Measurement Explained: The Sensorial Interpretation of QM (faulty version)

The infamous “measurement problem” in quantum mechanics remains, as of 2022, an unresolved and embarrassing issue in the field of physics, the most influential and prestigious science of our time. Astonishingly, there is no consensus among theoretical physicists about the very meaning of the concept of “measurement”. This is particularly mortifying, since physics itself can be defined (in more than one sense, as we will see) as “the science of measurement”.

In the last century, since the very beginnings of quantum physics, various attempts have been made at explaining what constitutes a measurement. None of them have been successful. The purpose of this short essay is to propose a new, quite simple theory of measurement, one that will also clear up much of the apparent “weirdness” of quantum reality.

The idea I’m proposing is ridiculously simple: the reason why physicists have so far failed to understand the apparent mysteries of quantum measurement is that they have overlooked a very obvious, undeniable fact: physicists have physical bodies.

Physicists, like most scientists, like to see themselves as disembodied minds, observing reality in an objective, detached way. But the truth is that physicists, like all humans, and like all living organisms, do have physical bodies. Every time physicists carry out a measurement, their physical bodies (more precisely, their sensory systems) have to be considered as part of the measurement apparatus. An indispensable, crucial part.

It is interesting to observe how, while forgetting their physical bodies, some physicists have on the other hand come to believe that their minds could play some fundamental role in physical reality. This is, of course, absurd. The human mind doesn’t belong to the physical world. It doesn’t have any physical properties we can measure. The human body, on the contrary, does.

Measurement: a new definition

Without any further preambles, we can now introduce our new definition of measurement:

1. Measurement is the interaction of a physical system with the sensory system of a living organism.

We are proposing here a broader concept of measurement, which encompasses all living organisms. Measurements carried out by human scientists constitute only a particular kind. In this sense, we can say that plants measure the light irradiating from the sun, microscopic algae measure the temperature of water, and so on. The mathematical precision applied by modern human scientists to their measurements doesn’t constitute a fundamental difference. In fact, the whole notion that physical reality is fundamentally mathematical in nature is only an illusion derived from the fact that modern scientists use mathematical equations in their measurements. Other living organisms don’t.

To clarify our definition, we need to elucidate what we mean by “interaction” and “physical system”:

2. A physical system is a wave of probabilities of observation.

This wave of probabilities is described in mathematical terms by the wave function (Schrödinger’s equation). Any physical system (a photon, an electron, an atom, a star, the whole universe) is nothing but a wave of probabilities of observation.

3. A physical interaction between two distinct physical systems is a correlated change in the wave of probabilities of both systems.

Any change in the wave of probabilities of a physical system is expressed mathematically as an update of the wave function.

It might be objected that we are incurring in a circular definition, since we are including the concept of “observation” within the definition of measurement, and both terms are usually considered synonymous. Sadly, some circularity is unavoidable if we want to remain within the realm of physics. Physics is, indeed, the science of measurement. The physical world is nothing but measurement.

Electrons, quarks, atoms, stars, are not self-existing objects. There are no “things” out there. There is nothing but measurements and observations, and the probabilities of observation.

Still, we can further clarify the notion of measurement by proposing a distinction between measurement and observation, and by making explicit what separates sensory systems from other physical systems. But in order to do that, we need to step for a moment outside the realm of physics:

4. An observation is the physical component of an instance of sensing.

We are introducing here a subtle but crucial distinction between the related concepts of measurement and observation. Measurement is a purely physical phenomenon. Observation is also a physical phenomenon, but it is inseparably (and mysteriously) linked to a non-physical phenomenon: sensing.

What do we mean by “the physical component of an instance of sensing? Well, for example, if a human eye is sensing red light, the physical component is the particular state of the photoreceptors in the retina. This is a purely physical state that can be measured. In the case of scientific observations involving instruments like telescopes, microscopes, detectors, particle accelerators, computers, etc., the physical component would include the particular state of all of these.

