Friday, April 26, 2013

Science cannot rule out free will

Leftist-atheist-evolutionist professor Jerry Coyne blogs frequently about the evils of religion and the superiority of science, and one of his favorite arguments is to attack free will:
Al-Khalili seems to be a compatibilist — that is, he seems to find physical determinism compatible with free will, though he sees quantum mechanics as throwing a wrench into the determinism. I agree: if we reran the tape of the universe, or even the tape of life, I think things would come out differently, for in the origins of the universe, and probably in the origins of new species, true quantum indeterminism plays a role. In the case of life, for instance, it may have a hand in the production of mutations, which are the very fuel of evolution.

But Al-Khalili, unlike some other compatibilists, doesn’t see quantum indeterminacy as rescuing free will. And I don’t think others, do, either—even if that indeterminacy plays out in our brains so that at any given moment we could equally well make either of two decisions. That kind of “quantum” free will is based on pure physical randomness and, to paraphrase Dan Dennett, “is not the kind of free will worth wanting.”

No, Al-Khalili finds free will elsewhere: in unpredictability. That is, our brains are incredibly intricate—they contain roughly ten billion nerve cells, each cell connected to others through about 10,000 synapses (cell-to-cell connections made via chemical or electrical stimuli) — so predicting how a series of environmental inputs will result in a given behavioral output — a decision — can often be impossible. ...

Chaos theory, of course, is deterministic: it’s a theory that simply says that very slight alterations in the initial conditions of a complex system (say, weather patterns) can lead to very different outcomes (whether you get a hurricane). It’s all deterministic, playing out through the non-quantum laws of physics. It’s just that, like the three-body problem, we don’t know enough to work out such systems from first principles.

What baffles me is how you can derive “free will”, if that term has any meaning, from unpredictability. Yes, we can’t predict our decisions, but they still are, according to Al-Khalili, determined by the laws of physics. How does that add up to “freedom” in any meaningful sense? His statement that the choices are “real” choices is ambiguous.
Coyne's argument is essentially that whether the laws of physics are deterministic, chaotic, or random, there cannot be free because all possibilities are contrary to free will.

Ignoring his politics, philosophy, and theology, his physics is wrong. Quantum mechanics teaches that electrons have free will. Physics do not agree on the Interpretations of quantum mechanics, as different philosophies are possible. But there is certain no scientific evidence for ruling out free will.

Wednesday, April 24, 2013

Maybe no mathematical double of the universe

Peter Woit trashes a new book by physicist Lee Smolin, and quotes Smolin saying:
The most radical suggestion arising from this direction of thought is the insistence on the reality of the present moment and, beyond that, the principle that all that is real is so in the present moment. To the extent that this is a fruitful idea, physics can no longer be understood as the search for a precisely identical mathematical double of the universe. That dream must be seen now as a metaphysical fantasy that may have inspired generations of theorists but is now blocking the path to further progress. Mathematics will continue to be a handmaiden to science, but she can no longer be the Queen.
I doubt that I will agree with Smolin's main claims, but I do agree that it is a mistake to believe that physics should be a precisely identical mathematical double of the universe. My FQXi essay explains why.

An Amazon review says:
But it was much worse than that when I realized that the author was leading up to a type of "hidden variables" interpretation of Quantum Mechanics (QM). If you don't know what that term means, don't worry about it, it is just physics jargon for theories that try to replace QM by deterministic approaches that avoid the probabilistic interpretation of it. Based on personal philosophy and even religion, countless people (many of them very prominent physicists themselves) objected to the standard probabilistic interpretation of QM in the last 90 years. Hundreds of alternative deterministic approaches were proposed to replace QM. These theories are termed "hidden variable theories." The better ones actually reproduce most of the predictions of QM. But no hidden variable theory has ever produced identical results to QM for all test cases. When the differences arose in predictions, the experiments backed the predictions by QM irrefutably. As of today, there is not one single hidden variable theory that produces the same results as QM for all experiments. It may yet happen some day, but based on how hard some of the smartest people on Earth have tried and failed for 90 years (including most notably Albert Einstein) to make hidden variable theories work, the prospects are rather dim.

