Richard Phillips Feynman (May 11, 1918 – February 15, 1988) was an American… Read Full Bio ↴Richard Phillips Feynman (May 11, 1918 – February 15, 1988) was an American theoretical physicist known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics, and the physics of the superfluidity of supercooled liquid helium, as well as in particle physics (he proposed the parton model). For his contributions to the development of quantum electrodynamics, Feynman, jointly with Julian Schwinger and Sin-Itiro Tomonaga, received the Nobel Prize in Physics in 1965. He developed a widely used pictorial representation scheme for the mathematical expressions governing the behavior of subatomic particles, which later became known as Feynman diagrams. During his lifetime, Feynman became one of the best-known scientists in the world. In a 1999 poll of 130 leading physicists worldwide by the British journal Physics World he was ranked as one of the ten greatest physicists of all time.

He assisted in the development of the atomic bomb during World War II and became known to a wide public in the 1980s as a member of the Rogers Commission, the panel that investigated the Space Shuttle Challenger disaster. In addition to his work in theoretical physics, Feynman has been credited with pioneering the field of quantum computing, and introducing the concept of nanotechnology. He held the Richard Chace Tolman professorship in theoretical physics at the California Institute of Technology.

Feynman was a keen popularizer of physics through both books and lectures, notably a 1959 talk on top-down nanotechnology called There's Plenty of Room at the Bottom, and the three-volume publication of his undergraduate lectures, The Feynman Lectures on Physics. Feynman also became known through his semi-autobiographical books Surely You're Joking, Mr. Feynman! and What Do You Care What Other People Think? and books written about him, such as Tuva or Bust!.

Space-Time
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Most interesting comment from YouTube:

BobC

7y↑372

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!

All comments from YouTube:

Stunt Panda

2y↑46

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."

Lil' Black Duc

42w

She sounds like a practical wife!

gorog

7y↑81

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!

DasItMane

4y↑5

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.

Artur de Sousa Rocha

6y↑8

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!

EE Ehrenberg

7y↑485

I'm getting the impression that this Feynmen guy was kind of a smart guy.

Entraya Crosshill

5y↑9

im glad gell-mann named the quarks tho, thats a whole 'nother level of genius

cristi g

5y↑7

Maybe... He certainly painted some beautiful drawings

Craig Smith

5y↑11

The Nobel committee seemed to think so. He won it in 1965 along with two other physicists.

MarsJenkar

4y↑12

Seth Martin If that's the case, then Feynman may well be the most underrated physicist of all time.

hinta Raaz

4y↑16

@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".

20 More Replies...

Anonymous

1y↑2

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.)

Walt S

7y↑81

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!

James Samples

2y↑1

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?

BobC

7y↑372

Aneesh Prasobhan

7y↑4

Hi Bob,
Im an Electrical Engineer and I dont remember learning Feinman diagrams in Physics. Can you suggest a good book to learn it ?

Julien S

7y↑18

This is probably the best comment I've seen on YouTube so far. The best to you sir !

alarcon99

7y↑5

BobC you took the words out of my mouth! I'm less than 5 min into video and its so beautifully simple!

The Mandalorian

7y↑6

you're a gentleman. thank you sir

BobC

7y↑2

UCSD? ME TOO! Were we extremely fortunate to go there, or what?

11 More Replies...

TimmacTR

7y↑14

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...

Robert Schlesinger

4y↑9

Excellent elementary overview on Feynman Diagrams. A must see for all beginning Physics students.

Jared Mulconry

7y↑8

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. :)

Deeponjit Bose

4y↑2

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

hoogmonster

3y↑1

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.

Akshat Saxena

7y↑98

I got really excited when he said that we're going to do some quantum field theory ourselves

The Angry Intellect

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.

Toby Brooks

3y↑14

@The Angry Intellect I could spend another 10 hours reading this comment and still have no idea what it means or why you posted it

Jacob 1

3y↑2

@Toby Brooks lmao

Nijara Deka

Just nothing but a very very humble thank you Matt!! This is by far one of the best channels across all genre on YouTube!

