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# How much does a shadow weigh ?

Michael attempts to determine whether or not it is possible to actually weigh a shadow.

#15 Posted by Taun on 5 February, 2014, 20:22
We don't know that, so far we only know is that a photon ( the main component of light) is smaller than 1×10?18 eV/c2. That is about the smallest measurement we can make. But given that gravitation affects light it must have a mass. I think 029b10 was making a reference to the old song "He ain't heavy, He's my Brother"...
#16 Posted by 029b10 on 6 February, 2014, 11:26
give him a few minutes, he might figure it out....
#17 Posted by lightly on 6 February, 2014, 11:53
all electromagnetic radiation produces radiant pressure ... it pushes on things... i wonder if that force adds to the weight of an object being pushed/radiated on? But i would guess that the shadow itself weighs Nothing... the space between the object casting the shadow and the shadow itself is where we might find a 'difference'? ... as compared to the surrounding areas containing more light? ... if a tree casts a shadow, but there is no one there to see it, does the sun exist? LOL ( just a silly twist on that old thing.)
#18 Posted by freetoroam on 6 February, 2014, 21:17
Ok, do not know much about photons, so this may sound like a silly question, but hey, I still want to ask it: Without a background surface, there would be no shadow in the first place, whatever the light, so IF a shadow did have a weight, would it not have to be based on the weight of the surface it has shadowed on? Eg: a shadow on a curtain would weight less than a shadow against concrete.
#19 Posted by Eldorado on 6 February, 2014, 22:53
Am thinking I'll put my shadow on a diet.
#20 Posted by toast on 6 February, 2014, 23:04
Ok, do not know much about photons, so this may sound like a silly question, but hey, I still want to ask it: Without a background surface, there would be no shadow in the first place, whatever the light, so IF a shadow did have a weight, would it not have to be based on the weight of the surface it has shadowed on? Eg: a shadow on a curtain would weight less than a shadow against concrete. Nope. Imagine the weight of a noise and you will have an idea about the weight of a shadow.
#21 Posted by acute on 7 February, 2014, 0:01
hmmm ... in that case .. do we weigh less in the dark?! (lol) You mean... instead of dieting and exercise, I can lose weight by closing my eyes? .
#22 Posted by Leonardo on 7 February, 2014, 13:58
The question "How much does a shadow weigh?" misrepresents the concept of weight in a scientific context. Weight is how we measure gravity's affect on a mass, and photon's have no mass. The reason photon's are able to exert a force is due to their having momentum*, which is quite different to mass. Measuring the difference in that force between light and unlit (shadowed) areas would simply provide the proof of this momentum, it would not indicate the shadow has 'weight'. * While momentum is calculated using the equation p = mv, which would imply the photon does have mass (the m in the equation... [More]
#23 Posted by Frank Merton on 7 February, 2014, 14:04
Thanks for that; photons can't have mass and still travel at light speed. But that is all beside the point. We are talking not about the weight of the light hitting the object casting the shadow but the weight of the absence of photons behind it. I hope that makes the answer obvious.
#24 Posted by Leonardo on 7 February, 2014, 14:20
Thanks for that; photons can't have mass and still travel at light speed. But that is all beside the point. We are talking not about the weight of the light hitting the object casting the shadow but the weight of the absence of photons behind it. I hope that makes the answer obvious. It's still a misunderstanding of weight. Take this scenario. You weigh yourself while standing in sunlight, and then weigh yourself while in shadow. If the scales were accurate enough, you would notice a difference between the two measurements. Does that mean a shadow has 'negative weight'? No. Sunlight applies a ... [More]

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