Quantum Mechanics: the Problem of Superposition

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It is the central question in quantum mechanics, and no one knows the answer: What really happens in a superposition—the peculiar circumstance in which particles seem to be in two or more places or states at once? Now, in a forthcoming paper a team of researchers in Israel and Japan has proposed an experiment that could finally let us say something for sure about the nature of this puzzling phenomenon.Their experiment, which the researchers say could be carried out within a few months, should enable scientists to sneak a glance at where an object — in this case a particle of light, called a photon — actually resides when it is placed in a superposition. And the researchers predict the answer will be even stranger and more shocking than “two places at once.”The classic example of a superposition involves firing photons at two parallel slits in a barrier. One fundamental aspect of quantum mechanics is that tiny particles can behave like waves, so that those passing through one slit “interfere” with those going through the other, their wavy ripples either boosting or canceling one another to create a characteristic pattern on a detector screen. The odd thing, though, is this interference occurs even if only one particle is fired at a time. The particle seems somehow to pass through both slits at once, interfering with itself. That’s a superposition.

And it gets weirder: Measuring which slit such a particle goes through will invariably indicate it only goes through one—but then the wavelike interference (the “quantumness,” if you will) vanishes. The very act of measurement seems to “collapse” the superposition. “We know something fishy is going on in a superposition,” says physicist Avshalom Elitzur of the Israeli Institute for Advanced Research. “But you’re not allowed to measure it. This is what makes quantum mechanics so diabolical.”

For decades researchers have stalled at this apparent impasse. They cannot say exactly what a superposition is without looking at it; but if they try to look at it, it disappears. One potential solution—developed by Elitzur’s former mentor, Israeli physicist Yakir Aharonov, now at Chapman University, and his collaborators—suggests a way to deduce something about quantum particles before measuring them. Aharonov’s approach is called the two-state-vector formalism (TSVF) of quantum mechanics, and postulates quantum events are in some sense determined by quantum states not just in the past—but also in the future. That is, the TSVF assumes quantum mechanics works the same way both forward and backward in time. From this perspective, causes can seem to propagate backward in time, occurring after their effects.

But one needn’t take this strange notion literally. Rather, in the TSVF one can gain retrospective knowledge of what happened in a quantum system by selecting the outcome: Instead of simply measuring where a particle ends up, a researcher chooses a particular location in which to look for it. This is called post-selection, and it supplies more information than any unconditional peek at outcomes ever could. This is because the particle’s state at any instant is being evaluated retrospectively in light of its entire history, up to and including measurement. The oddness comes in because it looks as if the researcher—simply by choosing to look for a particular outcome—then causes that outcome to happen. But this is a bit like concluding that if you turn on your television when your favorite program is scheduled, your action causes that program to be broadcast at that very moment. “It’s generally accepted that the TSVF is mathematically equivalent to standard quantum mechanics,” says David Wallace, a philosopher of science at the University of Southern California who specializes in interpretations of quantum mechanics. “But it does lead to seeing certain things one wouldn’t otherwise have seen.”

Take, for instance, a version of the double-slit experiment devised by Aharonov and co-worker Lev Vaidman in 2003, which they interpreted with the TSVF. The pair described (but did not build) an optical system in which a single photon acts as a “shutter” that closes a slit by causing another “probe” photon approaching the slit to be reflected back the way it came. By applying post-selection to the measurements of the probe photon, Aharonov and Vaidman showed, one could discern a shutter photon in a superposition closing both (or indeed arbitrarily many) slits simultaneously. In other words, this thought experiment would in theory allow one to say with confidence the shutter photon is both “here” and “there” at once. Although this situation seems paradoxical from our everyday experience, it is one well-studied aspect of the so-called “nonlocal” properties of quantum particles, where the whole notion of a well-defined location in space dissolves.

In 2016 physicists Ryo Okamoto and Shigeki Takeuchi of Kyoto University verified Aharonov and Vaidman’s predictions experimentally using a light-carrying circuit in which the shutter photon is created using a quantum router, a device that lets one photon control the route taken by another. “This was a pioneering experiment that allowed one to infer the simultaneous position of a particle in two places,” says Elitzur’s colleague Eliahu Cohen of the University of Ottawa in Ontario.

Now Elitzur and Cohen have teamed up with Okamoto and Takeuchi to concoct an even more mind-boggling experiment. They believe it will enable researchers to say with certainty something about the location of a particle in a superposition at a series of different points in time—before any actual measurement has been made.

