# Black holes with nothing left to ingest.

## Main Question or Discussion Point

If a black hole sucks everything into it that is near it, but at the same time has an event horizon, then everything between the event horizon and the black hole will get sucked it, but then there would be nothing else to suck in once it's all gone, unless the event horizon expands. Is this correct, or is it not? Thoughts?

## Answers and Replies

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chroot
Staff Emeritus
Gold Member
1) All black holes have event horizons.

2) A black hole can certainly "suck everything in" around it. The problem you might be having is in your phrase "suck things in." Black holes don't really do that! Black holes exert gravitational forces on bodies near them, exactly as the Earth does to you, only stronger. You can orbit a black hole in your starship just as you can orbit the Earth in a space shuttle. Black holes act gravitationally just like any other massive body.

3) As a black hole captures matter and grows more massive, its event horizon expands. The radius of the event horizon is given by:

$$r_s = \frac{2 G M}{c^2}$$

- Warren

Phobos
Staff Emeritus
Gold Member
Welcome to Physics Forums, Nibles!

The event horizon is simply the distance around the black hole singularity at which nothing can escape. The black hole singularity and event horizon exist whether or not the black hole is actively consuming nearby material. Some current black holes are active (consuming stuff) others are not...just adrift in space orbiting the galactic center like any other star. Of course, they can become active again if something crosses their path.

black holes are weird We cant prove anything about the black holes, because they just dont work with our physics. once u enter a black hole, the speed of light will be different, etc. We can only assume that there is an event horizon, because we have no proof (apart from very distant photos) that the horizon is there or doesnt exist

Phobos
Staff Emeritus
Gold Member
Originally posted by chemical
We cant prove anything about the black holes, because they just dont work with our physics.
The physics work just fine at a good distance from the singularity. The closer to the singularity, the weirder the physics. The singularity itself cannot be accurately described by modern physics.

We can only assume that there is an event horizon, because we have no proof (apart from very distant photos) that the horizon is there or doesnt exist
Based on the math, it should exist. (the EH is not a physical thing...it's merely a distance from the singularity)

chroot
Staff Emeritus
Gold Member
Originally posted by chemical
We cant prove anything about the black holes, because they just dont work with our physics.
Well, you can't really prove anything in science, so that's not a big deal. Black holes actually work just fine with our physics all the way down to their heart. Classical theory predicts a spacetime singularity, while quantum gravity predicts something quite different (though no one is exactly sure what yet). In any case, the only part of the black hole we don't fully understand is that part within a Planck length (10^-43 m) of its center.
once u enter a black hole, the speed of light will be different, etc.
False. Nothing exceptional happens inside a black hole's event horizon (which is, I asssume, what you mean by being "inside" one), except that all worldlines terminate at the singularity. Once you're inside the event horizon, you can zoom around in your spaceship all you want, and everything will seem normal -- you just can't ever get back out of the event horizon. Once you run out of fuel, though, you're lunch.
We can only assume that there is an event horizon, because we have no proof (apart from very distant photos) that the horizon is there or doesnt exist
We have no photographic evidence of any sort of black holes, since they are not detected photographically. They are detected via their gravitational interaction with nearby luminous objects.

- Warren