What quantum feature killed the classical picture?

David Jennings and Matthew Leifer just updated their No Return to Classical Reality

At a fundamental level, the classical picture of the world is dead, and has been dead now for almost a century. Pinning down exactly which quantum phenomena are responsible for this has proved to be a tricky and controversial question, but a lot of progress has been made in the past few decades. We now have a range of precise statements showing that whatever the ultimate laws of Nature are, they cannot be classical. In this article, we review results on the fundamental phenomena of quantum theory that cannot be understood in classical terms. We proceed by first granting quite a broad notion of classicality, describe a range of quantum phenomena (such as randomness, discreteness, the indistinguishability of states, measurement-uncertainty, measurement-disturbance, complementarity, noncommutativity, interference, the no-cloning theorem, and the collapse of the wave-packet) that do fall under its liberal scope, and then finally describe some aspects of quantum physics that can never admit a classical understanding -- the intrinsically quantum mechanical aspects of Nature. The most famous of these is Bell's theorem, but we also review two more recent results in this area.

I agree with them that those other things are not so radically different from classical mechanics, but then they go nuts with the profundity of Bell's Theorem.

The departure of quantum mechanics from classicality was put into a very sharp and powerful form by John Bell [2, 20], who showed that some aspects of quantum entanglement can never fit into a model in which systems possess objective properties prior to measurement and that also obeys a principle of locality. Since the result only depends on certain empirically observed predictions of quantum theory, rather than the structure of the theory itself, any future theory beyond quantum theory will be subject to the same argument, so there can be no going back to a conception of the world that is both classical and local. ...

In the literature, this is often referred to by saying that either “locality” or “realism” must be given up. However you wish to parse the dilemma, it is clear that Bell inequality violations imply a radical departure from classical physics. ...

To sum up, we have shown that many phenomena that are traditionally viewed as intrinsically quantum-mechanical; such as randomness, discreteness, the indistinguishability of states, measurement-uncertainty, measurement-disturbance, complementarity, non-commutativity, interference, the no-cloning theorem, and the collapse of the wave-packet; all appear within classical statistical mechanics under reversible dynamics. These serve to map out classical fragments of quantum physics, in a search for the genuinely strange aspects of the theory. In addition to Bell’s theorem on the failure of local causality at a fundamental level, we have described two less well-known results that reveal further deep and subtle insights into the quantum realm.

Bell's theorem is a consequence of non-commutivity and those other principles. It does not contradict local causality, unless you are using contrived definitions (as the paper does).

If two observables do not commute, then measuring one leaves some uncertainty in the other one. That is the quantum behavior at the core of Bell's theorem. Measuring the position of an electron has the effect of localizing it, and that creates uncertainty in momentum.

Bell wanted to believe that the act of measuring an electron did not necessarily disturb it, and gave the value of some hidden variable that was determined all along. If he were right, then quantum mechanics would be proved wrong. So all he really had was an argument that some alternative theory of hidden variables is wrong.

Lubos Motl's latest rant is about some Christian videos explain quantum mechanics better than the atheist videos. Along the way, he says:

quantum mechanics teaches us that (especially before a measurement) there is no "objective truth about the state of Nature" from which all the knowledge of all observers would be derived as projections or a subset.

Yes, that is right. I am not sure about the religious implications, but that is a core view of quantum mechanics that goes back to Heisenberg, von Neumann, and Dirac. It is not something that Bell discovered decades later.

Saying that Bell's Theorem rules out "realism" seems very profound, until you learn that realism is just defined as some particular hidden variable theory.

John Conway's Life

There is a new book out about a mathematician, and as usual he is portrayed as a mentally ill misfit. Here is the WSJ review:

Even Mr. Conway’s darkest points somehow take a sharp veer into whimsy. In 1993, suffering from heart disease and somehow flat broke on a Princeton salary, he attempted suicide by pills. Upon recovering, he wore a T-shirt around campus that read “SUICIDE” in large block letters, apparently with the intent of diffusing rather than generating awkwardness. (“I wore it for 2 or 3 days until it got too sweaty,” he recalls.) ...

