coderage9100

Could Black Holes be the hottest things in the Universe?

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Hello!

Does it make sense for Black Holes to be the hottest things in the Universe?  Since nothing escapes it, light included, we can't actually directly observe any data.  But since energy equals heat and the incredible amount of mass and energy packed into them, wouldn't that make them the hottest things in the Universe?

And a follow up question.  IF we could see a Black Hole, what color would it be?

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7 minutes ago, coderage9100 said:

Does it make sense for Black Holes to be the hottest things in the Universe?  Since nothing escapes it, light included, we can't actually directly observe any data.  But since energy equals heat and the incredible amount of mass and energy packed into them, wouldn't that make them the hottest things in the Universe?

The temperature of a black hole is inversely proportional to its mass. In other words, smaller black holes are hotter than large ones.

7 minutes ago, coderage9100 said:

IF we could see a Black Hole, what color would it be?

Uhm ... black? (Not that simple ... more later ...)

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23 minutes ago, QuantumT said:

It basically says that the event horizon is super hot, and the inside is colder the bigger it gets.

Only for really small black holes.

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25 minutes ago, QuantumT said:

You might find this article useful:

https://phys.org/news/2016-09-cold-black-holes.html

It basically says that the event horizon is super hot, and the inside is colder the bigger it gets.

That seems so backwards!  haha  And yeah, I was reading that article.  How do we know that it's cold?  We can't see anything one way or the other.  They don't really explain why in the few articles that I've read.

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31 minutes ago, coderage9100 said:

IF we could see a Black Hole, what color would it be?

Because of Hawking radiation, a black hole with the size of an asteroid (say, Janus) would be white hot (but very, very faint). One that was twice as massive would be a dull red glow.

Any realistic black holes (ie the mass of the sun or more) would be black.

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4 minutes ago, Strange said:

Only for really small black holes.

2 minutes ago, coderage9100 said:

That seems so backwards!  haha  And yeah, I was reading that article.  How do we know that it's cold?  We can't see anything one way or the other.  They don't really explain why in the few articles that I've read.

Temperature is a matter of atomic movement, if I'm not mistaken. And the denser the singularity gets, the less movement it allows - thus colder. Or?

 

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Just now, QuantumT said:

Temperature is a matter of atomic movement

Not in the case of black holes. There are no atoms!

The temperature comes from the concept of Hawking radiation. This is a result of quantum effects near the extreme conditions of the event horizon. It is often explained in terms of virtual particles - but I'm not sure that is a very accurate analogy. 

5 minutes ago, QuantumT said:

And the denser the singularity gets, the less movement it allows - thus colder. Or?

The radiation comes from outside the event horizon, to from the singularity. It is possible that matter falling towards the event horizon gets heated to extremely high temperatures by friction and pressure but, if so, that would have no effect externally.

And, related to that, the Hawking radiation from real black holes would not be measurable. However, most real black holes will have matter falling into them at some time. In this case, the infalling material will create high temperatures and lots of radiation. Some of the brightest things in the universe (quasars) are the result of matter falling into black holes.

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14 minutes ago, Strange said:

Not in the case of black holes. There are no atoms!

The temperature comes from the concept of Hawking radiation. This is a result of quantum effects near the extreme conditions of the event horizon. It is often explained in terms of virtual particles - but I'm not sure that is a very accurate analogy. 

The radiation comes from outside the event horizon, to from the singularity. It is possible that matter falling towards the event horizon gets heated to extremely high temperatures by friction and pressure but, if so, that would have no effect externally.

And, related to that, the Hawking radiation from real black holes would not be measurable. However, most real black holes will have matter falling into them at some time. In this case, the infalling material will create high temperatures and lots of radiation. Some of the brightest things in the universe (quasars) are the result of matter falling into black holes.

Very informative, thank you!

Could the EH be a fermion field?

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2 minutes ago, QuantumT said:

Could the EH be a fermion field?

No. It is define by the curvature of spacetime.

We only have approximations of quantum theory under these conditions. One of these leads to the concept of Hawking radiation. Others show some paradoxical things so are probably not correct. More work needed...

 

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If I remember correctly, a documentary about black holes suggested that just inside the event horizon are the most violent conditions in the universe.  The black hole totally encapsulates that chaos, nothing escapes.  Would that not also be the hottest place in the universe?

