Category Archives: Quantum Mechanics

Nobel Laureates on the QM Interpretation Mess

Update:  Perusing the web I noticed that John Preskill [not yet a Nobel laureate 🙂 ] also blogged on the same survey.  Certainly another prominent voice to add to the mix.


In the LinkedIn discussion to my earlier blog post that was lamenting the disparate landscape of QM interpretation, I had Nobel laureate Gerard ‘t Hooft weighing in:

Don’t worry, there’s nothing rotten. The point is that we all agree about how to calculate something in qm. The interpretation question is something like: what language should one use to express what it is that is going on? The answer to the question has no effect on the calculations you would do with qm, and thus no effect on our predictions what the outcome of an experiment should be. The only thing is that the language might become important when we try to answer some of the questions that are still wide open: how do we quantize gravity? And: where does the Cosmological Constant come from? And a few more. It is conceivable that the answer(s) here might be easy to phrase in one language but difficult in another. Since no-one has answered these difficult questions, the issue about the proper language is still open.

His name certainly seemed familiar, yet due to some very long hours I am currently working, it was not until now that I realized that it was that ‘t Hooft.  So I answered with this, in hindsight, slightly cocky response:

Beg to differ, the interpretations are not more language, but try to answer what constitutes the measurement process. Or, with apologies to Ken, what “collapses the wave function”: The latter is obviously a physical process. There has been some yeoman’s work to better understand decoherence, but ultimately what I want to highlight is that this sate of affairs, of competing QM interpretation should be considered unsatisfactory. IMHO there should be an emphasis on trying to find ways to decide experimentally between them.

My point is we need another John Bell.  And I am happy to see papers like this that may allow us to rule out some many world interpretations that rely on classical probabilities.

So why does this matter?  It is one thing to argue that there can be only one correct QM interpretation, and that it is important to identify that one in order to be able to develop a better intuition for the quantum realities (if such a thing is possible at all).

But I think there are wider implications, and so I want to quote yet another Nobel laureate, Julian Schwinger, to give testament to how this haunted us when the effective theory of quantum electrodynamics was first developed (preface selected papers on QED 1956):

Thus also the starting point of the theory is the independent assignment of properties to the two fields, they can never be disengaged to give those properties immediate observational significance. It seems that we have reached the limits of the quantum theory of measurement, which asserts the possibility of instantaneous observations, without reference to specific agencies.  The localization of charge with indefinite precision requires for its realization a coupling with the electromagnetic field that can attain arbitrarily large magnitudes. The resulting appearance of divergences, and contradictions, serves to deny the basic measurement hypothesis.

John Bell never got one of these, because of his untimely death.

Something is Rotten in the State of Physics.

How else to explain that almost a century after the most successful modern physics theory has been coined leading experts in the field can still not agree on how to interpret it?

Exhibit (A) this bar chart from a survey taken at a quantum foundations meeting.  It has been called the most embarrassing  graph of modern physics (and rightly so).

Screen Shot 2013-02-23 at 11.25.29 AM
















Unsurprisingly, my favorite interpretation of QM, Ulrich Mohrhoff’s Pondicherry Interpretation, is such a dark horse candidate it did not even make the list.

In accordance with this main confusion, the view on the role of the observer is also all over the map:

Screen Shot 2013-02-23 at 11.46.52 AMThe majority settles on a statement that no matter how I try to parse it, doesn’t make any sense to me:  If our formalism describes nature correctly, and the observer plays a fundamental role in the latter, how is it supposed to not occupy a distinguished physical role? The cognitive dissonance to take this stance is dizzying. At least the quantum hippie choice of option (d) has some internal consistency.

So it shouldn’t come as a surprise that with regard to quantum computing these experts are as ignorant as the public at large and completely ignore that D-Wave is already shipping a quantum computer (if the phrasing was about a universal quantum computer these results would have been easier to tolerate).  Invited to opine on the availability of the first working and useful quantum computer this was the verdict:








The paper contains another graph that could almost parse as a work of art, it visualizes the medium to strong correlation between the survey answers.  To me it is the perfect illustration for the current State of Physics with regards to the interpretation of quantum mechanics:

It is a mess.

