The Secrets of Feynman Diagrams
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- Опубліковано 21 вер 2024
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Unlock the secrets of Feynman Diagrams. Part 5 in our Quantum Field Theory series. And if you're submitting an answer to our challenge question email your answer by August 2nd to pbsspacetime [AT] gmail.com with the subject line "Feynman Diagram Challenge."
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Feynman’s path integral shows us that, to properly calculate the probability of a particle traveling from point A to point B, we need to add up the contributions from all conceivable paths between those points - including the impossible paths! In fact we can go even further: according to Feynman’s approach, every conceivable happening that leads from a measured initial state to a measured final state DOES in a sense happen. To calculate the probability of any quantum system evolving from one state into any other state we need to sum over every conceivable intermediate state. This is impossible because there are infinite possible intermediate states.
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I love the story about Feynman's wife, Gwyneth Howarth, driving the Dodge Tradesman van with Feynman diagrams all over it and some CalTech physics students were shocked so asked her why she had Feynman diagrams on her van. Her response was something like, "Because my husband invented them."
She sounds like a practical wife!
I hear Gwyneth, I think Iron Man's wife 😂. Quantum Feynman Fusion ARC reactor anyone?
OMFG! I was exposed to Feynman Diagrams in 1983, toward the end of a very intensive 2-year undergraduate physics sequence. Back then, I learned the math, turned the crank, and passed the exams.
Only today, 34 years later, did the light bulb light, the "Eureka!" get shouted, and the Zen-like state of the "Wonder of the Universe Explained" inhabit my mind.
Matt, thanks so much for communicating so extraordinary well! I think it was the "rotations of a single vertex diagram" that broke my mental block.
I knocked out the challenges in literally 10 minutes, delighting and amazing myself in the process. But I won't submit them, since, ideally, I really should have been able to do them 34 years ago.
I've already got my prize. Thanks!
Hi Bob,
Im an Electrical Engineer and I dont remember learning Feinman diagrams in Physics. Can you suggest a good book to learn it ?
This is probably the best comment I've seen on UA-cam so far. The best to you sir !
BobC you took the words out of my mouth! I'm less than 5 min into video and its so beautifully simple!
you're a gentleman. thank you sir
UCSD? *ME TOO!* Were we extremely fortunate to go there, or what?
I'd get a Feynman Diagram tattoo if i wasn't so unsure about getting one.
dabebop lol I don't know if you were being literal or punny, but I actually was thinking that they would make a great tattoo.
I got a math tattoo, because if something is true right now in math, it is true forever and ever in the whole universe. Math is put logic. But physics is too volatile, stuff changes "quickly". I'd hate to make a physics tattoo, and have someone discover a new/better way that makes whatever I tattooed obsolete :(
I was being both, at the same time!
dabebop even better!
Do your own henna "tattoo" of a Feynman Diagram. It will last only a week or two.
This is amazing. This is basically a free semi-formal lecture with sharp and engaging production value -- I am really getting a lot out of this. Thank you!
gorog in a way this is better than a lecture because you don't have to wait for the prof to draw each diagram. You can just focus on the ideas themselves.
you should make a "I am made of particles of questionable reality" shirt
I'm getting the impression that this Feynmen guy was kind of a smart guy.
im glad gell-mann named the quarks tho, thats a whole 'nother level of genius
Maybe... He certainly painted some beautiful drawings
The Nobel committee seemed to think so. He won it in 1965 along with two other physicists.
@Seth Martin If that's the case, then Feynman may well be the most _underrated_ physicist of all time.
@@MarsJenkar Dude... I think you haven't heard about him that much because his work mostly comes in Graduation Physics. If there had been social media like today, he'd be most interesting guy of all time. More than half of today's physicists say Feynman is their primary inspiration. I suggest you read Feynman Lectures Vol 1,2,3 and "Surely you're joking Mr. Feynman".
