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Einstein and Quantum Mechanics

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  • Einstein and Quantum Mechanics

    THE FABRIC of the COSMOS, Brian Greene, 2004
    ```(annotated and with added bold highlights by Epsilon=One)
    Chapter 4 – Entangling Space
    Section: Einstein and Quantum Mechanics

    ~~~~~in reality, it was at or very near X a moment before the measurement was carried out? And if so, Einstein prodded, doesn't quantum mechanics' reliance on the probability wave — a wave that, in this example, says the electron had some probability to have been far from — X reflect the theory's inadequacy to describe the true underlying reality?

    Einstein's viewpoint is simple and compelling. What could be more natural than to expect a particle to be located at, or, at the very least, near where it's found a moment later? If that's the case, a deeper understanding of physics should provide that information and dispense with the coarser framework of probabilities. But the Danish physicist Niels Bohr and his entourage of quantum mechanics defenders disagreed. Such reasoning, they argued, is rooted in conventional thinking, according to which each electron follows a single, definite path as it wanders to and fro. And this thinking is strongly challenged by Figure 4.4, since if each electron did follow one definite path — like the classical image of a bullet fired from a gun — it would be extremely hard to explain the observed interference pattern: what would be interfering with what? Ordinary bullets fired one by one from a single gun certainly can't interfere with each other, so if electrons did travel like bullets, how would we explain the pattern in Figure 4.4?

    Instead, according to Bohr and the Copenhagen interpretation of quantum mechanics he forcefully championed, before one measures the electron's position there is no sense in even asking where it is. It does not have a definite position. The probability wave encodes the likelihood that the electron, when examined suitably, will be found here or there, and that truly is all that can be said about its position. Period. The electron has a definite position in the usual intuitive sense only at the moment we "look" at it — at the moment when we measure its position — identifying its location with certainty. But before (and after) we do that, all it has are potential positions described by a probability wave that, like any wave, is subject to interference effects. It's not that the electron has a position and that we don't know the position before we do our measurement. Rather, contrary to what you'd expect, the electron simply does not have a definite position before the measurement is taken.

    This is a radically strange reality. In this view, when we measure the electron's position we are not measuring an objective, preexisting feature of reality. Rather, the act of measurement is deeply enmeshed in creating the very reality it is measuring. Scaling this up from electrons to everyday life, Einstein quipped, "Do you really believe that the moon is not there unless we are looking at it?" The adherents of quantum mechanics responded with a version of the old saw about a tree falling in a forest: if no one is looking at the moon — if no one is "measuring its location by seeing it" — then there is no way for us to know whether it's there, so there is no point in asking the question. Einstein found this deeply unsatisfying. It was wildly at odds with his conception of reality; he firmly believed that the moon is there, whether or not anyone is looking. But the quantum stalwarts were unconvinced.

    Einstein's second challenge, raised at the Solvay conference in 1930, followed closely on the first. He described a hypothetical device, which (through a clever combination of a scale, a clock, and a cameralike shutter) seemed to establish that a particle like an electron must have definite features — before it is measured or examined — that quantum mechanics said it couldn't. The details are not essential but the resolution is particularly ironic. When Bohr learned of Einstein's challenge, he was knocked back on his heels — at first, he couldn't see a flaw in Einstein's argument. Yet, within days, he bounced back and fully refuted Einstein's claim. And the surprising thing is that the key to Bohr's response was general relativity! Bohr realized that Einstein had failed to take account of his own discovery that gravity warps time — that a clock ticks at a rate dependent on the gravitational field it experiences. When this complication was included, Einstein was forced to admit that his conclusions fell right in line with orthodox quantum theory.

    Even though his objections were shot down, [I]Einstein remained deeply uncomfortable with quantum mechanics[/B]. In the following years he kept Bohr and his colleagues on their toes, leveling one new challenge after another. His most potent and far-reaching attack focused on something known as the uncertainty principle, a direct consequence of quantum mechanics, enunciated in 1927 by Werner Heisenberg.

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    Last edited by Reviewer; 09-30-2012, 05:58 PM.