty program
for exploring special relativity:

http://realtimerelativity.org/

 - Ricky

Post a reply to this message

triple_r wrote:
> I may be guilty of having implied that the curvature is infinite at the event
> horizon, but it looks like only the redshift to an outside observer is.  Kinda
> learning this as I go, so you'll have to forgive me.

Ha! You're teaching me as you go! :-)

-- 
   Darren New, San Diego CA, USA (PST)
   "Ouch ouch ouch!"
   "What's wrong? Noodles too hot?"
   "No, I have Chopstick Tunnel Syndrome."

Post a reply to this message

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?

Yes - they'll just seem to come at ever longer intervals (and with an ever
stronger redshift).

Of course, there is only a limited number of photons the particle can emit
before hitting the EH. However, from the observer's point of view, the very
last such photon will arrive at (roughly) infinite time after the second to
last.

Roughly, if that last photon is emitted a finite time before the particle passes
the EH.

Exactly, if the last photon is emitted at the very moment the particle passes
the EH.


>
> > it sounds logical to me that
> > in fact the event horizon *is* the singularity...
>
> But it's not.

Sure? Were you there and had a look? Do you grok the formulae? I don't ;)

>
> > 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. :-

No, tidal forces are a difference in gravitational *force* - which (possibly
very simply speaking, as I'm not perfectly familiar with the terms) *is* the
gravity gradient. And they don't jsut rip apart your head and feet

If it was just your feet being pulled away from your head, you would be torn in
two because the force would pull on the whole chain of electromagnetic forces
tying your feet to your head, and some "weakest link" will give way.

This is not the case for tidal forces due to a gravity gradient: These will pull
on each link in the chain of electromagnetic forces separately - each link
subject to its own little gradient.

This will allow you to sustain a lot more gradient between your head and your
feet, possibly even without much adverse effect. But when it will ultimately
rip the skin off your feet, it will just as well rip apart the grains of sand
in your shoes.

Oh, and BTW, once you hit the EH, I guess there'll probably be no floating
around... with so much mass in so little space, there'll probably be not enough
room to swing a cat...

.... but then again, that place may be quite crowded already even outside the EH.

Post a reply to this message

Darren New <dne### [at] sanrrcom> wrote:
> Ha! You're teaching me as you go! :-)

If it helps, I'm trying to be pretty conservative in the insight I offer.  Hey -
it could be worse.  I could have written a book on the subject like this guy
did.  Now he probably knows and understands exactly what he's doing, but there
are errors left and right.  I followed the derivation, then plotted the
equations.  Then plotted them again.  And again.  Then derived it myself and
figured out that, indeed, he had a blatant sign error (one of many
errors/typos).  Perhaps I should have waited for the second edition.  Anyway,
didn't your mom tell you not to trust newsgroup physics debates?

 - Ricky

Post a reply to this message

Darren New <dne### [at] sanrrcom> wrote:
> 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.

Well, from an outsider's perspective - given that everything falling into the
black hole comes to a standstill at the EH... doesn't this qualify for a
singularity?

Okay, granted, from the poor victim's POV the singularity is still a few yards
away...


BTW, I've looked at the illustration with the "bent" ray by now, and the
illustration of what somebody falling into a black hole would see, and am now
coming to yet another conclusion:

I think that as a person closes in to the event horizon, (a) from his
perspective the *universe* will begin to collapse to a singularity (although
not a point, but rather a line), and (b) the event horizon will appear to
recede - until he hits the singularity, which in this sense is again identical
to the event horizon.

What happens then? Well - I guess that poor sod just falls through.
Back into the universe - which by then has aged by an infinite amount of time.
The only thing the poor guy suffered will be that he'll be turned around by 180
degrees when emerging from the singularity...

From an observer's point of view, the guy should thus appear at the other side
of the EH (and again take infinite time to get out of it).


You remember the famous greek thought experiment about Achilleus trying to catch
up with a turtle, and being unable to - because whenever he reaches the point
where the turtle was, the turtle will have crawled yet another bit ahead?

I guess we have a similar paradoxon here: An outside observer will see things
from the greek philosophers' point of view, in which a person heading for the
EH will never reach it. The poor victim, however, sees things from the classic
everyday perspective we know: He can just run past the turtle, no problem.

Post a reply to this message

Darren New <dne### [at] sanrrcom> wrote:
> 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.

Well, same with me - but that's the fun part about it: Nobody *expects* us to
know what we're talking about, so we can let our imagination run wild ;)

> > 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.

