Quantum Computing – The Einstein-Bohr Debates – Extra History – #3
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Quantum Computing – The Einstein-Bohr Debates – Extra History – #3


At last we have reached the Solvay conference of 1927, where the two most powerful voices
in physics Einstein and Bohr began an intellectual sparring match to determine
the very way we understand reality while aspects of these debates
continue even to today, Einstein’s main goal was to take down the idea
of complementarity and the uncertainty principle, concepts upon which many of the most unsettling aspects
of quantum mechanics hang. It was like an elaborate game. Einstein would offer
brilliant thought experiments that violated complementarity
and the uncertainty principle and Bohr would have
to dismantle them to keep his understanding
of quantum mechanics alive. So get your brains ready. This is gonna get science-y. Brussels, October 27th, the fifth Solvay conference begins. This may be the greatest meeting
of minds humanity has ever assembled. Heisenberg, Dirac, Curie, De Broglie, Planck, Pauli, Lawrence, Born, and of course, Bohr and Einstein are all in attendance. One by one, lectures are given and long stretches of discussion
while away the hours Then, on the third day
of the conference, its Heisenberg
and Born’s turn to speak. As they near the close of their remarks they include a short statement
saying that they considered the fundamental tenets of quantum mechanics
to be established to the point where no modification
to them would occur. Everyone knows Einstein famously disagrees. But he remained silent. Before the conference began, Einstein had said that he would
probably just be an observer, citing that quantum mechanics,
the theme of the conference, was not his area
of expertise. And so, he kept
his objections to himself. But then later, Bohr restates the idea that the fundamentals
of quantum mechanics are settled. At this, Einstein gets up and walks over
to Lorentz, who was running
the conference, and says: “Despite being conscious
of the fact that I have not entered deeply enough
into the essence of quantum mechanics, nevertheless, I’d like to present some general remarks.” by which he means: “I would very much like to lay
the smack-down, please.” He walks up to a chalkboard and proposes a mechanism that utilizes the idea of conservation of momentum to violate the uncertainty principle. His argument went like this: imagine our double slit experiment
from the first episode. Now put another barrier
with a single slit in it between our beam of light
and our double slit wall. When we fire
our light through that slit, we have to treat it as a wave form which means just as in our original experiment, when that wave form passes through
the two slits on the second wall the two wave forms
it produces will interfere
with each other and give us
the interference pattern we are so used to. But since light’s
also a particle, the only thing
that could have possibly deflected the particle-like nature
of the light would be bouncing off
the first wall. There are no other variables
or components in the experiment. So that’s
the only possibility. But now, imagine that first wall is
on ball bearings and can move. By the laws
of conservation of momentum, if we measure which way
and by how much the first wall was moved by the impact
of the photon bouncing off it, we should be able to trace its path and tell which slit the photon will eventually
go through on the second wall. If the wall was pushed
a little bit south, then we know the photon
bounced off it and got reflected north, heading towards
the northernly slit on the second wall, right ? But that can’t be the case because if you remember from our discussion of wave- particle duality while basically everything operates like a wave and a particle, things don’t operate as both at once. Things like light are a possibility wave until that wave is collapsed by someone trying to measure it; at which point, stuff like the location of the photon within the space of possibility has to be resolved. So, by that logic, we shouldn’t be able to tell which slit on the second wall a photon will go through, which is particle-like behavior, and get an interference pattern for where those photons land on the screen behind it. Because, in order for the wave pattern to interfere with each other on the other side, the light would have to be acting like it was a wave as it passed through both slits. If we could observe both the wave aspects and of the particle aspects of light simultaneously, that would violate indeterminacy and thus unseat the uncertainty principle. But Bohr had an answer. He basically said that Einstein had set up a strawman here. He hadn’t removed uncertainty; he had just moved the uncertainty from one place to another without realizing it. You see, in order to measure how much rolling that first wall did as a result of being hit with one photon, you’d have to know the exact location and the movement of the wall itself before the photon hit it. And because we’re dealing with one photon here, you’d have to be incredibly precise: precise to a level that quantum rules kick in. And dealing at the quantum level means we run into indeterminancy, You can’t know both the momentum and the location of the wall with perfect accuracy. So, yes, Einstein’s example would violate indeterminacy, but the only way his example works is if you ignore indeterminacy first. Q. E. D. Not bad for a first round. Over the rest of the conference, with a spirit of camaraderie, Einstein would assail Bohr with other, less intense thought experiments and Bohr would respond parrying and thrusting back. Soon, the real conference was going on in the dining halls in the hotel hallways, with everyone getting in on the discussion. And so, with the spirit of inquiry and joy, the first real test of quantum mechanics was overcome. But Einstein wasn’t totally satisfied. The next Solvay Conference would come in three year’s time and this time, Einstein would come bearing gifts. More specifically, he would bring Bohr a box of light. Einstein shows up to the next conference and walks up to Bohr and he says, “Imagine a box whose walls are perfectly reflective mirrors. Inside the box, there’s light bouncing around. There’s also a clock built into the box and at a specific time, this clock will open a hole in the side of the box for however long it takes for one photon to get out.” Einstein then waits to see if Bohr will see what’s coming. Then, he says, “Weigh the box.” Bohr goes pale. He starts wandering around the conference like a man in the aftermath of a natural disaster, going from person to person, saying, “It can’t be true.
