Exchange Particles and Feynman Diagrams - A Level Physics
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- čas přidán 27. 07. 2024
- Continuing the A Level Physics revision series looking at Exchange Particles and Feynman Diagrams, including gauge bosons, the virtual photon, the gluon and the W and Z bosons.
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I stand to be corrected, but for A level I would adopt the tip: Take the W away from the baryon (and towards the lepton). Whether it is a + or a - will then be determined by what is needed to conserve charge. If a p is changing to a n then you will need a W+ away from the proton. If it is a n changing to a p then you need a W- away from the neutron.
Great video. There is only one thing that is teached differently in our university: If there is an antiparticle involved, you should mark it as an arrow backwards in a diagram.
university??? we have to learn this in our first year of a levels.
That's just stupid to be honest. There is no way that you can truly understand that at that level. Students just end up learning a set of arbitrary formulas and rules.
Really we should be learning more of this earlier on. Nobody studying physics is exploring Newtonian mechanics, Quantum Physics is where the future lies, yet senior schooling is stuck in the 1600's-1930's.
Aetrew While quantum physics is the new frontier it is NOT complete. And while with quantum mechanics you can predict a few things, it is not practical. Whereas Newtonian physics works and is practical to use in situations where it applies. Every theory of physics can be used at different scenarios and if you can not grasp this concept you should stop and think about it.
Yes he is wrong.
On Amazon the best reviews on products are usually from users who've had the product for at least a year. Seen this video a year ago and have gone farther into the subject. Must say this video is remarkable and "stays with you". This instructor knows what is relevant, applying a unique approach focusing on the provocative things scientific minds appreciate. The video's value is that it is introductory -yes but at the same time provides artful preparation for advanced learning. Having seen other videos, the instructor's approach stands out by focusing on forces as a means to understanding particles. It is a very effective approach helpful in understanding Feynman diagrams.
Great Video, Sir!
Thank you for taking the time to reply to my question- your videos are very helpful and what you're doing is great, my exam is tomorrow and it would be a disaster without your videos!
Thank you again.
Yes. Anything that can happen does happen. Its a different reaction but both can occur.
Some Feynman diagrams show antiparticles apparently moving backwards in time. This is a convention. But as far as I know, no one is seriously suggesting that antiparticles travel through time in the opposite direction. An antiparticle may be created and destroyed but in the interval between the two, time is moving forward.
Thank you, your videos make understanding everything so much easier.
When a neutron change into a proton in this reaction electron and anti neutrino produce. In its Feynman diagram the direction of anti neutrino should be inward.
If there is a difference in mass between input particles and output particles then the difference is usually accounted for by E=mc**2. But in my previous comment I pointed out that much heavier particles can exist for a very short time if they borrow energy (and hence mass) from nothing provided they pay it back very very quickly. The diagrams dont explicitly say anything about KE, tho energy must be conserved in some form.
Thanks for the video it really helped me get my head round feynman diagrams unlike any of the other videos I've watched :)
Remember the adage - anything that can happen does happen. So I am just showing one outcome. The outcome with the W- is also a possibility.
Have you found my nuclear physics playlist on my home page. Are there particular issues you would like covered?
Best explanation I have seen so far on the diagrams.
This is gold!!! I love your videos !!
Because the strong nuclear force is confined. I deal with this in my current series on particle physics. The energy associated with the force between two quarks increases as you try to separate the quarks. It soon reaches a level where pair production arises and two new particles are created all within the radius of a nucleus.
The same argument applies as below. At 6:48 the positron and the neutrino both travel in the same direction to conserve total lepton number. But a Feynman diagram with say a neutrino going one way can also be drawn with an antineutrino going the other way. Compare the diagrams at 6:02 and 8:58. They are virtually the same except in the former the anti-neutrino goes out, in the latter the neutrino goes in. A W- in the former going to the right has same effect as the W+ in the latter going left.
Bless this guy!! What a lifesaver, thank you!!
I was confused on this but your video cleared things up!!!! Thank you sm 🙌🏾🙌🏾 keep up the good work :)))
I got completely confused by my textbook. The exchange particle lines had no arrows, so i presumed they went both ways so i couldn't get my head around it, and then they also had some particles on the right hand side with an arrow pointing back down the page, which I now understand is impossible as the cannot travel back in time. I have been stuck for ages trying to work this out! Very helpfull video again.
Jon
The basic special relativity formula for mass is that the mass (at speed) = rest mass divided by the square root of (1 - v2/c2). When v approaches c, the mass approached infinity.
