Light Beam fired 1 AU into space

[Molloy]

Thanks for being here, this is my first post.
Imagine on a dark night a very powerful beam of light was aimed at the sun from earth
and switched on, at 23:00.
I am observing from side on, in space, at a distance far enough away to get a good view of the beam
as it travels out into space on its 7-minute journey.
My question is: Would I be able to see the beam actually moving as it headed along?
My only comparison would be from a distance watching a train moving along a track.
Would this perspective, perched in space, allow me to "watch" that solid chunk of light
moving along ? I am trying to vizualize what a very powerful light beam "moving through space" would actually look like.
Thank you.

 

[Nugatory]

You only see light when it enters your eyes, so you have to be in its path to see it.

If there is a cloud of dust, water vapor, raindrops or the like in its path, then some of the light will be scattered off to side and may get to your eyes. That's how we can see the beams of searchlights at night. Even then we don't see the light moving, but that's because it moves so fast. Switch the searchlight on and we immediately see a solid column of light maybe three kilometers high; switch it off and the column disappears. That's because light travels three kilometers in about ten microseconds, much too short of a time for us to catch the moment when the searchlight has switched on but the first light from it hasn't yet traveled three kilometers.

 

[Asymptotic]

"Dark night" means from that position on the Earth's surface the sun is outside of your line-of-sight. How can the light beam be aimed at it?

 

[davenn]

"Dark night" means from that position on the Earth's surface the sun is outside of your line-of-sight. How can the light beam be aimed at it?

exactly

tho what the OP said is that they are not on the Earth ... they are at a point somewhere between the Earth and the Sun

I am observing from side on, in space, at a distance far enough away to get a good view of the beam
as it travels out into space on its 7-minute journey.

But your point still stands ... On a dark nite on earth, where the light is ... it cannot be aimed at the sun ... It's going to go out into the solar system somewhere

@Molloy needs to rethink his/her question

I am observing from side on, in space, at a distance far enough away to get a good view of the beam
as it travels out into space on its 7-minute journey

you need to think about that statement as well
Nugatory was referring to atmospheric dust etc ... here isn't likely to be enough of anything to reflect some of the light in your direction off to the side of the beam
Dave

 

[sophiecentaur]

This is an essentially practical question. In principle, of course, such an experiment could work but you would need a powerful enough light source to ensure sufficient scattered light gets back to be measured. This is a catch 22 situation because you need some scattering of light on the way (in order to see it) and that means that the beam would be attenuated and would have less power for it to be seen when scattered at the far end.
I could propose an alternative experiment, which could use the light from a Supernova, being reflected by gas clouds at various distances from it. The light from many nebulae is due to light from nearby stars and a good sized supernova would easily be seen as a flash of increased intensity. All you would need would be to be looking at the right time to see 1. The Nova, 2, Reflection from one nebula, 3. Later reflection from another nebula (or a distant part of the same nebula). If the light path is more or less straight across our field of vision, the timing differences would be observable. Distances out there will be many AU which would be large enough to see the difference time profile of the observed flash.
Of course, you have to ready with your Nova spotter and have time to look at likely nebula. The principle of this is so well established that I doubt it would be judged a worthwhile experiment to tie up a suitable (satellite?) telescope.

That's because light travels three kilometers in about ten microseconds, much too short of a time for us to catch the moment when the searchlight has switched on but the first light from it hasn't yet traveled three kilometers.

The sort of response time that a detector would need to see that motion would probably be practical. It only requires a slew rate of, say 0.1 μ (ball park) and a suitable position from which to observe the passing pulse of light. (Assuming that your laser pulse can be short enough, which is not a problem.)

 

[anorlunda]

I speculate that you can do this experiment, but at an entirely different scale of distance and time.

I am thinking of a supernova explosion. Some of the light reaches us on a direct line. But some of the light illuminates nebula some distance from the event, we see the reflected light from the nebula some time later; perhaps years or centuries later.

PF members better at astronomy than I am might be able to point to some photos of such reflected light.

