Paper says all probability is quantum

NewScientist reports:

WHY is there a 1 in 2 chance of getting a tail when you flip a coin? It may seem like a simple question, but the humble coin toss is now at the heart of a lively row about the multiverse. At stake is the ability to calculate which, of an infinite number of parallel universes, is the one that we inhabit.

The debate comes in the wake of a paper posted online a couple of weeks ago by cosmologists Andreas Albrecht and Daniel Phillips, both at the University of California, Davis. They argue that conventional probability theory, the tool we all use to quantify uncertainty in the real world, has no basis in reality (arxiv.org/abs/1212.0953). Instead, all problems in probability are ultimately about quantum mechanics. "Every single time we use probability successfully, that use actually comes from quantum mechanics," says Albrecht.

This controversial claim traces back to the uncertainty principle, which says that it is impossible to know both a quantum particle's exact position and its momentum.

Albrecht and Phillips think particle collisions within gases and liquids amplify this uncertainty to the scale of everyday objects. This, they say, is what drives all events, including the outcome of a coin toss. Conventional probability - which says the outcome simply arises from two equally likely possibilities - is just a useful proxy for measuring the underlying quantum uncertainties. ...

There is just one problem. The Born rule breaks down in some situations. The latest theories in cosmology say that our universe is just one part of a vast multiverse containing a large or even infinite number of other "pocket" universes. Some of those universes will be exact copies of our own, right down to a duplicate you. The mathematics behind the Born rule can't cope with this.

"In these situations, the quantum wave function can tell you nothing about which pocket you are in," says Albrecht. That's a problem if we want to predict the properties of our universe, which will look identical to many others at a given point in time, but which can eventually evolve differently due to quantum uncertainty.

Until now, physicists seeking to predict the properties and behaviour of the multiverse have added a sprinkling of conventional probability to reflect the chance of us being in a particular universe. For example, in a multiverse with just two universes, you might add a 50-50 chance of being in either one, just as we instinctively assign the same odds to a coin toss.

It is baffling how someone could think that probability is some sort of physical thing. Probability is just a mathematical interpretation, and the claims of this paper do not make any sense. And even if probabilistic aspects of observable events are attributable to quantum mechanics, that still would not say anything about the multiverse.

Quantum computing is an open question

Complexity theorist Scott Aaronson reaffirmed that I am banned from his blog, and blocked this comment about quantum computing (QC):

Scott, you are backtracking. Maybe the truth is that no scalable QC is possible. It seems to me that either (1) scalable QC is a consequence of currently established physics, and will be demonstrated someday if civilization does not collapse; (2) scalable QC is a speculative extrapolation of current physics, and its possibility is a open question; or (3) current physics is fundamentally wrong for reasons that are not understood. I thought that you were in camp (2) but now you seem to be in camp (1).

He justifies quantum computing research as being objectively important because he says that an "advanced extraterrestrial civilization" would be doing it also.

I say that scalable QC is an open scientific question. It might be true, and it might be false. Aaronson does not seem to want to accept the idea that it is an open question. He will agree that no one has shown it to possible, but then he will deny that it could be impossible.

New poll on quantum mechanics

Maximilian Schlosshauer, Johannes Kofler, Anton Zeilinger have posted A Snapshot of Foundational Attitudes Toward Quantum Mechanics:

Foundational investigations in quantum mechanics, both experimental and theoretical, gave birth to the field of quantum information science. Nevertheless, the foundations of quantum mechanics themselves remain hotly debated in the scientific community, and no consensus on essential questions has been reached. Here, we present the results of a poll carried out among 33 participants of a conference on the foundations of quantum mechanics. The participants completed a questionnaire containing 16 multiple-choice questions probing opinions on quantum-foundational issues. Participants included physicists, philosophers, and mathematicians. We describe our findings, identify commonly held views, and determine strong, medium, and weak correlations between the answers. Our study provides a unique snapshot of current views in the field of quantum foundations, as well as an analysis of the relationships between these views. ...

The statements that found the support of a majority -- i.e., answers checked by more than half of the participants -- were, in order of the number of votes received:
1. Quantum information is a breath of fresh air for quantum foundations (76%).
2. Superpositions of macroscopically distinct states are in principle possible (67%).
3. Randomness is a fundamental concept in nature (64%).
4. Einstein's view of quantum theory is wrong (64%).
5. The message of the observed violations of Bell's inequalities is that local realism is untenable (64%).
6. Personal philosophical prejudice plays a large role in the choice of interpretation (58%).
7. The observer plays a fundamental role in the application of the formalism but plays no distin-guished physical role (55%).
8. Physical objects have their properties well defined prior to and independent of measurement in some cases (52%).
9. The message of the observed violations of Bell's inequalities is that unperformed measurements have no results (52%). ...

