# Thread: At what distance/mass would light orbit?

1. I've found two answers to this, and so I don't know which one to go with, and I would appreciate any insight anyone can offer into the matter.

One answer is that it's the Schwartzchild radius (radius for the event horizon of a black hole), because it's theorized that light could be captured at the event horizon of a black hole, and stay there for some period of time before either being thrown out, or drawn in.

So: 2*G*M/C^2

Originally Posted by Arcane_Mathematician
it's the radius of the event horizon. Where the escape velocity is the speed of light. A photon will not orbit a black hole at the event horizon. it would skirt the hole and speed away.

some equations:

basic force equation.

The approximate equation for the force of gravity between 2 objects.

The equation for centripetal force.

the equation for the Schwartzchild radius.

Where F is the force, a is acceleration, is the mass of the object you are attracted to, is your mass, and is the schwartzchild radius of the object you are attracted to.
Given these useful tools, we can calculate the radius at which an object will endlessly orbit an object by playing with the first three equations:

and

so

notice that they tie into the fourth, like so
His reasoning is very sound, but it results in a radius exactly half the Schwartzchild radius.

So: G*M/C^2

Are there any other sensible approaches I could try?

2.

3. I believe that what your looking for is called something like the schwarzchild radius, i came across it in Kaku's book on parallel universes. He explains it as how black holes work. Eventually the gravity from the instance becomes so intense that light cannot escape. you should look it up

4. I haven't done the math myself, so I'm just guessing, but I wouldn't automatically think that light would orbit at the Schwartzchild radius. That's defined as the radius at which a beam of light could no longer escape, but an orbit would not be an escape trajectory, so while I doubt it'd be bigger than the Schwartzchild radius, I can't think of any immediately obvious reasons it wouldn't be smaller.

5. I say everything with very limited knowlege of the subjects so anything I say is likely wrong, but I will say it anyway.

From what it sounds like to me, the schwarzchild radius is the maximum distance from a massive object (black hole) that light will orbit. This doesn't negate the possibility that there is also a minimum distance from the massive object that light can orbit. There is likely a range, based on the mass of the massive object where you will find light orbiting. Any closer to this object, and the light will inevitably fall into it, any further and it will escape.

Similar how we have the habitable zone around stars. Any closer and it's too hot for life as we know it, and any further and it is too cold for life as we know it. But within the two parameters life can exist.

Too close, light falls in. Too far, light escapes, between these area's light orbits.

But again, I really don't know what I am talking about, just an idea that makes sense in my head.

6. Since light moves at a fixed speed, it could only possibly orbit a given mass at a fixed distance. There are multiple possible orbits for massive bodies because they can move slower as the move away.

Actually, a thought occurs to me... If the light had to orbit at a radius less than the Schwartzchild radius, then anything outside of that orbit would try to drift outward. But it's inside the Schwartzchild radius, so it couldn't. Unless there's a lot more to it than I'm imagining, that makes me think it would have to orbit at the Schwartzchild radius.

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