How can a photon have momentum?
ฝัง
- เผยแพร่เมื่อ 20 พ.ค. 2024
- Physics students often ask how it is that a massless photon can have momentum. In this video, Fermilab’s Dr. Don Lincoln shows that the question arises from a misuse of equations and also shows that, when you think about it, it’s not surprising at all.
Why E = mc2 is wrong:
• Why E=mc² is wrong
The origins of mass:
• The Origins of Mass
Further reading:
opentextbc.ca/universityphysi...
hyperphysics.phy-astr.gsu.edu/...
Fermilab physics 101:
www.fnal.gov/pub/science/part...
Fermilab home page:
fnal.gov - วิทยาศาสตร์และเทคโนโลยี
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Thank you for all you do. Please NEVER "apologize" for including math in your videos. We WANT the math. Just be sure to explain what arcane symbols mean so those unfamiliar with those symbols can understand.
And of course make it accessible for those of us who struggle with the math. Math doesn't intimidate me, REQUIRING math intimidates me.
AMEN, g! Agreed!
Math is good for calculating, but I hate how math is misused by many to show how physics works; It gives people the illusion of understanding. Same way people nowadays just google whatever they don't know and end up feeling like they know more than they actually do.
"Everyone" knows that E = mc². But how many know WHY? That is what's important to me at least; understanding how the universe works.
Question your unconscious assumptions.
@Science Revolution because the flame is transparent, and also because the laser used to move drones does it with heat transfer, not momentum.
I was just gonna comment that
*I never knew the Kinetic Energy equation was actually a summation series. I have never found it in any physics textbook. Thank you*
According to Wikipedia it's actually mc^2/sqrt(1-v^2/c^2). The approximations given are a Taylor series for the inverse square root
But have you read Landau and Lifshitz Theoretical Physics?!
me too
@pyropulse Yes I learned that in undergrad.
@@chair547 that equation is total energy.
Kinetic energy one is mc^2*(1/sqrt(1-v^2/c^2)-1)
At least light pushes me out of bed in the morning....😆
I absolutely loved this. I started my undergraduate in Manchester 2005, and probably the most important lesson I took away was that these top physicists were incredibly humble and willing to say they don't know, or that they would need to look into it more before answering. The general public has this misconception that professors are arrogant, certain of themselves, and think they know everything. Nothing could be further from the truth.
The most common sentiment was everything is more complicated than it looks, we don't fully understand even if we can measure and model it fairly accurately. That as you said at the start, the intuitive explanation, even after decades of their research and study is still not all that intuitive.
Much of the general public dislike the fact that scientists won't give absolute predictions and answers, they demand certainty and dislike the unknown complexities. But physicists love that uncertainly, they love how strange and counter intuitive the universe is. If reality was just Newton's laws et. al the world we find ourselves in would be so boring. Instead we are confronted with something strange which when we begin to understand we unearth beneath strange layered on strange on top of strange. It is truly beautiful and fun. I'm glad I wasn't born into a time where we truly knew everything.
So beautifully put!
A great example of Dunning Kruger in action too, especially the ironic certainty about arrogance and looking down on people.
Excellent video as always. Fermilab has always been one of the coolest youtube channels out there. Thanks Fermilab and Don for making science more approachable for everyone!
Thank you for the ongoing uploads.
Haven't looked on any physics videos in a while, but I feel a sense of pure joy, and relief from the crazy stuff going on in the world, as soon as I do 🙏
Great Video. I love how clearly Don explains things. We live in an amazing time. LOVE FERMILAB!!!
Wonderful material...please, DO NOT STOP, we love the videos, they have been informing us for years. Thank you for all that you do!
I've missed these. These videos are always so well explained.
@Science Revolution may be it had thermal convection
@Science Revolution one reply ok but replying on all comments? I'm reporting all of your replies.
@@esajpsasipes2822 spamming can be annoying but at least he'll get a few different replies.
@@alwaysdisputin9930 he is very likely a bot
@@esajpsasipes2822 It's a decent question. I think maybe the flame is made out of plasma & it's mostly empty so the laser does push some particles but only a few.
I'm so glad I found this channel by chance. First I was trying to understand refraction of light and now I'm trying to figure out if mass is even real. Thanks for explaining concepts in a fun and clear manner!
Mass is an emergent characteristic of structures in our Universe; analogous to how life is an emergent characteristic of certain configurations of matter. What we refer to as (quantum) particles is a particular kind of fluctuation in a quantum field. Mass emerges as a measurable quantity associated with any structure that forms as a result of interactions between a collection of particles. Even though a single photon is massless, a collection of photons confined within an internally mirrored box adds mass to the box due to the (confinement of) photons. The mass of most quarks and leptons is due to the interactions of their underlying quantum fields with the Higgs field.
