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Experimental set-up

In air, when there is light propagating in a direction, we can still see it even when it is not primarily travelling in our direction, because a small part of the light hits the air molecules, and changes its direction; it travels towards us.

Does this mean that, in a vacuum, you would not be able to see light which is not travelling towards you?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – tpg2114 Apr 30 at 12:34
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If the light has nothing to scatter off of to reach your eyes you won't see anything.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – David Z May 1 at 8:32
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No. Light has to physically interact with the sensors in your eyes for you to be able to see it, and likewise for cameras, too.

The reason you can see "light beams" in a terrestrial environment is that in the atmosphere, some of the light can be scattered so it does get into your eyes. In vacuum, this does not happen.

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  • $\begingroup$ It should be noted that very little light is scattered by clear air. It's air containing dust and moisture particles that will scatter the light. $\endgroup$ – Hot Licks Apr 29 at 0:30
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In a perfect vacuum, not only can you not see light that isn't traveling toward you, you can't even see light that is traveling toward you until it actually reaches your eyes.

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    $\begingroup$ "...you can't even see light that is traveling toward you until it actually reaches your eyes." Isn't that the case even when there's no vacuum? $\endgroup$ – user258881 Apr 26 at 18:34
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    $\begingroup$ @FakeMod True, but I felt it needed to be emphasized in this case, that it's the photons actually reaching the back of your eyeball that cause you to be able to "see". $\endgroup$ – HiddenWindshield Apr 26 at 18:40
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    $\begingroup$ @FakeMod pretty sure although nothing can travel faster than the speed of light in a vacuum, being faster than the speed of light in a particular medium is possible (see Cherenkov radiation). So it's in principle possible to see that the light is traveling towards you before it actually reaches your eyes. $\endgroup$ – Allure Apr 27 at 4:54
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    $\begingroup$ "it's the photons actually reaching the back of your eyeball that cause you to be able to see". Once again : as opposed to what? How else can you see anything? Sorry, I don't see the added value of your answer or comment, given that the question specifically mentions vacuum and the properties you talk about don't appear to be linked to vacuum. $\endgroup$ – Eric Duminil Apr 27 at 8:37
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    $\begingroup$ @Allure Wait, but wouldn’t the mysterious phenomenon travelling faster than light in a particular medium either have to be a) light in which case it is the light travelling towards you hitting your eyes or b) not light, in which case you wouldn’t able to see the light travelling towards you before it hit your eyes? $\endgroup$ – 11684 Apr 28 at 11:25
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Light must reach your eyes/detectors one way or another - no matter vacuum or not. It may be directed there in the first place or scattered somehow.

Then again, scattering is a complex matter.

Both your eyes and the light source have some edge diffraction so SOME light will get into your eyes (given enough time to propagate) no matter where everything is directed. If the source is strong enough and the eyes sensitive enough, you will see something.

"Vacuum" is relative, too. Even the intergalactic space has some atoms flying around. There is also a cosmic microwave background, so you can expect some photon-photon scattering (really hard to observe, but has rather strong theory backing it).

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You can only see light that arrives at your eye, whether in a vacuum or not. In air, when you 'see' sunbeams, for example, you are actually seeing light that has been scattered from the sunbeam towards your eye. In a perfect vacuum there would be nothing to scatter light, so you would see only light that has travelled directly to you from its source.

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  • $\begingroup$ And the light that is scattered by the sunbeam is so scattered because it's hitting water droplets in the air. $\endgroup$ – Hot Licks Apr 29 at 0:31
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Thinking through this by attempting to distill it to its essence.

"Seeing light" is another way of saying, "photons are striking my eye." That is, in order to see light, photons must strike my retina.

Now, consider a single photon within the beam. In order for my eye to detect it, the photon must change direction and strike my retina.

This could happen if there's matter for the photon to bounce off of. But, in this case, it's traveling through a vacuum, so there is no matter, and the photon will never strike my retina.

Perhaps there's one way this could happen - if the photon passes through a mass' gravity well, veering toward my retina. There could hypothetically be a vacuum surrounding the mass, but in practice the vacuum won't be perfect. But of course, at that point the light is now traveling toward my eye.

So, the answer is "no." Light consists of photons, a photon must strike my eye to be seen, and a photon that strikes my eye is light traveling toward my eye.

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In a universe that is shaped like the three-dimensional surface of a basketball whose space would be expanding at a rate initially almost exponential but eventually only quasi-inertial, like each of the local universes in at least one "bouncing" and inflationary cosmology (Nikodem J. Poplawski's "cosmology with torsion", described in numerous papers written between 2010 and 2020 that are available free on Arxiv), photons would (since they each have an infinitesimal relativistic mass) be orbiting through the curvature of the aforementioned surface's volume, so that you would eventually be able to see even light that had been emitted from the curved region to your rear, if you'd somehow wait (and survive) long enough after having somehow made an appearance on the scene.

However, regarding a question (at https://astronomy.stackexchange.com/questions/19013/statistical-techniques-for-estimating-distribution-of-mass) on the Astronomy Stack Exchange, Pela's answer describing the most standard procedure for estimating mass distributions shows a disproportionately large distribution of mass in the brightest and most red-shifted clusters of galaxies, which suggests that the probability that any photon you might see would not have been involved in one or more refractions might be very low.

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As the other answers state, the answer is no, if there is nothing for light to scatter off of, to change its direction towards you, you will not see anything.

I would like to add, that it might be interesting to consider two things:

There is no perfect vacuum, and the real vacuum in our universe does have certain particles, some of which might scatter light towards you.

Does vacuum (empty space) exist?

Curvature (extreme) can exist even in vacuum. You might be surprised to realize that light that originally seems in 3D space to not be moving towards you initially, might end up actually moving towards you because of spacetime curvature. Examples are gravitational lensing, or photons orbiting black holes (like the photon sphere).

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    $\begingroup$ The curvature argument is very clever but still boils down to light directed at you. $\endgroup$ – candied_orange Apr 26 at 22:27
  • $\begingroup$ @candied_orange correct, we should have defined what we mean by "directed at you". It might have a different meaning in curved spacetime. I will edit. $\endgroup$ – Árpád Szendrei Apr 26 at 22:28
  • $\begingroup$ It could be argued that all spacetime is curved. It's just a question of how much. $\endgroup$ – candied_orange Apr 26 at 22:29
  • $\begingroup$ @candied_orange of course, I meant extreme curvature. $\endgroup$ – Árpád Szendrei Apr 26 at 22:30
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    $\begingroup$ In both cases light is traveling towards you. $\endgroup$ – user76284 Apr 27 at 0:30

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