What *is* a photon?

[@LookingGlassUniverse]

Quantum experiments at home: kzbin.info/aero/PLg-OiIIbfPj3mDFx5zjVPtgiGwZMM4Erw 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: kzbin.info/www/bejne/q6CWlaZme7ujjtUsi=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!

[@DampeS8N]

Re: the failed demonstration. I noticed nothing was happening to your hair in the demo; I assume that because you were going to be on camera, you combed your hair before or something similar? I've noticed that you can temporarily deplete your head with similar demonstrations. Did you, maybe, brush out all the static?

[@zverh]

Great video but the failed experiment in the beginning renders the entire video kind of dubious.

[@classicalmechanic8914]

I always thought photons are like small balls with smiley faces and I have real existential crisis after realising photons don't have smiley faces 😂

[@patrickmeehan6856]

@@zverh "Failed" experiments are a part of science. She linked to videos which showed her experiment "successfully" demonstrated. The video's point was to explain that the photoelectric effect has quantum influences driving it and by understanding this, that light is fundamentally a particle. I don't doubt for a second that she rendered this video much more precisely and effectively than your misuse of the words "failed,", "renders", and "dubious" trying to shade her work.

[@SteveTose]

Thanks for the video. Sorry about you're experiment but I'm glad you explained it and it was neat hearing about it. I feel like I understand photons better now, somewhat anyway. And the infinite length and variable power explanations make cosmological redshift somehow less crazy ...

[@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!

[@TheMemesofDestruction]

We are eternal Students. ^.^

[@carpdog42]

Not an EE but a hobbiest. I just went back and watched the previous video about light moving through water, and spent half of it with a big smirk on my face thinking "Water is starting to look a lot like an inductor and so yes, the phase really does lag"

[@EngRMP]

@@carpdog42 Yup, I like the way you're thinking about this. A recent 3blue1brown about prisms and springs was especially pleasing (on the same topic) to me who works with antennas, radar and signal processing (Fourier transforms). We are so lucky to have these folks who can explain things so well, and will take the enormous amount of time required to create these videos.

[@BitwiseMobile]

I took CSE which ls like a dual major of CS and EE. This was in the 90s, and quantum theory was just starting to be taught. We had to take a required course on quantum mechanics. Honestly I didn't need it until semiconductors, and then it really helped dispel a lot of confusion. I was an AT in the Navy before I went to college, and so I was troubleshooting to component level while working in AIMD. I was completely lost when they discussed holes and electrons, but it doesn't matter for a technician. I just need to know what the voltage should be at TP1 :) - it's more complicated than that, but troubleshooting really is a process of elimination. As an engineer though you very much have to know these things, since it can impact your design at the end of the day.

[@kojo7485]

I studied Electrical Engineering at Wake Technical Community College, but I didn't finish. However, I plan to work at RedHat as a Software engineer 😀 this 🎬 year 🎉❤ Wish 🤞 me Luck 🎉😊

[@mbmurphy777]

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.

[@ГеоргиГеоргиев-с3г]

It's weird to people who use euclidean space-time, once you go wavy everything falls into place, cursory read of "Heisenberg uncertainty principle" repeated until you understand the concept, a moderate understanding of Furrier transforms and knowing that quantum states can be represented as a 3D normalised complex vector, is pretty much all you need to understand everything in quantum mechanics, only thing is quantum tunneling is achievable but you need to remember the part with H.U.P. and that you are allowed to apply it freely everywhere, so if you know the velocity with perfect accuracy you forget about the position and boom you have teleported on the other side of a perfect barrier.

[@alexbenzie6585]

How is any of that related to the comment you replied to lol​@@ГеоргиГеоргиев-с3г

[@LuisAldamiz]

@@ГеоргиГеоргиев-с3г - Quantum Mechanics uses Euclidean/Newtonian separate notions of space and time. And that's probably its limitation or one of its major limitations. QM needs to be reframed in terms of Einsteinian physics and stop all that Heisenbergian nonsense about casino physics.

[@carlosgaspar8447]

especially when you look at the original experiment there wasn't a double slit but more like a hair dividing the light beam.

[@Leland-y1d]

@@LuisAldamiz Wrong, dirac already combined qm with relativity

[@phillupson8561]

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.

[@garak55]

And extremely funny. Watching theoreticians struggle to interact with the physical world always puts a smile on an experimentalist's face :)

[@nathanieljames7462]

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.

[@KipIngram]

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

[@ChristopherSLucas-hv7nz]

A great idea

[@gauravarora4823]

or a bundle of energy

[@AB-nu5we]

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).

[@Khwartz]

Thanks for having Share Such Live Experience and Experiment 😊

[@LookingGlassUniverse]

I had no idea it was so finicky! How did people do these experiments originally? It seems like you need to be very precise and know what you're looking for

[@wbeaty]

@@LookingGlassUniverse some years after Volta they were using fancy Quadrant Electrometers, suspended by long vertical glass fiber, perhaps with a tiny mirror-chip, so a sunbeam can become a lever-arm. The fancy labs had powered electrometers, with 5,000V "zamboni piles" to charge the field-plates. Foil mass is critical, and in classrooms, kids get some stick-type chewing gum, to peel off the microns-scale aluminum foil from the wax paper. (Is that even sold anymore? Or imported cigarettes, wax paper packs w/foil.) Or, try a local artist-supply store, which usually sells packs of actual silver-leaf. First get it working with a UV lamp and clean zinc. The zinc will not work unless it has been recently sanded, bright exposed metal. Probably the same is true of any metal surface: ...atomically clean, no slight oil film, no oxide at all. (I bet a reactive metal will only work under dry nitrogen. In air, even the sanded zinc plate won't last long, before needing to be scoured again.)

[@wbeaty]

@@LookingGlassUniverse Everything starts working fine in the winter, with humidity down below 15%. Also, back in early 1800s, everything was brass, or sheet lead, sheet zinc, etc. You might notice that your electroscope would unexpectedly be discharged by bright sunshine, but only if the top electrode was zinc, and only if it was charged negative. Track it down, and you'd find that only the short ultraviolet part of sunlight will do it. (In classrooms they'll use a UV germicidal lamp, and freshly-sanded zinc plate. There must be no oil film, no oxide layer. Calcium would need to be freshly scraped under dry nitrogen, I suspect.)

[@solconcordia4315]

Oh, yes, here's the reason explaining why my undergraduate photoelectric effect laboratory experiment was one of just taking measurements *WITHOUT* doing any setup for the experiment.

[@WestOfEarth]

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

[@hatpeach1]

SO EXCITED to hear you reference Huygens Optics because... that's been the only KZbin channel discussing light that has made any sense to me. The slit experiments, in particular, seem to be a jumbled mess of contradictions when KZbin 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.

[@peterfireflylund]

3blue1brown also has some good videos on electromagnetism/photons.

[@LookingGlassUniverse]

I love his videos!! Some of the very best content on light available. Maybe one day we could make a video together :)

[@wbeaty]

@@LookingGlassUniverse The difference between Huygens Optics and everyone else is ...that guy doesn't worship textbooks. Instead, he'd rather be writing his own. He's realized that contemporary textbooks are full of misconceptions (mostly it's the high-school books, but I've seen some questionable things even in undergrad texts.) Also, track down Art Hobson's paper in AJP, called "There are no particles, there are only fields." He complains that the undergrad QM texts are filling students minds with wrong concepts about the nature of quanta ...which have to be unlearned when they take graduate-level courses. Quanta are not "particles."

