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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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2 Answers2

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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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