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"...)

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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 ;)

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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."

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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...

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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."

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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."

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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

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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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