Given this state of affairs it’s small wonder that one of my heros, Carver Mead, recently described the QM revolution that started in the early last century as an aborted one. It is indeed time to kick-start it again.

The Wave Particle Duality – A Deadly Divide

A particle and its associated wave function.

The curious fact that matter can exhibit wave-like properties (or should this rather be waves acting like particles?) is now referred to as the wave particle duality.  In old times it was often believed that there was some magic in giving something a name, and that it will take some of the christened’s power. Here is to hoping that there may be some truth to this, as this obvious incompatibility has claimed at least one prominent life.

It was Einstein who first made this two-faced character of matter explicit when publishing on the photo electric effect, assigning particle-like characteristics to light that up to this point was firmly understood to be an electromagnetic wave phenomenon.

But just like the question of the true nature of reality, the source of this dychotomy is almost as old as science itself, and arguably already inherent in the very idea of atomism as the other extrem of an all encompassing holism. The latter is often regarded as the philosophical consequence of Schroedinger’s wave mechanics, since a wave phenomenon has no sharp and clear boundaries, and in this sense is often presented as connecting the entirety of the material world. Taken to the extreme, this holistic view finds its perfect expression in Everett’s universal wavefunction (an interpretation happily  embraced by Quantum Hippies of all ages) which gave rise to the now quite popular many worlds interpretation of quantum mechanics.

While atomism proved to be extremely fruitful in the development of physics, it was never popular with religious authorities.  You can find echoes of this to this day if you look up this term at the Catholic Encyclopaedia:

Scholastic philosophy finds nothing in the scientific theory of atomism which it cannot harmonize with its principles, though it must reject the mechanical explanation, often proposed in the name of science, …

Or at this site of religious physicists:

Atomism is incompatible with Judeo-Christian principles because atomism views matter as independent of God, …

Religion of course really doesn’t have a choice in the matter as it can hardly maintain doctrine without some holistic principle.  It is no coincidence that physics only progressed after the cultural revolution of the Renaissance loosened the church’s dominance over the sheeple’s  minds. But history never moves in a straight line.  For instance, with Romanticism the pendulum swung back with a vengeance. It was at the height of this period that Ludwig Boltzmann achieved the greatest scientific breakthrough of atomism when developing statistical mechanics as the proper foundation of thermodynamics. It was not received well. With James Clerk Maxwell having seemingly established a holistic ether that explained all radiation as a wave phenomenon, atomism had thoroughly fallen out of favour.  Boltzmann vigorously defended his work and was no stranger to polemic exchanges to make his point, yet he was beset by clinical depression and feared in the end that his life’s work was for naught. He committed suicide while on a summer retreat that was supposed to help his ailing health.

He must have missed the significance of Einstein’s publication on Brownian Motion just a year early.  It is the least famous of his Annus Mirabelis papers, but it lay the foundation for experimentalists to once and for all settle the debate in Boltzmann’s favor, just a few years after his tragic death.

Thermodynamics made no sense to me before I learned statistical mechanics, and it is befitting that his most elegant equation for the entropy of a system graces the memorial at his grave site (the k denoting the Boltzmann constant).

A physicist can't ask for more to be remembered by than his most fundamental equation.
Ludwig Boltzmann Tombstone in Vienna.

Blog Hole Memory Rescue and Lost Papers that were Really Lost

There is more than one path to classical mechanics.

So much to do, so little time.  My own lost paper work (i.e. the translation of some of Hilbert’s old papers that are not available in English) is commencing at a snail’s pace, but at Kingsley Jones’ blog we can learn about some papers that were truly lost and misplaced and that he only recovered because throughout all the moves and ups and downs of life, his parents have been hanging on to copies of the unpublished  pre-prints.  Kingsley’s post affected me on a deeper level than the usual blog fare, because this is such a parent thing to do.  Having (young) kids myself, I know exactly the emotional tug to never throw away anything they produce, even if they have seemingly moved on and abandoned it.  On the other hand, the recollection of how he found these papers when going through his parent’s belongings after they passed away, brings into sharp relief the fact that I have already begun this process for my father,  who has Alzheimer’s.  So many of his things (such as his piano sheet music) are now just stark reminders of all the things he can no longer do.