5. The sensory systems of living organisms are physical systems capable of sensing.

And that’s it. We are outside the realm of physics now. We can’t define what “sensing” means, using strictly physical terms. We are stepping into the realms of biology and psychology. We all know what sensing means, but we know it in a purely subjective manner. Sensing itself cannot be measured.

This shouldn’t be construed as a shortcoming in our approach, however. On the contrary, it is the reductionism of the current paradigm that is hampering the progress of physical science. The reductionist notion of a hierarchy between the natural sciences, with physics as the most fundamental, followed by chemistry and then biology and then psychology, is a misleading illusion. There are no such hierarchies in nature. Everything is interwoven in a much more organic, holistic way. It’s true that there can be no psychology without biology, and no biology without chemistry, and no chemistry without physics, but it’s also true that there can be no biology without psychology, and no physics or chemistry without biology and psychology.

The physical world is the world we sense. The world we touch and taste and smell and hear and see. This is such an obvious statement it seems ridiculous. And yet, most modern physicists have become strangely oblivious to it.

What we are doing with this new approach is simply bringing back the science of physics to the realm of sensing, where it belongs, and away from the realm of thinking, where it has gone astray. All aberrations in modern physics, like absurd “consciousness causes collapse” interpretations, or the various attempts at introducing abstract notions of “knowledge” or “information” into physics, or the infamous “many-worlds” interpretation of quantum mechanics, are the result of losing touch with our sensed reality (the only physical reality) to wander aimlessly into the realm of thought and fantasy.

Quantum weirdness explained

This new definition of measurement sheds a new light on the apparent “weirdness” of quantum phenomena. We can illustrate this using four famous thought experiments: the double-slit experiment (Feynman’s version), Schrödinger’s cat, the Elitzur-Vaidman bomb tester and the EPR-Bohm-Bell experiment.

A. The double-slit experiment explained

Let’s imagine a single photon passing through a double slit. And let’s suppose that this double slit is placed in a room with several experimental physicists present. There is no interaction between that photon as it passes through the double slit and the sensory system of the human experimenters (the only living organisms that are relevant in this particular case). In other words, there is no correlation between the wave function of the photon and the wave function of the human experimenters. No measurement is happening. The photon behaves as a wave of probabilities.

(We need to remember that photons, like any other physical system, are not self-existing objects. It is absurd to say that photons can “go through both slits at the same time”, or that subatomic particles can somehow “be at two locations at the same time”. In the physical world, there is only measurement and probabilities of observation.)

We now introduce some kind of device capable of detecting the photon at one slit or the other. The moment we add this detector to the experimental setting, we create an interaction between the photon and the sensory system of the human experimenters. There is a correlation between the probabilities of observing the photon at one slit or the other, and the probabilities of observing the sensory system of the human experimenters in one particular state or another.

Sensory systems of living organisms (physicists, for example) are physical systems. As such, they are waves of probabilities of observation. In practice, it would be impossible to calculate the wave function of a human eye. It would be mind-bogglingly complex. But in principle, a human eye behaves like any other physical system.

To simplify, let’s imagine that we have a detector that will flash a red light if the photon goes through the left slit, and a green light if the photon goes through the right slit. This detector generates a change in the probabilities of the eyes of the experimenters (or any other non-colour-blind person present in the room) sensing red or green lights, which are correlated with the probabilities of the photon going through one slit or the other.

The crucial point here is that the measurement happens the moment the experimenters introduce the detector at the double slit, not at the moment they look at the detector. In fact, it doesn’t matter if the experimenters pay attention to the detector or decide to take a nap instead, or look through the window at the passing clouds. It is also irrelevant if they stay in the room or not. The interaction has already happened: the probabilities of observing the sensory system of the experimenters at certain states (sensing red or green light, say) have changed, in correlation with the probabilities of observing the photon at one slit or the other. In traditional terms: the wave function of the photon has “collapsed”. It can no longer interfere with itself. There will be no interference pattern on the screen.

This illustrates the key distinction between measurement and observation. It isn’t any particular instance of observation that causes the collapse. It is the measurement, which encompasses all the probabilities of observation, that does it.