This time the probe photon’s route would be split into three by partial mirrors. Along each of those paths it may interact with a shutter photon in a superposition. These interactions can be considered to take place within boxes labeled A, B and C, one of which is situated along each of the photon’s three possible routes. By looking at the self-interference of the probe photon, one can retrospectively conclude with certainty the shutter particle was in a given box at a specific time.

superposition-graphic

Credit: Amanda Montañez

The experiment is designed so the probe photon can only show interference if it interacted with the shutter photon in a particular sequence of places and times: Namely, if the shutter photon was in both boxes A and C at some time (t1), then at a later time (t2) only in C, and at a still later time (t3) in both B and C. So interference in the probe photon would be a definitive sign the shutter photon made this bizarre, logic-defying sequence of disjointed appearances among the boxes at different times—an idea Elitzur, Cohen and Aharonov proposed as a possibility last year for a single particle spread across three boxes. “I like the way this paper frames questions about what is happening in terms of entire histories rather than instantaneous states,” says physicist Ken Wharton of San Jose State University, who is not involved in the new project. “Talking about ‘states’ is an old pervasive bias whereas full histories are generally far more rich and interesting.”

That richness, Elitzur and colleagues argue, is what the TSVF gives access to. The apparent vanishing of particles in one place at one time—and their reappearance in other times and places—suggests a new and extraordinary vision of the underlying processes involved in the nonlocal existence of quantum particles. Through the lens of the TSVF, Elitzur says, this flickering, ever-changing existence can be understood as a series of events in which a particle’s presence in one place is somehow “canceled” by its own “counterparticle” in the same location. He compares this with the notion introduced by British physicist Paul Dirac in the 1920s who argued particles possess antiparticles, and if brought together, a particle and antiparticle can annihilate each other. This picture at first seemed just a manner of speaking but soon led to the discovery of antimatter. The disappearance of quantum particles is not “annihilation” in this same sense but it is somewhat analogous—these putative counterparticles, Elitzur posits, should possess negative energy and negative mass, allowing them to cancel their counterparts.

So although the traditional “two places at once” view of superposition might seem odd enough, “it’s possible a superposition is a collection of states that are even crazier,” Elitzur says. “Quantum mechanics just tells you about their average.” Post-selection then allows one to isolate and inspect just some of those states at greater resolution, he suggests. Such an interpretation of quantum behavior would be, he says, “revolutionary” — because it would entail a hitherto unguessed menagerie of real (but very odd) states underlying counterintuitive quantum phenomena.

The researchers say conducting the actual experiment will require fine-tuning the performance of their quantum routers, but they hope to have their system ready to roll in three to five months. For now some outside observers are not exactly waiting with bated breath. “The experiment is bound to work,” says Wharton — but he adds it “won’t convince anyone of anything, since the results are predicted by standard quantum mechanics.” In other words, there would be no compelling reason to interpret the outcome in terms of the TSVF rather than one of the many other ways that researchers interpret quantum behavior.

Elitzur agrees their experiment could have been conceived using the conventional view of quantum mechanics that prevailed decades ago — but it never was. “Isn’t that a good indication of the soundness of the TSVF?” he asks. And if someone thinks they can formulate a different picture of “what is really going on” in this experiment using standard quantum mechanics, he adds, “well, let them go ahead!”

Meditation  for those who want to grok more deeply

An Inconvenient Sequel: Truth to Power – Review

My wife and I attended an event last night at the Greek Theater in Los Angeles, featuring a screening of the new film and an appearance by Al Gore, who gave in an interview and update of what’s been happening since the release of An Inconvenient Truth in 2006.  What I describe below are features and conclusions expressed in the film and by Al Gore during his interview.

The essential truth is that climate change and the warming of the Planet Earth is not a political issue.  It is not hypothetical.  It is not a projection for the future.  It is here.  It is reality.  It has been made a political issue in America by the fossil fuel industry and well-healed and powerful people who are being made richer by denying that reality.  It’s that simple.

The political ploys that they have used are nearly identical to those that were employed by the tobacco industry in the suppression and obfuscation of smoking related health data in the 1980’s.  The results have also been similar.  They have spent over two billion dollars on their campaign to sway public opinion on the issue.  Their efforts have have been somewhat successful.  A significant segment of the population has been successfully misled, which has produced a widespread apathy to to the urgency of the situation and to the issue itself.

