Why light has momentum even without mass?

Light (or photons) is massless. Yet, photons have momentum given by the equation
P = E/c. Where E is the energy the photon enegy and c is the speed of light.
The goal of this video is to gain a deep intuition behind it.
First to understand where the light's momentum really comes from. Second, to use that intuition to derive the equation without using anything quantum. But, by connecting all the fundamental equations of high school electromagnetism.
I first learnt about this from Feynman lectures. You can find his actual derivation in the last paragraph of his Volume 2, chapter 34 - "Relativistic effects in radiation".
www.feynmanlectures.caltech.e....

[@Mahesh_Shenoy]

5 FAQs (and counting) and references! First and foremost, here is the source. www.feynmanlectures.caltech.edu/I_34.html#:~:text=We%20now%20appreciate%20that%20light,%CF%89%2Cp%3D%E2%84%8Fk Check out the last paragraph. Now, let's get to some FAQs 1. Wouldn't protons experience a force in the opposite direction? No, when charge flips (from negative to positive), the velocity due to electric field also flips. Hence, the magnetic force direction stays the same. Another way to see it is, F = q (v X B). If both q and v become negative, their negatives cancels out. So, the force direction is independent of the sign. 2. Can EM waves push neutral particles, like neutrons? Neutrons are made of quarks that have charge. So, if you consider that and the fact that force direction stays the same for both positive and negative charge, it should be able to push it. But, I think it would be stretching it. Classical physics is wrong. And we really shouldn't be using at the quantum level. But, the point was to not sell it short. It is still a powerful tool to gain some intuition. My perspective is to use classical physics as a tool to generate some insights. Without these, it would be hard to study any physics. 3. Why would velocity flip when E field flips? What about electron's inertia? We are assuming the electrons are damped oscillators. Wait, how in world can we imagine electrons to be oscillators, let alone be damped oscillators? I am yet to understand that myself. But, I just took Feynman's word for it. :D. [If I try to understand every single nuance, I will never publish any video on such topics. So bare with me]. But, here's the excerpt from Vol 2. Chapter 32, Paragraph 3 👇 "We use a model of an atom or molecule in which the electron is bound with a force proportional to its displacement (as though the electron were held in place by a spring). We emphasized that this was not a legitimate classical model of an atom, but we will show later that the correct quantum mechanical theory gives results equivalent to this model (in simple cases). In our earlier treatment, we did not include the possibility of a damping force in the atomic oscillators, but we will do so now. Such a force corresponds to a resistance to the motion, that is, to a force proportional to the velocity of the electron." 4. Comet tail is caused by Solar wind, not from Sun's light! There are two comet tails. The ion tail (which I didn't mention) is indeed caused by the wind. But, the second tail - the dust tail - is apparently caused by the sunlight. (Radiation pressure). Here are a couple of resources that dig deep into this articles.adsabs.harvard.edu//full/1968ARA%26A...6..267B/0000267.000.html www.sciencedirect.com/topics/earth-and-planetary-sciences/comet-tails 5. Aren't electric and magnetic field 90 degrees out of phase? No! Any EM wave should always have E and B field in phase. Please check out this amazing video that explains this misconception kzbin.info/www/bejne/jWLGhaOngp53l7csi=T2buT-rhWLQGDDZx I will update this list as I see more FAQs. You folks rock. You push me to think and research more! Love it ❤

[@gyro5d]

Mediated to center of everything is Nothing. Nothing has Zero pressure, where the Aether pressure field heads = Gravity.

[@-danR]

A neutrino has no magnetic field and no magnetic moment. Why would it have momentum when it (rarely) bangs into another particle? A 𝛎 with enough energy will even blow a nucleus to bits if it hits a quark dead-on.

[@michaelharrison1093]

Another question that has turned up a lot (and I have tried to answer each time) is in regard to the E and B waves being in phase. Many people have commented that they question if they should not be 90 degrees out of phase? The fact is that fot a traveling EM wave then the E and B waves are phase aligned, but for a standing EM wave there is a 90 degree phase shift which then changes the force from being in a unipolar direction to being oscillitory. You might want to do a video explaining why this difference occurs for traveling Vs standing EM waves

[@AveragePearEnjoyer]

​@@-danR m = 1.4x10^-36 kg

[@owenwilson25]

NO, a comet's tail is blown by the Solar Wind not light, and a shadow effect that can amplify the visual affect as light gases condense in the shade rather than continuing to expand while exposed to strong sunlight.

[@itcantbetrueable]

45 years ago high school physics taught me that photons had momentum but not mass. Thank you for explaining how this is possible and bless you for your enthusiasm 😊

[@satyambehera413]

how old r u

[@pythondrink]

​@@satyambehera413 at least 60 years old, I guess

[@pythondrink]

I was never taught this in high school

[@loodstroh4634]

Yes but does photons really exist?

[@pythondrink]

@@loodstroh4634 wdym by this question?

[@i.b.blithe3263]

This was an excellent discussion of by Floathead explaining Feynman who in turn was explaining Maxwell... well done. Thank you Floathead.