N Brown

Great explanations. I join others here to say you did a better job than my physics profs did on this topic 38 years ago.

bryan roland

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.

VenomStryker

So crazy! All these possible interactions and we are trying to diagram every possible one!

Vacuum Diagrams

7y↑52

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.

pierrecurie

yay furry's theorem

thedeemon

7y↑1

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...

Vacuum Diagrams

7y↑3

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.

Manasv Chhabra

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...

PBS Space Time

7y↑34

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.

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Michael

7y↑22

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.

Basic Common Sense

7y↑11

I thought Compton scattering was when the po po breaks up the party.

Jared Mulconry

7y↑1

I Listen to Lucid Planet You read my mind. X3

B. Xoit

*effect (unless you mean emotion)

Yuvraj

so, are you going to give the email?

3 More Replies...

Lewis Leslie

That on-shell diagram you showed and explained briefly taught me more than my relativity lecturer ever did. My grades thank you.

Joye Colbeck

7y↑1

I do love space time. It's very relaxing. Thanks for doing what you do.

Mike B

Man I remember doing Feynman diagrams late in my graduate classes, and once I learned about them I wondered why we bothered all the other stuff, like all the wave equations we had to setup in undergrad qunatum mechanics classes... so frustrating to know there was a much easier method (at least to me) to visually how set everything up.

Interesting math

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

Vegan Evolution

3y↑1

So.. how theoretically possible is it that an electron and positron each give off their own photons, which go on to interact with the other symmetrically? Would the particles change in direction after emitting them, so that they are both moving away from one another, and the resultant photons would never catch up?

Noneother80

7y↑4

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.

Everett01

7y↑1

I love that they have the balls to ask a challenge question after these videos.

reynaldo velasco

5y↑1

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.

animist channel

Finally! Been bugging you people with dual-directional time for a while now. I know it's hard to record with machines, but science communicators could at least point it out more often. It does have larger scale effects that can affect forward-time dynamic systems. The ancients personified it as the Norns or Fates and put it in their mythologies.

Physics Videos by Eugene Khutoryansky

7y↑227

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.

Spec

7y↑6

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?

William Dye

7y↑46

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.

Neil

7y↑35

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.

Fandom guy

7y↑3

We know they exist, due to their effects, we just can't know what paths they take.

Rbn Srkr

Physics Videos by Eugene Khutoryansky f

26 More Replies...

Troy Planner

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

Benjamin Crom

7y↑3

Keep up the great work !!! Thanks for another fantastic video upload : )

Logan Fisher

Could you clarify the rules of virtual particles? From what you said in this video, I don't understand why they can't pass the event horizon of black holes, allowing for particle interactions through event horizons.

Cinema Ipswich

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.

Dennis Lubert

Did you ever read about mathematical periods and motives and their possible impact on feynman diagrams/amplitudes in conjunction with the zeta function? I really wish the research comes out with the zeta function being the base of their period, unfortunately all that is mathematically way over my head :(

John Cornwell

Imagine if Einstein, Dirac and Feynman somehow de aged to their 20 year old versions of themselves in 1950. What path would modern physics have gone in the search for the Grand Unified Theory? Would it still follow the current timeline of superstring theory or would it have gone in a completely different direction?

Jackson Burek

7y↑23

Wow. Kind of reminds me of control systems diagrams, and how they can be converted to the system diff EQs

Timfamy Hampson

I thought that too

Samir Zouaoui

same

Bezbez Zebbyson

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

dabebop

7y↑586

I'd get a Feynman Diagram tattoo if i wasn't so unsure about getting one.

James Rosenblum

7y↑26

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.

André Guimarães

7y↑33

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 :(

dabebop

7y↑18

I was being both, at the same time!

James Rosenblum

dabebop even better!

NGC6144

7y↑5

Do your own henna "tattoo" of a Feynman Diagram. It will last only a week or two.

15 More Replies...

Gaemlin Sidoharthi

3y↑1

I always wonder how Matt manages to keep missing the plural from mathematics in abbreviation, US style, and still sleep at night.

