Dirac on photon polarization

1. Jul 8, 2015

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

2. Jul 8, 2015

stevendaryl

Staff Emeritus
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.

3. Jul 8, 2015

Staff: Mentor

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.