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We know any object which reflects light back(or obstructs its straight path) casts a shadow. We know the light is of dual nature i.e, both wave and a particle. So, is it possible to have a light particle bounce off of another light particle to cast its shadow? In short, can photons cast a shadow?
A Big No #
Yes, Light cannot cast shadows directly by bouncing off of each other. It is because light particles don’t interact with each other directly. In fact, this non-interactive nature of light is what makes our vision clear, our telecommunications and radio stations function without much interference. If they interact all these might not be possible. But fear not, for we have a few indirect ways by which light can cast shadows. Let us look into them.
Indirect Means #
Case 1: #
Light particles or photons can interact with electrons in space which can deflect another photon thereby casting a shadow of the former photon. But this is not very reliable because where in the seemingly infinite space can we expect an electron hanging around to interact with a photon? But this interaction can help cast shadows and is neatly summarized by the Feynman diagram depicted below.
Case 2: #
Light particles contain energy and momentum in them. This can cause the gravitational attraction between photons possible. But it is very minute that the heavily focused high energy beam of light crossing each other hardly interacts in such a manner. It is quite tricky for light to interact in this case, but the probability is still finite if not infinitesimal.
Case 3: #
High energy gamma rays can emit an electron-positron pair and annihilate them back and change back to photons. Now, these high energy photons can interact with weak photons available in CMBR present all over the universe left out by the light emitted during Big Bang. The below Feynman diagram can summarise such a possible interaction.
But this makes us think that perhaps it is because of this casting of Shadow that the space is very dark looking? Is it the availability of CMBR and these high energy photons which is casting a shadow and making the space look dark? How will the space look if we remove this phenomenon out? More on this in a future article.