Measurement Explained: The Sensorial Interpretation of QM

I’m proposing here a new definition of measurement, which I think can solve the measurement problem and clarify much of the apparent “weirdness” of quantum phenomena.

This new definition of what constitutes a measurement (not only in quantum mechanics, but in a more general sense) provides the basis for a new interpretation of quantum mechanics, the “sensorial interpretation”.

The basic idea is that, when physicists carry out a measurement, their physical bodies (more precisely, their sensory receptors) have to be considered as part of the measurement apparatus.

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 receptors 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.

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.

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

3. A physical interaction between two 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.

We can further clarify the notion of measurement by making explicit what constitutes an observation, and what differentiates sensory receptors of living organisms from other physical systems:

4. The observation of a physical system is a change in the state of the sensory receptors of a living organism, correlated with the state that physical system is in.

We are introducing here a crucial distinction between measurement and observation. Measurement is a change in the wave of probabilities that defines the probable future states of the sensory receptors of any living organism, accompanied by the reduction of the measured physical system to a definite state. Observation is an actual change in the state of the sensory receptors of a particular living organism, at a particular location in space-time.

Measurement and observation can happen simultaneously, but in most cases the measurement precedes the observation. Every observation implies a previous or simultaneous measurement. Conversely, not every measurement results in an observation. The mere possibility of an observation is enough to make a measurement happen.

5. The sensory receptors of living organisms are physical systems that always exist in a definite state.

This is the main postulate of the sensorial hypothesis: that the sensory receptors of living organisms are fundamentally different from other physical systems, in that they can never be found in a superposition of states.

The question of why this should be the case lies outside the realm of physics (for a philosophical discussion of this postulate, see my essay “The Law of Unity”). What interests physicists is that this hypothesis, at least in principle, can be tested experimentally.

In this view, since the sensory receptors of living organisms are always in a definite state, it is the interaction with these sensory receptors what causes the collapse of the wave function: in other words, what gives rise to physical reality.

Physics and biology

The special role ascribed to living organisms by the sensorial hypothesis implies a radical rethinking of our scientific paradigm. The reductionist notion of a hierarchy between the natural sciences, with physics as the most fundamental, followed by chemistry and then biology, can no longer be maintained.

According to the sensorial view, there are no such hierarchies in nature. Everything is interwoven in an organic, holistic way. It’s true that there can be no biology without chemistry, and no chemistry without physics, but it’s also true that there can be no physics or chemistry without biology.

The physical world is the world we sense. The world we touch and taste and smell and hear and see. The sensorial view guides us away from the purely mental lucubrations of other interpretations, back to our direct, sensed experience of the physical world. It rescues physics from the realm of thinking, where it has gone astray, and brings it back to the realm of sensing, where it belongs.

Quantum weirdness explained

The sensorial interpretation of quantum mechanics sheds a new light on the apparent “weirdness” of quantum phenomena. We can illustrate this using four famous thought experiments: the double-slit experiment, 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 receptors 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 experimenters’ sensory receptors. No measurement is happening. The photon behaves as a wave of probabilities.

We now introduce some kind of device capable of detecting if the photon goes through 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 receptors of the human experimenters. There is a correlation between the probabilities of observing the photon going through one slit or the other, and the probabilities of observing the sensory receptors of the human experimenters in one particular state or another.

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 retinas 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 (more precisely, the moment the photon reaches the detector at the double-slit), not at the moment they look at the detector. 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 receptors (photoreceptor cells in their retinas) 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 (the update or reduction) of the wave function. 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 (more precisely: the moment the Geiger counter’s detector is in a position where a potential particle emitted by the radioactive substance would reach it), the measurement is done. The wave-function collapses. There is no “superposition of states” anymore.

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 (in range of potential particles emitted by the radioactive substance) and switch it on, the wave function describing the state of their retinas 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 and it hits the detector) you’ll see a green light on the Geiger counter; if you look after that, you’ll see a red light”.

