What *is* a photon?

This video was supported by Screen Australia and Google through the Skip Ahead initiative.

Links to other photoelectric effect videos:
There's so many other good ones! If you see one you think I should add to this list, please leave the name of the video and the channel in a comment!

Confining the light causes it to have more colours. This is explained well in an excellent video by Ben Miles. But at first glance, the experiment seems to be a totally different one than what I explained in the video- so I'll explain what the connection is. In particular, it's only the "single slit" version of the experiment that's relevant for us. I said that the researchers confined a laser to a small space. The technique they used to do this was to have two lasers- one which is the source, and the other which is used to turn on and off a "switch" of sorts. What the switch does is it makes the material in the experiment go from transparent to reflective very quickly, then back. The source laser is shining continuously at the material. But the idea is that for the short while that the material is reflective a little section of the laser beam is reflected. That's the "confined" light- they took a laser beam that's always on and constant and isolated a small section, confining the whereabouts of the light. They then measured the colours of that light and find it's spread out. (This result is at the 8 minute mark)

[LookingGlassUniverse]

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!

[EngRMP]

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!

[mbmurphy]

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.

[phillupson]

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.

[nathanieljames]

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.

[hatpeach]

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.

[kmunson]

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!

[WestOfEarth]

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

[SanderKonijnenberg]

For historical context at 1:32 : Einstein referred in 1905 to the photoelectric effect as one piece of evidence that light may be quantized, but the scientific community didn't generally accept the photon hypothesis until Compton scattering was discovered in 1923.

[AB-nuwe]

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).

[erikziak]

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.

[KipIngram]

Call it a quantum of light instead. The word "particle" really does mislead people who don't know how to think about it.

[marcodebruin]

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.

[reneejones]

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

[marcfruchtman]

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?)

[twonil]

This truly an award winning video. You are allowing other scienctific experts like myself (Chem PhD) to get insights into physics we did not have. We "know" how to use the photon concepts to understand our experiments. But this is truly understanding "near reality" model stuff. Thank you!

[InductorMan]

Very enjoyable video! The statements about a photon's spatial extent are incorrect and I hope I can add some constructive feedback.

A single photon state can absolutely exhibit a spatially/temporally short gaussian wave packet, and really can look just like the cartoon "wave packet photon". Think about using a femtosecond laser in a pump/probe experiment: the laser probes a transient chemical reaction (or whatever) during only an extremely short period of time. The video asserts that this type of confined temporal extent can only come from multiple photons (13:30). But what if we add neutral density filters in the femtosecond beam until only one photon per second on average comes through? Does the experiment still work, and still only probe the reaction during a brief time? Absolutely! Temporal extent and spectral width are complementary variables in QM (they embody longitudinal position and momentum of a photon), and Heisenberg's uncertainty principle tells us that a real (non-temporally-infinite) single photon state needs to occupy both limited temporal extend and some finite spectral extent. Every real single photon state effectively MUST have somewhat uncertain color!

Interestingly, Fourier analysis theory tells us something similar about classical waves: a temporally short classical wave packet must also have increased spectral breadth. I think the confusing part is that QM is telling us that energy is discrete, and we are very familiar with spectrally narrow sources in physics experiments, and so we tend to think of these discrete energy packets as having one color (and correspondingly large temporally extent). But QM never said that: it just says that the energy is discrete, and once you add Heisenberg into the mix it's natural that we don't actually know the amount of energy (color) with perfect certainty. It's just as valid to apply QM to a temporally short wave packet, to say that there are multiple photons' worth of energy contained in it, and to say that it necessarily doesn't have a single color. We just lose certainty about the exact energy of a photon (and maybe also certainty about the photon number? unsure on that one, but "squeezed light" is a very interesting thing to read about, and in particular the tortuous things which the LIGO collaboration are doing to light for gravitational wave detection).

There's another incorrect statement that needs to be addressed about plane waves (around 20:00). This is the second video in which this statement was made: something to the effect that a plane wave can only correspond to a photon of infinite temporal duration. Here, we're conflating transverse momentum and longitudinal momentum. The complementarity principle is a bit weird in this case. While the longitudinal momentum uncertainty (which is what we called spectral width above) is complementary with longitudinal position uncertainty (what we meant above when we talked about temporal extent), the transverse momentum (or beam direction) is complementary with the transverse position (or beam width). This particular complementarity actually doesn't differ between classical and quantum mechanics, QM doesn't actually have anything to add on this point. A plane wave is a photon state (or classical wave) whose direction is perfectly known, but whose transverse position is completely unknown: it's a beam that's infinitely WIDE, not a beam of infinite duration. This is all lumped under the heading of beam diffraction.

Anyway, hope this feedback is helpful, keep making these great videos and I'll keep enjoying them!

[johnpawlicki]

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).

[agxryt]

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.

[juanraa]

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