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August 25 - September 14, 2020
There is nothing more surreal, nothing more abstract than reality. —Giorgio Morandi
Do light and electrons show wavelike behavior (like water does)? Or do they act like particles (like grains of sand do)?
For any given frequency or color of light, a photon of light is the smallest unit of energy, and it cannot be divided any further: the light cannot come with any less energy than contained in one photon.
Nature, at its deepest, seems inherently nondeterministic. Or is it merely hiding its secrets, and we haven’t dug deep enough yet?
The experiment hasn’t changed in its conceptual simplicity for more than two hundred years, but it has become technologically more and more sophisticated, as experimenters keep thinking of clever ways to trick nature into revealing its profoundest secrets.
The idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them . . . is impossible. —Werner Heisenberg
The experiment involved building a transmitter of electromagnetic waves, and a receiver—and showing that these invisible waves did indeed exist and could propagate through air. Hertz had inadvertently discovered radio waves.
When asked about the usefulness of such waves, Hertz reportedly said, “It is of no use whatsoever. This is just an experiment that proves Maestro Maxwell was right. We just have these mysterious electromagnetic waves that we cannot see with the naked eye. But they are there.”
“To be sure, it is a discovery, because it deals with a completely new and very puzzling phenomenon. I am of course less capable of judging whether it is a beautiful discovery, but of course it does please me to hear others call it that; it seems to be that only the future can tell whether it is important or unimportant.”
In the midst of terrible social unrest and unhinged ideologies across Europe, the quantum revolution was set in motion.
In an analogy to the way electrons behave in orbits, the car jumps from 10 mph to 60 mph in chunks of 10 mph, without going through any of the intermediate speeds.
The upshot of this somewhat arbitrary postulate was that another property of electrons, their angular momentum, was also quantized: it could have certain values and not others.
Nature, it seems, did not discriminate: everything had wavelike behavior and particle-like behavior.
Discreteness, or jumps from one state to another, is baked into matrix mechanics.
How could the physical world be represented by things that could only be imagined?
An element of randomness, or stochasticity, became an integral part of the laws of nature.
As Born put it, “The motion of particles follows probability laws but the probability itself propagates according to the law of causality.”
The advantage of wave mechanics, in Schrödinger’s opinion, was the idea that nature even at the smallest scales was continuous, not discrete. There were no quantum jumps.
As Walter Moore writes in his book Schrödinger: Life and Thought, “Schrödinger was a ‘visualizer’ and Bohr was a ‘nonvisualizer,’ one thought in terms of images and the other in terms of abstractions.”
A cartoon captioned “At home with the Heisenbergs” was stuck on the bathroom door outside the apartment, with Mrs. Heisenberg saying, “I can’t find my car keys,” and Mr. Heisenberg replying, “You probably know too much about their momentum.”)
This was a grand battle of ideas, the likes of which occur infrequently enough in science to be etched in cultural memory as moments that changed our understanding of our place in the universe.
“It is fair to state that we are not going to experiment with single particles any more than we will raise dinosaurs in the zoo.”
The Hitachi team’s film of the electrons hitting the screen (the actual elapsed time was twenty minutes, but the film is sped up) is one of the most fascinating short films in the history of physics.
Think of sitting in a train that’s racing through the countryside at night. If it’s completely dark outside and you are looking at the windowpane, you will see the inside of the carriage reflected in the glass. But when the train passes by some lighted buildings outside, you see the buildings while simultaneously seeing your own reflection in the glass. The windowpane is both reflecting and transmitting light.
As philosopher David Albert of Columbia University writes in his book Quantum Mechanics and Experience, the term superposition is “just a name for something we don’t understand.” (And the above analysis is inspired by a similar analysis of a slightly different system in Albert’s book.)
“There fails to be a fact of the matter about which slit the particle went through. Asking which slit the particle went through is [like] asking about the marital status of the number five or the weight in grams of Catholicism. This is something philosophers call a category mistake.”
What?!
