# Does a black hole really have mass?

1. Jan 10, 2012

### Jimmy9

Do black holes really have literal mass or is that just an expression of the amount of space curvature? I would think on collapse to infinite gravity all actual mass would be turned into energy and transformed into space curvature. I mean is there really a little pebble somewhere in the well pulling down on the space around it?

2. Jan 11, 2012

### mathman

What is inside a black hole is an open question. However to the outside it acts like a massive thing, swallowing up nearby gas and stars.

3. Jan 11, 2012

### Drakkith

Staff Emeritus
The mass of a black hole acts just like the mass from any other object. The curvature of space, which results from mass, is the underlying cause of gravitation in General Relativity. Nothing is "transformed" into curvature, it simply exists.

The Event Horizon around a black hole is merely the point in space where the gravity finally reaches the strength needed to trap light. It has a finite diameter that increases when the mass of the black hole increases.

4. Jan 11, 2012

### Ryan_m_b

Staff Emeritus
Consider it this way: the Earth is orbiting the Sun. If the Sun was suddenly compressed into a blackhole it would not affect Earth's orbit at all.

As others have said we don't have good enough models to accurately describe the conditions beyond a event horizon.

5. Jan 11, 2012

### DaveC426913

except that we know the gravitational curvature due to mass is the one thing that remains.

6. Jan 11, 2012

### Chronos

Sounds like another victim of the 'mass falling into a black hole 'freezes' at the event horizon' analogy. The 'singularity' at the core of an event horizon merely represents the inability of our mathematical models to yield logical results.

7. Jan 12, 2012

### Jimmy9

"Nothing is "transformed" into curvature, it simply exists."

Transformed was a poor choice of words. What I mean is the "mass", as in a object with defined measurements and in which the mass is contained ceases to exist and all that is left behind is the deformed spacetime.

8. Jan 12, 2012

### rollcast

But the mass or energy of the object must remain as otherwise spacetime would not be deformed by the gravity of the object

9. Jan 12, 2012

### Drakkith

Staff Emeritus
The mass does not disappear. As Rollcast said the mass must remain, and if it didn't spacetime would no longer be curved. Also, in general relativity mass and energy both affect spacetime the same way. IE they both cause gravity. Even photons, which are massless, contribute to gravity.

10. Jan 13, 2012

### aayushgsa

I think the massy thing of black hole can be explained like this
density of blackhole is infinite...
density= mass/meter cube
∞= mass/meter cube
let the edge of the cube be 1unit..
∞= mass/1....
which shows
∞=mass
means that the mass of black hole is infinite!!!

If I had posted wrong things I am sorry

11. Jan 13, 2012

### Chronos

You might get an argument on the infinite density thing. Most physicists reject that idea. No doubt it is really, really dense, but, probably not infinite. There is widespread agreement that the 'singularity' is merely the point at which our models cease to be predictive.

12. Jan 14, 2012

### rollcast

The problem with what you are saying is that in Quantum Mechanics a particle cannot inhabit a space smaller than its wavelength, so therefore the centre of a black hole cannot be an infinitely dense, infinitely small point(a entity which can be given a definite location in space but doesn't have any length, area or volume).

13. Jan 14, 2012

### Drakkith

Staff Emeritus
A black hole with infinite density would have a set amount of mass inside a volume of nothing, leading to infinite density. And as explained above, the consensus is that it's just our math that is wrong since we don't know what goes on behind the event horizon.

14. Jan 14, 2012

### DaveC426913

As I said in my response to your PM:

Your logic has a flaw in it. You start with the premise that the density of a black hole is infinite, which is fine. Density is mass divided by volume. The BH's mass is known and it is *finite*. And its volume is assumed to be zero. So your premise is an finite mass and a zero volume. That's where the infinite density comes from.

Yet a few steps later you decide to set the volume to some other number (i.e. 1). Well, you can't. Your premise was that volume is zero.

OK ... you *could* set the volume to 1 but now you have to recalculate the density - which is now no longer m/0, but m/1, which is certainly finite.

See?