[@acrylix3073]

Maxwell explaining Faraday.

[@everythingisalllies2141]

Shame that light still has no momentum, because it has no Mass, and no one has ever proved that Light is made of two waves, one electrical and the other magnetic. That claim is actually not even rational, and direct experiments cant support this claim.

[@cosmic_gate476]

​@@acrylix3073 Faraday explaining Archimides.

[@bardsamok9221]

​@@cosmic_gate476Archimedes explaining Pythagoras

[@sqlexp]

I say that this explanation is bs. E^2 = (mc^2)^2 + (Pc)^2 is fabricated just to gaslight people into accepting massless photons. For one, the equation fails for object with rest mass because it gives Pc = (mv^2)/2, i.e. P = (mv^2)/(2c), which is wrong. The last equation he uses only shows that E = Pc, which is true even for massive photons, and it is how the "patched" energy equation was originally fabricated.

[@keithwilliams2547]

After all these years, someone finally explains this! You are quite a teacher!

[@chrisoakey9841]

I think your first problem is the assumption that light has no mass. Remember it acts both as wave and particle. Not just wave.

[@_ranko]

@@chrisoakey9841 It's not an assumption, it literally has no mass within an incredibly tiny uncertainty range. Just because it's a "particle" (which it technically isn't based on QFT), doesn't mean it has to have mass. You're the one making an assumption here.

[@chrisoakey9841]

you must truly be part of mensa. not an assumption, it has no mass, would be an assumption. if you cant even fully figure out if it is a particle or a wave then it seem pretty stupid to suggest we know if the whatever has mass. after all, it has momentum. it is affected by gravity. its just the light speed equation that heads to infinity that goes no mass. its either no mass or the math doesn't quite work as we know right now. but since it isn't fully categorized, let alone understood how it travels sure, im the one making the assumption. does work, probably has mass. but since we still are using "photon" as a concept because we know so little, maybe it doesnt have mass. lets just accept without thought because einstein..... @@_ranko dont mind that general relativity doesnt quite work on the cosmos or quantum levels.

[@alonewanderer4697]

@@chrisoakey9841idk about the latter half, but i would like to point out that our current understanding of gravity is that it isn’t a force, but more a thing that curves spacetime that means that a photon doesn’t have to have mass to be affected by gravity, as it is not a force :)

[@chrisoakey9841]

@@alonewanderer4697 that is true, that the current understanding is the curvature of space time. however what force makes stuff change direction. space time is never defined. the ether concept seems to be disproven by nicholson morley ether exp. so to change somethings direction takes a force. so what in space time forces us to change direction? general relativity says time, but the force is the same on both sides of the earth. if it were time it would have a relationship to the movement of the earth around the sun. and therefore would be different amounts of acceleration depending on which part of the earth you are on and whether your spin is in the same direction as the earths, or the opposite direction to its rotation around the sun. also it would require the earth to be accelerating outward in all directions at the same time. not a speed but a continued acceleration of 9.8/s/s at sea level. so the curvature of space time says nothing that would create a force. just like if i draw a curved line on a table, when i roll a ball on the table the ball will roll straight unless i draw the line with a thick material that physically acts on the ball. just a curve doesn't curve the ball.

[@wayneyadams]

1:33 Feynman blows everybody's mind. I have never seen someone who can simplify complex topics like he can. He is probably the greatest teacher of Physics of all time. His language is simple, like saying things bounce around as he makes hand gestures rather than saying vibrations.

[@wayneyadams]

@@IsaacNewton818 I have eclectic tastes, political commentary, Physics (my M.S., which I taught for 33 years), Chemistry (my B.S., which I worked at for 9 years), magic, railfanning, flying model airplanes, painting, woodworking, etc. It's a good chance you will see me on dozens if not hundreds of channels.

[@everythingisalllies2141]

Except that time when he was asked to explain how Magnetism worked. He was actually clueless but pretended that it was the person asking the question that was incapable of 'understanding". This a a common fallacy of logic.

[@wayneyadams]

@@everythingisalllies2141 There is always one person who has some contrary comment to make about any comment. I could write, "It's a beautiful day today." and a yahoo like you would have something negative to say.

[@everythingisalllies2141]

@@wayneyadams Is that your best defense of Feynman? You obviously have not watched that interview. I can't help it if famous people cant afford to admit that they just don't know something. But having "A story" to explain something is not the same as having a rational story. The point is, that people are taught to just accept what people like Feynman say, and they don't bother to consider if its true or rational or not. Einsteins mad beliefs are a great example, probably the most silly claims ever made, and everyone just laps it up without thinking.

[@wayneyadams]

@@everythingisalllies2141 I don't need to defend Feynman. I take him at his word since he probably was unable to dumb down magnetism question to a level where the questioner could understand his answer. Why don't you give me the link to interview and I will watch it myself and decide.

