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 between photons. 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. It’s sponsor time! Which means Particle Fever promotion time! I’ve recommended it before because it’s
an amazing documentary about particle physics, and I’m gonna recommend it again because
I know not all of you have watched it on Curiosity Stream yet - otherwise why would they still
be sponsoring MinutePhysics videos? Anyway, in addition to Particle Fever, Curiosity
Stream has a deal going on right now - if you get a curiosity stream subscription (which
costs less than a cup of coffee), you now also get a bundled subscription to Nebula,
the streaming service made by and for educational youtube video creators, with ad-free viewing
and exclusive originals, like Real Engineering’s series on the insane logistics of D-Day. So go to curiositystream.com/minutephysics
(and use offer code minutephysics) to get your first month of curiosity stream and nebula
for free. Just think how many times you can watch Particle
Fever in a month!