Jai Bellare

The best explanation of Feynman diagrams I have ever seen

T3CHN01200

Question: If the math of the full diagrams is impossible to solve, but you have 6 fundamental interactions, why not just model the math around that one interaction? Does the math representing those 6 interactions break down with virtual particles, or is there another limitation?

bobshouse123

This channel is fantastic. Thank you so much.

musicalfringe

Great video. I never quite understood the conceptual framework of Feynman diagrams and you fixed that. Also: epic pun.

Romaji

7y↑145

you should make a "I am made of particles of questionable reality" shirt

Josh N

Thank you for explaining Feynman diagrams for the first half of the video which offers an entirely new perspective of nature.

JDavis2000

7y↑1

Josh Neubert wait a minute I recognize that profile pic... I just can't put my finger on it...

Aske Mervig Bahnson

Is it possible for an on-shell electron to annihilate with an off-shell one? If an electron changes momentum by emitting a photon, and that photon then creates an electron-positron pair, is it then possible for the positron to annihilate with the original electron, letting the virtual electron escape?

Koen Van Damme

I like how these recent videos are supporting my current reading material (The Particle Zoo, great read).

cellovid

These diagrams seem to me to describe particles approaching and diverging, as if in a void, and I’m left confused over what happens if the particles CAN’T scatter, like if one electron is part of your rear end, and the other is part of a park bench. So they can’t scatter, and they just sit there exchanging virtual photons, and where does the energy for that come from?

Alejop1995

Hey, use formulas for something like this. It would be wonderful to draw and calculate

Nate Underwood

7y↑6

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?

Vacuum Diagrams

7y↑1

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.

Gareth Dean

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.

1 More Replies...

ericjane747

In another video with John Archibald Wheeler, He mentioned asking Feynman what he thought about the possibility that the universe was just one electron.

Naimul Haq

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.

Planci Acanthaster

7y↑1

How about an in-depth video about what life would be like if space had significant curvature in either direction. I saw a minutephysics video that mentioned that negative curvature would make sports impossible to play.

Steve Hinkle

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.

Ronald de Rooij

I love this. Why are all genius ideas so simple? I could have thought of this. But I didn't.. :-(

Kevin J

7y↑3

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?

Iago Silva

7y↑4

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 :)

Indigofrost

hey spacetime i have a question. at 6:23 you stated that the particles between the shells are virtual particles, and stated that they are unmeasurable. couldnt you just break down the diagram into 4 separate vertex?

CaptainConcerned Sr.

Why are we not using this method in schools? Im more into mechanics and this channel is spelling out the known universe to me

Quinson

At 9:00 to 9:30,
wouldn't the two inner-shell interaction be dictated by the energy of the photon? An electron can emit just about any energetic photon, but for a electron-positron pair to be created would mean that the energy of the photon would have to be at least twice the energy-mass of the electron. Although, I suppose it could happen fast enough to allow for such energies by the Heisenberg uncertainty.

David Thompson

If hawking radiation turns out to really be a thing, wouldn't this be a way of measuring virtual particles?

Q.E.D.

7y↑1

Love the show. Love everything about it.

I wanted to ask about the Feynman description of antiparticles. When you say the antiparticle is just the particle moving backwards in time, what does that mean for the concept of causality?
And do modern physicists prefer to think about particles and antiparticles as objects moving along paths in space-time or as curves with endpoints at the particle's creation and annihilation? Because doesn't the idea of movement suggest a kind of instantaneous position in space-time as defined by a concept of time outside of time? Am I just misinterpreting the idea of a direction in space-time?

Kevin Peterson

Q.E.D.

Florence Bacus

7y↑3

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.

Basic Common Sense

Florence B if they have no momentum, then how would they come together?

Florence Bacus

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.

Badly Drawn Turtle

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.

Florence Bacus

You can always find a reference frame in which total momentum is zero.

Vacuum Diagrams

7y↑5

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.

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Jwb52z

Has anyone ever tried to develop a way to distinguish between the same type of particles so that you would be able to know more specifically about the reactions going on that can't be directly measured??

