Particle Spinning: Why and What Happens If They Stop?

[Kanishka Toshniwal]

we all know that particles do spin. But my question is why it does so why do they spin? what will happen if they stop spinning? what makes the particle to do so?

 

[Omega0]

we all know that particles do spin. But my question is why it does so why do they spin? what will happen if they stop spinning? what makes the particle to do so?

Well, I learned a lession in a book about thermodynamics where the author said, more or less, free translation: "If you are asking yourself what entropy means, have you ever asked what momentum means? If you just write the equation this is just an equation. "
The answer is: They spin because they spin. Precisely: They spin because it has been measured that they spin.

 

[LURCH]

Also, I’ve read in a few different sources that the the property known as “spin” may not be exactly the same as the spin with which we are familiar in every day life, like a billiard ball spinning in place after a rough hit. It’s really just a name given to describe the fact that the particles have angular momentum. I actually found that quite helpful, because I had an easier time accepting it as simply “a property they have”, instead of some in-powered movement.

There’s a bit of a description in this article from Forbes:
https://www.forbes.com/sites/starts...ysics-holds-a-surprising-answer/#320974a12c3a

Not a pier-reviewed publication, but probably ok for this level of conversation.

 

[Omega0]

Also, I’ve read in a few different sources that the the property known as “spin” may not be exactly the same as the spin with which we are familiar in every day life

It is absolutely not what you would expect from your day life. This is what quantum mechanics is: Something strange in "the quantum world", a world which you will never understand but which is there.
In theoretical physics as well as in mathematics you need to use names for things. It is needed to speak about it.
Unfortunately, this is a trap.
When you give things names they may be in your mind as beeings, as things you didn't want to create. Feynman was super cool in my eyes, he said more or less "I give it this name if I need this picture and I give it that name if need another picture".
The property called "spin" in quantum mechanics has the same units as angular momentum but it is not the same.
The trick is - and this seems to be very complicated - not to mix in your mind the quantum world with the daily experience.

 

[Kanishka Toshniwal]

okay. I know that classical spin has nothing to do with quantum spin. Its totally different. Then why are they charged? How do we define that it has negative or positive charge?

 

[Nugatory]

okay. I know that classical spin has nothing to do with quantum spin. Its totally different. Then why are they charged? How do we define that it has negative or positive charge?

I'm not sure I understand the question. Quantum mechanical spin and charge are pretty much unrelated.

 

[DaveE]

Then why are they charged? How do we define that it has negative or positive charge?

Same answer as spin. We can't know why they are charged, that is just a property that we measure consistently. The definition of negative and positive was quite arbitrary. Most everyone wishes Ben Franklin had done it the other way, I think. We just observe that particles have a property called charge, some react in an opposite fashion than others, so we called that negative vs. positive. Like many definitions in physics, it has been standardized because it makes the math easier.

 

[Kanishka Toshniwal]

okay. But still nobody knows what's really happening in quantum level.

 

[weirdoguy]

what's really happening

Define "really happening" and how it differs from "happening", or "really truly happening". Hint: you can't do that in a meaningfull way.

 

[Kanishka Toshniwal]

I meant that we all predict things for quantum level. But we can't assure it like we do in classical mechanics.

 

[weirdoguy]

Why do you think so? If outcomes of thousands experiments in quantum physics tell us something, then I don't see why we can't be as sure about it as we would be in classical mechanics.

 

[Kanishka Toshniwal]

Because in classical mechanics we can observe things. In quantum mechanics who is the observer?

 

[weirdoguy]

Define "observe" and "observer" and tell us why there is none in quantum physics. I think you have a big misunderstanding about what observation is.

 

[Kanishka Toshniwal]

The thing is in classical mechanics we observe things. But in particle level, we only get the outcomes after our experiment and thus we conclude something. We can never know the reason. For example,light behaves as wave as well as particle. Because we just got the outcomes like that. But we don't know why it happens. Because quantum mechanics is beyond our imagination, we can observe every little thing there. Can we?