Conservation rules. For example, baryon and lepton numbers have to be conserved. So when a proton changes to a neutron, positron and neutrino, the baryon number is conserved (proton in - neutron out) and the lepton number is conserved since a lepton and an anti lepton cancel out so lepton in =0 and lepton out = 1 + -1 = 0
Why are they always interacting by weak force? How do you know if the exchanged particle is a gluon a photon or a W+-/Z?? How I know wich is the force that is interacting??
this helped me so much in my exam today! THANKYOU
Great videos I can understand you very well and these videos have been useful to me to study physics and concepts are wonderful thank you very much mister DrPhysicsA.
I'm assuming you mean an experiment where you fire particles (eg protons) at a target (eg a thin layer of gold) and a situation where you have two beams of protons travelling in opposite directions and made to collide. The latter gives more energy because each photon can be travelling at very close to c making the relative speed of collision very very close to c. Thus more energy.
Hi, the video really helped me! I just have one question about the n+V --> p+e- . why does the boson direction go backwards (to the left) ?
A Neutrino may be massless but current thinking is that it has a very small mass (much much smaller than that of an electron). Nevertheless your question is key. Indeed one might also ask how a W boson can be created from a proton/neutron which is 90x lighter. The answer is a version of Heisenberg's uncertainty principle which says that energy can be borrowed (from nothing) as long as it is paid back in a very short time. That is why the W boson exists for such a short time.
Thanks for the quick responses.
You explain things so well!!
Rest mass energy is the inherent energy of a particle which has no kinetic energy ie is at rest. In non relativistic equations, the total energy = rest mass energy (mc^2 - where m is rest mass) + KE.
Thank you i got my as exam tomorrow this really helped!
Such a useful video, thank you very much !
The gauge boson column has nothing to do specifically with the fermions in the associated rows. Its just done like that for aesthetic reasons. If they find the Higgs, someone will have to put in another box. My advice would be to put the W and the Z in the same box and then use the spare box for the Higgs.
they found higgs bro
Thank you so much for the lesson!!
Thank you so much. I think I finally get this!
Quick question:
how do you know which product goes on which side of the squiggly line?
You are usually given the starting products. But in general anything can happen as long as you obey the rules of conservation (charge, baryon, lepton etc)
Thank you x 100! This video helped me tremendously!
Not necessarily. For example a W+ particle going one way is equivalent to a W- going in the other direction.
This video was very helpful thanks a lot!
No. Its a W+ because it is traveling from right to left. You could draw the same diagram with a W- traveling from left to right.
Thank you for educating me.
For the bottom example at 8:20, why is it not the other way around with a W- coming from the electron side?
What boson is present when 2 atoms go to fusion. I know some reactions can create muons, which implies a boson
Can you consider a video on discussing how the diagrammed results from particle accelerators led to the concepts we are fed today about the nature of fundamental particles? Specifically, how are the traces, loops, squiggly lines, etc., interpreted as charge, mass, momentum, types of particles, etc. And how did they infer partial characteristics of quarks? Thanks.
thanks so much for these videos, so much help and clarity
No. The diagram at 6:02 for example has both the electron and the anti-neutrino traveling in the same direction (upwards). This is necessary to conserve the lepton number. No leptons come in. An electron (lepton number = +1) and an anti-neutrino (lepton number =
-1) go out.
Hey, I didn't understand this lesson from my teacher but you explained it very well! Hopefully I ace my test. Thanks
Did you ace your test?
Should we write the arrow downwards for antiparticles such as positrons?
sir u r superb, really good explanation
thank you so much, my exams on friday and i am bricking it, i get all the electronics, but the particle physics stuff is not quite there yet but this helped massively, thanks
Also thanks for a video with an explanation that makes sense!
for the nuetron-neutrino interaction, my textbook has the W- Boson instead of the w+ you displayed. Is this a mistake in the textbook, or can it be either w+ or w- depending on direction?
In reaction of, let's say p + e- -> n + ve, how do we know that it's W+ going from neutron to electron and not W- going from electron to neutron? As far as I understand it, Feynman diagrams are supposed to be symetrical with respect to time.
One more issue. On those diagrams, after exchanging the boson, both particles change direction a little bit. As far as I understand it, it's because exchange particle also carries some momentum and kinetic energy. Is that correct?
Question, when you have a field and say you have a static situation where there is no motion or its in equilibrium, do you need a photon being exchanged to feel the electrostatic force? I mean, protons are everywhere and they all contribute to the final field, but that does not mean you need a photon from every proton to establish the field. There are photons but there is also the field. So isnt it true that you can have a force from the field but no photon exchanges required. Is there not a field and a photon?
how do you tell which particles will give and which will receive the exchange particle
how do you know which way your arrow should point with the W+/- bosons? also are there any interactions with Z bosons? Thank you.
