If photons have no mass, how are black-holes able to suck in light?



If photons have no mass, how are black-holes able to suck in light?

In: Physics

They are still bound to space-time, and since black holes can bend it as well, photons are also affected.

Photons have no *rest* mass. If you could get them to stand still, they’d literally weigh nothing.

But they do have energy, because they’re moving, and mass and energy are related and in certain ways equivalent. A moving photon has mass.

Before Einstein, we assumed that gravity was like an invisible rope between objects with mass that pulled them together. Under this sort of understanding, light wouldn’t be affected by gravity at all.

One of Einstein’s biggest contributions to science is what we now call “general relativity”. Put very simply, he theorized that objects with mass are actually fundamentally curving space around them slightly in the 4th dimension. This means that as objects move through this space in a straight line, they will end up moving towards the center of this curve, kind of like rolling a marble into a funnel. Incredibly fast moving things like light are still absurdly hard to trap; using the marble analogy, if you throw a marble into that funnel as hard as you can it’ll just bounce out. However, with a big and steep enough funnel it can be done, and that’s what a black hole is.

Gravity as mass attracts mass is the model in Newtonian mechanics.In general relativity, gravity is the curvature of space-time caused by mass deforming it.

We know that general relativity explains our observation of the universe better than Newtonian mechanics. The bending of light because of gravity is in fact one way that special relativity was tested. The [https://en.wikipedia.org/wiki/Eddington_experiment](https://en.wikipedia.org/wiki/Eddington_experiment) is an observation during a total solar eclipse in 1919. They observed a star that was close to the edge of the sun observed that their position had changed because the gravity of the sun bent the light. You had to do it during an eclipse else the glare of the sun would make an observation of a star close to it impossible. The result is an observation that Newtonian mechanics can’t explain but general relativity

This was front-page news on most major newspapers around the world, it made Einstein and his theory of general relativity world-famous.

We still use Newtonian mechanics in most situations. Is a lot simpler to use and the result is almost identical on the human scale and the solar system scale.

On the solar system scale, the [Precession of the perihelion of Mercury](https://aether.lbl.gov/www/classes/p10/gr/PrecessionperihelionMercury.htm) is the change in position in the obit is it farthest from the sun. The difference between the two explanations is 0.012 degrees per century. This is the most significant difference for obits in our solar system.

You can observe the effects on earth too if you set up very accurate experiments. But for everyday usage the difference is negligible.

So Newtonian mechanics is used most of the time because it is simpler and the difference will be less the uncertainty of any other measurement. In some situations like you need to use general relativity. Blackholes or just light passing close to our sun are examples of where it is needed.

We also know that general relativity can’t explain everything because on a small scale quantum mechanics and general relativity do not match up. There is a lot of recharges that tires to combine them.