# Quantum entanglement. -- Spin up, spin down, isn't it the same?

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Surely the direction of spin of any type of sphere depends on the orientation of the observer. Please clarify, how does that work for entangled quantum particles?
If I were viewing the Earth from high above the North pole, I would notice it spinning in an anti clockwise direction BUT when viewed from the South pole it would be spinning in a clockwise direction. If I were high above the equator oriented in a "North up" position I would observe the globe spinning from "left" to "right" but if I were oriented in a "South-up" ("upside down") position, the Earth would be spinning from right to left. My point is that the direction of spin of a sperically shaped obect depends solely on the direction of view and orientation of the observer.

My question is thus when I am observing a quantum particle does the same principle (as above) still apply and, either way, how the heck does that work exactly?

Please explain as if for a 10 year old.
:D

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You pick an axis on which you want to observe the spin. Then "up" means parallel to that axis and "down" means anti-parallel to that axis. If you rotate your axis 180 deg then what was "up" is now "down".

Many thanks for your reply Dale. Am I then correct in assuming that spin must always be observed along the axis of spin and then depending on whether you are observing the spin from either the "North" or "South" pole, the spin will be either the same or opposite to the spin of the entangled particle? Furthermore, it is thus strictly incorrect to say that two entangled particles will always have similar or opposite spins because it solely depends on which pole you are observing?

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Furthermore, it is thus strictly incorrect to say that two entangled particles will always have similar or opposite spins because it solely depends on which pole you are observing?

For measurements on pairs of entangled particles: the direction you call "North" is arbitrary. For such North: if you measure both particles at that North, they will be exactly correlated (or anti-correlated) as you would expect.

You mentioned "the direction of spin of a spherically shaped object" in the OP. Quantum particles should not be considered in the manner you describe for many reasons: primarily because they don't behave like that. Quantum particles have a property called "spin" but that does not successfully map to the properties of a spinning ball, for example. An electron has spin in the X axis and Y axis (where X is arbitrary and Y is 90 degrees offset from X). But they are NOT completely independent (when observed) as would be true of a spinning ball. Specifically, they do not "commute" as would be the case with a classical object. Further, a measurement of the spin of either the X or Y axis (or any axis for that matter) always has an observed value of +1/2 or -1/2 - and no other value. A spinning ball can have virtually any spin value.

For measurements on pairs of entangled particles: the direction you call "North" is arbitrary. For such North: if you measure both particles at that North, they will be exactly correlated (or anti-correlated) as you would expect.

You mentioned "the direction of spin of a spherically shaped object" in the OP. Quantum particles should not be considered in the manner you describe for many reasons: primarily because they don't behave like that. Quantum particles have a property called "spin" but that does not successfully map to the properties of a spinning ball, for example. An electron has spin in the X axis and Y axis (where X is arbitrary and Y is 90 degrees offset from X). But they are NOT completely independent (when observed) as would be true of a spinning ball. Specifically, they do not "commute" as would be the case with a classical object. Further, a measurement of the spin of either the X or Y axis (or any axis for that matter) always has an observed value of +1/2 or -1/2 - and no other value. A spinning ball can have virtually any spin value.
Thank you for your reply DrChinese. Yes, you are absolutely right, "North" for both particles are completely arbitrary and my understanding is that their spin would be perfectly correlated or anti-correlated depending on the viewing position and that essentially answers my original question, I think.

I also understand that quantum particles should be considered differently except I don't fully understand "how" they should be considered. I suppose I have a lot of trouble with this "concept of quantum spin but not spin as you know it", are they spinning or aren't they? I have read the literature but, from the little that I understand, none of it has helped me gain an intuitive understanding of how it actually works.

I guess I still have a lot to learn. :)

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I also understand that quantum particles should be considered differently except I don't fully understand "how" they should be considered. I suppose I have a lot of trouble with this "concept of quantum spin but not spin as you know it", are they spinning or aren't they? I have read the literature but, from the little that I understand, none of it has helped me gain an intuitive understanding of how it actually works.

Some of the terms to describe quantum properties should not be taken too literally, and spin is one of those. Ditto with the term particle. These are primarily used to allow us to converse. And you are correct that our understanding of quantum behavior is not "intuitive" at many levels. We like to say instead that if you follow the mathematical representations, you don't run into problems - but that's not completely true either. Otherwise we wouldn't have the so-called "interpretations" of QM - which are basically attempts to provide a more intuitive explanation of the quantum world.