I don't think we really understand the precise mechanics of exchange particles - and indeed it is misleading to describe quantum effects by analogy with the world we observe, but in A Level physics attractive forces are sometimes likened to two people throwing a boomerang to each other. The boomerang spins round and approaches the other person from behind thus pushing him towards to thrower (ie attractive force).
A neutron star results from the gravitational collapse of a large star at the end of its life. The gravitational forces are sufficient to crush the atom and force the protons and electrons to merge into neutrons. So something similar to the situation at 8:00 must be happening.
Question about arrow directions:
Don't the arrows for anti particles point in the opposite direction of particles (ie. negative time)? Time points towards the address bar of this window. Therefore, the last two figures, the anti neutrino should point downwards, rather than upwards like the neutrino in the previous figure?
It's a question that has got me very confused with Feynman Diagrams
It depends on the convention you use. Many people draw Feynman diagrams with antiparticles shown as travelling backwards in time. That is a convention. Antiparticles do not move backwards in time. I personally prefer therefore to show them travelling in the forward direction of time.
Unfortunately, nothing is gained by thinking of anti-particles as traveling back in time. Just because one can do it doesn't mean that one should. Like the new convention of electronics, teaching that electrons run from a relatively negative ground towards the direction of the positive voltage source. The desire to teach electrons rather than "holes" is just an additional and needless confusion factor for young students, now forced to learn the old heuristically developed convention running backwards. I would say that it's an additional stupidity factor, since electrons vs "holes" are really irrelevant to electronics engineering. The old model was developed for the purpose of wrapping ones mind around the subject, so why force new students to learn that model counterintuitively backwards? "Smart" professors are sometimes quite stupid.
DrPhysicsA i think this means that Feynman Diagrams simply don't have an absolute arrow of time. we can read them either way.
Fantastic Video. Onto the next video ...
About the electron capture: as the weak interaction only works at very short ranges and as electrons must be at a certain distance from the nucleus due to the uncertainty principle (and also because they cannot have less energy than the allowed by quantization), how can the weak interaction between the proton and electron occur? from that, I think the proton and the electron couldn't come close enough so they are within the range needed for the weak interaction to take place... also, the interaction couldn't occur from afar because the boson would decay before it met the electron due to the boson's lifespan and thus would not turn the electron in a neutrino...
Great video, thanks.
how the anti-neutrino in Beta minus and the positron in the Beta plus decay diagrams go up ? isn't anti particle suppose to go backwards in time?? the arrows are confusing ...
Is it antielectron neutrino or electron antineutrino?
How can there be a graviton if gravity is caused by the warping of space? Thanks for the interesting clip!
You see for 8:45 the W+ Bosons has traveled from electron-neutrino to neutron thus being able to produce proton and electron. Wouldn't it be possible for the W- Boson to travel from neutron to electron-neutrino thus producing proton and electron. Just kinda confused
hi. very nice video, thank you.
by the way, shouldn't the positron (in onw of the diagrams), have an arrow flipped back (downwards) ?
+Samixuos Lopes This. This was confusing when looking at diagrams after having done the A Level syllabus. Yes, it should go into the junction, but the A Level syllabus doesn't teach this, and so any arrows are ignored as long as the direction of time is apparent. In fact, it's kind of assumed that the student assumes that the arrows represent motion rather than anything else.
+Samixuos Lopes Some diagrams show the antimatter appearing to travel backwards in time. In my videos I wanted to keep it simple so I show all processes in terms of what produces what.
I'm really enjoying this series. I have a few questions...I'll spit them into comments so i dont run out of room.
1) Is the diagram you drew at 8:00 the exchange that takes place when a neutron star is formed?
how does a supermassive W+ boson combine with a light electron to create a massless particle?
Sir what about the direction of the anti particles??? plz answer you mention they have the same direction as those of particles...
Its so awsome we can find such highly educational and so well explained information free on you tube . I need to study harder though , I don't have much of an education but hope I can get it all to sink in , this stuff is amazing , and you make it seem so easy . keep up the good work .
Jack Specht keep learning dude 👍
Beautiful.
If photons are the force carriers of the EM force, therefore cause the interaction between charges, why is it that the W+ or W- transfers the postive/negative charge from the proton away into the electron/positron and the neutrino?
so thats how you get an electron and positron both since they are existing in a hypothetical superstate?
when we say any particle with mass (be it electron or nutrino or protons) moving close to the spead of light, we say its mass increases , is there any equation for this to find out the mass in that instance?.
In this scenario, what happens to the size of these partiles when mass increases?
One more question...
5) why are the guage bosons in the column on the right? Do they share some property with the associated rows? If the Higgs is discovered, where would it go?