I got really excited when he said that we're going to do some quantum field theory ourselves
Are you excited to pretend to be smart? 🤔
I don't understand most of this, which is why I watch and listen intently. Makes my brain tingle to learn of things I never knew of. Unlike watching a Hollywood movie.
@@TheAngryIntellect- I could spend another 10 hours reading this comment and still have no idea what it means or why you posted it
@@tobyzxcd lmao
How come a popular science channel just gave a more in-depth and clearer explanation of Feynman Diagrams than what I got at the university where I majored in physics? I watch your videos from time to time because I've been working in a different area for the last two decades and I often discover stuff here that wasn't even known at the time (plus astrophysics was just one optional lecture - one I took and liked). Now I got a new appreciation of the quality of the work you do!
The "questionable reality" of virtual particles should make us all very nervous. Our only measure for evaluating a theory is how precise its predictions are, and in this respect QED is the most successful theory in history, but this unfortunately still doesn’t tell us if it is an accurate description of reality. Some have the point of view that science is "only" about making accurate predictions. But the reason for getting interested in science, at least for me, is wanting to know what is actually going on.
But The problem is the fact that you just can't measure virtual particles so if we know that positron exists then considering conservation laws there must be this exchange things in feynman diagram right? What's the other possibility besides such theories containing non measurable things?
Eugene: When trying to discover "what is actually going on" in physics, maybe we should avoid thinking in terms of particles or even fields -- things which have a position in space and time. When we cut apart a piece of wood, for example, we get two pieces of wood, but the process doesn't go on forever. Eventually we get pieces of things which are not themselves "wood", but instead consist of the more fundamental things which can combine to form wood. Just so with particles. For a while we can keep splitting them into smaller particles, but the process eventually ends when we have things which are not themselves particles, but consist of more the more fundamental things which combine to form particles.
If a particle (or field) can be thought of as a bundle of characteristics in a given location in time and space, then what is more fundamental than "location"? Whatever it is, it cannot be things which themselves have a location. Certain elements of QM might seem counter-intuitive, but maybe the fix is to stop thinking that anything which is "real" must always have a definite position in spacetime. Field theory works, but what gives rise to fields? What gives rise to the complexities of "location" itself, or "energy"? When we start to deal with things that cannot themselves be described in those terms, feel happy! We're not avoiding reality, we're finally breaking into the next level of understanding it.
I'm sure this is obvious to anyone who reads this but surely the problem lies in the meaning of the expression, 'what's actually going on'? When I first entered physics, some 40 years ago, I too felt the need to find out 'what's really going on' and would agree that such need was a main drive. But increasingly I felt that the answer was less and less of a real goal. I no longer believe that physics will ever know, 'what's really going on' but we will develop increasingly sophisticated models which will enable us to create our own, human based, version of what reality is. To be honest, for what it is worth, I have dropped the idea of knowing 'what's going on'. I am no longer convinced that it is within our brains ability to answer, or understand such a question. In many ways we may have already crossed the threshold at which we can understand reality - hence the division between our beautiful, accurate mathematical models and our failure to be able cognise the physical reality that the maths describes.
We know they exist, due to their effects, we just can't know what paths they take.
Physics Videos by Eugene Khutoryansky f
I just wanted to stop and say that we live in an incredible time, where we have direct interaction, not with our anti-particle, but with experts in the field that can explain to us in simple and entertaining terms how the fundamental mechanics of existance work..
My brain interacted with this information and the result was a blown brain...
"you can make your quantum field theories yourself" *throws glitter in the air*
This video is especially good. Discussion of the six orientations of the basic vertex is quite clear and usually muddled in most field theory textbooks. Great job!
In these diagrams he has arrows pointing straight up and in others straight down. Does this mean that the respective Positron, Electron, and/or photon neither moves forward or backward in time?
I just looked up a bio on Dr. Matt and found out that he writes all of these scripts himself. Just when I thought that talent might have been left to someone else.