Mr. Conway typifies a popular stereotype of the mathematician: prone to wild enthusiasms, sweaty and wild-bearded, inattentive to the mundanities. Ms. Roberts, to her credit, reminds us that he is as much a social outlier among his colleagues as he would be in the general public; that when he forgets to show up to deliver a lecture, it’s annoying, not charming; that the sincere and profound admiration Mr. Conway enjoys is often tinted with exasperation. This is most notable in the only slightly touched-on subject of his romantic life. “I think John is the most selfish, childlike person I have ever met,” one of his three ex-wives tells Ms. Roberts. “One of the reasons I find that so intolerable is that I know damn well he can be human if he cares enough to bother.”

Hollywood usually treats mathematicians as mentally ill also.

A new physics biographical essay writes:

In the early 1970s, Yuri Golfand was among the discoverers of theoretical supersymmetry, a concept which completely changed mathematical physics in the 21st century. After his discovery, his research institution in Moscow fired him. He knew the humiliations of the Brezhnev regime firsthand, blacklisted and unemployed for the rest of the decade due to his desire to emigrate to Israel.

It calls supersymmetry a "revolutionary concept in theoretical physics". Supersymmetry certainly caused a lot of excitement, but it has been a gigantic dead end. Nothing has come out of that work that has any bearing on the real world. No Nobel Prizes have been given for any work related to supersymmetry. The world is not supersymmetric.

I don't want to minimize his hardships under Communism, but no one else got exit visas either.

Deutsch defends many-worlds philosophy

David Deutsch is one of the chief gurus of the many-worlds interpretation (MWI) of quantum mechanics, and of quantum computing. He says that quantum computing will work because of the efficiency of parallel computation being done in alternate universes.

I suspect that he was considered a crackpot at first, but now that we have 100s of millions of dollars being spent on these dead-ends of physics, he is revered as an insightful genius. He has been mentioned as a candidate for a Nobel Prize, if anyone ever finds any evidence for anything he says.

The MWI has two fatal flaws. First, there is no empirical evidence for it, and there can never be any such evidence. Second, it destroys the probabilistic predictions that are at the heart of quantum mechanics and every other science.

Deutsch has posted a new paper addressing these issues. There is no physics in the article; just philosophical hand-waving:

Claims that the standard methodology of scientific testing is inapplicable to Everettian quantum theory, and hence that the theory is untestable, are due to misconceptions about probability and about the logic of experimental testing. Refuting those claims by correcting those misconceptions leads to various simplifications, notably the elimination of everything probabilistic from fundamental physics (stochastic processes) and from the methodology of testing ('Bayesian' credences).

By "Everettian", he means MWI, and he shortens it to just "quantum theory", as if that were the most sensible interpretation. Copenhagen and other textbook interpretation are called "collapse" variants. The collapse is the idea that you refuse to consider the alternative (unobservable) universes.

Deutsch flips the arguments with a philosophical sleight-of-hand. He credits Karl Popper's rejection of positivism that a good scientific explanation is much more important than a crucial experiment. He agrees with the philosophers who say that there is no such thing as the crucial experiment. MWI doesn't explain any experiments but it does give a good explanation, so he says that it is philosophically superior to collapses.

He goes further and denies that any probabilistic theories are truly testable, and only something like MWI, which says that anything can happen without any probability estimates, should be considered testable. He concludes:

By adopting Popper’s explanatory, conjectural conception of science, and his objective, problem-based methodology of scientific testing (instead of ones that are subjective, inductivist, positivist, ‘Bayesian’ etc.), and bearing in mind the decision-theoretic argument, we can eliminate the perceived problems about testing Everettian quantum theory and arrive at several simplifications of methodological issues in general.

In particular, I have shown that the claim that the standard methods of testing are invalid for Everettian quantum theory depends on adopting a positivist or instrumentalist view of what the theory is about. The claim evaporates, given that science is about explaining the physical world.

