# Temperature of a black hole....observed from the singularity

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DiscoProJoe
I'm just a layman here, who enjoys science and astronomy. I was reading about the temperatures of the cores of black holes being extremely cold, and how time at the singularity...progresses super, super slowly.

But this is only as measured from an outside observer's perspective (such as from a telescope on Earth), correct?

So I have a cool and strange question: Just after the core collapse of a star of 25 solar masses, if you could somehow stand on the singularity of the newly-formed black hole without dying (and without any harm or discomfort) from the extreme temperatures, gravity, tidal forces, or the extreme background radiation from outer space being sucked in toward you,...then what temperatures would you end up measuring of both the singularity, and of the rest of outer space outside the black hole?

Correct me if I'm wrong (which I very well could be), but here's my guess:

Initially, the temperature of the singularity would be roughly the same (or slightly higher) than it was when the star core collapsed, which is around 3 billion Kelvin. But when looking at the surrounding outer space outside the black hole, the temperature of outer space would appear to be quadrillions, quintillions, or even 10^30 degrees Kelvin. But this would only seem to last for a nanosecond (from your perspective), as all the stars in the universe would quickly burn themselves out and become extinct. From that point forward, the temperature of outer space would quickly -- and then gradually -- decline from septillions of degrees Kelvin...down to a few billion degrees. As soon as the outside temperature drops below that of the three-billion-degree singularity you're standing on, the singularity would begin to age...by emitting its own radiation into space and then eventually, suddenly evaporating by leaving behind a bright cloud of hot dust and gases. At which point, the year would be something like 10^60 A.D.

Again, from your perspective, this would all appear to happen very quickly.

Any expert thoughts on this?

Is this completely false, or am I on to something, here?

Mentor
and how time at the singularity...progresses super, super slowly.
Time is a meaningless concept at a singularity.
if you could somehow stand on the singularity of the newly-formed black hole
You cannot. It is not just a practical issue. It's like trying to "stand on Sunday" here on Earth. If you are inside a black hole you cannot avoid reaching the singularity in the same way you cannot avoid reaching the next Monday here. You also cannot observe the singularity. In the short time that you still have, you'll measure some radiation from whatever else falls into the black hole.
Initially, the temperature of the singularity would be roughly the same (or slightly higher) than it was when the star core collapsed, which is around 3 billion Kelvin. But when looking at the surrounding outer space outside the black hole, the temperature of outer space would appear to be quadrillions, quintillions, or even 10^30 degrees Kelvin. But this would only seem to last for a nanosecond (from your perspective), as all the stars in the universe would quickly burn themselves out and become extinct.
You are just making up random things with no basis in science here.

Ilythiiri
Singularity is not a type of star. It's a concept, approximately meaning here be dragons "our formulas/models don't work there".

DiscoProJoe
If you are inside a black hole you cannot avoid reaching the singularity.

In theory, that's correct. But in practice, you (or probably...the vapors of your remains) could experience a time dilation effect that's so extreme that the black hole could evaporate into space (in 10^50 to 10^100 years) before your remains actually reach the singularity. So depending on the mass of the black hole and other factors, your remains may "come out of" the black hole before actually reaching the core...when it evaporates in the far future.

Is this a possibility?

In the short time that you still have, you'll measure some radiation from whatever else falls into the black hole.

Due to the extreme time dilation and blue-shift effects, I would think that this radiation falling in behind you...would be so strong (from your perspective) that it would vaporize you. For example, if the events in the outside universe appear to be happening a billion times faster, then it would seem like a billion times more photons would be hitting you per second (from your time-dilated perspective), right? So this outside radiation would seem a lot more bright, hot, and blue-shifted as a result,...true?

In other words, I'm wondering if there's a relationship between time dilation effects, and perception of temperature. I'm really curious about this.

You are just making up random things with no basis in science here.

I'll admit that I'm making up some random numbers, here. But I'm mostly interested in the concepts, not the numbers.

