Is the Moon still there when Einstein stops looking?

So two professors from the University of California at Santa Cruz decided to write a book on physics. Sounds like the beginning of a joke, right? A book on the physics of surfing, maybe? No, actually the book is on quantum mechanics. But that is not the worst part – the book is based on physics classes they taught to California liberal arts students!

At this point, most of us would be thinking about moving on down the bookshelf to see what else is available – but that would be a mistake. The professors have written an excellent, useful, thought-provoking book, aimed at making a complex issue accessible to a broad non-specialist audience:

Quantum Enigma: Physics Encounters Consciousness”, by B. Rosenblum & F. Kuttner, ISBN 978-0-19-975381-9 (2011).

The professors point out that the teaching of quantum mechanics to physics students focuses principally on how to apply the complex mathematical formulation of the theory; little attention gets paid to what it all means. When trying to explain quantum mechanics to liberal arts students, that gets reversed.

After some preliminaries, the authors take the reader on a quick tour through the history of the development of the scientific method and its triumph in the testable predictions of Isaac Newton’s classical mechanics. (Interestingly, the determinism of classical mechanics implies there can be no such thing as free will – but that is usually ignored in polite company). The commonsense intuition underlying classical mechanics is that physical reality exists “out there”, independent of the observer. However, classical mechanics could not explain certain observations about the behavior of matter at the atomic scale, as experiments on that became possible around the turn of the 20th Century. Attempts to explain those puzzling experimental observations led eventually to the development of quantum mechanics.

The authors argue that quantum mechanics has been stunningly successful; not a single prediction of the theory has ever been wrong. And it is a key foundation for about one third of the modern economy, through such technologies as lasers, transistors, Charge Coupled Devices in cameras, and Magnetic Resonance Imaging machines. Nevertheless, quantum mechanics has some very odd implications.

The authors discuss the well-known wave-particle duality which has repeatedly been demonstrated for photons, electrons, and even assemblages of atoms. If an experiment is set up to observe a particle, then the observer sees a particle; if instead the experiment is set up to observe a wave, then the observer sees a wave. The real physical state of the entity apparently depends upon the conscious expectation of the observer.

Given the intended audience, there is a good slice of material in this book on the “human interest” angle – the people who developed quantum theory like Max Planck, Albert Einstein, Niels Bohr, John von Neumann, Louis de Broglie, Arthur Compton, Erwin Schrodinger, Werner Heisenberg, Max Born, John Bell. The authors provide a fascinating well-told history. The end result of the years of struggle trying to explain various experimental observations is a quantum theory based on the mathematics of wavefunctions. Such wavefunctions describe all scales of matter, not just the atomic level. Quantum mechanics encompasses classical mechanics as a special case, being a good approximation for larger aggregations of matter.

The enigma about quantum mechanics is that it tells us the reality of the physical world somehow depends on our observation of it – an entirely non-intuitive conclusion. This quantum enigma comes from demonstrable experiments – NOT from quantum theory. And those experiments show us something even less plausible than entities which can sometimes behave as waves and sometimes behave as particles. To quote the authors:

Quantum theory also tells us that an object can be in two places at the same time. Its existence at the particular place where it happens to be found becomes an actuality only upon its observation. Quantum theory thus denies the existence of a physically real world independent of its observation.

Although observation is an essential part of quantum mechanics, what constitutes “observation” is not explained within quantum theory and remains controversial. Einstein had particular concerns about quantum theory’s need for an observer; hence his remark, “I like to think the moon is there even if I am not looking at it”.

For a long time, the conventional “Copenhagen Interpretation” of quantum mechanics swept that enigma under the carpet and did not brook questioning; nowadays, interest in the enigma is no longer a career-limiting move for a young physicist, but there are no accepted answers to the various controversies. It seems that quantum theory inevitably leads to questions about the nature of consciousness and about the existence of free will. The authors try to explain ten of the various approaches which have been proposed to resolve the puzzles.

For this reader, John Bell’s assessment seems the most reasonable: quantum mechanics is not wrong, simply incomplete – a view which is consistent with that advanced by Einstein, Podolsky & Rosen in their 1935 EPR paper. But your mileage may vary.

There have been very few books where my first inclination on reaching the end was to turn back to the beginning and start re-reading it. This fascinating volume is one of those rare books.


A common view today, developed mostly since the 1970s, after Einstein’s time, is that quantum decoherence explains the classical appearance of the macroscopic universe even in the absence of intelligent observers.

