If light doesn't have mass, then how can it be trapped/affected by the gravity in a black hole?

Question

At least that's what I heard... that light doesn't have mass.

Great responses so far, but a few of you are saying that photons have mass. But, I've been reading questions about the speed of light with so many answers stating that photons have no mass.

Answer 1

Black Holes and the Speed of Light
Black holes almost certainly exist, and one of their basic properties is that they trap light. However, it is also true that nothing exceeds the speed of light. In fact, the theoretical prediction of black holes is due to the General Theory of Relativity, which is built on the principle that the speed of light in a vacuum is constant. The analogy of a cannonball falling back to Earth with the trapping of light in a black hole is only a crude and suggestive one that is not correct at a fundamental level (for one thing, the cannonball has mass, but light does not; it turns out that this difference is critical, because massless particles MUST travel at light velocity, but massive particles CANNOT travel at light velocity).

To understand fully why a black hole can trap light but the light still always travels at constant velocity requires an understanding of the General Theory of Relativity, but the essential point is that the black hole curves spacetime back on itself, so that all paths in the interior of the black hole lead back to the singularity at the center, no matter which direction you go (an analogy in two dimensions is that no matter which direction you go on the surface of the Earth in a "straight line" (what mathematicians call a "geodesic" or a "great circle"), you never escape the Earth but instead return to the same point. Imagine extending that analogy to the 4 dimensions of spacetime and you have a rough explanation for why light travels at light speed, but cannot escape the interior of a black hole.

Answer 2

Photons do not have rest mass.
Newton's Law of Gravitation:
F = GMm/r^2
would lead you to believe that a massive body (like a black hole) would not exert any force on a photon, since m=0 in the above equation.

Einstein changed everything with General Relativity. In GR, mass bends the spacetime metric. Light has to travel along paths in spacetime, and so is affected as well.

Answer 3

You should study relativity, Einstein proved that mass and energy are really the same things,if something has energy in any form, than it means it has mass, light can act as both particles and waves, so as particles, even though if they are mass less, they are moving with some velocity and therefore they possess kinetic energy which serves as the mass of that particle. You know e=mc^2, now find m by, m=e/c^2, e is the kinetic energy possessed by the particle and c is the speed of light, so in other words, the energy of the body gives a certain mass to it, though it would be too small, but it does have some mass due to its energy, and therefore gravity can pull it.

Answer 4

First, what is light? Light, to us, is what we are able to sense by means of sight. But, this is a very limited scope of its actual existence. Light is electromagnetic energy. It is a form of radiation that moves at the speed of light, and has energy values ranging from radio waves to cosmic radiation. The greater the frequency of the waves in a given distance, the greater the energy of that bit of light (photon).

Normally photons move about our world in a very business-like manner, minding their own task and ignoring everything else. Occasionally a high energy photon will enter the nucleus of an atom, and the energy of that photon will divide into a negative and positive electron. At this time the energy of the photon actually becomes "three dimensional light".

Mass is also able to be reduced to photons, as in a nuclear explosion. Photons are able to form physical mass and are subject to the obvious limits of gravity, and they are able to be influenced by a strong gravitational field in a slight manner. Because they travel so fast, the time period for being influenced is very short.

So, light does not have mass (3 dimensions) as we think of it, but this form of energy is able to be formed into mass, so it is under the same limitation.

Answer 5

The problem here is visual and not related only to mass. Definitely one factor is that photons have mass and when any body travels at a speed so high there is an increase in its mass and that is why it is trapped

Answer 6

Particles like proton and electron has a mass called rest mass, which is mass when they are rest.

And from Einstein’s theory of relativity, mass is equivalent to energy. The rest mass of a particle of matter is the internal energy of that particle. The equivalent energy is m0 CC. m0 is its rest mass.

When the particle has some speed, it has kinetic energy.

When the speed of a particle increases its kinetic energy increases.

When the particle’s speed increases its mass also increases.

This kinetic energy is calculated by INCREASE in mass x CC. {C is the speed of light}.

This is different from the classical method of calculating kinetic energy which is ½ m v v (v is the speed of the particle}. This formula cannot be used, since in this formula m is a constant. But mass increases with speed.

Therefore in calculating the kinetic energy we are not bothered about its speed v. This v is incorporated in the increase in mass.

It is to be noted that the kinetic energy of a particle is calculated only with the increase in mass times the square of the speed of light.

Also we are not bothered about its rest mass m0, if we know the increase in mass.

Now if we consider the photon, it is energy transmission. It is energy. It is having only kinetic energy; it has no internal (stock) energy.

What is the kinetic energy of a photon? Since kinetic energy is increase in mass times the square of the speed of light, ‘the increase in mass’ of a photon is its kinetic energy divided by the square of the speed of light.

Though it has no rest mass it has an “increase in mass”.

Since a photon is energy and as it has no (stock) rest mass energy and since its speed cannot be slowed down at our will in vacuum, we can by all means take the increase in mass as the mass of photon.

Answer 7

Light does has mass, consisting of photons. Therefore the weight (though negligible) enables it to be drawn to a Black Hole.

Answer 8

R u sure, the mass of photon is zero!. The "rest mass of photon is zero",i.e, whenever it comes to rest there will be no photon,means it may have mass while in motion.Then mass particle has some gravity.I hope u got it.

Answer 9

Light is bent by any significant source of gravity, not just black holes. In particle theory, light must have mass. In wave theory it doesn't. The real question is "what is gravity?" (we haven't answered that one properly yet).

Answer 10

because black holes bend space time so much that a ray of light follows the curvature of space into a black hole's center. it has nothing to do with having or not having mass