How QED Unites Relativity, Quantum Mechanics & Electromagnetism | Quantum Electrodynamics

0:00 video start
3:10 Hard math
7:14 Visual explanation
8:26 Feynman Diagrams
Quantum theory solved many problems with classical theory - the photoelectric effect solved by Einstein, atomic model solved by Niels Bohr. But quantum theory had a high price - determinism. it introduced randomness. Events were not deterministic anymore but probabilistic. We don’t see this in our macro world because it gets smoothed out.

Small things move at very high speeds. And so to describe them at velocities near the speed of light, Einstein’s Special relativity must apply. The integration of quantum mechanics with electromagnetism is called Quantum electrodynamics, or QED. QED replaces the classical theory of electromagnetism, which involves continuous electromagnetic fields and puts it in terms of discrete quantities.

In 1928 the first big step to QED was taken by British physicist Paul Dirac, when he published the Dirac equation. It blended quantum mechanics and special relativity. His equation was similar to the Schrodinger equation. Psi which is the wave function is present in both equations. Dirac made a Hamiltonian, which is the sum of all the energy, in terms of space-time. It also has the terms MC^2, and p the momentum on the left side - from special relativity.

Schrodinger equation treats time and space as independent coordinates, but Dirac integrated space-time, into the equation. There are some formulations that result in negative solutions. His equation was predicting anti-matter. No one had ever thought of that before. Dirac believed antimatter had to exist based simply on the math. 4 years later, 1932 the positron was discovered by American physicist Carl Anderson.

Later, three scientists began formulating a quantum theory of electromagnetism based on Dirac’s equation, and formulated QED - Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga. They won the 1965 Nobel prize in physics. The result was a Lagrangian. This equation holds information about the electromagnetic field and about whatever charged fermions one wants to investigate, like electrons and quarks.

In the 19th century, it was thought that two electrons near each other would simply repel similar to the way that two like poles of a magnet repel. Paul Dirac and others showed that there is no continuous field, but that the forces are mediated by the exchange of discrete photons.

When two charged particles travel near each other toward a colliding path, as they get close they will repel each other. In classical theory, this would be thought of as the particles repelling due to a continuous field. But in QED, the path can be simplified to look like a Feynman diagram. These diagrams were developed by American physicist Richard Feynman around 1949 and they help to intuitively understand the mathematics of QED.

The direction of the arrow on the fermion line tells us if it's a matter, or the anti-matter particle. If the arrow goes forward in time (to the right), it is the matter, and if it goes to the left, it’s antimatter. From this simplest Feynman diagram, you can build all other diagrams for QED.

The lines with the arrows represent fermions, and they must be continuous. This is because of the law of conservation. The same number of fermions coming into a process must also come out. Note that a photon is a boson, not a fermion. But if a positron and electron come in, and only a photon comes out, then how are things conserved? The reason is because a matter particle is considered a +1 fermion and an antimatter particle is considered a -1 fermion. So an electron and positron coming in works out to be +1-1 = 0, so the rule still works.

The Key to understanding Feynman diagrams is to know that each diagram is really an equation. One of the simplest diagrams is where two electrons come in, two electrons coming out and a photon is being exchanged – this equation has 7 components. Each of these 7 components has an associated element in an equation.

Note that when two charged particles actually come near each other, they exchange many photons, not just one. Many different types of potential exchanges can occur. But by simplifying the equation to the exchange of a single photon, we can get a good approximation.

The lessons from the development of QED were later used for the development of Quantum Chromodynamics (QCD) & Quantum Field Theories starting in the 1960’s, like elecroweak theory.
#QED
#quantumelectrodynamics
Electromagnetism was the first force to be expressed in terms of a quantum field theory.. QED is electromagnetism expressed in terms of quantum fields.

[ArvinAsh]

I see that there are a lot of questions about virtual photons. Sorry, I did not spend more time on this. Here are a few points that might answer some of your questions:
1) In the Feynman diagram showing the interaction of two electrons with a photon exchange in between - this is a virtual photon. It is important to remember that these are not real photons. They cannot actually be detected. They exist only as the force carrier between charged particles. They allow QED quantization in terms of field excitations.
2) In the Feynman diagram showing electron-positron annihilation, this is a real photon. It can be measured.
4) Regarding why the charges exchange photons or what exactly a charge is -- no one knows. It is possible that a future TOE may provide these answers.

