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“You’ve observed the robbers. They know it. That will change their actions,” says Charlie Eppes, the math savant who helps detectives on television’s “Numbers.” Eppes claims that this insight follows from quantum physics, in particular, Werner Heisenberg’s infamous “uncertainty principle.” Not all mischaracterizations of Heisenberg’s principle are as innocent as Eppes’s. The film “What the Bleep Do We Know!?” uses it to justify many articles of faith in New Age philosophy. Asserting that observing water molecules changes their molecular structure, the film reasons that since we are 90 percent water, physics therefore tells us that we can fundamentally change our nature via mental energy. Fundamentally inaccurate uses of the principle are also common in the academy, especially among social theorists, who often argue that it undermines science’s claims to objectivity and completeness. As Jim Holt has written, “No scientific idea from the last century is more fetishized, abused and misunderstood — by the vulgar and the learned alike — than Heisenberg’s uncertainty principle.”

Why exactly is the uncertainty principle so misused? No doubt our sensationalist and mystery-mongering culture is partly responsible. But much of the blame should be reserved for the founders of quantum physics themselves, Heisenberg and Niels Bohr. Though neither physicist would have sanctioned the above nonsense, it’s easy to imagine how such misapprehensions arise, given the things they do say about the principle, and especially the central place they both give to the concept of measurement.

Heisenberg’s uncertainty principle is not quite as strange as we think.



Heisenberg vividly explained uncertainty with the example of taking a picture of an electron. To photograph an electron’s position – its location in space – one needs to reflect light off the particle. But bouncing light off an electron imparts energy to it, causing it to move, thereby making uncertain its velocity. To know velocity with certainty would then require another measurement. And so on. While this “disturbance” picture of measurement is intuitive – and no doubt what inspires the common understanding exemplified in “Numbers” – it leaves the reason for uncertainty mysterious. Measurement always disturbs, yet that didn’t stop classical physicists from in principle knowing position and velocity simultaneously.

For this reason Heisenberg supplemented this picture with a theory in which measurement figures prominently. It’s not simply that we can’t simultaneously measure definite values of position and momentum, he thought. It’s that before measurement those values don’t simultaneously exist. The act of observation brings into existence the properties of the world. Here we find the seeds of the claims made by some social theorists and found in “What the Bleep Do We Know!?” If reality depends on interaction with us, it’s natural to suppose that objectivity is undermined and that we, from the outside, make reality, possibly with some kind of mental energy.

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Bohr, for his part, explained uncertainty by pointing out that answering certain questions necessitates not answering others. To measure position, we need a stationary measuring object, like a fixed photographic plate. This plate defines a fixed frame of reference. To measure velocity, by contrast, we need an apparatus that allows for some recoil, and hence moveable parts. This experiment requires a movable frame. Testing one therefore means not testing the other. Here we find inspiration for the idea that the principle shows that science can never answer everything.

But as interpretations of the principle, both views are baffling, most of all for the undue weight they give to the idea of measurement.

To understand what the uncertainty principle actually says, one needs to understand the broader physical theory in which it figures: quantum mechanics. It’s a complex theory, but its basic structure is simple. It represents physical systems – particles, cats, planets – with abstract quantum states. These quantum states provide the chances for various things happening. Think of quantum mechanics as an oddsmaker. You consult the theory, and it provides the odds of something definite happening. You ask, “Oddsmaker, what are the chances of finding this particle’s location in this interval?” and the equations of the theory answer, “25 percent.” Or “Oddsmaker, what are the chances of finding the particle’s energy in this range?” and they answer, “50 percent.”

The quantum oddsmaker can answer these questions for every conceivable property of the system. Sometimes it really narrows down what might happen: for instance, “There is a 100 percent chance the particle is located here, and zero percent chance elsewhere.” Other times it spreads out its chances to varying degrees: “There is a 1 percent chance the particle is located here, a 2 percent change it is located there, a 1 percent chance over there and so on.”

The uncertainty principle simply says that for some pairs of questions to the oddsmaker, the answers may be interrelated. Famously, the answer to the question of a particle’s position is constrained by the answer to the question of its velocity, and vice versa. In particular, if we have a huge ensemble of systems each prepared in the same quantum state, the more the position is narrowed down, the less the velocity is, and vice versa. In other words, the oddsmaker is stingy: it won’t give us good odds on both position and velocity at once.

Note that nowhere in my explanation of the principle did I mention anything about measurement. The principle is about quantum states and what odds follow from these states. To add the notion of measurement is to import extra content. And as the great physicist John S. Bell has said, formulations of quantum mechanics invoking measurement as basic are “unprofessionally vague and ambiguous.”

After all, why is a concept as fuzzy as measurement part of a fundamental theory? Interactions abound. What qualifies some as measurements? Inasmuch as disturbance is related to uncertainty, it’s hardly surprising that observing something causes it to change, since one observes by interacting. But a clear and complete physical theory should describe the physical interaction in its own terms.

Today there are several interpretations of quantum mechanics that do just that. Each gives its own account of interactions, and hence gives different meaning to the principle.

Consider the theory invented by the physicists Louis de Broglie and David Bohm, commonly referred to as the de Broglie-Bohm view. It supplements the quantum state with particles that always have determinate positions, contra Heisenberg. Measurement interactions are simply a species of particle interaction. Uncertainty still exists. The laws of motion of this theory imply that one can’t know everything, for example, that no perfectly accurate measurement of the particle’s velocity exists.

This is still surprising and nonclassical, yes, but the limitation to our knowledge is only temporary. It’s perfectly compatible with the uncertainty principle as it functions in this theory that I measure position exactly and then later calculate the system’s velocity exactly. But the bigger point is that because of the underlying physical picture, we here know exactly why uncertainty exists.

Related More From The Stone Read previous contributions to this series.

Other interpretations exist. For example, there are the “collapse” theories associated with the physicist Giancarlo Ghirardi. In these theories the quantum state abruptly changes its development (“collapses”) when big things interact with small things. Here fields of mass interact with one another. And in the “many worlds” picture of the physicist Hugh Everett III, all the possibilities given odds by the oddsmaker come to fruition, but in parallel worlds. Here the abstract quantum state is regarded as physical, and interactions are connections that develop between different bits of this strange reality.

All of these interpretations have their pros and cons, but in none do observers play a fundamental role. You and I are big clumps or aspects of the basic stuff. Measurement is simply a type of interaction among those types of stuff, no different than a basketball’s redirection when bounced off a patch of uneven gym floor.

Once one removes the “unprofessional vagueness” surrounding the notion of measurement in quantum physics, the principle falls out as a clear corollary of quantum physics. Weirdness remains, of course. The stingy quantum oddsmaker is genuinely odd. But all the truly wild claims – that observers are metaphysically important, that objectivity is impossible, that we posses a special kind of mental energy – are the result of foggy interpretations made even less sharp by those wanting to validate their pet metaphysical claims with quantum physics.

To prevent future temptation to misuse, I urge that we demote the uncertainty principle. If Pluto can be reclassified as a dwarf planet, then surely we can do something similar here. Going forward, let’s agree to call the uncertainty principle the “uncertainty relations” or even the less provocative “quantum standard deviation constraints.” Not many people outside of a lab are likely to invoke a principle with a name like this.

And that’s probably a good thing.



Craig Callender is chair of the philosophy department at the University of California, San Diego. He recently edited “The Oxford Handbook for the Philosophy of Time,” and he is finishing a book on time and physics entitled “What Makes Time Special.”