Bell showed, essentially, that when pairs of polarization-entangled photons are measured for linear polarization in particular directions, quantum mechanics predicts that the coincidence rates between detections of the entangled pair vs. the angle between the polarization measurements can be used to generate a quantity that has a value of 2.8 according to quantum mechanics, while “realistic local” theories predict that the same quantity of 2.0 must be less than. This difference occurs because the coincidence rate predicted by quantum mechanics (and the classical Malus Law of polarization) falls off as the square of angle between the measured linear-polarization directions, while all local theories predict a linear falloff. The mathematical expression of this dichotomy is called Bell's Inequalities or Bell's Theorem.
Since the 1970s, experimentalists have performed many “EPR experiments” based on Bell's Theorem. (See my AV Column, “Einstein's Spooks and Bell's Theorem,"Analog, January 1990.) These experiments have consistently found with high statistical precision that Bell's quantity has a value of 2.8, demonstrating the validity of the nonlocal predictions of quantum mechanics and falsifying alternative theories that are local and “realistic” (see below).
* * * *
But do such EPR experiments actually demonstrate the existence of quantum nonlocality? As it turns out, there is more than one way of interpreting the implications of the EPR experimental results, and there is a dispute as to whether it is locality or “realism” (the objective observer-independent reality of external events) that has been refuted by the EPR measurements. To put it another way, to accommodate the results of the EPR experiments, either one has to accept that there is some mysterious “nonlocal influence” that acts across space-time to force the results of separated measurements to be consistent with each other and with conservation laws, or one has to relinquish the concept of objective reality, the idea that the universe exists with well-defined properties, independent of what we choose to observe and measure. Local realistic theories have been falsified by the EPR experiments based on Bell's Theorem, but is it locality or realism (or both) that has been eliminated?
The reaction of the general physics community to these Bell's Inequality test results has been either (a) to ignore them altogether (as the majority of working physicists seem consistently to do) or (b) to assume objective reality is okay and to admit grudgingly that nonlocality is perhaps an inseparable aspect of quantum mechanics.
However, Noble Laureate Tony Leggett of the University of Illinois has recently pushed this issue somewhat farther. He has demonstrated that by focusing on the falloff of correlations with elliptical polarization rather than the linear polarization of the Bell Inequality EPR experiments, one can compare the predictions of quantum mechanics with a class of nonlocal realistic theories. The resulting Leggett Inequalities can be used in the same way as the Bell Inequalities, but to test nonlocal realism instead of local realism.
A group of experimentalists at the Institute for Quantum Optics and Quantum Information (IQOQI) in Vienna has now performed an EPR experiment that is a definitive test of the Leggett Inequalities, and their results have recently been published in the British science journal Nature. They show that in EPR measurements with elliptically polarized entangled photons, the Leggett Inequalities in two observables are violated by 3.6 and by 9 standard deviations. This in interpreted as a statistically significant falsification of the whole class of nonlocal realistic theories studied by Leggett.
The group summarizes the implications of their results with the statement, “We believe that our results lend strong support to the view that any future extension of quantum theory that is in agreement with experiments must abandon certain features of realistic descriptions.” In other words, quantum mechanics and reality appear to be incompatible.
* * * *
Is the case against objective reality truly so strong? To answer this question, we must examine in more detail the nonlocal realistic theories that Leggett studied. This class of theories assumes that when entangled photons emerge from their emission source, they are in a definite state of polarization. It is well know that when that assumption (and no others) is made, one does not observe the quantum mechanical prediction of Malus's Law for the correlations of the photon pair.
However, Leggett cures that problem by assuming an unspecified nonlocal connection mechanism between the detection systems that fixes the discrepancy. In effect, the two measurements talk to each other nonlocally in such a way that the detected linearly polarized photons obey Malus’ Law and produce the same linear polarization correlations predicted by quantum mechanics calculations. Leggett then shows that this nonlocal “fix” cannot be extended into the realm of elliptical polarization, and that quantum mechanics and this type of nonlocal realistic theories give differing predictions for the elliptic polarization correlations. In other words, the “reality” that is being tested is whether the photon source is initially emitting the entangled photons in a definite state of polarization. It is this version of reality that has been falsified by the IQOQI measurements.
* * * *
We can clarify what is going on in these experimental tests by applying the Transactional Interpretation (TI) of quantum mechanics (See my column, “The Quantum Handshake,” in Analog, November 1986) to these Leggett Inequality tests. As some of you readers may know, I originated the TI in 1986, and it is considered to be one of the leading alternatives to the orthodox Copenhagen Interpretation of quantum mechanics.