[@NemoFilHimry]

I love it that you explain your way of thought, which is almost exactly like mine. That voice in the head that self-criticizes every claim, that asks all the time "but why". I too was frustrated by many explanations that only partially answer the question (like most explanations we encounter), but don't really answer the deeper question. This really gave me a deeper understanding, and that itch that existed because of those unsatisfying explanations finally itches less.

[@Mahesh_Shenoy]

I am so glad to see you resonate, buddy!

[@VersaceUnderwear]

@@Mahesh_Shenoy please reply sir, everywhere i see the energy of photon in place of the energy per second that we've got (which i guess is the power) and just momentum in place of momentum per second... so is it like we can use momentum transferred per second=energy transferred per second / c for photons too? ie momentum transferred by a photon=energy of a photon / c which i think should be correct because in this if we put the energy of photon as hc/lambda it gives us a predefined formula momentum of a photon=h/lambda take this question A radiation of energy ‘E’ falls normally on a perfectly reflecting surface. The momentum transferred to the surface is (C = Velocity of light) :- (1) 2E/C (2) 2E/C^2 (3) E/C^2 (4) E/C in this if with our formula (ie momentum transferred per second=energy transferred per second / c) do we solve it like, the change in momentum would be double the momentum the radiation has because it perfectly reflects 2*energy transfered by the radiation per second/c or take it all for photons like 2*energy of photon/c (while making the comment) the answers are same which means i was confused in what the question meant by "energy of radiation"

[@ryanbaker7404]

Your excitement is contagious! I'm pleasantly learning a lot of new concepts from you, sir. Well done!

[@iplaypocketfjords]

But his physics is poor, and induces an authority figure - generally a fallacy, but sure, for narrative sake, I’ll give it a pass but you can’t deny the shocking absence of imagination. Think about it, it’s just a back and forth - where the beginning, middle and end are like a template pasted three times over with the old black swan psychological bias thrown in that isn't even appropriate here because the uncertainty in the physical phenomenon is not adequately rare to solicit that very explicit example.

[@arjunarun9147]

@@iplaypocketfjords quit yapping you don't know physics.

[@perkinscurry8665]

Finally someone has called BS on the so-called explanations using E^2 = (stuff) which have always struck me as mind-bogglingly circular. You've given a brilliant explanation that goes to the heart of the matter. Bravo!

[@omarazami7377]

I got my masters a few years ago and I stopped there, I think I want to go back. This video has pushed me in that direction. Feynman delivered and so did you.

[@brijeshpatel5017]

You and feynman bring unique and captivating energy to science explanations. Thanks for being so awesome!

[@Alex-nq7uh]

I can't explain how amazing this video was. You are such a natural teacher, nothing was left unanswered. I pray you go far so more people can see what you have to offer.

[@robbannstrom]

Well, yes, "it's the magnetic field which gives rise to momentum", but that's only the case since the electron is being pushed - i.e. is *_caused to move up and down_* by the electric component of the EM wave which enables the force equation to come into play. It's the interplay betweeen the electric and magnetic fields in the photon / light wave which allow it to impart momentum on the electron. Having said that, this is the clearest explanation for this phenomenon I've ever seen, so a big thanks to FHP for posting this video! Great work, man!

[@DivineMisterAdVentures]

If you have a question about these as quantum mechanics, see my commentary for a possible solution about 20 comments up.

[@robbannstrom]

@@DivineMisterAdVentures No, I'll refer to the books on quantum mechanics I used while studying QM as part of an astrophysics degree. Thanks anyway...

[@sohamadak6811]

The enthusiasm in your eyes gives me immense joy. Always refreshing to see someone so passionate about physics

[@casualphysics840]

Hello! I just wanted to say a big thank you for being one of the best educators on khan academy! A lot of your physics videos have been really helpful to me this year and I can feel the effort and thought you put into making these videos. From the smooth animations and the analogies to the crisp delivery of the material, you really make sure that quality doesn't take a hit anywhere. You may feel like I'm flattering you but that's not the case, your contagious enthusiasm for the subject matter manages to sell anything & everything you teach in a way that sometimes even Sal khan struggles with! Keep doing what's your doing sire!

[@Mahesh_Shenoy]

Wow, the message truly made my day. Thanks for putting it so beautifully!

[@casualphysics840]

@@Mahesh_Shenoy You use sketchbook to make videos right? Are there any plugins you use to help with the process or are the extra things like animation added in post

[@victorbirgisson2957]

@@Mahesh_Shenoy was going to say thank you but @casualphysics840 said it so beautifully. I haven't seen this video yet (seen some of your wonderful others) and as former undergrad in Physics, look forward to dusting off any special relativity knowledge, hiding somewhere in my brain, beneath the blanket of beer cars. ;P

[@Mahesh_Shenoy]

@@casualphysics840 Yes, sketchbook (with loads of layers). For this video, I did every single visual on sketchbook itself. Nothing else. The only post was zooming in and out at places. I do that in Camtasia. It's my screen capture + video editing tool. Camtasia also has baby animations that we can add in the post. I use it sparingly (didn't use it here). I hate editing.

