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Posts Tagged ‘John Stewart Bell

“It can be argued that in trying to see behind the formal predictions of quantum theory we are just making trouble for ourselves”*…

Context, it seems, is everthing…

… What is reality? Nope. There’s no way we are going through that philosophical minefield. Let’s focus instead on scientific realism, the idea that a world of things exists independent of the minds that might perceive it and it is the world slowly revealed by progress in science. Scientific realism is the belief that the true nature of reality is the subject of scientific investigation and while we may not completely understand it at any given moment, each experiment gets us a little bit closer. This is a popular philosophical position among scientists and science enthusiasts.

A typical scientific realist might believe, for example, that fundamental particles exist even though we cannot perceive them directly with our senses. Particles are real and their properties — whatever they may be — form part of the state of the world. A slightly more extreme view is that this state of the world can be specified with mathematical quantities and these, in turn, obey equations we call physical laws. In this view, the ultimate goal of science is to discover these laws. So what are the consequences of quantum physics on these views?

As I mentioned above, quantum physics is not a realistic model of the world — that is, it does not specify quantities for states of the world. An obvious question is then can we supplement or otherwise replace quantum physics with a deeper set of laws about real states of the world? This is the question Einstein first asked with colleagues Podolski and Rosen, making headlines in 1935. The hypothetical real states of the world came to be called hidden variables since an experiment does not reveal them — at least not yet.

In the decades that followed quantum physics rapidly turned into applied science and the textbooks which became canon demonstrated only how to use the recipes of quantum physics. In textbooks that are still used today, no mention is made of the progress in the foundational aspects of quantum physics since the mathematics was cemented almost one hundred years ago. But, in the 1960s, the most important and fundamental aspect of quantum physics was discovered and it put serious restrictions on scientific realism. Some go as far as to say the entire nature of independent reality is questionable due to it. What was discovered is now called contextuality, and its inevitability is referred to as the Bell-Kochen-Specker theorem.

John Bell is the most famous of the trio Bell, Kochen, and Specker, and is credited with proving that quantum physics contained so-called nonlocal correlations, a consequence of quantum entanglement. Feel free to read about those over here.

It was Bell’s ideas and notions that stuck and eventually led to popular quantum phenomena such as teleportation. Nonlocality itself is wildly popular these days in science magazines with reported testing of the concept in delicately engineered experiments that span continents and sometimes involve research satellites. But nonlocality is just one type of contextuality, which is the real game in town.

In the most succinct sentence possible, contextuality is the name for the fact that any real states of the world giving rise to the rules of quantum physics must depend on contexts that no experiment can distinguish. That’s a lot to unpack. Remember that there are lots of ways to prepare the same experiment — and by the same experiment, I mean many different experiments with completely indistinguishable results. Doing the exact same thing as yesterday in the lab, but having had a different breakfast, will give the same experimental results. But there are things in the lab and very close to the system under investigation that don’t seem to affect the results either. An example might be mixing laser light in two different ways.

There are different types of laser light that, once mixed together, are completely indistinguishable from one another no matter what experiments are performed on the mixtures. You could spend a trillion dollars on scientific equipment and never be able to tell the two mixtures apart. Moreover, knowing only the resultant mixture — and not the way it was mixed — is sufficient to accurately predict the outcomes of any experiment performed with the light. So, in quantum physics, the mathematical theory has a variable that refers to the mixture and not the way the mixture was made — it’s Occam’s razor in practice.

Now let’s try to invent a deeper theory of reality underpinning quantum physics. Surely, if we are going to respect Occam’s razor, the states in our model should only depend on contexts with observable consequences, right? If there is no possible experiment that can distinguish how the laser light is mixed, then the underlying state of reality should only depend on the mixture and not the context in which it was made, which, remember, might include my breakfast choices. Alas, this is just not possible in quantum physics — it’s a mathematical impossibility in the theory and has been confirmed by many experiments.

