# What does it mean that two particles have opposite spins?

if spin is a term used to describe a particles angular momentum, then how can something have an "opposite" angular momentum than another. spin has me confused honestly. i barely know what it is or how its measured or why it isnt just called angular momentum

Staff Emeritus
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Spin has direction - it can be clockwise or counterclockwise, for example. Does that clear things up?

dx
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if spin is a term used to describe a particles angular momentum, then how can something have an "opposite" angular momentum than another.

In quantum mechanics, angular momentum is quantized. A spin half particle can have only two possible values for it's spin; they are opposite to each other. Opposite here just means negative.

spin has me confused honestly. i barely know what it is or how its measured or why it isnt just called angular momentum

Spin can be measured using a Stern-Gerlach apparatus. Look it up on wikipedia.

Matterwave
Gold Member
Spin has direction - it can be clockwise or counterclockwise, for example. Does that clear things up?

That statement needs to be quantified since point particles (as far as we know) shouldn't be able to "rotate" about an axis. (so, clockwise, and counterclockwise may not be the best modifiers) Indeed, if you take and upper bound of the radius of an electron and use L=Iw=hbar/2 on it, and find the "velocity" of a point on the equator of the electron, you get a value that is over 10 times the speed of light.

nicksauce
Homework Helper
That statement needs to be quantified since point particles (as far as we know) shouldn't be able to "rotate" about an axis. (so, clockwise, and counterclockwise may not be the best modifiers) Indeed, if you take and upper bound of the radius of an electron and use L=Iw=hbar/2 on it, and find the "velocity" of a point on the equator of the electron, you get a value that is over 10 times the speed of light.

There's an angular momentum vector associated with spin (hence a direction), but that doesn't mean it's associated with the rotation of the point particle.

Matterwave
Gold Member
Yes, that's true, but my point was that "clockwise" and "counterclockwise" implies a rotation about an axis. Which demands further comments.

DrChinese
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Yes, that's true, but my point was that "clockwise" and "counterclockwise" implies a rotation about an axis. Which demands further comments.

I wouldn't take that analogy any further. That is a backwards step because it implies particles are spinning balls, which they are not. Electron spin - for example - is always one direction or the other in 3 dimensions. However, the axes can be set arbitrarily and the value is always quantized as +1/2 or -1/2. As you mention, electrons are point particles. It doesn't make sense to say they spin faster at the equator or similar.

If you prefer, you can think of spin as being a mathematical device. As far as I know, the clockwise and counterclockwise designations are arbitrary as well and don't really represent any kind of "corkscrew"-like physical structure.

Matterwave
Gold Member
That's exactly my point. :)

DrChinese
Gold Member
so now if theyre not rotating about an axis then how can something have opposite directions of spin. ughh what is it meant by "direction". is it the "direction" of spin which determines which way the particle curves in the magnetic field? even though it has nothing to do with rotation? if thats the case then "direction" doesnt even mean direction. it just means that "something" is different about electron A that makes it take one path while electron b takes the other... right?

Yes, one has a spin value of +1/2 and the other is -1/2. It is confusing, because it is so tempting to think of a spinning golf ball or similar (you can probably guess what major tournament I was watching yesterday).

Keep in mind that positively and negatively charged particles also exhibit movement (attraction) and you don't need a picture of a spinning ball to picture that. Instead, you picture a magnet. Now, really, what does a magnet explain in this case? (Answer: nothing, it is circular logic as magnets are created by a special arrangement of their atoms.) The point is: with all particle phenomena, you CANNOT let the picture dominate your understanding. The picture is just a shorthand, nothing more, and even that only as long as it doesn't hold you back.

Yes, one has a spin value of +1/2 and the other is -1/2. It is confusing, because it is so tempting to think of a spinning golf ball or similar (you can probably guess what major tournament I was watching yesterday).

Keep in mind that positively and negatively charged particles also exhibit movement (attraction) and you don't need a picture of a spinning ball to picture that. Instead, you picture a magnet. Now, really, what does a magnet explain in this case? (Answer: nothing, it is circular logic as magnets are created by a special arrangement of their atoms.) The point is: with all particle phenomena, you CANNOT let the picture dominate your understanding. The picture is just a shorthand, nothing more, and even that only as long as it doesn't hold you back.
are there any other ways to determine spin or is it just the stern-gerlach experiment which tells you that? thingd are starting to make sense