[@casualphysics840]

@@Mahesh_Shenoy Your layer work is really good :D

[@Mahesh_Shenoy]

The next video will be the most challenging, yet. I want to show how to re discover special theory of relativity - specifically time dilation (although, you could re discover any other consequence as well) - using key concepts of classical physics and thought experiments only. No math! But, with incredible rigour! Will make it live next Thursday.

[@dbtest117]

The devil is always in the details we ignore.

[@mridulacharya8250]

exactly! Going through the whole thought process, the whole journey behind the discovery is all we should seek for...everybody in classroom should rediscover...that's the key!

[@bare827]

Can't wait 😊😊😊

[@petevenuti7355]

I'm confused, the way I understood what you were saying about momentum, does that mean the way it's imparted to a proton results in the proton moving towards the source of light‽ Not away like an electron?

[@charlesstewart4436]

Why light has momentum even without mass? The gist of your explanation is a moving electric charge has a magnetic field and when this is parallel to the magnetic field of an incident electro-magnetic wave the charge is repelled (as if struck by a body possessing momentum). NB in the scenario you are describing the electric field of the electro-magnetic wave is the field dictating the motion of the electric charge. Without this condition it's possible for other possible interesting outcomes. So far as E^2 = (mc^2)^2 + (pc)^2 is concerned this is the same quantity of total energy as is referenced in E = Mc^2. The latter equation is using M which means relativistic mass whereas the former uses m which means rest mass. The latter equation doesn't need to square terms on either side of the equals sign to achieve equality. Each equation can be transformed into the other as the latter is only more compact, while the former uses one term to represent the energy bound up in the mass of a body at rest and a second term to represent its energy of motion (relativistic mass captures in one term all the energy in a body). Plainly if a body is at rest we can see they agree as then m=M and pc=0. However Einstein pointed out these equations make sense even if m=o but pc does not, if we can find a particle which travels at c. Special relativity requires any such particle to have no rest mass but posses momentum p. I think physics buffs need to know both of these quite different reasons why/how we understand light to carry momentum.

[@DFPercush]

Wow, that actually makes total sense! I've tried to explain this using just algebra before, and it never really felt satisfying. Amazing explanation. Your enthusiasm is contagious, too. :D +1 sub

[@nigeldepledge3790]

Yes. This was indeed a surprise. I never realised that I had the tools to work out the momentum of light all along. Thank you for making this make sense.

[@vwcanter]

Hey, Floathead, you're doing an awesome job with these videos, keep it up. This is the part of the subject that is hard to come by- looking closely at what actually happens with the fields and the charges, and having a clear picture in your head, in addition to the mathematical relations. This is in fact why people liked Feynman's lectures so much.

[@lauri268]

Amazing video. This feels way more intuitive than the E=mc^2 derivation that most textbooks use. Thank you.

[@danielfarcas751]

This video did what 3 years of undergraduate theoretical physics could not, thank you 🙏

[@scottfranco1962]

The problem is: Feynman is deceased. How are you getting replies from him?

[@THE_FORESAKEN_GAURDIAN]

Defenitely something to do with edo tensai...

[@rishabhsingh2389]

😂

[@Blade.5786]

Just hire a medium

[@paulklee5790]

His energy is conserved… obviously.

[@umapatibhandarifunallover1832]

Heaven

[@L2p2]

Awesome ! This question was bothering me for a long while! Finally you have a video that answers it. Phew! I don't have to take grad school physics classes now just to find this answer. I have only a high school knowledge of physics and I have been trying to understand grad level physics by reading and using intuition to develop a better understading of physics. This channel seems to have the same intent but in reverse that it tries to deliver this understanding. If there are more peoplelike me out there then this will help them. Do keep up this work ! 👏👏👏👏👏

[@deformityy264]

The amount of energy you used making this video I had to subscribe. That's passion for science

[@scottm5425]

Yes, this problem has bugged me for years. Finally someone explains its properly in a really great way. Thanks. Subscribed.

[@optimusmaximus9646]

Love the enthusiasm! I have to say, if anyone get so excited about Lorentz's Law, imagine how excited they will be when they come across the relativistic form of the Lorentz force 👍

[@georgerevell5643]

its awesome an so original how you explain your own learning process rather than just state the solutions like a seasoned professor. Also how your not afraid to show you full enthusiasm for this amazing physics, it all adds a layer of mutual intrigue and personal connection with the audience.😎

[@eriktempelman2097]

Love your enthusiasm, and your obvious admiration of Feynman. Great one ❤

[@corsaircaruso471]

This was the first derivation video I ever understood. Thank you so much. Instantly subscribed.

[@StefanoBusnelliGuru]

I'm astonished about this explaination, great work!

[@kingplunger6033]

This video is just absolutely great ! Glad I found your channel and I should finally check out Feynman's Lectures.

[@Mahesh_Shenoy]

Thanks man. Everyone should check out Feynman's lectures.

[@kingplunger6033]

​@@Mahesh_ShenoyJust ordered them in book form as I like physical books more than digital

[@mridulacharya8250]

@@Mahesh_Shenoy by feynman lectures u mean the pdf thats there on internet right??