 

[Molloy]

Thanks for all these great replies.
The point about it being "dark night" and therefore "how could you fire it at the sun" made me laugh, and shows the kind of mistakes laypeople make when coming up with these questions. In fact, I didn't need to mention the sun at all. It seems that by talking about "dispersed light" and the fact I can only see light when it is reflected off other particles has answered my question and I didn't know this fact. Is this the same for all light? That if we are not in the direct path of the beam (torch light in eyes for example), we can only see it because it illuminates other particles and reflects off them?

Yes, I was not on earth. I was "side on" standing as a spectator in space. My idea (and hope) was to get some real concrete idea of "light speed", by watching an incredibly focused beam of light from a sufficient distance to be able to see the actual "beam" as it heads like a train along a track through space. Because of my distance, I would be able to see it moving slowly and steadily towards, say, point B in space, say 1 AU away, doesn't have to be the sun. Could be anything.

Let's change the light source to a very powerful concentrated laser beam, in this case. Would that make any difference? I know we can observe light from Supernovas, at great distances, and the light scattered on gas for light years around, thank you. But I am focusing (no pun) on the idea of being able to watch a concentrated beam "moving" in space, not reflected light. If you are saying that I can only see this beam if there is some dust for it to reflect off, then I suppose your answer to my question will be no, it's not possible,

Thanks.

 

[A.T.]

If you are saying that I can only see this beam if there is some dust for it to reflect off...

... or if the beam goes directly into your eye. Or how did you think our eyes work?

 

[Molloy]

... or if the beam goes directly into your eye. Or how did you think our eyes work?

When I point a small laser beam from a simple laser torch at night, I can see the beam clearly. I thought a light beam was a stream of photons. If there is no "dust" in space for that beam to reflect off, will that beam be invisible to me, if I am watching from side on, in space?

 

[anorlunda]

If there is no "dust" in space for that beam to reflect off, will that beam be invisible to me, if I am watching from side on, in space?

Yes

 

[PeroK]

When I point a small laser beam from a simple laser torch at night, I can see the beam clearly. I thought a light beam was a stream of photons. If there is no "dust" in space for that beam to reflect off, will that beam be invisible to me, if I am watching from side on, in space?

Suppose it is a dark night. You can see the moon and the stars, some of which are planets. You can see the moon and the planets because the Sun's light reflects off them. But the Sun's light is everywhere else as well, where there is no moon and no planets. If you could simply "see" the Sun's light without its being reflected or scattered towards your eye, then the whole night sky would be lit up, except in a region where the Earth would cast a shadow.

The whole inner solar system is constantly bathed in the Sun's light, as bright as it is on Earth on a clear day.

 

[sophiecentaur]

When I point a small laser beam from a simple laser torch at night, I can see the beam clearly. I thought a light beam was a stream of photons. If there is no "dust" in space for that beam to reflect off, will that beam be invisible to me, if I am watching from side on, in space?

Yes.
Also you don’t see the sunlight streaming from behind the Earth until it hits the Full Moon. The sky in between looks Black on a cloudless night with no street lights. Something has to get in the way in order to direct some light into your eye

 

[Nugatory]

I thought a light beam was a stream of photons.

It is not, at least if you're thinking it's analogous to the way a river is a stream of water molecules moving along. Nothing awful will happen if you use that model in this particular problem (although it will mislead you in many other problems) but nothing is gained either. Light is electromagnetic radiation, and photons only come into the picture when you do a quantum mechanical treatment of the electromagnetic field.

If there is no "dust" in space for that beam to reflect off, will that beam be invisible to me, if I am watching from side on, in space?

That is correct. On the other hand, the human eye is remarkably sensitive so it doesn't take a lot of dust to scatter enough light for you to see it.

 

[davenn]

. It seems that by talking about "dispersed light" and the fact I can only see light when it is reflected off other particles has answered my question and I didn't know this fact. Is this the same for all light?

