Spinors – Understanding Wikipedia’s Definition of a Spinor

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I originally asked this question on math SE but I'm asking it again here due to the lack of responses. I should note that I come from a mathematical background and not a physics one so I am not comfortable defining things in terms of how they transform and I have a hard time understanding spinors as introduced in many physics books. Here is the original post:

I am trying to understand spinors from a mathematical view. I've seen similar questions on this website but I'm still unclear on what they are exactly. On Wikipedia they state:

Although spinors can be defined purely as elements of a representation space of the spin group (or its Lie algebra of infinitesimal rotations), they are typically defined as elements of a vector space that carries a linear representation of the Clifford algebra.

What confuses me is say we are working on space time so that $\mathbb{R}^4$ is our vector space. Going by the above passage, a spinor would be an element of $\mathbb{R}^4$ that carries a representation of the spin group, let us denote this pair as $(x, \rho)$ where $\rho$ is the representation. However $x$ is itself also a vector since $x \in \mathbb{R}^4$. So why are vectors and spinors referred to as two different objects? In other words, how does the representation play any role in describing the spinor itself (which as I understand is simply a vector in the underlying vector space)?

My current guess is that spinors are always to be taken as a pair consisting of a vector and a representation, for example $(x, \rho)$ above. Thus, spinors are actually also vectors, but the difference is that when you talk about rotations (i.e. the action of O$(3)$) then spinors "rotate" differently than an ordinary vector in $\mathbb{R}^4$. However I am not sure if this is right since most textbooks do not describe anything along these lines. For example, no physics textbook describes a spinor as a pair $(x, \rho)$. Is this implicitly assumed?

In the example above involving $\mathbb{R}^4$, since $4 = 2n + 1 \implies n = 3/2$, this spinor is said to be spin 3/2. Thus using common physics terminology, a Dirac spinor is the unique 2-dimensional vector space (up to isomorphism) representing the action of $SO(3)$. Is this right?

Best Answer

The spin group has a multiple irreducible representations of dimension 4. Two of them are the left- and right-handed spin-3/2 representations. The other one is called the vector representation. "Ordinary" 4-vectors belong to (copies of) the latter representation, which is not considered one of the "spinor representations". Elements of these representations do all "rotate differently" from each other under elements of the spin group. In general, a representation of the spin group is labelled $(p,q)$ for $p,q$ nonnegative half-integers, with $n=p+q$ being the spin and the dimensionality being $(2p+1)(2q+1).$

In math, "vector" just means "element of a vector space". Spinors are by definition always "vectors" in the math sense. But in physics, "vector" usually means more than it does in math. Intuitively, a physical vector is "a magnitude with a direction" in space(time). An element of $\mathbb R^{100}$ is not going to be a physical vector. Even an element of $\mathbb R^4$ should only be considered a physical vector if you've fixed a coordinate system etc. to interpret it as a magnitude and a direction in spacetime. In fact, one structure you'd like to see on any space of physical vectors is a vector representation of the spin group on it, which tells you how to rotate the elements. (Again, the vector representation is the $\left(\frac12,\frac12\right)$ one, not the $\left(\frac32,0\right)$ or $\left(0,\frac32\right)$ spinor ones.) Hopefully this clarifies why in physics we distinguish "vectors" from "spinors".

Note that treating a spinor $v$ as a pair $(v,\rho)$ where $\rho$ is the representation is much like treating a (math) vector $v$ as a pair $(v, V)$ where $V$ is a vector space. It's somewhat wrong-headed. The word "vector" in math means "member of a vector space" and says nothing about what the object actually is. If you want to talk about vectors, you first fix your vector space and then you talk about its elements. The same is true for spinors. If you want to talk about spinors, you pick some spinor representation, perhaps postulated by some physical theory, and then you start talking about its elements. You don't consider the representation to be part of the spinor; it's the other way around.

The Dirac equation is a differential equation for a field $\psi:M^{3,1}\to V,$ where $V$ is the space of "Dirac spinors". Specifically, we write $V=\left(\frac12,0\right)\oplus\left(0,\frac12\right)$ to denote that $V$ is a four-dimensional space carrying a reducible representation of the spin group, where two of the dimensions transform under the (irreducible) $\left(\frac12,0\right)$ representation (left-handed spin-1/2) and the other two independently transform under the $\left(0,\frac12\right)$ irrep (right-handed spin-1/2), such that $V$ is a direct sum of two smaller spaces with the specified representations.

  • Note that $V$ carries yet another 4-dimensional representation of the spin group that is distinct from the 4-vector representation. Even though the Dirac spinor has four components, you can't apply a Lorentz transform with the usual $\mathbf\Lambda$ matrices we use for 4-vectors, but must come up with a new ones. The Dirac spinor rotates differently to a 4-vector.
  • Note that there is no such thing as the unique 2-dimensional representation of the spin group. There are two. The members of these 2-D representations are called Weyl spinors.