As if that was not bad enough, in the last few chapters the author rejects the concept of "identical particles" in QM.
I haven't seen the book, but pursuit of hidden variable theories is severely misguided, as I have argued here, and criticized Smolin here.

Lubos Motl calls the book An incredible pile of unscientific gibberish. Motl also trashes Aaronson's book here and here, and if you just read those posts, you would get the impression that Motl has a very low opinion of Aaronson. See also the comments, which did not display in my preferred browsers. No, those posts are only mild criticisms compared to the venom for Smolin.

Aaronson is really a smart theoretical computer scientist with a warped view of physics. I am sure that his book is mostly correct, even if it is over-enthusiastic about some ideas. But Smolin subscribes to a philosophy that is antagonistic to modern science. Smolin once gave this definition of science:
Science is not about what's true, or what might be true. Science is about what people with originally diverse viewpoints can be forced to believe by the weight of public evidence.

Sunday, April 21, 2013

Failing to make the inductive leap

Science writer George Johnson raves about The Best Science Book Ever Written, and adds:
Meanwhile I was reminded of a remarkable section in Judson’s The Eighth Day of Creation where he recreates, through Franklin’s journal and other sources, what she knew and when she knew it, every step along the way. He writes of her “grievous slowness of intuitive response,” of her working “head down and doggedly, ingeniously struggling in the wrong direction.” “It is easy to feel great sympathy with Franklin,” he concludes. “The fact remains that she never made the inductive leap.”
Rosalind Franklin did extremely important research on the structure of DNA. She discovered that DNA was a helix with the backbones on the outside, she specified the water content of DNA, and she did the crucial experiment with X-ray crystallography. According to Watson's 1968 memoir, The Double Helix, he and Crick made essential use of her work without her knowledge or permission, and they would have been helpless without it. Their famous 1953 papers avoided explaining how they got their DNA model and gave the false impression that Franklin's work was only to verify what Crick and Watson had already done. The 1962 Nobel Prize went to Crick, Watson, and another guy who hated Franklin and schemed to devalue what she had done. As of last year, Watson was still badmouthing Franklin.

Some people say that Franklin has been maligned because she was a woman. I don't buy it. Lots of women are properly credited. That does not explain why a well-regard science history book would say something so foolish as, “The fact remains that she never made the inductive leap.”

Whatever she may have failed to do is irrelevant to crediting her for what she did do. Her contributions are documented, and there is no dispute about what she did, as far as I know.

Einstein would never be credited for relativity if he were criticize for failing to make the inductive leap. He completely missed the most essential parts of the theory, such as the spacetime geometry and the electromagnetic covariance, and did not understand these concepts were published by others.

Update: Johnson posted a followup, drawing this comment:
Franklin deserved credit for the discovery of the structure of DNA to the degree that a lab tech deserves authorship on a paper. She did essential work that produced the breakthrough. Having said that, she did not make the intellectual breakthrough, and she did not take part in making the intellectual breakthrough. Anyone saying that she deserves to be treated on the same basis as Crick and Watson is doing so for non-scientific reasons - transparently non-scientific reasons. Franklin did excellent lab work. She failed to understand the product of that work. Crick and Watson did. She-would-have, she-could-have hypotheticals are good for novels, but for history, not so much.
I cannot explain this antagonism against Franklin. The annotated Watson Crick paper says:
(5) Here, the young scientists Watson and Crick call their model “radically different” to strongly set it apart from the model proposed by science powerhouse Linus Pauling. This claim was justified. While Pauling’s model was a triple helix with the bases sticking out, the Watson-Crick model was a double helix with the bases pointing in and forming pairs of adenine (A) with thymine (T), and cytosine (C) with guanine (G).
Watson-Crick got that double helix and inward bases from Franklin.

Thursday, April 18, 2013

Physicists in a groupthink bubble

The distinguished physicist Philip Anderson reviews a biography of Freeman Dyson:
But we did not meet until the first energy crisis, when we both attended a workshop on energy that was sponsored by the American Physical Society. Afterwards, we met at disarmament seminars at Princeton University in New Jersey, which is where I first sensed his ambiguity about conventional liberal positions on subjects such as the "Star Wars" defence initiative – most of which I hold unambiguously. ...