@@RockBrentwood I appreciate the detailed response! I was referring to the fact that he says mass is an illusion at 8:20 but thanks for your explanation as well :)
@@RockBrentwood Wow, I hadn't really thought about the math that way, but it's quite illuminating in relation to the ideas behind special relativity. Thank you.
@@RockBrentwood side note: 1. for a photon V is always slower than C (except for an imaginary vaccume) from an outside reference frame.
2. its m=m0/√(1-(v/C)²) NOT (1 + √(1-(v/c)²)) (m0 is the rest mass)
@@Patrik6920 isn't the rest mass of photons equal to zero?
Dr Don, you are the best teacher and your most important quality is you love what you do! Watching is always a pleasure.
Thanks for this very clear explanation, I like that you show the nuts and bolts of how things work at the subatomic level, without resorting to a lot of analogies, which usually just confuse me further.
This, along with and followed up by "The origins of mass" are a truly enlightening combo. Thank you for them.
I've always seen you so calm and collected. I really wasn't expecting these "danger" segments!
@Science Revolution Dunno but that has absolutely nothing to do with my comment.
@@mugwump7049 yeah I'm wondering if it's a bot. They're putting the exact same reply under people's comments.
i had the momentum to watch and the mass to sit and learn with my energy and the velocity to go check out more of doc don’s videos. i actually understood this pretty clearly - thank you.
I came across this channel just right now. I'm 67. It's soooo beautiful!!! I can't stop crying. There are so much cool stuff to learn and my time is almost up...
Blessings!!!❤
Loved this presentation, thank you so much!
I think it would be a good idea if our HS and Physics 101 instructors did a better job of explaining that the basic stuff they ae teaching only applies in certain circumstances.
Learning things as complex as physics is hard enough, UNlearning things is much harder.
Great point. I wish my instructors would have continued their studies by watching these videos. Then I may have been better informed from the get go.
@Science Revolution Buzz off Bozo
Your instructors probably didn't know in the first place. How can they teach what they don't already know themselves? And by the way, just because you watched a few videos on a subject, it doesn't mean you know anything about it really. In fact, you likely just know enough to make yourself think you know the topic when you really don't. In something like this, it's harmless. If the topic was auto repair, you could easily get yourself in a lot of expensive trouble.
Great video as always. I really appreciate Don's sense-of-humor (or that of his writers, or both). I wasn't even expecting it to take a detour into the deeper nature of mass.
And I agree with another person's comment below: definitely no need to apologize for showing equations. Though I imagine you are trying to keep a balance, and avoid alienating those who are just getting started.
One of the things I feel gets glossed over a lot in explaining to mere mortals how there's this hierarchy of explanations for what mass is, is what gives rise to the inertial properties of energy-masquerading-as-mass. I would love a more in-depth video about that.
In broad strokes: if the mass of nucleons largely comes from the energy of confinement of quarks, and mass of everything *else* comes from interaction with the Higgs field (does that apply to quarks too, or only to leptons and gauge bosons?), then what determines *that* degree of interaction?
However, the explanation always seems to stop there, and never gets into what it is about confinement or the Higgs interaction that "binds" energy, and makes it "massy" (I'm using air-quotes, because I really lack the vocabulary to express it).
Since I've got an EE background, I've always assumed (probably wrongly) a mental image of "holes" in a semiconductor having effective mass, as a way of thinking of the Higgs field. But I should probably make the time to teach myself the the math for that, and learn what the actual model is.
Anyhow, apologies for my rambling. (And that was the brief version...)
These Fermilab videos are fantadtic.
This guy explains things very well.
Great to watch.
THANK YOU SO MUCH, this was my question and I feel so good when someone reads the comments to give such beautiful videos. I thought my comment would at most be featured in viewers thoughts section but I got a video to my question... And of course now I understand it so much better than before thanks a lot.
i envy you.. i asked all grear channels my question but got no reply so far😔
Someone’s been lying to us? Who woulda thunk it.
@@kwnorton5834 It was funny when he said ThEY'Re LYiNg to YoU SheEPle but then he said we're made out of mass
but science has never proven what we are. We could be singularities for all we know.
@@kwnorton5834 science isn't even close to say any absolute truth about the whole universe. But it's not claiming that it does either. It's the best thing we got though.
This was well thought out, well executed, humorous and educational at once (an incredibly difficult balance to strike). I came here after reading about the pellet propulsion spacecraft idea that university of California is working on, which uses photons as a "fuel sorce" of sorts. Good read if you're interested. Anyway, made me wonder about exactly the point of this video, and this did help put it all a bit into perspective. Thank you for this, and your presentation style. I look forward to burning through more videos from Ferilab as soon as I have the free time
Your genuine smiles and love of physics are a gift thanks for sharing your knowledge :)
A superb communicator,. These videos do a real service. Well done and thank you.