[@peetiegonzalez1845]

Huygens is a gem. Also 3b1b as mentioned. So many underrated channels it's great to see them shout out.

[@carlosgaspar8447]

@@wbeaty i seem to recall that photon terminology did not exist when the photoelectric effect was first discussed.

[@kmunson007]

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!

[@LookingGlassUniverse]

Thank you so much!! Which experiments did you try? I'd love to hear about it!

[@marcodebruin5370]

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.

[@kenfryer2090]

Simple things please simple minds

[@erikziak1249]

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.

[@marcfruchtman9473]

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?)

[@ADHJkvsNgsMBbTQe]

I came here to make the same observation. It doesn’t look her hair tries to chase the plastic bottle after rubbing. Perhaps not enough static charge?

[@thevikifalcon7670]

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.

[@AB-qx3pf]

Everyone draws a light wave in 2D but it's a 3D probability map with all the uncertainty principle stuff built into it.

[@pablossm4742]

Yeah. You can also think about it as a Time-Energy Uncertainty. If you assume the photon always traves at the speed of light, the indetermination in position comes from when the photon exited the lamp in the first place. The more you know about the actual wavelength of the photon (and therefore, its energy) the closer to a plane wave and the less defined its position is, which means it can be detected on a broader range of times.

[@johnpawlicki1184]

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).

[@amarissimus29]

There is nothing special about white. It's just another color. btw I'm a postgraduate at Harvard. So I know what I'm taking about.

[@willo7734]

The way I finally started thinking about it is that light is a wave, but when it interacts with something it doesn’t interact along the whole wave. All the energy contained in the wave gets “zapped” down to a single point. It may not be completely factually correct but it makes the whole wave-particle duality seem less weird and mysterious to me. It really starts to get trippy when you think about matter interacting the same way on a quantum level.

[@Bearrtracks]

Love this channel. It's one of the most down to earth, creative and insightful math/science series on KZbin. Thanks to Mithuna, and please keep the content coming!

[@davidshlivko9594]

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.

[@twonil1]

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!

[@Guido_XL]

This is probably the best comment to this video yet! I felt that the video's explanation about wave duration and the relationship with energy was not complete enough, but I couldn't immediately find a better explanation, before delving into it. Thank you!

[@juanra31a]

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

[@lepidoptera9337]

That's a completely classical result. Not sure what you want to test there. I also don't know what you mean by "single photon source". All photons are singles, it doesn't matter where they came from. ;-)

[@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.

[@gurmeet0108]

There seems to be some confusion here. Nothing in the video requires "particle"-like nature of light. This is well explained in semi-classical picture. Please don't mistake me for saying light don't act like particle at all, it's just it isn't the case here. 1) Photo-electric effect DO NOT need the "particle"-like behavior of light (more precisely and technically, EM wave to quantize) - the approach where you require the electrons to follow Schrodinger equation and study the interactions with EM field WITHOUT quantizing it, is called semi-classical model. It covers the photoelectric effect. 2) Reducing energy below "h nu" is not enough - way below one photon energy, EM waves still acts like just classical wave. Fully explainable using semi-classical model. 3) Single photon source was first created by John Clauser in 1974 ("Experimental distinction between the quantum and classical field-theoretic predictions for the photoelectric effect". Phys. Rev. D. 9 (4): 853-860.) using cascade transition of mercury atom. Which, arguably, resulted in the Nobel Prize 2022, jointly with Alain Aspect and Anton Zeilinger, who beyond any doubt showed that quantization of light was needed. That is, it is possible to prepare an EM field state which acts in a way which is NOT accounted in semi-classical model and requires fully quantum treatment of EM field. 4) It's pretty hard to prepare single photon state and equally hard, if not more, to establish non-classical nature of such a state (basically, what Clauser-Aspect-Zeilinger did was no small feat) - One way to establish non-classical nature of such a state is photon anti-bunching. Note, very low energy light (even a minuscule fraction of "h nu") still DO NOT generally exhibit anti-bunching BUT single photon source (single photon fock state) do. 5) If you are wondering, ultraviolet catastrophe and discrete spectrum of atoms like hydrogen also DO NOT requires EM field quantization, that is, DO NOT require "particle"-like nature of light, just the quantum mechanics of electrons with usual classical light already covers all these.

[@imnewtothistuff]

And where did you click and paste that out of?

[@QuantumPolyhedron]

I would do you one up and say that nothing here actually requires the "wave"-like nature of light. There is a rather common yet not seemingly well-known way of interpreting the wave function in the literature which is to interpret it dispositionally, that is to say, it merely describes the propensity of particles to behave in certain ways under certain conditions. The actual observed waves are weakly emergent properties of large collections of particles with these dispositions rather than the particles actually existing as a wave-like entity that "collapses" at all.

[@gurmeet0108]

@@imnewtothistuff 😅, I'm not sure what do you mean... But it seems I should take it as a compliment so I will... 🤣

[@richardspectacular5327]

It's not that difficult to create a single photon source. Single molecule fluorescence and spontaneous parametric down conversion are two methods that come to mind. It's also not that difficult to measure the non-classical nature of the photon. Photon anti-bunching just means that sending a stream of photons through a beam splitter with a single photon detector in each arm doesn't result in click in both detectors at the same time.

[@davidofearth]

Sad that you jump to this. It is very obvious to a physicist that he did not cut-paste any of this. I think this is all solid/accepted. @@imnewtothistuff

[@brothermine2292]

Although the photoelectric effect proved the classical EM model was flawed, it did NOT prove that light travels as a particle. It proved only that when light is absorbed, a quantum of its energy is absorbed (converted into potential energy, and typically some kinetic energy, of the electron that absorbed it). It should have been immediately obvious to Einstein and other physicists that the photoelectric effect does NOT imply the light TRAVELS as a particle, BEFORE its energy is absorbed. Unfortunately, they had mental baggage -- what we now call the Locality assumption ("under no circumstances can mass or energy travel to a region of spacetime outside its future lightcone," which is still a very popular assumption) -- which led them to reject the otherwise reasonable idea that the quantum of energy may have been widely distributed in space a moment before the quantum of energy is entirely absorbed at a small location (the location of the electron). The Compton Scattering effect was similarly misinterpreted. It falsified the "classical" wave model of light, but it didn't really falsify the possibility that light is a "nonclassical" wave that has the quantum absorption property. It's a false dichotomy to believe the only alternative to the "classical wave" model is a "quantum particle" model. Looking Glass Universe posted a video about 2 years ago that claimed to provide a theoretical proof that light cannot travel as a wave. That proof depended on the Locality assumption: that if the energy of the wave is widely distributed a moment before the absorption event, then the energy cannot be entirely absorbed instantaneously (or nearly instantaneously) at a small location. The video didn't mention that Locality is only an assumption, and that Locality has been undermined by Bell's Theorem and experimental confirmation that entanglement violates Bell's Inequality. (Note: By "undermined" I don't mean falsified. I mean physicists have a stronger reason to doubt the assumption than they used to have.) I posted a comment to that old LGU video, about its dependence on the (unproven) Locality assumption. It's nice to see that LGU's thinking has evolved from that video's "travels as a particle" model to this new video's "actually travels as a wave, but absorbed like a particle" model. I hope a future video will revisit the old video's "proof," discuss its assumption of Locality, and discuss physics consequences of absorption's strong violation of Locality in the "nonclassical wave" model. For example, does absorption nonlocality violate the "No FTL Signaling" theorem? My intuition is that it doesn't... which ought to partially mollify Einstein.