Most recently he has delighted in maintaining minority views on a number of topics such as climate, religion (his Christianity places him in the minority for his profession) and genetic modification.
Dyson is a liberal Democrat, and yet Anderson finds a way to criticize him for not being politically correct on all issues.

Dyson and Anderson are two of the most accomplished living 20th century theoretical physicists. This review gives a glimpse of how we might compare them. What I get out of this is that academic physicists live in a leftoid groupthink bubble, with very little deviation tolerated. Why does Anderson have to tell us that he holds conventional liberal positions unambiguously? Is he worried that his fellow might think that any reviewer should distance himself from Dyson's politics? He sounds like a Commie who might say, "I did not deviate from Kremlin policy, and you cannot trust those who do."

My guess is that a physicist would really be an outcast if he endorsed a Republican. While physicists are entitled to their political opinions, of course, it shows that the field is intolerant of critical thinking. They are not experts on global warming, but Anderson has to denounce "Dyson's dreadful misjudgment on the climate question".

Peter Woit explains how Anderson invented the Higgs mechanism, but the high-energy physicists do not like to credit him because of his role in the Democrat cancellation of the Texas Superconducting Super Collider.

Tuesday, April 16, 2013

Electrons have free will

The free will theorem of John H. Conway and Simon B. Kochen says under basic quantum postulates, electrons have free will if humans do. Here is a new interview on it:
Schleicher: Could you make a simple statement about what exactly, or intuitively, the Free Will Theorem says?

Conway: Yes. [Throws a piece of paper.] I just decided to throw that piece of paper on the floor. I don’t believe that that was determined at the start of the big bang, 14 billion years ago. I think it’s ludicrous to imagine that the entire development of the universe, including, say, this interview, was predetermined. For the Free Will Theorem, I assume that some of my actions are not given by predetermined functions of the past history of the universe. A rather big assumption to make, but most of us clearly make it. Now, what Simon and I proved is, if that is indeed true, then the same is true for elementary particles: some of their actions are not predetermined by the entire past history of the universe. That is a rather remarkable thing.

Newton’s theory was deterministic. In the 1920s, Einstein had difficulties believing that quantum mechanics was not deterministic. That was regarded as a defect of quantum mechanics. Certainly when I tried to learn quantum mechanics and didn’t succeed, I thought it was a defect. It’s not a defect. If the theory could predict what one of those particles could do, then that theory would be wrong, because, according to the Free Will Theorem — supposing we do have free will — a particle doesn’t make up its mind what it’s going to do until it does it or until shortly before it does it.

Let me describe the theorem this way. Suppose there is only a very tiny amount of free will in humans: you can press either button A or button B in a manner that is not predetermined. That is a very tiny part of what we normally consider free will for humans. And if we have that tiny amount of free will, so do the elementary particles, in a sense that a particle in response to some experiment can choose which path, C or D, that it follows. It has free action. It chooses C or D in a manner that is not a predetermined function of all the information in the past history of the universe. Schleicher: You believe that humans have free will. Conway: I do. Strict determinism tells us that all of our actions are predetermined by the past history of the universe. I don’t know, maybe it is. I can’t disprove it. I can prove that I can’t disprove it. I can prove that you [points to Schleicher] can’t disprove it either. But I believe anyway that humans have free will.

Schleicher: That is your belief.

Conway: And it is very strong. If you or somebody else doesn’t believe this, I am not going to argue with you, because I know that I can’t disprove the determinist’s position. After giving lectures on this subject in various places, sometimes I have asked whether there were any determinists in the audience. Usually in an audience of a hundred, twenty people put their hands up. They are usually among the most intelligent members of the audience, because it takes some intelligence to disbelieve something that everybody else feels is obvious or to believe something that everybody else feels is ludicrous. Several times people have come up to me and told me they were determinists and expected me to argue the matter. But since I’ve proved that nobody can disprove determinism, what is the point in trying to disprove determinism? I have no argument with determinists or, I should have said, I have no arguments with determinists.

