Saturday, June 07, 2008

Seed - The Reality Tests

A very cool article from Seed Magazine last week, The Reality Tests. In essence, it describes new research to study the idea that an observer is necessary for reality to exist (a notion I am more than a little skeptical about).

Here is a taste of what is a very lengthy article.

Some physicists still find quantum mechanics unpalatable, if not unbelievable, because of what it implies about the world beyond our senses. The theory's mathematics is simple enough to be taught to undergraduates, but the physical implications of that mathematics give rise to deep philosophical questions that remain unresolved. Quantum mechanics fundamentally concerns the way in which we observers connect to the universe we observe. The theory implies that when we measure particles and atoms, at least one of two long-held physical principles is untenable: Distant events do not affect one other, and properties we wish to observe exist before our measurements. One of these, locality or realism, must be fundamentally incorrect.

For more than 70 years, innumerable physicists have tried to disentangle the meaning of quantum mechanics through debate. Now Zeilinger and his collaborators have performed a series of experiments that, while neatly agreeing with the theory's predictions, are reinvigorating these historical dialogues. In Vienna experiments are testing whether quantum mechanics permits a fundamental physical reality. A new way of understanding an already powerful theory is beginning to take shape, one that could change the way we understand the world around us. Do we create what we observe through the act of our observations?

Most of us would agree that there exists a world outside our minds. At the classical level of our perceptions, this belief is almost certainly correct. If your couch is blue, you will observe it as such whether drunk, in high spirits, or depressed; the color is surely independent of the majority of your mental states. If you discovered your couch were suddenly red, you could be sure there was a cause. The classical world is real, and not only in your head. Solipsism hasn't really been a viable philosophical doctrine for decades, if not centuries.

But none of us perceives the world as it exists fundamentally. We do not observe the tiniest bits of matter, nor the forces that move them, individually through our senses. We evolved to experience the world in bulk, our faculties registering the net effect of trillions upon trillions of particles or atoms moving in concert. We are crude measurers. So divorced are we from the activity beneath our experience that physicists became relatively assured of the existence of atoms only about a century ago.

Physicists attribute a fundamental reality to what they do not directly perceive. Particles and atoms have observable effects that are well described by theories like quantum mechanics. Single atoms have been "seen" in measurements and presumably exist whether or not we observe them individually. The properties that define particles—mass, spin, etc.—are also thought to exist before we measure them. In physics this is how reality is defined; particles and atoms have measurable properties that exist prior to measurement. This is nothing stranger than your blue couch.

As a physical example, light consists of particles known as photons that each have a property called polarization. Measuring polarization is usually something like telling time; the property can be thought of like the direction of a second hand on a clock. For unpolarized light, the second hand can face any direction as with a normal clock; for polarized light the hand will face in only one or a few directions, as if the clock were broken. That photons can be polarized is, in fact, what allows some sunglasses to eliminate glare—the glasses block certain polarizations and let others through. In Vienna the polarization of light is also being used to test reality.

For a few months in 2006, Simon Gröblacher, who had started his PhD not long before, spent his Saturdays testing realism. Time in the labs at the IQOQI is precious, and during the week other experiments with priority were already underway. Zeilinger and the rest of their collaborators weren't too worried that this kind of experiment would get scooped. They were content to let Gröblacher test reality in the lab's spare time.

For those not familiar with the history behind the questions being addressed by Gröblacher's research, here is a fairly comprehensive account featuring all the major players in quantum theory's origins.
In the summer of 1925, Werner Heisenberg was stricken with hay fever and having trouble with math. He asked his advisor for two weeks off and left for a barren island in the North Sea. He spent his mornings swimming and hiking, but every evening Heisenberg tried to describe atoms in a theory that included only what could be measured. One night, feverish with insight, he calculated until dawn. After Heisenberg put down his pencil as the sun began to rise, he walked to the tip of the island, confident he had discovered quantum mechanics.

By this time a quarter century had passed since Max Planck first described energy as whole-number multiples of a basic unit, which he called the quantum. When two of the quantum's other leading progenitors, Niels Bohr and Albert Einstein, heard about Heisenberg's completion of the work they began, their reactions were almost immediate; Bohr was impressed, Einstein was not. Heisenberg's theory emphasized the discrete, particle-like nature of matter, and Einstein, who tended to think in images, could not picture it in his head.

In Switzerland, Erwin Schrödinger had also been "repelled" by Heisenberg's theory. In the fall of 1925, Schrödinger was 38 years old and rife with self-doubt, but when Einstein sent him an article describing a possible duality between particles and waves, Schrödinger had an idea. Over a period of six months, he published five papers outlining a wave theory of the atom. Though it proved difficult to physically interpret what his wave was, the theory felt familiar to Schrödinger. Heisenberg, who had moved to Copenhagen to become Bohr's assistant, thought the theory "disgusting."

Schrödinger and Heisenberg independently uncovered dual descriptions of particles and atoms. Later, the theories proved equivalent. Then in 1926 Heisenberg's previous advisor, Max Born, discovered why no one had found a physical interpretation for Schrödinger's wave function. They are not physical waves at all; rather the wave function includes all the possible states of a system. Before a measurement those states exist in superposition, wherein every possible outcome is described at the same time. Superposition is one of the defining qualities of quantum mechanics and implies that individual events cannot be predicted; only the probability of an experimental outcome can be derived.

