# B Can you change a quantum particle's spin?

1. Nov 27, 2016

### cj20x2

I've been doing some research on this but i've been seeing conflicting answers. 1 group of answers says an electron's spin can be changed using a magnetic field. Is the same thing true for quarks?(like the up down quarks in protons and neutrons?). Another set of answers are saying it's impossible to change a particles spin.

2. Nov 27, 2016

### Staff: Mentor

Where? Please give some specific references.

From where?

From where?

3. Nov 27, 2016

### Staff: Mentor

I have a guess as to what may be causing your confusion, and I suspect PeterDonis does also. However, if we tell you what we think is the problem, and we are wrong, we will probably confuse you even more than you probably are now. If you tell us exactly what the sources of your confusion are, it would help us a lot and probably prevent more confusion.

4. Nov 27, 2016

### hilbert2

If you have an electron that initially has spin +1/2 in the z direction and then you put it in a magnetic field that is directed parallel to the x or y axis, the expectation value of the spin's z component starts oscillating between values +1/2 and -1/2 with a sine/cosine like time dependence. The time development of spin depends on how the spin operator appears in the hamiltonian in the system (which depends on the strength and direction of the magnetic field). I'm not sure about the spin of quarks/baryons, as that's not something I've particularly studied very much.

5. Nov 29, 2016

### Khashishi

Baryons are similar. That's the basis of nuclear magnetic resonance.

6. Nov 30, 2016

### cj20x2

I think what was confusing me is what hilbert2 posted, where the spin of an electron can change from 1/2 to -1/2, I was wondering if the spin of other elementary particles (like up/ down quarks) could be changed in a similar way.

7. Nov 30, 2016

### Staff: Mentor

More precisely, the direction of an electron's spin can change, so that the probability of measuring +1/2 or -1/2 when we measure its spin about a given axis changes. For a spin-1/2 particle (where here "1/2" refers to the magnitude of the spin only) like an electron, its spin state can be thought of as an arrow pointing in a particular direction; the probability of measuring +1/2 when we measure the electron's spin about a particular axis is then $\cos \theta$, where $\theta$ is the angle between the measurement axis and the direction the electron's spin state arrow points. (The probability of measuring -1/2 is then 1 minus the probability of measuring +1/2, since they are the only two possibilities.) Various processes, like passing through a magnetic field, can cause the arrow to rotate, changing the spin state.

This simple intuitive model, unfortunately, only applies to spin-1/2 particles, like electrons, neutrinos, and quarks. (See below for more on quarks.) For spin-1 particles, like photons and the other "force carrier" particles, it's more complicated, but the general rule that various processes can change the spin state, while still keeping the magnitude of the spin (spin-1/2, spin-1, etc.) the same, still applies.

According to our current theory (the Standard Model of particle physics), yes, it can. However, we can't directly observe quarks, so we can't run the same sorts of experiments that we run on electrons to show the spin changing. We can only infer that the same quantum rules that apply to electron spins apply to quark spins, based on the fact that our theory which is built on that assumption makes correct predictions.

8. Dec 1, 2016

### odietrich

The direction can change, but it's one of the basic (and not really intuitive) properties of elementary particles that the magnitude of the spin cannot change. So, if we use for a moment the (at least inaccurately simplified, and physicists would generally use the word "wrong") model of a tiny spinning sphere for an elementary particle (like an electron), this means that the magnitude of its angular velocity is always the same: all electrons spin with exactly the same "spinning speed". It is not possible to increase or decrease this spinning speed (as it would be for a macroscopic object by applying a torque). In contrast, the axis of rotation (or, correspondingly, the direction of the spin) can be changed as explained above.

To be even more picturesque: Try imagining a bunch of oranges hovering in midair in front of you. All oranges spin (rotate) with exactly the same angular speed (say 1 revolution/second) around their own axis. You can arbitrarily and individually change the orientation of the axis of rotation of each orange, but the rotational speed stays always at exactly 1 revolution/second and there is no way to decelerate or accelerate it. This gives you a first (and very rough) approximation to the behavior of spin of elementary particles.

So, summarizing, the magnitude of the angular momentum of an elementary particle is fixed and is therefore (like its mass) called an intrinsic property of each (kind of) elementary particle.

(This is, by the way, somewhat different for composite particles such as the proton (with quark content uud) with spin 1/2; there is another baryon with the same quark content (uud) and spin 3/2 - namely the Delta resonance $\Delta^+$ - which could be interpreted cum grano salis as a kind of proton with increased spin.)

Last edited: Dec 1, 2016