(Nope. Apparently he has ALL of the talents. I really enjoy every SpaceTime video. I’m a complete layperson with a new interest in physics for personal reasons, and this channel is practically all I watch anymore.)
I'm super high stumbling through UA-cam and this comes up new. Great timing...
Toasted Fan Art haha... same
3rd hahaha
Toasted Fan Art I have a feeling you are going to win one of those shirts....
watch it again when you're not high. you'll probably realize you didn't understand it the first time. just felt like you did. weed is like that. unless you're talking about meth, in which case ..... go to rehab ;p
If it was heroin youd have only seen the first 20 seconds then nodded out and woke up again to see a space chicken.
Quantum mechanics joke:
"A photon walks into the bars."
Want a light?
Badum-tsh! 🤣
Bravo!
photons in QM?
Two things I regret; not continuing physics study being secondary school and not learning maths to a more advanced level too. But PBS Space-time perfectly feeds my continuing lay curiosity in these fields. Maybe when i retire I'll take some evening courses in physics and maths just for the fun of it. Meanwhile channels like this are a real treasure and joy to watch.
I am a Undergrad Physics Major student. Wish I have been learning physics from professors like you. So well and smartly explained sir. Many Many thanks Professor Matt for explaining these stuffs so simply and interestingly
To all of you having trouble with conservation laws and the vertex: you are right. The vertex by itself does not respect both conservation of energy and momentum and thus doesn't represent a real process. If you do the math, you can calculate the amplitude for, say, an electron and a positron to annihilate and produce a photon, and that probability is always zero.
It's best to think of the vertex as a building block you can use to build more complicated diagrams. For example, the diagram in which an electron and a positron exchange a virtual electron (or positron!), emitting *two* photons gives a nonzero amplitude.
yay furry's theorem
There are delta-functions over sums of momenta involved in complete equations, providing the conservation. Would be nice if someone explained in more details where exactly these delta-functions go (each vertex or the whole term of a single diagram or something else) and which kinds of momentum (3D or 4D) are used there...
thedeemon, the delta functions go on each vertex, and they refer to 4-momentum. This is because each "particle creation" (or destruction) event accompanies a factor of e^(±ip.x). You have to be careful with the signs, which depend both on your conventions and whether you're dealing with a particle or antiparticle. You then integrate over x (heuristically because the event can happen anywhere), and what you get is a known representation of the delta function.
The emit the photon and reabsorb it at the same time right? Because else the energy is lost.
But how can it take 0 seconds to emit and reabsorb??! Wow this is weird...
Thanks for pointing this out, Dr Diagrams. Vertices are building blocks, not valid diagrams in and of themselves. We'll be sure to note this and talk about why in the solution episode.
I wasn't sure how I would go with Feynman Diagrams. When you showed the example with an incoming electron and photon and showed the reverse-time electron, I immediately laughed and thought that was pretty cool.
3 months ago, I never thought I would be okay with all of this quantum weirdness. Now I'm just having a good time exploring all that goes on at this scale. :)
Wow. Kind of reminds me of control systems diagrams, and how they can be converted to the system diff EQs
I thought that too
same
Feynman diagrams are a pictorial representation of perturbation series terms. You can draw them for nonlinear PDEs perturbative solutions. See R.C. Helling solving classical field equations. This is pretty well known
What did one approaching electron say to the other?
I only want a photonic relationship.
is this a "light" joke?
The electron says to the photon: "you are so enlightening"
One electron asks the other "how much longer till we get to the egg?" The other electron answers "Not long now - we just passed the tonsils".
Oh - my mistake - that was sperm - not electrons. Sorry.
an electron and a neutrino walk into a bar. Who pays the bill? The electron. He says to the neutrino: "for you, no charge"
Mauricio Micoski worth the price of admission 😂
I don't always understand everything that you say. but you do, and it's very comforting.
So, now in addition to dark matter and dark energy, we have Dark Humor.
Heat Death Coming.
I love that they have the balls to ask a challenge question after these videos.
