Understanding Photon Polarization with Dirac's Principles of Quantum Mechanics

[forcefield]

Is this statement correct: ?

"The effect of making this observation is to force the photon entirely into the state of parallel or entirely into the state of perpendicular polarization." *

I don't see how you can talk about how the polarization of a photon changes if the photon gets absorbed (as is the case here if it does not get through with a perpendicular polarization).

* The Principles of Quantum Mechanics, 4th edition, pg 7

 

[stevendaryl]

Is this statement correct: ?

"The effect of making this observation is to force the photon entirely into the state of parallel or entirely into the state of perpendicular polarization." *

I don't see how you can talk about how the polarization of a photon changes if the photon gets absorbed (as is the case here if it does not get through with a perpendicular polarization).

* The Principles of Quantum Mechanics, 4th edition, pg 7

You're right--measuring the polarization of a photon is a destructive measurement--after the measurement, the photon is gone. However, Dirac's statement is still relevant in a case such as EPR where the polarizations of two different photons are correlated. In that case, measuring one photon's polarization tells you something definite about the other photon's polarization.

 

[Nugatory]

It's not perfect, for the reason that you've pointed out... But that's a chronic problem with English-language descriptions of physical processes that are more precisely described in more mathematical terms. Treat this statement as a way of getting some intuition about what the mathematical formalism is saying and it'll work just fine.