Yet, nearly 90 years after the theory's development, there is still no consensus in the scientific community regarding the interpretation of the theory's foundational building blocks. Our poll is an urgent reminder of this peculiar situation

Don't take the percentages too seriously -- apparently some people have two different ideas for the message of the Bell experiments.

Update: See Lumo and John Preskill for additional opinions. My take-away is that there is still no consensus on matters that I thought were settled 80 years ago.

Why solid matter is solid

The current Radiolab podcast says:

It's comforting to think that if you take an object -- a rock, let's say -- and break it down into tinier and tinier more elemental parts, that that's exactly what you end up with: smaller and smaller particles until you reach the smallest. And voila! Those are the building blocks of everything around us.

But as Jim Holt, author of Why Does the World Exist? points out... that's an old worldview that no longer jives with modern-day science. If you start slicing and sleuthing in subatomic particle land -- trying to get to the bottom of what makes matter -- you mostly find empty space. Your hand, your chair, the floor...it's all made up of mostly of nothing.

I say the old worldview is more accurate.

It is not true to say the modern-day science teaches that atoms are mostly empty space. First, there is no such thing as empty space, as modern physics teaches that it is filled with pervasive fields that used to be called the aether. Second, atoms are held together by nuclear and electromagnetic fields, and those fields fill up atoms and material objects.

Holt goes on to say that solid matter is solid because of the Pauli Exclusion Principle and the Heisenberg uncertainty principle, and not because of the electrical properties of electrons. This is only partially correct, and not a helpful explanation. For a more technical explanation, I suggest this review.

Holt's attitude seems to be that because we have mathematical descriptions of subatomic particles, matter is just ethereal math, and is not substantial. Physicists sometimes talk this way, so he is not just making it up. First, we have not reduced these atoms to pure math. Second, having mathematical approximations has nothing to do with matter being substantial.

Holt's book made the NY Times list of the 10 best books of 2012, and it was the only science book on the list. Maybe I should read his book to get his full story, as maybe he quotes prominent physicists for his goofy ideas.

Prizes for quantum physics

A reader refers to the 2012 Nobel Prize in Physics as proof that quantum decoherence can be measured, and others have claimed that it is progress towards quantum computing. For completeness, I quote the official citation:

On the verge of a new computer revolution
A possible application of ion traps that many scientists dream of is the quantum computer. In present-day classical computers the smallest unit of information is a bit that takes the value of either 1 or 0. In a quantum computer, however, the basic unit of information – a quantum bit or qubit – can be 1 and 0 at the same time. Two quantum bits can simultaneously take on four values – 00, 01, 10 and 11 – and each additional qubit doubles the amount of possible states. For n quantum bits there are 2ⁿ possible states, and a quantum computer of only 300 qubits could hold 2³⁰⁰ values simultaneously, more than the number of atoms in the universe.

Wineland’s group was the first in the world to demonstrate a quantum operation with two quantum bits. Since control operations have already been achieved with a few qubits, there is in principle no reason to believe that it should not be possible to achieve such operations with many more qubits. However, to build such a quantum computer is an enormous practical challenge. One has to satisfy two opposing requirements: the qubits need to be adequately isolated from their environment in order not to destroy their quantum properties, yet they must also be able to communicate with the outside world in order to pass on the results of their calculations. Perhaps the quantum computer will be built in this century. If so, it will change our lives in the same radical way as the classical computer transformed life in the last century.

And also this:

David Wineland and Serge Haroche have invented and implemented new technologies and methods allowing the measurement and control of individual quantum systems with high accuracy. Their work has enabled the investigation of decoherence through measurements of the evolution of Schrödinger’s cat-like states, the first steps towards the quantum computer, and the development of extremely accurate optical clocks.

I say that there is no way that the quantum computer will change our lives in the same radical way as the classical computer transformed life in the last century.

There has to be a better scientific term than "evolution of Schrödinger’s cat-like states".

A couple of other quantum physicists have just won the Israeli Wolf Prize. Here is praise for them and their work on ion traps for quantum computing. I do not know whether their work proves me wrong about anything. I look forward to finding out.