Note that this update in the wave function of the sensory receptors of the experimenters implies also an update 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 receptors of all human beings, and of all living organisms, share fundamental characteristics (explained by a common evolutionary origin) that cause them to interact with other physical systems in basically the same way. (See the contrast with the absurd “Wigner’s friend” scenario.)

The interaction with the sensory receptors of the human eye 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. Its wave function has been updated, indicating the exact moment in time when an atomic decay will happen.

For the human experimenters, however, this update in the wave function will only become manifest when they make an observation (when they “open the box”).

This famous thought experiment illustrates what constitutes a measurement apparatus. The box is not part of the measurement apparatus: it is completely irrelevant for the measurement process. The Geiger counter, on the other hand, is a necessary part of it.

C. The quantum bomb tester explained

In this light, the apparent weirdness of the quantum bomb tester disappears. If the bomb is live, it introduces a change in the wave of probabilities of the sensory receptors of the experimenters, correlated with the wave of probabilities of the photons (if the photon takes the lower path the experimenters will hear a bang; if it takes the upper path, they will hear a click at detector D; or whatever), therefore 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 inaccurate 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 (more precisely: the moment a potential photon would reach the bomb).

D. EPR-Bohm-Bell explained

Let’s say we have pairs of entangled particles being sent to two detectors placed at different distances from the source. The two detectors randomly and independently change between three different measurement settings. A measurement happens when one of the entangled particles reaches the first detector (the one positioned closer to the source). Its entangled pair automatically collapses to a definite and correlated state, before it reaches the second detector. This correlation will only become apparent if both detectors happen to be at the same setting. If the second detector is set to a different measurement setting, we will have two different uncorrelated measurements, instead of one.

In the sensorial interpretation, physical systems (particles, say) are nothing but waves of probabilities of observation. These waves extend indefinitely in space-time. All waves of probabilities of observation overlap at any given location in space-time. Measurements (like all physical interactions) happen locally, but they happen simultaneously at all locations where the probabilities of observation are above zero. There is no “spooky action at a distance”.

Cosmological and existential 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 organisms 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.

This will undoubtedly sound absurd at first glance. But if we take a closer look at how this would work, it may begin to make a little more sense.

Let’s begin with the sun. We all know that life on Earth (life as we know it, at least) would be impossible without the energy coming from the sun. In what sense, then, can we say that the sun was “created” by microscopic organisms?

The idea is that the photoreceptors in the first photosensitive bacteria and archaea, and/or in the first microalgae, were able to measure the position of the sun, and also its intensity, wavelength, etc. In this view, light and energy are fundamental phenomena, as fundamental as life itself. But the physical sun we now observe, with its particular location in space-time, is the result of measurement.

What about the outrageous idea that the universe at large, with all its billions of galaxies, has been created by modern scientists? We are talking here about the non-visible universe, which has been discovered in modern times with the help of advanced technology, like radio telescopes. The simple optical telescopes used by the founders of modern astronomy (Galileo discovering Jupiter’s moons, say) did nothing but amplify visible light coming from celestial objects. This visible light must have been unconsciously measured millions of years ago by organisms (dinosaurs, perhaps) equipped with eyes sensitive enough to detect photons reflected by the moons of Jupiter or the rings of Saturn, or coming from the distant stars composing a visible galaxy like Andromeda, etc. But the cosmic microwave background radiation (the main evidence for the Big Bang), for example, was never measured (and therefore, was never there) until humans developed radio technology. The same can be said of black holes, quasars, pulsars, etc.

In a recently published interview with Sabine Hossenfelder, Roger Penrose offers a beautiful illustration of the absurdity of the “consciousness causes collapse” interpretation of quantum mechanics: “Imagine a space probe going out looking at planets. The space probe visits a planet with no conscious being anywhere, not on that planet and not anywhere close, and it takes a photograph. Now, the weather is a chaotic system and ultimately depends on quantum effects. So the space probe sees a superposition of different kinds of weathers. It takes a photo and sends it back to Earth. After I-don’t-know-how-many years, someone sees the photo on a screen. And when that conscious being sees the photo, flip, suddenly it becomes one weather? That makes absolutely no sense to me. It seems to me that is not the right answer, surely.”1

Professor Penrose is absolutely right, of course. The “consciousness causes collapse” interpretation makes no sense. In the sensorial view, the weather in the distant planet collapses into a definite state at the exact moment the space probe takes its photograph. It is irrelevant when or if that photograph will be observed by human scientists back on Earth. By the time the radio signal carrying the information of the photograph reaches Earth, a catastrophic event might have happened that destroyed all advanced human technology. It doesn’t matter: the measurement has been already made.

