Fresh from his wrangles with Jauch, Bell returned to EPR's thought experiment. He wondered whether such "spooky actions at a distance" were endemic to quantum mechanics, or just one possible interpretation among many. Might some kind of hidden variable approach reproduce all the quantitative predictions of quantum theory, while still satisfying Einstein's (and Bell's) intuition about locality? He focused on a variation of EPR's set-up, introduced by David Bohm in his 1951 textbook on quantum mechanics. Bohm had suggested swapping the values of the particles' spins along the x- and y-axes for position and momentum.
"Spin" is a curious property that many quantum particles possess; its discovery in the mid-1920s added a cornerstone to the emerging edifice of quantum mechanics. Quantum spin is a discrete amount of angular momentum—that is, the tendency to rotate around a given direction in space. Of course many large-scale objects possess angular momentum, too: think of the planet Earth spinning around its axis to change night into day. Spin in the microworld, however, has a few quirks. For one thing, whereas large objects like the Earth can spin, in principle, at any rate whatsoever, quantum particles possess fixed amounts of it: either no spin at all, or one-half unit, or one whole unit, or three-halves units, and so on. The units are determined by a universal constant of nature known as Planck's constant, ubiquitous throughout the quantum realm. The particles that make up ordinary matter, such as electrons, protons, and neutrons, each possess one-half unit of spin; photons, or quanta of light, possesss one whole unit of spin.
In a further break from ordinary angular momentum, quantum spin can only be oriented in certain ways. A spin one-half particle, for example, can exist in only one of two states: either spin "up" or spin "down" with respect to a given direction in space. The two states become manifest when a stream of particles passes through a magnetic field: spin-up particles will be deflected upward, away from their previous direction of flight, while spin-down particles will be deflected downward. Choose some direction along which to align the magnets—say, the z-axis—and the spin of any electron will only ever be found to be up or down; no electron will ever be measured as three-quarters "up" along that direction. Now rotate the magnets, so that the magnetic field is pointing along some different direction. Send a new batch of electrons through; once again you will only find spin up or spin down along that new direction. For spin one-half particles like electrons, the spin along a given direction is always either +1 (up) or -1 (down), nothing in between. (Fig. 2.1.)
No matter which way the magnets are aligned, moreover, one-half of the incoming electrons will be deflected upward and one-half downward. In fact, you could replace the collecting screen (such as a photographic plate) downstream of the magnets with two Geiger counters, positioned where the spin-up and spin-down particles get deflected. Then tune down the intensity of the source so that only one particle gets shot out at a time. For any given run, only one Geiger counter will click: either the upper one (indicating passage of a spin-up particle) or the lower one (indicating spin-down). Each particle has a 50-50 chance of being measured as spin-up or spin-down; the sequence of clicks would be a random series of +1's (upper counter) and -1's (lower counter), averaging out over many runs to an equal number of clicks from each detector. Neither quantum theory nor any other scheme has yet produced a successful means of predicting in advance whether a given particle will be measured as spin-up or spin-down; only the probabilities for a large number of runs can be computed.
Bell realized that Bohm's variation of the EPR thought experiment, involving particles' spins, offered two main advantages over EPR's original version. First, the measurements always boiled down to either a +1 or a -1; no fuzzy continuum of values to worry about, as there would be when measuring position or momentum. Second, physicists had accumulated decades of experience building real machines that could manipulate and measure particles' spin; as far as thought experiments went, this one could be grounded on some well-earned confidence. And so Bell began to analyze the spin-based EPR arrangement. Because the particles emerged in a special way—spat out from a source that had zero spin before and after they were disgorged—the total spin of the two particles together likewise had to be zero. When measured along the same direction, therefore, their spins should always show perfect correlation: if A's spin were up then B's must be down, and vice versa. Back in the early days of quantum mechanics, Erwin Schrödinger had termed such perfect correlations "entanglement."
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11 Comments
Add CommentFor anyone who was interested in physics/cosmology in the 60s/70s this book is a fun read that will spark a lot of memories/nostalgia, and add a lot of background information on what was happening behind the headlines.
Reply | Report Abuse | Link to thisOne reason I did not complete ROTC was because commanding officers told me the prime objective was to stop communism. Not pro or con towards communism, I wondered why the communist party was not allowed an equal voice to the democrats or republicans.
Reply | Report Abuse | Link to thisA wonderful and thoroughly enjoyable article--thank you! I totally want to get the book.
Reply | Report Abuse | Link to thisGood point. I probably would have dropped out if the war was against the democrats or republicans also.
Reply | Report Abuse | Link to thisMaybe it's because they would kill both party's off and rule the roust as King.
Reply | Report Abuse | Link to thisYou have to step over the rotting corpses of seventy million people in order toget to Communism's justification. There is a reason Communism fell in the late eighties; it outlawed freedom.
Reply | Report Abuse | Link to thisNonsense. Communism the ideology had no more to do with those corpses than democracy the ideology was responsible for the deaths in Vietnam (you can look directly to corrupt US leaders for that particular death toll.) Those 70 million corpses were the result of NOT doing communism as the ideology specified, and almost entirely a consequence of being ruled by a series of bloodthirsty, paranoid, power-mad, corrupt dictators -- communist in name only -- who shoveled their caca downhill through a thoroughly corrupt power structure which again, was about as "communist" as you are.