Schleicher: The usual interpretation of quantum mechanics is that the behavior of the elementary particles is simply random.

Conway: You know, randomness doesn’t help. If the action of each particle were a predetermined function of its past plus a random string of bits, then we might as well suppose that this string of bits was produced just before the universe was created, and this is excluded just as well as totally deterministic behavior.
I would phrase it differently, because "free will" is one of those terms that drives philosophers nuts. But his theorem is correct, and restates longstanding understandings of quantum mechanics. Einstein's ideas about determinism are contrary to quantum mechanics, and so are various naive ideas about randomness. Saying that nature is deterministic or random are both very misleading, at best.

Thursday, April 11, 2013

Popper misquoted Einstein

The Indian physicist G. H. Keswani pointed out in 1965 that Poincare never credited Einstein for relativity, and argued that Lorentz and Poincare should be credited for their contributions. This prompted immediate rebuttals by Dingle and Popper. The exchange is summarized here. (See also I, III, reply, reply, "LTE" = Lorentz Transformation Equations)

Popper wrote:
Lorentz and Fitzgerald thought that the contraction of measuring rods is caused by the movement of matter through the ether, ...
Einstein suggested that the contraction is a question of "perspective" rather than a physical contraction in the moving system, and that it is mutual.
Here is what Lorentz said in 1895:
§ 91. As strange as this hypothesis would appear at first sight, nevertheless one must admit that it's not so far off, as soon as we assume that also the molecular forces, similarly as we now definitely can say it of the electrical and magnetic forces, are transmitted through the aether. If this is so, then the translation will change the action between two molecules or atoms most likely in a similar way, as the attraction or repulsion between charged particles. Now, since the shape and the dimensions of a fixed body are, in the last instance, determined by the intensity of the molecular effects, then also a change of the dimensions is inevitable.
Even tho Popper belittles it, this is a brilliant and correct statement. People knew that matter was made of atoms, but the atoms could have been solid objects in physical contact with each other. In fact, they are held together by electromagnetic forces, just as Lorentz predicted.

Karl Popper was a very famous and well-respected philosopher, but his defense of Einstein is based on misquoting him. Einstein never said that the contraction is a question of "perspective". You can check Einstein's famous 1905 paper for yourself. You will not find the word "perspective", and you will not find any disagreement with Lorentz's views. Einstein gave dozens of interviews all his life about how he (supposedly) discovered relativity, and while he acknowledged having read that 1895 Lorentz paper, he never denied that the contraction was a physical contraction.

There are a lot of Einstein idolizers who argue at great length that Einstein's 1905 view was somehow different from Lorentz's, but those arguments are never based on what Einstein actually says. Since Einstein spent a lot of time and effort into trying to explain the merits of his relativity work, it is a strange to claim that he had some view that he never claimed himself.

Einstein was a patent examiner. They are trained to separate work from what went before, and to judge an inventor's originality by his own claims. Inventors only get credit for what they claim for themselves.

Popper says:
Though Einstein appears to have known Poincaré's Science and Hypothesis prior to 1905, there is no theory like Einstein's in this great book. Einstein could not have known Poincaré's article of 1905; and even in this article there is only a most inspiring programme sketched for a relativity theory -- not the theory itself.
Poincare's 1902 book was written for the general public, and does not have any equations. It explains the principle of relativity, and denies the aether, but does not flesh out the theory, as Popper says. Poincare's 1905 article was really two articles -- a 5-page summary and a 50-page detailed paper. Einstein certainly could have known about Poincare's summary as it was published and delivered to Einstein's library three weeks before Einstein submitted his own 1905 relativity paper.

Popper was wrong to say that Poincare's 1905 paper was only a sketch and not a theory. Poincare had concepts like 4-dimensional spacetime geometry and covariance of Maxwell's equations, and these went way beyond what Einstein did.

Herbert Dingle was not so respected:
Ultimately Dingle re-focused his criticism to claim that special relativity was logically inconsistent: "The theory [special relativity] unavoidably requires that A works more slowly than B and B more slowly than A -- which it requires no super-intelligence to see is impossible."
This is indeed a paradox, but Poincare and Minkowski proved that there is no logical inconsistency, by showing that the Lorentz transformations form a group preserving a geometric structure.

