Fulfilling Einstein's dream

Einstein considered that quantum mechanics must only be an approximate theory which was derivable from a "classical" theory which did not have the same philosophical problems. In a 1949 essay, Reply to Criticisms published in response to the essays in Albert Einstein: Philosopher-Scientist he wrote

...Within the framework of statistical quantum theory there is no such thing as a complete description of the individual system. ....The attempt to conceive the quantum-theoretical description as the complete description of the individual systems leads to unnatural theoretical interpretations, which become immediately unnecessary if one accepts the interpretation that the description refers to ensembles of systems and not to individual systems. .... For if the statistical quantum theory does not pretend to describe the individual system (and its development in time) completely, it appears unavoidable to look elsewhere for a complete description of the individual system; in doing so it would be clear from the very beginning that the elements of such a description are not contained within the conceptual scheme of the statistical quantum theory....this scheme could not serve as the basis of theoretical physics. Assuming the success of efforts to accomplish a complete physical description, the statistical quantum theory would, within the framework of future physics, take an approximately analogous position to the statistical mechanics within the framework of classical mechanics. I am rather firmly convinced that the development of theoretical physics will be of this type; but the path will be lengthy and difficult.

Stephen Adler has attempted to fulfill this mission in his book,  Quantum Theory as an Emergent Phenomena: Statistical Mechanics of Matrix Models as the Precursor of Quantum Field Theory
A review of the book by Philip Pearle gives a very helpful summary.  More comments on that later...

Could Quantum Mechanics be wrong?

I find it interesting that there are some physicists who won't even entertain this question. I remember raising it in a "round table" discussion at a conference and people just laughed and did not want to engage with the question. Hence, it is nice that last year Science published a short piece, Is Quantum Theory Exact? by Stephen Adler and Angelo Bassi. 

They discuss a physical collapse  model called the continuous spontaneous localization (CSL) model which involves adding noise terms to the Schrodinger equation in order to produce spontaneous wave function collapse on "macroscopic" scales. It should be stressed that this is a radical proposal involving fundamental new "forces" in the universe. They discuss physical bounds from known experiments for the parameters in the model.

Unfortunately, there are several significant issues that the short article does not mention. 

Most of the dynamical equations and corresponding experimental signatures that these physical collapse models produce are identical to those for decoherence models. Hence, it will be difficult to experimentally distinguish the CSL model from the ubiquitous effects of decoherence from the environment .

Given that decoherence does not solve the measurement problem because of the problem of definite outcomes the physical collapse models seem to me to do only slightly better, invoking the "gamblers ruin" problem to derive the Born rule.

A broader perspective is given in a 2005 Reviews of Modern Physics article, Decoherence, the Measurement Problem, and the Interpretation of Quantum Mechanics by Max Schlosshauer. He reviews the physical collapse models discussing the above issues. His Summary and Outlook is:

Decoherence has the distinct advantage of being derived directly from the laws of standard quantum mechanics, whereas current collapse models are required to postulate their reduction mechanism as a new fundamental law of nature. On the other hand, collapse models yield, at least for all practical purposes, proper mixtures, so they are capable of providing an “objective” solution to the measurement problem. The formal similarity between the time evolution equations of the collapse and decoherence models nourishes hopes that the postulated reduction mechanisms of collapse models could possibly be derived from the ubiquituous and inevitable interaction of every physical system with its environment and the resulting decoherence effects. We may therefore regard collapse models and decoherence not as mutually exclusive alternatives for a solution to the measurement problem, but rather as potential candidates for a fruitful unification.

Proof read that application or paper!

If you are applying for something it is really worth proof reading your application a couple of times and getting someone else to as well. This is a nice and helpful thing that students and postdocs can do for each other. Don't rely on your busy supervisor.

At times I have to review large numbers of applications (jobs, grants, Ph.D proposals, or papers to referee ...). One thing I have noticed that quickly irritates me and some of my senior colleagues is the number of typos or incomplete information in many applications. Maybe it should not matter, but it does have an effect on how people perceive your application.

Also if you are reapplying (or resubmitting) do NOT assume that people will remember the last version and why they asked you to revise for re-submission or re-application. It really helps your case if people can see that you have taken on board the feedback given. Ignoring it can be the kiss of death....