QC is a failed pathological program

Scott Aaronson attacks this Dyakonov paper for its conclusion that quantum computing is a failed, pathological research program, which will soon die out and be of interest only to sociologists:

This is a brief review of the experimental and theoretical quantum computing. The hopes for eventually building a useful quantum computer rely entirely on the so-called "threshold theorem". In turn, this theorem is based on a number of assumptions, treated as axioms, i.e. as being satisfied exactly. Since in reality this is not possible, the prospects of scalable quantum computing will remain uncertain until the required precision, with which these assumptions should be approached, is established. Some related sociological aspects are also discussed.

I do think that QC is a failed research program.

QC starts with the hypothesis that it is impossible to efficiently simulate a quantum system with a classical (Turing) computer. I suspect that is correct. But the leap to scalable QC seems extremely doubtful to me.

Infiniphobia

My last 2012 issue of the NY Times has some new words not in any dictioary: xenointoxication (poisoning the guest) and infiniphobia (fear of infinity). Natalie Angier writes:

Given infinity’s potential for troublemaking, it’s small wonder the ancient Greeks abhorred the very notion of it. ...

On Pythagoras’ Table of Opposites, “the finite” was listed along with masculinity and other good things in life, while “the infinite” topped the column of bad traits like femininity. “They saw it as a cosmic fight,” Dr. Moore said, “with the finite constantly having to subjugate the infinite.”

Aristotle helped put an end to the rampant infiniphobia by drawing a distinction between what he called “actual” infinity, something that would exist all at once, at a given moment — which he declared an impossibility — and “potential” infinity, which would unfold over time and which he deemed perfectly intelligible. As a result, Dr. Moore said, “Aristotle believed in finite space and infinite time,” and his ideas held sway for the next 2,000 years.

Newton and Leibniz began monkeying with notions of infinity when they invented calculus, ...

With his majestic theory of relativity, Einstein knitted together time and space, quashing old Aristotelian distinctions between actual and potential infinity and ushering in the contemporary era of infinity seeking. Another advance came in the 1980s, when Alan Guth introduced the idea of cosmic inflation, a kind of vacuum energy that vastly expanded the size of the universe soon after its fiery birth. ...

Relativity and inflation theory, said Dr. Aguirre, “allow us to conceptualize things that would have seemed impossible before.”

Reading this, it appears that we have not made much progress since the Greeks. No, Einstein did not knit together time and space. That was done by Lorentz and Poincare, and Einstein did not even understand spacetime until after Minkowski's papers became popular. And Einstein had nothing to do with our understanding of infinity, as far as I know.

Guth's theory is just an interesting hypothesis with no hard evidence.

Ms. Angier raves about the mystical aspects of infinity, and sounds as if she is trying to match that Pythagorean image of women. She seems to think that relativity allows infinity because time is not absolute.

She gives the impression that believing in the multiverse is just like Cantor discovering infinite numbers. It is not.

Mathematical analysis is all about the study of the infinite. But the infinities are usually just shorthands for finitary arguments with precise meanings. When physicists talk about infinities, they are usually very sloppy about what is meant. There is no math to support the infinities of the multiverse.

String theorist Lumo explains:

People such as Sean Carroll or Brian Greene correctly notice that the microscopic laws of Nature are time-reversal-invariant (more precisely, CPT-invariant if we want to include subtle asymmetries of the weak nuclear force) but they're overinterpreting or misinterpreting this fact. This symmetry doesn't mean that every statement about the future and past may be simply reverted upside down. It only means that the microscopic evolution of particular microstates – pure states – to particular other microstates – pure states – may be reverted.

But no probabilistic statements may actually be reverted in this naive way.

I posted before that Unitarity is not a fundamental tenet of quantum mechanics, while a reader accused me of arguing from authority.

Belief in unitarity is rooted in the belief that predicting the future is just like predicting the past. That belief is entirely mistaken, as Lumo explains better than I do. I thought that this stuff was obvious, but prominent physicists keep saying crazy things.

Now you might notice that I sometimes quote authorities favorably, and sometimes unfavorably. There is no contradiction. If I am writing about how quantum mechanics has been understood for 80 years, then I quote authorities, because they are the one who define that understanding. But if an expert says something silly, then I criticize it.

In the case of unitarity, I would not mind so much if a physicist said that it was an interesting hypothesis, and wrote a paper exploring the consequences of the hypothesis. It might be true, but it is contrary to the textbooks and contrary to the most common interpretations of the popular experiments. But when a physicist says that it is an essential part of quantum mechanics, he is just wrong.

(Besides the NY Times words, this post has several other words that are not in my dictionary.)