[@kristianveselinov138]

I think it might be possible that your calcium is badly oxidized on the surface. It is a very reactive metal, and its surface is always covered with nonconducting CaO and CaCO3. You can try removing the top layer of the chunk with a knife (just be careful not to touch the metal or the scraps that you produce with bare hands) to expose the real calcium metal. It is very shiny and metal looking. Then, the photoelectric experiment might work. I am sorry if you have already tried this, and it didn't work. Nevertheless, your video was great and useful! I am a chemist who is also wondering what light is.

[@reneejones6330]

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

[@juliuseder]

Thank you for your explanation. Always wondered how a point-like photon particle emitted from a distant star is supposed to have no size but could potentially be observed at places that are light years away from each other.

[@dl5244]

For a single photon of light travelling (at c) across a distance, there is a time t0 when the intensity amplitude is zero (before the photon), a time t1 when the peak intensity amplitude is reached, and a time t2 when the intensity amplitude has returned to zero (end of photon). It is not the continuous (from the beginning of time until the end?) sinusoidal amplitude vs time function you showed. I think another confusion people have with these diagrams is they assume the x and y axis describe dimensions of space (ofc. the time axis is related to distance, but the intensity axis is not an indication of perpendicular movement)

[@ryanmccampbell7]

I think she was plotting the photos over distance, not time, although of course since they are traveling through space the time plot should be proportional. But also I guess her point about wave packets having impure frequencies also applies to changing over time; a photon with completely pure frequency would have to be infinite in space and time. I guess the Heisenberg uncertainty principle solves this.

[@bustercam199]

That's a good point as one can consider the amplitude as a function of time or its representation in the physical space of the phase. In either case, though, dl5244 brings up an important point in that even for a discrete value of the energy it has to start at zero, ramp up to a peak, and then return back to zero. She appears to be confusing a Fourier transform of a mixture of different frequencies with what is inevitable even for monochromatic light too.

[@ryanmccampbell7]

@@bustercam199 I think the only way it can be truly monochromatic though is if the amplitude transitions abruptly from 0 to 1 and back. Instantaneous frequency isn't really a thing for smooth mathematical functions, it's a property of the whole wave.

[@dl5244]

@@ryanmccampbell7 ofc. the distance in ONE dimension is proportional to the speed the light. But her animated diagrams implied at least two dimensions of movement in space as well as time. And two of those are wrong/misleading. The frequency of a packet (eg. photon) is always proportional to the time derivative of the amplitude function. You can tweak the signal (data) so that the abrupt change in frequency during the on-off and again during off-on is arbitrarily short.

[@anuman99ful]

That's not a photon

[@akaHarvesteR]

Holy cow this makes so much more sense than the usual 'mind boggling' explanations and graphics we all see everywhere. So then, could it be argued that, if the photon is only detectable from an electron interaction, that light itself maybe isn't necessarily quantised, and instead it's the energy required to pop electrons that is?

[@GreylanderTV]

You are unfortunately getting this wrong. A photon's momentum(i.e. it's frequency) can be as indefinite as it's position. If you precisely measure the position of the photon, it will be in a state such that it has no precise frequency. This does not mean it is "made up of many different photons of different frequencies". It is one photon, many [possible] frequencies. At most times the photon will be in the form of a wavelet, which has both indefinite position _and_ indefinite frequency.

[@narfwhals7843]

You're not being very clear about what you mean by the energy in a classical wave. Energy in what? One wavelength worth? One second worth? In classical EM you can really only talk about energy densities. And the energy _per time_ passing through some region will depend on the wavelength _and_ the amplitude. For a given amplitude and a given time green light will still deliver more energy than red light. In QM we have an inherent energy density formulation of energy per frequency=h. I think there are really two main things we can talk about as "photons". A ground state excitation, which is generally not physical. Or a single wave packet. But you talk about these wave packets as if they're made of many photons. They're not. There is only one photon worth of energy in there. The wave packet arises from a time/energy uncertainty. If this wave packet interacts and deposits one photon worth of energy(which, as you mentioned, _can have different values_ ) it will be gone. Consider what happens when an electron falls into a lower energy state and gives off electromagnetic radiation. The energy is quantized. It isn't infinitely spread out. Is this a photon? I don't think that, when you lower the amplitude of your wave you don't deterministically have to wait longer for a photon. That is again mixing classical views with QM. You lower the probability of getting a photon quickly. You, and Huygensoptics, seem to argue that there is no quantization in the EM field, but only in the interactions with electrons. I don't think that is a tenable position after photon correlation experiments starting from the 70s.

[@evilotis01]

this is *such* a good explanation of what being a "quantum of light" actually means. thank you.

[@charlesfranz9018]

I'm a 70 year old EE PhD doing same. It's interesting that bound electrons in materials are described by a discrete set of different standing wave frequencies, one for each energy level. I wonder if you could model them as cavities? Then there is thermal noise. The mysteries of the photon concept attract many students to physics and it is fun to reconcile as we see in these videos. Thanks for sharing. See "How big is a photon" on KZbin by Huygens Optics.

[@Halphase-xm4wl]

She gave a shout out to Huygens Optics near the end

[@charlesfranz9018]

"How Big Is A Photon" is directly related to this discussion, although most others are also highly worthwhile, the wave interference simulations in my own case.

[@lepidoptera9337]

@@charlesfranz9018 A photon is a small amount of energy. We don't teach anywhere that energy has physical size. Energy is like color... a property. Do colors have size? What is that even supposed to mean?

[@ArgumentumAdHominem]

This is really cool. But I must say I am even more confused than before, when I was blissfully ignorant on problems of reconciling the interpretations of classical and quantum electrodynamics. * One claim is that a single monochromatic photon is given by a plane wave, and thus does not have a position at all, as it is literally everywhere. Since that is not possible, you state that a single photon is a theoretical approximation that does not happen in reality. So what does happen in reality then? What are the fundamental building blocks of light in reality, if they are not photons? * I remember that for wave packets it was always instructive to look at water waves, as they exhibit exactly the same properties as electrodynamic and quantum waves, including Heisenberg's Uncertainty Principle. A wave on the surface of the sea can be decomposed into imaginary Fourier harmonics either in position-space, or velocity space. Each harmonic either has a fixed velocity and an undefined position, or vice-versa. Waves have a gaussian distribution of positions and velocities, and the relations between widths of those is given by a relation very similar to the uncertainty principle in QM. However, the interpretation here is rather simple. A water wave is a collective effect of a large amount of basic objects. These objects have different locations and have different velocities. One can attempt to define a single velocity or a single position for the collective object we call a water wave. What the uncertainty relations tell us is that any such definition will be more or less meaningful depending on how spread out the wave is in position or velocity space. Most importantly, the water wave has fundamental building blocks - the atoms of water, that oscillate in space, and thus the wave emerges from their collective behaviour. This is yet another reason why theories that prescribe wave-like behaviour to its building blocks make no sense to me. If waves are composite effects, how can a basic building block of a theory be a wave or behave like one?

[@DDranks]

I dont think photons are generally monohromatic, they have some distribution, uncertainty in time when they were emitted, and uncertainty in energy. I'll think a physicist trained in QFT would say that a fundamental building block is the field, which is something that can take different values at different positions. The wave function is spread over time and space, so a single photon can be spread all over and there's no particular restrictions how "much" or "little" of it can be here or there. It's when the field interacts with another field (photon field with electron field, for example) when the amount of energy transferred between fields is a quantum.