But if it all collapses again, then we couldn't have such an event horizon,
right? After all, it would all "boil down" (literally ;)) to a single point
again, where all the stuff will be able to interact once more...

(Then again, currently scientists claim we'll all be ripped to pieces one day by
the ever-faster expanding space within us all... hmmm... "the space within us" -
sounds a bit hippie-ish ;)

Post a reply to this message

triple_r wrote:
> didn't your mom tell you not to trust newsgroup physics debates?

Definitely.  I'll try not to fall into any singularities before checking 
with an actual knowledgeable physicist. :-)

-- 
   Darren New, San Diego CA, USA (PST)
   "Ouch ouch ouch!"
   "What's wrong? Noodles too hot?"
   "No, I have Chopstick Tunnel Syndrome."

Post a reply to this message

"triple_r" <nomail@nomail> wrote:
> "In this case, we will find that while the surface r=2m has some unusual
> properties, the singularity is due to the choice of coordinates, and so is a
> *coordinate singularity*.  Simply put, by using a different set of coordinates
> we can write the metric in such a way that the singularity at r=2m is removed.
> However, we will see that the point r=0 is due to infinite curvature and cannot
> be removed by a change in the coordinates."

Hm... yeah, maybe that's a mathematical way to describe what I'm currently
pondering: If we choose the falling victim's co-ordinates (location, time,
speed and all) a reference frame instead of that of a "fixed" outside observer,
we move the singularity from r=2m towards r=0 - from the Schwartzschild-Radius
(the "classic" EH) towards the "classic" singularity...

So I'd say the EH is the projection of the singularity into the reference frame
of an observer...

> I may be guilty of having implied that the curvature is infinite at the event
> horizon, but it looks like only the redshift to an outside observer is.  Kinda
> learning this as I go, so you'll have to forgive me.

Not a problem at all - as it seems, we're all in that same boat here ;)

Post a reply to this message

clipka escreveu:
> Darren New <dne### [at] sanrrcom> wrote:
> I think that as a person closes in to the event horizon, (a) from his
> perspective the *universe* will begin to collapse to a singularity (although
> not a point, but rather a line), and (b) the event horizon will appear to
> recede - until he hits the singularity, which in this sense is again identical
> to the event horizon.

Will he or any matter draw into the blackhole ever hit the singularity? 
  Does the singularity or blackholes even exist or are merely the point 
where GR equations break?

We're doing no better than ancient philosophers trying to split the 
matter and thinking about its outcome without a shred of evidence or 
measurements... which is the point of the GM guy in the article this 
thread links to:

"We start with effects actually seen in the lab, which I think gives it 
more credibility than black holes"

evidence.

> What happens then? Well - I guess that poor sod just falls through.
> Back into the universe - which by then has aged by an infinite amount of time.
> The only thing the poor guy suffered will be that he'll be turned around by 180
> degrees when emerging from the singularity...
> 
> From an observer's point of view, the guy should thus appear at the other side
> of the EH (and again take infinite time to get out of it).

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...

> You remember the famous greek thought experiment about Achilleus trying to catch
> up with a turtle, and being unable to - because whenever he reaches the point
> where the turtle was, the turtle will have crawled yet another bit ahead?
> 
> I guess we have a similar paradoxon here: An outside observer will see things
> from the greek philosophers' point of view, in which a person heading for the
> EH will never reach it. The poor victim, however, sees things from the classic
> everyday perspective we know: He can just run past the turtle, no problem.

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

Post a reply to this message

clipka escreveu:
> Darren New <dne### [at] sanrrcom> wrote:
>> 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.
> 
> Well, same with me - but that's the fun part about it: Nobody *expects* us to
> know what we're talking about, so we can let our imagination run wild ;)

Don't worry, you guys!  Even scientists let their imaginations run wild 
too...

This was a very interesting thread, despite being a very difficult and 
misunderstood subject among laymen.  But I'm actually amazed by how few 
were drawn to it.  I thought most geeks love space talk and povray is 
full of geeks after all! :D

>>> 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.
> 
> But if it all collapses again, then we couldn't have such an event horizon,
> right? After all, it would all "boil down" (literally ;)) to a single point
> again, where all the stuff will be able to interact once more...