B. Schrödinger’s cat explained

The same understanding applies to Schrödinger’s famous cat. The moment we introduce the Geiger counter into the box, the measurement is done. The wave-function collapses. There is no “superposition of states”.

It doesn’t matter when we open the box, or if we open the box or not. Let’s say that the Geiger counter, while connected, displays a green light. If it detects a radioactive particle, the green light changes to red. The moment the experimenters put the Geiger counter inside the box and switch it on, the wave function describing the state of their eyes changes. Before introducing the Geiger counter, this wave function would say something like “if you look into the box, you’ll see a chunk of radioactive substance” (we can leave the poor cat out of this). The radioactive substance is in a quantum superposition of possible states. Those states aren’t detectable by the human eye.

With the Geiger counter in place, the wave function will say “if you look into the box before this particular moment in time (the moment in time when the radioactive substance emits a particle) you’ll see a green light on the Geiger counter; if you look after that, you’ll see a red light”.

Note that this change in the wave function of the eyes of the experimenters implies also a change in the wave function of all (non-colour blind) humans. If the pizza delivery guy happened to come into the lab and peak into the box, he would see the same thing: green light or red, depending on the moment in time. There is nothing mysterious in this. The sensory systems of all human beings, and, in fact, of all living organisms, are inextricably entangled. (See the contrast with the absurd “Wigner’s friend” scenario.)

The interaction with the sensory systems of the whole of humanity is what “collapses” the wave function of the radioactive substance. The moment the Geiger counter is in place, the radioactive substance is no longer in a superposition. It is in a definite state. It’s wave function has changed, indicating the exact moment in time when an atomic decay will happen.

The bad news for physicists is that this change in the wave function of the radioactive substance, since it is ultimately linked to a non-physical, subjective phenomenon (sensing), can’t be predicted using mathematical equations. All we have are the probabilities given by the wave function itself.

C. The quantum bomb tester explained

In this light, the apparent weirdness of the quantum bomb tester disappears. If the bomb is live, it functions as a detector, causing the collapse of the quantum superposition. If the bomb is a dud, there is no interaction and the quantum superposition (of possible paths for the photons) remains.

In other words, it’s incorrect to call this an “interaction-free measurement”. According to our definition of measurement, the interaction occurs the moment we put the live bomb into place.

D. EPR-Bohm-Bell explained

Without going into much detail, the same applies here. The measurement happens when the detectors are switched to a new position, not when the “particles” or whatever1 reach the detectors. No matter the distance the entangled particles have to travel, the measurement happens instantly. But it happens locally. (For the measurement to happen, there must be an interaction between the wave function of the detectors and the wave function of the particles. In other words, the detectors have to be placed in the particles’ trajectories.) There is no “spooky action at a distance”. The particles are in a definite state the moment the detectors are set in place.

The sensorial interpretation of quantum mechanics

I’m presenting here a completely new (as far as I know) interpretation of quantum mechanics. From my limited perspective, it’s such a ridiculously simple interpretation that it seems self-evident.

At the same time, it offers a radically new view of the physical universe. If I’m correct, this is the culmination of the scientific revolution begun roughly a century ago by giants like Einstein and Bohr.

It also offers a few characteristics that set it apart from all other interpretations (I think):

Beyond local and nonlocal

The sensorial interpretation transcends the traditional opposition between local and nonlocal phenomena. Measurements happen instantaneously, involving all living organisms capable of sensing a particular phenomenon (from a red light flashing on a Geiger counter to a solar eclipse), no matter their location. But measurements happen locally, at a particular location in space-time. The wave functions describing the states of the sensory systems of all those living organisms are purely local (something like: “if you happen to go into that lab and look into that box at that particular time, you will see a red light”).

The key to understanding this is to remember that physical systems are not things (“particles”, or whatever) located at a particular point, but waves of probabilities. These waves extend indefinitely in space-time.

