Show that rotations and boosts lead to a combined boost

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Homework Help Overview

The problem involves proving a relationship between infinitesimal rotations and boosts in the context of special relativity, specifically showing that a sequence of transformations leads to a combined boost. The subject area is primarily focused on the mathematical representation of these transformations.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss expanding the exponential of infinitesimal rotations and simplifying the resulting expressions. There are attempts to equate the left-hand side of the equation to a single boost on the right-hand side, with some participants questioning the correctness of their expansions and the consistency of variables used.

Discussion Status

The discussion is active, with participants providing guidance on how to approach the problem, particularly regarding the expansion of terms and the identification of errors. There is a recognition of the need to find a new variable to satisfy the equation up to a certain order, indicating a productive direction in the exploration.

Contextual Notes

Participants note potential errors in the expansions and the need for consistency in the variables used throughout the problem. There is an emphasis on ensuring that the boosts are along the correct directions, which may affect the overall approach to the solution.

ma18
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Homework Statement


Prove that applying an infinitesimal rotation of angle k<<1 around the axis x1, then a boost of speed -k along the axis x2 then the inverses of these is equal to a single boost of speed k^2 along the axis x3

The Attempt at a Solution



Putting this into mathematical terms I get

e-i*k*Jx1*ei*k*Gx2*ei*k*Jx1*e-i*k*Jx2 = e-i*k2*Gx3

However I don't really know where to go from here, the example we did in class only involved the rotations and we just used the matrix representations around the axis, but I don't know how exactly do it with the boosts.

Any help would be much appreciated.
 
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You can expand the exponential of an infinitesimal rotation and then simplify.
 
mfb said:
You can expand the exponential of an infinitesimal rotation and then simplify.
Doing this I get to the equation:

upload_2015-10-27_0-24-32.png


Which after expanding leads me to

upload_2015-10-27_0-27-30.png


Taking only the terms up to e^2 reduces this to

upload_2015-10-27_0-29-57.png


Which is not equal to the right hand side with only a single boost which expands to

upload_2015-10-27_0-31-21.png


What am I missing?
 

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Where does e come from and where did k go?
Your boost is along a different directions, the G on the different sides are not the same.
Also, I think there is some error in the expansion of the right hand side.
 
mfb said:
Where does e come from and where did k go?
Your boost is along a different directions, the G on the different sides are not the same.
Also, I think there is some error in the expansion of the right hand side.
Ah sorry, I used e in the equation instead of k, they are the same. I see, as the G is different what would be my next step?
 
Find a new G' such that the equation is true up to order k2.
If you fix the expansion of the RHS, you are just one line away from this.
 
ma18 said:
Ah sorry, I used e in the equation instead of k, they are the same.
mfb said:
Find a new G' such that the equation is true up to order k2.
If you fix the expansion of the RHS, you are just one line away from this.

Ah I see, the RHS is supposed to be (where G_x represents the new boost around x_3)

upload_2015-10-27_18-22-1.png


and then taking out the e^4 term this reduces to

upload_2015-10-27_18-22-42.png


and solving for G_x we get

upload_2015-10-27_18-23-14.png


Correct?
 

Attachments

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Should be right.
 
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Likes   Reactions: ma18
mfb said:
Should be right.
Great, thank you so much for your help
 

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