Science is a major proponent of truth in our civilization.  It is not inherently wise, but it can tell us when something is broken, and often, how to fix it.  Climate science has already shown us how to fix the climate problem physically.  It has fallen short of helping enough people in power to develop the will to do something about it.

In order to fix that, it has taken a serious advocate, in the person of Al Gore, to champion the endeavor to educate and otherwise shift the awareness of people toward the truth.

The most prominent example of this is Gore’s negotiations with the Indian Prime Minister Narenda Modi during around the time of the global climate conference in 2015.  Modi was reluctant to lend his support for a climate accord because he felt that the Indian economy was not strong enough to shift resources toward the development of new alternative energy technologies and industries.  The need for the developing nation’s increase in energy production was immediate, with no room for mistakes. Modi and Parliament were on the verge of authorizing the building of 400 new coal based generation plants.  Gore realized that that would be a climate catastrophe.  He also realized that it was not their fault for their thinking that way.  India was in a really tough spot.

So, Gore contacted Solar City’s CEO Lyndon Rive about negotiating an agreement with India for the transfer of solar photovoltaic technology to India that would benefit both India and Solar City.  After intense negotiations, the deal was done, and Modi agreed to join the Paris climate accord.  Now that’s deal making.

Gore admitted, both to the audience and in the film, that he was often on the verge of despair regarding the trend of American politics on the matter.  Still, he persevered.  The history of setbacks is long.

One of the first was the cancellation of the NASA DSCOVR project.  It was one of the first casualties of George W Bush’s new administration.  The project was intended to launch a satellite into a solar orbit that is synchronized with the orbit of the earth in order to observe the earth from a constant “full earth” perspective.  It could make make measurements of the earth 24/7 which could then be analyzed to yield useful climate data.  For example, it would be a constant monitor over time of the ratio of incident and reflected energy on the earth.  That would yield an accurate measurement of how much energy is being absorbed by greenhouse gasses and the rise of global temperatures.

Many setbacks have occurred in America with the election of climate denying politicians to government office, and most recently, the appointment of many of them to federal executive cabinet and other high ranking positions.

On the other hand, there seems to be a global trend for the adoption of renewable energy sources.  Even in the US, in Texas, no less, one town proudly touts its 100% reliance on these resources. Some states have nearly reached 100% fossil fuel independence.  Across the world, the adoption and use of renewable energy is accelerating dramatically.  In Chile, in the last year or so, the production of renewable energy has grown by several thousand percent.  China has committed to the movement.

When asked whether a tipping point has been reached in the industrial and political adoption of renewable energy, Gore did not state unequivocally that it has, but he indicated that he thinks it’s inevitable.  He remains hopeful.

There were many examples shown in the movie of devastating events that have occurred worldwide since the last movie that are directly and unequivocally attributable to temperature and climate. Amid all the massive devastation, one event really stood out as a surprising and disturbing data point.

In 2015 a massive deluge was recorded in Tucson.  It seemed to be an aerial view of clouds dumping water (billions of gallons?) on Tucson as if a giant barrel in the sky tipped over.  You could actually make out the splash  of the water on the ground. Not drops. Barrels.

I don’t recall any mention of tipping points with regard to climate change itself.  That is, the point at which the planet will not recover sufficiently to stop the warming progression.  This was a prominent topic earlier on in the discussion.

See movie trailer

See interview at Greek Theater (video shaky, audio good)

 

The Milky Way Galaxy

On the platform of our planet Earth we can view the milky extending across the sky.  Mostly, we see the edge of the flat disk that is the galaxy in which we live.

Published on Apr 28, 2017

This video takes a close look at a new image of the Milky Way released to mark the completion of the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). The APEX telescope in Chile has mapped the full area of the Galactic Plane visible from the southern hemisphere for the first time at submillimetre wavelengths — between infrared light and radio waves — and in finer detail than recent space-based surveys.

 

Albert Einstein: The Negro Question (1946)

LJ_36AALBERT EINSTEIN: THE NEGRO QUESTION (1946)

I am writing as one who has lived among you in America only a little more than ten years. And I am writing seriously and warningly. Many readers may ask:

“What right has he to speak about things which concern us alone, and which no newcomer should touch?”

Continue reading “Albert Einstein: The Negro Question (1946)”