Even ‘everything-possible-happens’ theories can be testable. But Everettian quantum theory is more than an everything-possible-happens theory. Because of its explanatory structure (exploited by, for instance, the decision-theoretic argument) it is testable in all the standard ways. It is the predictions of its ‘collapse’ variants (and any theory predicting literally stochastic processes in nature) that are not genuinely testable: their ‘tests’ depend on scientists conforming to a rule of behaviour, and not solely on reality conforming to explanations.

I cannot make any sense of this. All of science involves some sort of comparison of theory with experiment. The measurements never match up exactly, so we are always left with the problem of deciding whether the observations are within the margins of what the theory said was likely. There is no other way to do science, as far as I know.

If a theory gives probabilities and error estimates, as all good scientific theories do, then Deutsch says that it is not testable. If a theory says that everything possible happens, as MWI does, then Deutsch says that it is testable.

There is no merit to anything Deutsch says. Popper was wrong in his rejection of positivism. Duhem-Quine were wrong in their rejection of the crucial experiment. MWI is incoherent. Quantum computing is a pipe dream. You can reverse almost everything he says, and get closer to the truth.

Where are the extraterrestrials?

Dennis Overbye writes in the NY Times that not everyone is excited by the possibility of primitive life on Mars or elsewhere:

In an article published in Technology Review in 2008, Professor Bostrom declared that it would be a really bad sign for the future of humanity if we found even a microbe clinging to a rock on Mars. “Dead rocks and lifeless sands would lift my spirit,” he wrote.

Why?

It goes back to a lunch in 1950 in Los Alamos, N.M., the birthplace of the atomic bomb. The subject was flying saucers and interstellar travel. The physicist Enrico Fermi blurted out a question that has become famous among astronomers: “Where is everybody?”

The fact that there was no evidence outside supermarket tabloids that aliens had ever visited Earth convinced Fermi that interstellar travel was impossible. It would simply take too long to get anywhere.

The argument was expanded by scientists like Michael Hart and Frank Tipler, who concluded that extraterrestrial technological civilizations simply didn’t exist.

The logic is simple. Imagine that one million years from now Earthlings launch a robot to Alpha Centauri, the closest star system to our own. It gets there in a few years, and a million years later sends off probes to two other star systems. A million years after that, each of those sends off two more probes. Even allowing for generous travel times, in 100 million years roughly a nonillion stars (10³⁰) could be visited. The galaxy contains maybe 200 billion stars, so each could be visited more than a trillion times in this robot crisscrossing.

I think that this is correct. If Earth-like planets are common, then it would only take 100M years for an advanced civilization to colonize the galaxy.

It seems reasonable to assume that primitive life has evolved on 100s of other planets in our galaxy. But it is doubtful that any of them evolved into an advanced civilization.

Earth has many strange features that have been essential to human life, and are unlikely elsewhere. We have a single sun, a single large moon to cause tides and stabilize the orbit, a Jupiter to clear out other junk, water covering 2/3 the Earth so sea and land life is possible, etc. We do not know where the water comes from.

Newton studied alchemy

Alchemy is frequently cited as an example of pre-modern pseudo-science, like astrology. This always seemed unfair to me, as alchemists presumably spent most of their time studying properties of materials, and was hence legitimate early chemistry.

Even transmutation, such as trying to turn lead into gold, is not inherently crazy. As we now know, all matter is made of the same quarks and electrons, and there is no law of nature to prevent convert one kind of atom to another. It is just extremely difficult, and only possible today in very tiny quantities in giant particle accelerators.

In a new collection of essays on The Unknown Newton, William R. Newman writes on Newton and alchemy:

For the tercentenary celebration of Newton’s birth, Keynes famously wrote in an address that:

Newton was not the first of the age of reason. He was the last of the magicians, the last of the Babylonians and Sumerians, the last great mind which looked out on the visible and intellectual world with the same eyes as those who began to build our intellectual inheritance rather less than 10,000 years ago.

The thrust of Keynes’s address was that the conventional view of Newton as a “rationalist, one who had taught us to think on the lines of cold and untinctured reason,” was not quite right and that the truth was more complicated: one of the greatest scientists of all time spent a large part of his most creative years on various unscientific quests, including a search for that most elusive of alchemical substances, the philosophers’ stone. ...

