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I know it sounds like a foolish question but I have a reason for asking and I'm hoping someone here, can give a convincing response.

Here is why I pose the question...it seems to me that all this discussion of photons, and other sub-atomic particles possessing mass or negative mass is really a discussion about the mass of shadows.

Although we can't really weigh shadow we can see it, we can measure its area, observe its outline. a photon- which is massless - may seem heavier than shadow. It is really driving me crazy when I think about it.

Mohammad
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    You first have to define what is the shadow. For example, if the shadow is a surface, then we don't actually define a mass for surface in physics. – fffred Jun 14 '13 at 19:24
  • Maybe, a shadow in the wall makes it weigh less actually. Wondering the usage of the QM tag, I think the negligible value (though it's GR) can be taken into account :) – Waffle's Crazy Peanut Jun 14 '13 at 19:37
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    Bad ghosts have negative norm. – Trimok Jun 14 '13 at 20:02
  • This question mixes apples and oranges. subatomic particles are not the shadow of anything. – anna v Jun 15 '13 at 08:36
  • I guess this is what you are looking for http://m.youtube.com/#watch?v=Do1lm9IevYE&desktop_uri=%2Fwatch%3Fv%3DDo1lm9IevYE – Mark Jun 15 '13 at 07:50
  • What is interesting about this question is that objects with light on them experience force from the photons hitting them, this is how solar sails work. The larger the area, the more light it can capture, the further from the light source, the less it can capture. Objects in complete shadow do not experience this force, and thus may "weigh" less than the same object with light shining on it. The difference would be small, and it doesn't fundamentally change the object's mass, but since the force of a photon hit is measurable, the force on the object will differ in shadow vs not. – Adam Davis Jan 15 '14 at 15:16
  • @AdamDavis I know I learned that from the video #neomhmd shared, too bad guys here didn't like the topic. – Mohammad Jan 15 '14 at 17:09
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    @user689 I think the problem is that you didn't ask an answerable physics question. If you changed it to "how significant is the force of photons on an object" and then gave details about the size of the object, distance from light source, strength of light source, and any medium the light must pass through between the source and target, you'll probably get an answer. Even asking how much light or shadow changes an object's mass is unanswerable since it doesn't change mass. But going further and asking if shadow itself has mass is like asking if cold has mass. – Adam Davis Jan 15 '14 at 17:22
  • Cold and Darkness are merely the absence of something. So of course they don't have mass, they aren't objects. They are explicitly the lack or reduction of radiation. So you might need to change your question around a bit to make it fit, but I suspect it can be made to fit. – Adam Davis Jan 15 '14 at 17:24
  • @AdamDavis yes I don't think it would be reworded to fit the rules here but it was nice to know some information about this topic. I think it was worth getting downvoted – Mohammad Jan 15 '14 at 18:13

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The shadow of a body is the region where the light of the source doesn't arrive so since their isn't any photon (assuming that the body is 100% opaque) in the shadow, we shouldn't be able to see it. The only reason we see the shadow is by the contrast effect with the surrounding which reflects or scatters light. This is analogue to the way we see black holes in astrophotographs.

Another problem is what do you exactly mean by shadow? The definition I gave above is mathematical and we can't talk about mass until you define "shadow" physically. Note also that if a body is opaque for visible light, it isn't opaque for the rest of the electromagnetic spectrum and so there's a lot of invisible photons hitting the shadow area and if you meant by the mass of the shadow the mass of the photons present in this region, yes the shadow have a mass but I don't think you want this definition.

user5402
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I'll give only a funny answer:

I will prove by contradiction that mass of the shadow cannot be proportional to its area.

Take light source, ball and paper in empty space, this system as whole is closed and hence its total mass is constant. The ball makes a circular shadow on the paper. Now move the paper further from ball so make the shadow bigger. Now the shadow has higher area thus it has higher mass :O. Total mass of the system got higher but that is contradiction.

Edit: Ok I add the conclusion :D. Shadow of zero area should have zero mass right? But shadow's mass cannot depend on its area(as proved above) therefore shadow of any area should have zero mass.

Tom
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  • This only proves that the mass of the shadow is not proportional to its area. Where is the answer? – Mohammad Jun 15 '13 at 07:43
  • It does show that to first order a shadow does not affect mass by its size, as it should by any definition of mass. – anna v Jun 15 '13 at 08:34
  • One would expect that shadow's mass would be proportional to its area like paper's mass is proportional to its area. So I show that this cannot be happening. Paper don't get bigger and smaller when you move it around but shadow does! – Tom Jun 15 '13 at 08:58