Love the fact that you shared the assumptions under which they apply. Learnt something new. Thanx 😊
Math is easy, as long as, it's explained well. Like this one
I had to rewatch this video a bunch. Great stuff! More videos with math would be great! Analogies are fine but it's wonderful to see the actual explanations and equations.
@Science Revolution what the utter fuck are you talking about...take your nonsense elsewhere
Thank you so much for helping my mind parse out these equations in a more intuitive way!! This is a wonderful explanation!
This is the first video I've seen from this channel. Thank you so much for the excellent explanation!
One of your funniest and most engaging videos to date! Thank you and keep 'em coming!
@Science Revolution What?
Wow, amazingly well explained video. I just didn't think that the math was that complicated as warned haha.
You know how sometimes you wish you could rewatch a movie, reread a book, or go play a game like it was your first time? I kind of wish I rediscover the wonders of science again. For me to get these kinds of wow moments again, I need to really dig into the actual math of wave functions and stuff, and I'm honestly not at that level, either. I'm in this weird in-between.
@@kindlin Read up on biology. Evolution has all these predictions that boggled my mind when I first learned them, well after I finished taking school biology. It's a shame we don't teach it in school. Totally changed my outlook on the living world.
It was indeed very well explained.
@Science Revolution How would it move a flame? What does that even mean? You need to define your question properly before you can get a clear answer.
@@kindlin the crackpots are everywhere. Even though it's not 100% perfect, it's best if we take for granted what is consensus first. Very rarely do crackpots have reason. Crackpots don't contradict just this result, there's always a crackpot denying the most basic universally accepted ideas, and when they deny them it's always with a lot of non- sense, like, "hey, I am an uncomprehended genius, I am right while all you doofus are wrong."
I started reading the New Scientist in the 1950s and have studied Chemistry, Metallurgy, Psychology and other science subjects at graduate level, dipping into every known science along the way, with many of the popular ''usual suspects'' in science paperbacks. I've seen all those equations (no particularly advanced maths) there and I was always a duffer at maths. For my secondary school Physics Final Exam (''A'' Level) I derived the Ideal Gas Equation... from memory of the steps rather than by mathematical genius.
I do not recall ever seeing ANYWHERE that Einstein's famous equation only applies to STATIC objects!!! That piece of information alone answers at least one of my fundamental questions about mass at luminal velocities... great stuff... thank you.
E=mc² is very often not clearly communicated. Sometimes people use the m to mean relativistic mass(ym), in which case the equation holds at any velocity below c.
In that case one should write E0=m0c² with E0 and m0 being the "rest" energy and mass of an object.
But since relativistic mass has fallen out of favor in modern physics the equation m is basically synonymous with m0 and mass is always rest mass.
Then the clear way to write the equation would be E0=mc².
But people just write E=mc² without specifying what they mean.
Even though this is just a scratch on the surface of quantum mechanics, it still blew my mind after you provided the example of protons with internal motion vs a photon. This ties energy and matter together quite brilliantly in my head.
Literally, lol
Such an elegant, beautiful explanation. Well done!
If I’d have seen this as a kid, I’d have really wanted to become a physicist.
This is an awesome explanation and I can tell you have a real passion for the science and for sharing it with others! I totally randomly found this channel but I'm subscribing for sure :)
I appreciate the work you did here.
I remember how much of a revelation it was, when I learned about mass / energy equivalence. Cool stuff!
Speaking of total BS...
@Science Revolution I know photons can interact with particles such as electrons, but do photons interact with other photons? I'm genuinely curious, but I somehow doubt you're the right person to ask, given your comment..
@Science Revolution - Let me guess, next you're going to say there's this stuff called "luminiferous aether" that carries the light waves?
@@malachiwiens2455 they don't, at least not directly.
There're higher-order mechanisms where they create another virtual particles which can then interact and return into photons, see en.wikipedia.org/wiki/Two-photon_physics
@Science Revolution First, change your handle, because THAT is the only BS around here. Second, stop trolling, because either you know better and are just trying to provoke people into arguments, or you DON'T know better and are too stupid to learn. In either case, shut up.
Classic shirt; relatively silly but you can really see his energy.
The story about that picture that I heard is that he was annoyed at always being photographed, so he decided to ruin the picture by sticking his tongue out. He was even more annoyed when the picture became so popular.
Underrated comment
Generally speaking, it’s special!
@Ratnasambhav Sahu relativistic shirt*
@Ratnasambhav Sahu idk just said something.