To summarise a complicated and mathematically involved analysis, “quantum weirdness” such as a particle being in two places at once, only occurs when the wave function of the particle is not collapsed due to interaction and entanglement with other particles. The requires very special conditions which generally only occur in the microworld or in very specially prepared laboratory conditions. An object like the Moon is interacting innumerable times per second with photons and neutrinos of the cosmic background radiation, photons from the Sun and other stars, galactic cosmic rays, interplanetary and interstellar dust particles. Thus, Einstein doesn’t have to observe the Moon—it is being “observed” quintillions of times a second by interactions with its environment. This is true of almost all macroscopic objects.

Thus, we can ignore quantum mechanics except for the very special conditions in which decoherence does not apply.


That is indeed one of the many theories trying to explain the peculiarities thrown up by quantum mechanics. Whether it is right – time will tell.

Personal “gut feel” view on the margins of physics these days is that we have a shortfall of humility. Our current theories have to be correct. The movement of stars around the galactic core is anomalous – so we invoke “dark matter” (which we cannot observe) to make the observations consistent with the theory. The expansion of the universe is anomalously accelerating – so we invoke “dark energy” (which we cannot explain) to make the observations consistent with the theory. Is quantum decoherence cut from the same cloth? I for one do not have the knowledge or understanding to say.

We do know that science has been corrupted – Catastrophic Anthropogenic Global Warming is proof positive of that. It is quite possible that the problem has spread to other areas, especially those parts of science which rely on massive government funding.

Let’s all keep open minds. We can be proud of what the human race has learned, but still accept that we may not yet have reached the end of the road.


I don’t see much evidence for a lack of humility or blindness to alternative theories. After all, the surest way to win a Nobel prize is to propose an alternative theory that makes a novel prediction which is subsequently confirmed by experiment. In the case of the rotation of galaxies, Mordehai Milgrom’s Modified Newtonian Dynamics (MOND) was published in 1982 and remains in contention today as an explanation of a variety of phenomena for which dark matter has been proposed. It models gravitational rotation curves very well, but it has trouble explaining the dynamics of galaxy clusters and some gravitational lensing phenomena, and so remains a minority view, but is in no sense fringe science.

Similarly, there are numerous theories which explain the apparent observation of accelerated expansion of the universe without resort to dark energy. Over the years, however, precision measurements of the cosmic background radiation have ruled out or severely constrained the ability of many of these theories to explain what is observed.

This is how science is supposed to work: a curious observation is made, multiple theories are proposed to explain it, and subsequent observation and/or experiment tests those theories, many of which fall by the wayside as they fail to agree with observation. I don’t think there is a cabal promoting dark matter and energy to the detriment of other theories, but simply a majority who believe they are the best explanation for the observational data we have collected to date.

Before the Type 1A supernova data provided evidence for acceleration of the expansion of the universe, I don’t think there was one in a thousand people working in cosmology who expected or predicted that result. There was a universal consensus that gravity was slowly decelerating cosmic expansion, and the only question was how fast. And yet when the data came in, from two independent collaborations of researchers, with a little more than a year the new phenomenon was acknowledged as existing and then the puzzle became how to explain it.

I think the best evidence for humility among astrophysicists and cosmologists is that they readily acknowledge that around 95% of the mass-energy of the universe is made of up of stuff about whose origin and composition they know absolutely nothing. Arrogance would be denying the evidence because it conflicts with their theories. Humility is accepting the evidence and admitting you don’t have a theory to explain it.


That is the scientific standard. I should be careful not to tar the whole scientific community with the same brush with which the climate changers and Covid fear-mongers have daubed themselves. And we should all be careful about subscribing to majority views in science – that way lies Algore’s fatuous “settled science”.

To return to this fascinating book, the authors point out that Einstein’s proposal of the quantized photon as an explanation for the puzzling photo-electric effect was widely derided, even by physicists of the caliber of Robert Millikan. When Planck nominated Einstein for membership in the Prussian Academy of Science, he begged the committee not to hold Einstein’s hypothesis of the light quantum against him. Even when Einstein was awarded the 1923 Nobel Prize for his work on the photelectric effect, the citation avoided explicit mention of the controversial photon. The authors note that for 18 years from 1905 to 1923, Einstein was a man apart in being the only one, or almost the only one, to take the light-quantum seriously.

