What is the mass of a photon? If it has no mass...?

Question

If it has no mass, why can't it escape from a black hole?

Answer 1

The first three answers are correct, in that they are different ways of looking at the same problem:

1) Any path taken by the photon eventually leads back to the black hole (because the universe is heavily curved).

2) a photon "at rest" has a mass of zero. However, a photon is never at rest. It carries energy: by Einstein's famous equation ( E = mc^2 ), this energy is equivalent to mass (a very tiny mass -- depends on the wavelength).

3) The warping of space is too much (same as 1, with different words).

Let me add yet another way of looking at the same problem:

5) As the mass of the black hole is collapsing to a singularity (unbounded density), it "sinks' deeper and deeper from the event horizon. The event horizon is always the same size (seen from the outside -- as long as the black hole stays at the same mass), therefore the fabric of space must expand inside the black hole (sort of a local expansion of the universe).

The distance between the photon and the event horizon keeps increasing because of the rapid rate of expansion.

There are also other ways to see this same problem.
4) One of them involves the interchange of space and time inside the event horizon: the photon is now free to move in time but its movement in space is limited to moving towards the centre. (The same way that we are free to move in space, but we can only move towards the future in time).

(PS: I just saw Uncle Al's answer and it makes the last one number 4; so mine above is a number 5)


All these things are actually analogies (since we do not really know what goes on inside the event horizon); these analogies are based on the different mathematical approaches taken to solve the puzzle.

Answer 2

http://prola.aps.org/abstract/PRL/v90/i8/e081801
Phys. Rev. Lett. 90 081801 (2003)
Phys. Rev. Lett. 80 1826 (1998)
Phys. Rev. Lett. 68 23 3383 (1992)
Phys. Rev. D 8 2349 (1973)

Photon rest mass is less than 10^(-51) grams or 7x10^(-19) electron volts. If the photon had non-zero rest mass then electromagnetism would have a finite range and force proportional to 1/r^2 would not obtain.

A photon cannot escape from **within** a black hole's event horizon for two reasons:

1) Escape velocity exceeds lightspeed, and
2) All directions point toward the singularity.

Answer 3

A photon has no mass. It can't escape a black hole because the black hole curves spacetime so that there is no path the photon can take that leads out of the hole.

Answer 4

I don't think the photon has actual mass, but I believe the mass of the energy it carries has mass.

Answer 5

a photon tends to be deflected from its original path by gravity,this is proved by Einstein's theory of general relativity.in such a case the photon is considered to be exhibiting a gravitational mass which can be calculated with the equation
m=(h*frequency)/c^2
and you will obtain an answer in the unit eV/c^2
hence the gravitational mass it exhibits is quite large and is comparable to that of an electron.

Answer 6

< 10 to the minus 51 grams or 7 x 10 to the minus19 electron volts

Answer 7

The space through which the photon is traveling is warped (as far as we know) and it is therefore unable to escape.

Answer 8

Gravity is poorly understood force. By far. So it just means gravity can act on something with no mass. Far as I can see from that. Newtonian gravity stuff is very general stuff. No one has much detailed understanding of gravity at much smaller scale.

Answer 9

It has a minimal mass, as such it can't escape from -186,000^2 gravity

Answer 10

hot pavment will warp light , anything moveing at the speed of light has no mass if a mass managed to get to light speed it would become energy light.