So, does this mean the universe cares about what I have for breakfast? Not necessarily. But, to believe the universe doesn’t care what I had for breakfast means you must also give up reality. You may be inclined to believe that when you observe something in the world, you are passively looking at it just the way it would have been had you not been there. But quantum contextuality rules this out. There is no way to define a reality that is independent of the way we choose to look at it…

Why is no one taught the one concept in quantum physics which denies reality?” It’s called contextuality and it is the essence of quantum physics. From Chris Ferrie (@csferrie).

* “It can be argued that in trying to see behind the formal predictions of quantum theory we are just making trouble for ourselves. Was not precisely this the lesson that had to be learned before quantum mechanics could be constructed, that it is futile to try to see behind the observed phenomena?” – John Stewart Bell

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As still we try, we might relatively hearty birthday greetings to Sir Marcus Laurence Elwin “Mark” Oliphant; he was born on this date in 1901. An Australian physicist who trained and did much of his work in England (where he studied under Sir Ernest Rutherford at the University of Cambridge’s Cavendish Laboratory), Oliphant was deeply involved in the Allied war effort during World War II. He helped develop microwave radar, and– by helping to start the Manhattan Project and then working with his friend Ernest Lawrence at the Radiation Laboratory in Berkeley, California, helped develop the atomic bomb.

After the war, Oliphant returned to Australia as the first director of the Research School of Physical Sciences and Engineering at the new Australian National University (ANU); on his retirement, he became Governor of South Australia and helped found the Australian Democrats political party.

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“Bohr was inconsistent, unclear, willfully obscure, and right. Einstein was consistent, clear, down-to-earth, and wrong.”*…

The founders of quantum mechanics understood it to be deeply, profoundly weird. Albert Einstein, for one, went to his grave convinced that the theory had to be just a steppingstone to a more complete description of nature, one that would do away with the disturbing quirks of the quantum.

Then in 1964, John Stewart Bell proved a theorem that would test whether quantum theory was obscuring a full description of reality, as Einstein claimed. Experimenters have since used Bell’s theorem to rule out the possibility that beneath all the apparent quantum craziness — the randomness and the spooky action at a distance — is a hidden deterministic reality that obeys the laws of relativity.

Now a new theorem has taken Bell’s work a step further. The theorem makes some reasonable-sounding assumptions about physical reality. It then shows that if a certain experiment were carried out — one that is, to be fair, extravagantly complicated — the expected results according to the rules of quantum theory would force us to reject one of those assumptions.

According to Matthew Leifer, a quantum physicist at Chapman University who did not participate in the research, the new work focuses attention on a class of interpretations of quantum mechanics that until now have managed to escape serious scrutiny from similar “no-go” theorems.

Broadly speaking, these interpretations argue that quantum states reflect our own knowledge of physical reality, rather than being faithful representations of something that exists out in the world. The exemplar of this group of ideas is the Copenhagen interpretation, the textbook version of quantum theory, which is most popularly understood to suggest that particles don’t have definite properties until those properties are measured. Other Copenhagen-like quantum interpretations go even further, characterizing quantum states as subjective to each observer…

… which has, as you will see as you read on in the piece excerpted above, some pretty profoundly weird implications. Either the rules of quantum mechanics don’t always apply, or at least one basic assumption about reality must be wrong: “A New Theorem Maps Out the Limits of Quantum Physics.”

See also “Reality is that which, when you stop believing in it, doesn’t go away.”

* John Stewart Bell

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As we pity Schrödinger’s cat, we might we might send penetrating birthday greetings to Henry Way Kendall; he was born on this date in 1926. A particle physicist, he shared the Nobel Prize in Physics in 1990 (with Jerome Isaac Friedman and Richard E. Taylor) “for their pioneering investigations concerning deep inelastic scattering of electrons on protons and bound neutrons, which have been of essential importance for the development of the quark model in particle physics.”

In 1969, Kendall helped found the Union of Concerned Scientists. In 1997, in connection with the Kyoto Climate Summit, he helped produce a statement signed by 2,000 scientists calling for action on global warming.

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