The weight of my shadow

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Hello dear friends.

Although the title of this post is almost poetic, the content of it has nothing of poetry haha, and is that the shadow is something that accompanies us almost all the time, but it is something that we do not really think about until we see a horror movie and then any shadow scares us. But is it something that weighs us down? It seems like a silly question since we don't even realize if it is present or not, how can it be heavy? A shadow is simply the absence of light that should reach a place but was blocked by something.


Does our shadow have weight?. Source: pixabay.com.

Put this way it makes no sense to think that the shadow has weight, it would be the same to think that light weighs, and light does not weigh, right? Perhaps the question is how we interpret the meaning of weight, since we always associate it with a number on a scale or what keeps us glued to the ground, and so it is difficult to imagine that light or shadow weighs, it is not that we can put our shadow on a scale.

Light is composed of particles called photons, which have no mass per se, but they do have energy and momentum, and when the light from a light source reaches us all the photons that compose it hit us as if it were a gentle rain of subatomic particles.


A photon is a particle of light that propagates in a vacuum. Source: pxhere.com

But the momentum of these particles is not very large, a body is able to transfer motion to another in a collision if the product of mass by velocity is sufficient to make the other move, so when the sun illuminates us, photons can barely exert a very slight pressure on us, which is called radiation pressure, then, when a body is exposed to a light source, this slight pressure is exerted on the illuminated area, while the shadowed side of the object, because the light photons do not reach it, does not perceive it.

The weight of the shadow

So, this difference in pressure can be related to the weight, remember that weight is a force, specifically the force applied by our body against the ground, so when our body is illuminated from above by the sun we apply more force on the ground product to the thrust of photons, in a way, an object weighs more when it is illuminated than when it is in the dark.

presion fotones.png
Photons exert a certain pressure on an object. Source: image elaborated in powerpoint.

In this way, the surface on which our shadow is cast feels a lower radiation pressure than the rest of the surface that receives the light that our body does not block, in other words, that pressure difference between the shadow zone and the illuminated zone is the effect of the pressure caused by our shadow.

This radiation pressure (in N/m2) is generally calculated with the following equation:

imagen.png

Where, I is the intensity of light, c is the speed of light and α is a parameter that takes into account the absorption capacity of the body, taking values between 1 and 2, being 1 for a perfect absorber (black body) and 2 for a reflecting body, it is considered that the human body absorbs half of the radiation it receives, so it is assigned a value of 1.5.

In the region where I live the average sunlight intensity is 6 KW/m2, and the body surface of a person like me (1.7 m high and 40 cm wide) is 0.68 m2, then the radiation pressure on my body is:

ec sombra.png

Then, the force of light would be equivalent to:

ec sombra2.png

If we convert this expression into mass:

ec sombra3.png

That is, illuminated by the sun and assuming that the area of my shadow has the dimensions of my body, the pressure defect caused by my shadow is equivalent to that produced by a mass of 0.0000002 kg.

It is an interesting fact to discover that the amount of light, or lack of it, could have an effect on the weight of objects, but beyond knowing this interesting fact is the applications it has, and is that it has been thought in the push of photons to move spacecraft using giant sails, just as the wind drives the ships with sails, solar sails would take advantage of the pressure exerted by the photons to move the ships in space. NASA will test this means of propulsion with the ACS3 spacecraft.


Ships with solar sails could be the future of space travel. Source: wikimedia.org.

In less futuristic uses, a team of scientists won the Nobel Prize in physics in 2018 for the invention of "optical tweezers" a method used to manipulate tiny objects using the pressure of radiation originating from a laser, being used to trap and move these objects with great precision without touching them, something that could have many applications in the research of biological processes.

So you know, whether to build the spacecraft of the future or ways to manipulate objects without touching them, knowing the pressure exerted by photons of light has more applications than just knowing the weight of light, or our shadow.


Thanks for coming by to read friends, I hope you liked the information. See you next time.



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6 comments
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Hello friend @emiliomoron

Independent to any scientific interpretation, yes I believe that our shadow has weight, I liked the way you approached this content with your academic fundamentals, great post.

Best regards, be well.

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Hi friend @lupafilotaxia, thanks for stopping by to read, glad you liked the content, and yes, I guess I hadn't thought about it until reading about it, certainly interesting. Regards my friend.

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This was a cool and interesting reading. Thanks for sharing.

Please do not hesitate to add more references. This is something we enjoy a lot at STEMsocial. Back to the topic, ACS3 will be interesting to follow (I didn't know about it). I guess "staying tuned" is the word here ;)

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Hi friend, glad you enjoyed the post, I'll keep your suggestion in mind, thanks. And yes, we will keep an eye on ACS3's performance, it will be interesting to find out what it can achieve. Best regards!

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