I'm proud to say that I visited Fermilab a few years after it was ignited. I became a new law student in 1976 near Chicago, but I also worked as a part-time flight instructor at the fledgling DuPage Co. Airport where DuPage aviation was the largest Beechcraft dealership in the midwest at the time. I worked Fridays, Saturdays, and Sundays 8 hours per day as a primary flight instructor. One of my brand new flight students was an employee at the new "Fermilab". We used the lab as an aerial reporting point for the control tower. We would say something like: "Dupage Tower, Beechcraft N12345 at the atom smasher, landing with Bravo (the ATIS or automatic terminal information service) which was a tape-recorded broadcast of the weather conditions and runway usage at the time. Anyway, one of my new flight students was a "computer genius" who worked at the "Lab" at the time. We became friends and he took me on the "inside tour" of the lab. The tour was so sophisticated that today, it would have beyond normal security protocols. But back in the day, it was permissible. This was about 1978 or 79. I bet you guys who work there today can't imagine the archaic computers that were being used back in the late seventies.
I remember saying to one of my other flight students who was a software developer: "someday, instead of having analog flight instruments, I'll bet we'll have little TV screens on our flight panels". He said: "We already have the technology". It's too bad it took so many years for the aviation tech to become commonplace.
Thank you so much! The concepts are well explained and easy to understand
I love your videos, they are helping me in understanding our reality better. I have got a comment from the sound engineer perspective. Loudness of your videos is exceptionally high, could you please try to master audio at lower RMS/LUFS levels? I would suggest to try the range -3 to -6dB. TH-cam audio compression is probably generating some chaos too.
Agreed. I had to turn down my volume to comfortable listen to this video. Otherwise the content and presentation is great.
Thanks for that deep explanation. I would like to know about another variety that I suppose is even more fundamental i.e. angular momentum and spin of massless particles. Is it related to the concepts of "charge", "Colour" in subatomic particles?
That's great. Never thought of it in this way. Thanks for sharing!
I always learn fascinating things with your videos. Thank you so much!
A beautiful articulation for something I have believed for 40+ years. Never seen it so nicely explained by equation, but thank you for confirming my thoughts
Excellent and informative presentation!
So informative and amusing! Great video!
Thank you for another beautiful lecture, you are TEACHING and REMINDING in beautiful way .Many times people ask me I DIRECT them to YOU.
Thanks for this video. I just learned about this in my physics class not very long ago. I'd like to see a video explaining the de Broglie wavelength next, and how this is related to photon momentum.
@Science Revolution Please provide a reference to this. Or start over with whatever you consider is knowledge.
@Science Revolution seriously are you a bot? You just have the same copy/pasted reply under people's comments.
@Science Revolution Because light things that move very fast can have tiny momentum and a crap ton of energy.
@@malachiwiens2455 I've seen that a few times recently, 'regular' comments posted more than once under different names.
Science Revolution, tell us if you are human or not
@@alexrobomind yeah, but for EM waves, photons, the energy is directly proportional to their momentum, isn't it?
Ultimately, at the subatomic level there is no momentum, however it can be useful to think in these terms. -just as it is useful at some level to accept GR as true, knowing it has its limitations. At subatomic level everything is about 'pushing and pulling' (classical description of nature's fundamental dynamic).
In his famous (to me) book QED The Strange Theory of Light and Matter Richard Feynman attempts to describe their best and most complete theory, but what he really accomplishes brilliantly and eloquently is to frame and explain a puzzle of what they do not understand. It taught me that mathematics does not mean understanding although it can solves problems and find solutions. It taught me that imagination is the door way to understanding. In the end the book showed me where the answers are to the puzzle and inspired me to search for the answers.
Nature is not complicated at the fundamental level and easy to understand. What is astonishing about nature is that from just one fundamental principle nature can create infinite complexity, and that's when maths comes into its own.
Ah, the old conflict between quantum physicists who want to understand the fundamentals of the universe, and the other side whose motto is "shut up and calculate".
@Science Revolution Because ... that has nothing to do with it?
Yes, fundamentally mass is a tension- the tension of gluons pulled by quarks. Tension is a very important concept for the imagination, as GR is also the study of tension (although tension of dimensions and time).
Indeed, physics is a model. Intuition is the tool to understand the underlying reality. Imagination is the key to intuition.
(I enjoyed your post. You're clearly a philosopher in the original context- "lover of knowledge".)
@Science Revolution What is a flame? A flame is not a "thing", but individual atoms at high velocity (plus light). "Lasers can't move a flame"? Well, that's not correct. Lasers are how we trap individual atoms, and move them around. All we need do is match the frequency of laser light to the absorption frequency of the atom. When those frequencies match, the atom slows it's motion toward the laser. Add lasers from multiple directions, and you have an "atom trap". Magnets then help contain that atom from hitting the container when it loses momentum.
You want to stop a "flame"? Well, that's a motion stream of many individual atoms. Can you stop a river by throwing a tennis ball into it? Nope. So let's do apples to apples: Can you lower the temperature of specific atoms in that stream with a laser? You bet 'cha. Just find the absorption frequency, point 'n shoot- voila. Yes, lasers can "move" a flame by transferring their momentum. You just need the right frequency of laser light.