It is cool to see a Feynman diagram of Compton scattering. I work in radiology and Compton scatter of x-rays is an issue we have to deal with when performing exams that contributes to image degradation as x-rays are scattered off into alternate paths from the primary beam. We use specialized angled grids made of lead to absorb and minimize the affect of these scattered x-rays to help improve image quality.
I thought Compton scattering was when the po po breaks up the party.
I Listen to Lucid Planet You read my mind. X3
Really? You work with some kind of precision work, or is the idiot typing this too naïve in assuming only the lowest-order (AKA classical) contribution is relevant to radiology? LOL me, LOL my
*effect (unless you mean emotion)
so, are you going to give the email?
I color-coded these diagrams! Quarks: red, jade, or sky blue according to the colors. Antiquarks: maroon, pine green, or blue according to the anti-colors. Gluons: A mix of a color and an anti-color. Leptons and anti-leptons: yellow. Photons: purple. W+ and W-: orange. Z: brown. Higgs Boson: pink.
Excellent elementary overview on Feynman Diagrams. A must see for all beginning Physics students.
‘On-shell’ sounds like a new synonym of ‘cool’. Example usage:
‘Have you seen the film I recommended?’
‘Yeah, you were right, it was on-shell.’
‘Off-shell’ could be used as well to mean something wasn’t cool.
I always wonder how Matt manages to keep missing the plural from mathematics in abbreviation, US style, and still sleep at night.
That on-shell diagram you showed and explained briefly taught me more than my relativity lecturer ever did. My grades thank you.
I love the "do it yourself" things! Could you give us more of them? I think that I got the diagram correctly, but I'm not sure... I've followed you for the past year or so, and I cannot get enough of these videos :D I watch them like a fat guy watches a donut shop.
I have no 'Feynman Diagram Challenge', but I would like to propose scooping up space debris. Unfurling a large, strongly knit magnetized net, from a special crafts, can stop the smaller pieces, like the metal nets by the road side stopping large falling rocks, from the hill by the side of the road, during mud slides. It might catch big pieces even.
_People-who-wait-for-Spacetime-to-upload-on-Wednesdays_ Squad!
And-then-go-to-sleep-waiting-for-Isaac-Arthur-on-Thursday-morning Squad!
Bryan Wilson yes!
yeash
I dont wait, I show up right on time.
How can everybody not find this stuff interesting‽
xxxooo
dc
Astonishingly beautiful explanation. I wish you had been my first QED prof (instead of the old, crusty....well...).
Matt is the best but sometimes I miss Gabe, what is he up to?
matt can science me anyday!
Cindy Yamaguchi Gabe Is great but he talks too fast for people like me that hardly understand english.
Instead I understand Matt much better then Gabe. Anyway, both are good teachers! Excuse me for my bad english but we Italians have problems with languages :)
He got a job at the National Science Foundation and had no more time to devote to this project according to a Geek Alabama article posted in 2015.
I saw a couple of Gabe vids the other day, definitely talks too fast for you to absorb the information in your first viewing.
matteo conz there are chrome plugins that allow for slower video replay. I like x0.9 for a more chilled science kick and better understanding
Just nothing but a very very humble thank you Matt!! This is by far one of the best channels across all genre on UA-cam!
I noticed something when drawing Feynman diagrams a few weeks ago, on an event horizon of a black hole, Hawking radiation looks like a positron falling into a black hole and an electron traveling away, but if positrons are time reversed electrons, then would the same electron fall out of the black hole through backwards time, then change history so it can travel away from the black hole? More importantly, is this universe Star Trek or Dr. Who?
This article is basically what your saying but scaled up to macro scale.
massgap.wordpress.com/2017/04/17/clever-demons-and-hungry-black-holes/
Hawking radiation is overwhelmingly composed of photons, actually, at least up to the very last moments of the black hole's life when it becomes hot enough for more particles to be produced. The usual explanation of Hawking radiation in terms of pair of particles being generated near the horizon, where one falls in and the other escapes, is completely wrong.
