Commuting the Lorentz transformation with derivative

In the process of proving that the d'Alembert operator

View attachment 31306

is invariant under Lorentz transformations, it was required to commute two terms in the following expression for the transformed operator, which was obtained by switching the index on the first del to the contravariant position, applying the Lorentz transformation to each del, then switching the index on the second del to the covariant position:

View attachment 31307

Now, so long as the last lambda and the second last del terms can commute, then all the Lorentz and metric terms collapse into the Kronecker delta, and the proof is complete (after switching the index positions to obtain the original form). But what justifies the ability to commute them, i.e. why is the following true (or is it at all?):

View attachment 31308

If the Lorentz transformation is just a boost in some arbitrary direction, then I see why it holds - the components of lambda then do not depend explicitly on the time or space coordinates, thus the derivative can commute with that term. However if the transformation involves a rotation, the components of lambda will depend explicitly on the coordinates, so it seems that commutativity would fail.

What am I missing?

EDIT:
It seems these attachments failed to upload...here are the intended equations, in order:
http://www.use.com/ccd724f4259a55704e5f?tc=no#photo=1
 
Last edited:

haushofer

Science Advisor
Insights Author
2,227
562
I don't understand your argument why a rotation from SO(3) would depend on the coordinates, but a boost from SO(3,1) not. You should pay attention to that, because it will solve your problem.

The Lorentz group SO(3,1) or Poincare group ISO(3,1) is a global symmetry group. That means that the elements don't involve the coordinates. So, performing a Lorentz rotation (whether a boost or spatial rotation) and taking a derivative commute.
 
A silly mistake of mine - that's true, the components Lambda are just constants that depend on the speed of light, rotation angle, and relative velocity of the two frames. Thanks. Problem solved.
 

Related Threads for: Commuting the Lorentz transformation with derivative

Replies
27
Views
3K
Replies
4
Views
481
Replies
1
Views
6K
Replies
16
Views
5K
Replies
28
Views
2K
Replies
1
Views
433
  • Posted
2 3 4
Replies
78
Views
4K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top