# Spin

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I find this subject interesting , yet at the heart of quantum theory. If anyone has any eureka feelings about " spin" at the quantum level please could you share them !

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If you look at energy quanta and light waves, they have wavelength and frequency. Wavelength is connected to distance and frequency is connected to time. For energy, the product of the two is connected to C (speed of light).

Spin contains loosely similar characteristics. The spin is connected to frequency or time, but the wavelength or distance is not related in the same way to C, since any size object (radius, diameter or circumference) can spin at different frequencies. However, there will be a sweet spot where the size and spin frequency can actually equal C, for inertial spin. That is a theoretical possibility, which I would assume creates a state where inertial spin becomes more like energy. But as long as the time and distance characteristics of the spin are not C, we have inertial spin. Just a theory.

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I find this subject interesting , yet at the heart of quantum theory. If anyone has any eureka feelings about " spin" at the quantum level please could you share them !

It represents spin as the energy flow of a wave packet of limited size around z-axis. However the spin does not depend on the packet size. Factually it is shown that a vector field has spin 1 and spinor field does 1/2. We knew this without wave packets.

Edited by Bob_for_short

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If you look at energy quanta and light waves, they have wavelength and frequency. Wavelength is connected to distance and frequency is connected to time. For energy, the product of the two is connected to C (speed of light).

Spin contains loosely similar characteristics. The spin is connected to frequency or time, but the wavelength or distance is not related in the same way to C, since any size object (radius, diameter or circumference) can spin at different frequencies. However, there will be a sweet spot where the size and spin frequency can actually equal C, for inertial spin. That is a theoretical possibility, which I would assume creates a state where inertial spin becomes more like energy. But as long as the time and distance characteristics of the spin are not C, we have inertial spin. Just a theory.

!

Moderator Note

Hypothetical musings belong in speculations. The are not appropriate as answers to questions outside of speculations.

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I have been absent over the Xmas and new year, but am returning to the subject of spin. Surely the early founders of quantum mechanics such as Pauli used spin to derive the exclusion principal necessary for orbital or energy band structure for all ( the entire universe ) atoms. If the spin was not there, surely the universe would fall apart ! Although spin appears in all studies on quantum theory and is indeed used in ideas leading to quantum computing. It seems less spoken about in connection with quantum mechanics than perhaps the other aspects. Is this due to the fact that there is less understood about spin in the way that "wave particle duality " seems to leave many mystified ?

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In standard non-relativistic quantum mechanics spin is simply "bolted-on" to the theory. It appears as a requirement from nature with little mathematical justification.

Only when one considers relativistic systems does spin appear naturally. Loosely, you can think of spin as a relativistic counterpart to angular momentum. Making the last statement requires a bit of knowledge about Lie groups, their Lie algebras and Casimir operators.

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Thank you for your comments. I, perhaps foolishly, would like to do some research around the subject of "spin". Perhaps as Frank Wilczec voiced I am making trouble for myself. However I have a hunch ( eek ! ) that some goodies lay deep within this particular aspect of quantum mechanical arena.

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I don't know exactly what it is, but from what I can derive it sounds like how the poles of an electron or proton are oriented or the way its actually spinning. Someone else care to explain?

Edited by steevey

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I find this subject interesting , yet at the heart of quantum theory. If anyone has any eureka feelings about " spin" at the quantum level please could you share them !

!

Moderator Note

Hypothetical musings belong in speculations. The are not appropriate as answers to questions outside of speculations.

The OP's call for "eureka feelings" somewhat invites a broad spectrum of expressions. Perhaps it is the OP that belongs in speculations, along with any earnest responses that describe such "eureka feelings." If the OP wants non-speculative responses, I think they should ask more specific questions with more concrete parameters.

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The OP's call for "eureka feelings" somewhat invites a broad spectrum of expressions. Perhaps it is the OP that belongs in speculations, along with any earnest responses that describe such "eureka feelings." If the OP wants non-speculative responses, I think they should ask more specific questions with more concrete parameters.

It involves people's own opinions/"feelings", so no all the things in here can be considered scientific consensus.

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The OP's call for "eureka feelings" somewhat invites a broad spectrum of expressions. Perhaps it is the OP that belongs in speculations, along with any earnest responses that describe such "eureka feelings." If the OP wants non-speculative responses, I think they should ask more specific questions with more concrete parameters.

It involves people's own opinions/"feelings", so no all the things in here can be considered scientific consensus.

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It involves people's own opinions/"feelings", so no all the things in here can be considered scientific consensus.

The OP was asking for speculations in the way of phrasing the question, imo. They basically expressed a vague sense of strong potential in the concept of "spin" and asked for validation. Such a question lends itself to musings about how significant "spin" could be or become. How can someone, purely on the basis of established facts, confirm or deny such a "feeling" about the (potential) significance of "spin?" If they didn't want speculation, they should have stated their "feeling" and then asked for delineation of the extent to which existing knowledge supports or contradicts such a feeling. Then someone with expertise could have said either, "no, current discourse does not hold any potential for further relevance for "spin," or "yes, there is a lot of discursive attention paid to 'spin' and its spectrum of potential implications." Beyond that, all people can do is speculate, based on their own knowledge and thinking, about the potential future applications of the concept of "spin," right?

Edited by lemur

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I don't know exactly what it is, but from what I can derive it sounds like how the poles of an electron or proton are oriented or the way its actually spinning. Someone else care to explain?