Newton’s alchemy fits neither the Keynesian picture of the English natural philosopher as “the last of the magicians” nor the Dobbsian view of his alchemy as a religious quest. Instead, Newton’s alchemical studies reveal an early modern scholar and experimenter hard at work in deciphering extraordinarily difficult texts and a natural philosopher attempting to integrate the fruits of this research into his overall reform of scientific knowledge. Although this view of Newton’s alchemical scholarship and experimentation may be less evocative than Keynes’s or Dobbs’s, it conforms more closely to the depiction of Newton familiar to scholars of his physics, mathematics, and biblical studies. Throughout his divergent activities, Newton remained wedded to techniques of analysis and understanding that would be familiar to most of us today. The apparent incongruity between Newton the scientist and Newton the alchemist dissolves when we acquire a deeper understanding of alchemy and of the man himself.

The other essays examine Newton's religious investigations.

Theories of Everything, Mapped

Quanta magazine reports:

“Ever since the dawn of civilization,” Stephen Hawking wrote in his international bestseller A Brief History of Time, “people have not been content to see events as unconnected and inexplicable. They have craved an understanding of the underlying order in the world.”

In the quest for a unified, coherent description of all of nature — a “theory of everything” — physicists have unearthed the taproots linking ever more disparate phenomena.

The vast majority of these efforts are foolish and misguided, in my opinion, but please click on the link and click "START" for a snazzy map of all these theories on your screen. It looks great. You can waive your mouse over it, and feel as if you are touching some grand idea, all tied in together.

It is just a stupid collection of failed buzzwords, of course, but it looks great.

Wave function can be just our knowledge

Interpretations of quantum mechanics can disagree about whether the wave function Psi is a direct reflection of reality (ontology, ontic) or just a representation of our knowledge (epistemology, epistemic).

Some would also say to "shut up and calculate", and would be dismissive of such philosophical distinctions. Bohr would say that anytime you write formulas on paper, you are just trying to express our knowledge about a system.

A new paper comments on the PBR theorem:

Building upon the Harrigan‐Spekkens analysis, the PBR paper (Pusey, Barrett and Rudolph 2012) raises the question of whether a Ψ‐epistemic interpretation of the Ψ‐function is consistent with QM. According to the theorem proved in the paper, it is not, namely, if the epistemic interpretation is accepted and an overlap of the supports of two distinct probability distributions (corresponding to two distinct quantum states) is allowed, a violation of the predictions QM follows. PBR conclude that QM is not amenable to the epistemic interpretation. This surprising result has immediately attracted a great deal of attention. Most readers have taken the theorem at face value: The Ψ‐epistemic interpretation is indeed ruled out by the theorem and consequently, the remaining option is the Ψ‐ontic interpretations. In other words, the PBR theorem has been advertised as supporting a realist interpretation rather than an epistemic interpretation of Ψ. ...

What if we go radically epistemic and deny the assumption of definite physical states? In that case we take QM to be mute about the physical state of the system, interpreting it instead along the lines of Schrodinger, Pitowsky, Bub, Fuchs and others have suggested, as a maximal catalog of possible measurement results, a betting algorithm, a book‐keeping device. This option is left untouched by the PBR theorem. Not only is it not undermined by it, to the contrary, in ruling out a more classical probabilistic interpretation, which presupposes the existence of the ‘real’ state of the system, the PBR theorem in fact strengthens the radical epistemic interpretation.

So this paper made a big splash because the authors claimed to be disproving an epistemic interpretation, but it does nothing of the kind. It only gives an argument against a hidden variable theory that no one believed in anyway.

The original title to the PBR paper was The quantum state cannot be interpreted statistically, and was accepted for publication in Nature, a very high status journal. This raised eyebrows as the quantum state wave function has been interpreted statistically for 80+ years, and nothing short of a startling Nobel-Prize-winning discovery can change that.

But the title was incorrect use of terminology, and all they had was an argument against replacing quantum mechanics with a hidden variable theory of the type that had been considered and rejected 80 years ago. The argument had nothing to do with statistical interpretations. The title had to be changed, and the paper was published in a lesser journal.