@@onebylandtwoifbysearunifby5475 No, I am no philosopher, however it can be fun. I would describe myself as a self educated budding fundamental physicist by necessity (there is a bigger story here). The reason that my comments seem philosophical is because I cannot discuss what I know publicly. My comments are designed to capture Don's attention as I know that he has a security clearance and I would know with whom I am talking to. My knowledge has the potential to transform human society with exponential speed, however it cannot fall into the wrong hands due to potential of misuse.
I have a problem of not being able to pass this information to liberal democracies, including my own country, because I am thwarted by a 'party' who has already achieved this and is in manipulative control of governments and its agencies. Hence, my condition of passing the information is through intelligence agencies with at least six individuals face to face and repeated for at least three consecutive days. Some authority needs to be ceded to me in case of unexpected circumstances. Verification of credentials is essential.
Warning: all means of electromagnetic communication is manipulated or prevented.
Problem for me: the more I warn you about the said third 'party' the more you consider me a conspiracy theorist, and the less I tell you about the third 'party' the easier you will be manipulated and the communication effort derailed.
My personal information:
Jerry Mlinarevic
9 Dryden Court
Bundoora Victoria 3083
Australia
Mob. +61 403 447 155
VoIP. +61 3 9466 8023
email. jerry.mlinarevic777@gmail.com
Remember that just as I am being thwarted that you too will be manipulated to ensure that your effort to communicate with me or an attempt to inform others about this will be interfered with.
This has been painstakingly typed on a TV set top box which is more secure than computers or phones.
FloatHeadPhysics recently did an episode about the same question. His intuitive explanation is amazing, and he also derives formulas. What gives light the ability to push things is its magnetic component. The E field vibrates the electrons in a material, and that moving charge is acted upon by the B field. It makes so much sense even in a classical way.
FLP is pretty much making videos about the feynman lectures at this point. If you'd like a deeper understanding about their topics you can read those.
Dr. Lincoln is one of the greatest science communicators alive today.
Thanks, Dr. Don, for another informative, and dare I say... entertaining video😀
Brilliant. There's a beauty to the idea of mass being largely the result of kinetic energy in sub-atomic particles, but I'm not clever enough to think what it is .... but it's mind-blowing. Another fantastic, and lucid presentation.
Same here..... at fundamental level if Atoms are made up of quarks then why they derive most of their mass due to some kind of elusive binding energy.... just beyond my capacity to grasp
@@rajkumardhakad8773 Maybe it's like trying to comprehend infinity, we're not really wired up to truly understand it ....
Idk, I have been watching these videos for years and things make a lot of sense now. Still mind blowing but more in an “of course” way. What helped me to get into the right mindset was the video about quantum field theory, always by dr. Lincoln. When you grasp and keep in the back of your mind the idea that what we perceive as reality at the fundamental level is just a cobweb of buzzing energy fields everything else, not only is not so surprising, but gets easier to understand.
Of course speed is important niels borh was waiting for als traffic Light and investeren the atom bomb danish he was yunis
Explanation is crystal clear!
Great video.
I was drawn in by the title in hopes that it would touch on and help explain something that I observed experimentally. That being light (laser light in this case) transferring energy to particulate impurities introduced into a near vacuum environment as a laser passed through striking a target.
When doing this, it is possible to lay down (basically an atom at a time) the vapor of copper and other materials as a doughnut shaped deposition on the target.
My guess was that the material was surfing the laser and finally being annelid to the target but I'd love to hear your take on this.
Great explanation. You do a good job of simplifying these complex topics. Thanks for sharing
Love the math vids. It's the cosmos' language. It's amazing to learn something new about literally EVERYTHING across our universe. No matter how big or small.
If you want to watch math vids go watch redpenbluepen He's pretty good and you can work on the problems with him.
No... Math is not the language of the cosmos. It is our language. Do not think for a second that the universe "communicates" or "orchestrates" through math. The universe just is what it is, and WE use math to describe what we see. And it's flawed too. Just take a common example of pi. We will never be able to calculate pi to perfection. We can get it "good enough" for our engineers, but we'll never be able to describe it perfectly. Our numbers, our math, falls short and the universe clearly hasn't ordained itself such.
Really well explained. Thanks, very interesting.
For anyone who's confused about the equation E² = (pc)² + (mc²)² and the fact that you still have to know the momentum in order to use it, and every way of calculating momentum that was shown in the video included mass, here's a transformation that solves this problem, i.e. allows you to calculate momentum for something that has zero mass: p = (sqrt(E² - m²c⁴)) / c
If m = 0, then m²c⁴ = 0, and the whole thing can be simplified to: p = E / c
You can also get the momentum from the de broigle relation p=h/wavelength.
