Can anyone please expand on white holes?

[Nalar]

"It is theoretically possible for a traveler to enter a rotating black hole, avoid the singularity, and travel into a rotating white hole which allows the traveler to escape into another universe."

Can anyone please expand on white holes?

 

[Chronos]

No suspects have been observed to date. That is sufficient to question their existence.

 

[zhermes]

"It is theoretically possible for a traveler to enter a rotating black hole, avoid the singularity, and travel into a rotating white hole which allows the traveler to escape into another universe."

My understanding is that for the classical Einstein-Rosen worm-hole, both 'sides' look like a black-hole (with an event horizon). I'm not familiar with this quote, but it sounds like combining the worm-hole metric with that of a kerr black-hole, and setting the spin coefficient to above the 'maximal' value---in which case you can end up with a 'naked' (excapable) singularity.

The general concept behind a 'white-hole' is that somehow (usually magically) you end up with a naked-singularity that emits particles instead of consuming them (like a black-hole). People like to talk about white-holes being the source of the big-bang, or the counter-parts to black-holes in other universes. There is no real scientific backing for these ideas---as Chronos suggests.

This is my understanding.

In a new sense a black hole is a white hole and like a black holes, white holes have properties like mass, charge and angular momentum.

What?!

 

[Misericorde]

No White hole or effects expected to be generated by one has ever been observed, and I don't think they're a necessary part of any theory either. If I recall correctly, white holes don't really make any logical sense, and don't arise like black hole singularities in Einstein's theories. I could be wrong, I'm not a scientist.

 

[AstrophysicsX]

As soon as you hit the event horizon of a black hole you would be ripped to shreds.

 

[twofish-quant]

As soon as you hit the event horizon of a black hole you would be ripped to shreds.

No you wouldn't if the black hole is large enough. This is another first year astronomy homework problem, which is *cool* to do. :-) :-) :-)

You can calculate the tidal forces from a black hole, and you'll find that it has nothing to do with the event horizon. For a small BH, the place where you get ripped to shreds is outside the event horizon, but for a big black hole, you can cross the event horizon, and you wouldn't notice at all.

 

[twofish-quant]

"It is theoretically possible for a traveler to enter a rotating black hole, avoid the singularity, and travel into a rotating white hole which allows the traveler to escape into another universe."

Can anyone please expand on white holes?

The idea is that you might be able to find a solution of Einstein's gravity equations that allows you to travel through a wormhole.

Current thinking is "probably not."

The problem is that no one has found a solution that works. People have come up with a lot of solutions of the equations that look like white holes, but you end up assuming things that don't make sense (like negative energy). On the other hand, no one has mathematically proved that it is impossible (and people have tried).

 

[twofish-quant]

People like to talk about white-holes being the source of the big-bang, or the counter-parts to black-holes in other universes. There is no real scientific backing for these ideas---as Chronos suggests.

On the other hand, no one has been able to mathematically prove that it can't happen, and there are some people that are playing with this idea (google for cosmological natural selection).

The idea is that every time a black hole forms, you end up with a new universe somewhere, so ultimately most universes are those in which you can form black holes.

 

[phinds]

The idea is that every time a black hole forms, you end up with a new universe somewhere, so ultimately most universes are those in which you can form black holes.

What is the basis for the joining of these two concepts? That is, I understand you to be saying that the theory (right or wrong) that you are describing says that

• black holes form universes
• those newly formed universes contain black holes

Why does the second follow from the first? I'm not arguing with either statement, just questioning the apparent lack of some rather massive connective tissue between the two. And I hasten to add that I am coming from a POV of pretty much total ignorance, so I grant that it might be obvious, but not to me.

 

[narrator]

No you wouldn't if the black hole is large enough. This is another first year astronomy homework problem, which is *cool* to do. :-) :-) :-)

You can calculate the tidal forces from a black hole, and you'll find that it has nothing to do with the event horizon. For a small BH, the place where you get ripped to shreds is outside the event horizon, but for a big black hole, you can cross the event horizon, and you wouldn't notice at all.

Oh this is cool. I didn't know this aspect of size. Can you explain why size matters? (lol.. sorry) What causes this change in the event horizon with size?

Side question: With CERN's experiments in creating black holes, how much mass can they hold before they become dangerous? (not worried, just curious)

As for white holes, you only have to look at Alice for proof, but is there a variable which accounts for the Mad Hatter?

 

[Misericorde]

Oh this is cool. I didn't know this aspect of size. Can you explain why size matters? (lol.. sorry) What causes this change in the event horizon with size?

Side question: With CERN's experiments in creating black holes, how much mass can they hold before they become dangerous? (not worried, just curious)

As for white holes, you only have to look at Alice for proof, but is there a variable which accounts for the Mad Hatter?

The event horizon of a black hole is proportional to the black hole's mass; a stellar mass black hole has a relatively small event horizon. An AGN (Active Galactic Nuclei) is orders of magnitude more massive, with a commensurately larger event horizon. The old way of looking at this would be that you could cross such an event horizon and never know you had done it, unlike the rapid death from tidal stresses and radiation that you would in a smaller hole.

