What happens to spin as a particle increases in velocity?

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Discussion Overview

The discussion revolves around the behavior of an electron's spin as it approaches relativistic speeds, particularly from the perspective of a laboratory observer. Participants explore the implications of relativistic effects on spin and magnetic moment, addressing both classical and quantum interpretations.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants question the validity of treating electron spin as a classical spin, suggesting that such an approach leads to nonsensical conclusions.
  • Others assert that the spin of a particle is invariant and does not change with velocity, emphasizing that spin is a quantized unit of angular momentum without a classical analogue.
  • One participant proposes that while the magnitude of spin remains invariant, the direction of the spin vector may align with the direction of motion as velocity increases.
  • Another viewpoint suggests that from a laboratory frame, an electron's spin may appear to "slow down" due to relativistic effects, depending on the observer's perspective.
  • Concerns are raised about the implications of time dilation on the perception of the electron's spin and magnetic moment at high velocities.
  • A hypothetical scenario is presented where classical spinning objects are compared to electrons, suggesting that changes in spinning rates could provide insights into electron behavior at relativistic speeds.

Areas of Agreement / Disagreement

Participants express differing views on the nature of electron spin and its behavior at relativistic speeds. There is no consensus on whether spin changes or remains invariant, and the discussion includes multiple competing interpretations.

Contextual Notes

Participants reference concepts such as Lorentz transformations and the distinction between spin and orbital angular momentum, indicating a reliance on advanced theoretical frameworks. The discussion also highlights the complexity of relating classical and quantum perspectives.

Who May Find This Useful

This discussion may be of interest to those studying quantum mechanics, relativity, or the foundational concepts of particle physics, particularly in understanding the nuances of spin and its relativistic implications.

idea2000
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Hi,

If we think of an electron's spin as just being a classical kind of a spin, what happens as it speeds up close to the speed of light relative to a laboratory frame of reference? Does the lab frame see the spin of the particle "slow down"?
 
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There's no reasonable answer to your question, because the electron's spin is not a classical spin - it's sort of like asking "If 2+2=5, then what happens when..."

However, you may be asking a different question that does have a reasonable answer: "If a very small (but not so small that quantum mechanics dominates) spinning ball is accelerated to close to the speed of light, what happens to the spin as observed in the lab frame?". If that's what you're asking, you may get a better answer in the relativity forum.
 
The spin of a particle is invariant, it does not change. As Nugatory said, spin is not a rotation about an axis. It is a quantized unit of angular momentum, that is given by [itex]S = \frac {n} {2}.[/itex] So, allowed values for spin are [itex]\frac {1} {2}[/itex], [itex]1[/itex], [itex]\frac {3} {2}[/itex], [itex]2[/itex], and so on. There is no classical analogue to spin - there is a form of angular momentum in quantum mechanics, known as orbital angular momentum, that is essentially the same concept as classical angular momentum. But, spin does not have any apparent classical meaning since particles such as electrons do not have internal structures.
 
Spin does change and it remains static at the same time just like all the other things, gravity, time, mass etc... its all about perspective.

If you could somehow observe the particle spin from Earth as it travels close to the speed of light around our solar system it will barely be spinning, it will slow down.

If you travel with the particle near the speed of light and observe you will notice no difference.
 
what happens to electron spin and magnetic moment at high velocity?

Hi,

I read that an electron's spin and magnetic moment are invariant. What does this mean exactly from the perspective of a laboratory observer? Would the laboratory frame see the magnetic moment of an electron moving at near the speed of light as being completely unchanged? And, would the effects of time dilation make the electron seem as if it were not spinning at all? Thanks in advance for any help anyone can provide...
 


idea2000 said:
Hi,

I read that an electron's spin and magnetic moment are invariant. What does this mean exactly from the perspective of a laboratory observer? Would the laboratory frame see the magnetic moment of an electron moving at near the speed of light as being completely unchanged? And, would the effects of time dilation make the electron seem as if it were not spinning at all? Thanks in advance for any help anyone can provide...

The magnitude of a particle's spin is an invariant, but the direction is not. So the lab observer would continue to measure spin 1/2 for an electron, but as the velocity increases, he'd find that the spin vector starts to become aligned with the direction of motion. To really understand this, you need to know something about Lorentz transformations and how spinors transform under them http://en.wikipedia.org/wiki/Spin_(physics)#Spin_and_Lorentz_transformations
 
idea2000 said:
Hi,

If we think of an electron's spin as just being a classical kind of a spin, what happens as it speeds up close to the speed of light relative to a laboratory frame of reference? Does the lab frame see the spin of the particle "slow down"?


If we have small classical objects spinning at different rates, and we shoot one electron into each of the objects, then the objects experience various changes of spinning rates.

Some of the objects experience no change of spinning rate at all. We could perhaps say that these objects have the same spinning rate as an electron.

Now if we accelerate all of this experimental setup to great speed, we know that the spinning classical objects will spin more slowly, while we also know that the result of this experiment does not change. This suggest that electrons also spin more slowly.
 

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