# D'Alembertian in Sakurai's Advanced QM book

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## Main Question or Discussion Point

In Sakurai's book "Advanced QM", he writes the Dirac equation (equation 3.31 to be exact) as: $$\left(\gamma _\mu \ \dfrac{\partial}{\partial\ x_\mu} + \frac{m\ c}{\hbar}\right) \ \psi= 0$$ which we can write as $$\left(\gamma _\mu \ \partial ^\mu \ + \frac{m\ c}{\hbar}\right) \ \psi= 0$$

Next, we go to the section 3-3, and subsection "Free particles at rest".

The author multiples the Dirac equation from the left by ##\gamma _\nu \ \partial ^\nu##. We get the form $$\left[ \partial ^\nu \ \partial ^\mu \ \gamma_\nu \ \gamma_\mu \ - \left(\frac{m\ c}{\hbar}\right)^2 \ \right]\psi = 0$$ If we interchange the indices ##\mu## and ##\nu##, and add to the above equation, we get $$\left[ \partial ^\nu \ \partial ^\mu\ \left( \gamma_\nu \ \gamma_\mu + \gamma_\mu \ \gamma_\nu\right) \ - 2 \ \left(\frac{m\ c}{\hbar}\right)^2 \ \right]\psi = 0$$ which reduces to $$\Box \ \psi - \left(\frac{m\ c}{\hbar}\right)^2 \ \psi = 0$$ using the anticommutation relations of the ##\gamma## - matrices.

Till now, I have just quoted from the book (except changing the ##\partial / \partial x_\mu## to ##\partial ^\mu##).

I knew the D'Alembertian operator to be ##\partial ^\mu \ \partial _\mu##, but Sakurai seems to be suggesting that it is ##\partial ^\mu \ \partial ^\nu## instead. The two can be related: $$\partial _\mu \ \partial ^\mu \ = \ g_{\mu \nu} \ \partial ^\nu \ \partial ^\mu\ ,$$ but that's not what the author has written. What am I missing?

Here is a picture of the page:

atyy

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One of the properties of ##\gamma## matrices is $$\left\{\gamma^\mu,\gamma^\nu\right\}=2g^{\mu\nu}$$

vanhees71, atyy and Wrichik Basu
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One of the properties of ##\gamma## matrices is $$\left\{\gamma^\mu,\gamma^\nu\right\}=2g^{\mu\nu}$$
Thank you, that perfectly solves the problem.

I don't know why Sakurai is reluctant to use the metric tensor. He just writes ##\{ \gamma_\mu , \ \gamma_\nu \} = 2 \delta _{\mu \nu}##. In fact, in chapter 1, he says,
Note that we make no distinction between a covariant and a contravariant vector, nor do we define the metric tensor. These complications are absolutely unnecessary in the special theory of relativity. (It is regrettable that many textbook writers do not emphasize this elementary point.)

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Well, this is possible by defining 4-vectors as ##x^\mu=(ct,ix,iy,iz)##. Then $$x^\mu x_\mu=\delta_{\mu\nu}x^{\mu}x^{\nu}=c^2t^2-x^2-y^2-z^2$$

atyy
Gold Member
Well, this is possible by defining 4-vectors as ##x^\mu=(ct,ix,iy,iz)##. Then $$x^\mu x_\mu=\delta_{\mu\nu}x^{\mu}x^{\nu}=c^2t^2-x^2-y^2-z^2$$
OK, but that's not what most other books use. At least I have learnt till date ##x^\mu=(ct,x,y,z)##. I am really getting confused with the notation in this book.

OK, but that's not what most other books use. At least I have learnt till date ##x^\mu=(ct,x,y,z)##. I am really getting confused with the notation in this book.
Yes, I think nowadays using ##(t,x,y,z)## with ##c=1## is the most common way to proceed, so maybe you want to change your book... Sakurai is from 1967.

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so maybe you want to change your book... Sakurai is from 1967.
Sakurai is quite a recommended book, that's why I started reading it. But I'll not continue with this book any longer.

Well, I've never read Sakurai's book, but it's true it has a lot of popularity. You need to think if it's worth for you. Maybe you can start with something else and then return to Sakurai when you think you'll be able to be comfortable with the notation.

Wrichik Basu
George Jones
Staff Emeritus
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Sakurai is quite a recommended book, that's why I started reading it. But I'll not continue with this book any longer.
Sakurai has written (at least) two different books, "Advanced Quantum Mechanics" and "Modern Quantum Mechanics". I have seen both books recommended, but I have seen "Modern Quantum Mechanics" recommended much more often than "Advanced Quantum Mechanics".

"Modern Quantum Mechanics" is probably the text used most often in North America for graduate quantum mechanics courses.

Wrichik Basu
atyy
OK, but that's not what most other books use. At least I have learnt till date ##x^\mu=(ct,x,y,z)##. I am really getting confused with the notation in this book.
It is old notation for special relativity. It doesn't carry over (as far as I know) into general relativity. You can see an example of the change of notation from SR to GR also in 't Hooft's notes http://www.staff.science.uu.nl/~hooft101/lectures/genrel_2013.pdf.

Wrichik Basu
vanhees71