Transition Probability | Bransden & Joachain | 4.38 & 4.39 Dimension

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Homework Help Overview

The discussion revolves around the transition probability expressions found in Bransden & Joachain, specifically equations (4.38) and (4.39). Participants are examining the dimensional consistency of these expressions, questioning whether they can be dimensionally equivalent given the presence of the differential term dω.

Discussion Character

  • Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants are analyzing the dimensional properties of the transition probability and its amplitude, noting that while (4.38) is an amplitude and (4.39) is its squared form, there seems to be a discrepancy in dimensions due to the dω term. Some participants express confusion over how both can be dimensionless if one appears to lack a necessary factor.

Discussion Status

The discussion is ongoing, with participants actively questioning the dimensional consistency of the equations. There is recognition of the need for clarity regarding the definitions and roles of the terms involved, particularly the relationship between the amplitude and the probability. Some guidance has been offered regarding the nature of probability as dimensionless, but the dimensional inconsistency remains a point of contention.

Contextual Notes

Participants are working from specific equations in a textbook and referencing additional literature to explore the dimensional aspects of the problem. There is an acknowledgment of potential assumptions regarding the units of ω, which is typically understood to be in s-1.

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At page 190 of Bransden & Joachain (see the page from http://books.google.com/books?id=i5...nsden,+Charles+Jean+Joachain&hl=da#PPA190,M1"), there are 2 expressions for the transition probability, (4.38) and it's absolute value squared in (4.39).
Is it just me or are the 2 term dimensionally different? Obviously everything from (4.38) is squared in (4.39) except the d\omega. Hence they can't be the same dimensionally. How can this be right?
 
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I am not sure what the question is, but they are both dimensionless. Probability doesn't have a unit.
 
Cyosis: Yes they are supposed to be dimensionless, but I just looked at the difference between the 2 expressions.

Note that every term from (4.38) appears squared in (4.39) except for the d\omega term which has the same power in both, hence (4.39) is short by a factor of 1/sec.
 
In any case (4.38) is not the probability, it's the amplitude.
 
But it's supposed to be dimensionless like the probability, hence must have the same dimensions.

EDIT:
Ok to make it more clear, then (4.38) has the form

c_b = \int_0^{\infty} f(\omega) \, d\omega

while (4.39) has the form

|c_b|^2 = \int_0^{\infty} |f(\omega)|^2 \, d\omega

which certainly can't be right. What's going on?
 
Last edited:
That's weird, it certainly does look dimensionally inconsistent, regardless of what cb represents.

I'm assuming ω means what it usually does and has units of s-1.
 

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