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# How much does a shadow weigh ?

## 25 posts in this topic

How much does a shadow weigh ?

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

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Get Hank Marvin in and just stand him on some scales! (lol.... sorry)

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I didn't watch the embedded video (no sound here at work) but it might be possible if you could "weigh" the photons (if that's the right word) that hit the lit areas and compare to the impacts in the shaded

areas... the difference would be the "weight" of the shadow...

(Unless I'm wrong of course. In which case - never mind)

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I didn't watch the embedded video (no sound here at work) but it might be possible if you could "weigh" the photons (if that's the right word) that hit the lit areas and compare to the impacts in the shaded

areas... the difference would be the "weight" of the shadow...

(Unless I'm wrong of course. In which case - never mind)

In that case I would think you'd get a negative number.
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I didn't watch the embedded video (no sound here at work) but it might be possible if you could "weigh" the photons (if that's the right word) that hit the lit areas and compare to the impacts in the shaded

areas... the difference would be the "weight" of the shadow...

(Unless I'm wrong of course. In which case - never mind)

Brilliant deduction, Watson

I agree with you, shawdows which are formed by the absence of visible light which is massless, you can't be wrong because if light had mass how could it pass through a window since no two bodies of mass can possess the same body of space at the same time, and seeing that they are in themselves a massless form of visible light which become visible from the absence of mass and energy. Therefore, massless being the absence of mass, shadows therefore have no weight. I don't know how you figured all that out but your absolutely brilliant.

Like Aloe says, your the man!

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In that case I would think you'd get a negative number.

Sure... just disregard the negative sign.... 3 from 2 is -1 ... 2 from 3 is 1.... same difference...

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hmmm ... in that case .. do we weigh less in the dark?! (lol)

Edited by lightly

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Let me rephrase what I wrote earlier... The difference would not be the "weight" of the shadow... The 'weight' would be the actual value of the photons hitting the shaded area (from other light

sources or reflections, etc - because a 0 count would be absolute darkness )... The difference would simply indicate how much the primary light source was putting out...

And while Photon's do not have a mass (for practical purposes), they may be able to be measured (counted) when they strike an object...

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after all that.. the weigh of michael's arm/hand shadow, weigh roughly the same as michael's brain

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Photons do exert a force and that force can be measured in small fractions of a gram. That is at least how I think certain theoretical spacecraft would travel between stars.

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A shadow cannot be weighted as it is not a physical body that is subject to the effects of gravitation/acceleration (+ or -).

The word shadow is just a description for an area that absorbs less photons than the area around it during observation.

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A shadow cannot be weighted as it is not a physical body that is subject to the effects of gravitation/acceleration (+ or -).

The word shadow is just a description for an area that absorbs less photons than the area around it during observation.

True... but you can quantify the amount of photons that strike an area and from that determine the "shadowiness" of an area... hence the "weight"... Sort of a "grey scale of shadows"...

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if a shadow has any influence on a scale at all it would be a negative one because less photons hit it. So, the right question would not be: "how much does a shadow weigh?" but "how much does light weigh?".

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The correct would still be nothing, cause Light ain't heavy, that's my brother!

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The correct would still be nothing, cause Light ain't heavy, that's my brother!

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.

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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"...

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give him a few minutes, he might figure it out....

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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.)

Edited by lightly
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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.

Edited by freetoroam
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Am thinking I'll put my shadow on a diet.

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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.

Edited by toast

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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?

.

Edited by acute
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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), that mass is virtual - understood when considering the mass equivalence of energy (which photon's have.)

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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.

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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 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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