On a more upbeat note: The content of these fortuitously recovered papers is quite remarkable.  They expand on a formalism that Steven Weinberg developed, one that essentially allows you to continuously deform quantum mechanics, making it ever less quantum.  In the limit, you end up with a wave equation that is equivalent to the Hamiltonian extremal principal–i.e. you recover  classical mechanics and have a “Schrödinger equation” that always fully satisfies the Ehrenfest Theorem. In this sense, this mechanism is another route to Hamilton mechanicsThe anecdote of Weinberg’s reaction when he learned about this news is priceless.

Ehrenfest’s Theorem, in a manner, is supposed to be common sense mathematically formulated: QM expectation values of a system should obey classical mechanics in the classical limit.  Within the normal QM frameworks this usually works, but  the problem is that sometimes it does not, as every QM textbook will point out (e.g. these lecture notes).  Ironically, at the time of writing, the Wikipedia entry on the Ehrenfest Theorem does not contain this key fact, which makes it kind of missing the point (just another example that one cannot blindly trust Wikipedia content). The above linked lecture notes illustrate this with a simple harmonic oscillator example and make this observation:

“…. according to Ehrenfest’s theorem, the expectation values of position for this cubic potential will only agree with the classical behaviour insofar as the dispersion in position is negligible (for all time) in the chosen state.”

So in a sense, this is what this “classic  Schrödinger equation” accomplishes: a wave equation that always produces this necessary constraint in the dispersion.  Another way to think about this is by invoking the analogy between Feynman’s path integral and the classical extremal principle.  Essentially, as the parameter lambda shrinks for Kingsley’s generalized Schrödinger equation, the paths will be forced ever closer to the classically allowed extremal trajectory.

A succinct summation of the key math behind these papers can be currently found in Wikipedia, but you had better hurry, as the article is marked for deletion by editors following rather mechanistic notability criteria, by simply counting how many times the underlying papers were cited.

Unfortunately, the sheer number of citations is not a reliable measure with which to judge quality. A good example of this is the Quantum Discord research that is quite en vogue these days. It has recently been taken to task on R.R. Tucci’s blog. Ironically, amongst many other aspects, it seem to me that Kingsley’s approach may be rather promising to better understand decoherence, and possibly even put some substance to the Quantum Discord metric.

Lies, Damned Lies, and Quantum Statistics?

Statistics has a bad reputation, and has had for a long time, as demonstrated by Mark Twain’s famous quote[1] that I paraphrased to use as the title of this blog post. Of course physics is supposed to be above the fudging of statistical numbers to make a point.  Well, on second thought, theoretical physics should be above fudging (in the experimental branch, things are not so clear cut).

Statistical physics is strictly about employing all mathematically sound methods to deal with uncertainty. This program turned out to be incredibly powerful, and gave a solid foundation to the thermodynamic laws.  The latter were empirically derived previously, but only really started to make sense once statistical mechanics came into its own, and temperature was understood to be due to the Brownian motion. Incidentally, this was also the field that first attracted a young Einstein’s attention. Among all his other accomplishments, his paper on the matter that finally settled the debate if atoms were for real or just a useful model is often overlooked. (It is mindboggling that within a short span 0f just 40 years (’05-’45) science went from completely accepting the reality of atoms, to splitting them and unleashing nuclear destruction).

Having early on cut his teeth on statistical mechanics, it shouldn’t come as a surprise that Einstein’s last great contribution to physics went back to this field. And it all started with fudging the numbers, in a far remote place, one that Einstein had probably never even heard of.