I don’t understand how this is an explanation: If you put p=E/c in the first equation you end up with E^2 = E^2. What am I missing in the thought process ?
I think this is how our alien friends are getting around
@@Wolfman4Jack There aren't two equations, just one equation rearranged. By doing that substitution, all you did was rearrange it in a different way. The fact that you got E^2 = E^2, which is definitely a true statement, just shows that the arrangement he gave you is consistent with the original equation. It doesn't "prove" anything, its just writing it in different ways that may or may not be easier to understand.
Loved this one especially ❤️ Don't worry so much about the Maths, it's very basic and easily understood. Thank you Dr.Don and Fermilab👍
In physics class they showed us this formula for momentum of photons which is Planck's constant over the wavelength of the photon. I was curious to know it's origins and took m=E/c^2 from Einsteins formula and somehow got that exact formula by substituting E/c^2 in the classic momentum one. Idk if it makes sense but I found it interesting.
Yes, I was thinking this would be easy to explain. Just use E=MC^2 which says that mass and energy are equivalent. Solve the equation for mass and you get M=E/(C^2). So a photon has energy and is moving at the speed of light. If you know the energy of the photon then you can calculate the mass equivalency and proceed to use this value to calculate momentum.
@@perrygershin3946 yeah, so I guess that the momentum of a photon is dependent of it's energy instead of it's mass, since it doesn't have.
Or you can just take E=pc which gives you the momentum of the photon rather directly.
@@tarmairon431 i don't understand where this comes from. Can u explain pls?
@@dinaangelia5145 It was in the video, but I can repeat it if you want.
I assume you've heard E= mc^2 ?
That's a simplified version for particles at rest. The complete equation is
E^2= (mc^2)^2+(pc)^2
Since photons don't have mass the equation for them is
E^2=(0c^2)^2+(pc)^2
The first half is all equal to 0 which leaves
E^2=(pc)^2
That also can be written as
E=pc
Sidenote: Mathematically you would also get E=-pc as a solution, but that doesn't make any sense in physics so it can be discarded.
You are the best teacher I've ever come across!!
Thank you so much !!!
So helpful
"You probably remember all those equations I just showed."
Me who failed physics classes and switched major to linguistics: *laughs nervously*
38 years ago I received an Associate in Science in Laser Electro Optics. I want to thank you for helping me relearn what's been forgotten. I was an engineer and did engineering work. No need for complex equations other than length × width or 2+2.
Kinetic energy is the sum of KE's of all molecules in a gas enclosed in a container.
This ultimately results in pressure of gas in the container.
P= 1/3( Rho).(c squared).
Hope it is now clear
@@krishnaraolingam4812 Bazinga
for the question "how light can push something?" The way how I imagined it is when you heat up gas using EM waves like light from the sun or microwave. The gas molecules heat up and move faster because the photons have transferred its energy into the molecules thus increasing the molecules kinetic energy and therefore moving faster. It gives as sense as if like those photos have collided or "gave push" to the molecules of the gas.
the photoelectric effect yes
@@orgaynigga8425 yeah that is included but i think for high energy photons like uv and above. radio photons cant knock electrons but merely vibrate the molecules causing heating effect
Excellent information as always. Worth mentioning that velocity is always relative to a frame of reference, so kinetic energy and momentum are relative to that frame of reference and will be different for a different frame of reference, except for photons I assume.
The energy and momentum of photons still depends on the reference frame. That's just redshift.
Very interesting video, thanks for sharing this important information
wow, thank you, may thousands of young people be inspired by your fascinating explanations. Two things occurred to me, should one better write m = E/c2 ? Looking out of the window, seeing the solid hills, there is nothing there?
The problem with you suggestion is that not all energy is mass (all mass is energy) and keep the definition of mass as is (m^2=E^2-p^2). Not sure why you think the hills are nothing.
@@kylelochlann5053 Thank you very much indeed. About the hills, .. I first thought about Rutherford, Atoms are nearly empty. Through this Video I thought that I learned that even the nucleus is not entirely a true mass, but derives its mass from Quarks on a somewhat confined race track. Kind regards, Carl
@@carlbrenninkmeijer8925 When we look into the nucleus (deep inelastic scattering) we see that quarks are point particles no different than electrons so the fundamental particles do not occupy any volume at all. However, these particles do interact and it's in the interaction is what we mean by existing and "being there".
I may guess there is a mass of the earth that is including us and everything else.
@@vitr1916 True, you are right, but I was suddenly realising that matter is empty, the atom is empty having only a tiny nucleus and now I learned that even the nucleus is almost empty.....
A photon may lack mass, but it doesn't lack momentum (p=Planck's Constant times the wave length) or kinetic energy. In fact when the mass is zero the total energy of a particle like a photon is kinetic energy (E=hv).
So a mass-less particle can certainly transfer energy or impart a force onto another particle or body.