[@ArgumentumAdHominem]

​@@DDranks Thanks :). One day I will sit down and understand what QFT really implies about the world we live in. When I was in uni, it flew over my head. All the Bogolyubov transformations and infinite series felt like I was getting closer to the zen of "shut up and calculate" and further away from tantra of understanding it with my skin as opposed to my head. The first thing you said I have never really understood. I do understand the Heisenberg's Uncertainty Principle, in the sense that, we have lack of knowledge about a particle, and measurement of one non-commuting degree of freedom of it results in disturbance along another non-commuting degree of freedom (e.g. position and momentum). But this, if anything, tells us something about our ability to know the true properties of the quantum objects, not about their inherent uncertainties. Should I interpret your comment through entanglement? As in, when an atom emits a photon, they would enter into an entangled superposition state, where the energy of the photon is mostly defined by the energy level transition of the atom, but also to some extent defined by other modes of the atom such as vibrational and translational movement, which remains entangled with the actual energy of the photon until that energy becomes exact through measurement or decoheres ad infinitum. This part I also do not fully understand, namely whether everything is always entangled with everything, or if there are some rules under which entanglement may emerge.

[@mattferrigno9750]

I really like this channel. She explains things well and shows your examples without just spitting out a bunch of words I'm suppose to believe by showing pictures or diagrams drawn by someone else or a computer.

[@TimTeatro]

Well done! A gentle and encouraging constructive criticism from a fellow physicist: it sounded to me (around 1:20) as if you were promising to explain the mechanism of wavefunction collapse. Of course, this is something nobody understands yet: we don't know what happens to go from a wavefunction to a discrete measurement. But I love your perspective, I love that you're sharing your playful, curious and humble learner's eye, and I love that you do the experiments. You earned a sub from me, and I'm excited to see more!

[@Holobrine]

Your explanation of the photoelectric effect really makes it sound like the quantization occurs in the effect itself as the electron is ejected, rather than it being a property of light waves themselves. It’s like you’re slowly adding energy to all the electrons at once, but when one gets ejected, it abruptly uses all the energy that was adding up to do it and the others have to start over. What is a photon? It’s the energy required to eject an electron.

[@jyothishkumar3098]

"What is a photon? It’s the energy required to eject an electron." - that's intuitive. But what about photons of EM waves generated by freely oscillating charges? (AC radio transmitters use currents but it is comparable). That's what's confusing to me. The Single Photon Avalanche Detectors she mentioned makes it sound like when a photon chooses a grid to intersect with, it just immediately sucks out all the energy of the wave into that point.. weird but also funny :p

[@Holobrine]

@@jyothishkumar3098 I’m pretty sure every measurement we’ve ever made in quantum mechanics involved the photoelectric effect because that avalanche you mentioned is how the detectors work. It seems likelier that the detectors simply cannot detect anything weaker than a photon. Perhaps a breakthrough will happen if/when we discover another effect to build detectors with, that can measure weaker energies.

[@jyothishkumar3098]

@@Holobrine Yeah that should be it. I believe right now we're stuck with only having metre scales and having to hypothesize about what goes on at the millimeter level.

[@orchidejczyk]

Not sure how to agree your proposition with a situation like this: imagine an atom in excited state which after some times relaxes by emitting light - supposedly a photon? It surely can't be plane wave and probably not even monochromatic but we usually call this a single photon. Should we then think that this type of emitted wave is some kind of special photon with special shape and so on who can be found in kinda random places with distribution given by the amplitude of calculated wave? I know I mix here a bit of a quantum and classical picture but I feel similar about what you have said so I hope this makes a sense to you.

[@FunkyDexter]

A single atom emission happens in "beats" (look up "Beat (acoustics)" on wikipedia for pictures). Basically, while the fundamental oscillation of the emission is NOT monochromatic as you'd expect from basic fourier analysis, the enveloping frequency (the beat frequency) is. Remember that the emitted photon has energy equal to the DIFFERENCE between the atom's energy levels, like an acoustic beat is created from the difference between two frequencies. In a laser, the emission is constantly stimulated among multiple atoms arranged in a regular crystal lattice: the result is a plane, coherent wave.

[@drdca8263]

@@FunkyDexterHm, but surely the excited atom must eventually have (mostly) finished falling back down to the lower energy state? I imagine the beats you are talking about are due to the relative changes in phase between the “excited” and “not excited” (or “less excited”) states of the atom. So, is the idea that, the expected value of the photon number is gradually increasing as the expected value of “the atom is in the excited state” is gradually decreasing? And... hmm... so, I guess then there’s a larger non-repeating envelope (that maybe in theory takes an infinite amount of time to fully go to zero, but practically speaking gets very close to zero within a reasonable amount of time?) and within this envelope, there’s something emitted at a frequency of the difference of the atom energy levels? Is there a nice toy model of this which I could read about, where instead of an atom we have a quantum simple harmonic oscillator, going down an energy level, and emitting a particle? (By “emitting a particle” I mean “particle” in a sense such that it would be accurate to describe a photon as a “particle”.)

[@FunkyDexter]

@@drdca8263I am not sure what you're confused about, but I didn't talk about "expected values". The emitted photon is not monochromatic because the emission happens in a finite amount of time. You may think about it as a "pulse" instead, spreading in all directions. This pulse has a beat frequency. I suggest watching Huygens Optics for the experiment where he shows a macroscopic analog, i believe the video is called "Coherence part 3: This is not a wave"

[@drdca8263]

@@FunkyDexter I mentioned expected values because I was talking about observables in the context of a state which is not an eigenstate of those observables. And, I think I probably have seen that video? I’ve at least seen previous videos in that series. I don’t think a macroscopic picture will really answer the questions I have, because the questions I have are about like, mathematical details of the wavefunction

[@FunkyDexter]

@@drdca8263 the mathematical wavefunction won't dispel your doubts either. In fact, quantum mechanics doesn't describe light matter interactions, for that you need quantum electrodynamics, where the photon is seen as a "real particle". In normal QM photons are simply there to conserve energy and angular momentum between state transitions.

[@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.

[@eckligt]

It's great to hear the shout-out for Huygens Optics, I've enjoyed his channel immensely as well as yours! A collab between you two would be a dream to watch. The one thing I'm left wondering, having watched your video now, is how _time_ figures into the understanding of what a photon is and isn't. If an idealised laser creates single-wavelength light, and the wave coming out of it is infinite in space, then the photons exist along the whole path the light is moving along, and are thus not bound in space along the direction of travel. But you turn the laser _on_ at some point, and you turn it _off_ at some point, and if your laser is blue, then it might cause a photo-electric effect in whatever you are pointing it at. If you keep this blue laser running for a while, this photo-electric effect is pushing off electron after electron. Clearly this means there must be several wave-function collapses, and thus several photons in quick succession. How are the photons delineated in time?

[@LookingGlassUniverse]

Huygen's Optics is fantastic, totally agree! I love this question about time- I found this very confusing too. I don't think I've fully grasped it, but this is my current understanding. When you do the measurement of the approximate position (using the photoelectric effect or a SPAD), you're also doing an approximate time measurement too, since you know roughly when it arrived. In a similar way to how the position measurement collapsed the wave into a small part of space, the time measurement collapses it into a small bit of time too. Previously, each "photon" was totally delocalised in time and space, because it was spread everywhere and every time. Something like that... But I'm not sure.

