 |
|
|
|
|
|
| |
| |
|
|
|
|
| |
| |
|
|
> 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
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
scott <sco### [at] scott com> 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
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
"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
|
|
| |
| |
|
|
|
|
| |
|
|
