Thread: what happens to light after you shine it?

if the area of spacetime curved back on itself and was empty, it would just loop around forever. In the real universe, Its intensity at a point will be inversely proportional to the surface area of a sphere whose radius is from the point measured to the point of emission. Also, photons that hit atoms will be absorbed into those atoms and bump electrons into higher orbits.

If you really want to get down into it, light doesn't actually do anything until it is absorbed. If you had a photon that was destined to never be observed, then you don't really have anything at all.

Originally Posted by NeoSioType

Like from a flash light? Of course it bounces off everything but it doesn't do that forever. Does it escape or dissipate?

Let's say I had a small area of curved space, a vacumm with nothing in it. Would light ever go away once it's shined? (Not that it wouldn't have anything to illuminate)

The light in this case would not go away if the space is curved back onto itself. The problem with this scenario is that whatever you are using to shine the light would be made of atoms and absorb some of the light.

Originally Posted by Xaqaria

If you really want to get down into it, light doesn't actually do anything until it is absorbed. If you had a photon that was destined to never be observed, then you don't really have anything at all.

I'm not sure I agree with that. You still have a wave function and the assurance that if you were to measure it, your measurements would be predicted by the wave function.

We're probably arguing semantics though.

I don't think any of you guys understood the question. I think the OP means what happens to the light when it heads towards infinity, when it heads towards a place without any matter anymore. Would it just go on eternally anyway? Would it not propagate there at all?

It was once an idea of mine: light only propagates if there is matter for it to propagate to. Light is a wave, and it behaves like a whip would. As long as there is still a transmittable piece of whip ahead, the wave carries on. If the transmittable area ends (end of the whip) the wave turns back on itself.

Like from a flash light? Of course it bounces off everything but it doesn't do that forever. Does it escape or dissipate?

It doesn't bounce off everything; nothing is a perfect mirror, and eventually it gets absorbed. Though parts of it can conceivably be retransmitted.

Let's say I had a small area of curved space, a vacumm with nothing in it. Would light ever go away once it's shined? (Not that it wouldn't have anything to illuminate)

Whilst it couldn't be observed, it would continue to propagate until it was absorbed. With nothing to absorb it, it would continue indefinitely.

when it heads towards a place without any matter anymore. Would it just go on eternally anyway? Would it not propagate there at all?

You don't need matter, you need space.

It was once an idea of mine: light only propagates if there is matter for it to propagate to. Light is a wave, and it behaves like a whip would. As long as there is still a transmittable piece of whip ahead, the wave carries on. If the transmittable area ends (end of the whip) the wave turns back on itself.

I appreciate you're using the past tense here, but obviously light does not behave in this manner.

Originally Posted by Photolysis

I appreciate you're using the past tense here, but obviously light does not behave in this manner.

Well, any other wave behaves in that manner...

The difference is that in such waves, it's the motion of matter that propagates. In this case, it's the oscillations of the photon itself as it moves through space. Hence why photons can continue to propagate without matter, whereas other waves, such as water waves, sound, or even those in a whip cannot.