[@JonBrase]

Time and energy are non-commuting observables, so there is an uncertainty principle for time and energy just like for position and momentum. The less time your laser is on for, the less monochromatic it will be (photons may be observed at different frequencies). Photons are only totally and uniformly delocalized in time and space if they are perfectly monochromatic. If the laser is on only briefly, so that the line width is very large, there's a probability for some photons that strike the target to fail to eject electrons because they have insufficient energy.

[@triffid0hunter]

@@ThePowerLoverHeisenberg uncertainty is an innate behaviour of physics that interacts with other physics all by itself even in the absence of quantum wavefunction collapse, there's no hidden variable that we're simply unable to measure properly.

[@Dilshad-gu7je]

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.

[@iulia1690]

Very beautifuly explained. Thank You!

[@quietackshon]

When generating static electricity you shouldn't be grounded. NOTE: Rubber soled shoes, in most cases, won't be of any use regarding high voltages. The current flows like water and will happily overflow into the ground, and then you're dead.

[@Self-Duality]

Lovely presentation!

[@FunkyDexter]

The photoelectric effect is fundamentally an experiment that shows how the EM field interacts with matter. I don't understand why we jump directly to "the EM field must be quantized!" instead of simply stopping at "matter-field interactions are quantized". The energy to make a guitar string vibrate at different octaves is quantized; does that mean my plucking finger is quantized? Also think for a moment what you are doing when you're measuring that electron on a grid like at 18:04. To have that kind of measurement (single electron ejections) you need a very, VERY low intensity light. This is usually achieved with attenuators, which are basically screens that absorb radiation. See the problem? You're simply making it very, very unlikely that enough radiation ever goes through the attenuator, it doesn't mean you've isolated single photons. And of course, attenuators simply lower the amplitude of an incoming wave by an amount that depends on their thickness, which makes the results at 19:08 trivially obvious. As for the photoelectric effect's dependance on wavelength, many macroscopic resonance systems exhibit the same behaviour. It has to do with pure frequencies having sharp fourier transforms. It doesn't mean my radiation is "chopped" into bits.

[@danamulter]

Favorite new nerd channel. Love to see it.

[@worldnotworld]

Great work; using this for private lessons! One small criticism: It should be made clear early on that the frequency of light is proportionally related to its energy, and that this was anticipated before quantum theory (by classical mechanics/Maxwell). Though you wouldn't want to get into that mathematically here, it should be acknowledged. The puzzle was, "why doesn't doubling the frequency have the same effect as doubling the intensity?"

[@dougmarkham8576]

Possibly the calcium had an impure surface might have worked has you cut the surface layer off with a scalpel blade.

[@LookingGlassUniverse]

Yes, this could have been it. But I got it stored in mineral oil and did the experiment straight away. The calcium is rock solid so I couldn’t cut it unfortunately

[@dougmarkham8576]

Ah, OK. I guess the mineral oil was there to keep water off the calcium. Maybe others have another method to get to a purer calcium layer.

[@adityakelkar6860]

For the longest time, I’ve had more questions about the setup of the double slit experiment than the results themselves. This video and the Huygens optics callout do a fantastic job of explaining the experiment that the theories are based on. To me this also answers some questions on pilot wave theory.

[@DellHell1]

Your hair may have anti- static conditioner on it.

[@jokeyxero]

I love your channel and the videos in this series. I've gone through Susskind lectures, Feynman lectures, videos and lectures from all over for nearly two decades now as a quantum physics hobbyist. In all that, I've had many of your exact same questions and could never find anything satisfactory. Your stuff is taking just enough of those extra steps to explain the reality of it all without relying on all the math as a crutch. I still have questions, but I feel many of my understandings have been validated and so much more of it makes so much more sense now. Thank you for all your work.

[@lepidoptera9337]

Sadly none of those people are teaching quantum mechanics very well. More precisely, we have been teaching quantum mechanics wrong for almost a century now. The topic itself is extremely straight forward, but the way we approach it makes it nearly incomprehensible for anybody who isn't exposed to its phenomenological simplicity in the laboratory.

[@ebog4841]

i'm loving the endless crackpot commenters on this vid lol

[@docopoper]

If we're saying that EM waves can have any amplitude but the amount of energy in a photon is discrete. Then wouldn't an alternative explanation be that the electron needs to collect a certain amount of light of a high enough frequency? That the discrete part of the setup is in the electron's absorption rather than the waves themselves? Doing away with photons as a concept.

[@jyothishkumar3098]

I have this doubt too, but when we are doing measurements, we have no other way than to make use of electron shell transitions. So any other theory will have to be purely theoretical. I guess we can make an alternate theory without violating any experimental observations, but by citing the reason this theory was formed in the first place.

[@docopoper]

@@jyothishkumar3098 An alternative version of this I've been thinking about for quite a while too is that we have a sea of EM waves in the universe that are below the threshold for interacting with matter. Sometimes those waves constructively interfere with each other enough that we detect spurious measurements. We label these as cosmic rays and dark detections in our experiments. When we shine a low intensity light in the direction of our detection surface, we make it more likely for these constructive interferences to pass the threshold required to interact with the electron. Thus we detect electrons being interacted with seemingly at random. It's not actually random, it's background noise.

[@B1g_Salad]

I’m building a Flight Radio communications / navigation course and have been watching videos like this to understand electromagnetic energy. I know that a simplified explanation of these concepts would suffice for the level that I’m teaching, but I can’t help but try to understand fully what is happening. This has brought me to your videos. It started with trying to understand refraction because all of the explanations seem to contradict themselves. Now I’m here and I can’t stop. I really appreciate the effort you put into these videos. I feel like we are alike in that we seek to fully understand things and it will drive us crazy until we see it with our own eyes.

[@nitroxide17]

This amazing video made a breakthrough in helping me understand the nature of photons. Everyone who is curious about physics should watch this video.

[@FireStormOOO_]

Part of the confusion is that everyone talks about light as if we're proving things about the light itself rather than the discrete interactions it's involved in. The atom has discrete energy levels that interactions with light have to conform to - the light itself is not so constrained.

[@ManyHeavens42]

Wait the most important thing Is energy follows geometric Pockets, 100 percent probability.There are only 7 Master frequencies, your can rejuvenate skin cells.

[@samsade2689]

Outstanding fashion to facilitate complex subjects to comprehend. You r gifted.....carry on.......Prof. Hassane Saadeh

[@universalsoup630]

This is fantastic! Really amazing to get a proper explanation that bridges the gap between what we learn at uni and what we see in popular science content.

[@darrellee8194]

It seems to me that the photo-electric effect can be interpreted as saying nothing about what light is, but only about how it interacts with electrons. Travels as a wave. Interacts as a particle. But the particleness is not necessarily a property of the photon, rather it is a property of the interaction, or the measurement system. 1:45

[@hawgokutai]

This is the view that I always had (after doing my physics degree). I found your way very didatic. I always talked about modes. I also strugled with the mesurement. This makes lots of sense. Thank you!

[@andrearaimondi882]

This is an amazing video more for the failed attempts than the successful explanations: the reason is that very rarely do we get to see the “work” behind an experiment and the many unintuitive ways they can fail. Also, please don’t stop making experiments because they are the key to understanding. When they work you suddenly realise why they previously didn’t and gain a ton of knowledge as a result.