In the city that is now the capital of Bangladesh, a brilliant but entirely unknown scholar named Satyendra Nath Bose made a mistake when trying to demonstrate to his students that the contemporary theory of radiation was inadequate and contradicted experimental evidence.  It was a trivial mistake, simply a matter of not counting correctly. What added insult to injury, it led to a result that was in accordance with the the correct electromagnetic radiation spectrum. A lesser person may have just erased the blackboard and dismissed the class, but Bose realized that there was some deeper truth lurking beneath the seemingly trivial oversight.

What Bose stumbled upon was a new way of counting quantum particles.  Conventionally, if you have two particles that can only take on two states, you can model them as you would the probabilities for a coin toss. Lets say you toss two coins at the same time; the following table shows the possible outcomes:

    Coin 1
     Head  Tail
 Coin 2  Head  HH  HT
   Tail  TH  TT

It is immediate obvious that if you throw two coins the combination head-head will have a likelihood of one in four.  But if you have the kind of “quantum coins” that Bose stumbled upon then nature behaves rather different.  Nature does not distinguish between the states tails-head and head-tails i.e. the two states marked green in the table.  Rather it just treats these two states as one and the same.

In the quantum domain nature plays the ultimate shell game. If these shells were bosons the universe would not allow you to notice if they switch places.

This means, rather than four possible outcomes in the quantum world, we only have three, and the probability for them is evenly spread, i.e. assigning a one-third chance to our heads-heads quantum coin toss.

Bose found out the hard way that if you try to publish something that completely goes against the  conventional wisdom, and you have to go through a peer review process, your chances of having your paper accepted are almost nil (some things never change).

That’s where Einstein came into the picture.  Bose penned a very respectful letter to Einstein, who at the time was already the most famous scientist of all time, and well on his way to becoming a pop icon (think Lady Gaga of Science).  Yet, against all odds, Einstein read his paper and immediately recognized its merits.  The rest is history.

In his subsequent paper on Quantum Theory of Ideal Monoatomic Gases, Einstein clearly delineated these new statistics, and highlighted the contrast to the classical one that produces unphysical results in the form of an ultraviolet catastrophe. He then applied it to the ideal gas model, uncovering a new quantum state of matter that would only become apparent at extremely low temperatures.

His audacious work set the state for the discovery of yet another fundamental quantum statistic that governs fermions, and set experimental physics on the track to achieving ever lower temperature records in order to find the elusive Bose-Einstein condensate.

This in turn gave additional motivation to the development of better particle traps and laser cooling. Key technologies that are still at the heart of the NIST quantum simulator.

All because of one lousy counting mistake …

[1] Actually the source of the quote is somewhat murky – yet clearly inducted into popular culture thanks to Twain  (h/t to my fact checking commenters).

UPDATE: For some reason Because this site got slashdotted new comments are currently not showing up in my heavily customized WordPress installation – I get to see them in the admin view and can approve them but they are still missing here.

My apologies to everybody who took the time to write a comment! Like most bloggers I love comments so I’ll try to get this fixed ASAP.


For the time being, if you want to leave a comment please just use the associated slashdot story.

The comment functionality has been restored.

Information is Physical

Even when the headlines are not gut-wrenching

Information processing is seldom that physical.

One of the most astounding theoretical predictions of the late 20th century was Landauer’s discovery that erasing memory is linked to entropy i.e. heat is produced whenever a bit is fully and irrevocably erased.  As far as theoretical work goes this is even somewhat intuitively understandable: After all increasing entropy essentially means moving to a less ordered phase state (technically a micro-ensemble that is less special). And what could be possibly be more ordered than a computer memory register?

Recently this prediction has been confirmed by a very clever experiment.  Reason enough to celebrate this with another “blog memory-hole rescue”:

If you ever wondered what the term “adiabatic” in conjunction with quantum computing means, Perry Hooker provides the answer in this succinct explanation. His logic gate discussion shows why Landauer’s principle has implications far beyond the memory chips, and in a sense, undermines the entire foundation of classical information processing.

Truly required reading if you want to appreciate why quantum computing matters.

The Rise of the Quantum Hippies

… and why I blame Niels Bohr.