Yes

My idea (and hope) was to get some real concrete idea of "light speed", by watching an incredibly focused beam of light from a sufficient distance to be able to see the actual "beam" as it heads like a train along a track through space. Because of my distance, I would be able to see it moving slowly and steadily towards,

no, you won't see the beam of light ( when positioned in its path) till it arrives at your location ... till then, nothing from the source.

Let's change the light source to a very powerful concentrated laser beam, in this case. Would that make any difference? I know we can observe light from Supernovas, at great distances, and the light scattered on gas for light years around, thank you. But I am focusing (no pun) on the idea of being able to watch a concentrated beam "moving" in space, not reflected light. If you are saying that I can only see this beam if there is some dust for it to reflect off, then I suppose your answer to my question will be no, it's not possible,
Thanks.

That only makes it more difficult because the "beam" of light is more concentrated and you will need to be more accurately positioned within the beam path so you see it when it arrives at your location
Dave

 

[Redbelly98]

Thanks for being here, this is my first post.

Welcome to PF.

My question is: Would I be able to see the beam actually moving as it headed along?

Now that we've cleared up that you would only see light that gets scattered from dust or other particles in the beam's path, let's think about the phrase "as it headed along".

It turns out you would not see the scattered light from the beam as it headed along. There would be a time delay from when the light hits any particular particle to when it reaches your eye, in accordance with the time it takes light to travel the distance from that particle to your eye.

If we are presuming this distance is much longer than 1 AU, then this time delay will be much longer than 8 minutes. If the distance is comparable to, say the distance between the sun and Saturn, the delay would be over an hour.

 

[Molloy]

very interesting replies thank you, though to me (as a layman) some of them counter-intuitive. :-) It looks as if I am never going to see my light beam traveling through space, unless...some dust is scattered all the way along its path in space, and in that case, we will see it...I hope

Doesn't the last post by RedBelly98 only mean that there would be a delay when I saw it, but I would still perceive it, when it reached me, as long as it was moving through dust or gas etc?

 

[Molloy]

Welcome to PF.
Now that we've cleared up that you would only see light that gets scattered from dust or other particles in the beam's path, let's think about the phrase "as it headed along".

It turns out you would not see the scattered light from the beam as it headed along. There would be a time delay from when the light hits any particular particle to when it reaches your eye, in accordance with the time it takes light to travel the distance from that particle to your eye.

If we are presuming this distance is much longer than 1 AU, then this time delay will be much longer than 8 minutes. If the distance is comparable to, say the distance between the sun and Saturn, the delay would be over an hour.

I think I understand now about light only being visible if it is reflected off dust and gas. The same way I can see the rays of sunlight when they hit the smoke from my camp fire or the dust in my room.

But, even if the light takes some time to reach me, as i sit side on in space, waiting for the powerful concentrated beam of light to be switched on, even if I only perceive it later, it will reach me and when it does, as long as there is enough dust and gas in its path, I will see this beam moving in a straight line through space. No ? :-)

The whole point of me theorizing about this, is because I was hoping that my distance from the beam, in space, would give me the thrill of not only seeing light when it "reached" me, as we do normally, but to be able to see the beam itself "as it moves" and as it cuts its way through that gas and dust.

 

[Redbelly98]

But, even if the light takes some time to reach me, as i sit side on in space, waiting for the powerful concentrated beam of light to be switched on, even if I only perceive it later, it will reach me and when it does, as long as there is enough dust and gas in its path, I will see this beam moving in a straight line through space. No ? :-)

Yes. :-)

 

[A.T.]

I think I understand now about light only being visible if it is reflected off dust and gas. The same way I can see the rays of sunlight when they hit the smoke from my camp fire or the dust in my room.

It is generally true, that you can only see light that hits your eyes.

But, even if the light takes some time to reach me, as i sit side on in space, waiting for the powerful concentrated beam of light to be switched on, even if I only perceive it later, it will reach me and when it does, as long as there is enough dust and gas in its path, I will see this beam moving in a straight line through space. No ?

Yes, but there can be some distortion due to the different times the scattered light needs to reach your eyes from different points on the path.