The Copenhagen interpretation, while it does not invoke the need for human consciousness, nonetheless demands a classical measurement. The corollary is that the quantum state of a system is adequately and indeed completely captured by the wavefunction, and since the wavefunction lets you calculate only the probability of finding the system in some state, and does not correspond to, say, where the photon actually is, reality does not exist in any meaningful sense independent of measurement with a classical apparatus.
Quantum mechanics asks us to suspend disbelief and hold on to some counterintuitive notions of reality for long enough to be able to appreciate the bizarreness of the subatomic world.
“The world is not as simple as one could think. But physicists were smart enough to develop mathematical tools to render an account of what happens.”
And what they observed was that there was no fooling the photon. If the second beam splitter was not there, it behaved like a particle, otherwise it acted like a wave. It did not matter when the second beam splitter was inserted.
“Bohr’s statement that it is the measurement that determines what you observe etc. . . . should not be taken in a too naive [manner]. It is more subtle than that,” said Aspect.
Nonlocality forces us to extend the conceptual toolbox we use to talk about nature’s inner workings. —Nicolas Gisin
It’s a violation of the principle of locality, which says that if something is happening in one region of spacetime, it cannot influence something else happening in another region of spacetime any faster than the speed of light. The collapse of the wavefunction, in this way of thinking about it, is instantaneous and patently nonlocal.
“If The New York Times is the secular press, it follows that the sacred text is the Physical Review.”)
Haha
“Any serious consideration of a physical theory must take into account the distinction between the objective reality, which is independent of any theory, and the physical concepts with which the theory operates. These concepts are intended to correspond with the objective reality, and by means of these concepts we picture this reality to ourselves.”
For Einstein, the real world exists independent of our observations.
Hermann straddled the worlds of philosophy and mathematics with ease. In 1935, she published a paper in a German journal in which she showed that von Neumann’s proof was incorrect. “A thorough examination of the proof of von Neumann reveals . . . that in his argumentation he makes an assumption which is equivalent to the statement he wants to prove,” she wrote. “Therefore, the proof is circular.”
He never lived to see the results of these experiments, and one can only wonder how he’d react to the growing realization among many followers of standard quantum mechanics that reality is nonlocal.
He’d write to Max Born in a letter dated March 3, 1947, “I cannot seriously believe in it [quantum theory] because the theory cannot be reconciled with the idea that physics should represent a reality in time and space, free from spooky actions at a distance.” Einstein died in 1955.
Say the combined wavefunction hits a photographic plate. The particle appears somewhere on that plate: it gets localized. But at all the other locations on the photographic plate where the particle had a nonzero probability of existing, nothing happens. These are simultaneous events and nonlocal.
“How puzzling is that?” said Maudlin.
But to Maudlin, the mystery of the double slit is even more pronounced when one tries to detect which slit the particle goes through. The interference pattern goes away. But why? It’s because the system being used to detect the particle as it goes through the double slit becomes entangled with the particle. “Schrödinger said that what was really new about quantum mechanics was entanglement,” said Maudlin. “And so from that point of view, the really [surprising] quantum mechanical effect is the disappearance of the interference.”
These experiments are a magnificent affront to our conventional notions of space and time. Something that takes place long after and far away from something else nevertheless is vital to our description of that something else. By any classical—commonsense—reckoning, that’s, well, crazy. Of course, that’s the point: classical reckoning is the wrong kind of reckoning to use in a quantum universe. —Brian Greene
“How can that be? How can we just have probability distributions and nothing behind it?”
“The probabilities are the reality we have. There is nothing behind it. The probability is not about a hidden reality . . . full stop.”
“A dumb kid from Wyoming, I didn’t know that the Nobel Prize physicist at Yale wasn’t there for me.”
What if this information is erased? Will the interference pattern come back?
But if you look at the pattern made only by those system photons whose corresponding environment photons were detected at D1, something strange happens: you see interference fringes.
If the idea of waiting and waiting before choosing what to do with the environment photons seems like theoretical fantasy, no one bothered to tell some physicists in Austria.
The delayed-choice part comes in because the decision to erase or not to erase is made only when the environment photon reaches Tenerife—well after the partner system photon has been detected at La Palma, and thus well after it has ostensibly already behaved like a wave or a particle.