[@Mahesh_Shenoy]

@@mridulacharya8250 Yes!

[@spacejunky4380]

I love your video. I love how you broke down a complicated idea into a basic equation. When you broke down the equation, the physics made more sense to me. I would love more in-depth videos

[@cubic_regent]

Im a mechanical engineering student and your videos are always so beautiful. I love your enthusiasm and storyline you put into the material. Im grateful for the age of information that allows me to learn from your lessons like this.

[@tanmantacomama8851]

Wow! Had this question in class recently. What a beautiful explanation.

[@helbertrodriguez6449]

Ur energy makes me want to learn and learn. I also find this amusing and fun to try to explain n think about it then give it an explanation. I’m just really bad at math and I lack foundations that are essential for this topics. I don’t plan to give up I can learn and learn I wish my professors were like u

[@danishumer6392]

What a great video! Your enthusiasm and excitement is contagious.

[@hinesification]

That was really great. My undergrad physics courses were based in the Feynman Lecture Series. Most intuitive courses ever

[@hardpanchal2821]

Its always a good day when you upload, and OMG this was absolutely amazing! This took me to a another world, and the way you explain is already out of this world, FAR FAR better than anyone I have looked upon. I truly love physics and you are just increasing my love for it. Saying Thankyou is an understatement... I truly appreciate all your efforts, and I would absolutely LOVE more content on quantum physics and Relativity. Much Love!

[@Nuovoswiss]

I think this deserves a follow-up video on how the velocity of an electron induced by the photon's E-field varies in phase relative to that E-field (and by extension, also the photon's B-field). The imparted momentum of a photon can be 0 (transparency), E/c (absorption), or 2E/c (reflection). Or anywhere in-between for various types of scattering.

[@DFPercush]

That would be amazing. It was always a mystery to me why some materials are transparent to different wavelengths. I feel like all the explanations I've heard are kind of hand-wavy. All I know is that electrical conductors tend to reflect.

[@Nuovoswiss]

@@DFPercush In this video he depicted the electron's velocity as being proportional to the photon's E-field, (and also the photon's B-field having no phase lag behind its E-field). Consider a bound electron (in a covalent bond, as in an insulator) exposed to a photon. Rather than its *velocity* being proportional to the photon's E-field, its *displacement* would be proportional. Velocity and displacement are a quarter wave out of phase in sinusoidal motion, so the effect of the photon's B-field would cancel out and impart no (net) momentum.

[@robbannstrom]

@@Nuovoswiss True enough, but don't lose sight of the fact that many electrons in a metal are free electrons, forming a degenerate electron "gas", and it's the free electrons which will tend to be mostly freely accelerated by the electric component of the photon/light wave.

[@MecanicaCuanticaQRAGC]

"Momentum" is a disruption of our space into parallel (Plank lengths), which creates energy, as a parallel divergent effect, which determines the speed of light (without quantum decoherence).

[@Nuovoswiss]

@@robbannstrom Right, that why metals reflect light, though I'm curious where the extra factor of 2 comes in (a reflected photon imparts twice as much momentum as one just being absorbed).

[@akioasakura3624]

This yt is amazing big up sir 🔥🔥. Actual logic, HD, easy to understand, nice animations, no annoying music. I listen to some of these when I want to relax or sleep thank u for uploading sir

[@thephilosopher7173]

Wow, that was an excellent explanation!! I was able to follow along without even having to pause to think about it. Instant sub and I look forward to more of these!

[@leonhardtkristensen4093]

In a previous video you appeared to me to want to explain every thing with electricity and basically removed magnetism. Here however you need it for your explanation. In my oppinion they are both needed and equally important. By the way I have subscribed a month or so back after having seen a few of your videos as I find you quite good at explaning. I am a retired Electronic engineer and now dabble in trying to understand especially the part of Physics that has to do with understanding and explaning what I have noticed and experienced through my working life.

[@Mahesh_Shenoy]

Great call out. I think I got quite some heat for saying that. :D. In my defence, I did put some disclaimer at the end :D. I agree with you!

[@vegitoblue7063]

Yeah!!!!our favorite physics teacher is back with his cool videos❤

[@AyushSingh-be2nm]

Have you studied with him?

[@vegitoblue7063]

@@AyushSingh-be2nm yes he is my favorite teacher, I wish I had the opportunity to study from him in class 11th.

[@thomasherbig]

The happiness I can hear in your voice when you explain this makes my day!

[@mbrothers123]

This is INCREDIBLE; your personality is AMAZING and you make learning soo infectuous

[@Dekoherence-ii8pw]

OH MY GOD, AMAAAZING!!! 🙂 I've wondering about this for YEARS. Finally found an answer that makes sense. And it's COMPLETELY UNMYSTERIOUS. It just makes total sense!

