# Black hole Vs Light

1. Jun 3, 2007

### Manu Vincent

Hi all,
We know that the black hole is able to pull every thing that get near to it, even light because of its high gravitational force. But, we know that light is mass less. Then how will black hole attract light????

2. Jun 3, 2007

### Danger

A gravitational source doesn't directly act upon the light. Rather, it curves the space-time that the light is travelling through. The light 'thinks' that it's still going in a straight line.

3. Jun 3, 2007

### angel 42

I agree with Danger, but does it only curves space-time???

4. Jun 3, 2007

### MeJennifer

Gravitation = Spacetime curvature.

5. Jun 3, 2007

### Chris Hillman

Well, what else do you think a massive object should "curve", in addition to spacetime?

Don't confuse path curvature with sectional curvature. Path curvature is the magnitude of the acceleration vector of a curve, and has relativistic units of reciprocal length. The Riemann curvature tensor, which measures the curvature of spacetime, has components which are sectional curvatures, and they have relativistic units of reciprocal area (so do mass and energy density).

As Danger said, in gtr, in the geometric optics approximation, a laser pulse has a world line which is a null geodesic of the spacetime (a Lorentzian manifold), so its acceleration vector and thus its path curvature vanishes identically. This does not contradict the notion of "light bending", just implies that "light bending" means something a little different from what you might think at first, er, sight

Last edited: Jun 3, 2007
6. Jun 3, 2007

### angel 42

let's (curve) this subject a bit, does the black hole only could do the curve thing? and is it posibel to get ((some thing)) that would have the same properities that the blak hole has??

7. Jun 3, 2007

### angel 42

just thinking loud

8. Jun 3, 2007

### Chris Hillman

Acoustic and other analogs of black holes

A not terribly successful attempt to explain in nontechnical terms a currently popular idea in gravitation physics:
http://en.wikipedia.org/w/index.php?title=Acoustic_metric&oldid=26629648
An excellent and authoritative technical survey:
http://relativity.livingreviews.org/Articles/lrr-2005-12/index.html

Last edited: Jun 3, 2007
9. Jun 3, 2007

### cesiumfrog

If you were going to take a Newtonian approximation (treating gravity as a force), I think it's fair to say you'd use relativistic mass (which is non-zero for light) rather than rest mass.

10. Jun 3, 2007

### prasannapakkiam

LOOK AT ENCARTA'S BRILLIANT GR ANALOGY!!!! It can easily explain this problem...

11. Jun 5, 2007

### Danger

So you're the one who's been screwing with my reputation. :tongue:
You start with 'As Danger said', and proceed with an explanation that I can't begin to understand after reading it a dozen times. Now I know why I keep getting PM's from people who think that I know what I'm talking about. Stop that!