How much does my shadow weigh? – Ambientum Environmental Leader Portal


In the world of the ephemeral, few are the elements that accompany us wherever we go, every day of our lives, in almost all circumstances. Something we hardly notice is our shadow. The shadow is nothing but the absence of a light that was expected, but which does not reach its destination because it has been blocked by an object. Explaining what light is is not so easy.

In a simplified way we can say that it is made up of photons, elementary particles without mass but with energy and with “momentum”. This “momentum” is the ability of physical objects to push each other. When the photons that make up a ray of light shine on an object, they push it by exerting on it a slight pressure called “radiation pressure”. When we put ourselves in the sun, our body feels this pressure, while the area that we shade, that the photons do not reach, does not feel it.


weight of your shadow

We can quantify this pressure difference with weight, which is the force we exert on the ground, or on a scale. When we are illuminated, we exert a greater force than when we are in the dark, because the moment transferred by the photons that collide with us must be added to the force of our body. Thus, we can say that an object weighs more when it is illuminated than when it is not.

In the same way, the region where our shadow is is feeling less radiation pressure than if we weren’t there, blocking out the light. In other words, the excess weight we feel when lit is a defect in our shadow weight.

In the case of an adult person of average height, located under the sun at the latitude of Madrid, and assuming that the dimensions of his shadow are the same as those of his body, this weight defect in the shadow will be equivalent to that which would be exercised by a mass of about 0.00000004 kilograms.

(Here is the detailed calculation of the weight of the lampshade as well as the verification of the values ​​which can be found in other similar articles).

Beyond white light and mirrors

That’s not all: photons of light of different colors have different moments, so their energy and the pressure they exert will be different. This means that if we illuminate ourselves with red light, we will weigh less than if we do so with the same number of blue light photons.

On the other hand, just because we don’t see something doesn’t mean it doesn’t exist. When it comes to light, most of it is invisible to the human eye. This is the case of ultraviolet photons, like those of the Sun, which, in addition to tanning us, are more energetic than visible photons and, therefore, subject our body to a greater thrust.

In this way, the difference in weight with respect to the illuminated object is greater for the shadow that we do not see than for the one that we see. Curious, right? Do all objects react in the same way to radiation pressure, regardless of their properties? Of course not.

An object’s ability to absorb, transmit or reflect photons will also affect its shadow: if it is perfectly transparent, it will let the photons through and therefore not be too heavy. On the other hand, a reflecting object, a mirror, will feel twice as much thrust as an object that totally absorbs radiation (black body), by reflecting the photons that reach it.

From ladder to Nobel (and in space)

Our calculations on the weight of shadow and light are fun, but are they useful? The difference in weight between an illuminated object and an unilluminated object is tiny: one hundredth of the weight of a single grain of sugar. As a miracle diet, it looks poor.

However, these considerations were behind the 2018 Nobel Prize in Physics awarded to Arthur Ashkin, Gérard Moureau and Donna Strickland, for the development of “optical tweezers”, a method for catching and manipulating tiny objects using the radiation pressure of a laser. .

A laser light source, in which photons move coherently, as if coordinated, can be used to move objects with great precision.

The first experiments were performed in the 1960s by Ashkin’s team at Bell Labs. Researchers shined tiny, partially transparent spheres with a laser to move and levitate them, counteracting their weight with radiation pressure. .

Moreover, by focusing the beam with a lens at a point, they were able to trap the particles, thus creating the first optical tweezers. Over the following decades, they were perfected and made it possible to observe, rotate, cut and push the objects studied without touching or modifying them. Therefore, they are ideal for studying biological processes. Isn’t that enough? There is another field in which radiation pressure is also used, but on a large scale: space exploration.

As photon thrust depends on the size of the surface they strike, it may become relevant when considering a sufficiently large region. This is how the “solar sails” were designed: a revolutionary way of propelling aircraft into space, consisting only of a large surface that reflects sunlight.


Like the sails of a ship when the wind blows, these solar sails take advantage of the radiation pressure of the photons that hit them to make the plane move.

One of the great advantages of this propulsion system is the high speed that ships using it can achieve. Also, since they don’t have to store fuel to move around, they are lighter and can travel longer. Therefore, they are one of the few technologies that could be used for interstellar travel.

While these ideas may sound like science fiction, the first aircraft to use sunlight to alter its orbit around Earth was launched in June 2019 as part of an aerospace project called LightSail. NASA also plans to experiment with these new propulsion technologies in space with the 2022 launch of ACS3, a toaster-sized spacecraft that will use these sails for orbit changes.

Whether for futuristic applications or fundamental questions, what is clear is that if Peter Pan had been aware of his relevance, he would have been much more careful before losing his shadow. Luckily, we still can’t get rid of it.

Character font: Celia González Sánchez / Javier Rodríguez Laguna / THE CONVERSATION

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