 

[narrator]

Thanks Misericorde.. That makes sense. Kind of like getting dumped by a small wave close to shore, compared to riding our a high swell further out to sea, I guess. :)

 

[phinds]

Thanks Misericorde.. That makes sense. Kind of like getting dumped by a small wave close to shore, compared to riding our a high swell further out to sea, I guess. :)

Narrator, here's the way I learned to think about it that makes sense to me. Think about being suspended in the air about 20 feet off the ground and then released. Earth's gravity is going to pull on you pretty hard because you're only a few thousand miles from the center of the Earth and when you hit the ground you're going to hit pretty hard.

Now think about being suspended about 10,000 mile above the surface of the Earth and kept from moving towards the earth. Then you are released. After you have moved 20 feet, you hit a wall that is NOT moving vertically relative to the Earth (hey, this is a THOUGHT experiment ... I don't know how you DO these things). When you hit the wall, which will take a while, you'll hardly feel a thing because the weak gravity out there doesn't move you rapidly. If you kept going, you would pick up speed because the force would increase as you get closer to the center of the earth.

This is the difference in gravitational effect depending on distance, and with a big black hole, your head and your feet get similar effects because both of them are close to the same distance from the CENTER of the gravitational attraction (the effective point source) but for a small one, as you get close, you are close to the CENTER of the gravitational attraction, so your head and feet do NOT feel anywhere near the same forces and you are sphagettified.

The event horizon is not any kind of physical event, it's just that place where the gravity is so strong that even light can't escape. A little in from the EH, the graivity is just a little more and a little out from the EH the gravity is a little less, so the EH only has to do with its effect on light, not on your body. The only thing that matters to your body is the DIFFERENCE between the force on your head and the force on your feet.

 

[narrator]

Thanks phinds.. that makes sense too ;)
I guess the event horizon is a bit of a non-event.. hehe

 

[Misericorde]

Thanks phinds.. that makes sense too ;)
I guess the event horizon is a bit of a non-event.. hehe

Yes! This is really a good way of thinking about it; it's not really a thing anyway, just a demarcation between what can and cannot escape the gravity of the hole. You learn very quickly, it's a pleasure to help; ask any questions you like. For me, black holes were my first love in physics, and I'm still learning about them today.

 

[Polyrhythmic]

Narrator, here's the way I learned to think about it that makes sense to me. Think about being suspended in the air about 20 feet off the ground and then released. Earth's gravity is going to pull on you pretty hard because you're only a few thousand miles from the center of the Earth and when you hit the ground you're going to hit pretty hard.

Now think about being suspended about 10,000 mile above the surface of the Earth and kept from moving towards the earth. Then you are released. After you have moved 20 feet, you hit a wall that is NOT moving vertically relative to the Earth (hey, this is a THOUGHT experiment ... I don't know how you DO these things). When you hit the wall, which will take a while, you'll hardly feel a thing because the weak gravity out there doesn't move you rapidly. If you kept going, you would pick up speed because the force would increase as you get closer to the center of the earth.

This is the difference in gravitational effect depending on distance, and with a big black hole, your head and your feet get similar effects because both of them are close to the same distance from the CENTER of the gravitational attraction (the effective point source) but for a small one, as you get close, you are close to the CENTER of the gravitational attraction, so your head and feet do NOT feel anywhere near the same forces and you are sphagettified.

The analogy has one flaw: When released from 20 feet above the ground, you're not going to get hurt because anything tears you apart due to the different force at head and foot, but because you simply hit the ground. A small black hole will definitely tear you apart.

A little in from the EH, the graivity is just a little more and a little out from the EH the gravity is a little less, so the EH only has to do with its effect on light, not on your body.

Apart from the fact that your body wouldn't be able to escape from inside the horizon. ;)

 

[phinds]

Polyrythmic, I agree with everything you have said but I think you missed the POINT of my post which was purely to help an understanding of why the EH has no meaning for the human body, not to suggest that getting up close and personal with a BH is a good idea.

 

[Finbar]

A white hole is just the time reversal of a black hole. If you take any solution to Einstein's equations and replace t by -t then this too satisfies the Einstein's equations. Hence white holes are solutions in general relativity just as black holes are. The difference is that to form a black hole you only need a sufficient amount of matter to be collapsing as your initial condition. For a white hole you would need a finely tuned set of initial conditions which are likely to be very unstable. Put another way it is the second law of thermodynamics that actually forbids white holes from existing in nature. (one might think of the big bang as a finely tuned set of initial conditions however its not a white hole because there is not time reversed event horizon rather a cosmological horizon)

If you analytical extend the kerr solutions you indeed fine that you can find a geodesic that goes from outside the BH, crosses the event horizon and the cauchy(inner) horizon, misses the singularity and flies out into another region of spacetime for which it reaches infinity. This region is just the time reversal of the black hole hence it is the corresponding white hole solution. However it can be shown that if one tries to send real real matter through this "worm hole" the solutions is unstable. So the analytical extended kerr solution is not a transverable worm hole.

 

[Misericorde]

Analogies are rarely perfect when pressed to their extremes.

 

[Polyrhythmic]

Polyrythmic, I agree with everything you have said but I think you missed the POINT of my post which was purely to help an understanding of why the EH has no meaning for the human body, not to suggest that getting up close and personal with a BH is a good idea.

Sorry then, I thought you wanted to give an analogy to explain why surface gravity would tear you apart.