[@joan4jays]

While all the high school equations were taught, the comet tail to sun relation was never taught 😅

[@Mahesh_Shenoy]

There are some things you learn in school. But, for everything else, there is KZbin :D

[@thecritiquer9407]

​@@Mahesh_Shenoy true

[@prapanthebachelorette6803]

They don’t help you link things together while you’re in school 😂

[@susilgunaratne4267]

Most texts don't go beyond the basic equation that with PC. Your derivation make a whole experience by ourselves traveling with the EM wave. That's truly a physical intuition far removed from the mathematical symbolism & argumentation. That's why Feynman always said knowledge & understanding are two things. Thanks!

[@bariumselenided5152]

Idk what it is about your presenting style, but it holds my attention so well. Always love your videos

[@sushantbagale.02202]

It can be also perceived from DeBroglie's matter waves concept which shows momentum=Planck's constant/wavelength

[@bardsamok9221]

Yes I'm surprised the quantum explanation was ignored, considering it's so key to understanding photons and physics. Perhaps he doesn't understand it, or why its important.

[@wayneyadams]

It is the combination of the forces generated by BOTH fields that gives light momentum. In addition, think about what happens when one mass exerts a force on another mass. Nothing actually touches in the laymen's understanding of touch, the force is the result of the repulsion of electrons surrounding the atoms. So, it should not be surprising that an EM wave exerts force on electrons.

[@hrperformance]

I really appreciate the reference to Feynman's lectures in the description 👍🏼

[@darrellee8194]

Well done! I don't have the wherewithal to tackle the Feynman lectures for myself. Thanks for making this so accessible.

[@joe_ninety_one5076]

I enjoy your content, the questions that you ask and answer and your enthusiasm for physics. Apart from the solar wind issue, which has been discussed below, I had a few thoughts about this subject: 1. When drawing an electromagnetic wave you follow a near-universal practice of making the magnetic wave in-phase with the electric one. But, isn't the magnetic wave a result of the electric wave and determined by its rate of change. If so, they should be ninety degrees out of phase. Isn't this also the only way of conserving energy continuously (i.e. by the exchange of electric field energy with magnetic field energy components). Granted, the drawing is just a schematic, but does the pictorial representation affect the explanation in any way? 2. If the magnetic field carries the momentum as described then presumably a positron would be pushed in the opposite direction. Is this the case? I havent found an answer to this, but the Compton scattering formula does not contain charge explicitly, not does its derivation (see Wikipedia). 3. Electrons have inertia, so they don't move instantaneously in a field. They start to accelerate. If the frequency is high, they won't move much at all. If it is very low, they might gain a lot of speed. However, their direction will not be instantly reversed when the field is reversed, they will just be de-accelerated. So, there are a lot of complications in this interaction that your qualitative explanation omits. Is it the case that high frequency photons transfer less momentum to an electron because the electron's mass prevents it from gaining any speed? 4. If the effect can be derived from classical electrodynamics in this way, why wasn't Compton scattering of electrons by photons predicted and fully understood well before Compton received the nobel prize for it in the 1920s, over 20 years after the discovery of the electron? Maybe it was. I'm not arguing with Feynman, by the way, just a bit unsure about what exactly he meant

[@Mahesh_Shenoy]

Love the deep dive. 1. This is an incredible question. What I love about this question is it's actually counter-intuitive. But, it would only be counter-intuitive provided you know enough about the subject. So, the fact that it is counter-intuitive to you is a great sign. (it wasn't for me for a long time :-/). Physics by Eugene has tackled this in great details. Check out his videos. [His channel is famous] 2. Interesting question! And interestingly, the force on a positive charge would be in the same direction as well. Do it do it! 3. All classical derivations are bound to fail at some point. Because, at the end of the day, classical physics is just wrong. And you can't use it explain stuff in the realm of quantum. I think light momentum qualifies to be the realm of quantum. But, the deeper point is that that doesn't mean we sell the classical theory short. What I find absolutely mind boggling is that it is still excellent at providing intuition. 4. My history is rusty here. But, Compton effect is way more interesting. It's a type of inelastic scattering.

[@martinfenner3222]

Over 40 years ago my physics teacher taught us, the drawing of the electromagnetic field in our physics book is wrong, because electric and magnetic waves were shown in phase ignoring completely the 90° phase shift between them. IMHO this is a well known but not corrected fact in nearly all books i know of for such a long time. Not to mention that it's highly misleading about the nature of light and radiation.

[@shubhamkumar-nw1ui]

It is so fascinating to think that Light is a self sustaining unit. When I leaned about this my mind just blew away.

[@MostlyIC]

I really appreciate Joe's questions, I'll add another 5. this explanation doesn't seem to mesh well with light that gets reflected, as this explanation seems to require quite a long time and distance for the light waves to interact with the electrons.

[@jimroth7927]

Will the derivation you described also work for positively charged particles? If not, the whole argument falls apart, at least for electrically neutral (non-ionized) atoms.

[@gordonlocke5937]

Another “Thought Experiment” might be to consider the effect on a single isolated positron in space?

[@carlosdgutierrez6570]

Unless we are talking about light with extremely small wavelength like x-ray and gamma rays, the photons of light don't interact with the nucleus of the atom, it only interacts with the electron cloud around the atom, it doesn't get to interact with the nucleus.

