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ThinksMarkedly wrote:Black holes have three properties: mass, charge, and spin. The mass is what defines it as a black hole and creates the event horizon. The charge doesn't matter really because a charged black hole will attract the opposite charge and neutralise itself quickly. The spin is what creates the frame dragging.

Replying to myself to clarify: I was referring to the tangential frame-dragging.

The mass of the black hole creates the radial frame-dragging: towards the centre. That's true for every mass, actually. The difference is that, as markus wrote above, at the event horizon, the frame dragging towards the singularity is great enough that even photons and gravitons travelling at the speed of light directly away from the singularity are still in a net movement towards the singularity.

The mass spinning creates the tangential frame dragging.

markusschaber wrote:As far as I found out, there's about 30 million G at the event horizon. So just outside of the event horizon, there's still a small area with millions of G, and a bigger area with thousands of g, until it gradually fades out. That should be quite measurable by grav sensors.

That depends on the size of the black hole (or its mass, which are directly proportional to one another).

Using Viktor T. Toth's Black Hole Calculator, if you input 30 million Earth gravities for surface gravity, it tells you the BH would have a mass of 51708 solar masses and a radius of 152,000 km. That's a supermassive blackhole, though not so massive as the ones found at the centres of galaxies. The one in the centre of ours has 4 million solar masses, which gives it a radius of 11 million km and its surface gravity decreases to 387,000 Earth gravities.

The website also gives this information in the asterisk to the surface gravity: "The surface gravity calculated here is the Newtonian value. The actual surface gravity is infinite at the horizon; that is to say, an infinitely powerful rocket would be needed for an object to maintain its position precisely at the event horizon." What this means is that the surface gravity is actually irrelevant: you're accelerating towards the singularity, and that's all.

What matters really is the surface tide: that is, how much the acceleration is changing per unit of distance from the event horizon. Going back to your BH of 51708 solar masses, the calculator tells us the tide is a mere 3.85 m/s² per metre. That is, if you were 2 metres tall (Honor is 1.87), your feet would be feeling a gravity 7.7 m/s² stronger than your head, which isn't enough to rip you apart in spaghettification. That means entry into this 50,000-solar-mass black hole is survivable... at least for 0.8 seconds, the time it would take you to reach the singularity thereafter.

(Note: the calculator also said the surface tide was 3852 Earth gravities/m, which makes no sense with the other number, so there has to be something wrong with the units)

ThinksMarkedly wrote:The mass of the black hole creates the radial frame-dragging: towards the centre. That's true for every mass, actually. The difference is that, as markus wrote above, at the event horizon, the frame dragging towards the singularity is great enough that even photons and gravitons travelling at the speed of light directly away from the singularity are still in a net movement towards the singularity.

The mass spinning creates the tangential frame dragging.

I thought you and I were in agreement that the black hole has a gravitational field outside of the event horizon; you even said this:

Gravity "escapes" a black hole. That's how we know it is there.

But then you make the highlighted statement which seems to be in agreement with Penny. Please clarify your clarification.

ThinksMarkedly wrote:

markusschaber wrote:As far as I found out, there's about 30 million G at the event horizon. So just outside of the event horizon, there's still a small area with millions of G, and a bigger area with thousands of g, until it gradually fades out. That should be quite measurable by grav sensors.

That depends on the size of the black hole (or its mass, which are directly proportional to one another).

The text I found calculated that 30 million something G is the G needed to not let photons escape. But probably, you're right. I'm not an expert in relativity. Thanks for your detailed explanation.

tlb wrote:

ThinksMarkedly wrote:The mass of the black hole creates the radial frame-dragging: towards the centre. That's true for every mass, actually. The difference is that, as markus wrote above, at the event horizon, the frame dragging towards the singularity is great enough that even photons and gravitons travelling at the speed of light directly away from the singularity are still in a net movement towards the singularity.

The mass spinning creates the tangential frame dragging.

I thought you and I were in agreement that the black hole has a gravitational field outside of the event horizon; you even said this:

Gravity "escapes" a black hole. That's how we know it is there.

But then you make the highlighted statement which seems to be in agreement with Penny. Please clarify your clarification.

Maybe it's about the "at the event horizon" just before the text highlighted by you?

Been about 50 years since my science degree, although I have tried to keep current. In relativity terms could it be possible that the picosecond on the cusp of the event horizon in the black hole, would feel like an eternity to those experiencing it?
Meanwhile the discussion about tides does remind me of Larry Niven's Known Universe series where space ship crews were torn apart by tidal forces.

ThinksMarkedly wrote:The mass of the black hole creates the radial frame-dragging: towards the centre. That's true for every mass, actually. The difference is that, as markus wrote above, at the event horizon, the frame dragging towards the singularity is great enough that even photons and gravitons travelling at the speed of light directly away from the singularity are still in a net movement towards the singularity.

The mass spinning creates the tangential frame dragging.

tlb wrote:I thought you and I were in agreement that the black hole has a gravitational field outside of the event horizon; you even said this:

Gravity "escapes" a black hole. That's how we know it is there.

But then you make the highlighted statement which seems to be in agreement with Penny. Please clarify your clarification.

markusschaber wrote:Maybe it's about the "at the event horizon" just before the text highlighted by you?

Of course the critical point is at the event horizon and there the highlighted text is saying that photons and gravitons now have movement back toward the singularity. Which suggests that they cannot escape, but there is a gravitation field outside the event horizon. So what is the explanation?

Gravity does NOT escape a black hole. Hence, a black hole is imploding gravity. We know it is there because we can see a big black hole in space.

penny wrote:Gravity does NOT escape a black hole. Hence, a black hole is imploding gravity. We know it is there because we can see a big black hole in space.

Go tell that to NASA, since you know so much more than them:

NASA wrote:GRAVITY'S THE SAME.
If you replaced the Sun with a black hole of the same mass, the solar system would get a lot colder, but the planets would stay in their orbits.

From: Essential Black Hole Facts - NASA

Do you just ignore the NASA finding of a binary pair composed of a regular star and a black hole? Also occlusion is not the primary way that black holes are found.

tlb wrote:

penny wrote:Gravity does NOT escape a black hole. Hence, a black hole is imploding gravity. We know it is there because we can see a big black hole in space.

Go tell that to NASA, since you know so much more than them:

NASA wrote:GRAVITY'S THE SAME.
If you replaced the Sun with a black hole of the same mass, the solar system would get a lot colder, but the planets would stay in their orbits.

From: Essential Black Hole Facts - NASA

Do you just ignore the NASA finding of a binary pair composed of a regular star and a black hole? Also occlusion is not the primary way that black holes are found.

I will say it one more time.

GRAVITY DOES NOT ESCAPE A BLACK HOLE!

IMPLODING GRAVITY!

Are you putting words in NASA's mouth? A black hole causes a huge gravitational effect, yes. The accretion disk of dust and matter is evidence of that, yes.

Your binary pair is irrelevant. There can also be a binary black hole. Still irrelevant until they collide.