Composing a few transformations

  • Thread starter etotheipi
  • Start date
  • #1
Homework Statement:
See below
Relevant Equations:
N/A
I messed up somewhere, but don't know why! We consider this sequence of infinitesimal transformations,$$U = e^{i\varepsilon K_{\mu}}e^{i\varepsilon K_{\nu}}e^{-i\varepsilon K_{\mu}}e^{-i\varepsilon K_{\nu}}$$with ##K_{\mu}## and ##K_{\nu}## being two generators. I said, this simplifies to$$\begin{align*}

e^{i\varepsilon K_{\mu}}e^{i\varepsilon K_{\nu}}e^{-i\varepsilon K_{\mu}}e^{-i\varepsilon K_{\nu}} &= (I+i\varepsilon K_{\mu})(I+i\varepsilon K_{\nu})(I-i\varepsilon K_{\mu})(I-i\varepsilon K_{\nu}) + \mathcal{O}(\varepsilon^5) \\

&= \left[I + i\varepsilon(K_{\mu} + K_{\nu}) - \varepsilon^2 K_{\mu} K_{\nu} \right]\left[I - i\varepsilon(K_{\mu} + K_{\nu}) - \varepsilon^2 K_{\mu} K_{\nu} \right] + \mathcal{O}(\varepsilon^5) \\

&= I+ \varepsilon^2 (K_{\mu}^2 + K_{\mu} K_{\nu} + K_{\nu} K_{\mu} + K_{\nu}^2) -2 \varepsilon^2 K_{\mu} K_{\nu} + \mathcal{O}(\varepsilon^3) \\
&= I + \varepsilon^2 (K_{\nu} K_{\mu} - K_{\mu} K_{\nu}) + \varepsilon^2(K_{\mu}^2 + K_{\nu}^2) + \mathcal{O}(\varepsilon^3)

\end{align*}$$but the textbook only quotes$$U = I + \varepsilon^2 (K_{\nu} K_{\mu} - K_{\mu} K_{\nu}) + \mathcal{O}(\varepsilon^3)$$I wondered why I ended up with an extra term? Thank you.
 
Last edited by a moderator:

Answers and Replies

  • #2
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
2022 Award
23,786
15,400
If you have an answer to second order ##\epsilon^2## then you ought to have quadratic terms in your original expansion.
 
  • #3
If you have an answer to second order ##\epsilon^2## then you ought to have quadratic terms in your original expansion.

Ohhh, you're right! Guess I'll get started on expanding all that out, then...
 

Suggested for: Composing a few transformations

  • Last Post
Replies
24
Views
608
Replies
35
Views
1K
Replies
30
Views
924
Replies
2
Views
363
  • Last Post
Replies
7
Views
312
Replies
7
Views
394
Replies
1
Views
706
Replies
1
Views
394
  • Last Post
Replies
11
Views
505
  • Last Post
2
Replies
35
Views
785
Top