Be careful here, all other objections aside a positron looks like an electron moving back in time DOING WHAT THE POSITRON DOES BACKWARDS.
So the positron CAN be an electron LAVING the black hole BUT its path would lead it to the electron outside the hole. (Which would look like a positron falling into the hole to it.) It would then be destroyed. It couldn't go back further than that because the positron's world line starts at the pair creation.
In effect this effect is similar to rewinding a video, you can see what's happened in reverse, but you don't get a whole new movie out of it.
My favourite channel. The explanations are clear, even to an innumerate viewer like me, and the way they are delivered is always easy to listen to. Thanks, PBS and guy with cool T-shirts, whatever your name is.
So, Feynman diagrams are *quantum memes*?
OMG I just came from an Alex Jones meme super mix
Perhaps they are also the definition of faith ;).
PlayTheMind oh my fucking god mang, I came here for physics to blow my mind, not UA-cam comments!
No...
You keep using that word. I don't think you know what it means.
My favourite science teacher.
Who needs notifications when you live on youtube?
Yet, here you are replying instead of doing something else. Personally, I love his presentation style and, obviously, so do a very large number of other people.
One of my favorite youtubers made a little song about living on youtube... bigclivedotcom So much UA-cam. (No time for sleep.) and in that video he mentions other youtubers to which i am subscribed to.
Oh it's 3 o'clock in the morning, i have to go to bed. N8.
v=tX0lKqguw7s
DaeNight Who needs original comments when you can just copy paste
The Earth is flat and you can see Chicago from Michigan! There are many places to view the Chicago skyline, cause its a big lake! Second of all there are air bourn particles that cause interference , fact. Third, globe heads try telling me that some days you can see the skyline really good proving its flat and on the other day when you can't it's cause of the curvature lol. Water is level and seeks it own level always not just when it feels like it. I seek truth and facts repeatable and observable and am not interested in sitting by while people like you brain wash your followers with bullshit, stay in your cognitive dissonance I don't care, someday you will wake up.
So can anybody here address my argument or no?
@Derp Jesus
>Meanwhile, subscribed to TYT because I'm so interested in truth and provable facts.
Pathetic troll is pathetic. The least you could do is go to a video that's even slightly related to the globe.
I do love space time. It's very relaxing. Thanks for doing what you do.
Legit instant clicked
Thank you for explaining Feynman diagrams for the first half of the video which offers an entirely new perspective of nature.
Josh Neubert wait a minute I recognize that profile pic... I just can't put my finger on it...
Keep up the great work !!! Thanks for another fantastic video upload : )
I like how these recent videos are supporting my current reading material (The Particle Zoo, great read).
NB: there is a typo of the email address in the description.
The best explanation of Feynman diagrams I have ever seen
I don't understand how you do it but I've been reading "beyond Einstein" by Dr.Michio Kaku, and you have been uploading videos about exactly on what I'm reading this helps me understand it a lot more considering I'm only 15.
DishRag so is the book good
DishRag I'm 17 and ive been watching/reading all these things since i was 10...
This is all easily solvable and easy to understand. Considering I'm an 8 month old baby...
15 and 10?! I've been reading about Einstein ever since I was a fetus. Get on my level plebs.
You waited until you were conceived before you started studying quantum physics? Pfft.
This made me look up what wavelength of light is given off by positron-electron annihilation.
Gamma. And you get two photons per collision and they go off in opposite directions.
4:23 Shouldn't it be impossible for a positron and an electron to annihilate to produce a single photon? If we look at a reference frame where the electron-positron pair has zero momentum, then momentum cannot be conserved, since a single photon always has nonzero momentum.
Florence B if they have no momentum, then how would they come together?
1. Conservation of momentum is definitely a thing? He mentions it at 4:41.
2. The momentum of the positron-electron system being zero doesn't mean that the positron and electron individually have no momentum. Basically, I'm talking about a scenario where the two momenta are nonzero, but have opposite sign and cancel each other out.
