Quantum Theory and Common Sense: It's Complicated

There's more of it in quantum physics than we think

Physical theories can open new vistas of human thought, suggesting that the world is not what it seems to be. This situation presents itself as a conflict between the scientific account of the world and “common sense”, a conflict that scientists sometimes gleefully portray as the defeat of common sense. There are clear historical episodes of this character.

Copernicus, for example, proposed that instead of the Earth being fixed in place with the Sun, planets and stars whirling around it, it is the Earth itself that is spinning on its axis and orbiting the Sun. A quick calculation shows that locations at the equator would then be moving at about a thousand miles an hour due to the rotation of the Earth, and the whole Earth moving over million miles a day in its orbit around the Sun. In a world in which a speed of 10 miles an hour would be considered fast, these sorts of apparently unnoticed motions defied common sense. If the Earth is spinning so fast, one wonders, how could the birds in flight possibly keep up?

Common sense is a collection of widely shared beliefs that spontaneously arise from everyday interaction with the world together with some seemingly obvious inferences from that experience. In the case of the birds, the conflict between Copernicus and common sense was only resolved by the introduction of the concept of inertia, first by Galileo and then by Newton. Inertia meant that objects put in motion tend to remain in motion without any further ado, so the birds resting on trees and therefore being carried along with the rotation of the Earth would tend to maintain that motion without any additional effort, even when they left the trees.


"Common sense plays a dual, and almost self-contradictory, role in scientific practice" 



Common sense plays a dual, and almost self-contradictory, role in scientific practice. As we have seen, common sense is sometimes the foil—if not the adversary—of scientific theorising. Making progress in understanding the world sometimes requires overcoming or limiting the authority ascribed to common sense. But on the other hand, it is logically impossible for any empirical science to break free completely of common sense. For empirical theories ultimately appeal to experimental outcomes for their justification, and claims about experimental outcomes themselves draw their authority from common sense. The experimentalist informs us that 28% of the electrons in a certain experiment have been deflected upwards and 72% downwards. Why should we believe her? Because it is part of common sense to accept what we take to be the plain evidence of our senses in appropriate everyday situations.

Niels Bohr made precisely this point in one of his discussions of quantum theory, using “classical terms” to mean what we are calling “common sense”: “it is decisive to recognise that, however far the phenomena transcend the scope of classical physical explanation, the account of all evidence must be expressed in classical terms”. Speaking of the numbers of marks that formed in different locations on a screen is exactly expressing the evidence in classical terms.

Granting that the evidence for the theory must be stated in a way that accords with common sense, just how far can quantum theory go in undermining the more extensive everyday picture of the world? Many physicists seem to positively delight in pushing the scope of “quantum weirdness” as far as possible. An iconic example of this is the so-called “paradox of Schrödinger’s cat”.

In his intellectual struggles with the physical significance of quantum theory, Erwin Schrödinger described a certain easily realisable (but ethically unacceptable!) experimental situation. Take a single atom of a radioactive element. Quantum theory makes only probabilistic predictions about such a system: it ascribes a definite chance to the atom that it will decay in a given period of time, but makes no further precise prediction about whether or not the decay will actually occur, even given complete information about the initial “quantum state” of the atom. This failure to make precise predictions was one of the great innovations of quantum theory: in classical physics, complete information about the initial state of a system, together with the laws of physics, entail exactly how the system will behave at all times. The failure of quantum theory to do better than merely probabilistic or statistical predictions was taken by Bohr and his school as an indication that the laws of physics themselves fail to be deterministic. It was, in contrast, taken by Einstein as an indication that the quantum-mechanical description of a system is not a complete description: there are further physical characteristics of the individual system, including perhaps characteristics that pre-determine the precise moment of decay.

Schrödinger was considering just how far Bohr’s account of the matter could be pushed. The quantum state of an electron in an atom does not ascribe it a particular location: the “wave function” of the electron is “smeared out” around the atomic nucleus. If the wave function is a complete description of the electron, then the electron itself must also be “smeared out”: it is not a point particle with a definite precise location but rather something more like a mist or a fog. Schrödinger acknowledged that such a description of the electron might be accurate: maybe electrons are “smeary” at subatomic scale. (It is essential to this picture that the probabilities associated with different locations for the electron are not a matter of us not knowing where the electron “really” is, they rather somehow arise from the electron not actually being in any precise location. As Schrödinger memorably put it in the cat paper: “there is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks”.) If the quantum wave function is complete, then the “smeariness” of the electron wave function reflects that of the electron itself: sub-atomically, an electron is more like a cloud bank than a billiard ball.