A solar sail wouldn't work otherwise.
Next step: There are no particles. It's just a wave, until it hits something.
@@Mp57navy not a wave.
Physics deals with fields.
Particles are perturbations in the field. Collisions are incidental
I accept your apology
@@PetraKann that's.. a stupid distinction...
That's like saying sound isn't a wave, it's perturbation in the atmosphere.
Very interesting and informative. Thank you! I subscribed. 😊
I really enjoyed this presentation.
Thanks for this. But there’s still one an unanswered question: for a photon with zero mass, the complete Einstein equation boils down to E=pc, but how is p defined in this case? It cannot be p=mv=mc both because m=0 and because it contradicts E=pc=mc2 which would bring us back to square one. And even the relativistic definition of momentum doesn’t help here, since it is p = m0v/sqrt(1-v2/c2). So how is momentum defined for a photon without just going in a loop and saying p=E/c? Something seems to have been left out here! Thanks for any further insights!
Thats not a loop. Or m=p/v is also a loop. You can solve the equation for p if you know E (for example from E=hf) or for E if you know p, for example by measuring it.
The same goes for the Newtonian equation. We just usually expect to know m. So by that token we should expect to know E.
@@narfwhals7843 I still don't get that. In the classical sense that wouldn't solve for m=0 because you can't divide by 0. Math was never my strong point but I still can't get that m out of the equation. ( I know, physics isn't wrong, my understanding of it is wrong but I hoped I would understand after this video and I'm still stuck in the same place)
@@rienkhoek4169 If you know the E for E=pc (which only specifically applies to massless particles) you can solve for p=E/c . There is no mass there anywhere. You can also get p from another relation, p=h/λ where h is plancks constant and lambda is the wavelength of the photon. There is also no mass in there.
Momentum is the ability to exert a force. And that does not require mass.
The issue is that the special case p=mv is so ingrained in our intuition that our brains often refuse to let go of it. But our intuition is built on our extremely limited experience in the everyday world.
@@narfwhals7843 thanks. Appreciate it. According to Wikipedia p is in kg*m/s. Or is this also limited to non-zero mass?
@@rienkhoek4169 No, but the units don't matter. Units are essentially arbitrary historical artifacts. In SI units momentum is given in terms of kg*m/s and the unit for energy, joule, is given as kg*m²/² . Both of them are "derived" units from the base units we chose. We could also chose a different system of units in which they are given as different basis or are base units themselves. We can chose units where velocity is a base unit and the speed of light is set to 1 to simplify our equations. en.wikipedia.org/wiki/Planck_units
3:51
Wake up, Sheeple! 🤣😆😂😅
Thank you for your video.
Thank you, good teaching.
If mass is just moving energy, and photons are moving energy, why don't we just say photons have mass?
Excellent explanation
I never took physics and I understand very little of what you're saying. But, I enjoy listening and hope that a little bit each time will stick.
an honest answer leads to real learning.
I have a Question.
When an electron absorbs a photon, does its velocity/momentum change instantly?
Photons have descrete energies and momentums, and I think photons are emmitted and aborbed at once and as a whole. Instant change in velocity would mean instant acelleration and that doesnt sound right. So what am I missing?
Electron is not accelerated by the photon, but it goes to a higher energy state. (Its simplified representation in school is that it goes to a higher orbit).
Oh, I heard from some where that accelerating charges make light and vice versa
Nothing happens instantaneously. If a free electron (one not confined in an atom) absorbs a photon it will accelerate over a time period that is proportional to the energy of the photon. The higher the energy, the quicker the change of momentum (acceleration). For example, an electron at rest absorbing a photon of visible light with a wavelength of around 600 nanometres (red light) would accelerate in the time the photon would take to cross that distance which is around 10^-15 seconds, which is also the inverse of the frequency of that photon. An electron moving to another energy level in an atom will also take some time.
What I've written above is a classical explanation (and so a rough estimate) but they are quantum objects and so the real answer is more complicated, but the general rule is that nothing can change faster than light takes to travel across the distance of the interaction.
@@tonywells6990 isn't there a whole conundrum about light itself being instantaneous in the absence of other mass?
It's not proven, because we can never examine it, since everything we are, and could create would have mass. So we can't create a device to measure the speed of light in the absence of it. But it is the natural outcome of taking the idea of relativity into account. Since mass causes gravity, and gravity distorts time. Greater gravity= greater time distortion. Then the inverse is also true. Less gravity = less time distortion.
And in the complete absence of gravity time doesn't exist, and everything happens all at once. But the only thing that can exist in the absence of gravity is light due to its lack of any mass, and therefor all light travel occurs instantly under these conditions.
@@anthonypolonkay2681 Light still travels at the speed of light in a flat region of spacetime, and in our universe gravity exists everywhere.