Personally, I'm a huge fan of "hypothetical astronomy/geology/geography" videos, such as from YouTube channels RealLifeLore, Dreksler Astral, V101 Science, and Isaac Arthur. Hypotheticals such as, "If you could stand on the sun without it killing you, what would you see and experience?" It makes learning a lot more interesting and fun.

Ilythiiri
Hmm. If you want to learn things, video is totally inefficient compared to text - goes at speed of speech and loads of waste information (:

There were discussions relevant to your questions in pf.

Suppose we threw a very bright clock into a black hole. We knew where the black hole was and we knew the velocity and acceleration on the clock. We tossed at 12:00 and it should be in the hole at 12:10. Later (hours, or years) we look through a powerful telescope and detect photons radiating from just outside the event horizon. The clock display will show less than 12:10. These photons were emitted before the clock went into the hole. This observation does not change the location of the clock.

I'm currently doing a project on black holes and need some input on what you believe to be the biggest misconceptions about black holes, thanks

Mentor
In theory, that's correct.
And we don't have experimental results. Theory is all we have.
But in practice, you (or probably...the vapors of your remains) could experience a time dilation effect that's so extreme that the black hole could evaporate into space (in 10^50 to 10^100 years) before your remains actually reach the singularity.
You never experience time dilation effects for yourself. You only see them in others. Which is irrelevant here - you fall in and hit the singularity, in microseconds to hours of your time depending on the size of the black hole.
Due to the extreme time dilation and blue-shift effects, I would think that this radiation falling in behind you...would be so strong (from your perspective) that it would vaporize you.
You would not. For a free-falling observer, blueshift is limited to a factor 2. That makes some stars a bit brighter, but it wouldn't vaporize anything.
For example, if the events in the outside universe appear to be happening a billion times faster
They do not. At most they will appear a factor 2 faster.
That is fine, but avoid stating things as fact if they are just speculation. Or, better, avoid the speculation and ask questions.

DiscoProJoe
For a free-falling observer, blueshift is limited to a factor 2. That makes some stars a bit brighter, but it wouldn't vaporize anything.They do not. At most they will appear a factor 2 faster.

Wow, very interesting. For a long time, I've also been under the impression that if someone travels in a spaceship at 99.99999999999999999999999999% the speed of light, then they'd experience a time dilation effect so huge that they'd feel as if they're traveling at trillions of times the speed of light, and with a tiny, compressed, ultra-blue-shifted dot of a view of everything in front of them that contains much radiation that it would kill them, and vaporize them and their spaceship. Is this also true, or is the time dilation in this scenario also limited to a factor of 2, as well?

That is fine, but avoid stating things as fact if they are just speculation. Or, better, avoid the speculation and ask questions.

That's what I'm here to do: ask questions to see if my concepts are right or not, and to separate fact from fiction. Replies are appreciated.

Mentor
Wow, very interesting. For a long time, I've also been under the impression that if someone travels in a spaceship at 99.99999999999999999999999999% the speed of light, then they'd experience a time dilation effect so huge that they'd feel as if they're traveling at trillions of times the speed of light, and with a tiny, compressed, ultra-blue-shifted dot of a view of everything in front of them that contains much radiation that it would kill them, and vaporize them and their spaceship.
If we consider this speed to be relative to most matter in our universe then you'll indeed see everything coming directly towards you at a huge blueshift, but that has nothing to do with the black hole scenario. You also don't experience time dilation, you'll calculate that the rest of the universe is nearly frozen, although it won't look like that to you.

Wow, very interesting. For a long time, I've also been under the impression that if someone travels in a spaceship at 99.99999999999999999999999999% the speed of light, then they'd experience a time dilation effect so huge that they'd feel as if they're traveling at trillions of times the speed of light

Wrong. In flat Minkovski spacetime, no one ever "feels" (more accurately, can measure) as if they're traveling at light speed or more. In your case, astronaut will see all distances in the direction of travel very shortened. Thus, if he flies to Andromeda, he will see it to be much closer to him than a stationary Milky Way observer would (and it approaches him at nearly speed of light, and it is very blueshifted).