This account of the nature of an electron cannot conflict with common sense because common sense has no access at all to the microscopic structure of things. That’s the whole point of calling the structure “microscopic”. Maybe individual electrons are “smeary”: who knows? Our best counsel is attend to what our best physics tells us.

But—and this was Schrödinger’s point—the quantum theory of Bohr had no principled means of confining the smeariness to microscopic scale. Schrödinger proposed placing the radioactive atom near a Geiger counter, and then hooking the Geiger counter up with a device that would smash a flask of hydrocyanic acid if the atomic decay is detected, thereby killing a cat. If the wave function of the system always evolves in accord with Schrödinger’s equation and if the wave function provides a complete description of the system, then the smeariness of the electron will inevitably be amplified to macroscopic scale into a smeariness of the cat itself: just as the electron was not in any particular location, the cat would end up objectively “smeared out” between being alive and dead! And that would, indeed, be in the most severe possible conflict with common sense beliefs about cats!

It is essential to note that Schrödinger was not proposing that one accept such a bizarre conclusion. He describes the example as a “ridiculous case”: it does not show that cats can end up smeared out between alive and dead, it shows that Bohr’s understanding of quantum theory cannot be correct. That does not tell us how to correct Bohr’s views, but it does show that they must be corrected somehow. Either the quantum wave function does not provide a complete physical description of the cat (so the cat can have a definite state of health despite the noncommittal nature of the wave function) or else the Schrödinger evolution of the wave function must break down at some point. As the physicist John Bell put it: “Either the wave function, as given by the Schrödinger equation, is not everything or it is not right”.

But for some obscure reason, physicists have taken to presenting Schrödinger’s example as an illustration of the very thing he found ridiculous: that cats can indeed be in a state of indefinite health! Further, they sometimes assert that this peculiar state persists until someone looks: the very act of observation somehow magically forces the cat to make the literal life-or-death decision. This peculiar physical power of observers to make the physical state of systems jump would also violate common sense, and pretty much all other sense as well. Einstein used to mock the idea by asking just how sophisticated an observer had to be to acquire this magical power: can a mouse do it, for example? (If a cat can do it, then presumably Schrödinger’s cat will avoid such an indeterminate situation by simply observing its own state of health!).

It is impossible to overstate how extreme physicists have sometimes been in insisting on this bizarre view. The Cornell physicist David Mermin, for example, once wrote “We now know that the moon is demonstrably not there when nobody looks”. Now if we did know that then that would be the most extreme and radical rejection of common sense in the history of mankind. But we know no such thing. And I am sure that Professor Mermin regrets having written that particular claim. But he did write it, and publish it, and it certainly illustrates that physicists discussing quantum theory feel no compunction about letting common sense get in the way of the astonishing claims they are making. But let the public beware: what the average physicist has to say on this topic in not at all reliable. Indeed, although there are several quite different clear and coherent ways make sense of quantum theory, none of them suggests that the moon does not exist when no one is looking at it.

But if quantum theory does not tell us there are cats that are neither alive nor dead, and quantum theory does not tell us that the moon’s very existence depends on it being observed, what does it tell us, and how does that comport with common sense?

The direct answer, in most cases, is that “quantum theory” as it is taught in physics texts simply does not make definite claims one way or the other about many aspects of the physical world. What goes by the name “quantum physics” is a predictive algorithm: it allows you to calculate probabilistic predictions about experimental outcomes. It could, for example, tell you to expect that in the long run about 72% of the marks formed in a particular experiment will be on the upper part of the screen and about 28% on the lower. But whether that is because “God plays dice” (i.e. experiments that begin in exactly the same state sometimes come out differently) or because we simply have not accounted for the physical conditions that determine each particular outcome, “quantum theory” has nothing to say about. To answer questions like that you need what is commonly called an “interpretation” of quantum theory, i.e. a precisely articulated physical theory that makes the same predictions as the predictive algorithm.

