By Bernard D'Espagnat
Qu'est-ce que l. a. réalité?
Le physicien Bernard d'Espagnat aborde ici l. a. query du "réel", défendant l'idée qu'il serait intellectuellement discutable de prétendre l. a. traiter sans tenir compte des leçons de los angeles body moderne. Il apporte l'éclairage de los angeles philosophie classique, et celui de l. a. technology contemporaine. Ce livre est donc une brillante initiation aux problématiques de los angeles body, technology qui entre toutes a connu récemment les plus grands bouleversements.
"Alors que j'étais étudiant à l'École centrale, un ami m'offrit À los angeles recherche du réel, qui venait de paraître. Je le dévorais aussitôt en annotant chacune de ses pages." Etienne Klein.
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Additional resources for A la recherche du réel
In the foregoing text it has been emphasized that the quantum mechanical state concept applies to ensembles. They allow us to testify the probabilistic quantum mechanical predictions of what will happen in measuring a given observable. In particular, statements saying that a certain observable has a sharp value can be veriﬁed on an ensemble. Consider, for example, an ensemble of photons that is asserted to be in the x polarization state. 2, namely a polarizing prism with a separate detector in each exit.
Indeed, when measuring any observable on an ensemble that is in a pure state, this ensemble will become split into subensembles that are in eigenstates of the observable. When we do not separate those ensembles, or simply take no notice of the outcomes of the measurement, we have an ensemble that is in a mixed state. 3 Mixed states 25 ‘decomposing’ the mixed state into pure states. Normally, the measuring apparatus does this separation by itself. Think of the polarizing prism mentioned in the beginning of this section or a Stern–Gerlach apparatus that splits an incoming beam of spin 12 atoms into two diverging beams corresponding to the spin components 12 and − 12 , respectively.
Now, the intensity is nothing but the number of incident photons per second, multiplied by hν, and so we can specify the quantum mechanical probabilities as w1 = cos2 and w2 = sin2 . It is interesting to note that our simple device allows us, in fact, to perform a variety of measurements; we need only change the angle . A classical physicist would see the reason for the different behaviour of the incoming photons in a difference of their physical states. He would postulate the existence of a physical parameter that can take different values, thus determining whether an individual photon takes one way or the other in the polarizing prism.
A la recherche du réel by Bernard D'Espagnat