Beyond objective vs. subjective

The sensorial interpretation of quantum mechanics transcends the objective-subjective dichotomy. Physical reality is objective, in the sense of being the same for all living organisms, but it includes as a fundamental element a purely subjective quality: sensing. The physical world is real because we can touch it, taste it, smell it, hear it, see it. Because we can sense it.

Beyond mechanistic causality

The sensorial interpretation offers a radically new way of thinking about the physical world. We no longer find ourselves in a world of things, of particles bumping into each other like tiny billiard balls. Instead, the physical world reveals itself as nothing but measurement and observation and probabilities of observation. Ultimately, there is nothing to the physical world but sensing and probabilities of sensing.

We can no longer expect to find some sort of “mechanism” to explain the collapse of the wave function. The ultimate reason the wave function of a physical system collapses is quite simple: sensing is always definite; we either see a red light, or a green one. But this doesn’t mean that sensing itself causes the collapse. It is the mere possibility of being sensed that forces a physical system (a photon, say) into a definite state. Nature is always a step ahead of us. We will never catch her with her pants down.

Nature doesn’t improvise, either. She isn’t waiting for us to look, before coming up with a definite result. She has all the answers ready, before we happen to ask. This is actually what Einstein meant, I believe, with his famous phrase “God does not play dice”.

Beyond the Einstein-Bohr debate

The sensorial interpretation of quantum mechanics provides a satisfactory resolution (in my view, at least) to the celebrated debate between Einstein and Bohr: both were right, in their own way. Measurement constitutes the only physical reality (Bohr). But physical reality is objective, and independent of any particular act of observation (Einstein).

We now can at last answer Einstein’s famous question about the moon: The moon is always there, because it is being measured all the time… even when no one is looking at it!

Philosophical implications

If correct, the sensorial interpretation of quantum mechanics has far-reaching implications. The trite comparison with the Copernican Revolution seems inevitable. The irony is, the sensorial interpretation brings us back to a fundamentally geocentric worldview.

If physical reality relies on the existence of living organisms, it becomes obvious that the physical universe as we know it is the result of the evolution of life on Earth. The sun was created by microscopic plants in the beginnings of life. And the universe at large, with its billions of galaxies, and black holes, and the Big Bang at the bottom of it all, has been created by modern human scientists. Earth is the centre of the universe.

The centre of this physical universe, at least. We can speculate about the existence of other universes out there. But one thing is certain: there is no one living in all those planets we can observe through our telescopes. All those planets have been created by us (more precisely, they have been created by our measurements). In other words: Earth is not a planet.

(In this view, the UFO phenomenon acquires a new meaning. If UFOs are real (I’m agnostic on this), there are only two options. Option A: they are coming from a different universe, in which case they are non-physical in nature. This would explain their apparent defiance of physical laws. In short, the aliens described in many UFO sightings are non-physical creatures, like fairies or elves. Option B: UFOs are indeed physical phenomena, in which case they originate in Earth. In both cases, the hypothesis of “visitors from other planets” has to be discarded.)

These novel ideas, like the outrageous notion that plants created the sun, will sound absurd to most people. This is due to the common belief that physical reality is the fundamental, ultimate reality. If the sensorial interpretation is correct, that belief is false. In this view, life is more fundamental than the physical universe. This would be a comforting discovery indeed. It would mean that even if humans end up destroying the physical Earth (what we call “planet Earth”), they can’t destroy Earth itself. This physical universe might come to an end some day, but life will go on.

The experiment (The undetectable detector)

This new interpretation of quantum mechanics may appear compelling to some, preposterous to others. The good news is that, since in some special cases it makes different predictions than other interpretations, it can be tested experimentally.

This could be done with a relatively simple double-slit type experiment. I propose here a variation of a well-known experiment published in Physical review, March 2002, by S. P. Walborn, M. O. Terra Cunha, S. Padua and C. H. Monken2.

An argon ion pump laser is set up to emit single photons. Each photon passes through a special nonlinear crystal called beta-barium borate (BBO), where it is converted to two entangled, longer wavelength photons. The two entangled photons go off in two different directions, p and s.

