Equivalent representations for Dirac algebra

In summary, the author searched for a matrix that represents the change of basis from the Cartesian coordinate system to the Weyl-Dirac representation, but didn't find an explanation or derivation for it. He found the matrix in an appendix of a book, tested for its existence, and solved for its eigenvalues. He then recommended waiting until after substitution of the definition of ##D_\mu## before trying to find the matrix.
  • #1
Wledig
69
1
Homework Statement
Consider the following representations that satisfy the Dirac algebra:

$$ \gamma^0 =
\begin{pmatrix}
1 & 0 \\
0 & -1
\end{pmatrix}
$$

$$
\gamma^i=
\begin{pmatrix}
0 & \sigma^i \\
-\sigma^i & 0
\end{pmatrix}
$$

and

$$ \gamma^0 =
\begin{pmatrix}
0 & 1 \\
1 & 0
\end{pmatrix}
$$


$$ \gamma^i=
\begin{pmatrix}
0 & \sigma^i \\
-\sigma^i & 0
\end{pmatrix}
$$

Show that they are equivalent, that is write a 4x4 unitary matrix U such that:
$$\gamma^{\mu}_B = U\gamma^{\mu}_A U^\dagger$$
Relevant Equations
Dirac algebra: ##\{ \gamma^\mu , \gamma^\nu \} = 2\eta^{\mu \nu}##
Where ##\eta^{\mu \nu} ## is the metric tensor from special relativity.
One thing I was thinking about doing was to consider these representations as a basis for the gamma matrices vector space, then try to determine what the change of basis from one to the other would be. However I'm unsure if it's correct to treat the representations as a basis, or whether this is the right approach at all.
 
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  • #2
Have you looked up the Weyl- and Dirac representation, e.g. on Wikipedia?
(Hint: check the other languages)
 
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  • #3
Alright, so after searching a bit I managed to find U in an appendix in the book by Itzykson:
$$ U = \dfrac{1}{\sqrt{2}}(1+\gamma_5\gamma_0) = \dfrac{1}{\sqrt{2}}
\begin{pmatrix}
1 & -1 \\
1 & 1
\end{pmatrix}
$$
I've tested for ##\gamma^0##, so I'm convinced it works, but I still don't know how to reach this matrix. Like I've said it was found in an appendix, without much explanation to go along with it.
 
  • #4
Forget it, I figured it out. Just needed to find the eigenvalues of the Weyl representation then normalize the eigenvector matrix to make it unitary, it's simpler than I thought.
 
  • #5
Wledig said:
Alright, so after searching a bit I managed to find U in an appendix in the book by Itzykson:
$$ U = \dfrac{1}{\sqrt{2}}(1+\gamma_5\gamma_0) = \dfrac{1}{\sqrt{2}}
\begin{pmatrix}
1 & -1 \\
1 & 1
\end{pmatrix}
$$
I've tested for ##\gamma^0##, so I'm convinced it works, but I still don't know how to reach this matrix. Like I've said it was found in an appendix, without much explanation to go along with it.
It's hard to tell how to find if you've seen the answer, which I did when I looked up the definitions. The spatial ##\gamma^i## should remain unchanged, so it's probably an idea to focus on ##\gamma^0##.

The hard way to find ##U## is probably to solve the equations ##U(\{\,\gamma^i,\gamma^j\,\})=\{\,U(\gamma^i),U(\gamma^j)\,\}##.
 
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  • #7
I'm afraid I have no idea. I am no physicist and don't know what the other variables are or how they multiply. But you didn't use the given hint. I would wait as long as possible, before I'd substituted the definition of ##D_\mu##.
 
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  • #8
That's alright, thanks for the tip.
 

FAQ: Equivalent representations for Dirac algebra

1. What is the Dirac algebra?

The Dirac algebra is a set of mathematical rules and properties that describe the behavior of quantum particles, specifically fermions. It was developed by physicist Paul Dirac in the 1920s and is a fundamental part of quantum mechanics.

2. What are equivalent representations for Dirac algebra?

Equivalent representations for Dirac algebra refer to different ways of mathematically expressing the same underlying principles and properties of the Dirac algebra. These representations can vary in notation and form, but ultimately represent the same physical concepts.

3. Why are equivalent representations important?

Equivalent representations are important because they allow us to describe and understand the same physical phenomena in different ways. This can provide new insights and perspectives, as well as make complex calculations more manageable.

4. How are equivalent representations for Dirac algebra derived?

Equivalent representations for Dirac algebra are derived through mathematical transformations and manipulations of the original equations and properties. These transformations must preserve the fundamental principles and rules of the Dirac algebra.

5. What are some examples of equivalent representations for Dirac algebra?

Some examples of equivalent representations for Dirac algebra include the bra-ket notation used in quantum mechanics, the matrix representation used in linear algebra, and the spinor notation used in particle physics. These all describe the same underlying principles of the Dirac algebra but in different mathematical forms.

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