Popper has his critics also. A recent paper by Alan B. Whiting argues that in "his most famous work he displays misunderstandings of science and mathematics at a basic level." Popper understood science much better than most philosophers, but maybe that is not saying much.

Much of this is explained in How Einstein Ruined Physics.

Wednesday, April 10, 2013

New Poincare biographies

A biography was published last year, titled: Henri Poincaré: Impatient Genius:
This book describes the life and work of Henri Poincaré, detailing most of his unique achievements in mathematics and physics. It is divided into two parts—the first on Poincaré’s life, and the second on his contributions to the mathematical sciences. Apart from biographical details, attention is given to Poincaré’s results on automorphic functions; differential equations and dynamical systems; celestial mechanics; mathematical physics, in particular the theory of the electron and relativity; and topology (analysis situs). A chapter on philosophy explains Poincaré’s conventionalism in mathematics and his view of conventionalism in physics. The book shows how Poincaré reached his fundamentally new results in many different fields, how he thought about problems, and how one should read his work. Simultaneously, it is made clear how analysis and geometry are intertwined in Poincaré’s thinking and work. In dynamical systems, this becomes clear in his description of invariant manifolds, his association of differential equation flow with mappings, and his fixed-point theory. There is no comparable book on Poincaré presenting such a relatively complete vision of his life and the working of his very original mind. Scientists and engineers as well as general readers interested in the history of science will find this book of interest. Reviews of this book:"The title of this biography is particularly well chosen : Henri Poincaré was a true genius, and he was impatient. It gives a fair picture of both the man and the scientist, completed by particularly well chosen illustrations. Jean Mawhin, Université Catholique de Louvain, Belgium "Ferdinand Verhulst has written a true scientific biography, introducing Poincaré the man, his cultural milieu, and his mathematics. This book shows why, a century after his death, Poincaré's ideas still shape a substantial part of the mathematical sciences." Philip J Holmes, Princeton University, USA
The discovery of relativity is considered one of his minor accomplishments.

Albert Einstein is considered the greatest genius who ever lived, primarily for his 1905 relativity paper. But he got the Lorentz transformations, constancy of the speed of light, local time, relativistic mass, and theorem of corresponding states from Lorentz, and the relativity principle, clock synchronization method, and E=mc2 from Poincare. Meanwhile, Poincare's 1905 relativity paper discovered the Lorentz group, covariance of Maxwell's equations, spacetime geometry, and relativistic gravity. Einstein completely missed these points, and did not even understand them until years later. Today relativity is taught based on Poincare's ideas.

The biography says:
The main priority controversy regarding the new mechanics, replacing Newtonian classical mechanics by relativity, is over special relativity, with prominent candidates Einstein, Lorentz, and Poincaré. [p.62] ...

In this respect, it is difficult to understand why Einstein, when describing the development of relativity in 1949 [Einstein 1950], mentions many scientists, in particular Lorentz, but omits Poincaré. [p.64]
That is a polite way of saying that Einstein lied about the relativity story all his life. The obvious answer is that there was nothing that Einstein could have said about Poincare without diminishing his own status. He could not get away with saying that Poincare's work was deficient, or wrong, or unknown, or not influential, or not original, or anything like that. He just had to pretend that it did not exist.

Another biography last year was Henri Poincaré: A Scientific Biography. A recent AAAS Science magazine review (behind a paywall) says:
Despite his many brilliant interventions and mathematical virtuosity, Poincaré, Gray argues, made no great discovery in physics. Nonetheless, Gray emphasizes that for Poincaré “there is no valid or clear distinction to be made between mathematics and physics because the two are so intimately entangled.”
The book seems to take the view that Poincare's discovery of special relativity as spacetime non-Euclidean geometry was a mathematical discovery, not a physical one, and it was perfected by Minkowski, not Poincare or Einstein. The book says:
He [Poincare] preferred a theory in which space and time were separate and Lorentz contractions really occurred, and argued, quite correctly, that because there could never be conclusions on this view that were incompatible with conclusions derived from a theory of space-time it was simple a matter of convenience which theory was adopted. [p.529]
The book also discusses Poincare's many contributions to celestial mechanics and quantum mechanics.