Visual_Vexing

I thought I didn't understand Feynman Diagrams before because I was aware that an infinite amount of possibilities could be made up. Apparently, that's the whole point!

Usually such behaviour produces meaningless information.

James Samples

In your diagrams you have 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? 3:10

Dean Fuqua

7y↑1

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?

William Dye

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.

Dean Fuqua

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.

Jocelyn Joseph

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.

Yokatelo Amorim

6y↑8

"you can make your quantum field theories yourself" throws glitter in the air

Viswanathan Sankar

3y↑1

I do not know who would dislike these videos or why??? It is soo informative video

The Real Flenuan

I remember first reading about this model in A Universe from Nothing…
To this day few things have ever blown my mind more

Peter Sage

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.

Afshar ali: The Great Iranian

just remembered the day I subscribed this channel. Not at all regretting now! Love this channel!!

Pavlos Papageorgiou

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

Atlas Reburdened

7y↑219

What did one approaching electron say to the other?

I only want a photonic relationship.

Thapelo Afrika

7y↑22

is this a "light" joke?

Mauricio Micoski

7y↑12

The electron says to the photon: "you are so enlightening"

William Richards

7y↑5

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.

Mauricio Micoski

7y↑14

an electron and a neutrino walk into a bar. Who pays the bill? The electron. He says to the neutrino: "for you, no charge"

Jeffrey Lardizabal

7y↑3

Mauricio Micoski worth the price of admission 😂

9 More Replies...

Guillaume Verdon

7y↑1

This was a really good explanation

Daniel

But if virtual particles are capable of going back in time, on a space-time diagram, would it have gone back in time to then be absorbed by another particle or would it have just been emitted by that particle and not go back in time??

Mangua

3y↑1

My favourite science teacher.

sam

7y↑1

Would rotations of the Feynman diagrams qualify as different diagrams?

Possum Crotch

Really, Feynman diagrams are super easy. It’s mostly just straight lines, which be drawn with a ruler, and squiggly lines can be done free-hand. Add a few arrows and circles with letters. I could draw these all day. No big deal. What it all means is probably complicated though.

J M

7y↑4

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.

Byron Watkins

It seems to me that a vertical photon is impossible entering or exiting a Feynman diagrams; should they not ALWAYS have slope c or -c?

Pablo Spreafico

Nice video! The interactions you describe between electrons and photons are only for electroms bound in atoms, correct? For free moving electrons wouldn't the particles just "bounce" off each other due to Compton scattering?
Love the show, keep on making awesome videos!

thedeemon

7y↑1

No, not just for atoms. This theory is mainly used to describe collisions in particle accelerators where you have free flying particles with known energies/momenta. You know what goes in, you measure what goes out, and compare observed counts with calculated probabilities. So it's mainly for free particles, not bound ones. Applying this theory for bound particles in atoms is really hard. "Cluster decomposition" is a relevant principle to look up here.

Pablo Spreafico

Thanks for explaining :)!

juakofz

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

Nadav Morad

I'm procrastinating from studying for my particle physics test by watching this

Ákos Tárkányi

It is a pity that positrons never go backward in time.

Ayoobe Wonders

7y↑87

Quantum mechanics joke:

"A photon walks into the bars."

Brock Williams

4y↑2

Want a light?

Docta Osiris

Badum-tsh! 🤣

Possum Crotch

Bravo!

Bezbez Zebbyson

photons in QM?

Sergio Giacomo Sammartano

Do astrophysicists keep in consideration the electromagnetic forces generated by the spinning galaxies when calculating the orbits of the stars and their velocities about their axis of rotation, i.e. the center of the galaxy? I always hear about gravitational effects on astronomical scales, but what about the elctromagnetic force?

Brian Crane

7y↑1

Astonishingly beautiful explanation. I wish you had been my first QED prof (instead of the old, crusty....well...).

DoridTheBlue

Great video. Keep it up.

Austin Elwyn

I have a unrelated question to the video but if there was a multiverse wouldn't there be a universe that had someone so powerful that they could destroy the multiverse?

