# Black holes and Light

1. Aug 6, 2006

I was watching an interesting article on string theroy on TV the other night and they said (not sure if it was theroy or fact) that, the speed of light and gravity work at the same speed. eg gravity pulls light towards the source but the light is travelling fast enough to counter gravity, thus leading to the light being stationary.

This got me thinking if this statement is true how come we cant see black holes? Surly we would be able to see some light if both work at the same speeds.

Clarification on whether the statement is true or not would be of help aswell.
Thx heaps.

2. Aug 6, 2006

### George Jones

Staff Emeritus
Consider an observer who hovers a small distance d above the event horizon of a black hole. The observer throws a ball straight up. If the initial speed of the ball (with respect to the observer) is less than the escape velocity due to gravity, then the ball comes back down. As the hovering distance d approaches zero, the escape velocity approaches the speed of light.

3. Aug 7, 2006

### Jorrie

If you can lay your hand on Kip Thorne's "Black Holes and Time Warps: Einstein's Outrageous Legacy", read his version of the "Parable of the ants". It is a fun way to answer the question you asked.

4. Aug 10, 2006

### EP

Wait I thought light does bend when it encounters gravity. Doesnt a blackhole act as a lens bending the behind it.

5. Aug 11, 2006

### Phobos

Staff Emeritus
Doesn't sound right. Light can't be stationary. Can you clarify?

The speed of gravity really isn't the issue. The black hole will have a gravitational field around it that light will fall into and either escape or be trapped by. Like George was saying, the closer you are to the black hole (or any mass really) the higher the "escape velocity" is for something to get away from that black hole. Once inside the event horizon of the black hole, the escape velocity is greater than the speed of light, so not even light (the fastest thing there is) can escape. If no light escapes it, you can't see it directly. Now, light eminating from stuff falling into the black hole prior to the event horizon can be seen.

6. Aug 11, 2006

### Phobos

Staff Emeritus
Yes, but that's light that doesn't encounter the event horizon of the black hole. A light beam moving tangential to a black hole doesn't necessarily get drawn in, but does have its path deflected. A light beam going directly at a black hole is a goner.