# Are White Holes real?

1. Apr 28, 2013

### Astro-Anouar

The White Hole is Real ? or Just a hypothesis with a mathematical model ? and Why the White Hole is the Opposite Of the Black hole ?

2. Apr 28, 2013

### Staff: Mentor

Assuming that by a "white hole" you meant the thing described by the bottom wedge of a Kruskal-Szeres diagram...

None have ever been observed, and there are good reasons to doubt that any exist.

3. Apr 28, 2013

### Astro-Anouar

and Why the White Hole is the Opposite Of the Black hole ?

4. Apr 28, 2013

### phinds

There is this REALLY neat facility for doing a little of your own research ... it's called Google Search. I suggest you learn how to use it.

https://www.physicsforums.com/blog.php?b=3588 [Broken]

Last edited by a moderator: May 6, 2017
5. Apr 28, 2013

### Astro-Anouar

Last edited by a moderator: May 6, 2017
6. Apr 28, 2013

### phinds

Yes, and if you had done a simple search you would have found the following in wikipedia:

A white hole, in general relativity, is a hypothetical region of spacetime which cannot be entered from the outside, but from which matter and light have the ability to escape. In this sense, it is the reverse of a black hole, which can be entered from the outside, but from which nothing, including light, has the ability to escape

7. Apr 28, 2013

### Mordred

As phinds pointed out its easy to google search. Type in google "how do white holes form" will pull up numerous sites such as this one.

http://garrettmaster1.homeip.net:82/BlackHoleReview/WhiteHoles.htm [Broken]

Last edited by a moderator: May 6, 2017
8. Apr 28, 2013

### Astro-Anouar

Thank for search and Sorry For harassment But I red This Before and this didn't answer my question ' Why the White Hole push the matter and energy why matter and light have the ability to escape' for example: nothing even light could escape from Black Hole Why ? Because it had a Great Density and an incredible Gravitational Field.

9. Apr 28, 2013

### Astro-Anouar

Last edited by a moderator: May 6, 2017
10. Apr 29, 2013

### Chalnoth

A white hole is a time-reversed black hole, and is thus unphysical (it means you got the arrow of time wrong).

11. Dec 15, 2015

### taiyo

Actually finding out if they exist would be quite simple

12. Dec 15, 2015

### newjerseyrunner

A white hole is the opposite of a black hole because a black hole can be entered from the outside but once inside, it can never escape. A white hole is a region of space that can be exited, but never entered.

13. Dec 15, 2015

### ChrisVer

I don't see any problem with the repulsion from a white hole ... except for that you either need negative mass-squared matter (exotic matter) or something similar to dark energy (with this strange equation of state relationship leading to negative pressure)

14. Dec 15, 2015

### Chalnoth

The white hole has positive mass, just like a black hole. It's simply that the time coordinate is reversed. This is a perfectly valid solution to the field equations, it's just that it violates the second law of thermodynamics (i.e., a white hole's entropy decreases over time).

15. Dec 15, 2015

### ChrisVer

well, I connected the whitehole existence with wormhole and refered to its instability. So yes I think it wasn't the right way to answer that

16. Dec 15, 2015

### Staff: Mentor

How so?

17. Dec 15, 2015

### taiyo

Create a d
by building a device that detects anomalies in space when light is being pushed or pulled to or from an unknown source that another device can figure out if it is solid or a black or white hole

18. Dec 15, 2015

### Staff: Mentor

No, actually, its entropy is constant, since the area of its horizon is constant--at least if we're talking about an idealized white hole that never loses any mass from things getting out (the time reverse of an idealized black hole that never gains any mass from things falling in). So this solution doesn't violate the second law (but that's not to say it's physically reasonable--see below).

If we want to talk about a white hole with changing entropy, then the area of its horizon has to change with time, and that requires putting some nonzero stress-energy somewhere into the solution. For example, we could look at the outgoing Vaidya metric, which is basically a "white hole" that constantly emits null radiation and has a horizon that shrinks with time as a result of this. But this solution doesn't violate the second law, because we have to count the entropy of the emitted radiation as well as the entropy of the hole. Since the solution is exactly time reversible (the time reverse is just the ingoing Vaidya metric, which is basically a black hole that constantly absorbs null radiation and has a horizon that grows), the entropy does not change in either direction, as with any process that is exactly reversible.

The reason white holes are considered unphysical is the initial singularity. For black holes, the singularity is in the future, and we have an idealized model (the Oppenheimer-Snyder model) of how such an object could be formed by the gravitational collapse of a massive object. We know massive objects exist that could in principle undergo such a collapse, so it is at least physically reasonable that something similar to it has happened in our universe. (That still leaves the issue of whether the final singularity is still there when quantum gravity effects are taken into account, but that's a separate question that doesn't make the overall model unphysical--it just means there is an open question for further research.)

In the case of a white hole, however, there is no physically reasonable model of how such an object could come into existence, because the singularity is in the past, not the future--it would have to be part of the initial conditions, and nobody believes such initial conditions (an initial singularity with a past event horizon) are at all reasonable, because nothing like them has ever been observed and we don't even have an idealized model (analogous to the O-S model for black holes) to show how they might be satisfied even in principle.

19. Dec 15, 2015

### Staff: Mentor

Ok, and how would you build such a device, and how would you know that the unknown source was a white hole? Just waving your hands isn't enough.

20. Dec 15, 2015

### taiyo

Another machine determining if its solid and by launching light and matter if it's not solid and it gets pushed away from no where it is most like a white hole if the light and matter is absorbed and it is not solid it is most likely a black hole

21. Dec 15, 2015

### Chalnoth

Right, I was being a little bit non-specific. The classical black hole has non-decreasing entropy: its horizon size can never decrease. A white hole has the opposite issue: its horizon size can never increase. But the white hole's horizon can decrease if it expels matter (it can expel anything).

Hawking radiation, of course, complicates the whole situation, and has the effect of making a white hole seem even more unphysical, as the outgoing Hawking radiation from a black hole is purely thermal and therefore high in entropy, while the outgoing matter from a white hole would in general not be thermal.

22. Dec 15, 2015

### Staff: Mentor

How?

A white hole does not have repulsive gravity; it has attractive gravity, just like a black hole (or any other gravitating object). So things launched towards the white hole do not get "pushed away from nowhere".

23. Dec 15, 2015

### Staff: Mentor

But, again, as the example of the outgoing Vaidya metric shows, it's not clear that the entropy of the system as a whole (white hole + matter expelled) decreases in this process. One would have to look at an analysis such as Bekenstein's derivation of black hole entropy and see if a "time reverse" of it could be done for the case of a white hole expelling matter.

24. Dec 16, 2015

### taiyo

actually yes in a way because matter would be forced to go around like light because nothing can enter white hole

25. Dec 16, 2015

### Staff: Mentor

No, this is not correct. The reason nothing can enter a white hole is not because the white hole repels things; it attracts things, just like any other gravitating body. The reason nothing can enter a white hole is that its horizon is moving inward at the speed of light, so for anything to enter it, it would have to move inward faster than light.