Some such precise “interpretations” are indeterministic: according to them, God does “play dice”. But other “interpretations” are deterministic: according to them there is a reason each particular mark forms where it does. “Quantum theory” is silent on this question.

what is knowledge a quantum perspective juha staasi SUGGESTED READING What Do We Know? A Quantum Perspective By Juha Saatsi There is, however, one common sense belief that quantum theory—and the world itself!—does violate. This is the principle John Bell called “locality”, and whose violation Einstein called “spooky action-at-a-distance”. In fact, the reason Einstein rejected Bohr’s account of quantum theory was not because it is committed to indeterminism—to God playing dice—but because it is committed to spooky-action-at-a-distance. Even Newton rejected unmediated action at a distance between objects. In a letter to Richard Bentley he wrote: “It is inconceivable, that inanimate brute Matter should, without the Mediation of something else, which is not material, operate upon, and effect other Matter without mutual Contact, as it must be, if Gravitation in the Sense of Epicurus, be essential and inherent in it. And this is one Reason why I desired you would not ascribe innate Gravity to me. That Gravity should be innate, inherent and essential to Matter, so that one Body may act upon another at a distance thro’ a Vacuum, without the Mediation of anything else, by and through which their Action and Force may be conveyed from one to another, is to me so great an Absurdity, that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it.”

It may be that “common sense” is not quite as unequivocal about this as it once was. Newton would not have had much experience of what seems to be instantaneous action at a distance, but anyone who uses a TV remote control has experiences suggestive of it every day. Even so, it may well rank as a dictate of common sense that in for system A to have an influence on system B, some sort of physical entity must at least be capable of passing continuously from A to B.

And once the Theory of Relativity comes on the scene one can be even more precise: if nothing can go faster than light, then an intervention on System A can have no influence on system B until at least as much time has passed for something travelling at the speed of light to get from A to B. It was precisely this principle that Bell subjected to investigation.

What Bell showed that if A and B are governed by local physics—no spooky-action-at-a-distance—then certain sorts of correlations between the behaviours of the systems cannot be predicted or explained by any local physics. It is this universal character of Bell’s proof that allows one to draw conclusions without having to settle on a particular interpretation of quantum theory. What Bell further showed is that the quantum predictive formalism entails violations of his constraint—a violation of Bell’s inequalities—which means that it predicts behaviour that no local physics could account for. And the absolute kicker is that experimentalists have shown that the quantum-mechanical predictions are correct. That is, nature itself violates Bell’s inequalities and so must—one way or another—employ some superluminal physics. Further, this spooky-action-at-a-distance does not appear to be mediated by any sort of particle or wave that passes continuously from one system to the other, even at greater than the speed of light.

That surely violates common sense.

So quantum theory and common sense are indeed in tension, and given the experimental proofs common sense should just modestly withdraw.

But quantum theory does not suggest—as many physicists have asserted—that cats can be neither dead nor alive or that physical reality depends crucially on being observed. Further, there are limits to how far any empirical science can, even in principle, undermine common sense. For if our confidence in what common sense asserts concerning experimental outcomes were to be undercut, our confidence in the accuracy of our physical theories would soon follow. As Democritus had the senses admonish the mind over two millennia ago:

Poor mind, do you take your evidence from us and then try to overthrow us? Our overthrow is your fall.

Latest Releases
Join the conversation

ida sanka 2 September 2021

This is such a great help! I've been struggling for days already, soo glad i found this site! thanks for sharing! mold removal

killer smile 2 September 2021

It was so good to see you acknowledging this topic, it really feels great. Thanks for sharing such a valuable information which is very hard to find normally. I have subscribed to your website and will be promoting it to my friends and other people as well.
boudoir photography san antonio

Xinhang Shen 30 December 2019

It is just the same to argue whether the speed of light is constant relative to all inertial reference frames in special relativity as to argue whether Schrodinger's cat is alive or dead in quantum mechanics because these theories themselves have assumed the claims. If you want to refute these claims, you should examine whether their very postulates are correct or not.

In special relativity, Einstein used the change of definition of time and space to claim a physical fact, just like a claim that everybody has the same height if the measuring ruler is an elastic band, which does not make any discovery at all. When we talk about the speed of light, it's the speed of light measured by rulers and clocks, not by randomly defined mathematical variables.