But what if the catastrophic event destroying human civilization happened before the space probe reached the distant planet and took the photograph? In that case, no measurement would be made. The weather of the distant planet would remain in a quantum superposition. As for the space probe, it would dissipate into nothingness: the probability of it being observed by any living organism on Earth would be zero.

According to the conventional physicalist worldview, the laws of physics are fundamental, while life is a mere accident. In the sensorial view it’s the other way round: life is fundamental, and the physical laws we currently observe, with all their apparent arbitrariness and inexplicable constants, are the accidental result of the evolution of life on Earth.

This is without doubt the deepest implication of the sensorial interpretation: life is more fundamental than the physical universe. In this view, even if humans end up destroying the physical Earth (what we nowadays call “planet Earth” even if it has little in common with the celestial planets out there), they can’t destroy life itself. This physical universe might come to an end some day, but life will go on.

Experimental challenges

This new interpretation of quantum mechanics will appear compelling to some, preposterous to others. The good news is that, since in some special cases it makes different predictions to other interpretations, it can be, in principle, tested experimentally. The bad news is that this experimental falsification might be quite difficult in practice, maybe impossible.

A theoretically possible experimental test that springs to mind would be a simple double-slit experiment involving an “undetectable detector”: a device capable of acquiring “which-path information” without interacting in any way with the sensory receptors of the human experimenters. I haven’t been able to find in the literature an example of this theoretical “undetectable detector”.

In some experimental demonstrations of the double-slit experiment, polarizing crystals (quarter-wave plates) are put in front of each of the slits, altering the polarization of the photon as it goes through one slit or the other. These crystals destroy the interference pattern, acting as “which-path markers”. According to the sensorial interpretation, the quantum superposition (and therefore the interference) is destroyed because the sensory receptors of the human experimenters are capable of detecting the polarization of single photons. This isn’t a far-fetched assumption: the human eye is (at least to some degree) sensitive to polarization.

It’s important to remember that the sensorial interpretation says nothing about perception. Perception is a complex process that happens in the brain, involving the organization and interpretation of sensory information coming from the sensory receptors via the nervous system. Human perception is famously unreliable. All kinds of errors can happen during the processing of sensory data, as has been often demonstrated with different kinds of optical illusions. But the sensory receptors themselves are never wrong. When a photoreceptor in our retina detects a photon, we can be sure that the photon is there. Sensory receptors provide a reliable basis for physical reality.

It is well known that the photoreceptors in the human retina can respond to a single photon. But neural filters only allow a signal to pass to the brain to trigger a conscious response when at least about five to nine arrive within less than 100 milliseconds.2 Again, the sensorial interpretation has nothing to do with human consciousness.

Human scientists can make mistakes in the way they interpret their observations. They can get a glimpse of Saturn’s rings and take them to be moons, for example. But their sensory receptors are never wrong. It’s the sensory receptors that actually perform the observation, not the scientists’ mind.

In short, it doesn’t seem easy to find a way to falsify the sensorial interpretation by performing a quantum measurement without any sort of interaction with the sensory receptors of the experimenters. It may very well be an impossible task.

This leaves us with yet another unfalsifiable interpretation of quantum mechanics. (As if we didn’t have enough already!) Still, the sensorial interpretation presents us a refreshing, intuitive and meaningful view of the physical universe, one that sees life not as a mere accident, but as the fundamental ground and source of physical reality.

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1Sabine Hossenfelder. (2022) Existential Physics. Viking. Penguin Publishing Group.

2Philip Gibbs, 1996. Physics FAQ.