What *is* a photon?

Quantum experiments at home: youtube.com/playlist?list=PLg-OiIIbfPj3mDFx5zjVPtg… Clarification: I didn't mean to imply that the squiggly wave (gaussian wavepacket) isn't real light. It is! Pulses of light are actually way more realistic than (approximations of) the infinite "photon" states in real life. All I'm saying is that these aren't photons. That's because photons can only have a single wavelength (i.e. colour). In this video I talk more about realistic waves of light versus plane waves: https://youtu.be/uo3ds0FVpXs?si=l_ygoQ9Jh-etGlGR Here’s an update about my “why light slows down in water” videos in the series. At the end of that video I was optimistic that my simulation kind of showed light slowing down- but it was hard to tell. A lot of extremely kind people offered to improve my code to see if the effect was real. It turned out when they ran it for much larger times, that the simulation didn’t show light slowing down. That means something is fundamentally wrong with my simulation, but I don’t know what. Separately, a bunch of people suggested I look up the “Ewald–Oseen extinction theorem”. That looks very very promising, but not super easy to understand. (If you understand it, I’d love to hear about it!) All up, I’ve decided to put that question out of my mind for a few months, since I’ve spent a lot of time on it. I do want to revisit it though. Thanks everyone for being so supportive!

I'm a 67 yo electrical engineer, going back to try to learn all of the physics that I was supposed to learn in college. You're asking the exact questions that I've been asking. But, you're making MUCH more progress than I am. Keep up the good work... this will serve you well... and you're doing a great job helping the rest of us... young and old!

I appreciate the fact that you are presenting material that isn’t the same recycled rehashed “quantum mechanics is weird, look at this double slit experiment etc“ that lots of other people just present over and over.

Please don't abandon the experiments, they're amazing to see when they work as you journey from theory to reality! Love this series so much, got me thinking and kept me up late more than once.

I get excited when you tell us about a way you WANTED to do a thing because I know I'm about to see critical thinking in action and that's one of my favourite things about your channel. Too many people are too ashamed of not getting a thing right the first time to show their diagnostic processes and that's a real disservice to the world when you're bright enough to solve problems and would rather act like you're so smart that you never have problems to solve instead of showing how to think through things and never give up on the learning process.

SO EXCITED to hear you reference Huygens Optics because... that's been the only YouTube channel discussing light that has made any sense to me. The slit experiments, in particular, seem to be a jumbled mess of contradictions when YouTube animators try to explain them. I'd love to see you and Jeroen collaborate; at the very least, would love to see you walk through some of his videos to tease out the harder points.

I did that photoelectric effect lab for undergraduate physics lab. Had to be done overnight as you're measuring pico amps and nothing, nothing can be allowed to interfere with the measurement process. The first time, at 2:30 am, a large (in the US) semi-truck (lorry) went by the building, vibrations ruined the trials for the night. The following weekend, at around 3:30 am, some grad students decided to stop by their offices to pick up some books. Vibrations from the building elevators killed the experiment. Third time's a charm. Finally. Tough lab. Not as invasive as the Rutherford experiment. One entire floor of physics building had to be cleared for most of the day to do that lab (I didn't choose that one).

You're always so chipper, it was kinda nice to see the frustration that comes with setting up and attempting an experiment.

Nothing original here, just emphasizing what others have said in the comments - I love this series. I can't get enough of these videos. Unlike other physics channels, I feel like I am going on the journey with you, not getting blasted with look-what-I-know content. Your channel is pure gold! I actually recreated a couple of those experiments at home and while simple, they do bring about this feeling of awe like look, its really true!

I studied Theoretical Physics in uni, but never finished (got too distracted by too many interests across all of science), and ended up as a software developer. Fast forward 30ish years and my interests in all sorts of sciences never disappeared, including Quantum Mechanics. I definitely accept the results of QM theory shown in counter-intuitive experiments - but I never "got it". And then you come along and with some simple diagrams and a few steps of explaination, make the whole wave/particle duality "click" in my mind.

I love Feynman's "it comes in lumps."

I am genuinely happy to see that you give us your own explanations, using words and phrases that you came to by your own reasoning and experimenting (even if the experiments "failed"). Most people only repeat textbook phrases like a parrot, without really understanding them (yet they think they understand them). This series is a very fresh look into the topic and I highly commend you for the work you put into it.

Regarding the hair experiment... I didn't notice a lot of "frizz" of your hair. Perhaps your hair has a protectant on it? Sometimes it can be a hair product, or perhaps the humidity at your location is preventing static build up? (Are you sure you are making a good static build up?)

22:05 Another good explanation for why is because of Heisenberg's Uncertainty Principle. This states that the uncertainty in the energy and position of a photon are related, and the more certain you are about one, the less certain you become about the other. It's related to the usual principle between momentum and position because, when factoring in special relativity, a photon's momentum entirely determines its energy by E = pc (energy-momentum relation in SR), so the uncertainties for both properties coincide. So the reason why you get more colors after a measurement is because, when measuring photon position, you decrease the uncertainty in position and increase the uncertainty in energy as a result. Since you are less certain about which energy the photon is at, you get a superposition state of all possible energies that photon could be at, weighted by the probability the photon is in each energy state, which is where the extra colors come in. It even also explains why an infinite light wave is not practically possible, because this would imply perfect certainty about the energy of a photon and thus infinite uncertainty in the position (which is why it is then distributed across the whole universe), but this is an impossibility by Heisenberg's principle, which states there is a minimum amount of combined uncertainty.

It looked like the room was lighted with essentially white light. There will be some blue which may cause the electrons to simply go away. Try again in the dark or under a darkroom light (red).

Subbed. This vid has a very "veritasium" vibe to it. Have never heard things explained like this, and you answered questions that have lived in the back of my mind for years.

I really appreciate the clarity and enthusiasm you have put in this video. Your description of the detector provides the same explanation of why the Hanbury-Brown and Twiss experiment works, which is the way in which we test for a single photon source in the lab! Very very cool video

Very beautifuly explained. Thank You!

I know that single-particle states are usually defined as momentum eigenstates, but iirc you can have single-particle states that are smeared across different momenta with varying amplitudes such that you have, for example, a single-particle state with definite position.

I really appreciate the summary of your understanding of light. That helped me consolidate what you taught us and left me feeling confident that I understand light a little better now.