The fatal error of special relativity is that it redefines time and space with Lorentz Transformation and equates the newly defined relativistic time with physical time defined by a physical clock, which are two totally different things as shown in the following:

It is known that a physical clock is a physical process such as the rotation of the earth around the sun in which the physical time is recorded by the status change of the process. The status change of a physical process is always represented by the product of the lapse of the theoretical time and its progressing rate divided by a calibrate constant in either Newtonian mechanics or special relativity. That is, the relationship between the theoretical time t and the physical time T measured by a physical clock is: T = tf/k where t is the theoretical time, f is the frequency of the clock and k is a calibration constant. We have to verify that T = t before using a clock to measure the theoretical time.

In Newtonian mechanics, since the theoretical time is the absolute Galilean time and thus the frequency is a frame independent constant. We can set k = f in all reference frames to make T = tf/k = tf/f = t in all reference frames, which means a physical clock does measure the absolute Galilean time, and confirms that our physical time is absolute too.

Now let’s look at physical clocks in special relativity. The situation becomes more complicated because time is relative to the inertial reference frame. We don't have the simple relationship as T = t for all reference frames any longer. Thus, we take another approach to study the properties of clock time T and relativistic time t between different inertial reference frames as shown in the following:

If you have a clock (clock1) with you and watch my clock (clock2) in motion and both clocks are set to be synchronized to show the same physical time T relative to your inertial reference frame, you will see your clock time: T1 = tf1/k1 = T and my clock time: T2 = tf2/k2 = T where t is relativistic time, f1 and f2 are the frequencies of clock1 and clock2 respectively observed in your inertial reference frame, k1 and k2 are calibration constants of the clocks. The two events (Clock1, T1=T, x1=0, y=0, z=0, t) and (Clock2, T2=T, x2=vt, y=0, z=0, t) are simultaneous measured with both relativistic time t and clock time T. Now we want to see how these simultaneous events will behave after Lorentz Transformation. When these two clocks are observed by me in the moving inertial reference frame, according to special relativity, we can use Lorentz Transformation to get the same events in the frame of (x', y', z', t'): (clock1, T1′, x1′=-vt1', y’=0, z’=0, t1′) and (clock2, T2′, x2′=0, y’=0, z’=0, t2′), i.e., I will see T1′ = t1’f1’/k1 = (γt)(f1/γ)/k1 = tf1/k1 = T1 = T and T2′ = t2’f2’/k2 = (t/γ)(γf2)/k2 = tf2/k2 = T2 = T, where γ = 1/sqrt(1-v^2/c^2). That is, no matter observed from which inertial reference frame, the events are still simultaneous measured with clock time T i.e. the two clocks are always synchronized measured with clock time T, but no longer synchronized measured with relativistic time t’. Therefore, clock time and relativistic time behave differently in Lorentz Transformation. The change of the reference frame only makes changes of the relativistic time from t to t’ and the frequency from f to f’, which cancel the change of each other in the formula: T= tf/k and thus makes the physical time unchanged. In a real experiment, we actually only have clock time T1 = T1' = T2 = T2' = T and can never see relativistic time t or t'. Thus, relativistic time t and t' are just irrelevant to real experiments. Current mainstream physicists in the world do not realize that there are two changes in special relativity (time expansion and frequency decrease) happened in any moving physical clock and wrongly interpret the slowdown of its frequency as the slowdown of clock time shown on the moving clock, missing the effect of the expansion of the relativistic time of the moving frame. Actually, all what special relativity does is the introduction of a new definition of time and space as shown in the following.

In Newtonian mechanics, time is absolute and space is rigid, following Galilean Transformation between inertial reference frames (X, Y, Z, T) and (X', Y', Z', T') with a relative velocity v:

T' = T

X' = X - vT

Y' = Y

Z' = Z

where (X, Y, Z, T) is set to be the frame with isotropic speed of light and (X', Y', Z', T') is the frame moving at speed v relative to (X, Y, Z, T).