I think you're picturing a positron and an electron hitting each other head on, going in exact opposite directions; in which case, yes, conservation of momentum might be a problem. But that's a very specific and highly unlikely scenario; more likely, when the electron-positron pair come together, their trajectories will have a (non-straight) angle between them, in which case a zero-sum momentum is impossible and conservation of momentum isn't an issue for producing a photon. It also isn't an issue if they come in head on at different speeds.
You can always find a reference frame in which total momentum is zero.
That's right, it's not possible. All the diagrams with only one vertex have amplitude zero because they fail to conserve both energy and momentum simultaneously.
"Within a week of the release if this video"
Me watching it 5 years later: ahh, great
So, if literally ANYTHING can be in between the ingoing and outgoing, as long as the end products are the same. And since faster than light and all directions in time are valid. Would that mean you could have two electrons enter, and the big bang happens, the entire universe pops into existence, lives out its entire existence, and in the end degrades into a single pair of electrons out the other side?
Nope - the Universe is made up of _real_, measurable particles, not _virtual_ ones.
But I can see you're a guy who likes fun, so kudos :)
Very good exposition. No need to drag your feet across tons of literature about Feynman diagrams. It reveals the very characters of the alphabet for building QED
That t-shirt would be SO much better if it just said "Heat death is Coming." It looks like you tried to cram too much into it.
Yeah, but 'heat death' applies to a lot of things. YOUR heat death is coming, quite soon on cosmic timescales.
Gareth Dean but that's not heat death
Well that depends. You sustain yourself on various biological processes that in the end vent heat. When you start to break down (Saying you'll die at some exact moment is a tricky thing to do.) these processes will likewise break down until the generation of heat by your body ceases. (You may or may not wish to include decay in this as your gut bacteria are a part of you too.) At this point your body will have minimum thermodynamic free energy relative to its surroundings, a relative heat death. Like wise will happen to things like individual stars long before the Ultimate Heat Death.
My Feynman diagrams have these two ways for the Bhabha scattering:
1. A positron gives off a photon and turn into an electron, then an electron absorbs the photon and becomes a positron.
2. An electron and a positron annihilates and creates a photon. The photon then turns into an electron and a positron.
Probably a misunderstanding but, could you see into the future by very closely measuring anti matter if it is in fact time reversed matter?
"Time reversed" doesn't mean that "causality" works differently for antimatter. Think of it like this: I can film myself bringing buckets of water from a well to a city. If I play the film backwards, it'll look like I'm bringing water from the city to the well. This "backward flux" of water is analogous to how we think of positrons as being a "backwards flux" of electrons.
Time reversed matter specifically only "happens" to virtual particles, which are by definition unmeasurable. So unfortunately no.
No, you can't. Once you try to look at the process the virtual particles must materialise and that will result in a different outcome.
No, the universe will not allow prediction to arbitrary time scales. It is not possible to observe enough data. Uncertainty sucks.
Put it this way: you could travel into the past by jumping into a vat of antimatter, because the annihilation of your particles with the antimatter particles is mathematically identical to your particles turning around in time. The problem is, "you" would arrive in the past as the scattering of antimatter particles that were collected together into the vat you-in-the-future jumped into, and then when your particles turned back around in time, they would just be the scattered particles that were created along with those antiparticles, because backward-travelling particles turning around in time is mathematically identical to particle-antiparticlel pair creation.
Any message you might possibly want to send back in time that way would suffer the same effect: it would arrive in the past as noise. Any message from the future unintentionally sent back in time via interaction with antimatter would also suffer the same effect: it would arrive in the past as noise. So we here in the past of some future cannot glean any useful messages from that future out of antimatter -- what arrives here in the past is just noise.
