There are plenty of objects that are good at blocking light and casting shadows; clouds,
trees, birds, tables, people, the moon.
But what about light itself?
Can photons cast shadows?
At first glance, the answer appears to be no.
Light - photons - are electromagnetic waves, and electromagnetic waves don’t directly
interact with themselves.
Other waves in nature - waves in shallow water, for example - can directly bounce off of or
at least influence each other . But not photons - they just pass right through.
That’s why sunlight doesn’t block cell phone signals, or human vision.
However, there are three indirect ways that photons CAN interact with other photons.
First, if a photon bumps into, say, an electron, and that electron bumps into another photon,
the photons will technically have redirected each other.
But this requires an electron to be in just the right place at the right time, so I don’t
think this really counts as a way for a photon by itself to make a shadow.
Second, just like how photons passing close to massive objects like the sun or a black
hole follow paths curved by gravity, a photon itself has energy and momentum and would technically
gravitationally deflect another passing photon.
But the gravity from a photon is ridiculously tiny - even the most energetic photon we’ve
ever measured had a smaller gravitational field than a strand of (virus) DNA . Which
won’t allow a photon to make a noticeable shadow.
But, third and finally, super high energy photons can spontaneously turn into particle-antiparticle
pairs (like an electron and positron), and then back again - and these particles can
deflect or absorb other photons, resulting in legit photon-on-photon scattering.
I say “legit” because the key here is that you don’t need to luck out and have
an electron happen to be passing by - two solitary (if high-energy) photons can spontaneously
generate their own means of crashing into or bouncing off each other.
So what kind of shadow do we get?
Well, photons only bounce off each other exceedingly rarely Even very very carefully controlled
experiments with ridiculously high powered lasers have a hard time observing any interaction
Which doesn’t sound promising for noticing a shadow.
But, there is one very real way that photons cast shadows.
Because space is so huge, super high energy photons traveling through it DO eventually
crash into one of the many (low energy) photons of the cosmic microwave background radiation
that are present pretty much everywhere in the universe.
And so, right now, you are literally being shadowed from ultra high energy gamma ray
photons by the photons left over from the big bang.
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