Why Is the Quantum World so Strange?

This page is the first in a series of pages that belong to the intersection of the author’s topics in physics, biology, and cognitive science.
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Is this a legitimate question? Most people (physicists included), seem to think that it is. Why do quanta behave so strangely, so unlike what we are familiar with in our macro-world? Why do they defy our laws of logic? I assume the reader is familiar with the fact of the strangeness of the quantum world, so I don’t plan to bore you by listing cases of such strangeness. (But if you do wonder what I refer to by “strangeness”, please read footnote zero (0).) In this introduction I will only support my claim that physicists agree that the world of quanta is strange, and also that, most of them, don’t know why. The main text in this page attempts to answer the question of the title. If you are a physicist and think you already know the answer I give below and agree with me, I’d think you belong to a minority. If you have a different opinion, whether you are a physicist or not, let me know what you think.

Let’s start by substantiating the claim that many physicists don’t know why quanta appear so strange.

I was motivated to write this page after reading the book “The Quantum World” (2004, Harvard University Press) by physicist Kenneth W. Ford, retired director of the American Institute of Physics. By the way, this is a wonderfully written book, and an excellent introduction to quantum physics for anyone wanting to read down-to-earth, introductory material to a world filled with mysteries and deep questions. In the first few pages of Chapter 1, “Beneath the Surface of Things”, we read: (Ford’s italics, my highlighting and comments in brackets.)

“Let me [...] pause to ask why the subatomic world is so strange, why it is so weird and wonderful. Why do the laws governing the very small and the very swift defy common sense? Why do they stretch our minds to the limit?” (p. 4) “In fact, the physics of the past hundred years has taught us that common sense is a poor guide in the new realms of knowledge. [He means mainly quantum physics and relativity.] No one could have predicted this outcome, but no one should be surprised by it. Everyday experience shapes your opinions about matter and motion and space and time. Common sense says that solid matter is solid, that all accurate watches keep the same time, that the mass of material after a collision is the same as it was before, and that nature is predictable: sufficiently accurate input information yields reliable prediction of outcomes. But when science moves outside the range of ordinary experience—into the subatomic world, for instance—things prove to be very different.[...]” (pp. 4–5) “Why is this? We don’t know why. Common sense could have extended beyond the range of our senses, but it didn’t. Our everyday worldview, it turns out, is a limited one, based on what we directly perceive. We can only echo the parting words of the respected old TV news anchor Walter Cronkite: “That’s the way it is.” You can be enchanted, you can be amazed, you can be befuddled, but you shouldn’t be surprised.” (p. 5)

Who are “we” who “don’t know why”? Ford’s “we” apparently means “we, people, human beings”. It could also be interpreted in a narrower sense: “we, physicists”. But when physicists say “we know/don’t know” about a matter concerning physics, they usually mean “we people”, because, after all, if physicists don’t know something regarding matters physical, who else is supposed to know? Certainly not my grandpa, who used to run a grocery shop. Isn’t that just common sense?

It is, except that physicists like Ford do not realize that some of the questions they ask, although seemingly physical, are not in fact answerable within the domain of physics. The question “Why the quantum world appears strange?” is not a physics question. To answer it, we need some help from other sciences; specifically, cognitive science and — mainly — biology. Why cognitive science? Because it is we, human beings, whose minds think that the quantum world appears strange. “Appearances” exist in minds, not in rocks or rockets.⁽¹⁾ “Strangeness” is a subjective, psychological state. What is strange for me doesn’t have to be strange for a chimp, a computer, or an alien. And why biology? Because, ultimately, the answer follows naturally only in an evolutionary context.

I will not tire the reader with long and needless introductions to the principles of biology and cognitive science, because understanding the answer is very easy once one allows their view to encompass, besides physics, facts that we all know from those other sciences. Without further ado, I proceed now to explain what I mean.

We can understand why the quantum world appears strange if we first understand the answer for another question: why does chocolate taste good?

Bear with me please, I am not kidding. Yes, the answer for the serious, important, philosophical, and seemingly physics question “Why the quantum world is so strange” is the same like the answer for the seemingly superficial and unimportant question “Why chocolate tastes so good” (and also similar to a number of other questions, as we’ll see later, some of them seemingly important, others seemingly superficial). If you have the patience to read why chocolate tastes good, the answer for our main question will follow naturally.