It can’t be true.” You see, he realized what Einstein is saying. If you weigh the box,
before the photon leaves it, and then weigh it again after, you’ll know the mass of the photon. And thanks to Einstein’s own insight that E=mc², if you know the mass of a thing, you know its energy. When the uncertainty principle and complementarity are batted around in science fiction novels or Popular Mechanics articles, usually all anyone ever talks about is the fact that you can’t find the position and momentum of a particle with absolute certainty. But there are a number of other complementary pieces of information which these principles also say you can’t know both aspects of absolutely. One of them is energy and time. According to the uncertainty principle. you can’t know the exact energy
of a particle at an exact time. But that’s what Einstein’s box
appears to successfully do. The clock gives
an absolute time and the weight of the photon allows us
to determine its energy at that exact moment. So Bohr retreats to his room, stunned, muttering that, “It’s the end
of physics if Einstein’s right.” Sleepless, he works the problem through the night. He draws a diagram of the box; he imagines the experiment as
if he was actually going to do it. Then, as dawn nears, it hits him. With a smile, he lays down to catch
a few hours of sleep. He’ll be ready to unveil
his total triumph in the morning. And so, it was with
mounting excitement in the crowd he addressed the gathering
the next morning. He explained that, if a photon escapes the box, then the box itself must recoil
by the laws of conservation of momentum. By how much? Well, due to the uncertainty principle, we can’t know precisely. But this fact itself proved nothing. This was an issue of momentum and Einstein’s argument was dealing
specifically with energy and time. But then, Bohr administered
the coup de grâce: in order to weigh the box, that box would have to be within a gravitational field. After all, weight means nothing without gravity. But, by Einstein’s very own theory of relativity, we know that gravity distorts spacetime and, for an object moving
through a gravitational field, how quickly time moves is affected by that distortion. It’s why you’ll hear people talk about
how time moves faster on a mountaintop or why time might stop near the center of a black hole. Thanks to gravity’s distortion of spacetime, we can’t tell exactly how much the clock within it was affected by that movement. HENCE, UNCERTAINTY. Now that was a gross generalization of Bohr’s answer; and there are plenty of physicists, who over the years, have suggested
that Bohr’s solution here was flawed or that they have a better solution. But the principle still stood: Bohr had bested Einstein
using his own theory. And so, Bohr’s understanding
of the quantum world came to be accepted by the majority
of the scientific community. Basically, through today. So why walk through the Solvay conferences? Well, because, without seeing the pieces
being put into place, it’s hard to fully accept what comes next. The real thing that Einstein found
so upsetting about quantum theory was something he called, “spooky action at a distance”, which we know today is the violation of local realism. Because once we have that and the uncertainty principle, the idea that energy comes in quanta and the fact you can superimpose two quantum states and have them interfere with each other, like we saw
with the waves in the double slit experiment; once we’ve got all that stuff, which you’ve hopefully come to
at least grudgingly accept here, then, we can come to understand the power and the challenges of the quantum computer. I Know Zoe, my head’s spinning, too.

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