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- A few years after writing the preface of that book, Popper fell into an opposite, and equally serious error about an "EPR situation," On this occasion, contrary to the preceding one, there is an over- rather than an underevaluation of the EPR analysis. On p. 27 of the same book, Popper proposes an experiment that constitutes a variant of the EPR argument, asserting that if the the Copenhagen interpretation is correct, the experiment just analyzed would allow for sending signals faster than the speed of light. This work is one of a lengthy series we will discuss later, in which it is maintained that quantum formalism would permit us to use the reduction of the wave packet to violate one of the postulates at the basis of relativity (i.e., that the speed of light cannot be exceeded). Now, despite the peculiarity of the situation addressed by EPR, this conclusion is fundamentally erroneous and arise from an incorrect use of quantum formalism.
I recall a spirited discussion I once had with Popper at the International Center for Theoretical Physics at Miramare in 1983. Professor Abdus Salam informed me that on the occasion of Popper's visit (for delivering a lecture on the foundations of quantum mechanics), he would be very pleased if the Center would have on hand some competent person in the field, and asked me to take mart in the discussion. I knew Popper's work well and told Professor Salam that my intervention could be critical. Salam's reply was simple: "I have full confidence in you, and if you think you are right, you should explain your position without any fear." Popper presented his thought experiment (a variant of the EPR argument), which, according to him, left us with only two alternatives: either the orthodox interpretation was correct, and it would then be possible to send signals faster than the speed of light, or there would not be any action at a distance and the experiment would constitute a falsification of quantum theory. At the end of the conference I explained to him in simple, but mathematically precise term,s the reasons why his point of departure was erroneous: he had not correctly applied the rules of the theory and in fact, the impossibility of sending superluminal signals would confirm the theory rather than falsify it—the exact opposite of what he maintained. At the end of my intervention he only said that he could not answer my objection since he did not have a mastery of the mathematics of the formalism, but was still convinced that the theory implied the possibility of superluminal signals. This strange, and, as we shall see, fundamentally erroneous idea has been supported by various researchers in various scientific works, and published in prestigious journals.
- Giancarlo Ghirardi, Sneaking a Look at God's Cards: Unraveling the Mysteries of Quantum Mechanics (2005), Ch. 8 : A Bolt from the Blue: The Einstein-Podolski-Rosen Argument