# 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)? Dec 20, 2016 at 17:37

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 Dec 20, 2016 at 18:58
• @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. Dec 20, 2016 at 19:36
• @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. Dec 20, 2016 at 20:49
• @LaserYeti and what's the fundamental difference? Dec 20, 2016 at 21:04
• @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. Dec 20, 2016 at 22:14

What, like this? Resolved astrophysical jets are not unusual. In this example, showing the region near the centre of the active galactic nucleus of M87, relativistic particles are emitted in a collimated beam from bear the central black hole. The particles both produce radiation and interact with material to produce more radiation.

Whilst originating from a jet of particles, there is certainly an intense beam of radiation emitted along the path you can see.

• I see nothing on this image relating to the question... he is asking whether reality is conforming with seeing those star wars like laser beams at sufficient distance...
– AnoE
Dec 20, 2016 at 21:43
• @AnoE This is astronomy SE. If the question isn't asking for astronomical examples of cases where one can see the beginning and end of a beam going from A to B (in this case what you see does trace the path of beam of synchrotron radiation, though not it's end), then it is off topic and would be better on Physics SE. Dec 20, 2016 at 21:59
• True, @RobJeffries.
– AnoE
Dec 20, 2016 at 22:17

Answers above will explain seeing with naked eyes.
But you can film light beam for sure
See this amazing TED video of a light beam, filmed at trillion frames per second.