### Can you see the starting and the ending of a light beam passing in the distance?

• This question arose to me when I saw a SciFi movie where they shot with laser guns and you clearly could see the dashes of light beams travelling from the shooter to the target. Nonsense of course, reality is much more boring.

But I wondered if you would just need more distance:

• Given a limited light beam traveling from A to B, where "limited" means that there is a start and / or end of the emission source causing that beam.

• Given enough distance from us to A, B and the line between them

• And add some kind of nebular smoke in between just enough to disperse as much light as needed so we can see it here but not as much to prevent it from reaching B.

Couldn't we observe the starting and / or the ending of that light beam, progressing on it's way from A to B, like a jet trail in the sky?

If it is possible, where can this be observed? Or do the laws of physics forbid such a thing?

Do you mean like a gamma-ray burst (GRB)?

6 years ago

Nothing forbids this, and it is actually observed astronomically. You need a very bright source of light: such as a supernova (which isn't a beam, but radiates in all directions) and very large distances. The flash of light can be seen spreading out from supernova in a circle, as it illuminates the dust and gas ejected by the supernova progenitor star in the years before

The effect is known as a light echo. Over time the "beams" of light move further out, and so the echo widens, as shown in this series of images (from an Australian telescope so you have to read from the bottom up, 507 days means 507 days after the supernova in 1987.) For scale, 5 arcmin is quite small: The moon is about 30 arcmin. source

Good thought, however nothing about a spherical shell resembles a "beam" as described by the OP

@LaserYeti I have to disagree with you, there's no real difference for the purpose of the question. If we want to consider a certain direction only, we could consider a sector of the spherical shell only, but it doesn't seem worth making the distinction.

@JohnDavis, given that the original question is "Couldn't we observe the starting and / or the ending of that light beam, progressing on it's way from A to B, like a jet trail in the sky?", I am going to say that it does.

@LaserYeti and what's the fundamental difference?

@LaserYeti, if we had placed an opaque screen around the supernova just before she went off, with a single hole in it, we would have had a beam, and that would be visible on these photos. q.e.d.

@AnoE, yeah, but that isn't practical or an answer to the question. See the answer from Bill K

@AnoE There would be a sequence of points of light, because the light from a supernova resembles a pulse rather than a continuous beam. At no point would you see something resembling a beam.

@RobJeffries The key point is that there should be a "start and / or end of the emission source", explicitly not a continuous beam. Reading the question I think that the shape of the emission is not key, but the "pulse" nature of it, so that a clear front and rear edge of the light is visible.