Do Photons Cast Shadows?

Do Photons Cast Shadows?


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!

100 comments

  1. light is always a wave never a particle, because wave can interfere with itself, but not particle
    light wave can support each other and also cancel each other

  2. Awesome and informative video, but man, the compression is way too high on vocals… we can hear the interactions of all particles in your throat.

  3. Wait, photons pass through eachother without interacting? So two photons can occupy the same space at the same time?

  4. "electromagnetic waves can't directly interact with themselves" I thought it was electromagnetic waves interacting with themselves that explains Young's double slit experiment. What am I missing?

  5. But the rate at which earth is hit by gamma rays shouldn’t be changed because rays that otherwise wouldn’t hit the earth are also deflected toward the earth.

    At least I think

  6. — Are you coming to bed ?
    — I can't. This is important !
    — What ?
    — MinutePhysics is POSTING on the Internet !

  7. Guys you do realise science programs existed before Vsauce right?
    Anyhoo more importantly, I'm troubled by the statement that photons would have a gravitational force. How is that possible since they don't have mass, am I missing something?

  8. Can you please ask your brother if he can stop exercising playing guitar when you record a new video?
    And yes, photons are so fast that you have to apply relativity. The photon is long gone when its effects become active.

  9. Pretty sure that since what whe normally think of as a shadow can only exist because of light, which is made up of photons, they cast shadows by definition, but whatever

  10. This is incorrect. Light acts as a wave and that wave can interact with its surroundings causing and interference pattern. This does not mean light produces a shadow. A shadow is created by an object blocking a photon from hitting a surface. Photons do not cause the wave to interfere with each other meaning there would be no visible shadow behind the photon on the surface. This is true for all wavelengths of light.

  11. Better question… do photons only project light in their direction of travel..? Or does the back of a photon emit light traveling at lightspeed in the opposite direction of travel..?

  12. Its not clear what you mean when you say they "crash into" a low energy photon. If you're referring to the same kind of particle/anti-particle "bounce" that you said can create shadows, then you would expect there to be as many "bounces" directed toward us as away from us, as the universe is isotropic on the largest scales where the CMB pervades, meaning there should be no preference for the away-from-us bounces and therefore any shadowing effect should be exactly canceled out by the "lighting" effect for the toward-us bounces.

  13. Slightly off topic & all over the pace but does anyone know for sure off hand ? : So, can/do photons interact with other photons ? I had thought no, but it seems you suggested yes but extremely rarely/weakly ? & If yes, does that ever go for photons of the same energy/wavelength. I was aware of extremely high energy gamma rays spontaneously creating electron positron pairs, does that also occur at lower energies, but perhaps less frequently, like with hard X-rays for instance, soft, UV etc. ? The microwaves from the CMB are just "blocked" by the spontaneously created e-/e+ from high energy EM & not really their photons ?

  14. Wait, if photons don't experience time, how is it that they "spontaneously" anything? What is it interacting with that allows it to undergo a change?

  15. "There are plenty of objects that are good at blocking light and casting shadows . . ."

    I know there's a Yo Mama joke in there somewhere.

  16. Before I even watch this redundant video. Does fire cast a shadow? Come on, how many people here couldn't work that out?

  17. To get a bit metaphysical and philosophical, the source of Light doesn't cast a shadow on anything. Instead, it radiates its light on all things so they may be revealed

  18. 2nd example: Light bends around the sun due to mass of the sun.

    Light or photons will not bend around another photon as photons have zero mass.

  19. How do photons move in the sun? I've heard it can take an excessively long time for a photon to travel out of the sun once generated. How is it interacting long enough to do that?

  20. Hmmm… How about the two-slot interference? The destructive interference at least to me appears a better shadow than any of the mentioned 3 candidates.

  21. Particle light doesn’t work…. particle gravity doesn’t work.

    First rule of physics is you need and object. What is an object?

  22. Light is not electromagnetic wave ! Light can only repeat it self on a medium (electromagnetic wave is such a medium among the many other ! So no light can not cast shadow ! The medium tho can

  23. As for light speed ? It doesn’t exist ! If light had to move from point a to point b ? There wouldn’t be enough energy in the universe to power all the light !nope light repeat it self on a medium ! What is the max repeat distance ? I can assure you it’s very long ! this is why science thing black hole is what it is. ! Here is what I say ! close to black hole there is no medium , light can’t repeat it self without medium ! You end up with black hole ! Given the amount of energy light would need to move ? There would be a mass or a weight something ? And there is nothing

  24. hey friend I think you have made an error. pair production begins for ~1MeV photons but this is for the field of a nucleus. it’s around 2MeV for electron field. Pair production for my understanding cannot occur in a vacuum since there is a reference frame for which conversation of momentum would be violated there (the photons move at the speed of light for all reference frames but the resulting pair does not, so you need another particle to balance the momentum for all reference frames). Perhaps my understanding is off – always willing to admit I don’t know things. Here is an easy reference for energies at which processes dominate – http://rcwww.kek.jp/research/shield/photon_r.pdf – so in your words you always need a “convenient” particle located at the right place no matter what, which means it doesn’t cast a shadow by your definition. (This is your definition of course, feel free to change it)

  25. Particle fever looks neat, certainly going to check it out!
    Also, I came across a neat book: Particle Physics Brick by Brick: Atomic and Subatomic Physics Explained… in LEGO!
    Figured folks here might also get a kick out of it.
    Amazon link below, but I'd also suggest going to your local bookshop if you can, because they're awesome 😉
    https://www.amazon.com/Particle-Physics-Brick-Subatomic-Explained/dp/0228100127

  26. What about Kugelblitz black holes? They might not be technically feasible, and probably don't exist yet in our observable universe, but they theoretically could cast shadows.

  27. Yeah….. Nope. That all too common picture of a "photon" being a "little packet of waves" scooting across space is incorrect. The fundamental premise behind this video is therefore flawed. There is no distance and no time in the boson frame between an emitting fermion and the absorbing one in the electromagnetic (or any other) bosonic interaction. Which happens to be one of the fundamental consistencies between relativity and QM transtemporality.

  28. but dont photons that have opposite waves from eachother cancel eachother out? wouldn't that also count as casting a shadow?

  29. I could cross light paths n make a shadow in the light streaming through of photons bouncing off each other being the only source of the shadow. Using only a varnished door and door frame at the right angle… For anyone wondering the smaller the gap the darker the shadow but the shadow is always the same width of the gap.

Leave a Reply

Your email address will not be published. Required fields are marked *