Tuesday, April 9, 2013

No randomness on the fly

The MyCQstate blog gives a nice summary of attempts to use entanglement for cryptography, and
This credo is best exemplified by the use of entanglement in quantum information: ... This paradigm shift ...

This semester (or what remains of it) at MIT I am organizing a small reading group whose focus will be the study of entanglement as a resource for cryptography. ...

Bell identified the following three reasonable assumptions, that I’ll refer to as in the following:

1. Measurement independence (“free will”):...
2. No-signaling (“no violation of special relativity”): ...
3. Outcome independence (“local realism”):...

Although it was explicitly made only relatively recently (see Chapter 5 in Roger Colbeck’s 2009 Ph.D. thesis), the sole violation of any Bell inequality already has one striking consequence: any physical process whose input/output behavior generates said violation cannot by definition satisfy all three basic assumptions . Provided that we believe in free will (inputs in the experiment are chosen independently of the system’s internal state) and e.g. special relativity (as a way to enforce the no-signaling condition between the system’s two parts), then I claim that the physical process — whatever it is, quantum mechanical or not — must generate randomness on the fly.
Random numbers are useful in cryptography, so if quantum mechanics can find a way to generate and transmit randomness on the fly, then maybe there would be some use to cryptography.

Unfortunately no such application has been found. The problem is in the assumption of local realism, which means that observations must faithfully reflect hidden mathematical variables, and no such variables have been found. The concept of hidden variables is appealing to some people, but it is contrary to how quantum mechanics has been understood for 80 years.

Entanglement is not even necessary for quantum cryptography. See BB84 for an explanation of it without entanglement. So the emphasis on entanglement is misleading.

Quantum cryptography does not solve any problems that are not better solved with conventional cryptography, and has no practical use. The main drawbacks of quantum cryptography are its inability to use routers or provide authentication, in addition to the questionable security. I guess it is being used here as a way of explaining quantum mechanics, but even that it not correct if it is claimed that it generates randomness on the fly.

Testability of many-worlds interpretation

I got this comment below:
You mention the many worlds interpretation, which is an untestable idea about the interpretation of quantum mechanics that pretty much nobody is working on. So I'm not sure that we need to "throw out" many worlds.
Here is Scott Aaronson plugging his new quantum computing book on Lumo's blog, of all places:
Why does David Deutsch (one of the originators of QC) think that a scalable quantum computer would be a powerful demonstration of the truth of the many-worlds interpretation? What are the counterarguments to Deutsch's position?
I haven't read the book yet, but I cite this to show that Deutsch believes many-worlds is testable. Furthermore, many-worlds appears prominently in popular explanations of modern physics, such as Brian Greene's latest book.

I agree that many-worlds is untestable, but it is worse than that, and should really be thrown out like yesterday's newspaper. It has no theoretical value and it explains nothing. It only causes confusion, largely because it makes probabilities meaningless.

I will probably buy the print edition of Aaronson's book, when it comes out in a couple of months. He is an expert on the subject, and explains things well, even if he has trashed me personally. He is a quantum computing enthusiast, and I am a skeptic, but he actually agrees with me that scalable quantum computing is an unproven concept. If you cannot wait for the book, some draft lecture notes are freely available. There is also a funny video commercial with models summarizing his opinion that quantum mechanics is "about information, probabilities, and observables, and how they relate to each other."

He likes to explain quantum mechanics in terms of negative probability. I don't find this view particularly helpful, but maybe I should reserve criticism until I see the book.

Update: It is funny how Aaronson cannot mention Motl without a digression into a denunciation of his politics:
“Let’s be clear: the work of science has nothing whatever to do with consensus. Consensus is the business of politics. Science, on the contrary, requires only one investigator who happens to be right, which means that he or she has results that are verifiable by reference to the real world. In science, consensus is irrelevant. What is relevant is reproducible results. The greatest scientists in history are great precisely because they broke with the consensus.” -Michael Crichton

That Michael Crichton quote is one of the dumbest things I’ve ever read. ...

