# Why does A black hole need a massive center

1. Jun 15, 2014

### russinnj2000

Was thinking about this why is it a requirement that the center of a black hole be a singularity? If the event horizon contained all the mass of a black hole would it still not have the same effect on it's surroundings? Why could it not be similar to a a crystal where as each molecular link or in this case each piece of mass was pulling as a whole? Further as the mass accelerated towards the speed of light unless it was directly heading at the exact center of the black whole would it not orbit? Or be constantly approaching an escape velocity? If it was Orbiting at near the speed of light would it not increase the likelihood of other particles orbiting in a similar fashion? also even if it were to per say have a dead center pass on the central point of the black whole would the gravitational field not get weaker not stronger as it approached the absolute center? As all the mass surrounding as it approached the center would be pulling it away from the center equally? (Sort of the Gravitational Bungee Jump)

2. Jun 15, 2014

### Matterwave

In the framework of general relativity, once an object is inside the event horizon, there is no world line, no matter how accelerated, that does not end up in the singularity.

Whether a singularity actually exists or not inside a black hole is up for interpretation. We can only view and observe a black hole from the outside of the event horizon. Once we are inside, we can't get information out, so there's no way to make observations. All we can say is that according to the currently generally accepted theory of gravity, once inside the black hole, the eventual future of any object is in a singularity.

3. Jun 15, 2014

### mathman

The main problem with saying there is a singularity at the center of a black hole is that current physics is undecided. General relativity leads to the singularity. Quantum theory says there is no such thing. The question remains unresolved.

4. Jun 15, 2014

### russinnj2000

So my next question would be if you were approaching the event horizon approaching the speed of light would as you approached it and other objects which fell in approached it as well behind you observe your acceleration as an expansion regardless. It may as though appear to you nearing the horizon as if you were in an ever expanding space without explanation as to what was pulling you apart?

5. Jun 20, 2014

### Damo ET

To my layperson understanding, that would appear to be happening if you were looking directly in front or behind you, but not if you were looking in any other direction! To visually 'see' what you suggest, 'you' would have to be shrinking.

Damo

6. Jun 20, 2014

### Bill_K

russinnj2000,

The geometry of a black hole is different from what you're imagining. Dramatically different!

A black hole does not have a "center" in the usual sense. As you enter the hole, the singularity is not something you can see in front of you. Inside of the hole, the coordinates we call r and t switch roles - r becomes the time coordinate, while t is one of the space coordinates. So the singularity at r = 0 is nowhere beside you, it is in your future. The geometry inside the hole is collapsing, taking you with it, and eventually (at time r = 0) everything inside reaches the singularity.

Any infalling matter cannot remain at the event horizon. Also, you seem to be imagining the mass is spread out inside, filling the hole. In a Schwarzschild black hole, the interior of the event horizon is vacuum. There is no matter inside. If matter does fall into the hole from outside, the matter that preceded you will not be in front of you, it will be in your past! The matter that falls after you do will be in your future and you cannot be affected by it.

There's a limit. The only circular orbits lie outside the hole. The smallest circular orbit is at r = 3M (the horizon is at r = 2M). Matter nearing the speed of light (or light itself) could orbit at this radius, r = 3M, but once inside this limit it would death spiral inward and fall through the horizon.

No.

No. the strength increases all the way to r = 0, where it is infinite.

As I said above, that does not happen.

7. Jun 20, 2014

### Janus

Staff Emeritus
The distance at which something must be traveling the speed of light in order to maintain an orbit is is known as the photon sphere. Inside the photon sphere orbits are not possible. The photon sphere actually lies outside] of the event horizon.

8. Jul 7, 2014

### stevebd1

The following link gives a good description of what happens within the event horizon of a black hole using the Schwarzschild metric-

Spacetime Geometry Inside a Black Hole

Scroll down to the sub headings The Schwarzschild Metric and Inside the Black Hole