2) You said any charge conserving combination _can_ happen, but what causes it to happen? For example in the exchange at 8:00 it looks like a proton must convert into a neutron + w boson before the w boson can combine with the electron. Is this due to random decay, or can it be provoked?
I have a question sir? Why do higgs boson decompose even if it is an elementary particle? As we all know, elementary particles do not compose into another smaller particles.
has the graviton been measured?
I prefer to show particles moving forward in time. Its not clear what moving back in time could mean. It's more intuitive to show an antiparticle being created and moving forward in time.
Its random and actually low probability - which is why the sun burns for 10 billion years. A W boson is 80x heavier than a proton and arises from a variation of Heisenberg's uncertainty principle which says that you can borrow energy for a very short amount of time. So a W boson can be created but it can only last for 10**−25 secs.
is there a feyman diagram for z bosons?
4) at 8:45 you show that the equations are symetric. This is probably related to one of my other questions...what stops it from just toggling back and forth forever?
Why, in the diagrams, was there always a W+ exchange instead of perhaps a W- in the opposite direction?
if the range of a force is inversely proportional to the mass of the gauge boson, then why is the range of the strong N force so short (gluons are massless)....
Very useful, thank you
It might well toggle. But that will be determined by the probability of the exchange taking place. Think about the sun. Some protons on the sun must have been converted to neutrona on day 1 yet other protons might not be converted till 10 billion years later. So the probability for a lot of the potential toggling is very small.
In your earlier comment 7 months ago, you advise to generally to take the W away from the baryon, towards the leptons. In the neutron-neutrino interaction at 8:45, the W particle goes from the leptons to the baryons, with the opposite W particle. Are both correct? i.e. a W- particle from the neutron to the leptons, or a W+ particle from the leptons? Thanks in advance. I really appreciate the videos - thankyou!
Its a case of "If it can happen (i.e. within the rules of conservation of baryon number, lepton number, charge etc) it will happen" So you can have an interaction with the W- moving one way or the W+ moving the other. The probability of each occurring may differ though.
3) The input mass doesn't equal the output mass. I assume the difference goes into the velocities. Do the diagrams say anything about kinetic energy?
Hello professor, what about the penguin diagram?? Tried to find something about here on youtube, but the only results it gave me is about origamis...
At 9:17, on the neuton-electron neutrino interaction, i think it should be the W- with an arrow pointing at the right. Can you please check that? Sorry if i made a mistake
I dont think you made a mistake, I think it just depends on how you want to draw it. Your way should still work too. ~ The Astronomer
So can we say that the electro-magnetic field is nothing but photons?
Yes
don't the arrows have to face the same way along the line..?
Plz Dr. We need more lectures of nuclear physics, coz I am currently taking a nuclear physics course at the university, but I didn't find any detailed lectures
brilliant video thanks
6:55 I think it should be p -> n + e+ + anti-positronneutrino to conserve the lepton number
les bosons W et Z comment obtiennent ses masses
This analogy works for repulsive forces, if two electrons 'send out' virtual particles in all directions into its sourroundings, and the exchange between them transfers momentum between both electrons, then you would have to imagine a positron as a particle 'recieving' virtual particles from directions and disappearing in the positron, of course remember that they're called virtual because they represent the possibility of an interaction and not an interaction itself.
6:56 shouldn't the arrow for the positron be pointing backwards because it is an anti-particle?
even the electron anti-neutrino
thats what i thought too.
yeah that should also be the case for anti-neutrino in the beta minus decay
in a-level we're just taught to draw the arrows the same for both to make things simple i guess
If the neutrinos are neutral, how do you distinguish between the matter and antimatter versions? Apologies if you already covered this in another video.
From the direction of spin and lepton number, both are reversed.
hope this help.
Ramy Haddad i thought i heard that neutrinos might be their own antiparticle. how would this work in terms of spin and lepton number?
N Marbletoe As far as i know, All Leptons including neutrinos have lepton number of +1 and spin of ±1/2. So for example we have neutrino X and we want to distinguish between it and its anti-neutrino, the neutrino X will be having lepton number +1 and spin -1/2, then its anti-neutrino will be having Lepton number -1 and spin +1/2. For more info visit this link
hyperphysics.phy-astr.gsu.edu/hbase/particles/neutrino3.html
Ramy Haddad cool link. it does seem to have an inconsistency, though: since neutrinos do appear to have some mass, they don't travel at c, thus it is possible to travel faster than a neutrino and see it's spin flip directions.
i'm not sure if this means that a particle must be massless (and travel at c) to be it's own antiparticle, like the photon is its own antiparticle. i can't quite wrap my mind around Majorana vs Dirac fermions.
all a level students should watch and learn