All right, so why does chocolate — or sweets in general — appear “nice”, “good” to our taste? (Even if you are one of those rare people who dislike sweets, I hope you agree you are an exception: the average human being likes sweets, and that’s what concerns us here.) Is sweetness absolutely good? That is, is it good for all living beings on this planet? Of course not. To most living creatures sweetness cannot even be sensed: plants and bacteria do not have taste buds, for example, and even many mammals are indifferent to sugars. The goodness of sweet things is subjective, restricted to a class of animals that have taste buds to taste them, and a neural apparatus complex enough to fathom the concept “good!”. Among those animals are the primates, the biological order in which we belong. Some of our primate ancestors happened to be fructivorous: they were fed with fruits. Most fruits have a lot of sugar. So, our ape ancestors evolved the ability to detect certain chemicals — the sugars — in the food they were eating. In other words, they evolved to “like”⁽²⁾ what promoted their survival: food with that kind of taste: sugary, the one we describe as “sweet”. Lions, on the other hand, have not evolved to perceive sweetness as “good”, because raw flesh does not taste sweet (I don’t think). Quite likely, some types of raw flesh evoke a feeling of “goodness” to lions, analogous to our feeling when we taste chocolate.

“Good” is the thing that, ultimately, increases your chances for survival, either directly, or indirectly — and, sometimes, very-very indirectly, so much that it’s hard to see where the benefit is, or was, in our evolutionary past. Take the feeling of watching a beautiful landscape. You’re sitting on the grass under a large shadowy tree in the countryside; sunshine, deep blue sky, a light breeze, and in front of you stretches a meadow, the greenness of which is interrupted only by thousands of wild daisies; the only sounds you can hear are the natural ones in this setting: chirps of birds, and the buzz of insects working on the daisies. Why is this setting good? It’s because we evolved in it. Had we evolved in the depths of the ocean, we would find the oceanic abyss beautiful, and the terrestrial meadows scary and hostile — much as now we find the oceanic abyss hostile, perhaps even scary. Had we evolved on Mars, we would find that red is the natural color for a beautiful sky, whereas a blue sky would probably irritate us. Some other “good things” are harder to trace to their evolutionary roots. Good music, for example: why is music good (some of it, anyway)? Because chanting in groups and dancing in tribal rituals played their role in holding some of our ancestors together in a unit, thus increasing their chances of survival — there are plenty of anthropological studies on this subject, and it is out of the scope of this text to delve deep into it. In general, show to a biologist one thing perceived as “good”, and they’ll tell you how this has roots to something that increased the chances of survival of some (perhaps remote) ancestors of ours.⁽³⁾

We can now move closer to the issue of strangeness in the quantum world if we generalize the concept “good”, and put it in one category together with: “natural”, “fitting”, “right”, “the right thing to expect”, “the unsurprising thing”, “the logical thing”, “the familiar”. Did you notice that the concept “strange” does not belong to this category, but instead belongs to a different category of concepts antithetical to the previous ones? Does the answer to our original question now become easier to reach?

Instead, we evolved in the macroworld, which is governed by the physical laws of Newton and the logical laws of Aristotle and Boole,⁽⁴⁾ and is modeled most intuitively by the geometry of Euclid. Now, one might claim that since we are made out of quanta, in a sense we also live in the quantum world, and we evolved in it. But, no, the quantum laws are irrelevant for our survival. We do not need to perceive the particle–wave duality in order to find more food; we do not need to appreciate the tunneling effect in order to have more descendants; nor are we required to understand quantum indeterminacy to avoid our predators. Although it is true that we would not exist without all these quantum phenomena — indeed, nothing would exist — the point is that we do not perceive them in order to survive. They are irrelevant as far as our cognition is concerned. And they happen to be not just different from the laws of the macroworld, but to contradict them. Some examples will suffice to illustrate where the contradictions are.

Developmental psychologists have shown in relatively recent experiments that even very young children — infants! — have some expectations about how the world works. This is called a “naïve physics”, and is not a derogatory term, but means things like the following: babies expect a screen that fell and covered a single ball to reveal again a single ball when it is lifted; if the screen reveals two balls instead, babies are surprised.⁽⁵⁾ This means that our naïve physics leads us to expect the constancy of solid objects: things cannot simply appear in our view, or disappear, as if by magic. And yet this is what seems to be possible in the quantum world: particles can appear seemingly out of the void or disappear into the void. In other experiments, psychologists show an object moving at a constant speed along a straight line, until it disappears behind a screen. But the moment it disappears, the psychologist makes it look as if it instantly reappears at the other side of the screen (thereafter continuing along its trajectory), as if it made a “quantum leap” forward. Babies are surprised again. According to our naïve physics objects do not jump instantly to new locations, but are expected to continue along their trajectories in the same way as they started. Yet quantum particles can do their famous “tunneling effect” routine, in which they disappear and reappear at a new location instantly. Other violations of macroworld principles include our conviction that if something is A, it cannot be B at the same time, if A and B are qualitatively very different. But quantum particles can be simultaneously both particles and waves.