The s photons (those which go down path s) travel through a double-slit to detector Ds. The p photons travel directly to detector Dp. Detector Dp is configured to measure the polarization of each registered photon. Detector Ds is configured to measure both the polarization and the location of each photon.

With this set-up we should observe an interference pattern on detector Ds.

(Source: Wikipedia)

A quarter wave plate (QWP) is now put in front of each slit. This device is a special crystal that changes linearly polarized light into circularly polarized light. The two wave plates are set so that given a photon with a particular linear polarization (x/y), one wave plate will change it to right circular polarization while the other will change it to left circular polarization.3

Because p and s photons form entangled pairs, by measuring the linear polarization of each p photon, we can know the polarization of the corresponding s photon before it reaches the quarter wave plates. With this configuration, it is possible to figure out which slit the s photon went through, without disturbing the s photon in any way.

(Source: Wikipedia)

It is not necessary to actually observe the polarization of p and figure out what slit s passed through. Once the quarter wave plates are introduced, the mere possibility of making that observation is enough to make the interference pattern disappear.4

According to the sensorial interpretation, the explanation for this “weird” effect is that, by introducing the quarter wave plates, we are creating an interaction between the probabilities of an s photon passing through slit 1 or slit 2 and the probabilities of observing the sensory systems of the experimenters (or any other human beings) in certain corresponding states.

To test the validity of this interpretation, let’s replace detector Dp with an “undetectable detector” (UDp): a device that is configured to measure the polarization of each photon, exactly like the original detector Dp, but with a crucial difference: detector UDp is designed to show the results of its measurements in a display undetectable by the human sensory system. For example, by using ultraviolet light on a black screen (I leave it to experimental physicists to figure out the technical details).

With this new configuration, the “which-path information” would still be there. It just would be invisible to the human experimenters, undetectable by the human sensory system. The interaction between the sensory system of the experimenters and the path taken by the photons would be broken.

According to the sensorial interpretation, with detector UDp in place the interference pattern would appear again on detector Ds. All other interpretations (as far as I can tell) would make a different prediction: the result should be the same as with detector Dp in place (no interference pattern).5

If the result of this experiment agreed with the sensorial interpretation, it would open up a whole new field of scientific exploration: that of the central role played by the sensory system of living organisms in the generation of physical reality.

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1Mermin, N. David (1985). “Is the moon there when nobody looks? Reality and the quantum theory”. Physics Today. 38.

2Walborn, S. P.; et al. (2002). "Double-Slit Quantum Eraser". Phys. Rev. A. 65.

3Specifically, the first wave plate (QWP1), placed before slit 1, will change an x polarized photon (that is, a photon linearly polarized along the x axis) to a left circularly polarized photon (L), while the second wave plate (QWP2), placed before slit 2, will change it to a right circularly polarized photon (R). With a y polarized photon, the reverse will happen: QWP1 will change it to R, while QWP2 will change it to L.

4Walborn et al. explain this with the formula |Ψi = 1/√2(|ψ1(r)〉|M1〉+|ψ2(r)〉|M2〉), where |Mj〉 is the state of the which-path marker corresponding to the possibility of passage through the path j and |M1〉 is orthogonal to |M2〉. The only interpretation they provide is “The which-path marker’s presence alone is sufficient to make the two terms on the right-hand side of equation (2) orthogonal and thus there will be no cross terms in |〈r|Ψ〉|². Therefore, it is enough that the which-path information is available to destroy interference.” (Walborn, S. P.; et al. (2002). "Double-Slit Quantum Eraser". Phys. Rev. A. 65 (1-2).) One could ask the question: available to whom? And why should the “availability of information” change the behaviour of a physical system?

5I’m not familiar with interpretations based on “availability of information”. It may well be that, according to some information-based interpretations, the replacement of detector Dp with the “undetectable detector” UDp would effectively destroy the information available to the experimenters, thus bringing back the interference pattern. I have no idea why, in these hypothetical interpretations, the information available to human experimenters should play such a crucial role in the behaviour of physical systems.