[@jimbuono2404]

When talking about photon - particle or wave it's good to remember Einstein. He showed that matter and energy are basically, the same thing. Matter is energy that has been compressed. A photon, when moving, stretches it's energy out across it's path. When we get wave collapse, that energy stops spreading out behind the electron on path because the path is blocked, in a manner of speaking. The photon is not stopped but reflected. Think of silly string being shot from a height to the floor. The string will stretch until it reaches the floor, but there it piles up.

[@TedToal_TedToal]

There is a technique model railroaders use to make grass on their model layouts using an applicator that applies an electric charge to tiny plastic bits. My brother had a devil of a time getting this to work, and after extensive experimentation he determined that static electricity is impossible to generate unless the humidity is high enough.

[@DrDeuteron]

Low enough?

[@TedToal_TedToal]

@@DrDeuteron I originally wrote "low enough", and then was talking to my brother, and it turns out it was "high enough". Oddly enough, the grass itself was too dry, and he had to place the plastic grass in bags with humidor packets to add moisture to them.

[@jamiehuntingdon521]

Thanks. First explanation of photons that actually made sense. Your style is awesome. My kids love it.

[@LookingGlassUniverse]

Thank you!!

[@smile768]

This is new to me. I know the double slit experiments, partical wave duality etc but they have never made sense before. I think you've managed to present things that celebrity physicists gloss over. Well done.

[@gator1984atcomcast]

Light isn’t a single wave but an electromagnetic dual wave. It can be polarized at the same time. Don’t forget the Planck constant. Each packet of energy is in the form of momentum even though photons are massless.

[@lepidoptera9337]

There are no energy packets. There are only people who don't understand physics. ;-)

[@projectartichoke]

You need very thin foil, like gold leaf. Don't feel bad, it's extremely touchy to properly construct a vane galvanometer. Even the humidity in the air can interfere with its operation. A completely sealed jar is necessary, but you have to be careful what material you use to seal it and the overall mass since many materials will hold charges. A mason jar with a rubber ring seal can be a good choice, but you have to drill a hole in the glass lid for your conductor and cement in place. A desiccant inside the jar is helpful. Thin foils can be very difficult to handle without destroying them.

[@Drone256]

Excellent! My eyes are now open. So many KZbin videos depicting single photons in the double slit experiment are absolutely wrong. Single photons never pass through one slit or another. And when the wave function collapses that’s not a particle either. It’s just where we detected the energy from the wave.

[@lepidoptera9337]

Which is exactly what we are teaching in school. People aren't paying any attention, though. ;-)

[@letMeSayThatInIrish]

What an excellent video! I have seen that Gaussian wave packet illustration in so many contexts that I thought that's how light 'really' was like. This completely changed my mind.

[@BrianMcCartney-u6s]

Great explanation at the end of the video about what light appears to be… it’s a wave but interacts with matter like a particle. Keep up the good work!

[@Me-kq4dp]

Hey! What I like about your videos is that you just don't jump on conclusions but you focus on the process. Anyways I am 15 years old and I thought you might be the best one to ask that how do you know that you love physics? I have done lots of over thinking on that and still not sure whether I truly love physics and would i be able to invest a lot of time studying it. I also find myself doubting that what if even I love physics but then do I really have the abilities and enough IQ to pursue physics. Waiting for your thoughts on this! PS:I don't really score that much in my highschool physics(or maths)exams.

[@lepidoptera9337]

You don't truly love physics and that's that. You aren't going to be able to maintain the 24/7/365 work schedule that an academic career requires. It's pretty simple, really. If your parents are rich, then they can finance a good time for you at university, though, while you are pretending to become a scientist (or whatever). ;-)

[@Me-kq4dp]

@@lepidoptera9337 I have never asked your opinion please keep it to yourself.But I would LOVE to prove your wrong,remember your would surely hear my name one day! And it is also disheartening to see that still some people watch scientific videos without any scientific temperament.

[@lepidoptera9337]

@@Me-kq4dp If you are getting emotional over somebody giving you good advice, then you are definitely not cut out for science. You have to learn 24/7 to stand on the shoulders of giants, but you are already way too full of yourself to accept even the most trivial piece of advice from somebody who was in science. You will either need a lot of money or a lot of luck with that attitude. Good luck to you! :-)

[@Me-kq4dp]

@@lepidoptera9337 No,It's not like that.My point of view was that you can't directly tell a person that you can't be a scientist without a good explanation/reason. Anyways,I am definitely learning and after reading my own reply to your previous comment I can see what you are pointing at. Well,thank you for a different perspective.

[@lepidoptera9337]

@@Me-kq4dp Anybody can be a scientist, with the right attitude. You just don't have the right attitude. ;-)

[@spencergains7346]

Very good video. It’s nice to hear someone ask questions that might not always “make sense” if you know the theory.

[@kishfoo]

Thank you for this. I didn't really get everything, but now I thoroughly know the difference between amplitude and wavelength.

[@twilightknight123]

A single photon doesn't have to be a single frequency and in fact is almost always a superposition of a spectral function (which also leads to spatial and temporal confinement). You can see this with Hong-Ou-Mandel (HOM) interference using something like a Mach-Zender interferometer and a source of spontaneous parametric down converted (SPDC) light. I'm saying this because while it may have originally been defined that way and maybe that's what you're getting at with the video, that's not how it's used by 99% of the physics community and given how nacent the quantum optics field still is, it's important not to cause confusion just for semantics. But on the topic of semantics, an attenuated coherent state (like you mentioned with "single photon per unit time" experiments) is not the same as a single photon.

[@lepidoptera9337]

Photons don't have a frequency. Only classical electromagnetic waves can have a frequency if they have a sufficiently monochromatic spectrum. Photons are also not in superposition. I would suggest you go back to the basics. I do not believe that you understand the basics of quantum mechanics at this point. All of this was ontologically well understood in 1927. Please try to read up on the Copenhagen interpretation before you pretend to understand quantum optics.

[@twilightknight123]

@@lepidoptera9337 I am a quantum optics research engineer working on networking and photonic computing and have been doing this for over a decade. I'm not going to say I know more than you, but I at least think I know what I'm talking about when it comes to single photons. How can a photon have an energy if it doesn't have a frequency? How can photons not be in a superposition when polarization states exist? I'm not sure how you think photons work, but I don't think they work the way you think they do

[@lepidoptera9337]

@@twilightknight123 When was the last time you measured the frequency of a single photon? What experimental setup did you use? Superposition is even easier: Superposition of which base states? Every state (even of an ensemble) can be written in an infinite number of ways as superpositions of different bases. Superposition is a completely meaningless concept at the level of physics. It's a mathematical property of the ensemble theory, just like coordinates are linear combinations of base vectors in linear geometry. Chose different base vectors and the same physical vector becomes a different "superposition" of a different base. There is no physical meaning in that.

[@twilightknight123]

​@@lepidoptera9337 You can easily measure the frequency of single photons using either a homodyne setup or, how is more frequently done, just with a spectrometer sensitive to single photons. Each time the spectrometer detects a photons, it is making a measurement of its frequency. (Edit to add) Last time I performed this measurement was at work this week. As for photons being in superpositions, what about bell states? The psi minus state, for example, is rotationally invariant and will always be in a superposition of orthogonal states independent of the basis. So while I'll concede it is technically a two-photon state, it is still a single photon exhibiting this property during measurements. Similarly, non-degenerate SPDC can create photons in many types of superpositions such as frequency, position, momentum, polarization, and/or time.