The following year, in 1927, Heisenberg discovered the uncertainty principle, which placed a fundamental limit on certain measurements. Pairs of specific quantities are incompatible observables; momentum and position, energy and time, and other measurable pairs cannot be known together with absolute accuracy. Measuring one restricts knowledge of the other. With this quantum mechanics had become a full theory. But what physicists ended up with was a world divided. There was an inherent distinction between atoms unseen and their collective motion we witness with our eyes—the quantum versus the classical. While the distinction appeared physical, many, like Bohr, thought it philosophical; the theory lacked a proper interpretation.

According to Bohr every measuring device affects what it is used to observe. The quantum world is discrete and so there can never be absolute precision during a measurement. To know about quantum mechanics, we rely on classical devices. To Bohr this implied that the hierarchy between observer and observed had no meaning; they were nonseparable. Concepts once thought to be mutually exclusive, such as waves and particles, were also complements. The difference was only language.

By contrast Einstein was a realist who believed in a world independent of the way it is measured. During a set of conferences at the Hotel Metropole in Brussels, he and Bohr argued famously over the validity of quantum mechanics and Einstein presented a number of thought experiments intended to show the theory incorrect. But when Bohr used Einstein's own theory of relativity to evade one of these thought experiments, Einstein was so stung he never tried to disprove quantum mechanics again, though he continued to criticize it.

In 1935, from an idyllic corner of New Jersey, Einstein and two young collaborators began a different assault on quantum mechanics. Einstein, Podolsky, and Rosen (EPR) did not question the theory's correctness, but rather its completeness. More than the notion that god might play dice, what most bothered Einstein were quantum mechanics' implications for reality. As Einstein prosaically inquired once of a walking companion, "Do you really believe that the moon exists only when you look at it?"

The EPR paper begins by asserting that there's a real world outside theories. "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." If quantum mechanics is complete, then "every element of physical reality must have a counterpart in the physical theory." EPR argued that objects must have preexisting values for measurable quantities and that this implied that certain elements of reality could not be determined by quantum mechanics.

Einstein and his colleagues imagined two electrons that collide and fly apart. After the collision the electrons exist in a state of superposition of the possible values for their momenta. Mathematically and physically, it makes no sense to say that either electron has a definite momentum independent of the other before measurement; they are "entangled." But when one electron's momentum is measured, the value of the other's is instantly known and the superpositions collapse. Once the momentum is known for a particle, we cannot measure its position. This element of reality is denied us by the uncertainty principle. Even stranger is that this occurs even when the electrons fly vast distances apart before measurement. Quantum mechanics still describes the electrons as a single system across space. Einstein could never stomach that an experiment at one electron would instantaneously affect the other.

In Copenhagen Bohr began an immediate response. It didn't matter if particles might affect one another over vast distances, or that particles had no observable properties before they are observed. As Bohr later said, "There is no quantum world. There is only an abstract quantum physical description." Physicists' discourse on reality began just as the world slid inexorably toward war. During WWII physicists once interested in philosophy worried about other issues. David Bohm, however, did worry. After the war Bohm was a professor at Princeton, where he wrote a famous textbook on quantum mechanics. Einstein thought it was the best presentation of quantum mechanics he had read, and when Bohm began to challenge the theory, Einstein said, "If anyone can do it, then it will be Bohm."

In 1952, during the Red Scare, Bohm moved to Brazil. There he discovered a theory in which a particle's position was determined by a "hidden variable" even when its momentum was absolutely known. To Bohm reality was important, and so to preserve it, he was willing to abandon locality and accept that entangled particles influenced one another over vast distances. However, Bohm's hidden variables theory made the same predictions as quantum mechanics, which already worked.

In America Bohm's theory was ignored. But when the Irishman John Bell read Bohm's idea, he said, "I saw the impossible done." Bell thought hidden variables might show quantum mechanics incomplete. Starting from Bohm's work, Bell derived another kind of hidden variables theory that could make predictions different from those of quantum mechanics. The theories could be tested against one another in an EPR-type experiment. But Bell made two assumptions that quantum mechanics does not; the world is local (no distant influences) and real (preexisting properties). If quantum mechanics were correct, one or both of these assumptions were false, though Bell's theorem could not determine which.

Bell's work on local hidden variables theory stirred little interest until the 1970s, when groups lead by John Clauser, Abner Shimony, and others devised experimental schemes in which the idea could be tested with light's polarizations instead of electrons' momentum. Then in 1982 a young Frenchman named Alain Aspect performed a rigorous test of Bell's theory on which most physicists finally agreed. Quantum mechanics was correct, and either locality or realism was fundamentally wrong.

During the 1980s and 1990s, the foundations of quantum mechanics slowly returned to vogue. The theory had been shown, with high certainty, to be true, though loopholes in experiments still left some small hope for disbelievers. However, even to believers, nagging questions remained: Was the problem with quantum mechanics locality, realism, or both? Could the two be tested?

I tend to side, intuitively since I can't possibly understand the math, with the EPR theory. This position is essential in Buddhism. There is an objective reality outside of all our experience and theories about our experience. As Einstein pointed out, the moon does not cease to exist simply because we are not looking at it.

On the other hand, there is something magical about the notion of quantum entanglement.

You can read the whole Seed article here.

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