Thus, quantum phenomena conflict with the expectations that we have either built into us by evolution, or acquired through our interactions with the macroworld. So they appear to us strange, unfamiliar, illogical, surprising, unexpected, wrong, unfitting, unnatural — to the uninitiated at least, if not to the trained physicist.

Parenthetically, the previous discussion also hints at the answer for these related questions: “Why is relativity counter-intuitive?” and “Why do we perceive a world of three, instead of four dimensions?”

Simple: relativity is counter-intuitive to us because we evolved in a world of speeds that are too slow compared to the speed of light. If we run at speed v₁, and throw a stone at speed v₂ relative to us, the combined speed of the stone relative to the ground is perceived as v₁+v₂, and we need to be able to appreciate that, if we use the stone as a weapon. Now, it turns out that in reality the speed of the stone relative to the ground is (v₁+v₂)/(1 + v₁v₂/c²). But the speeds involved are so small compared to c that the difference between the “correct” (relativistic) and “incorrect” (intuitive) formula is so minute that no biological perceptual mechanism can evolve to detect it. And even if such a mechanism could have evolved, what difference would it make to perceive according to the right formula? The difference of right from wrong at such slow speeds is simply negligible. So we evolved to perceive the simple addition of velocities. From this, it follows that the relativistic principle “The speed of light in vacuo is constant, independent of the observer’s speed” appears counter-intuitive to us, bordering on the absurd.

Similarly, our world seems to have only three spatial (macro-) dimensions because along the fourth one (time), our world is too flat for our kinds of velocities, and our weak gravitational fields. Our world appears to us flat, Euclidean, because it is too hard to perceive its minute curvature. It would appear curved only if we either moved at very high speeds with respect to our perceived surroundings, or lived in a world of extremely high gravity. Neither is the case, so our world is essentially not curved along the three spatial dimensions in our vicinity; or, to put it otherwise, its curvature is so negligible that either it wasn’t biologically possible to evolve a mechanism to detect it, or even if it was, this ability wouldn’t make any difference for survival purposes. As for time, our temporal motions⁽⁶⁾ are so minute that we can’t detect them without special modern equipment, and so we did not evolve to perceive time as a geometric dimension, on par with the other three ones.

In conclusion, Dr. Ford’s suggestion that “Common sense could have extended beyond the range of our senses, but it didn’t”, is misled. What we call common sense can only extend to the domain of laws that have been crucial for our evolution, which are the laws of the macroworld. Also, his next statement, “Our everyday worldview, it turns out, is a limited one, based on what we directly perceive”, is a truism: of course our everyday worldview is limited to what we directly perceive, by definition. It couldn’t have been otherwise.

Footnotes: (Clicking on the footnote number brings back to the text)

(0) Quanta can behave like waves or particles, but whenever we observe them directly they appear to us as particles. After such interactions (traditionally called “observations”) their wave-like nature is assumed to “collapse” to a particle, but no one knows what mechanism lies behind this collapsing event, how fast does it take place (instantly?), etc. A quantum particle can be considered to be at two places at the same time. A radioactively decaying nucleus of an atom can spontaneously break down into other nuclei — “spontaneously” meaning that no one has any clue what causes the decay, while many physicists believe there is no cause for such events. A particle can spontaneously come into existence out of nothingness, as well as disappear into nothingness. A particle can “tunnel” (jump) instantaneously from one place to another, bypassing the limit of the speed of light; how far the particle goes after this “tunneling effect” is limited only by probabilities: the farther, the less probable. Last, but not least, according to the laws of quantum physics, there is a non-zero probability (though “astronomically small” would be a gross overestimate) that Lake Michigan (or any other body of water on Earth) could freeze in midsummer; or that the Statue of Liberty suddenly bends her raised arm (you know, the one holding the torch) at the elbow. No magic is required for such events, only the vanishingly small probability that all quanta involved suddenly “tunnel” to all the right places (to make up ice crystals, to form the arm in its new position, etc.). (Why do we never observe such “miracles”? Because the probability of even a small number of particles tunneling simultaneously is extremely small, let alone ending up at all the right places to reconstitute a material object, let alone this happening for the vast numbers of particles included in any visible material object.)

(1) I am not expressing any dualistic view here, sharply separating mind from matter. I simply refer to the fact that the human mind is governed by its own laws, which are studied by psychology and cognitive science. The cognitive laws, although emergent from a substratum of different laws (the biological ones, which in turn emerge from the chemical ones, emergent eventually from the quantum laws), exist in a domain of their own (“cognition”), which is so remote from the domain of quanta that their tenuous relationship is hard to realize. The tenuousness of this relationship is what gives rise to the ancient mind–body problem in philosophy.