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. ;)

> (Then again, currently scientists claim we'll all be ripped to pieces one day by
> the ever-faster expanding space within us all... hmmm... "the space within us" -
> sounds a bit hippie-ish ;)

I don't want to live to see proton decay!  well, not that I would 
anyway... :P

Post a reply to this message

Darren New <dne### [at] sanrrcom> wrote:
> Ha! You're teaching me as you go! :-)

Scratch that.  It could be  LOT worse.  I could be this guy (worth at listen at
about 31:00):

http://www.thisamericanlife.org/Radio_Episode.aspx?episode=293

Also interesting/entertaining:

http://insti.physics.sunysb.edu/~siegel/quack.html
http://math.ucr.edu/home/baez/crackpot.html

 - Ricky

Post a reply to this message

clipka escreveu:
> 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...

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?

Post a reply to this message

nemesis wrote:
> "We start with effects actually seen in the lab, which I think gives it 
> more credibility than black holes"
> 
> evidence.

The effects of what appear to be black holes are seen in astronomy. Just not 
in the lab as such.

> If we assume blackholes exist, without much evidence so far,

A fair amount of evidence, actually.

> Perhaps the blackhole is then just a curve along 
> this surface and the poor fellow ends up in another region of 
> space-time.

Basically, yes. Some types of black hole structures are thought to work that 
way. Like, a rotating black hole can be traversed like that without hitting 
the singularity, supposedly.

It's all math, tho, with no actual physics evidence beyond what the math 
implies. If reality isn't isomorphic to the math after all, it won't work.

-- 
   Darren New, San Diego CA, USA (PST)
   "Ouch ouch ouch!"
   "What's wrong? Noodles too hot?"
   "No, I have Chopstick Tunnel Syndrome."

Post a reply to this message

Darren New <dne### [at] sanrrcom> wrote:
> nemesis wrote:
> > "We start with effects actually seen in the lab, which I think gives it
> > more credibility than black holes"
> >
> > evidence.
>
> The effects of what appear to be black holes are seen in astronomy. Just not
> in the lab as such.
>
> > If we assume blackholes exist, without much evidence so far,
>
> A fair amount of evidence, actually.

Yes, but according to the article you linked, QM physicists are giving another
explanation for the same effects without the paradoxes blackholes carry, the
same ones we were avidly discussing about previously:  the "lost information"
and the "after-image" effect of something crossing the EH as seen from an
external observer.  Of course, they are trying to explain it by relying on a
supposed dark matter star that has never been detected before either. :P

Surely the Dark side is very sneaky!

OTOH, will we ever witness such object, blackhole or dark matter star?  Such
humongous gravitational force surely has many objects orbiting it,
"overcrowded" as clipka mentioned, and most of it being gas and stars, so what
is the chance of seeing what isn't supposed to be seen except against a clear
starry background?

Post a reply to this message

Darren New <dne### [at] sanrrcom> wrote:
> A fair amount of evidence, actually.


Absolutely fascinating:

http://www.eso.org/public/outreach/press-rel/pr-2008/phot-46-08.html

 - Ricky

Post a reply to this message

triple_r wrote:
> http://www.eso.org/public/outreach/press-rel/pr-2008/phot-46-08.html

Kewl. I was thinking of this one.
http://en.wikipedia.org/wiki/Cygnus_X-1

-- 
   Darren New, San Diego CA, USA (PST)
   "Ouch ouch ouch!"
   "What's wrong? Noodles too hot?"
   "No, I have Chopstick Tunnel Syndrome."

Post a reply to this message

Darren New <dne### [at] sanrrcom> wrote:
> triple_r wrote:
> > http://www.eso.org/public/outreach/press-rel/pr-2008/phot-46-08.html
>
> Kewl. I was thinking of this one.
> http://en.wikipedia.org/wiki/Cygnus_X-1

Convincing evidence, but I'm a sucker for the visually spectacular ones:

http://en.wikipedia.org/wiki/Elliptical_Galaxy_M87

 - Ricky

Post a reply to this message

"triple_r" <nomail@nomail> wrote:
> Darren New <dne### [at] sanrrcom> wrote:
> > A fair amount of evidence, actually.
>
>
> Absolutely fascinating:
>
> http://www.eso.org/public/outreach/press-rel/pr-2008/phot-46-08.html

Yes.  Look the last video, it's a journey to the center of the galaxy through a
multilayered collection of ever zooming galaxy shots.  It ends with both the
collected frames aquired over 16 years and accelerated by a few million times
and a CG detailing what's going on.

Truth be told, I don't think the video gives the exact scale of the thing.  I
mean, I thought the central blackhole, or whatever it is those stars are
orbitting around, would be much bigger.  Yes, I know it's hyperdense, but even
still...