I head a talk by Prof. Alex Filippenko, who worked on one of the two separate teams which did the studies on the distant Class 1A supernovas. He said it was very fortunate that (coincidentally) a second team independently reported similar data at the same time. Had only one team reported those observations, he suspected the resistance to their awkward findings would have been high.

At its best, the scientific community keeps an open mind and recognizes that science is never settled. That is the approach to which we should all aspire.



Thank you for this excellent review. I’m glad you found the book fascinating, as I did.

The invention of “dark matter” seems similar to Ptolemy’s creation of epicycles to explain the retrograde motion of planets. The Ptolemaic system can predict the movement of planets from the vantage point of Earth pretty well, but it is not how the planets actually move. Likewise, dark matter might help us explain other eccentricities we observe in outer space, but it does not mean that dark matter actually exists. Is this a fair comparison?

Edit: Despite relying on epicycles, the Ptolemaic system lasted a long time because its predictions were good enough for most practical purposes. Even Copernicus used epicycles in his heliocentric model of the solar system. It was not until Kepler supplanted circular orbits with elliptical ones that epicycles were abandoned. Perhaps our contrived “dark matter” will last a long time, too. I suppose it will depend on whether it is capable of explaining newly discovered anomalies or not. When the anomaly to explainable phenomenon ratio grows uncomfortably large, we might be due for a paradigm shift.


Not really. Epicycles were introduced to explain why the orbits of planets were not the perfect circles philosophers expected them to be. By adding an epicycle, you can closely approximate the real, elliptical orbits. In fact, Copernicus retained epicycles in his system because he had not discovered elliptical orbits. It was only after Kepler worked out the laws of planetary motion that epicycles were discarded.

Dark matter is not a theoretical patch but rather based upon observational discrepancies between the motions of objects at large scales and the predictions of gravitational theory. There are only two possible resolutions of these discrepancies: either there is some invisible matter out there or our theory of gravitation is wrong at these scales. Many theorists have explored modifications to gravity which might explain the observations: MOND, discussed in an earlier comment, is just one of them. The problem is that general relativity has been tested to great precision on scales ranging from the laboratory to cosmology with no disagreement with experiment. It is extraordinarily difficult to modify the theory to accommodate the discrepant observations without breaking it in a way that it doesn’t work at already well-tested scales. This is the primary motivation for suggesting the presence of unseen matter.

The effects of dark matter are seen at widely different scales: galactic rotation, the motion of galaxies within clusters, structure formation in the early universe, and anisotropies in the cosmic background radiation. Most alternative explanations fail to explain all of these, while cold dark matter is in perfect agreement with them. Thus, even though we have not yet detected dark matter in the laboratory (numerous experiment are underway, some with ambiguous results), it seems the most parsimonious explanation for the observations.

Situations like this have happened before. In 1930, Wolfgang Pauli proposed the neutrino to explain what appeared to be violation of conservation laws in nuclear beta decay. The solution was so “sweet” and explained so many other subsequently-discovered phenomena that almost all physicists considered it a real particle. But because its interaction with other matter is so weak, it was not until 1956 that the neutrino was detected being generated by a nuclear reactor, which did not exist when Pauli proposed the particle. The detection of the neutrino won the 1995 Nobel Prize in physics, 65 years after it was introduced in theory.


Thank you, Magus – that really was an excellent recommendation. I greatly enjoyed reading the book – twice!

Book recommendations are always tricky, because what one person finds fascinating another may find dull. And I am not just thinking about Stephen King! If there are other books that have caught your fancy, please do not hesitate to share.


Indeed. And when observation fails to match theory, it will always be a challenge to find out whether the issue is a failing in the theory or some hidden variable which would make the data fit the theoretical model correctly if it were included. One could argue that epicycles were the hidden variable which had to be incorporated to make observations match the theory that every object in the perfect heavens had to move in a perfect circle.

The issue of paradigms suggests that we human beings tenaciously (and often reasonably) hang on to theories which have worked in the past. Being aware of that history, we should always be cautious when we have to invoke a hidden variable to make reality fit our accepted theory. And we should make every effort to “unhide” that hidden variable.


And a number of such efforts are underway. For example, eight tonnes of liquid xenon beneath a mountain in Italy or the DAMA/LIBRA experiment discussed here on 2022-06-03.

Here are summaries of experiments for:

In addition, collisions at the Large Hadron Collider are analysed for evidence of missing mass or momentum which would be the signature for the creation of a dark matter particle which escaped without triggering the detectors. Here is a paper on “Dark Matter Searches at Colliders”.