# How Much Does A Shadow Weigh?

Submitted by: uatme 4 years ago in
108

More than I thought (well less).
108
Male 5,811
[quote]So, to answer pumba62`s question, yes; if the light was below you, you would weigh less as it would be slightly lifting you up.[/quote]

This is technically true, light does exert a force, but not at the same speed as the light, rather, with the same momentum as the light. The momentum (p) of a single photon of light is equal its energy divided by the speed of light (E = c p). So how much momentum does a beam of light carry? Well, suppose we shine a 100 Watt spotlight. We are producing 100 Joules of light energy per second. So every second we are giving that light a momentum of (100J)/(300,000,000m/s) = .00000033 kg m/s.

NASA actually has plans for prototypes of a "solar sail" to push space probes out of the solar system using no propulsion other than that provided by reflected sunlight.
Male 164
I LOVE VSAUCE
Male 553
Cool post. Enjoyed it.
Male 3,462
Yo` mama`s so fat, her shadow weighs 42 pounds!
Male 4,746
auburnjunky:
"@Patch:
The light would have to be concentrated, because if it hit you anywhere else, in any other direction, it would apply force there as well, right?"
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Partially true. The reflected light would have lost a large amount of its energy, after hitting the surface it reflected off of. Consequently, the energy pushing down would be more than the energy pushing up. So, it would still be pushing you down.
Male 13,624
More of this IAB!!
Male 4,893

Good post. Good change of pace for IAB. Lately it seems there is too much religious or left vs. right political flame war fuel.
Male 10,339
@Patch:

The light would have to be concentrated, because if it hit you anywhere else, in any other direction, it would apply force there as well, right?

Male 4,746
Tiredofnicks:
"Patchy: You ARE wrong. ;)

I`m not quite sure about the details and I can`t create fancy examples like yours, but I can post a link to someone who can:

The speed of sound in other materials"
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Well, there you go.

I hadn`t committed any effort into thinking about the problem. Good thing is though, my analogy with the magnetic marbles still stands up, even if the duration is influenced by the density.
Male 5,094
Patchy: You ARE wrong. ;)

I`m not quite sure about the details and I can`t create fancy examples like yours, but I can post a link to someone who can:

The speed of sound in other materials
Male 2,711
patch "I`m not sure if the duration of time would be altered by the density of the material in question."

Using your own analogy, wouldn`t your magnetic marbles push each other at different speeds in different media? If they are in viscous oil, they would push much more slowly than in air, or water, no?
Male 4,746
auburnjunky:
"Secondly, light doesn`t push up per se. It surrounds and falls all over you, making you heavier."
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That`s not correct, auburnjunky. It does in fact "push" you. Light is a particle/wave that carries energy. When it hit`s your body, some of that energy would be lost and it would be "pushing" you. So, to answer pumba62`s question, yes; if the light was below you, you would weigh less as it would be slightly lifting you up.
Male 4,746
I`m not sure if the duration of time would be altered by the density of the material in question.

I`d explain it like this. Picture a long line of magnetic marbles. When you push one toward the other, the second one is repelled and pushed into the third which in turn repels the fourth etc. While the time it take for the marble to travel into the other marbles magnetic field, and influence it, is very small, it would be measurable. Now, scale that down into the atomic scale and, instead of several marbles, imagine trillions and trillions of them, all lined up in a row. The "wave" you`d create by pushing the first marble would take a very long time to reach the end of the line. Same rule applies when it`s particles in a board of wood or a diamond. More marbles means the wave would travel quicker but take the same amount of time to reach it`s destination.

Of course...I haven`t given this much thought so I very well could be wrong.
Male 10,339
@pumba:

It`s "weigh" first of all.

Secondly, light doesn`t push up per se. It surrounds and falls all over you, making you heavier.
Male 1,016
so ...I way more when the lights are on ?
If the light emits from below does that make me lighter , because it would push up !
Male 10,339
Right tiredofnicks. Like, if it were made of Titanium, or some other hard material. Like diamond.

I know it`s not feasible.
Male 5,094
About the board, how fast the push gets there all depends on what material it is made of, "sound" or compression waves travel at different speeds in different media.
Male 4,746
Remember, auburnjunky, objects are not solid but made up of particles with spaces in between.
Male 4,746
auburnjunky:
"There was an interesting question in that video.

If I make a board a light year long, and push it to push a button on the other end, it would take 900,000 years to push the button, because my push only travels at the speed of sound.

Huh?"
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With a board that long, when you pushed the one end, the "ripple" of that push would move along and take 900,000 years to reach to other end. As he explains in the video, the objects we push, here on Earth, are very small so the results of pushing a board seem instantaneous. In fact, there is an imperceivably small hesitation on the other end. A hesitation that, when magnified billions of times, becomes quite large.
Male 4,746
Another great science video here on I.A.B.! Thanks uatme!
Male 10,339
There was an interesting question in that video.

If I make a board a light year long, and push it to push a button on the other end, it would take 900,000 years to push the button, because my push only travels at the speed of sound.

Huh?
Male 5,094
Haven`t watched it, but I think I know the answer; a shadow is the absence of light, therefore "weighing" less than the surrounding area.
Male 1,084