Mermin defends Copenhagen Interpretation

N. David Mermin has written many popular essays explaining quantum mechanics, and now he summarizes his views on how to interpret the theory in Making better sense of quantum mechanics.

He prefers something called QBism, but nearly everything he says could be considered a defense of the Copenhagen interpretation.

Much of the ambiguity and confusion at the foundations of quantum mechanics stems from an almost universal refusal to recognize that individual personal experience is at the foundation of the story each of us tells about the world. Orthodox ("Copenhagen") thinking about quantum foundations overlooks this central role of private personal experience, seeking to replace it by impersonal features of a common "classical" external world.

He is drawing a fairly trivial distinction between his QBism view and Copenhagen. He illustrates with this famous (but possibly paraphrased) Bohr quote::

When asked whether the algorithm of quantum mechanics could be considered as somehow mirroring an underlying quantum world, Bohr would answer "There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature."

Mermin's only quibble with this is that he prefers "each of us can say" to "we can say". That is, he doesn't like the way Bohr lumps together everyone's observations and calls it the classical world.

Okay, I guess that distinction makes a difference when discussing Wigner's Friend, a thought experiment where one observer watches another. But for the most part, Mermin likes the Copenhagen interpretation, and successfully rebuts those who say that the interpretation is deficient somehow.

Correcting errors about EPR paradox

Blake C. Stacey writes about the Einstein–Podolsky–Rosen paradox:

Misreading EPR: Variations on an Incorrect Theme

Notwithstanding its great influence in modern physics, the EPR thought-experiment has been explained incorrectly a surprising number of times.

He then gives examples of famous authors who get EPR wrong.

He gets to the heart of the Bohr-Einstein dispute:

EPR write, near the end of their paper, "[O]ne would not arrive at our conclusion if one insisted that two or more physical quantities can be regarded as simultaneous elements of reality only when they can be simultaneously measured or predicted."

The response that Bohr could have made: "Yes."

EPR briefly considered the implications of this idea and then dismissed it with the remark, "No reasonable definition of reality could be expected to permit this."

But that is exactly what Bohr did. A possible reply in the Bohrian vein: "Could a `reasonable definition of reality' permit so basic a fact as the simultaneity of two events to be dependent on the observer's frame of reference? Many notions familiar from everyday life only become well-defined in relativity theory once we fix a Lorentz frame. Likewise, many statements in quantum theory only become well-defined once we have given a complete description of the experimental apparatus and its arrangement."

This is not a quote from anywhere in Bohr's writings, but it is fairly in the tradition of his Warsaw lecture, where he put considerable emphasis on what he felt to be "deepgoing analogies" between quantum theory and relativity.

In spite of all differences in the physical problems concerned, relativity theory and quantum theory possess striking similarities in a purely logical aspect. In both cases we are confronted with novel aspects of the observational problem, involving a revision of customary ideas of physical reality, and originating in the recognition of general laws of nature which do not directly affect practical experience. The impossibility of an unambiguous separation between space and time without reference to the observer, and the impossibility of a sharp separation between the behavior of objects and their interaction with the means of observation are, in fact, straightforward consequences of the existence of a maximum velocity of propagation of all actions and of a minimum quantity of any action, respectively.

This is well put. The aim of EPR is to explain a simple example of entangled particles, and to argue that no reasonable definition of reality would permit two observables that cannot be simultaneously measured.

And yet that is a core teaching of quantum mechanics, from about 10 years earlier. Two non-commuting observables cannot be simultaneously measured precisely. That is the Heisenberg uncertainty principle.

Theories that assign definite simultaneous values to observables are called hidden variable theories. All the reasonable ones have been ruled out by the Bell Test Experiments.

Complaining that the uncertainty principle violates pre-conceptions about reality is like complaining that relativity violates pre-conceptions about simultaneity. Of course it does. Get with the program.

There are crackpots who reject relativity because of the Twin Paradox, or some other such surprising effect. The physics community treats them as crackpots. And yet the community tolerates those who get excited by EPR, even tho EPR makes essentially the same mistake.

Saying "maximum velocity of propagation" is a way of saying the core of relativity theory, and saying "minimum quantity of any action" is a way of saying the core of quantum mechanics. The minimum is Planck's constant h, or h-bar. The Heisenberg uncertainties are proportional to this constant. That minimum makes it impossible to precisely measure position and momentum simultaneously, just as the finite speed of light makes it impossible to keep clocks simultaneous.

Joint Hubble Lemaitre credit is a bad idea

I mentioned renaming the Hubble Law, as a way to correct history, but it appears that they have made the matter worse.

The respected science historian Helge Kragh writes:

The Hubble law, widely considered the first observational basis for the expansion of the universe, may in the future be known as the Hubble-Lema\^itre law. This is what the General Assembly of the International Astronomical Union recommended at its recent meeting in Vienna. However, the resolution in favour of a renamed law is problematic in so far as concerns its arguments based on the history of cosmology in the relevant period from about 1927 to the early 1930s. A critical examination of the resolution reveals flaws of a non-trivial nature. The purpose of this note is to highlight these problems and to provide a better historically informed background for the voting among the union's members, which in a few months' time will result in either a confirmation or a rejection of the decision made by the General Assembly.

He notes:

Until the mid-1940s no astronomer or physicist seems to have clearly identified Hubble as the discoverer of the cosmic expansion. Indeed, when Hubble went into his grave in 1953 he was happily unaware that he had discovered the expansion of the universe.

He says the cited evidence that Hubble met with Lemaitre is wrong. Furthermore, there are really two discoveries being confused -- the cosmic expansion and the empirical redshift-distance law. Hubble had a role in the latter, but not the former.