From the point of view of the TI, Leggett's assumption that the entangled photons are emitted in definite states of polarization is wrong. The “offer wave” for each photon that emerges from the source includes all possible polarization states. These offer waves travel to downstream detectors, and time-reversed “confirmation waves” travel back up the time-stream to the source, arriving at the instant of emission. A three-way transaction then forms between the source and the two detections that matches the confirmation waves to a mutually consistent overall state that satisfies appropriate conservation laws (in this case, conservation of angular momentum). The final result is a completed transaction with the two photons in definite states, but this definite state was not present in the initial emission of the offer waves, and that is the part of the process described in detail by the wave-mechanics formalism of quantum mechanics. We note that the TI does not in itself make any predictions about the linear or elliptical polarization correlations of the entangled photon pair. It only describes the quantum formalism that is making the predictions that the IQOQI group has observed to be consistent with their experiment, but it clarifies what is going on in those predictions.
Does this mean that the TI (and the quantum formalism it describes) are not “realistic,” i.e. inconsistent with an objective reality that is independent of the observer's choice of measurements? I don't think so. The transactions that form in quantum processes arise from a “handshake” between the past and future across space-time, but they are not specifically the result of measurements or observer choices. The latter are only a small subset of the transactions that form as the universe evolves in space-time. The message of the Leggett Inequality tests, from the point of view of the TI, is that the assumption of emission in a definite polarization state is too restrictive. I would argue that initial emission without a definite polarization state is not inconsistent with objective reality and is consistent with the quantum formalism.
The TI description of the quantum formalism is realistic and nonlocal, in at least some definitions of those terms, and it is completely consistent with the IQOQI results. To put it another way, Leggett has set up a straw man that has been demolished by the IQOQI tests, but that is only an indication that his version of “realism” is too naive. And this theory and experiment can be viewed as another demonstration of the value and power of the TI in understanding the peculiar predictions and intrinsic weirdness of quantum mechanics.
* * * *
Since this is an SF magazine, we
should as usual consider the science-fictional implications of this work. Ignoring my remarks above about the TI, these experimental results could be viewed as reinforcement for the “observer-created quantum reality” (i.e., non-realism) that is an important theme in contemporary SF. This theme goes back at least as far as Greg Bear's pivotal novella Blood Music, a story that concludes with the formation of a planet-wide group mind, an entity that acts as the proverbial 1,000-pound gorilla and can collapse wave functions any way it damn well pleases. Perhaps the “ansible” used by LeGuin, Card, and others might be considered a SF use of nonlocality. With that exception, there has not been much SF written that uses the weirdness of quantum nonlocality in a central way (although I may write some of that soon). The IQOQI tests could be viewed as calling quantum nonlocality into question, but I would argue against that view using the transactional analysis of the IQOQI experiment described above. But, as usual, SF authors are free to work all sides of the street, when it comes to quantum phenomena, and we readers reap the benefits of that diversity.
* * * *
AV Columns Online: Electronic reprints of about 140 “The Alternate View” columns by John G. Cramer, previously published in Analog, are available online at:
www.npl.washington.edu/av
* * * *
References:
Reality Test:
"An experimental test of non-local realism", S. Groblacher, T. Paterek, R. Kaltenbaek, C. Brukner, M. Zukowski, M. Aspelmeyer, and A. Zeilinger, Nature 446, 871-875 (2007); available online at www.arxiv.org/pdf/0704.2529
Anthony J. Leggett, Foundations of Physics 33, 1469 (2003); available online at www.springerlink.com/content/r23275410u4p5q72/fulltext.pdf.
Transactional Interpretation:
John G. Cramer, Reviews of Modern Physics 58, 647 (1986); available online at: npl.washington.edu/npl/intrep/tiqm.
John G. Cramer, International Journal of Theoretical Physics 27, 227 (1988); available online at: npl.washington.edu/npl/intrep/tiover.
Copyright (c) 2007 John G. Cramer
[Back to Table of Contents]
Poetry: A CITY FORGED OF STEEL by GEOFFREY A. LANDIS
On the far edge of the lunar sphere they built strong domes of steel
A city of nickel-iron and platinum ore
A city built by immigrants, this city forged from steel
A city made of dreams from steel forged.
—
They left the earth for Luna, built this city to call home
This city they call Luna is their home.
—
The union men in hard hats stamp the steel from the forge
Iron men pour white-hot metal by the ton
Mass-drivers fling the ingots out; ion freighters bring in ore
The sparks of electric welding bright as sun.
—
And immigrants and union men came to make Luna home
They built a Lunar city to call home.
—
In the long dark lunar nighttime, ion rockets cross the sky
Freighters bring asteroid ore to Luna Field
While in the caverns far below the children learn to fly
In the city made of dreams and hot-rolled steel.
—
And families and children built this city to call home
They make this lunar city that's our home.
—
You see the round and sassy Earth from the cemetery on the hill,
Over a landscape gray with hills and piles of slag
But the reverberating furnaces that once turned ore into steel
One by one are shutting down their trade.