The reason we use c for the speed of light is because we can see it.
Also, Germans use k instead of c for their constants because the British were a naval power for so long that they controlled all the seas.
Groan.
@@a0cdhd Yeah, I know, they are real groaners. LOL
Keep up the great work.
I liked the final message...
If you study physics deeper and in deeper level, there is always something more to appreciate, and more you appreciate, the more you realize...more you realize that no one knows anything...and it's something for our future generations to laugh at us...when they realize more...
Thank you Don. It seems the photon is made of the 2 particles that are exactly what you claimed in 2013 in your article called "Whats the Point" where you claimed the tiniest particles are a Fixed big black particle that does not change size...... and a point particle that is bright and might have no mass at all.
We have photoed exactly these particles. A video called "Space Quantum Foam Seen here and as Fermilab says Does Exist" on TH-cam shows exactly the particles and credits you withe discovery.
th-cam.com/video/oupevm2Q7yU/w-d-xo.html
Wowsers! Go search for those never ending pockets of Discovery within this universe, that our minds couldn't even begin to fathom to be real....
Until we discover it. Then we know it's real...🙏🌿
@Science Revolution Explain how light gets to earth from the sun....I showed light particles in my experiments ...On my YT channel watch this video...it explains the Quantum Foam Don talks about and why we are overheating. "Cataclysmic pole shift hypothesis seems Likely Soon"
You told that quarks are unstable And leptons such as electrons are stable and Force between two charges is independent of mass Then why don't proton makes bond in Hydrogen atom ??
I don't understand the question.
Up quarks and Down quarks are pretty stable, but we don't find individual quarks because they bind together by the strong nuclear force. A proton is 3 quarks (two Up quarks and one Down quark) bound together. What kind of hydrogen bond are you asking about?
First of all if the speed of electron is more than that of a proton then it's relative mass should also be larger than the proton then why don't proton revolves around the electron ...
@@brothermine2292 If force between two charge particles is independent of mass that is given by F=kq1q2 divided by the square of distance
Then electron should also exert same force on proton as proton exerts on electron in Hydrogen atom where there is only one Proton and and one electron
Why don't protons revolve around the electron in an Hydrogen atom ....
@@brothermine2292 If magnetism and electricity are two different sides of a coin then why positive and negative charges exists independently more accurately electric charge exists happily but in case of magnetism why don't monopoles exist or why don't South pole and North pole exist independently or why Magnetic Charges don't exist ...
Why Universe breaks symmetry of electromagnetic force ....
If according to special relativity magnetism and electricity are the same thing from different frame of references then why don't magnetic charge or monopoles exist ...
Or in other words if Magnetism is Electricity from a different frame of reference then why don't....
If they both coexist as electromagnetic force then why don't ....
Great video I learned new things
I just discovered this channel great work and can you add more math?
Can't wait to show this to my kids...thanks for the great content.
This reminds me of something I've always told my friends: The numbers don't lie. You might be using the wrong equation, or you made an error, or you may not have accounted for every variable. If the answer is wrong, then you've made a mistake somewhere.
That's awesome. In a similar fashion, I learned as a programmer that the compiler is always right. When it seems as though it's wrong, it's because you didn't tell it to do what you thought you told it to do, or you assumed a variable would have a value that turns out to be missing at run time because you missed an edge case, etc. The compiler didn't make the mistake; the programmer did.
@Science Revolution Anyone who looks at your channel can see how full of misinformation you are. Please go away.
Hi. Could someone point me to link(s) that describes how the classical energy equation is in fact a series (~7min50sec) and where it comes from? I did not know that and am curious about it. Thanks.
Don't apologize for math - it's a good refresher. That's why I'm here. Been out of school too long and trying to pick this all up again.
7:53 Kinda pops into my head but does that equation need a (1/4)x((mv^2)/c) in between the first and second part ? It seems kinda odd.
Brilliant exposé ! 😳
Stumbled across these videos by accident, subscribed immediately, just awesome!,,
Great video. Although it makes me wonder why that equation for energy isn't taught to more university physics students. It's a relatively simple equation. The limitation for the standard momentum equation was never mentioned when I was in that class.
very good - i liked the part about the structure of the proton and its tornado of energy best - thanks
I LOVE how your video doesn't have music in the background. If I'm concentrating on what you're saying, I don't need or want to hear someone banging on a drum or playing a riff on a guitar (take note PBS Space Time).
I love the couloumb could you go into detail about it and what you can use it for.
I am very grateful to my first-year physics professor, who had integrated this explanation into our first-semester mechanics course.
That's how they do it, right there. A great teacher explains a subject subtlety so that the students will come to the conclusion on their own, thus increasing their understanding and retention. He can also see the eyeballs go wide or the eyebrows go up on the students who are lead just one step ahead.
I love your videos. Thank you.