Post a reply to this message

> 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?

Post a reply to this message

clipka <nomail@nomail> wrote:
> Well, from an outsider's perspective - given that everything falling into the
> black hole comes to a standstill at the EH...

  Slowing down asymptotically is not the same thing as being stopped.

  Also, as I have been saying, since it will emit constantly less and less
photons, which are more and more red-shifted, at some point it will be
practically impossible to observe the object. It will just fade to black.

-- 
                                                          - Warp

Post a reply to this message

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

Post a reply to this message

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

Post a reply to this message

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

Post a reply to this message

> 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).

Post a reply to this message

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

Post a reply to this message

>  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)?

Post a reply to this message

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

Post a reply to this message

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? :)

Post a reply to this message

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...

Post a reply to this message

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...)

Post a reply to this message

>  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.

Of course, are black holes known for not really having much momentum then 
compared to stars?

>  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).

I thought the idea of the slingshot was that you wait until you are at 
maximum velocity near the planet/star and then fire your rockets, firing 
them at this point gives you way more speed than firing them at much lower 
speeds, in effect meaning you need less fuel overall to get somewhere.  I 
didn't think about using movement of planets and stars as a slingshot.

>  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.

No, I think the path would be pretty symetrical, so you would speed up to 
100x as you passed the black hole, then slow back down to the same speed you 
set off with as you approach Earth.  Just like you were orbiting the black 
hole in a non-circular orbit, like a comet orbits the sun.

>  There are also some practical things to take into accout:

Hehe yes I imagined!

Post a reply to this message

clipka <nomail@nomail> wrote:
> 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...)

  Aren't you confusing gravity with acceleration?

-- 
                                                          - Warp

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scott <sco### [at] scottcom> wrote:
> I thought the idea of the slingshot was that you wait until you are at 
> maximum velocity near the planet/star and then fire your rockets

  No. That's a so-called powered slingshot and it can boost the effect
of the slingshot. However, the slingshot effect can be achieved with
pure inertia, without firing any rockets.

> I didn't think about using movement of planets and stars as a slingshot.

  Well, that's the whole idea. (The powered version is just a boost, but
not really the basic effect.)

-- 
                                                          - Warp

Post a reply to this message

>  Well, that's the whole idea. (The powered version is just a boost, but
> not really the basic effect.)

Hehe I guess it's easy to forget the planets are actually moving pretty damn 
quick :-)

Post a reply to this message

clipka <nomail@nomail> wrote:
> 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.

  The major objections against singularities are more or less philosophical,
rather than based on hard science. It's "hard to believe" that mass could
be compressed into a point of zero volume.

  However, if reading about quantum mechanics and general relativity has
taught me something, it's that the Universe doesn't always work intuitively,
and there are lots of things which are "hard to believe", yet demonstrably
happen.

  If someone objects to the notion of a singularity, he would have to show
some evidence that GR doesn't work as predicted in this case. There must be
some property of the Universe which makes GR not work in this situation,
something which actually stops the singularity from forming. What could this
phenomenon be? Has anyone ever measured such phenomenon to exist?

  If not, then any objection against singularities is more or less
philosophical. "It doesn't sound right" is not hard science.

  The thing is, if GR equations are right, and there is a lot of evidence
suggesting that they are, then there simply is no way for a singularity
to not to form when a mass collapses to be smaller than its Schwarzschild
radius. All geodesics inside the event horizon, including time geodesics,
point towards the center. There's no known way for matter/energy to stop
going towards this center. Just advancing in time makes it advance towards
this center. Even if you tried to apply some force to a particle to stop
it from getting closer to the center, you couldn't, not even if you applied
an infinite force.

  The only possible conclusion is that all the matter and energy compresses
into a point of zero volume. If the singularity does not form in reality,
then something must explain why. Something more than philosophical objections.

-- 
                                                          - Warp

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scott <sco### [at] scottcom> wrote:
> >  Well, that's the whole idea. (The powered version is just a boost, but
> > not really the basic effect.)

> Hehe I guess it's easy to forget the planets are actually moving pretty damn 
> quick :-)

  It helps understanding the gravity slingshot if you think of the effect
as a 100% elastic collision. If you do the math, you will see how the
larger object transfers momentum to the smaller object in such a collision.
And elastic collision is, AFAIK, exactly what the effects of a gravity
assist are, from an overall point of view.