 

[bobob]

okay. But still nobody knows what's really happening in quantum level.

What you are really saying is that you don't have an explanation given by your everyday classical idea of reality. This is backwards thinking, since you can't describe the components of something in terms of what you've built from them. Electrons, for example, are not classical things. We define an electron by certain properties we give it based on measurements we make that identiy it as a "thing" that is different from other "things" we describe with the same types of measurements. That's all the reality there is.

Spin is called spin because it adds to angular momentum like other angular momenta and what is conserved is the total angular momentum. That doesn't mean anything is actually spinning in any classical way. Spin has no classical analog. The "quantum level" to which you refer IS where it's reality lies.

 

[Kanishka Toshniwal]

okay!

 

[bobob]

The thing is in classical mechanics we observe things. But in particle level, we only get the outcomes after our experiment and thus we conclude something. We can never know the reason.

You should think about that a bit more carefully. You don't get any more in classical mechanics apart from your bias that what you "observe" is not in itself a measurement. On the contrary, what you observe is a measurement. If you observe two objects colliding, what you are observing is not the collision of two objects, but the light scattering from those objects into your eyes. Your brain is interpreting the signals in your retinas from the scattered light. What you observe is the same thing that you reer to as only the outcomes of experiments.

For example,light behaves as wave as well as particle.

Only because we are using two incommensurate classical concepts to describe something that isn't classical in origin.

Because we just got the outcomes like that. But we don't know why it happens. Because quantum mechanics is beyond our imagination, we can observe every little thing there. Can we?

Well, we do know why it happens. It happens because we have to arrange equipment in a certain way to measure what are called the observables in the theory. A theory describes the objects in it and the properties of those objects, even classical theories. You just are less aware of that because classical theories describe what humans can expeience directly. That is an anthropomorphic bias. Here is an example. Suppose you were blind. How would you understand color other than by measurements of some quantity? The reality is that your brain gives those measurements the meaning of color, but it's synonymous with wavelengths to which the human eye is sensitive and the meaning your brain attaches to it. If you were blind, you question would amount to the same thing as "What is color, really?"

 

[extranjero]

why it does so why do they spin?

This is the same question as "why do they are somewhere in a space (instead of being nowhere)"? Full description possible when we use extended Hilbert space: not this ##|x_1\rangle,\,|x_2\rangle,\,\dots|x_n\rangle,\,\dots##, but this ##|x_1, \uparrow\rangle,\,|x_1, \downarrow\rangle,\,|x_2, \uparrow\rangle,\,|x_2, \downarrow\rangle,\,\dots,|x_n, \uparrow\rangle,\,|x_n, \downarrow\rangle,\,\dots## (note, here I don't use words like "rotation")

 

[haushofer]

Spin is, like mass, an intrinsic aspect of a particle with which we can label them. We know that mass of fundamental particles is really an interaction with the Higgs mass, but because of technical reasons spin cannot likewise be dynamically obtained (at least I'm not aware of any attempts).

 

[Kanishka Toshniwal]

okay. I got it. Thank you.

 

[romsofia]

Even though you said you got it, I'd like to add one more thing: Look into the Stern-Gerlach experiment if you haven't yet. This gives a more physical idea of what spin is, and it's an experiment, so it isn't just an abstraction (i.e hilbert space...).

 

[David Lewis]

Spin is called spin because it adds to angular momentum like other angular momenta and what is conserved is the total angular momentum.

When quantum spin was first discovered, it was temporarily called "spin" for sake of discussion. They considered replacing "spin" with a less misleading term but by then it had already stuck.

 

[jtbell]

We have a good phrase for it: "intrinsic angular momentum." Its main problem is that it has nine syllables.

 

[A. Neumaier]

But still nobody knows what's really happening in quantum level.

Everyone who cares knows that the state changes according to the Schrödinger equations. This is enough to make very successful predictions.

