# Black holes, gravity, light.

The legend
As i know them, black holes are dead remains of stars where the gravitational pull is so great that not even light can escape.

Well, but light(electromagnetic radiation, in general) has no mass, then how does gravity effect light?

## Answers and Replies

Staff Emeritus
Gold Member
Gravity does not interact only due to mass, but also due to energy and momentum, both of which electromagnetic radiation has.

Another was to look at it is that gravity is a curvature of space, and that light follows geodesics in that space. At the black hole, the curvature of space is so great that all geodesics lead back to the black hole.

The legend
Gravity does not interact only due to mass, but also due to energy and momentum, both of which electromagnetic radiation has.

Another was to look at it is that gravity is a curvature of space, and that light follows geodesics in that space. At the black hole, the curvature of space is so great that all geodesics lead back to the black hole.

ok, thanks.

zketrouble
Gravity does not interact only due to mass, but also due to energy and momentum, both of which electromagnetic radiation has.

First let me say that I'm not an astrophysicist, and that I like being corrected if my picture of this is wrong. But anyway, although light and all forms of energy have no mass, the photon packets which carry light do have a tiny amount of mass. The statement that gravity reacts to momentum as well as mass is a strange thing to say, because momentum is = mass*velocity. If something has momentum, it must have mass, otherwise the p = mv equation would be p = 0*v and momentum would equal zero.

So, gravity can affect light because light travels in the form of photons. Is my perception of this accurate?

First let me say that I'm not an astrophysicist, and that I like being corrected if my picture of this is wrong. But anyway, although light and all forms of energy have no mass, the photon packets which carry light do have a tiny amount of mass. The statement that gravity reacts to momentum as well as mass is a strange thing to say, because momentum is = mass*velocity. If something has momentum, it must have mass, otherwise the p = mv equation would be p = 0*v and momentum would equal zero.

So, gravity can affect light because light travels in the form of photons. Is my perception of this accurate?

No. The photons have identically zero rest mass. The reason they can still have momentum is because photons are relativistic particles -- that is, the equations of classical mechanics, p=mv, do not apply to them. Specifically, we have:
$$p=\frac{h}{\lambda}$$

zketrouble
Gravity does not interact only due to mass, but also due to energy and momentum, both of which electromagnetic radiation has.

Another was to look at it is that gravity is a curvature of space, and that light follows geodesics in that space. At the black hole, the curvature of space is so great that all geodesics lead back to the black hole.

No. The photons have identically zero rest mass. The reason they can still have momentum is because photons are relativistic particles -- that is, the equations of classical mechanics, p=mv, do not apply to them. Specifically, we have:
$$p=\frac{h}{\lambda}$$

in the p=h/lambda expression you mentioned, this translates to:
Momentum = Planck's Constant/wavelength, or does the h stand for something else?
Thanks.

in the p=h/lambda expression you mentioned, this translates to:
Momentum = Planck's Constant/wavelength, or does the h stand for something else?
Thanks.

Correct.