—
And dissidents and exiles build this city we call home
We built this city Luna we call home.
—
The age of Earth is ending, the moon's shipyards slack their pace
The mills that built the colonies shutting down
Freighters still crawl orbital lanes, leviathans of space
But colonists now head for Jupiter and beyond.
—
And you and I together built this city that's our home
We built this Lunar city and called it home.
—
The lunar mills are closing, but the city they built remains
A working town, still working, hard annealed
And planning a new future, of new and better dreams
Forging new plans as hard and hot as steel.
—
Plans to move on outward, we will build new ships of steel
New ships of steel to head into the dark
On fusion flames we'll leave the outer planets in our contrail
Our ships of steel will head out for the stars.
—
And immigrants and dissidents will build starships of our own
And leave behind this city that is our home.
—
We'll launch out from the lunar sphere in new ships forged of steel
With habitats of rock and rebreathed air
New cities, built by dissidents, new cities forged from steel
Our homes are made from dreams and iron ore.
—
And all of us together will build this starship on our own
And bring with us this city that's our home.
—
—for the steelworkers of Cleveland
Copyright (c) 2007 Geoffrey A. Landis
[Back to Table of Contents]
Short Story: SALVATION by JERRY OLTION
Might there be more than one way?
The scientist was sweating. The conference room of the Universal Church of the Divine Revelation was air-conditioned against the muggy Florida heat, but no amount of cool air would comfort the supplicant seated before the board of regents. That suited the Reverend Billy Dickerson just fine. Scientists ought to sweat in the presence of clergy, just as they would sweat in the afterlife to atone for the multitude of sins they had committed against the Church over the past two millennia. Especially scientists like this William Winters, the type of snooty academic who would no more call himself “Billy” than he would get down on his knees to pray to the Lord. His kind were all equations and electrons and tensors and theories—theories presented as if they were facts, when everybody knew that the very word “theory” meant that it was just some egghead's crazy attempt to remove God from the explanation.
Oh yes, let this scientist, this precious William, with his refined mannerisms and his ridiculous ideas, sweat. Let him loosen his tie and wipe his palms on his polyester pants while he tried to impress the regents with his credentials. Let him squirm, because the words he spouted were insults to the Savior and to the men of faith who preached His word.
"You're used to buying scientists,” William was saying. “People who will rubber-stamp your preconceived notions of the way things work. All that does is call your own integrity into question. It's time you bought some actual science for a change."
That bought him a ripple of indignant frowns from the other six regents seated at the conference table, but Billy couldn't resist a smile. He had to admire the man's nerve, if nothing else. This blasphemer had walked into the lion's den on his own initiative, knowing the reception he would get, but he wasn't mincing words in an attempt to curry favor. He was sticking up for his principles, however misguided they might be.
"You're no doubt referring to our recent work on intelligent design,” Billy replied. “You may think what you like, but all our researchers are given complete autonomy in their investigations."
"After you hand-pick them for their adherence to the paradigm you support,” said William. “I'm talking about looking at the facts and accepting what those facts tell you about the way the universe works."
"And these supposed ‘facts’ would be what? Newtonian physics? But no, that was later proven wrong by Einstein. Lamarckian inheritance? No, that was disproven by Mendel and his peas. Evolution? Where are the inte
rmediate forms?"
"Probably in your lungs,” said William.
That caught Billy off guard. “What?"
"There are intermediate forms of viruses and bacteria all around us. They're constantly evolving. Why do you think you need a new flu shot every year?"
"I don't get flu shots,” said Billy. “Vaccination is an evil plot to destroy the minds of our children through mercury poisoning."
"Ah, yes, of course,” said William. “You've caught the A.M.A. red handed. How remarkable. But we were talking about your belief that there were no intermediate forms to prove the validity of evolution. So if you were to catch tuberculosis, may I assume that you would ask your doctor to treat it as if the bacillus hadn't evolved?"
Billy hesitated. That was a trick question. It was like asking a man if he had stopped beating his wife; there was no good answer to it. He considered how he should respond, but Roland Matson was already stepping in to cover his back.
"You didn't come here to lecture us on evolution,” Roland said in the thin, reedy voice that had kept him from developing his own television ministry. “You've asked us to buy science rather than scientists. Why don't you explain that statement instead? What kind of science would you have us purchase?"
"Physics,” said William. “Specifically M-theory, which posits ten spatial dimensions and one time dimension, any of which are accessible with the proper equipment. I'm asking you to fund the construction of that equipment."
Billy found his voice again. “What for?"
"To save humanity.” William let that statement hang for a second before he went on. “The energy level in these extra dimensions is wildly different from the three that we see. And any time you have an energy gradient, you can generate power from it. In this case, enough power to fuel all of human society a million times over, without pollution and without strife."
Analog SFF, December 2007 Page 12