You should not think of it as the electron spinning about some axis. Spin behaves like an "additional angular momentum", but it does not have such a simple interpretation.

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It's a vector so there is a direction, but it does not represent a physical spinning such as a top. But in a coordinate system, for a spin 1/2 particle you have "spin up" and "spin down" representing the projection of the angular momentum vector onto the axis.

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Is electron spin speed reduced in the magnetic field?

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Is electron spin speed reduced in the magnetic field?

Spin is an intrinsic part of a particle properties. It cannot be interpreted as a particle spinning or something similar. Fundamental particles have no internal structure, so it cannot be something to do with some substructure spinning around some axis. Therefore, I think your question is ill posed.

Spin can be viewed as part of a total angular momentum, and as swansont has said you can look at the components along an axis. For fermions, such as the electron this is always plus or minus 1/2 (in appropriate units).

For example, an electron "whizzing around" the hydrogen nucleus has orbital angular momentum. It also has an intrinsic angular momentum which we call spin. This spin is "built into" the electron and has nothing to do with its motion.

Edited by ajb

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Spin is an intrinsic part of a particle properties. It cannot be interpreted as a particle spinning or something similar. Fundamental particles have no internal structure, so it cannot be something to do with some substructure spinning around some axis. Therefore, I think your question is ill posed.

Spin can be viewed as part of a total angular momentum, and as swansont has said you can look at the components along an axis. For fermions, such as the electron this is always plus or minus 1/2 (in appropriate units).

For example, an electron "whizzing around" the hydrogen nucleus has orbital angular momentum. It also has an intrinsic angular momentum which we call spin. This spin is "built into" the electron and has nothing to do with its motion.

When iron bar is in the strong magnetic field, the iron become a magnetic bar. Does this case the magnetic field only change the electron turning direction?

And, is hydrogen 21CM line transition the same?

Edited by alpha2cen

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So, because of it's intrinsic spin an electron has a magnetic dipole moment. In most materials the spins are arranged in a random orientation and so no net magnetic field. In magnets the spins are aligned coursing an overall magnetic field.

It is the alignment of the spins that is important.

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Some famous maths person once said something to the effect that "if you cant jump off the back of a bus and go up to the first person you meet and explain in simple terms your theory, it is likely that it is not understood by yourself or others correctly " or something to that effect. Surely the fact that spin is so difficult to explain means , it is likely we don't yet fully and simply understand the subject of "Spin ". Is this not at the root of maths ( beautiful equations ) itself and symmetry.

I would still like a simple and beautiful explanation of " spin " as applied to quantum mechanics . If possible .

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Some famous maths person once said something to the effect that "if you cant jump off the back of a bus and go up to the first person you meet and explain in simple terms your theory, it is likely that it is not understood by yourself or others correctly " or something to that effect. Surely the fact that spin is so difficult to explain means , it is likely we don't yet fully and simply understand the subject of "Spin ". Is this not at the root of maths ( beautiful equations ) itself and symmetry.

I would still like a simple and beautiful explanation of " spin " as applied to quantum mechanics . If possible .

Spin is intrinsic angular momentum, but not physical spinning. That's pretty simple. The difficulty arises, I think, because people can't accept a model other than a physically spinning top, and that model fails when applied to QM spin; it's a problem of continuing to want a classical model or explanation for a nonclassical phenomenon.

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Spin is intrinsic angular momentum, but not physical spinning. That's pretty simple. The difficulty arises, I think, because people can't accept a model other than a physically spinning top, and that model fails when applied to QM spin; it's a problem of continuing to want a classical model or explanation for a nonclassical phenomenon.

I think the problem is more likely that a layman doesn't know what angular momentum is. Can you explain spin as if you were explaining it to a 12 year old? (I appreciate this might not be possible, just asking)

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I would still like a simple and beautiful explanation of " spin " as applied to quantum mechanics . If possible .

In the context of nonrelativistic quantum mechanics you won't find a beautiful explanation. You will need to consider the Poincare group to get at spin. I suggest you have a look at Ryder's Quantum Field Theory book, he devotes a few pages to this.

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I think the problem is more likely that a layman doesn't know what angular momentum is. Can you explain spin as if you were explaining it to a 12 year old? (I appreciate this might not be possible, just asking)

Classically it is related to the tendency for something that is spinning to keep spinning or rotating. To change it you must apply a torque. This is analogous to linear momentum being related to the tendency to move in a straight line at constant speed; to change it you must exert a force.

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In the Feynman video on the other thread, he mentions that iron atoms get magnetized because all the electrons are spinning together in the same direction. For some reason this led me to wonder if there was something special about iron. Then it occurred to me that iron is supposedly the cut off point for elements that can split and release rather than consume energy. Is it possible that these two attributes of iron are related? Should this question be posted as a new thread?

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Classically it is related to the tendency for something that is spinning to keep spinning or rotating. To change it you must apply a torque. This is analogous to linear momentum being related to the tendency to move in a straight line at constant speed; to change it you must exert a force.

you must know some clever 12 year olds... (jk).

OK, so if I understand correctly (unlikely), angular momentum in classical physics is rotational momentum, such that when you set some object spinning about its own axis, it tends to keep spinning unless some force acts upon it to slow it down or stop it.

In QM, it seems like spin is a measure of angular momentum of elementary particles but which can only hold specific values (quantised)? But elementary particles don't have an internal structure, so they don't actually rotate. Is that correct?

Can you point me in the right direction from here?

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