Cindy Yamaguchi

7y↑26

Matt is the best but sometimes I miss Gabe, what is he up to?

michelle reed

7y↑6

matt can science me anyday!

matteo conz

7y↑16

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 :)

Kowzorz

7y↑5

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.

BC3012

I saw a couple of Gabe vids the other day, definitely talks too fast for you to absorb the information in your first viewing.

Michb3ck

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

Andrew Kimmey

I want to speculate here a bit, and I want to hear opinions based on what physics might suggest. As I understand it, super fluids experience 0 friction. Now, what if you had a superconducting super fluid that was ALSO a ferrofluid. And this fluid was contained inside of a helical doughnut-like casing, much like a fusion reactor would be. If rotating magnetic fields were applied to the fluid causing it to start flowing, how fast could it go with no friction? If an insanely rapidly rotating mass of superconductive, magnetic superfluid was flowing in a closed loop near the speed of light, what sort of magnetic, or possible other forces could be generated by the fluid? Since it's magnetic I'm assuming it will act as a dynamo and create a magnetic field, but how strong? Could it do anything else?

Andrew Kay

7y↑1

Combining this with relativity, is it possible for e.g. Observer A to see an electron absorb a photon and then emit a new photon, while Observer B sees a photon produce an electron/positron pair, and then the new positron annihilate an incoming electron, producing a new photon? The Feynman diagrams for these two scenarios are topologically the same, is there ever a Lorentz transform between two different scenarios like this?

MikeRosoftJH

No, not really. The important thing is that virtual particles can't be directly observed - the only thing both the observers see is an electron and a photon deflecting each other.

Feynman diagrams do not distinguish between an outgoing virtual particle and an incoming virtual antiparticle. If what you are asking is if this were possible with real particles, i.e. one observer would see a particle and another an antiparticle moving in an opposite direction - again, the answer is no. (That is, unless you choose to believe that a positron is really an electron travelling backwards in time - but that would probably take the analogy too far.)

Suppose that a particle was emitted at space-time point A, and was then observed at point B. Because these events are causally linked, the interval between them is a time-like interval (or a light-like interval in case of a photon) - this corresponds to the observation that no particle can travel faster than the speed of light. All observers will agree on the temporal order of these events. (If you wanted to see the particle move from point B to point A and not vice versa, then the only way would be if you travelled faster than light yourself.)

Ricardo Siervi

7y↑1

That single vertex kinda looked like the "Flux Capacitor". Coincidence? I think not!

Anonymous User

7y↑2

At 5:20, shouldn't there be another circle where the second photon is emitted? There are only 9 circles, and I'm pretty sure there are supposed to be an even number, and that looks like a vertex.

Alan Snyder

7y↑1

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

Vacuum Diagrams

7y↑2

#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.

DishRag

7y↑91

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.

nikitha nayaer

7y↑1

DishRag so is the book good

Da real G

7y↑3

DishRag I'm 17 and ive been watching/reading all these things since i was 10...

Guilty Spark

7y↑78

This is all easily solvable and easy to understand. Considering I'm an 8 month old baby...

Berkan Yusein

7y↑50

15 and 10?! I've been reading about Einstein ever since I was a fetus. Get on my level plebs.

The Rogue Wolf

7y↑48

You waited until you were conceived before you started studying quantum physics? Pfft.

35 More Replies...

Existenceisillusion

This episode makes me want to go back to school to get my Ph.D.

Pancake Soup

After rewatching this for the 3rd time and looking at bhabha's scattering (cheating) I think I understood Feynman Diagrams correctly. I have a question: can we use Feynman Diagrams to see reactions between other particles than Electrons, Positrons and Photons? For example: Quarks combining into Neutrons and Protons and then Neutrons and Protons combining into a Nuclei?

ttul007

I don't always understand everything that you say. but you do, and it's very comforting.

M1K3 Droid

If virtual particles don't obey mass-energy equivalence, should we not count all these virtual particles as a contributor to the dark matter or dark energy (or both) mass energy balance of the universe? Why aren't virtual particles being considered as the cause of these?

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