 

[N88]

We have a good phrase for it: "intrinsic angular momentum." Its main problem is that it has nine syllables.

Thank you. So is there "extrinsic angular momentum" (sometimes) such that the combination of the two gives a particle's total angular momentum? Can that total be represented by vector in 3D?

 

[stevendaryl]

Thank you. So is there "extrinsic angular momentum" (sometimes) such that the combination of the two gives a particle's total angular momentum? Can that total be represented by vector in 3D?

Sure: ##\overrightarrow{J} = \overrightarrow{L} + \overrightarrow{S}##

 

[DrDu]

Spin is defined as the angular momentum in the rest frame of the particle. For a compound particle, this has at least a contribution of the usual classical angular momentum. Only for a point particle, it makes no sense to speak classically of angular momentum in the rest frame. On the other hand, we describe even electrons no longer as point particles but as quantized fields.

 

[N88]

Sure: ##\overrightarrow{J} = \overrightarrow{L} + \overrightarrow{S}##

Thank you. So if I wanted to show 2 particles flying apart in an EPRB setting [one toward Alice, the other toward Bob] with the total angular momentum conserved pairwise: what is the neatest way to show this relation pairwise? (i) For spin-half particles in the singlet-state? (ii) For photons as in Aspect's experiment?

And how do we take into account DrDu's comment, since I see particles as point-like but not points?

 

[stevendaryl]

Thank you. So if I wanted to show 2 particles flying apart in an EPRB setting [one toward Alice, the other toward Bob] with the total angular momentum conserved pairwise: what is the neatest way to show this relation pairwise? (i) For spin-half particles in the singlet-state? (ii) For photons as in Aspect's experiment?

And how do we take into account DrDu's comment, since I see particles as point-like but not points?

I'm not sure if this answers your question, but in EPR-type experiments, orbital angular momentum L doesn't much come into play. Spin S is not rigorously conserved by itself, but I'm assuming that it's close enough to being conserved as to not matter?

In the decay that produces an electron/positron pair, I think it's true that in the center-of-mass frame, the orbital angular momentum is zero, and there are no significant torques to change that between the time the pair is created and the spin is detected. But I don't know.

 

[N88]

I'm not sure if this answers your question, but in EPR-type experiments, orbital angular momentum L doesn't much come into play. Spin S is not rigorously conserved by itself, but I'm assuming that it's close enough to being conserved as to not matter?

In the decay that produces an electron/positron pair, I think it's true that in the center-of-mass frame, the orbital angular momentum is zero, and there are no significant torques to change that between the time the pair is created and the spin is detected. But I don't know.

I was hoping for some QM formulas and equations. With a need for subscripts A and B to distinguish Alice's particle from Bob's. I was also hoping that there'd be some simplification like

## \overrightarrow{L}_A = \overrightarrow{L}_B=0##

Why? Because if "orbital angular momentum" is the same as ## \overrightarrow{L}##, then it seems to me that the EPRB particles are not orbiting??

PS: I was happy with the terms extrinsic and intrinsic angular momentum because it allowed that a point-like particle could have angular momentum over and above the intrinsic when the particle was not orbiting.

My present thinking is maybe too much influenced by the fact that angular momentum (while tricky) is the stuff of everyday life. But then that is why I am seeking the QM formulations in the context of EPRB-type experiments.

 

[stevendaryl]

Why? Because if "orbital angular momentum" is the same as ## \overrightarrow{L}##, then it seems to me that the EPRB particles are not orbiting??.

I don't know the details of it,but no, I would think that, conceptually, you're creating an electron-positron pair that are heading off in opposite directions, so there is no orbital angular momentum in the center of mass frame.

 

[DrDu]

If it is only for the sake of understanding the EPR experiment, you could use pairs of compound particles. If the particles which make up the compound particle, have no spin of their own, the spin of the compound particle will be equal to the orbital angular momentum L. L will have more than two possible values, so the EPR experiment will turn out slightly more complicated.