Reply | Report Abuse | Link to thisYou'll notice communism hasn't produced a mass of corpses in the USA, though there are quite a few communists, and communist-leaning folks around. You might ask yourself why.
I'm not saying communism is any great shakes, but it isn't a formula for murder, either. Certainly no more so than democracy or a republic. It's all about responsible, restrained leadership.
Everywhere Communism was attempted in the twentieth century, mass murder ensued. "Doing" communism as specified involves a universal cooperative effort on the part of its adherents. In other words "to each according to his needs, from each according to his abilities.
Reply | Report Abuse | Link to thisThe Bourgeoisie, perfectly happy with the status quo wasn't the least bit interested in giving up power or wealth to become "equal" with the working class, and even Marx and Engels new that world wide bloody revolution was necessary to bring about the "workers Utopia". Lenin was a paranoid psychotic, but he was the perfect man to carry out the revolution. Indeed, as evidence for what I say, ask yourself this question:
Where in the history of Communism was there NOT the bloodthirsty Dictator you describe?
Think about that question for a minute; How could it be that every single Communist society in the twentieth Century was a bloody dictatorship? How could it be that there was not one place on earth during the entire century where Communism flourished peacefully and more important, abundantly for all of its citizens?
How else to realize the Socialist dream? How else do you get to full cooperation with this ideology?
The Communist Revolution failed not as a consequence of being ruled by a series of "bloodthirsty, paranoid, power-mad, corrupt dictators"; they were absolutely what the revolution called for in its infancy. The revolution failed precisely because they couldn't kill or imprison apostates fast enough.
Fyngyrz, I'm assuming you are of the impression that if we just tried it one more time, and did it correctly, we'll all be farting bluebirds and singing Kum Bye Ya in no time. Well, we won't.
I agree. I think the problem was that Communism explicitly excluded democracy, and therefore there was never any accountability. Bloodthirsty dictators never had to win an election. Same with incompetent governors, mayors, Party leaders, and garbage collectors. I personally think that the main reason democracy is successful is not that it allow the "best" people to get elected, but that it allows people to "fire" the abusive ones. It permits accountability, or in other words, a negative feedback loop in which an abuse of power results in the removal of power. Communism has a positive feedback loop, in which the worse the abuse of power, the more power the abuser acquires.
Reply | Report Abuse | Link to thisIf you have a tube full of a non-compressible liquid, and you start entering more liquid in the tube in one of its ends, the liquid you entroduce in the origin of the tube will take a measurable time to go from one end of the tube to the other end, but flow transmission, i.e. movement of liquid along the whole tube, would be instantaneous. Does this fact have any connection to "Spooky action at distance" ?. I'm not trying to resuscitate the concept of "ether", naturally. Salut +
Reply | Report Abuse | Link to thisWell, when it comes to Hippies they all look alike to me.
Reply | Report Abuse | Link to thisCERN's neutrinos exceeding the speed of light @ v-c/c=2.48 sec in 453.6 miles is an exact match to Stanford's SLAC E158 weak force asymmetry value showing the cause of 2.48e-5 is not the politics of a "loose fiber optic cable" checked countless times with the same "gain in space" shown by FERMI Lab's neutrinos supporting an observation that goes back to 1947 and is summed up in 2007 by G. Nimtz and A. A. Stahlhofen who believe it occurs outside the bounds of SR [arXiv:0708.0681v1] not aware of the simple explanation I showed E158 data provides with direct proof provided by SLAC's E158 data exposing a gain in time/space in 453.6 miles also @ 2.48e-5 with a .20 harmonic comma as I predicted for the asymmetry in the reverse arrow/phase of time the calculations reveal changing physics. SLAC's data comes from the distance light travels in 1000 years at the speed of light in a ratio to a 1 hour SOL gain making this comparative measurement the most spectacular ever made in physics and does so in regard to the most important observation in the History of Man.
This means E=m+{a}c2 outside the weak force where E=mc2 as that equation does not include the photon's force carrier space the neutrino provides to exit the weak force to create the strong force. This changes physics far more that E=mc2 did due to the infrastructure in place to take advantage of the energy tap {a} provides. It puts us back in sync with Newton's time but armed with E=mc2 corrected as well as Newton's law by adding the second reverse phase held by "imaginary" numbers in physics that never add up correct without {a} added to expose the second "real" reverse phase in time. And One more thing it shows, we live in a world of only 3.6 seconds a year due to time dilation and that's why atoms hang out so long but not their anti-matter reverse phases. This is what happens when you look at the Sun: a thousand years in the inertial frame of the week force is an hour of a day at the speed of light in time equal to distance at the speed of light relative to our body’s inertial frame of reference. Theata 13 shows there 3 points of observation within the weak force as I showed Erin Edwards in demonstrating the chemical source timing relative to a true inertial frame of reference that Mass location in time provides to create it’s placement within the DNA’s Gene Ensemble Instance Location. A new age in physics approaches awaiting your understanding to benefit Mankind.
And they all sound the same.