Edited by Airbrush

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For a distant observer the temperature of a BH is related to its entropy, which is directly proportional to the EH surface area.
Temperature is also a measure of energy, and energy is frame dependant.

The FoR of a distant observer is different for that at the EH, inside the EH, or at the possible singularity.
Talking about temperature of a BH makes no sense unless you specify your FoR.

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2 hours ago, Airbrush said:

If I remember correctly, a documentary about black holes suggested that just inside the event horizon are the most violent conditions in the universe.  The black hole totally encapsulates that chaos, nothing escapes.  Would that not also be the hottest place in the universe?

That sounds like the "firewall hypothesis". But that is a result of a contradiction that comes about when you try and combine GR and QM.

1 hour ago, MigL said:

For a distant observer the temperature of a BH is related to its entropy, which is directly proportional to the EH surface area.
Temperature is also a measure of energy, and energy is frame dependant.

The FoR of a distant observer is different for that at the EH, inside the EH, or at the possible singularity.
Talking about temperature of a BH makes no sense unless you specify your FoR.

Good point. An observer free falling into a black hole would not see any Hawking radiation so the concept of the BH's temperature becomes fairly meaningless.

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7 hours ago, Strange said:

That sounds like the "firewall hypothesis". But that is a result of a contradiction that comes about when you try and combine GR and QM.

Let's make a distinction between a solitary black hole with no matter inflow, with a feeding black hole.  The non-feeding black hole is just empty space inside the event horizon, no atoms, all mass is concentrated at the center, the singularity, which has no diameter, so it has infinite density.  All is quiet inside the black hole, and there is no temperature.  But when matter enters the event horizon all hell breaks loose.  The matter is accelerated to light speed towards the center, all trying to squeeze into zero space, which compresses it creating higher temperatures just inside the event horizon than outside, because the matter would be bouncing off itself at near light speed in a "hot witch's brew."  Why not?

Edited by Airbrush

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"The Inner Horizon of a Black Hole is a very chaotic place, matter falling into the black hole after crossing the Event Horizon, collides with matter being flung outwards by the centrifugal force caused by the Black Hole's rotation...."

https://www.quora.com/What-is-an-inner-event-horizon

When I googled about this subject (black hole interior) I found this on "Quora".  Anyone familiar with Quora?  How could matter be flung outwards against the much stronger force of gravity pulling it into the singularity?

Edited by Airbrush

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29 minutes ago, Airbrush said:

"The Inner Horizon of a Black Hole is a very chaotic place, matter falling into the black hole after crossing the Event Horizon, collides with matter being flung outwards by the centrifugal force caused by the Black Hole's rotation...."

https://www.quora.com/What-is-an-inner-event-horizon

When I googled about this subject (black hole interior) I found this on "Quora".  Anyone familiar with Quora?  How could matter be flung outwards against the much stronger force of gravity pulling it into the singularity?

 A Kerr BH, that is a spinning BH, will effectively have two horizons...the EH proper, and the static horizon, or static limit, which marks the boundary of spacetime that has been geometrically altered due to the angular momentum, much as the Earth is oblate due to its angular momentum or rotation.

Between the static limit/horizon and the EH proper, it is theoretically possible to extract energy for a sufficiently advanced civilisation, and matter caught within this region is dragged around with space, as per frame dragging effect.

Once the EH proper is reached, everything is back to what is normally envisaged with a BH...that is once any matter crosses that EH, it has only a one way path to the center.

Another point I'm not that familiar with is a so called "Cauchy horizon" or inner horizon, within the EH proper of the BH, and as best as I am able to describe, is a closed timelike curve. Mordred, Strange or one of our other renowned knowledge tanks may like to define that better then I am able to.  

Edited by beecee

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2 hours ago, Airbrush said:

"The Inner Horizon of a Black Hole is a very chaotic place, matter falling into the black hole after crossing the Event Horizon, collides with matter being flung outwards by the centrifugal force caused by the Black Hole's rotation...."

https://www.quora.com/What-is-an-inner-event-horizon

When I googled about this subject (black hole interior) I found this on "Quora".  Anyone familiar with Quora?  How could matter be flung outwards against the much stronger force of gravity pulling it into the singularity?

Most of those replies look pretty nonsensical. That is the trouble with sources like that, it is very hard to distinguish good replies from bad.

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