[@troypresley]

I love your enthusiasm! This was the first time I’ve heard this explained in a way I understand, so thank you!

[@olived9560]

This is an amazing explanation - I've been wondering why this was for a while and this explains it perfectly!! Thank you so much!

[@maulikaryan]

I want to ask from this from the video - What about electrically neutral particles like a neutron? Would it not experience any force?

[@prgnsean]

i just asked this same question. hope we get a response ...

[@mattgbarr]

Neutrons can be electrically polarised, so I'd assume that the large (local) electric field of the EM wave should allow for momentum to be transferred.

[@434mp]

No, the electric and magnetic field depend on a charge q, if that is zero the total forces are zero.

[@mattgbarr]

There is something that you are forgetting here. Neutrons are not fundamental, being Hadrons composed of three quarks which *do* have charge. Under a sufficiently high electric field the neutron should acquire an induced dipole moment; this imbalance of charge should (in principle) be able to interact (weakly) with light's EM field. In reality, a lone neutron should be pretty much transparent to light!

[@meartin]

Feynman rocks audience shocked😮

[@thesquatchdoctor3356]

And he's making sure he gets your wife's number for later

[@meartin]

@@thesquatchdoctor3356 yo wife in me dms

[@InsaneMetalSoldier]

This dude had me hooked from the first video. I started watching you today man, great energy, great graphics and great explanations!

[@Mahesh_Shenoy]

Thanks a lot for sharing that :)

[@Freddy18w]

Always feels good to Feynman a Maxwell equation. I love the joy you have for it.

[@charlesfreeman4147]

Very interesting! Does this mean the momentum of light is dependent on the electrical conductivity and magnetic permeability of the material?

[@triaswinter296]

I have the same question, sounds weird to me

[@ManishaGhogare-qw1wj]

If instead of electron there is uncharged particle then how it will transfer momentum

[@mbrusyda9437]

Photons don't interact with uncharged particles

[@DanMahoney-vq9yo]

A step by step and easy to follow ( even for someone like me without a strong physics background) explanstion of a rather complex subject. Well done.

[@user-xn9wv9hr5j]

Great job my indian friend U did a good job that other physics related KZbin channels can't deliver. Reasoning physical subjects using equations is what others can not do!

[@paaabl0.]

What about particles without charge? Awesome video, btw. Amazing energy!

[@tyruskarmesin5418]

I don't think particles without charge interact with electromagnetic waves.

[@skat9000]

Matter is made of atoms and atoms certainly have electron clouds, so this explains 99% of classical Physics. For more exotic no charged particles, lets wait for next videos :-)

[@golden_smiles]

@@tyruskarmesin5418 Meanwhile photons can pass a momentum to neutrons.

[@Derrekito]

Aren't the directions of comet tails affected by other types of radiation (e.g., electrons and protons) rather than light? Edit: I looked more into this. The ion tail (or gas tail) is influenced by charged particles and points directly away from the sun. The dust tail is influenced by light and doesn't usually point *directly* away from the sun due to a combination of the direction the comet is traveling and radiation pressure (i.e., light momentum?) - anyway I hope I got that right.

[@Native_love]

I love your approach to Physics! Thank you sir for explaining everything like a friend would! And for talking to Feynman!

[@harinayan7879]

Awesome job explaining this topic. Always wondered about it but never went beyond the equations.

[@fugitive6549]

Bro, I love you. Discovered you in the relativity video and now I'm hooked.

[@aquahood]

This is where my favorite physicist and speakers ever you can call him a philosopher a physicist and a science educator of the Master Class

[@puneetkumarsingh1484]

All these years and only now it all fits in place 😭. Thankyou very very much for making this video 🔥🔥🔥

[@hatfieldmain]

Superb explanation. Thank You for taking the time to make this.

[@markorletsky5976]

Best video I have seen in a long time - on any topic.

[@prapanthebachelorette6803]

Omfg you’re the coolest physics teacher I’ve ever seen. I’ve been traumatized to tears by math and physics when I was in school because nothing made sense to me and I just had to grind my way through in order to survive the system. Your enthusiasm is contagious I might consider revisiting this kind of stuff again someday 😊🎉

[@rjrampershad]

Thanks Mahesh. Beautifully demonstrated and elegantly explained !

[@tyapca7]

Very, very, very nice, clean and smooth lecture. Thank you so very much. Keep on, dear Sir!

[@milansalidummay]

Thanks for your straight forward explanations Mahesh. You have helped me understand many things in your video presentations! THANKS A MILLION!!

[@kelly4187]

Masterful. Subscribed. When i did this in my first degree (astrophys bsc) we used F=dp/dt as a method... which was basically your approach, in reverse, without the physical interpretation. I prefer yours :)

[@Epursimov]

Awesome. You're way of explaining physics is deeply intuitive. Please, continue.

[@Dtomper]

Pleaaaaaaaseee keep making these educational fun videos. I LOVE watching them and I learn a LOT from them, thank you so much!

[@alinsoncruz6674]

I hope you are a teacher. Everything makes sense and easy to follow!!