-- 
                                                          - Warp

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"nemesis" <nam### [at] gmailcom> wrote:
> Yes, but according to the article you linked, QM physicists are giving another
> explanation for the same effects without the paradoxes blackholes carry, the
> same ones we were avidly discussing about previously:  the "lost information"
> and the "after-image" effect of something crossing the EH as seen from an
> external observer.  Of course, they are trying to explain it by relying on a
> supposed dark matter star that has never been detected before either. :P

Interestingly though, the real-existing phenomenon of "black holes" was
originally a purely hypothetical prediction of GR - not some already known
effect that needed to be explained. And it did predict their properties quite
well as it seems. So this is strong evidence that the GR is a very good tool to
make predictions.

So in order to show that *their* theory is at least an equally good tool, they
would actually have to demonstrate how their own theory explains not only the
same *phenomenons* as GR, but *postulates GR itself* - or demonstrate how their
theory is just a different way of looking at GR.

> OTOH, will we ever witness such object, blackhole or dark matter star?  Such
> humongous gravitational force surely has many objects orbiting it,
> "overcrowded" as clipka mentioned, and most of it being gas and stars, so what
> is the chance of seeing what isn't supposed to be seen except against a clear
> starry background?

It would also probably an unhealthy place to stay even without being ripped
apart by tidal forces - with gamma ray jets and all that around.

BTW, speaking of it: When falling into a black hole I guess it may be best not
to try to fight the inevitable... suppose you manage to stay at a fixed
distance from the center - with all the blueshift of the photons falling in,
you're gonna get your daily maximum dose of gamma rays in a few split
seconds... so better fall in to experience the same blue shift (effectively
leaving the shift unchanged from your perspective) and hope that the
singularity is just a transition to a better life somewhere - um, I mean
someWHEN - else...

Post a reply to this message

"triple_r" <nomail@nomail> wrote:
> Convincing evidence, but I'm a sucker for the visually spectacular ones:
>
> http://en.wikipedia.org/wiki/Elliptical_Galaxy_M87

Yeah, definitely a sight!

Post a reply to this message

"scott" <sco### [at] scottcom> wrote:
> 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?

Exactly that way. Although maybe you'd need quite some acceleration to get there
- and back again - soon enough for 3050 ;)

And then again, speeding up, travelling a while at high speed, then putting the
gear shift to reverse and head back may do the job just as well. No need for a
black hole. You wouldn't want your Windows CE based guidance computer to crash
*there*...

Post a reply to this message

Warp <war### [at] tagpovrayorg> wrote:
> clipka <nomail@nomail> wrote:
> > Well, from an outsider's perspective - given that everything falling into the
> > black hole comes to a standstill at the EH...
>
>   Slowing down asymptotically is not the same thing as being stopped.

Why do geeks always have to be nitpickers? ;)

But then again, notice that I wrote "at the EH" - so what I wrote was actually
that lim[d->r] v(d) = 0...

(phew... last second save ;))


>   Also, as I have been saying, since it will emit constantly less and less
> photons, which are more and more red-shifted, at some point it will be
> practically impossible to observe the object. It will just fade to black.

"infra-brown", to be precise ;)

Post a reply to this message

Warp <war### [at] tagpovrayorg> wrote:
> > 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.

Allows for a much more "slender" parabola though... and, besides, a smaller
radius to the star's center, for more gravity and therefore more of the
time-dilating effect ;)


(someone sing "let's do the time-warp again"...)

Post a reply to this message

Warp <war### [at] tagpovrayorg> wrote:
>   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.

It does.

The trick is that the slingshot affects both bodies: That Huge Planet Over There
and yout Teeny Weeny Space Machiney.

Guess which one *seems* to be affected most...


So if you use a black hole to slingshot you around back home, at the same time
you sort of slingshot the black hole around your spacecraft into the opposite
direction... it's just that the black hole won't bother *much*, being the fat
lazy sucker it is.

> - 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.

Nah, I don't think so. Would tidal forces rip you apart at the surface of the
sun? I doubt. At half the star's radius you already get 4 times the
gravitational pull, but only about 2 times the tidal forces (gravity being
proportional to the inverse radius' square, so its gradient being only linearly
proportional to the inverse radius, if I get the math right).


> - 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.

Only if they are supermassive (those you'd expect to find at galaxy centers) or
happen to have a companion star.


> - 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.

