Quantum Electrodynamics (QED)

In our study of physics, we have become aware of four forces, and the fields that mediate them. Once we got deep into quantum theory, we started to realize that these forces are not mediated by fields at all, but rather by quanta. That's why it's called quantum theory, we are showing how particles, or quanta, run the whole show. The electromagnetic force was the first one to get a facelift, so let's check out how these quantum field theories work by looking at quantum electrodynamics!

Check out "Is This Wi-Fi Organic?", my book on disarming pseudoscience!

[kidzbopisstraightfire]

Feynmann was the man! I have a book of his, and it's amazing how he could explain quantum theory in such a easy way for a layman (like me) to understand. That's pure genius.

[zoltankurti]

Fields are very much still involved in QFT. The core assumption is that just as you made x and p into operators in basic quantum mechanics, you make the fields themselves into operators. That means that thatthere is an operator associated to fields at each point in spacetime or space, depending on if you are in the schrödinger or heisenberg picture. Particles emerge from these quantum fields, as the energy of free fields is as if they would represent multiple particles and add their energy together. And of course you can construct a particle number operator that is consistent with this. Feynman diagrams are in no way a fundamental building block of QFT, they are just a tool to represent certain terms in perturbative expansions, and they only work for weakly interacting theories. They emerge in the following way: you split the hamiltonian into two parts, the free hamiltonian and the interaction hamiltonian. You switch to the interaction picture, and in that picture the time evolution operator can be approximated by the dyson series. The nth term in the dyson series is associated with feynman diagrams with n vertices in them. Each term in the dyson series further splits into the sum of different terms using wicks theorem. It's these terms that are kn 1 to 1 correspondence with the feynman diagrams. Fields are still fundamental, feynman diagrams are just a tool to get terms in a series without doing the boring algebra required to obtain them otherwise.

[zoltankurti]

Ummm. No, fields are still not outdated as a concept. Particles are just a convinient way of describeing mathematically the state of a quantum field. The underlying idea behind all of this is still the field concept. This is illustrated by the use of lagrangian density in every quantum field theory.

[elnuraliyev]

However, how do then opposite charges interact? Do they also send out photons, and somehow still attract to each other?

[brynnagrace-]

I went from extreme confusion to a vague understanding in 3:54 how??? Thank you. I'm young so when I don't understand concepts in physics I end up convincing myself I can't. Guess maybe I can kind of learn some physics. (I read in a book if you think you understand quantum physics, you don't)

[ahmadzamanapu]

thanks for the explanation through the animation

[محمدالعمري-زو]

I don't get it:
First I doubt if someone can even imagine that two pieces are attracted to each other or push each other away because they exchange a package.
Secondly, I still don't understand what these two kinds of electric charges are. Different spices, salt and pepper?

[shinpark]

I'm confused. Shouldn't these "virtual photons" all have different frequencies? Therefore, different energy? If so, shouldn't the change in momentum between two electrons, due to virtual photons exchange be different each time? Or is it that the virtual photon being manifested by the electron like you said, already quantized so the change in momentum is always the same betwen two electrons?

[vivalibertasergovivitelibe]

A good explanation. A bit ruff around some edges but in general very good.
I mean we haven't truely merged quantum theory with general relativity and virtual photons are just a simplification for the exitations in its field and saying fields have come abundant for the transmission of forces when indeed the quanta that transmit forces are merely exitations in the regarding fiel feels a little weird but I guess this is nit picking.

[MrSunhare]

This material makes me really excited to learn about these phenomena in my physics class :) Thanks so much Professor Dave! Do you know what field of math is involved in order to understand the math behind these behaviors? I'd love to try my hand at perusing through them to get a scope of what is to come/what I'm missing haha

[pawankumar-wtfn]

You are doing a great job am from india and your videos are awesome

[primaryendo]

A physics professor once told me that quantum electrodynamics is the most beautifil thing the human intellect has ever achieved. I really wish i understood why.

[saqlainmalik]

Appreciate your work professer thank you

[hinkles]

Even though QED works just fine, quantum gravity doesn't work out because trying to merge quantum mechanics and general relativity produces a lot of infinities. Precisely, it produces an infinite number of infinities, which is annoying because it's tricky to get rid of that number.

[andreasklebrigtierschutzve]

I don’t really grasp how a photon, which has no mass, can divert the path of an electron, which has mass. To divert a path, a force is required and an object with no mass can not exert any force (as stated in Newton’s equation F=m*a).

[TheDVRS]

I understand how the transfer of momentum could cause repulsion but not attraction

[-OICU-]

Interesting to hear about getting something from nothing. I wonder if that may be due simply to, we humans not possessing all of the pieces to the quantum puzzle. Could this be linked back to the possibility of there being more dimensions than we can observe account for and understand? Is there a way to gain knowledge of unobservable and unverifiable dimensions of existence? This stuff seems pretty heady. Out of my head anyway, but very interesting.

[joaolucasgaldino]

Can someone help me? How do protons and electrons interact? Wouldn't the virtual photons emmited by the electron just push the proton away? How do the virtual photons make the electrons stay around the nucleus?

[astridmartin]

I don't get why the fields had to he discarded and exchange of virtual photons had to be used instead??

[kentkeatha]

How about the electromagnetism?
How the electric field generate this force.?