[@Figure-A]

At 3:16, it might help to ensure the plastic bottle is completely dry. Those tiny water droplets inside could be dissipating the charge. Just a guess though!

[@miinyoo]

Wow. You just explained something that has puzzled me for years. Threshold energy of photons in integers of a single photon relative to its wavelength. Now, _that_ I can completely picture in my head. Thanks!

[@rutger4131]

Great videos, thank you for being so open about your doubts and questions. It's essential for true learning and science! 18:35 very unlikely, but not impossible. If you focus a very strong source you can get a very small area where 2 photon absorption happens and you can make a microscope on this principle (TPEF).

[@mensaswede4028]

You explained the wave function “collapse” without ever referring to the word “wave function collapse”. Very well done video!

[@lepidoptera9337]

There is no wave function collapse. The wave function is like a probability distribution. It describes an infinite repetition of the same experiment. It's completely unchanged by an individual quantum event.

[@epicchocolate1866]

@@lepidoptera9337no? An act of measurement will collapse the wave function into a definite state, ie an infinite superposition takes on a single state, that’s QM 101

[@lepidoptera9337]

@@epicchocolate1866 No, that's not QM 101. That's just the nonsense you can find on the internet about quantum mechanics. A single physical system does not have a quantum state. Only the ensemble has a quantum state. The "final" state only exists after we have taken a quantum of energy out of the system, at which point the original system has been destroyed. This is no different in classical probability theory, by the way. Dice, as a physical system, can not be described by their outcome states. Individual rolling dice are not in some superposition of 1 to 6. An ensemble of rolling dice, however, is. OTOH resting dice (that had all of their kinetic energy removed) are in a well defined outcome state. But resting dice don't have a probability distribution... so it's either one or the other, but don't mix concepts that apply to the ensemble with concepts that apply to the individual system.

[@lepidoptera9337]

@@epicchocolate1866 No, that's not QM 101. That's just the nonsense you can find on the internet about quantum mechanics. A single physical system does not have a quantum state. Only the ensemble has a quantum state. The "final" state only exists after we have taken a quantum of energy out of the system, at which point the original system has been destroyed. This is no different in classical probability theory, by the way. Dice, as a physical system, can not be described by the outcome states. Individual rolling dice are not in some superposition of 1 to 6. Only resting dice (that had all of their kinetic energy removed) are in a well defined outcome state. But resting dice don't have a probability distribution... so it's either one or the other, but don't mix concepts that apply to the ensemble with concepts that apply to the individual system

[@quantum4everyone]

Your picture of a photon is approximately what is called a single mode photon. As you have observed, such photons are idealizations that cannot really exist. But you have not properly discussed multimode single photons, which are the little Gaussian wavepackets that you say are not photons--spoiler, they are. The key for single photons is to first have a single photon source of light. A laser is a classical source of light, which never, even if made very dim, can be a single photon source of light, because the photons in it are in a coherent state that bunch together. Single photon sources come from atomic transitions, transitions in quantum dots, or parametric down conversion. The classic way to measure them is to take your source, shine it one a beam splitter and measure the photons arriving in a detector on each output port of the beam splitter and seeing how many coincidences occur. Using this, if your g2 ratio is less than 1, you have a single photon source. It is very good if the g2 ratio is close to 0. I recommend Alain Aspects courses on quantum optics on Coursera as a great way to learn properly about what a photon really is. As for the measurement part, they are observed as dots, because that is how our detectors are made sensitive to the presence of a photon (and the detectors are spatially located somewhere). The notion of “travels like a wave but is detected as a particle” is not a great way to think about quanta. A wave-like detector of a photon is a diffraction grating, which measures the energy, or wave characteristic of the photon by transducing the frequency into a direction of propagation, which then can be counted by the single-photon detectors located at a specific location. You have some concepts correct, but others are in need of correction. Good luck with your future work in this area.

[@narfwhals7843]

I am honestly struggling to understand lasers in the quantum picture. I always thought the stimulated emission photon was in phase with "the photon that stimulated it", but that sounds like nonsense now that I type it out. But the coherent photons are not necessarily in phase. Is the in-phase nature of laser light just a feature of a many photon average? Don't they have to be in phase for useful double slit interference?

[@quantum4everyone]

@@narfwhals7843 Coherent photons share a uniform phase structure. Incoherent photons do not (incandescent light bulbs) which form a mixed state of photons.

[@mrjimjimjimmyjim9824]

one of the probing questions i have always had, is how does light travel in what seems to be a constant source of energy so that a person standing in one spot will see the light from an object (lets say a star), and a person standing elsewhere can also observe the same light, despite being elsewhere, meaning a constant flow of light can be constantly seen (even tho light waves are constantly overlapping each others waves - most of the time they dont affect each other too), i find the immense amount of energy that stays constant and spreads every where in space!

[@pk2712]

The energy of a photon is directly proportional to the frequency of light according to E=hf where h is Plank's constant and f=c/(wave length) where c is the speed of light .

[@minus1db]

Thank you, I have spent over 30 years first exploring Astro Physics then Quantum Physics, this was very interesting, I look forward to viewing your catalog. Again thank you for sharing your knowledge

[@dfoo75au]

That was a great explanation of how the photoelectric effect works! Thanks so much.

[@Chrome166]

I've never really understood the photoelectric effect before, but I think now I get why microwave ovens will only heat up organic compounds, but if you put metal inside of them it'll get a sparky and potentially destroy your oven. Because even if you're putting enough energy into the microwaves to boil water, things like carbon won't lose electrons from photons at such a low frequency, but certain metals absolutely will.

[@makeitreality457]

Good experimental setup. It appears that electrons are 'scaping through the paper plate, which is conductive at high voltages. Try a foam plate. Then other metals should work too.

[@eplumer]

Excellent video, exactly the explanation I was looking for.

[@jacobfreeman]

How do we know that the light is discreet and not just the matter that it interacts with? If we detect light with energized electrons and those electrons only take discreet amounts of energy, then of course the wave would lose discreet amounts of energy during measurement?

[@ronyraiser]

i love your way in teaching and explaining .. thank you so much

[@jojolafrite90]

I'm gonna be honest. I did watch a lot of scientific videos before (less now), but I still watch your channel from time to time because of how refreshing and charming I think you are.

[@markhuru]

Here's another point . The wave actually is not up and down this is a 2 d visual, the wave actually is like a cyclone, or cork screw, this is what keeps it in a straight line

[@anakimluke]

Thanks so much for the video! I absolutely loved this series!! I went to watch the videos you suggested, then came back and watched the original again. So, here is a dump of loose questions I have: How is the description of the 'eternal, infinite spread out, single wave length photon' different from that of a single-wavelength laser? Like, do lasers look like the gaussian wave packet(with many wavelengths) because they have to "end" at some point? Does the EM field value change if I get far away from the region the photon "is", or the difference in magnitude of the field is never measured, existing only in the wave function as a probability of where I'm going to find out the field to have the full value? From the moment a photon is created on, how does the probability of finding it in any particular spot change over time? I'd imagine it's probability 0 on time 0, then non-zero everywhere the bubble of the speed of causality has had time to touch? How far can you stretch Huygen's linked experiment and still see an interference? How to calculate that? Can I make up a gaussian wave packed so as to make Huygen's experiment not work when the distance is larger than a certain amount? Like, if there are many oscillations in the EM field, one after the other(which only gets detected *once*), even for the mathematical case of the "eternal infinite-photon", can I decrease the number of times the wave repeats before it tapers off? Are gaussian packets ever created as a fundamental field interaction(like I see in feyman diagrams), or they only exist then different parts of the field interact to as to cancel out on the right parts and leave you with a packet? Can a photon with the minimum number of energy for its wavelength every be measured? Why is it that this minimum amount of energy change with wavelength at all? Is the number of wavelengths quantized? Like, given any two wavelengths, there exist a wavelength between those? Considering the "classical wannabe" case where if I'd shine light on a CCD, every little bit would get a little bit of energy, instead of the observed case where a single atom interacts per time with full energy; does that mean that assuming the atoms can receive the energy, more energy is directly proportional to more likelihood of interaction? Like, the if field has twice as much energy here, that atom will be twice as likely to be interacted with? What happens if I shine two photons at each other perfectly out of phase? Or with different polarity? Will they annihilate? What if nobody is looking?