(2) I put “like” in quotes because an ape’s liking is not as complex a concept as a human’s liking, yet it’s more complex than a monkey’s liking, which is more complex than a frog’s liking, and so on.

(3) Sometimes the explanations proposed by biologists, and esp. paleoanthropologists (i.e., paleontologists who study the evolution of hominids, including our species), appear more like hand-waiving rather than solid scientific theories. This is natural, because the paleoanthropologist deals with events that happened in time past, so any conclusion must be based on evidence available at present, and we don’t always have all the necessary evidence to solidly back an idea. But, regarding the issue of “goodness”, nobody has proposed — to the best of my knowledge — an alternative explanation for why we feel some things are “good”, ignoring or bypassing the biological (evolutionary) explanations.

(4) A reader complained about the inclusion of Boolean logic as a foundational pillar of the world we live in. Please read the first comment below, under Readers’ Reactions.

(5) Psychologists do such tricks to measure the babies’ time of fixation on the scene, from which they determine whether babies are surprised. It is important that they measure the reactions of babies and not adults, because adults could pretend, try to guess the experimenter’s intention, etc. For a brief reference to such experiments see Steven Pinker’s How the Mind Works, pp. 317–319. More experiments on the perception of numerosity are reported in Lakoff and Núñez’s “Where Mathematics Comes From”, referenced below (see “Readers’ Reactions”).

(6) By “temporal motions” I refer to synchronized clocks going out of sync after observers carrying them move with respect to each other.

Readers’ Reactions:

(This page is not a blog; I hate the anarchy of blogs, so I am not willing to include here everything mailed to me, indiscriminately. But occasionally some reader will either correct me, or add something that I didn’t think of, or make me understand that I didn’t explain some idea well enough. In such cases I either modify my text, or add the reader’s view below and comment on it.)

A reader said:

“Lakoff and Nuñez and by now many other people would not agree with you at all that the macroworld is governed by the logical laws of Boole! I’d agree that the world of mathematical proofs is ruled by those, just that. I am not sure of what exactly Boole proposed, but for sure if he used the mathematical “if ..then” connective where false implies everything, there is nothing more away from human common sense than that.”

The reader refers to the book “Where Mathematics Comes From”, by George Lakoff and Rafael Núñez, in which the two authors claim (among other things) that the mathematics that we know of today is a product of the particular random direction our “Western” civilization took, and, should other civilizations prevail (e.g., Mayan, Indian, etc.), the world might have developed a different notion of mathematics, not based on ancient Greek roots and Boole’s logic.

First, let me quickly point out that when I wrote “the macroworld [...] is governed by the [...] logical laws of Aristotle and Boole”, what I had in mind was mainly the law of the excluded middle, first stated explicitly by Aristotle, and later codified by Boole as P v ~P, where P is any proposition. Although I agree that in the world in which we evolved it is seldom the case that something is either true or false, my view is that if any species evolved in our world so as to discover the quantum world one day, then that species could only have started with the foundations of mathematics as we know them, including the law of the excluded middle. (For the case of the “if...then” connective where the premise is false, I think that’s a mere unfortunate but unavoidable consequence of a greater system, i.e., Boolean logic: for the system to work properly, logical implication must be defined that way.) What I mean is this: how did we manage to learn about the quantum particles? Not by looking at them, but by having a solid mathematical foundation upon which to base our theories. A fundamental pillar of that mathematical foundation is the law of the excluded middle. Remove that pillar, and the entire edifice collapses. Without that pillar there would be no mathematical proofs; hence no Euclidean geometry, no Peano arithmetic, hence no calculus, hence no physics theories based on such math, such as classical mechanics, optics, thermodynamics, electromagnetism, etc., hence none of the technological advancements that led to 20th century physics (technological advancements that Lakoff and Núñez made heavy use of in order to create the freakish product of their labor.) Heck, there would be no computers, no internet, even this very discussion would not exist. To claim that we could have reached the quantum world without having knowledge of the mathematics on which we base quantum physics betrays a deep ignorance not only of quantum physics, but of the history of science itself. But it’s beyond the scope of this text to offer a full critique of Lakoff and Núñez’s unbelievably dumb conjecture that mathematics could have been different.

In short, the reader is right that the macroworld is not exactly “governed” by Boolean logic, as I wrote, but I would say, it affords the evolution of a cognitive species that, to have any hope to learn about quarks and leptons, it must start with Aristotelian and Boolean mathematical principles. (If there is some alternative way, the burden is on the shoulders of those who think that there is to show us explicitly what the alternative way could be.) Hence, for such a species, logical principles such as P v ~P appear normal, familiar, expected. Anything that violates them, such as the observation that an electron can pass through two slits at the same time, appears strange.

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