[Mr Skeptic]

There seem to be a few similarities between the universe and black holes. There's some differences too obviously. Not sure if there is any significance to this.

Similarities:
* Both have an "event horizon". For the black hole, its a spherical (ish) surface where anything that goes in doesn't come back out, ever. With the universe (if expanding), there is an event horizon where anything going past is regressing faster than the speed of light and so can't come back "in" (it's still in the universe, of course, just forever separated from a different point to which it was previously connected).

* Stuff may always be visible as it passes through the event horizon. It just gets redshifted to insignificance.


Differences:
* No singularity inside the universe (unless we have a Big Crunch universe)

* Universe is like an inside-out black hole?

[michel123456]

There are also similarities between BH and elementary particles.

Black Holes & elementary particles share three distinguishing characteristics: mass, charge, and spin .

The difference with your question is just a matter of scale.

But if it turned to be right that an elementary particle corresponds to a minuscule kind of BH, then your statement "No singularity inside the universe" would become false. The universe would be full of singularities.

[Airbrush]

What if the universe did not start from a singularity? Its' gravity would have been too great to allow any expansion. That is why I prefer M brane theory that the universe began with a collision of higher dimensions from a region of indefinite size to allow expansion. Black holes don't "explode" or expand like a big bang.

[emperor88]

Electromagnetic radiations are building blocks of matter in universe, everything is affected by them as they are almost everywhere in the galaxy.
The effect of electromagnetic radiation on humans as humans also have their own electromagnetic radiations.

for more information visit my Blog
http://physics-chemi...y.blogspot.com/

[michel123456]

I am afraid your thread don't go too well, Mr Skeptic. We are out of tracks.

[Mr Skeptic]

That's OK. It probably is just a coincidence then.

[Spyman]

Mr Skeptic said:

Differences:
* No singularity inside the universe

If the extreme densities at the initial condition of the Big Bang was without a singularity then why do you think that Black Holes still could have them?

Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This singularity signals the breakdown of general relativity.
http://en.wikipedia.org/wiki/Big_Bang


Also what do you think would happen to a Black Hole if it would consume so enormous much mass that the radius of the Event Horizon should exceed the Hubble Sphere?

[Mr Skeptic]

Well, I had mean a singularity like a black hole has, with everyone falling toward it. On the other hand, we are all "falling" away from the singularity at the BB.

Incidentally, what does a black hole look like if you reverse time?

[Spyman]

A White Hole, but it would differ from expansion of the Universe since there is no center placed inside space.

[caharris]

Mr Skeptic said:

Well, I had mean a singularity like a black hole has, with everyone falling toward it. On the other hand, we are all "falling" away from the singularity at the BB.

Incidentally, what does a black hole look like if you reverse time?

I don't know if this will answer your question directly, but I was watching a lecture from Roger Penrose ("Aeons Before the Big Bang") and he briefly mentioned black holes. He said that the black hole is a state of entropy, and that if you were to reverse that it would simply become larger in mass (in reference to the singularity) and would lose its strong gravitational pull on objects.
That is, if I understood him correctly of course :-p

[michel123456]

Mr Skeptic said:

(...)
Incidentally, what does a black hole look like if you reverse time?

Does gravity become repulsive when time is reversed?

[michel123456]

Is that a silly question?

[Sisyphus]

michel123456 said:

Does gravity become repulsive when time is reversed?

michel123456 said:

Is that a silly question?

Not a silly question. A straightforward one. The answer is no.

I'm having some trouble with a black hole "in reverse," though. (I guess because a black hole itself doesn't make any sense with just classical physics.)

[Mr Skeptic]

michel123456 said:

Does gravity become repulsive when time is reversed?

I don't think any fundamental force becomes repulsive when inverting time actually. All that changes is the directions and speeds stuff is going. If you have something like an explosion, then that might be a different story, but then that's an entropy thing.

I think it was me that asked the silly question, I was thinking of a black hole becoming repulsive when I already knew otherwise but just hadn't thought about it. I'm not even sure I know what inverting time on a black hole would mean, what with it being a 4D thing with time as one of its dimensions.

[eoinmac]

I dont understand how running time backwards on black hole would not make gravity seem repulsive? You would see the matter's history going into the black hole point from point and reversed it would be coming out

[Mr Skeptic]

Well, gravity as a force at least won't become repulsive. Suppose you see Newton under a tree and decide to play a joke on him. You throw an apple at his head, and it curves in the parabolic path common to projectiles under gravity. Now reverse time. The apple will fly away from his head backwards along the original path -- still curving downwards. Hence, gravity is still attractive under reverse gravity.

However, you do make an interesting point about the black hole and its event horizon. To reverse the trajectory means the object will have to exit the event horizon, which is supposed to be impossible. Maybe black holes aren't symmetrical under time reversal?

[eoinmac]

Well if space and time are so strongly woven together then time reversal would result in viewing the information or matter at a certain spacetime at a certain point, to the observer it would seem as if the particle never entered the black hole but only came out. Also if you think of gravity as the example of rubber sheet and ball, gravity would appear to be repulsive. Say a photon flew in from space toward a star, you reverse time and the particle would seen going away from the star. It would be interesting to apply this to the Big Crunch theory in which gravity would seem to be repulsive, turning into a chicken or the egg of crunch or expand. Could gravity's attractive force or repulsive force then rely on the direction of time we experience?

[caharris]

Mr Skeptic said:

However, you do make an interesting point about the black hole and its event horizon. To reverse the trajectory means the object will have to exit the event horizon, which is supposed to be impossible. Maybe black holes aren't symmetrical under time reversal?

Are you talking about Hawking's information paradox? I thought he finally said that was wrong?

[michel123456]

[quote name='Mr Skeptic']Well, gravity as a force at least won't become repulsive. Suppose you see Newton under a tree and decide to play a joke on him. You throw an apple at his head, and it curves in the parabolic path common to projectiles under gravity. Now reverse time. The apple will fly away from his head backwards along the original path -- still curving downwards. Hence, gravity is still attractive under reverse gravity.
QUOTE]

I am wondering for days, I am not quite sure...
When you throw the apple, you give an impulse. The apple goes up, then goes down, pulled by gravity. When it bumps Isaac's head, a shock wave travels through his body reaching the floor and expanding into Earth, spreading ultimately some heat.

By reversing, we see a reduce of heat that produces shock waves coming from all directions, rising into caasI's body, pushing the apple and expulsing it above. At some point of its trajectory, the apple is pulled by the hand, that emits at distance some reverse impulse, and the apple goes back in the hand, giving some heat raise (because in the first scenario, the first impulse was given by some body energy).

What I see is repulsive gravity,pushing the object. And attractive hands, acting at distance, using the same mysterious mechanism that gravity uses.

[Mr Skeptic]

michel123456 said:

What I see is repulsive gravity,pushing the object. And attractive hands, acting at distance, using the same mysterious mechanism that gravity uses.

If gravity had become repulsive, the acceleration of the apple would be upwards, so the curve would be upwards and into orbit. Also the atmosphere would all fly off into space, and you wouldn't end up where you started.