Another Question About Black Holes

[mgwisni]

My friends and I were talking about time travel the other day and black holes and I was wondering if Time goes faster or slower at black holes than everywhere else. Does it go faster or slower? I'd really like to know.

If it does travel slower there, then technically you could go and sit at the edge of a black hole, just far enough away to keep from getting sucked in, and you would go forward in time. If you sat there for like a year and then went back to Earth, you would have traveled time.

[eburacum45]

This is true enough- you would need to have a parabolic or hyperbolic orbit, to avoid getting captured... mind you, I have a feeling that getting close enough to a black hole to experience massive time dilation would also cause tidal effects within your spaceship and yourself. so you would reach the future as mincemeat.

Anyway, we are all travelling forward in time anyway, at roughly 1 second per second;
you might as well wait and see it all happen as it happens.
_________________
SF worldbuilding at
http://www.orionsarm.com/main.html

[Pete]

Yes, spending time deep in a gravity well and coming out again takes you further into the future than spending the same time outside.

The ratio of times is √[ k / (k-1) ] where k is your distance from the black hole center in schwarzschild radiuses.

k can obviously never be less than one if you intend coming back, and must never be less than 1.5 if you want to hang around for a while in a stable orbit. This puts a ceiling of around 1.76 on the time dilation factor, so it's not really a great time travel mechanism. You might do better with a hyperbolic orbit approaching the event horizon, but the time spent in close would get shorter the closer you got, so I don't think you'd get great gains.

I'm using very basic schwarzschild black hole figures. I don't know how a spinning black hole would change the picture.

You'd need to use a very large black hole to avoid being destroyed by tidal forces.

[Deepuz]

"You'd need to use a very large black hole to avoid being destroyed by tidal forces."

Does "size" matter in a Black Hole?

[Vortexx]

I would think so, small black holes (technically better said: small event horizons as the singularity is not to believed have any psysical size) are extremely curved, just compare standing at the beach and looking at the horizon and looking at a softball. Obviously the earth looks more flat, thus a larger black hole would allow a less curvy hyperbolic approach hence smoother tidal forces...????

[bigjnorman]

I think what vortexx is saying is analogous to saying that a person the size of a baseball would experience different tidal forces compared to a person the size of the earth.

I have heard before that larger black holes have less dangerous tidal forces but I have never heard a reason for this

[eburacum45]

Tidal forces are caused by the difference in gravity between different parts of the orbiting object; in a tight gravity field like a small black hole or neutron star the gravity gradient is so fierce that if you pass by in a spaceship your head and feet would experience different values of gravity; this would cause different parts of your body to attempt to orbit at different speeds, and this would almost certainly tear you apart;

however if the black hole was gigantic, like the one expected at the heart of a galaxy, the gravity gradient would be more gradual and the difference between the gravity field at the top and bottom of your spaceship would be small, so tidal forces would be less.
_________________
SF worldbuilding at
http://www.orionsarm.com/main.html

[Pete]

Does "size" matter in a Black Hole?It is conventional that 'black hole' refers to everything from the Event Horizon on in, not just the singularity.

[Greco]

What determines a black hole's spin?
Are all black holes spinnning in the same direction?
Does the galaxy that holds the black hole spin in the same direction?
Do some galaxies hold more than one black hole and if they do, do they spin the same direction?

more spin stuff

Do galaxies all spin the same direction? Does the universe spin?

What determines the direction of spin for a galaxy?

Does a clockwise spinning blackhole any different than a CCW spinning blackhole?

enough for now, any answers out there?

[Pete]

What determines a black hole's spin?The angular momentum of the mass that made it.

Are all black holes spinnning in the same direction?No

Does the galaxy that holds the black hole spin in the same direction?Not necessarily, but I think you'd find that galactic center black holes would generally spin in the same direction as the host galaxy, due to them originating from the same source. Just like the Sun spins in the same direction as the average planetary orbit in our solar system.

Do some galaxies hold more than one black hole...Yes. Ours does. Any star large enough will eventually become a black hole in a supernova event.... and if they do, do they spin the same direction?Not necessarily.

more spin stuff

Do galaxies all spin the same direction?No.

Does the universe spin?Not according to the map of the cosmic microwave background. (But I think there might be some tricky GR implications of rotaing reference frames that would make t impossible to determine if the universe is spinning or not. Or maybe I'm just full of it.)

What determines the direction of spin for a galaxy?The angular momentum of the originating gas cloud, plus effects of subsequent collisions.

Does a clockwise spinning blackhole any different than a CCW spinning blackhole?
No. CW and CCW is the same thing seen from different directions (eg the Earth spins clockwise if viewed from above the South Pole, and counter clockwise if viewed from above the North Pole).

[Greco]

No. CW and CCW is the same thing seen from different directions (eg the Earth spins clockwise if viewed from above the South Pole, and counter clockwise if viewed from above the North Pole). [/B] [/QUOTE]

So how do we differentiate spin direction on celestial objects? Do I understand you correctly that spin is a matter of view point?

[Pete]

So how do we differentiate spin direction on celestial objects? Do I understand you correctly that spin is a matter of view point?Spin direction is determined by the direction of the axis of spin, further refined by an arbitrary rule (such as a right-hand- rule) to determine which half of the axis you use.

If two objects have their spin axises pointing in the same direction, then they are spinning in the same direction.

[jcsd]

A couple of addendums to pete's post:

Yes. Ours does. Any star large enough will eventually become a black hole in a supernova event.

The actual mass for a star to defintely become a black hole is unknown, but it's linked more to the size of the star's core.

Not according to the map of the cosmic microwave background. (But I think there might be some tricky GR implications of rotaing reference frames that would make t impossible to determine if the universe is spinning or not. Or maybe I'm just full of it.)

Yep, the CMBR does confirm the universe is not spinning. Ther is actually a GR model for a spinning universe which has the suprising side effect of backwards in time travel, meaning you can travel to anywhere at any time you want!

[2inquisitive]

Can you accept a question from a virtual moron in physics? I am
trying to understand a little. I know this sounds stupid, but is
there a lot of evidence that a black hole continues to grow
after its initial formation? I do mean this as a question and
not a challenge to relativity, as I just don't know. I'm sure you
can see where my thoughts were headed if the answer was no.

[eburacum45]

Black holes attempt to grow, by attracting more mass and energy into themselves; however this has to be balanced against the Hawking Radiation, which allows energy to escape at the surface of the event horizon.
Small black holes lose energy more quickly than big ones, so they soon evaporate;

black holes formed from a mass bigger than a large asteroid will attract enough matter and energy to increase in size, so get bigger and bigger.
Eventually the black hole will have absorbed all the local matter, so settle down to just sucking in interstellar medium;
after a very long time the expansion of the universe will thin out the interstellar medium so much that there will be nothing for the black hole to absorb, and so the slow rate of Hawking radiation will become important again,
and eventually even the largest black holes will evaporate into the empty universe.
_________________
SF worldbuilding at
http://www.orionsarm.com/main.html

[2inquisitive]

Yes, thanks ebruracum45, I realize that is the accepted theory,
I guess I wasn't clear in my question.
I was having problems dealing with the singularity as predicted
by the maths. I have since found models that are more acceptable
to me personally, though I know not to everyone. Sorry for any
confusion I may have caused.

[Pete]

Hi jcsd,
Thanks for the clarifications.