In QCD (the dynamics of quarks and gluons that make up protons & neutrons): there are colors used to describe some properties, as well as things such as "charm", "strange", "up" and "down". These are just names, and bear no particular connections to the words themselves. So you can see that there is a need for a word, but the word itself is basically a placeholder for a deeper quantum concept.

PeroK
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Many thanks for your reply Dale. Am I then correct in assuming that spin must always be observed along the axis of spin and then depending on whether you are observing the spin from either the "North" or "South" pole, the spin will be either the same or opposite to the spin of the entangled particle? Furthermore, it is thus strictly incorrect to say that two entangled particles will always have similar or opposite spins because it solely depends on which pole you are observing?
The experiment that is often taken as a measurement of the spin of a particle is the Stern-Gerlach experiment, where silver atoms passing through a magnetic field are deflected in one of two directions, corresponding to spin-up and spin-down of the outermost electron along the direction of the magnetic field. So, there are definitely two experimental values for spin about an axis, not just one.

The measurement outcomes you call up and down are to some extent arbitrary, although there is a relationship with the magnetic field that corresponds to some extent with the relationship of a classical, charged spinning object with the same magnetic field. Hence quantum "spin".

One significant difference is that the gyromagnetic ratio of the electron is approximately twice that of a classically spinning object of the same mass and charge. This and the theory of QM states and measurement outcomes is why it's not particularly helpful to think of spin too literally.

Like many people, you have jumped into QM by looking at entangled pairs of particles. That means that you are missing the fundamentals of quantum spin that has nothing to do with entanglement.

To make any sense of QM you have to learn about the nature of quantum states. And, in fact, the quantum spin states are in many ways the best and simplest place to start.

Dale and DrChinese
Some of the terms to describe quantum properties should not be taken too literally, and spin is one of those. Ditto with the term particle. These are primarily used to allow us to converse. And you are correct that our understanding of quantum behavior is not "intuitive" at many levels. We like to say instead that if you follow the mathematical representations, you don't run into problems - but that's not completely true either. Otherwise we wouldn't have the so-called "interpretations" of QM - which are basically attempts to provide a more intuitive explanation of the quantum world.

In QCD (the dynamics of quarks and gluons that make up protons & neutrons): there are colors used to describe some properties, as well as things such as "charm", "strange", "up" and "down". These are just names, and bear no particular connections to the words themselves. So you can see that there is a need for a word, but the word itself is basically a placeholder for a deeper quantum concept.
Does that mean we should not take the angular momentum of fundamental particles literally either? Yes, I believe that even Richard Feynman followed the maths and proclaimed that no one really understood quantum mechanics.

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Does that mean we should not take the angular momentum of fundamental particles literally either? Yes, I believe that even Richard Feynman followed the maths and proclaimed that no one really understood quantum mechanics.
One of the most stunning parts of physics is the theory of QM angular momentum. Angular momentum at the quantum level obeys quite abstract mathematical rules.

I'd say the QM building blocks of angular momentum have fundamental properties that are not evident from macroscopic systems. A reasonable analogy, perhaps, is that what your computer is doing at the fundamental hardware level is very different from what those basic interactions produce in terms of the user interface and data processing.

I always thought Feynman was being a little provocative with that statement. What is perhaps true is that we cannot understand QM in the same way that we could if it looked like a microscopic version of the world we are used to. I'd say that many physicists have a deep understanding of QM and, in particular, how it shapes the atomic and subatomic phenomena. That said, there is probably yet a lot more to discover.

One of the most stunning parts of physics is the theory of QM angular momentum. Angular momentum at the quantum level obeys quite abstract mathematical rules.

I'd say the QM building blocks of angular momentum have fundamental properties that are not evident from macroscopic systems. A reasonable analogy, perhaps, is that what your computer is doing at the fundamental hardware level is very different from what those basic interactions produce in terms of the user interface and data processing.

I always thought Feynman was being a little provocative with that statement. What is perhaps true is that we cannot understand QM in the same way that we could if it looked like a microscopic version of the world we are used to. I'd say that many physicists have a deep understanding of QM and, in particular, how it shapes the atomic and subatomic phenomena. That said, there is probably yet a lot more to discover.
Thanks PeroK, I still have difficulty understanding the mathematics of angular velocity at the classical level so still much to learn. Thanks for your help. :)

PeroK
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Thanks PeroK, I still have difficulty understanding the mathematics of angular velocity at the classical level so still much to learn. Thanks for your help. :)

Yes, but your essential point is; are correlated or not, i.e. spin are contrary to each other. i.e. +1/2 or -1/2.