A satirical hyperbolic polemic

Recently there was a bit of a tempest in a teapot in the LinkedIn quantum physics group because it is very much over-run by members who I dub “Quantum Hippies”.  I.e. the kind of people who think they’ve read a quantum mechanics book after putting down Capra’s the Tao of Physics – you have probably encountered the type.

So this begs the question: Where did they spring from?

It certainly didn’t start with Capra, he was just a catalyst.

I blame this guy:

Niels Bohr stands accused.

If it wasn’t for him, and his side-kick Heisenberg, Bohr’s Copenhagen Interpretation would have never become the kind of dogma that it did.  We are still suffering the consequences.

Science is a competitive sport, even more so in the olden days when the myth of the lone genius reigned supreme.  Most of the founding fathers of quantum mechanics lacked many things but not ego. Much has been written about the struggle between Bohr and Einstein. The latter of course never stood a chance as he has been far removed from the development of the new theory. It didn’t help that he was old at the time and easily painted as a relic. Other challengers to the Copenhagen Interpretation were dealt with in various ways.

  • It was helpful that David Bohm could be vilified as a communist and nobody listened to de Broglie anyway.
  • Schrödinger mocked the new dogma with his famous cat in a box thought experiment but did not have it in him to put up a real fight.
  • Max Planck fell into the same geezer category as Einstein, but was even easier to dismiss due to his far less prominent name recognition.
  • Karl Popper was “just a philosopher”.
  • Others like Landé weren’t much of a challenge, falling into the “Landé who?” category.

Hence the Copenhagen Interpretation reigned supreme, and much energy was now devoted to keep its dirty little secret tucked away, in the closet, under the stairs with the key thrown away.

Maybe some of the energy directed at defending it against the other interpretations was in part motivated by the thought that it’ll be easier to keep this problematic aspect of the new theory under wraps. For whatever reason, Bohr and Heisenberg gave birth to a new physics omertà, the “shut-up and calculate” doctrine.  This would have far reaching consequences – way beyond the realm of physics.

The raison d’être of the hippie revolution was to challenge authority (that arguably was asking for it).

What a delightful gift Bohr had prepared for a counter-culture movement that was already high on half-understood Asian influenced mysticism and other more regulated substances. And so the Copenhagen Interpretation’s dirty little secret was dragged out of the closet and subsequently prostituted.  I am of course referring to the fact that the wave-collapse originally invented by Heisenberg requires an observer or observing mind. This was subsequently bastardized into the idea that “the conscious mind creates reality”. Just as Einstein’s Special and General Relativity entered popular culture as the spectacularly wrong premise that “everything is relative”,  cart blanche for magical thinking was brought to you courtesy of some of the greatest minds of the 20th century.  A more spectacular blow‑back is hard to imagine.

This was super-charged by Bell’s theorem that confirmed quantum mechanics’ essential non-locality.  This in turn was translated as the mystical certainty that “everything is instantaneously connected all the time”.  And so to this day you get spectacularly wrong pop science articles like this one. It completely ignores that these days entangled qbits (the essential ingredient in the thought experiment on which this article hinges) are very well understood as a quantum information resource, and that they cannot facilitate an instantaneous connection between distant events.  The term “instantaneous” has no absolute meaning when Special Relativity is taken into account. This is especially egregious when contemplating that this was published in the American Association of Physics Teacher’s journal.

Although it’s a well-established fact that the public American education system has hit rock bottom in the developed world I still would have expected better.

The Flower Power movement has been generally associated with the left political spectrum but it is in the nature of such powerful memes to eventually permeate the entire mainstream thinking.  Hence American journalists prescribe to a “he said she said” school of journalistic “objectivity”, after all everything’s relative, and so political operatives of all color feel fully justified in subscribing to a “Triumph of the Will” line of thinking.

When Ron Suskind interviewed inside staff from the Bush Jr. administration and questioned them as to how they thought they could just remake Iraq with the limited resources committed, the staffer famously answered: “… when we act, we create our own reality”.

Yes, I blame Niels Bohr for that too.