The break in the law of energy conservation during the big bang solves the cosmological constant problem
This mathematical/notational trick seems overly powerful. It reminds me of how the Geocentric model of the solar system was unknowingly using Fourier series to map the motions of planets. Little did the ancients know, Fourier series can be used to map ANY periodic path the planets could make. Thus, using such an overly powerful tool limited their knowledge, and made it difficult to accept the Copernican model
My point is that perhaps this summing over all possible paths (though a valid mathematical tool for attaining accurate predictions) is just limiting our view of the truth.
The times a second a photon passes a radio antenna creates phonons in the device, so we filter and amplify these for wireless audio reception.
The times a photon strikes each atom raises its kinetic energy and temperature.
Then photoelectric effect. Electron-positron annihilation.
Then the W and Z Bosons with mass. All these may be represented by the Feynman diagrams.
It's kind of silly but time being the y-axis makes this less intuitive for me.
rotate your head
just remembered the day I subscribed this channel. Not at all regretting now! Love this channel!!
Time to re-watch this for the entire week. I have been waiting.
All the patrons on Patreon in the world can't stop the Heat Death from coming.
there should be a PBS spacetime out of context video
you know what? i might just do that
It is good to see Matt before his hair went gray… PBS Spacetime now is… science history. The relaxed years before the plague and AGI.
Yay ! Bravo to the electric universe and it simplicity .
One thing that seems to be omitted in these discussions is the possibility that the electrons' wave functions simply pass through each other. There doesn't seem to be anything that prohibits this, so the Totalitarian Principle insists that it is compulsory.
What happens in the middle doesn’t matter. Only the result remains. This is the power of my King Crimson!
So crazy! All these possible interactions and we are trying to diagram every possible one!
Great explanations. I join others here to say you did a better job than my physics profs did on this topic 38 years ago.
I won't say I understand Feynman diagrams not but at least I can now kind of read them. One microscopic step toward my Nobel price for making a real TARDIS. I just wish future me would send the schematics to my current location in space time.
I guess your video came just in time for me to understand the latest XKCD comic!
So not gonna lie I’m probably going to watch this video like 50 times this week just because I love this so much and can’t believe no one taught it to me. I mean sure I didn’t major in physics but how can you know this and not work until everyone on the planet knows this ? How
I've wanted to delve more into Feynman diagrams for some time so this episode was excellent.
I now have a much greater understanding of how much I don't understand...Thanks!?
But seriously, a couple things I’d love for someone to clarify…
1. While rotation of a vertex obviously changes the interpretation of the “event”, does rotation of the entire diagram change anything, at least in the case of identical input/output components, or even different input/output?
I’ve seen annihilation diagrams drawn with the photon both parallel and orthogonal to the time axis. Furthermore, the prob amplitude is a product so would be commutative (Though I remember something about multiplication not being commutative in quantum calculations many many moons ago)
2. It seems that the “charge” notation is implied by the time component of the particle direction, i.e. electrons always move forward in time, positrons always backward but in the case of Moeller scattering it indicates an electron (noted as e-) traveling backward in time… How is that to be interpreted?
Matt says at 5:20 that the "bottom line" results are the inputs and outputs. If rotating the diagram keeps the inputs and outputs the same, then it's just a different path to the same end result. All the stuff in the middle with virtual particles cannot be measured, so it's not bound by mundane considerations like direction in time or locality. It doesn't matter if a virtual photon line is sloping up, sloping down, or even horizontal (implying that it briefly exists in multiple places at once). See 8:05. All that matters is the vertices.
William, thanks for your comment, it’s given me a lot to ponder! I’m still trying to come to grips with the nuances of the rules that define unique paths, i.e. distinct terms in the probability eq.
@5:20 “The overall interaction of a set of Feynman diagrams is *defined* by the i/o particles”. I get that the i/o is the “bottom line”, after all, the whole point is to determine the likelihood that specific inputs will produce a specific outputs and that each vertex must be one defined by the theory, (in QED just the one). The fact that the path I -> O is unmeasurable doesn’t infer it’s irrelevance. In fact, for an n-vertex diagram the is a finite number of possibilities which make up the “set of Feynman diagrams”. As I understand, mathematically, the probability is the (summation of |amplitude| for each distinct path)^2. If this is not correct, I’m really off base.