DiscoProJoe
If we consider this speed to be relative to most matter in our universe then you'll indeed see everything coming directly towards you at a huge blueshift, but that has nothing to do with the black hole scenario.

Wow, OK. Thanks for clearing this up.

In high school back in the mid '90s, my physics teacher told us that if we fell into a black hole, we would "reach the end of time" (as he put it), due to extreme time dilation. Since then, I'd always believed that about black holes, but it looks like he was wrong about that.

I think a lot of students and science enthusiasts will tend to assume (without being told otherwise) that the same type and extent of time dilation / blue shift that occurs when approaching the speed of light...will also apply when falling into a black hole.

Looks like it's actually totally different. So physics experts should be aware of this common tendency for people to assume this false connection, and should explain these differences to their students (and the public) more often.

A few years ago, I thought it was strange when reading an article about astronomers detecting the shock-wave effects of two black-hole singularities colliding with each other somewhere in our galaxy. I thought, "Wouldn't we have to wait something like 10^50 years until those two singularities end up colliding?"

Looks like it's primarily just hidden from view from the extreme gravity inside the black hole(s), and not extremely time dilated after all.

So, anyway, (getting back to my original question about the temperature), I have a new question: What causes the collapsing core of a massive star...to go from 3 billion degrees Kelvin...all the way down to 10^-20 Kelvin? To go from incredibly hot to incredibly cold? Is there a good article or video link that you can point me to about how this happens? Sounds pretty fascinating.

Thus, if he flies to Andromeda, he will see it to be much closer to him than a stationary Milky Way observer would.

I thought that as one approaches the speed of light, his view of everything will become more and more stretched in a forward direction, causing objects in front of him to appear to recede, even though he's actually approaching them.

There's a couple of videos that illustrate this effect:

Optical Effects of Special Relativity

Accelerating Toward the Speed of Light

Are these conceptually correct?

Ilythiiri
Related to your question "what's the deal with time at relativistic speeds?".
Read up on this, it's not long and clearly written.
Rnd quote:
"...Are the relativists just trying to bamboozle their opponents, like the defence attorney who just has to stir up doubt about the plaintiff's case without giving his own theory of events? Not at all; the physical theory should and does tell a single coherent story here. Relativity pays the price of permissiveness. It says to us, "Pick whichever frame you like to describe your results. They're all equivalent." No wonder that one analysis ends up looking like three or four. ..."

Mentor
So, anyway, (getting back to my original question about the temperature), I have a new question: What causes the collapsing core of a massive star...to go from 3 billion degrees Kelvin...all the way down to 10^-20 Kelvin? To go from incredibly hot to incredibly cold? Is there a good article or video link that you can point me to about how this happens? Sounds pretty fascinating.
There is no matter around any more that could be hot.

I thought that as one approaches the speed of light, his view of everything will become more and more stretched in a forward direction, causing objects in front of him to appear to recede, even though he's actually approaching them.

There's a couple of videos that illustrate this effect:

Optical Effects of Special Relativity

Accelerating Toward the Speed of Light

Are these conceptually correct?

Yes, the videos appear correct, but the correct measurements of length and distances are not achieved by just looking at objects. If your velocity is significant compared to speed of light, you need to correct for that fact. For example, if you fly past a 1 meter stick, you can see that it is contracted if you photograph it at the instant it is exactly 90 degrees away from your velocity vector:
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you |
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V

But it the stick is ahead or behind you, when you analyze its photo, you need to account for the unequal travel time for light emitted from either end of the stick, and for aberrations in directions of incoming light rays (similar how direction of falling snow is "distorted" if you observe it from a moving car).

In the second video, "floor tiles" below the observer are 5 light seconds on a side. Thus, naively, even at the speed of light one tile can be crossed in 5 seconds, no less. However, in the video, from moving observer's POV, we cross tile lines faster and faster, one in 3 secs, then one in 2 sec, then one per sec, then faster and faster.