Spin of an Electron: Why 'Spin Up' and 'Spin Down'? | Quantum Physics for Beginners

What is Spin? And what is the Spin of an Electron?

Hi guys, I'm back with a new video, all thanks to a brilliant comment on my previous one :) In this video, I wanted to discuss what the spin of an electron is, as well as why we call the two possible results of a spin measurement are called "Spin Up" and "Spin Down".

Spin is inherent angular momentum that a particle has, which comes about when we consider Quantum Mechanics together with Special Relativity. The reason it's called Spin is because this is angular momentum that a particle has AS IF it were spinning.

In my previous video I explained that quantum states can be represented as vectors in some abstract vector space ("Hilbert space") because there is a vector-like relationship between different quantum states in this space. But in order to represent them as vectors, we had to say that the "Spin Up" vector was at right angles to the "Spin Down" vector. John Pawlicki commented asking why we don't just call the two Spin states "Spin Up" and "Spin Right". It's a great question, so I wanted to explain exactly why we couldn't do that.

I hope you enjoy the video! If there's something unclear or if I made a mistake, do let me know in the comments below. Follow me on Instagram for my day-to-day shenanigans and vlogs - @parthvlogs

[MrJdcirbo]

For everyone wondering why physicists named the state spin-up or spin-down: I read about an experiment called the Stern-Gerlach experiment, in which electrons passing through a magnetic field were deflected either up or down. The weird thing is that there was no difference in the amount of up or down the particles were deflected. It is either all the way up to the top of the limit, or all the way down. Every time. This experiment was pivotal in demonstrating the existence of a quantized spin angular momentum state for subatomic particles. That's why they say spin-up or spin-down. This up/right stuff is about calculating the probability of the particle's spin angular momentum being one way or the other. Not the actual direction the particle will travel or spin.

Edit: added "I read about" to the beginning because I'm just a student and not an expert, and I feel it is disingenuous to portray myself as such, and I changed "atoms" to "electrons". I just now noticed that mistake...

[NaCl_AQ]

You've got the best YouTube channel about physics right now. Looking forward to your next videos!

[MXYM]

Really good explanations - thank you. Maybe, if you have time, you could ponder over how a wave of energy propagates through a wire, without radiating, (a transmission line) then how the wire ceases to be a transmission line and becomes a radiating antenna, then how the radiation leaves the antenna and launches into free space. [for instance you could ponder whether the 'electrons' are given a short ''jerk' (causing nature to wobble space) from the energy of the wave (or not)- [[is this combo electricity??]] Then, ponder how a distant antenna can 'pick-up' a wave (maybe concentrate on the magnetic and electric fields separately - and the respective geometry of each type of antenna?) and direct that wave back down a transmission line and into a detector (a good ol' radioset!). This would be a neat practical application of Maxwell in the real world. Maybe you could even do a series of practicals (unless of course you a theorist ;-). Thanks again.

[dnranjit]

Hey thanks for the video. I think "spin up" and "spin down" came from the Stern-Gerlach experiment where the electron from the silver atom either went up or down after the non-uniform magnetic field.Actually, even "spin" is a misnomer as an electon cannot spin on its own axis.It was suprising because the electron is supposed to be in the s-orbital for which orbital angular momentum is zero. Hence, it should not exhibit any magnetic moment and hence should not be affected by the non-uniform field. Instead, they found statistically the electrons exhibiting a quantized magnetic moment of two states. One set of electrons went up and the other went down depending on their spins and deflected by the magnetic field. Hence, S & G concluded that in there is another magnetic moment associated with the electron other than orbital angular momentum. This became a fundamental characteristic of particles like charge and mass.

[krawuppel]

Keep making these videos, please. Your enthusiasm for physics is a pleasure to watch

[Vegan_Cooking_Show]

Looking forward to the in-depth spin video.

[haljordan]

Good explanation. For anyone still struggling: It is important to understand that spin up and spin down is represented with two orthogonal vectors becouse once you measure some random Electron and it turns our to be spin up IT CAN'T BE spin down. This is how the word "orthogonal" translates into the abstract space. Orthogonal means that getting spin up is mutually exclusive with getting spin down. Just like two arrows forming 90 degree in the common R3 space are linearly independent.

[preetii]

I was a finance student with 0 knowledge on science ...but now I have developed interest in physics ..your videos are really good ...thank you 😁

[phy_dude]

I'm requesting for a topic .
" Bohr theroy and Energy levels of atom "
Why the enegry has to be discrete for a energy shell??

[sebastianloessl]

Thanks for making all these great videos. You've even better than my physics teacher

[Blazegaming]

Great quality video as always Parth! :D

[abhinandanangra]

Was waiting for this for long time
Finally got it...

[richardsmythe]

A visiting professor years ago mentioned to me that you merely need to follow through the formalism and see where it will take you. He referred to the electron spin as having two 'degrees of freedom'. This affects the deflection of electrons through a non homogeneous magnetic field - the Stern Gerlagh experiment. You cannot assign the spin to real space directions but it's effect is to cause a deflection in space. The abstract space of the spin 'vector' doesn't corrrespond as it is to anything real.

[johnpawlicki]

Thumbs up from me. The word "orthogonal" is what threw me. You said that spin up and down were orthogonal yet up and down are not orthogonal. Very good videos. Thanks.

[agb]

You do a really fantastic job of explaining complex topics in simple terms.
Could you please do a video relating spin to properties such as magnetism, and expanding on what the implications of an orbiting electron gaining more spin are.

[MohammedAbdullah-mxvg]

Great video Parth. I'm currently doing my A levels and this concept of spin is regularly skirted over in chemistry and physics A levels, but you explained this great clarity :) A quick request, could you please include more depth in terms of mathematical reasoning in your future videos. Thanks.

[shailendrakumar]

Is the vector you are talking about a spinor.and is that the reason for inversion of sign of spinor as the real vector is rotated through an angle of 360 degrees.( because of orthogonal states)

[sarafishman]

In elementary chemistry we are taught that two electrons can occupy the same orbital if their spins are "opposite, " i.e. one up, one down. This concept is further extended to explain the magnetic properties of atoms and molecules---that, for example, oxygen is paramagnetic because (according to molecular orbital models), it has unpaired electrons in separate molecular orbitals, whereas diamagnetic molecules have mostly paired electrons (everything being fundamentally diamagnetic). The beginning student is further encouraged to think of the "spinning" electron as a magnet. Opposite spins are then like two magnets with their N and S poles opposite to each other, and as with two real bar magnets placed together in that manner, the magnetic field is confined within the pair and does not affect space outside of the magnets. (As you might surmise, I've taught first-year chemistry.) Now this explanation of "spin up" and "spin down" in terms of imaginary magnets seems to work really well. Can you comment on how the orthogonal spin model relates to magnetism? How do these orthogonal angular momenta interact with each other so as to repel magnetic fields? Or am I speaking gibberish, reflecting the fact that I never had a really decent physics class?

[neurophilosophers]

Basically because of convention it has to be opposite up/own left/right in/out but what I didn’t know was why they don’t name it clockwise and counter clockwise. And that’s because it’s not really spinning in that simple way. But rather has properties of the underlying chirality describing all reality.

[EdgarUProductions]

Love your videos Parth! Hope you get better soon