So what I don’t fully understand is the rules that define uniqueness of a given two vertex interaction. Feynman was bloody brilliant so if some seemingly unique paths are accounted for by a single diagram/term, i’ll take it as fact.
Can you use Feynman diagrams to express:
* The Pauli exclusion principle where two electrons can't be at the same place (but photons can)
* How a laser works, by making atoms combinatorially more likely to emit more photons
it was such a long wait ....for this episode
This was painful on the brain but i managed to stay on the track
I have a tattoo of a feynman diagram of two virtual gluons from colliding protons interacting to produce a hypothetical Higgs boson, a top quark, and an antitop quark.
Then I changed my major to economics. Its still my favorite tattoo though.
For me, the Feynman Diagrams more simplified the action of particles. Like Math and the symmetry of quantum theory, the Feyman Diagrams are a simple tool to seek the answer to complex questions. Feyman may have been "groovy" but his explanation of science was a huge leap in the explanation of Quanta.
Thank the maker for this UA-cam channel!!!
I WANT BOTH T-SHIRTS PBS SPACETIME IS AWESOME!
this is why I assert that all the "missing antimatter" from the big bang isn't missing, it just went backwards in time into the previous iteration of this M-brane. At some point in the future we will encounter the antimatter coming backwards in time from the future big bang.....
Hey guys, there is a typo in the address in the description, it says pbsspsacetime, with an extra "s" in space! I just sent my answer. Very cool video and very interesting topic
So two questions.
#1) Are their useful Feynman diagrams for Hawkins radiation of particles near an event horizon?
#2) What about CP violating processes like some type of muon decay? Do Feynman diagrams help with those?
Alan
#1) Hawking radiation is not described by Feynman diagrams. Feynman diagrams are useful for computing interactions, but Hawking radiation happens even if there are no interactions whatsoever! It's like a quantum mechanical version of the twins paradox. It's a result of the different flow of time near the black hole.
#) When it comes to CP violation due to the weak nuclear force, yes. There are Feynman diagrams that describe such processes. When it comes to CP violation due to the _strong_ nuclear force, no. That is due to what are called "non-perturbative" effects, precisely because of their inability to be captured correctly by the perturbative expansion that Feynman diagrams represent.
"Ronald Raygun" was definitely a widespread verbal meme at the time, referring directly to SDI. "Star Wars" made it on the news, but Prez Raygun was well known
i don't want to start any trouble or anything but........the 2nd t-shirt is the coolest (pun intended)
best explanation of Feynman Diagrams I seen
Yes they can be separated by virtual thought
I should try taking a shot at one of these challenges when I'm high
QUESTION from a Fan: I have watched all your videos and some of it goes over my head as I am just a Software Architect, not a physicist. But I have a question for you:
All of these videos seem to lead me to the conclusion that all particles are just vibrations in space time and the actual manifestation of quarks, electrons and other particles are an emergent property of combinations of these vibrations. Am I right, wrong, crazy or in a superposition of all of them? :)
the sounds like more like string theory than QED
hey marc.. the marceting part of this clips get to high.. but big thanks for your show and the awesome discription ... defenetiv the best educ. channel on yt
I think you made the right decision to end the video after that pun.
Why are we not using this method in schools? Im more into mechanics and this channel is spelling out the known universe to me
I do not know who would dislike these videos or why??? It is soo informative video
Oh man I just submitted my answer a few moments ago :0 Let's hope that still counts as "by 2nd August"...
I like how they added the Stellaris leader death sound when they showed the electron-positron pair annihilating.
I love PBS Space Time.
I was a teacher w/ an old school method of teaching...i realized how ineffective and time-consuming my method was after watching these series.
Great video. I never quite understood the conceptual framework of Feynman diagrams and you fixed that. Also: epic pun.