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Le leggi della natura

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  Inviato da: Wakinian Tanka  Mostra tutti i messaggi di Wakinian Tanka
Titolo: Ha ancora senso introdurre l'effetto fotoelettrico...
Newsgroup: it.scienza.fisica
Data: 21/12/2017
Ora: 15:55:20
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  .... come uno degli indizi della natura quantistica della luce?<br /> <br /> http://www.mat.univie.ac.at/~neum/physfaq/topics/photodetection.html<br /> <br /> <br /> <br /> &lt;&lt;The photoelectric effect is usually explained (following Einstein, who received the Nobel price for this explanation) by saying that a sufficiently energetic photon falling on a photosensitive substance causes the latter to eject a single electron, which is then magnified by a photomultiplier to produce a macroscopic and hence observable effect - the ''click'' of the detector. This is commonly used in discussions of experiments on entangled photons carried out by Alice and Bob, who make statistics on clicks to prove or disprove things, or to communicate secret information.<br /> <br /> <br /> <br /> <br /> In the semiclassical picture known to Einstein 1905, currents are produced by discrete electrons. In 1905, when Einstein proposed his explanation, the photoelectric effect was a clear indication of the particle nature of light, since no other model was available that could have explained the process. Einstein's explanation was so important for the development of the subject that he got 1921 the Nobel prize for it, a few years before modern quantum mechanics was born. The modern concept of a photon was created only later (Lewis 1926, Dirac 1927).<br /> <br /> <br /> <br /> According to today's knowledge, just like Bohr's atomic model, Einstein's explanation of the photoeffect is too simplistic, and is not conclusive. Now, 100 years later, his picture is known to be approximate only, and that currents in metals are in fact produced by the continuous electron fields of QED. Discrete semiclassical particles are just very rough approximations.<br /> <br /> <br /> Indeed, the argument of Einstein put forward for the discrete nature of radiation is spurious, since it ignores the quantum nature of the detector (which was of course completely unknown at the time). As one can read in the standard reference for quantum optics,<br /> <br /> L. Mandel and E. Wolf, Optical Coherence and Quantum Optics, Cambridge University Press, 1995.<br /> <br /> the clicks in a photon detector are an artifact of photodetection caused by the quantum nature of matter, rather than proof of single photons arriving.<br /> <br /> Mandel and Wolf write (on p.629, in the context of localizing photons), about the temptation to associate with the clicks of a photodetector a concept of photon particles:<br /> <br /> <br /> ''Nevertheless, the temptation to interpret the electronic signal registered by a photodetector as due to a photon that is localized in some sense is quite strong.''<br /> <br /> <br /> The wording suggests that one should resist the temptation, although this advice is usually not heeded. However, the advice is sound since a photodetector clicks even when it detects only classical light! This follows from the standard analysis of a photodetector, which treats the light classically and only quantizes the detector.<br /> <br /> <br /> <br /> <br /> Sections 9.1-9.5 show that the electron field responds to a classical external electromagnetic radiation field by emitting electrons according to Poisson-law probabilities, very much like that interpreted by Einstein in terms of light particles. Thus the quantum detector produces discrete Poisson-distributed clicks, although the source is completely continuous, and there are no photons at all in the quantum mechanical model. The state space of this quantum system consists of multi-electron states only. So here the multi-electron system (followed by a macroscopic decoherence process that leads to the multiple dot localization of the emitted electron field) is responsible for the creation of the dot pattern. <br /> This proves that the clicks cannot be taken to be a proof of the existence of photons.&gt;&gt;<br /> <br /> --<br /> Wakinian Tanka  

Il thread:
da leggere Wakinian Tanka 21/12 15:55
Ha ancora senso introdurre l'effetto fotoelettrico...
   da leggere Giorgio Pastore 23/12 00:22
Re: Ha ancora senso introdurre l'effetto fotoelettrico...
      da leggere Wakinian Tanka 26/12 11:45
Re: Ha ancora senso introdurre l'effetto fotoelettrico...
         da leggere Giorgio Bibbiani 30/12 18:42
Re: Ha ancora senso introdurre l'effetto fotoelettrico...
         da leggere Giorgio Pastore 30/12 20:19
Re: Ha ancora senso introdurre l'effetto fotoelettrico...
            da leggere Wakinian Tanka 03/01 12:26
Re: Ha ancora senso introdurre l'effetto fotoelettrico...
               da leggere Giorgio Pastore 03/01 16:47
Re: Ha ancora senso introdurre l'effetto fotoelettrico...
                  da leggere Wakinian Tanka 12/01 00:22
Re: Ha ancora senso introdurre l'effetto fotoelettrico...
                     da leggere Luciano Buggio 14/01 13:56
Re: Ha ancora senso introdurre l'effetto fotoelettrico...
            da leggere Elio Fabri 08/01 21:50
Re: Ha ancora senso introdurre l'effetto fotoelettrico...
      da leggere Luciano Buggio 26/12 17:02
Re: Ha ancora senso introdurre l'effetto fotoelettrico...
      da leggere Elio Fabri 27/12 15:23
Re: Ha ancora senso introdurre l'effetto fotoelettrico...
         da leggere Wakinian Tanka 30/12 19:18
Re: Ha ancora senso introdurre l'effetto fotoelettrico...
         da leggere Luciano Buggio 03/01 21:28
Re: Ha ancora senso introdurre l'effetto fotoelettrico...
 

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