[@sharathkumark9692]

I am not physics student or something, and I love physics. This is the 1st video of yours I am watching. This one video itself was enough for me to subscribe you, you are awesome ❣

[@ugurunver2403]

I recently find out your channel and I'm so grateful for your work done here. You are a great teacher. I have two questions for you about the momentum of light. 1) Since when an object slows down when it transfers its momentum to another object in macro level, what should we do we expect from the light (or wave) to do? I mean, in classical perpective, it should slow down a bit but we know that is not the case. Does it change the direction? Does it completely be absorbed by the material? Or does it changes its its wave amplitude only, or maybe its wavelenght/frequence? What should we understand from a light wave that transfers a part of its momentum to an object? I mean, what happens to the light wave after the "collision"? 2) Maybe you already answered this in your other videos, or some physicist already explained it but i still want to ask. I think the first question before "how light has momentum?" should be "how light has velocity?". Since "F= m x a" and m = 0, how can universe manage to speed up a photon (or a wave) in first place? What pushes a photon at the begining of its journey, and how? Is there an acceleration and gradually changing velocity (from 0 to c) or does it start from c? In short, what happens to a photon at the exact moment of its birth, and how does it happen? (I wonder this because "the momentum of light" is induced onto a photon at that exact moment, and i think we could understand the momentum of light better if we could understand its origin). I would really appreciate if you could answer these questions because i'm really curious about it. Thank you in advance.

[@AlperShal]

Bro there is 3 days left for my university entrance exam and there isn't really much left for me to study. Not because I mastered everything but because I got totally burned out. My brain can't grasp nothing anymore as it's been 2 years since I have started studying. And somehow, I had forgotten the formula qBV. I had the right hand thing and what means what memorized but forgotten the q part. There really is a high chance of that being asked and somehow you made me remember thst because of my interest at understanding physics. I know this is just a coincidence but I just wanted to thank you for both this and making me understand how momentum happens to be without mass. Really appreciate it.

[@jackflash8756]

Excellent video and answered many of the mysteries and confusion I had studying physics.

[@nedanother9382]

Big thumbs up for your energy and enthusiasm about such a subject. Very Feynmanesque. I actually got it and I wouldn't graduate high school these days

[@tomkatsouleas6233]

You are an engaging teacher, and the analogies are excellent. The first part of the video is accurate, but it goes astray when you discuss the vxB force on the electron. The electron does indeed begin to move up in response to the downward electric field, but it takes a while to get up to speed - a quarter of a light cycle in fact to reach its maximum. By that time, it is still going up while the B and E fields are beginning to reverse direction. As a result, in the next quarter cycle, the vxB force is in the opposite direction and the forward momentum gained in the first quarter cycle is exactly given back in the next quarter cycle. Alas, this is why it is difficult to miniaturize a large particle accelerator by “pushing” electrons forward with light from a high-power laser. There are ways to overcome this quarter-cycle phase shift between v and B and achieve a net momentum gain by using light with an intensity gradient or electrons in a medium such as a plasma. But if it were that easy, those of us doing research on accelerators might be out of a job! PS. "Surely you must be joking (about your conversation with the late) Mr. Feynman." (Though I did have the privilege to discuss this topic with him as a graduate student many years ago).

[@ramonruilopezv.9457]

Great video and great performance. Clear explanations and examples, easy to follow, great analysis. Chapeaux

[@fantomstranger3965]

You have no idea how much I needed this video

[@atheistjourney]

I always wondered about this. Great explanation. Thank you!

[@LowellBoggs]

Thank you Mahesh for answering this question that has nagged me for decades!

[@mikesommer9866]

This is a great explanation. What makes it especially satisfying to me is that voltage (associated with electric field) has units of energy per charge, and vector potential (associated with magnetic field) has units of momentum per charge. Electric fields change energy and magnetic fields change momentum.

[@davidvernon3119]

I love your passion for this topic!

[@jamesmunroe6558]

Brilliantly clear explanation. Thanks so much!

[@catawbaisland]

Great video. A minor correction. In my set of Feynman lectures, it's Volume I (not II) Chapter 34. Thanks for directing me to this gem.

[@kryt7955]

You rightfully earned a sub. This video was amazing! Keep up the good work!

[@Macedonio414]

Great video! I finally understand how light could carry momentum. Every time I've asked that, I always got the same answer: "E=mc^2 is not the complete formula....". Ok, ok... but where does that momentum come from? NOW I UNDERSTAND. Thanks to Feynman and you for explaining it to us so darn well.

[@lesseirgpapers9245]

force is not Momentum

[@ClarkPotter]

You're great, my man. I can tell you grew up good at math, bc you ask WHY something works and won't rest until you know. That's all it takes to be good at math.

[@bharadwajkk6823]

Man, you're so excited this is actually so fun to listen to despite me supposed to be on JEE prep lmao Also, Feynman musta been a moma cuz he never fails to deliver

[@jayeshbhagchandani9798]

Mind blown! Thank you so much for the clear explanation.