But Einstein forces space and time to follow Lorentz Transformation which is equivalent to:

In the frame of (X, Y, Z, T):

t = T

x = X

y = Y

z = Z

and in the frame of (X', Y', Z', T'):

t' = T'/γ - γvX'/c^2

x' = γX'

y' = Y'

z' = Z'

You can easily verify that (x, y, z, t) and (x', y', z', t') follow Lorentz Transformation:

t' = T'/γ - γvX'/c^2 = T/γ - γv(X - vT)/c^2 = t/γ - γv(x - vt)/c^2 = (1/γ + γv^2/c^2)t - γvx/c^2

= γt - γvx/c^2 = γ(t - vx/c^2)

x' = γX' = γ(X - vT) = γ(x - vt)

y' = Y' = Y = y

z' = Z' = Z = z

i.e. Lorentz Transformation:

t' = γ(t - vx/c^2)

x' = γ(x - vt)

y' = y

z' = z

We can also see in the new coordinate system i.e. the relativistic coordinate system, the speed of light is

c = x/t =X/T = (X' + vT')/T' = [x'/γ + vγ(t' + vx'/c^2)/[γ(t' + vx'/c^2)] = [γ(x' + vt')]/[γ(t' + vx'/c^2)]

= (x'/t' + v)/(1 + vx'/t'/c^2)


c(1 + vx'/t'/c^2) = x'/t' + v


x'/t'(v/c - 1) = v - c


x'/t' = (v - c)/(v/c - 1) = c

i.e. the speeds of light defined by x/t and x'/t' are the same constant c in all inertial reference frames. Similarly, if you use the speed of sound to replace c in the above, you will get that the speed of sound defined by the new coordinate system is constant relative to all inertial reference frames too. But that does not change the fact that the real speed of sound measured with physical rulers and clocks is not constant relative to all inertial reference frames because clock time won't change with the change of the definition of the theoretical time and still absolute as shown above.

Now it is clear that all special relativity does is to redefine time and space. Just like the geometric property of a circle which is always a circle no matter whether you use a cartesian coordinate system or a polar coordinate system, the property of a physical clock should always be the same (i.e. clock time is always absolute) too no matter whether you use Galilean space and time or relativistic space and time. Therefore, the relativistic time is not clock time but a fake time and so is the relativistic space. Thus, all what special relativity describes is irrelevant to the physical reality.

As special relativity has been disproved and our physical time is absolute, there can be only one inertial reference frame relative to which the speed of light is isotropic. Since the speed of light after going through a lens can recover, unlike the speed of a bullet which will never recover after going through a wall, the speed of light only depends on the medium and thus light should be waves of a medium we call aether. Michelson-Morley experiment has denied the existence of a rigid aether, and thus aether must be a fluid. The very inertial reference frame with isotropic speed of light should be the frame moving with local aether similar to the frame moving with local air relative to which the speed of sound is isotropic. As light can exist everywhere in the visible part of the universe, aether should exist everywhere too. All electromagnetic phenomena are just the phenomena of aether dynamics. There is no electric field and no magnetic field in nature, but only different flows of aether. So-called electric force and magnetic force are forces exerted by the flows of aether, just like the resistance and lift exerting on an airplane, where there is no resistance field and no lift field, but only air flows. As aether exists everywhere, delivers all electromagnetic forces and plays critically important roles in all physical processes, quantum mechanics, without taking the effects of aether into account, should be wrong too. As every particle is bathed in aether, any motion of the particle will disturb aether and generate waves of aether to make the particle show the particle-wave duality. Thus, there is no probability wave in nature, not to mention the existence of wave function, superposition, entanglement and Schrodinger’s cat. Similarly, there is no such thing called spacetime in nature, not to mention the existence of expansion, curvature, ripples or singularities of spacetime. Thus, general relativity and big bang theory are wrong too.

Though common sense is not always reliable, physics theories are unreliable too. Now we know that all electromagnetic theories, quantum mechanics, relativity and their based physical theories are wrong. Many physical phenomena should be explained by aether dynamics which is starting a new era for modern physics.

Sydney Grimm 5 September 2019

If everyone is convinced that the earth is the centre of the universe, the heliocentric point of view is “violating” common sense. That’s why I suppose that one of these days quantum mechanics – actually quantum field theory – will become common sense too. We only need the right concept to understand the underlying mathematical structure of the basic quantum fields.

Ken Albert 4 September 2019

Typo/missing word in the article? 'The direct answer, in most cases, is that “quantum theory” as it is taught in physics texts simply does make definite claims one way or the other about many aspects of the physical world.' should be 'The direct answer, in most cases, is that “quantum theory” as it is taught in physics texts simply does **not** make definite claims one way or the other about many aspects of the physical world.', I think.