What do you make of the fact that the two theoretical physicists alive today who are arguably the most “Feynmannian” — Murray Gell-Mann and Steven Weinberg — are both on record strongly supporting the current scientific consensus on AGW, as is Stephen Hawking and pretty much every other celebrated physicist you’ve ever heard of, with Freeman Dyson the sole semi-skeptic that I know about? ...

No, I don’t demand “proof” that a new technology is safe before anyone can use it: “proof” outside of pure mathematics is a concept for crackpots. ... it seems obvious to me that the burden should lie squarely with the deniers, to convince the scientific community that the current emissions levels pose merely an “acceptable” degree of risk. So far, they’ve noticeably failed to do so.
Aaronson is a left-winger, and seems very annoyed that Motl and Dyson do not conform to the so-called consensus.

None of these folks are climate scientists, so they are out of their expertise. Motl frequently posts climate data and arguments on his blog. But Aaronson does not address the substance of any of those posts. He just doesn't like Motl's politics, and complains about a view against the consensus.

Aaronson makes a similar argument for quantum computing. He says that there is a consensus in favor of scalable quantum computing, and the burden of proof should be on the skeptics. That is a weak argument.

Saturday, April 6, 2013

Nonempty vacuum is not crazy

Physicist Lawrence Krauss writes:
Throughout my career I’ve been surprised. Perhaps the most amazing surprise to me was actually one that I ultimately proposed but it defied everything I'd thought before. And that is this amazing result that empty space has energy.

It is so weird to think that you can get rid of all the particles and all the radiation in space and it still weighs something. It seems crazy. And when I was a graduate student, we were all certain that the energy of empty space was zero. And ultimately we were dragged - in fact, I was dragged, kicking and screaming by the observations - to propose this incredibly crazy idea that empty space has energy.

In fact, it’s been observed to have energy. In fact, the people who observed it won the Nobel Prize last year. And I think for me that has changed everything about my understanding of the universe. Both its past, its present, and its future. And I think it’s a wonderful example of how scientists are willing to throw out ideas like yesterday’s newspaper.
This opinion is odd. The Wikipedia article on Zero-point energy says that the concept is a century old, and has been essential to our understanding of quantum mechanics. In the 19th century, it was called the aether. People were skeptical about it being cosmologically observable, and I agree with giving a Nobel Prize for the 1998 astronomy work, but it was not so contrary to conventional wisdom. It was not a crazy idea.

I agree with his point about throwing out failed ideas, but that means throwing out many-worlds interpretation, string theory, Bohm nonlocal mechanics, supersymmetry, black hole firewalls, proton decay, massless neutrinos, etc.

Update: Sabine Hossenfelder posts examples of papers before 1998 suggesting an accelerating cosmological expansion.

Wednesday, April 3, 2013

No entanglement of many qubits

Scott Aaronson is coming out with his own book on quantum computing, and criticizes another book:
Overall, Lance gives an admirably-accurate summary, and I was happy to see him throw cold water on breathless predictions about QC and other quantum-information technologies finding practical applications in the near future.  However, I think he goes beyond the truth when he writes:
[W]e do not know how to create a significant amount of entanglement in more than a handful of quantum bits.  It might be some fundamental rule of nature that prevents significant entanglement for any reasonable length of time.  Or it could just be a tricky engineering problem.  We’ll have to let the physicists sort that out.
The thing is, physicists do know how to create entanglement among many thousands or even millions of qubits — for example, in condensed-matter systems like spin lattices, and in superconducting Josephson junctions.  The problem is “merely” that they don’t know how to control the entanglement in the precise ways needed for quantum computing.  But as with much quantum computing skepticism, the passage above doesn’t seem to grapple with just how hard it is to kill off scalable QC.  How do you cook up a theory that can account for the massively-entangled states that have already been demonstrated, but that doesn’t give you all of BQP?
No, we cannot create entanglement of thousands of qubits. We can entangle thousands of electrons, or even atoms, but not qubits.

His main point is that scalable quantum computing appears to be very difficult or impossible in the lab, but there is no good theory explaining why it should be impossible.