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BadgerBadger92
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I read about quantum spin a while ago. If it’s not spinning, what is it doing?
Quantum Spin - If It’s Not Spinning, What Is It Doing?
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In summary, the concept of quantum spin is not related to the physical spinning of a particle, but rather an intrinsic angular momentum that it possesses due to its existence. This concept is similar to the role of mass in describing the linear momentum of a particle. Furthermore, the analogy between spin and mass is used in the application of representation theory to spacetime symmetries in quantum mechanics. The idea of angular momentum without physical spinning is not unique to quantum mechanics and can also be seen in classical electrodynamics and with point particles. This question has been previously discussed and answered in various forums, making it unnecessary to open new threads.
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It's evolving according to the Schrodinger equation. 
- #3
Nugatory
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Nothing.BadgerBadger92 said:
The word “spin” is used for historical reasons, but that property of quantum particles has nothing to the normal meaning of the word. It would have been better if we called it “intrinsic angular momentum” from the beginning, but it’s too late now - physicists used the word “spin” a century ago and it has stuck.
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Nugatory
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An off-topic digression has been removed. If any of the participants in the digression want it continued, let me know.
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Dale
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It has angular momentum. Is there any reason that it needs to be doing something with it?BadgerBadger92 said:
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Gaussian97
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As an analogy (which probably has some flaws) you can try to exchange the following concepts:
spinning <-> movement
angular momentum <-> energy
Then, it is true that when a particle is spinning, it has some angular momentum. Similarly, when a particle moves with some velocity is has some energy associated with that movement.
Now, this doesn't mean that any energy that the particle has is due to its movement, nor all the angular momentum is due to the spinning of the particle.
There is an intrinsic energy that the particle has only because it exists, is not due to any movement, we call it mass.
Similarly, there is an intrinsic angular momentum that the particle has only because it exists, not due to any spinning or anything else. We call it spin.
spinning <-> movement
angular momentum <-> energy
Then, it is true that when a particle is spinning, it has some angular momentum. Similarly, when a particle moves with some velocity is has some energy associated with that movement.
Now, this doesn't mean that any energy that the particle has is due to its movement, nor all the angular momentum is due to the spinning of the particle.
There is an intrinsic energy that the particle has only because it exists, is not due to any movement, we call it mass.
Similarly, there is an intrinsic angular momentum that the particle has only because it exists, not due to any spinning or anything else. We call it spin.
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docnet
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Leonard Susskind: "spin is an abstract mathematical quantity [intrinsic to particles] that transforms like angular momentum under rotation transformations."
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jtbell
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Angular momentum without "spinning" isn't unique to QM. In classical electrodynamics, an electromagnetic field can have angular momentum.
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kith
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The analogy given by @Gaussian97 points to spin and (invariant) mass being somewhat analogous.
But one could ask why choose energy and not (linear) momentum as comparison. After all, if there exists an intrinsic angular momentum for a massive particle without an angular movement, couldn't there also be an intrinsic linear momentum for a massive particle without linear movement?
Well, if we talk about the 3-momentum of classical mechanics, the answer is of course no. No linear movement means using the rest frame of the particle as reference for the description and there, the 3-momentum is zero. But even in this frame, the magnitude of the 4-momentum of relativity is equal to the invariant mass of the particle. So in this sense, mass plays the role of an intrinsic linear momentum which strengthens the analogy between spin and mass.
I think it's still a quite shaky analogy but there is a rigorous way in which spin and mass are analogous: both play similar roles in the application of representation theory to spacetime symmetries in QM. It's usually done within the relativistic context but does already work in non-relativistic QM.
But one could ask why choose energy and not (linear) momentum as comparison. After all, if there exists an intrinsic angular momentum for a massive particle without an angular movement, couldn't there also be an intrinsic linear momentum for a massive particle without linear movement?
Well, if we talk about the 3-momentum of classical mechanics, the answer is of course no. No linear movement means using the rest frame of the particle as reference for the description and there, the 3-momentum is zero. But even in this frame, the magnitude of the 4-momentum of relativity is equal to the invariant mass of the particle. So in this sense, mass plays the role of an intrinsic linear momentum which strengthens the analogy between spin and mass.
I think it's still a quite shaky analogy but there is a rigorous way in which spin and mass are analogous: both play similar roles in the application of representation theory to spacetime symmetries in QM. It's usually done within the relativistic context but does already work in non-relativistic QM.
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weirdoguy
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jtbell said:
One doesn't even need electrodynamics. Point particle moving uniformly along line can have non-zero angular momentum.
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andresB
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jtbell said:
There is even no necessity of fields, you can attach an intrinsic angular momentum to a classical point particle.
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otennert
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As anyone can see at the bottom of this very page, this question has been raised as well as answered many times before, e.g. here: https://www.physicsforums.com/threads/what-is-actually-spinning-in-quantum-spin.999883/
Is it worth opening just another thread?
Is it worth opening just another thread?
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Nugatory
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Indeed it has, so this thread is closedotennert said:
FAQ: Quantum Spin - If It’s Not Spinning, What Is It Doing?
What is quantum spin?
Quantum spin is a fundamental property of subatomic particles, such as electrons and protons, that causes them to behave like tiny magnets. It is not the same as the physical spinning motion of an object, but rather a type of intrinsic angular momentum that cannot be directly observed.
How is quantum spin measured?
Quantum spin is typically measured using a technique called Stern-Gerlach experiment, where a beam of particles is passed through a magnetic field and the deflection of the particles is observed. The amount of deflection indicates the spin of the particles.
Why is quantum spin important?
Quantum spin plays a crucial role in many areas of physics, including quantum mechanics and particle physics. It is also a key concept in technologies such as magnetic resonance imaging (MRI) and quantum computing.
Can quantum spin change?
Yes, quantum spin can change in certain situations, such as when particles interact with each other or with external forces. This change in spin can result in the emission or absorption of energy.
How is quantum spin related to other quantum properties?
Quantum spin is one of several quantum properties that particles possess, along with properties like charge and mass. These properties are all interconnected and can affect each other, making quantum spin an important factor in understanding the behavior of particles at the subatomic level.