# Why can't Light Escape Black Holes?

1. Aug 3, 2012

### DRC12

If light is transmitted by photons and photons are massless and gravity only affects particles with mass, then why can't light escape black holes' giant gravitational force?

2. Aug 3, 2012

### Mark M

Gravity doesn't only affect particles with mass. In general relativity, the stress-energy tensor is the source of gravitation. Since photons have energy, they also gravitate.

3. Aug 3, 2012

### DRC12

I swear a read that massless particles weren't affected by gravity but it must've just been that they don't create their own gravitational fields. Ether way that clears that up thanks

4. Aug 3, 2012

### Mark M

They don't gravitate or have gravitational fields in Newtonian gravity. In general relativity, gravity is equated with the geometry of spacetime. So, massless particles also curve spacetime and will gravitate to each other. Even though it is very small, two photons in empty space will attract each other.

5. Aug 3, 2012

### DRC12

But doesn't the mass of an object affect how much it warps spacetime? so how could photons curve spacetime, or is it based on their energy?

6. Aug 3, 2012

### Mark M

Energy. Remember that the rest energy associated with the mass of a particle is E = mc^2, so mass makes by far the largest contribution since c^2 is an enormous number. That's why Newtonian gravity is such a great approximation. However, in GR, other forms of energy make a small contribution, too. An object with more kinetic energy will have a stronger gravitational field (or it will curve spacetime more, if you like), although the difference is very negligible. So, photons still interact gravitationally because of their other forms of energy, but since they are massless this is much weaker than, say, a rock.

7. Aug 3, 2012

### Naty1

This is an ok description as a start but is technically incorrect; if it were correct, a particle traveling at sufficient velocity could turn into a black hole. Better to say:more KE, then more curvature, NOT more gravity.

A succinct description was given in another discussion:

It is worth mentioning that the source of gravity in general relativity is the stress energy momentum tensor {SET}. So mass, energy,stress, momentum and also pressure have gravitational effects. Only those component sources in the frame of the SET contribute to gravitational curvature; additional curvature, such as from KE, might loosely be considered 'gravity', but is technically a different type of spacetime curvature.
One especially interesting example of gravity is the negative pressure attributed to dark energy: such negative pressure is repulsive!! It is believed to be the cause of cosmological expansion.

An observational example of gravity bending light is 'gravitational lensing'...and was a major factor in confirming Einstein's prediction....Wikipedia discusses it.

Last edited: Aug 3, 2012
8. Aug 3, 2012

### Mark M

Thanks Naty. I should've been a bit mire careful with my terminology, and used momentum instead of KE. And thanks for the quote.

9. Aug 3, 2012

### CCWilson

Isn't it because massive gravitation deforms spacetime so much that with respect to an outside observer, time has almost stopped within a black hole? Since light's speed is constant per unit of time, that means its progress toward the outside world would be almost infinitely slow.

10. Aug 3, 2012

### Vorde

Not really, the 'time-slowing' way of looking at a black-hole is only valid outside the event horizon, and then only if you are really careful with your words.

The much better way to put it is that black holes bend space so much that within the event horizon, every possible path is pointed inwards.

The analogy is that it is a hill so steep that you cannot possible walk up it, no matter how strong you are, you must walk down.

11. Aug 3, 2012

### hover guy

First of all there is a difference between the rest mass and the relativistic MASS, in general relativity an object which moves with a velocity shall experience an in crease in its mass. Similarly the rest mass of a photon is zero but yet its energy provides its mass (m=hf/c2).The second thing is that , its more important to know how gravity actually works, gravity is nothing but curves and disturbances caused by the gravitational field with the space time, also see gravitational lensing, it will justify Ur questions .

12. Aug 3, 2012

### hover guy

i wonder what happens to a photon once it comes to rest and its rest mass becomes zero, such that no gravity can act on it, so should the photon have escaped , just after it was sucked?

13. Aug 3, 2012

### Vorde

Photons can't come to rest. They are always moving at c.

14. Aug 4, 2012

### CCWilson

But we are outside the event horizon. I assume that for a hardy observer inside the event horizon, time goes on as usual from his standpoint, and light travels at its usual speed. Right?

15. Aug 4, 2012

### Vorde

Time always goes on 'as usual' from any standpoint. It's just that you might see someone else's time differ from yours. Likewise, light always travels at the same speed c.

16. Aug 4, 2012

### Naty1

That 'curvature issue' is a subtle point I sure did not understand until Dr Greg here spent some time a few years ago explaining it to me in these forums. I would not even bother to mention it but there are frequent questions why a superfast particle can't become a black hole....

Here are a few more relevant comments I saved:

This sounds like pervect, but I did not record the poster:

DrGreg summarized a perspective:

And good separate discussion is here for those interested:

Does the speed of moving object curve spacetime?