Where exactly does probability enter the theory?

Peter Woit writes:

A central question of the interpretation of quantum mechanics is that of “where exactly does probability enter the theory?”. The simple question that has been bothering me is that of why one can’t just take as answer the same place as in the classical theory: in one’s lack of precise knowledge about the initial state.

Lee Smolin says he is writing a book, and there are 3 options: (1) orthodox quantum mechanics, (2) many-worlds, (3) hidden variable theories, like pilot waves. All attempts at (2) have failed, so he says "My personal view is that option 3) is the only way forward for physics."

This is a pretty crazy opinion. No one has been able to makes sense out of probabilities in a many-worlds theory, and Bell test experiments have ruled out all sensible hidden variable theories.

Lubos Motl posts a rant against them, as usual:

Quantum mechanics was born 93 years ago but it's still normal for people who essentially or literally claim to be theoretical physicists to admit that they misunderstand even the most basic questions about the field. As a kid, I was shocked that people could have doubted heliocentrism and other things pretty much a century after these things were convincingly justified. But in recent years, I saw it would be totally unfair to dismiss those folks as medieval morons. The "modern morons" (or perhaps "postmodern morons") keep on overlooking and denying the basic scientific discoveries for a century, too! And this centennial delay is arguably more embarrassing today because there exist faster tools to spread the knowledge than the tools in the Middle Ages.

Lumo is mostly right, but it is possible to blame uncertainties on lack of knowledge of the initial state. It is theoretically possible that if you had perfect knowledge about a radioactive nucleus, then you would know when it would decay.

However it is also true that measurements are not going to give you that knowledge, based on what we know about quantum mechanics. This is what makes determinism more of a philosophical question than a scientific one.

I agree with Lumo that deriving the Born rule is silly. The Born rule is part of quantum theory. Deriving it from something equivalent might please some theorists, but really is just a mathematical exercise with no scientific significance.

This question about the origin of probabilities only makes sense to those who view probably as the essential thing that makes quantum mechanics different from classical mechanics. I do not have that view. Probabilities enter into all of science. It is hard to imagine any scientific theory that can be tested without some resort to a probabilistic analysis. So I don't think that the appearance of probability requires any special explanation. How else would any theory work?

It is very strange that respectable physicists can have such bizarre views about things that were settled about a century ago. I agree with Lumo about that.

Another claim for QC real soon

Latest quantum computer hype:

Today the [Berkeley-based startup Rigetti] launched a project in the mold of Amazon Web Services (AWS) called Quantum Cloud Services. "What this platform achieves for the very first time is an integrated computing system that is the first quantum cloud services architecture," says Chad Rigetti, founder and CEO of his namesake company. The dozen initial users Rigetti has announced include biotech and chemistry companies harnessing quantum technology to study complex molecules in order to develop new drugs. The particular operations that the quantum end of the system can do, while still limited and error-prone, are nearly good enough to boost the performance of traditional computers beyond what they could do on their own -- a coming milestone called quantum advantage. "My guess is this could happen anytime from six to 36 months out," says Rigetti.

My guess is that their investors said that they require results in 6 to 36 months.

There is no chance that this company will have any success before the funding runs out.

Vote to rename law to Hubble-Lemaitre Law

Astronomers have long credited Hubble for discovering the expansion of the universe, even tho he had little to do with it.

If they can decide that Pluto is not a planet, then they can correct this error. Now they will vote on it:

Astronomers are engaged in a lively debate over plans to rename one of the laws of physics.

It emerged overnight at the 30th Meeting of the International Astronomical Union (IAU), in Vienna, where members of the general assembly considered a resolution on amending the name of the Hubble Law to the Hubble-Lemaître Law.

The resolution aims to credit the work of the Belgian astronomer Georges Lemaître and his contribution—along with the American astronomer Edwin Hubble — to our understanding of the expansion of the universe.

While most (but not all) members at the meeting were in favor of the resolution, a decision allowed all members of the International Astronomical Union a chance to vote. Subsequently, voting was downgraded to a straw vote and the resolution will formally be voted on by an electronic vote at a later date.

As the article explains, the Belgian Catholic priest published both the theory and the experimental evidence for it, before Hubble had a clue. Hubble did later publish some data confirming Lemaitre's paper as he had a better telescope, but the data was very crude and not really much better.

It is an amusing historical fact that Einstein, Eddington, and other leading cosmologists clung to the idea of a steady-state universe, while a Catholic priest and Vatican astronomers led the way to convincing everyone that the universe had a beginning in what is now called the Big Bang.

But Hubble was not the first. In 1927, Georges Lemaître had already published an article on the expansion of the universe. His article was written in French and published in a Belgian journal.

Lemaître presented a theoretical foundation for the expansion of the universe and used the astronomical data (the very same data that Hubble used in his 1929 article) to infer the rate at which the universe is expanding.

In 1928, the American mathematician and physicist Howard Robertson also published an article in Philosophical Magazine and Journal of Science, where he derived the formula for the expansion of the universe and inferred the rate of expansion from the same data that were used by Lemaître (a year before) and Hubble (a year after). ...

In January 1930 at the meeting of the Royal Astronomical Society in London, the English astronomer, physicist, and mathematician Arthur Eddington raised the problem of the expansion of the universe and the lack of any theory that would satisfactory explain this phenomenon.

When Lemaître found about this, he wrote to Eddington to remind him about his 1927 paper, where he laid theoretical foundation for the expansion of the universe.

It should be called the Lemaitre Law, or maybe the Lemaitre-Robertson Law, if you want to give an American some credit.