Again, only if there's an accretion disk. Remember: Black holes don't emit
anything - only their surroundings may ;)


However, maybe you also want to check the magnetic field of the black hole - I'm
not sure about the orders of magnitude, but I wouldn't want all my electrons go
spinning off to the left while the protons go off to the right - not to speak
of the resulting synchroton radiation ;)

Post a reply to this message

scott wrote:
> 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.

You can go forward in time by some arbitrary amount just by sitting still. 
:-)  It's just a question of whether you're still alive when you finish.

-- 
   Darren New, San Diego CA, USA (PST)
   "Ouch ouch ouch!"
   "What's wrong? Noodles too hot?"
   "No, I have Chopstick Tunnel Syndrome."

Post a reply to this message

Warp <war### [at] tagpovrayorg> wrote:
> clipka <nomail@nomail> wrote:
> > 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...)
>
>   Aren't you confusing gravity with acceleration?

Aren't you forgetting that Einstein postulated that you can't tell the
difference of the one from the other? ;)

After all, that's the *basic foundation* of GR...

Post a reply to this message

Warp wrote:
>   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.

I don't believe that's right.

Consider rolling a small ball at 10 kph on a level surface. It runs into a 
large heavy ball (say, 1000x the mass) rolling at 1 kph the other way and 
bounces off. Now the large heavy ball is rolling at 0.999 kph, and the small 
ball is rolling at 1000 kph (or 10,000 kph or something) back the way it came.

Clearly energy and momentum are conserved there.

You'd wind up changing the course of the black hole much less than you'd 
change the course of your spaceship, is all.

I also think the black hole would be better, because you can get closer and 
get a bigger time dilation from the gravity also.


>   There are also some practical things to take into accout:

We're talking about using black holes for time travel and you're considering 
practical things? :-)

> - The humongous amounts of radiation around a black hole would probably
> be enough to fry you to ashes in a fraction of a second, 

FWIW, it's the accretion disk that generates the radiation, so you could 
kill two birds with one stone by finding one without.

-- 
   Darren New, San Diego CA, USA (PST)
   "Ouch ouch ouch!"
   "What's wrong? Noodles too hot?"
   "No, I have Chopstick Tunnel Syndrome."

Post a reply to this message

clipka wrote:
> (someone sing "let's do the time-warp again"...)

With time dilation even:

http://www.youtube.com/watch?v=4GoTnd99rBo

-- 
   Darren New, San Diego CA, USA (PST)
   "Ouch ouch ouch!"
   "What's wrong? Noodles too hot?"
   "No, I have Chopstick Tunnel Syndrome."

Post a reply to this message

scott escreveu:
>>  Well, that's the whole idea. (The powered version is just a boost, but
>> not really the basic effect.)
> 
> Hehe I guess it's easy to forget the planets are actually moving pretty 
> damn quick :-)

Yeah, think about it:  Earth is rotating about the sun, which is 
rotating about the Milky Way nucleus, which is speeding in a cluster 
with many closer galaxies through ever accelerating expansive space.

We're actually cruising unknown frontiers at speeds vastly superior to 
200km/sec (which is about the speed of the sun as it rotates about the 
galaxy nucleus according to wikipedia).  Who needs a spaceship, huh?

and all this while you sit in front of your desk staring at the monitor, 
  holding your cup of coffee with a steady hand... :P

Post a reply to this message

Warp <war### [at] tagpovrayorg> wrote:
>   GR itself doesn't forbid the distance between two points in space growing
> faster than c.

  Btw, another situation where GR predicts relative motion larger than c
is inside the ergosphere of a rotating black hole.

-- 
                                                          - Warp

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clipka <nomail@nomail> wrote:
> >   Aren't you confusing gravity with acceleration?

> Aren't you forgetting that Einstein postulated that you can't tell the
> difference of the one from the other? ;)

  Just because you can't tell appart gravity from acceleration doesn't
mean that all accelerations are caused by the same phenomenon as gravity.

> After all, that's the *basic foundation* of GR...

  The foundation of GR is that gravity does not, in fact, cause
acceleration at all. An object in "free wall" is all about inertia
and nothing about acceleration (even though it may not look like it
to us).

  Just because gravity doesn't cause acceleration doesn't mean that
*nothing* can cause (true) acceleration.

-- 
                                                          - Warp

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nemesis <nam### [at] gmailcom> wrote:
> We're actually cruising unknown frontiers at speeds vastly superior to 
> 200km/sec

  With respect to what?

  Motion is always relative.

-- 
                                                          - Warp

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