[physicslover]

Dirac was a genius. He predicted antimatter so accurately. 💚💚💚 in the same way as Einstein predicted gravitational lensing, gravitational redshift and gravitational waves 💚💚

[BertoldSzekeres]

Arvin Ash: I cannot tell anything with absolute certainty.
QFT: Exactly.

[johnjamesbaldridge]

You are really the best science presenter out there, and that's saying a lot. What I REALLY like is your dissection of the equations, specifically what the terms represent. Mathematics is an art. Choosing symbols and deciding how much to compress into each one such that the result paints a concise picture.

[iron]

Wow, it’s awesome to see complicated ideas explained so simply

[papsaebus]

I love how you actually show equations and explain terms in it!

[theultimatereductionist]

Please DO NOT SKIP over the explanation of turning a Feynman diagram into a math equation!
That is the part we all desperately need to hear, because nobody does that!
There exist hundreds of videos already showing how to write out a Feynman diagram,
but NONE explaining how that becomes math.

[evoun]

That 2 min introduction in every of your videos was indeed worth to watch. I get a brief intro of what were you about to say on your main focus of the rest of your videos. Great job man.. Many thanks from Malaysia!!!

[Thedanishundertaker]

As always brilliant work from you guy’s. Love the smooth transition from the Diagrams to the equations awesome stuff guys. Even at 47 I’m still learning about this stuff and it’s hard to wrap your head around, so that why I love this channel, . Keep up the great work and I’ll see you in the next episode my friends. Stay save all from Denmark 🇩🇰

[smokey]

You know that part that you do at the end where you enthusiastically promote Square Space and sell a bit of your your soul so that we, your audience, can learn from you, even those of us who cannot afford a formal education… thank you for doing that. Your work is teaching an amateur physicist who is currently writing theories that will change current perceptions.

[LordandGodofYouTube]

Another great video Arvin! I must admit I had to watch this a couple of times to get my head around it just because it's not a simple subject, but once again you made it pretty easy to understand and very interesting. I am in awe of the people who are able to formulate these theories about things that most people don't even consider their existence. It's a shame we only get such a short time on this planet as I'd love to see what science has come up with in 500 years.

[NNiSYS]

Hi, Arvin! Ricardo again. Your clarity is beautiful. Thank you so much for sharing it. Thank you!

[vagl]

By the most accurate and comprehensive presentasion of QED on youtube.Keep up the great work!!!

[BangMaster]

General Relativity + Quantum Mechanics = General Mechanics
Which is an auto repair shop down the street i live on.

[fourkings]

I absolutely have no idea, but i really love watching it

[PaulHigginbothamSr]

Thank you Arvin for the clarity of these videos from the 40's which are still incredibly important. Feynman's most difficult mental computation was for spin angular momentum in a superfluid superconductor. He built mentally magnetic flux tubes through the super fluid pinning lines of flux inside neutron stars. The relative temperature is billions of degrees but to the free electrons approaches relative cold at the pressure down inside the neutron star, thus producing glitches in spin momentum and a slight shift of magnetic strength alters the spin momentum of the mass of spinning neutrons causing either a speed up, slow down, or gradual return to it's previous state of angular momentum. These tremendous forces will be found to produce "fast radio bursts", which happens through spin reduction of the angular momentum. The only area it can be created.

[tomgain]

Beautiful presentation I’ll rewatch this at least 10 times

[trewq]

Wow, great balance between being rigorous and explaining it intuitively! I really like that you show the equations and give an idea about what they mean.

[TheSaferHouse]

Thank you for making concepts like QED accessible to people like me who never took a physics class. Our universe is truly mind-blowing!

[rafanifischer]

I like these short instructive videos. I guess these are made for the intellectually curious and maybe for some serious students who find their professors boring.