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I'm new to physics and am just going through some of the free online classes at World Science U, and after watching this video on the nature of the speed of light and its constancy, a question came to mind about photons. (Video: YouTube Video, World Science U course)

I know that photons don't have mass, but what happens when photons — even the photons from distant stars — reach us? Are we merely observing the occurrence of photons moving through space relative to us, or are we really being "bathed" in photons? I know that when I observe rain, I can both observe it from a distance but could also be immersed in it as well if in the path of that rain. But with distant starlight, are we just observing it or are the photons actually reaching and penetrating the earth around us? If they are penetrating, does science tell us what is actually happening on an atomic or sub-atomic level?

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When you see a star, a tiny piece of it's energy (the photon) has been blasted out, flown for millions of years through space at half a billion miles per hour, and then hits you in the eye! When a particular spot in your eye absorbs enough of them, it sends a signal to the brain, which you interpret as "seeing a light". –  Mooing Duck Apr 21 at 17:58
    
@MooingDuck: if you see a star with the naked eye then the photon hasn't flown for millions of years through space. It's in another galaxy then at best you'll "see" the galaxy as a dot or fuzzy spot. Or I dunno, each photon came from a particle in the plasma at the surface of the star, so I guess you could claim to be able to see a proton 2 million light-years away if you like. Surprise your optician ;-) –  Steve Jessop Apr 22 at 2:44
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Obligatory xkcd: xkcd.com/1342 –  Twinkles Apr 22 at 11:27
    
Huh, big distance / big speed = 8 minutes? Bummer, I really thought it was a big number too :( –  Mooing Duck Apr 22 at 14:50
    
@MooingDuck that's awesome! I kinda wish I would have taken this stuff up in college way back when. I find this stuff really fascinating. –  michael t Apr 23 at 14:21

5 Answers 5

up vote 21 down vote accepted

Yes, the photons actually reach you, like rain falling on you, not like watching rain from a distance. When you see a star, photons from the star actually enter your eye.

In for example rods of your eye, the photon causes a molecule of retinal to react by change from cis to trans isomer.

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Oh wow, that's wild. So in a related question then, do the molecules in the rest of our body (apart from the retinal molecules in our eyes) have reasonably similar reactions as well? Sorry for the questions, just trying to wrap my mind around photons and what they do/are and how we react to them. –  michael t Apr 21 at 15:49
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@michaelt well molecules have a tendency to ionize when photons hit them. Really depends on how strong the photons are. Gamma rays will give you cancer for instance, visible light does nothing. It is really nothing special about our body, just usual chemistry and physics. –  Sabyasachi Apr 21 at 15:56
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Thanks so much to both of you. This just really ignites my curiosity even more. Much appreciated. –  michael t Apr 21 at 15:58
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after a photon is absorbed by a molecule, it no longer exists. Its energy is transferred to the molecule. –  DavePhD Apr 21 at 16:22
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michael t - And let's not forget the most common effect. Go outside during daylight, and feel the warmth of the sun on your face. That is the effect of photons from the sun. –  WhatRoughBeast Apr 21 at 16:37

A photon is emitted by a star, travels in a straight line* and enters your eye when you look at that star. You see the star continuously shining because photons are continuously being emitted, so you can't actually tell when each photon enters your eye. It's like a constant flow of water as opposed to dripping.

You can't see photons in the same way you can see rain from afar. You can only effectively feel the rain when it hits your skin, i.e. you can only see stuff when the photons emitted or reflected from that stuff enter your eye.

*Strictly speaking gravity can bend the path of light, but that's general relativity

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"Straight line" of a variety you describe has a name to it: en.wikipedia.org/wiki/Geodesic –  oakad Apr 22 at 1:37

I know that when I observe rain, I can both observe it from a distance but could also be immersed in it as well if in the path of that rain. But with distant starlight, are we just observing it or are the photons actually reaching and penetrating the earth around us?

When you're immersed in rain, you interact with it directly: you get wet. That's similar to what happens when we see light.

When you observe rain from a distance, you interact with it indirectly: you don't get wet. Instead, you're interacting with light which has previously interacted with the rain. If there were no light (or sound, etc.), you wouldn't know the rain is there until you immerse yourself.

So can we interact indirectly with light? It turns out that we can't. Light waves don't interact with each other; they pass straight through each other unchanged, for example if we shine one beam of light through another beam (although they do interfere with each other in the place where they meet!). In terms of particles, we say that photons are bosons. Compare this to, for example, spraying one stream of water through another: they'll crash together and the water will fly off in a different direction (we say the water particles are fermions).

I suppose I should mention that, when the light has enough energy, it is possible to get an interaction: known as pair production, but this is a rather unusual consequence of relativity and quantum mechanics.

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I know that photons don't have mass, but what happens when photons — even the photons from distant stars — reach us?

Small portion of photons is absorbed by atoms which are part of molecules which are part of retina. The energy of these absorbed photons is transformed to electromagnetic impulse. The produced impulses reach brain unit responsible for image recognition. After it recognized, for example, the star on the sky, the brain (mind) reacts correspondingly.

In your case, when photon reached your eye, you asked stackexchange which caused thousand of views :)

Some other photons are absorbed by body and their energy is transformed to heat (in case is star is Sun).

And lots of photons is dissipated and reflected by body. It means during daylight (star=Sun) or in the deep night (with no electric light) all you see is a reflected light from stars (including moonlight).

Are we merely observing the occurrence of photons moving through space relative to us, or are we really being "bathed" in photons?

We are really being "bathed" in photons. We are being bathed in visible and invisible spectrum of photons.

I know that when I observe rain, I can both observe it from a distance but could also be immersed in it as well if in the path of that rain. But with distant starlight, are we just observing it or are the photons actually reaching and penetrating the earth around us?

They are reaching and penetrating us and the world around us. Even you see object near you your eyes and brain react when photons reach you and the brain's image recognition unit creates picture for you and illusion that the object is 'there'. So in rain case there are two thing: rain itself (the other story) and light which corresponds to the rain visibility. In latter case it is just star (Sun) light reflected and refracted by water drops.

If they are penetrating, does science tell us what is actually happening on an atomic or sub-atomic level?

Science tells us the one kind of energy is transformed to another one. Light as electromagnetic kind of energy if transformed to heat when it is absorbed. Or it can be reflected or refracted. On atomic level, the atom changes its energy level to higher or lower if it takes or emits photon. When photon is absorbed it magically disappears and atom becomes hotter...

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Thanks a ton for answering! I really appreciated the in-depth answer and taking on each question. This stuff is all so fascinating to me. Wish I would have studied this in college. :) –  michael t Apr 23 at 14:27

If they are penetrating, does science tell us what is actually happening on an atomic or sub-atomic level?

You only have or see "photons" when the interaction process between the electromagnetic field (light) and matter (you) breaks spontaneously the time-reversal symmetry. This can be expressed in different ways, like saying that the vacuum state in not preserved by the interaction. The localization of energy in matter only arises because the interaction process is not reversible. This can be due for instance when a discrete atomic level is coupled to a continuum of levels (in the above molecular example). The time evolution will be given by Fermi-golden rule for instance, the time evolution is linear (with a constant rate), the process is irreversible and energy from the light field is being built and localized within the atom. A highly destructive process which does not conserve the energy in the field. This is different from the interaction with two identical discrete energy levels where the interaction is reversible for instance (Rabi oscillations) and as long nothing "perturbates" the process you will not see photons but oscillations of the field instead (the wave nature if you want).

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