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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.

   

Recent comments on this video
Comment icon #16 Posted by 029b10 on 6 February, 2014, 11:26
give him a few minutes, he might figure it out....
Comment icon #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.)
Comment icon #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.
Comment icon #19 Posted by Eldorado on 6 February, 2014, 22:53
Am thinking I'll put my shadow on a diet.
Comment icon #20 Posted by toast on 6 February, 2014, 23:04
Nope. Imagine the weight of a noise and you will have an idea about the weight of a shadow.
Comment icon #21 Posted by acute on 7 February, 2014, 0:01
You mean... instead of dieting and exercise, I can lose weight by closing my eyes? .
Comment icon #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 ha... [More]
Comment icon #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.
Comment icon #24 Posted by Leonardo on 7 February, 2014, 14:20
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 force on you in a certain direction because of the momentum of the photons. That force is not 'weight' - which is what we call the affect of gravity on a mass. The combination of forces affect the measurement, but that total is not your 'weight'.


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