[@LookingGlassUniverse]

Thank you for going back and gathering all these great questions!! I'll answer as best as I can 1. You're right, the laser is part way between the gaussian and the infinite plane wave. In the "middle" it looks plane wave ish, but to get the finite ends, you do need to add other colours in. So there is always a chance of measuring other colours in your "pure frequency" laser. 2. if you're talking about a finite pulse of light, then the EM field will only be different where the pulse is (assuming it drops to zero outside). If you have a plane wave, the wave is spread in at least line. Anywhere on that line, the EM field value is oscillating. 3. let's say a light emits light pulses. That pulse will move at the speed c. To find the probability of finding the light in a particular region and a particle time. you should look at how big the pulse is then and there. Eg, if the pulse has moved on already, the probability will be never 0. But if you have a true plane wave then the probability of finding it anywhere on the line is the same. 4. I think you can stretch that experiment far! Let's say path 1 is much shorter than path 2, and you turn on the laser and very very quickly measure for interference. In that case, the part of the light that went via path 2 might not have had time to get to the detector. But barring this effect, you'll always see interference. 5.That's right, Huygen's experiment will not work with pulses, because they will arrive at very different times via the two paths. And yes, you can make a wave that is a few pulses in a row before tapering off. 6. I don't know if gaussian packets ever created as a fundamental field interaction! 7. I don't know why the minimum energy changes with wavelength- it's strange 8. Wavelengths aren't quantized, you can have any length. 9. Yes, I think you're right about the CCD example 10. Absolutely, the light would be annihilated in any place where there is constructive interference. Please let me know if my answers didn't make sense

[@abdelhakkhalil7684]

There is a probability of getting different colors of light is due to the Uncertainty Principle: The more precise your measurement is the less uncertain you get about a property of the particle. In this case, the uncertainly is related to Energy. And since the energy is related to frequency, and we are uncertain about the energy, we get more colors.

[@UsefulAlien]

A photon is a pulsating spin wave in the Aether. When it travels it becomes an electromagnetic wave. You can slow a photon down to a standstill using a laser beam.

[@ps200306]

Loved this video. Sorry about the failed experiment -- I've made electroscopes at home and they can be finicky. One thing that occurred to me about the video is that you tend to show plane waves or longitudinal wave trains or even real laser light. The temptation for the viewer is to somehow imagine the photon riding that train somewhere. I know you went to pains to dispel that myth. It wouldn't be easy to show in a video, but I've always reminded myself that most light sources radiate isotropically so the waves are spherical. For me, an illustrative example is when we image faint astronomical sources with a telescope. With modern CCDs we capture incredibly faint sources where the integrated exposure is only a handful of photons (to the extent that we have to worry about things like shot noise and dark currents in the circuitry). Anyway, the point is that another observer millions of light years away could be observing the same source. In essence we two observers are like cells in the grid that you introduce at 16:35, only that we are separated by a vast distance. Ok, we think, we get our bunch of photons, the other observer gets theirs, what's the big deal? Well, all the photons arrive at random -- in the limit of very low flux I believe the time between arrivals is characterised by a Poisson distribution. And yet, the total flux for all observers (and in principle for the entire spherical shell surrounding the source) is still constrained by conservation of energy. We are all absorbing random photons from the same spherical field but those arrivals are somehow coordinated at a global level. To me that says something very deep about a non-local aspect of photon emission and absorption.

[@LookingGlassUniverse]

Wow, that's a very vivid example, thank you! Yes, that aspect of non locality is very very unsettling..

[@DrDeuteron]

Spherical waves have exact angular momentum…0, 1, 2…for monopole, dipole, quadrupole….radiation, so there is uncertainty in the angular coordinates. Just like momentum and position.

[@DrDeuteron]

Laser photons are poisson in time …totally random. Thermal light, like an Astro source, has bunched light, or super poission. Idk, that seems wrong, but second order correlation functions are not intuitive. Also, regarding the angular distribution of photons from a distant star….it’s not uniform, and arrivals are correlated depending on the angular diameter of the star. See Hamburg-Brown Twins effect….another totally 🤯 2nd order correlation things. Works for pions emitted in heavy ion collisions too, which is used to measure the size of a quark gluon plasma. Edit: spellcheck is killing me. Hanbury…not 🍔

[@RealNovgorod]

You can broaden the spectrum of your super narrow-band continuous wave laser by simply inserting a mechanical chopper into the beam. The time-bandwidth-product law applies even if you make "pulses" with your bare hands.

[@AllToDevNull]

Stoyan Sargs BSM-SG model has the best photon model I have seen so far. The photon is a boudle wave, one helical magnetic wave that encloses the electric wave - a round tube. Once you understand the electron system, you will understand how the electron creates photons and how the size of the electron defines the size of the photon width. The length of the quantum orbit, that is very precise, defines the length of the photon wavetrain, aka wavelength - color. What you arc showing is more like a group of entrangled photons.

[@jyothishkumar3098]

I was literally pondering over this question for the past few weeks, getting multiple closed questions on Physics SE for the question not being clear enough. Thanks for uploading! Edit: I didn't watch this yet, but my confusion was about whether they are the same as phonons in sound waves, and why exactly are they quantized apart from when they are generated by electron shell de-excitations (which are discrete jumps). I'll watch this in a while and see if my doubt is cleared. Edit 2: 16:46 That was exactly what's unsettling. I still have a doubt, what if the grid used was parabolid and the photons were emitted from the focus? My guess is, by assuming light moves like 3D sound waves, the wavefront that hits the metal first would be the one that's along the shortest path from the source to the point of intersection along the direction in which the photon was emitted.. and photons get equally sprayed in all directions, which explain the double slit interference pattern. But I haven't seen an experiment that's ever tried this. Wavefunction collapse would still happen due to lack of microcurrents observed, but perhaps it **could** be more intuitive..

[@DrDeuteron]

They’re quantized because they live in a simple harmonic oscillator potential, which has equally spaced eigenvalues, so the quantum number n can be considered as an occupation number. But it is? A plane wave doesn’t even have a fixed occupation number. See Glauber state.

[@insightfool]

Absolutely love your commitment to great explanations! thank you.

[@BenAlternate-zf9nr]

How does the idea that "ideal pure photons don't really exist" square with the idea that e.g. electron energy level transitions or gamma decay events have specific exact energy values corresponding to a single photon with that energy? Is there some chance of observing a decay photon with the "wrong" energy?