What is Spin?

Spin in quantum mechanics is an incredibly interesting property. However, it can be very difficult to understand what exactly it is. In this video, we dispel some misconceptions about spin as well as answer some of the more frequently asked questions about spin.

#physics #quantum

[lastchance]

Great video, thank you! Now I understand why I dont understand spin.

[PrettyMuchPhysics]

Comparing "spin as intrinsic angular momentum" to "mass as intrinsic energy" was great! 👍

[lcarliner]

What would be extremely helpful would be some information and examples how spin is observed from the output of detectors used by particle accelerators!

[qzamboni]

There is a developing classical intuition for spin nowadays. As well as all of the behaviours formerly thought of as "quantum"-only. Basically, the quantum wavefunction is (and always was) just the statistics of the state the particle is in, not a full description of the chaotic dynamical system which gives rise to that state. In particular for an intuition of spin, I've always liked the video "Spin lattices of walking droplets."

p.s. - I'm a PhD student and don't really have time or patience to respond to youtube comments, but just putting it out there. For anyone who happens to come across this and is interested, I recommend looking up walking droplets (there are more amazing videos) and/or reading the 2020 review paper by Bush, 'Hydrodynamic quantum analogs'.

[DrDeuteron]

one problem with understanding spin is thinking we already understand quantum momentum from classical physics. Momentum is that rate of change of phase w.r.t position, and as you mentioned: at zero momentum there is still intrinsic energy, or rate of change of phase w.r.t. time. Spin is rate of change of phase w.r.t to rotations. While that may admit no intuition, it does put E, p & S on equal quantum footing.

[SkyRiderJavelin]

Brilliant explanation you must really know your stuff to explain this concept in logical steps . I don't pretend to understand everything in this video but at least I am starting to appreciate the fundamental nature of the quantum property compared to other video's I have seen on this subject which gloss over the intrinsic quantum component. Thanks for posting

[ominollo]

Yet another great explanation!
I would be great if you good give some references (e.g textbooks) for self study 🙂

[odiram]

Trying to think about this in other terms, sorry if this is a dumb question, but when we talk about spin or inherent angular momentum are we basically talking about how much a particular particle seems to have an inherent tendency to swerve in a particular direction?

Like, we poke the particle to see where it moves, and instead of acting like a classical billiard ball that shoots in the direction of the summed forces, it swerves, and we analogize the tendency of the swerve to angular momentum though we know it can’t be spinning and isn’t orbiting anything. Is that kind of what we’re talking about, or is it some property unrelated to observations of motion?

[pkuvincentsu]

Great explanation! I've watched several vidoes on spin, and yours is the only one that conveyed the concept with clarity.

[Trizzer]

The earth doesnt actually have angular momentum from orbit. spacetime is warped, so earth is moving in a straight line

[brahimek]

Please make a course about classical electromagnetism

[Vrooten]

Would have been helpful for me to see some examples of how spin actually manifests itself irl!

[aydencook]

Fantastic video. I like it when a single video makes a lot of connections to other areas that we can research for ourselves.

[halnineooo]

I feel like using common language words like rotation, spin, wave, particle, to describe phenomena alien to our sensible experience only adds confusion.
I would rather coin complete new specific names that are as much possible rooted in maths without the macro world confusing imagery.

[narfwhals]

Hi Zap! Hope you're doing well.
Interesting video! I had no idea the spin statistics theorem followed from special relativity. I honestly was under the assumption it was, well, an assumption. Or an observation.
I tried to relate the spin 1/2 rotation symmetry it to the anti-symmetry of fermion wave functions but I realized that I was actually pretty confused what exactly _has_ the 720° rotational symmetry for a spin 1/2 particle. Is it the sign of the spin? The wave function? The entire system?

[munepuppie]

Excellent video! 😊 I’m constantly reminded of just how much good info is on YouTube; “appealing to the lowest common denominator” indeed! Personally I am a layman who does read a lot of science books (mostly popular ones) and who does watch science TV programs and YouTube videos and had come across quantum spin before but never truly had a handle on it until I just watched this video. Keep up the good work!

[StevenG]

Awesome stuff! I really admire your videos.

There’s a really nice visual proof of the spin-statistics theorem that I’ll try to share in words. Consider a process where two electron-positron pairs are created at t=0, and two electron-positron pairs are annihilated some time later. This can happen in two ways: the electrons can annihilate with the positrons they were created with, or they can switch and each annihilate with the other positron. Now, if you simply accept (from Special Relativity) that in a space-time diagram, you can treat a positron worldline as an electron worldline moving “backwards in time, ” you can consider the first process as two separate “loops” of electron worldline, and the second process as one big “loop” connected like a figure 8. Focus on the second process. Viewing it in terms of electrons and positrons, you note that two fermions switched places, giving an overall phase factor of -1. Viewing it as one electron going through a “loop” forwards and backwards in time, you can convince yourself (take a loop of ribbon and make a figure 8 out of it) that the electron rotates by 2π during its trip. What spin must it have to explain the -1 phase factor? A half integer.

[Live.SoapyYT]

I absolutely love particle physics. Quantum field theory to be precise.

[sethlawson]

What kind of experimental setup, regardless of whether it would actually be feasible/possible, could elegantly exemplify quantum spin and the different behavior of the fermions and bosons? I like the analogy of two particles being either red or blue and asking whether they are rb/br/rr/bb and eliminating two of those choices for fermions, but could we describe a similar thought experiment but in a more literal way?

I'm imagining a scenario of firing two identical particles down a T track and having them encounter a magnet prior to being measured at either side of the T and running the experiment a million times and asking whether there's a different distribution of results when firing photons vs electrons, something like that. I hope my question makes sense. This video and the "what are fermions and bosons" are really insightful!

[BiswajitBhattacharjee-upvv]

When you take any "m" value between 0and 1and multiply with "hand write down it using zero and decimal we will reach very two value of classical continuum.
And fraction is always an incomplete state
What is then expected symmetry???
But question is what is intrinsic.
What is the critical set up for intrinsic and extrinsic???