Again, you may not like this or agree with it. But, that is the way things are ....

An indefinite outcome for decoherence

Previously I asked a few fundamental questions about quantum theory. One was:

Why doesn't decoherence solve the quantum measurement problem?

This is a subtle question with subtle answers. I have hesitated on posting on this because the more I read the less sure I am of what the answer is. Basically, it seems there are a few key (distinct but related) aspects to the problem:

• how does a measurement convert a coherent state undergoing unitary dynamics to a "classical" mixed state for which we can talk about probabilities of outcomes?
• why is the outcome of an individual measurement is definite for the "pointer states" of the measuring apparatus?
• can one derive of the Born rule which gives the probability of a particular outcome?

It seems that decoherence only solves the first problem, but not the last two.

An accessible brief summary is given in a Book Review by Anton Zeilinger. He states:

Fullerenes .... showing quantum interference in two-slit experiments whereas they can be seen in a tunnelling electron microscope, for instance, at classically well-defined locations. This shifting boundary is confirmed by the decoherence mechanism. But to argue that this is evidence against the Copenhagen interpretation, as the author does, is unjustified: the Copenhagen interpretation itself says that whether an object is classical or quantum is a function of the chosen experimental set-up.

Decoherence is, to follow physicist John Bell, for all practical purposes sufficient to describe the loss of quantum features for large systems. There are still unanswered questions. It is well known, which Schlosshauer also stresses, that the interference terms never strictly vanish, so decoherence can tell us only that the interference terms disappear effectively but not rigorously. Even after accepting that approximation, we are still left with the system represented as a mixture of various possibilities, like being in two places at once. In the classical world, we know that the system is always at this place or at that place. To explain the two as equivalent is again, for all practical purposes, sufficient. Yet it involves, as Bell points out, another interpretive leap.

A more detailed technical discussion is in Why decoherence has not solved the measurement problem: a response to P.W. Anderson, by Stephen L. Adler.

The legality of physical Laws

Should Newton's laws, the laws of thermodynamics, and Ohm's law all be called Laws?

Phil Nelson gives a nice summary of The Character of Physical Law by Feynman in Biological Physics, Section 5.4. The common features of physical Laws (with a capital L) are:

• They have a very great degree of generality.
• Although they are general, they need not be, and generally cannot be, exact.
• They are intrinsically mathematical in their expression.
• Out of the simplicity of a Law, there always emerge subtle, unexpected, and true conclusions revealed by mathematical analysis.

Problems with impact factors

Seth Olsen brought to my attention an Editorial in the American Chemical Society journal Chemical Biology Deep Impact: Scientific Evaluation by the Numbers. It discusses some of the problems associated with the metric Impact Factors for journals:

Acta Crystallographica—Section A had an impact factor of 2.0 in 2008, which vaulted up to 49.9 in 2009 .... What is even more remarkable is that this rise can be predominantly attributed to citations to a single article published in 2008....

Nature calculated that 25% of published articles contributed to 89% of the journal’s 2005 impact factor.

While on the subject of research metrics the Wikipedia page on the h-index is worth reading.

Can elephants fly?

This week BIPH3001 is reading Life in the slow lane: The Low Reynolds-Number World,

chapter 5 in Biological Physics: Energy, Information, Life, by Phil Nelson.

He begins with the following great quote

Nobody is silly enough to think that an elephant will only fall under gravity if its genes tell it to do so, but the same underlying error can easily be made in less obvious circumstances. So [we must] distinguish between how much behavior, and what part, has a genetic origin, and how much comes solely because an organism lives in the physical universe and is therefore bound by physical laws.

– Ian Stewart, Life’s Other Secret

As with each chapter Nelson begins with a Biological question and a Physical idea:

Biological question: Why do bacteria swim differently from fish?

Physical idea: The equations of motion appropriate to the nanoworld behave differently under time reversal from those of the macroworld.

Figure 5.1 is a picture showing the peculiar character of laminar flow characteristic of a Reynolds number less than one. A really cool video of the same experiment is here.

I also enjoyed a video on Reynolds Number from Sixty Symbols which includes the image above of vortex-antivortex pairs created after a volcano eruption, taken by NASA.