Over the last handful of years, fueled by many dozens of books, lectures, videos, and papers, I’ve been pondering one of the biggest conundrums in quantum physics: What is measurement? It’s the keystone of an even deeper quantum mystery: Why is quantum mechanics so strangely different from classical mechanics?
I’ll say up front that I don’t have an answer. No one does. The greatest minds in science have chewed on the problem for almost 100 years, and all they’ve come up with are guesses — some of them pretty wild.
This post begins an exploration of the conundrum of measurement and the deeper mystery of quantum versus classical mechanics.
A simple starting point is the question: What is the difference between quantum physics and quantum mechanics? We could ask a similar question about classical physics versus classical mechanics. It’s a good starting point because it’s a question we can answer.
Physics is the more inclusive term. It’s the study of physical systems and the laws that apply to them. A given kind of physics, be it classical, quantum, nuclear, solid state, or whatever, delineates the kind of physical systems studied.
Mechanics is the subset of physics that studies the dynamics of the system under study, its motion and forces. The canonical example of classical physics is Newton’s laws of motion. In quantum mechanics, it’s the Schrödinger equation (or its relativistic equivalent).
This, therefore, is what classical and quantum mechanics have in common. In both, we solve equations of motion that tell us how a physical system evolves over time. In classical mechanics we have an actual physical object — a ball, a car, a planet — and we predict its future (or sometimes past) trajectory given that we know where it is now and how it’s moving.
Which brings us to the first strange difference about quantum mechanics. The Schrödinger equation tells us how a quantum state evolves. Part of the deep mystery is that we don’t know what a quantum state actually is — it’s not something we can directly access, like a ball, car, or planet.
What we do know is that a quantum state is something we can measure, and that brings us to another strange difference between classical and quantum mechanics. When we measure a quantum state, we get a definite result, but nothing in quantum mechanics tells us with certainty exactly what that result will be. All we can know is that there is some probability of getting certain result.
For example, we can solve the Schrödinger equation for the motion of the single electron in a hydrogen atom. What we get is a “cloud” of probability surrounding the nucleus that tells us a probability of finding the electron in a particular spot.
Another oddity of quantum mechanics (called the “long tail”) is that the Schrödinger equation provides, for that electron, a position probability for every point in the universe. There is some (vanishingly small) probability the electron could be detected in the Andromeda galaxy — or even in a vastly more distant galaxy. The probability may be indistinguishable from zero, but it’s not exactly zero. Contrast this with classical mechanics which does tell us exactly where things are.
Which is another difference between them. In classical mechanics we can, at least in theory, know all there is to know about a physical system. We can know its exact location as well as its exact momentum. (Momentum is a combination of where something is going and how forcefully it’s going there.) Further, knowing the position and momentum of a classical system is all we need to fully define that system.
But in quantum mechanics, because of the Heisenberg Uncertainty Principle (HUP), we can only know half the information, even in principle. We can know exactly what the momentum of a “particle” is, but then we know nothing about its position. Alternatively, if we know its exact position, we know nothing about its momentum. We can also choose to know something about both, but the more we know about one, the less we know about the other.
I usually put the word “particle” in quotes because it seems likely (although not certain) that there is no such thing. The classical notion of tiny balls of something, such as photons, electrons, or quarks, is very probably not the reality. (Unless one subscribes to the deBroglie-Bohm interpretation of quantum physics or something along those lines.)
The reason we think in terms of “particles” is that, when we “measure” a “particle” we see a point-like interaction. Ever since Louis deBroglie, we understand matter as having a wave-particle duality. This duality resolves as a “particle” — a definite point-like location — when one matter wave interacts with another matter wave.
For example, a photon in flight acts like a wave until it interacts with the electron (wave) of an atom (which raises the energy level of that electron). The photon’s wave can be extremely spread out. Consider a photon emitted from a distant star but absorbed by a specific electron here on Earth. That photon could have been absorbed anywhere along its path.
A big part of the conundrum is the question of what happens to that wave once the photon is “measured” here on Earth. The wave apparently instantly vanishes everywhere else because no other electron, here or there, can ever absorb that photon.
The main point of this post involves why “measure(ment)” is also a quoted word. It’s not because we don’t know what measurement is, or because quantum measurements only have probabilities.
Firstly, many authors prefer to replace the word “measurement” with observation (although that seems a word game to me). The intent seems to be bringing quantum experiments, with their formal measurements, into the greater scope of experience where we merely observe things. It’s often said, for instance, that matter acts like a wave until we observe it, in which case it acts like a particle.
As an aside, what I consider one of the silliest interpretations of quantum mechanics is the notion (such as the von Neumann-Wigner interpretation) that consciousness is a necessary aspect of measurement/observation. Under this (very arrogant) notion, the universe was in some vague quantum state (or maybe didn’t even exist?) until we humans, or at least something conscious, showed up. I don’t believe anyone takes this seriously anymore.
What inspired this post is what authors I’ve read recently (Roger Penrose, Lee Smolin, Peter Woit, Jim Baggott, and others) say about how the environment affects an actual or putative quantum system. I say “putative” because the deep mystery of quantum physics involves the question of whether large objects, such as cats, are, in fact, quantum systems.
On some level, they have to be because they’re made from quantum parts. The question is whether the classical reality that emerges from the parts still has a quantum nature. More pertinently, can they be treated as quantum systems. (For what it’s worth, my guess is no.)
Large systems are said to decohere and, thus, lose their quantum nature. It’s often said that the environment causes this decoherence. By “environment” we mean the surrounding air and the “heat bath” of infrared photons that comprise our day-to-day environment (not to mention all the RF photons that are also part of that environment).
In actual quantum systems — such as photons, isolated atoms, or quantum computing qubits — this is definitely the case. A huge aspect of engineering quantum computers is finding ways to isolate the qubits from the environment in order to retain the coherence of qubits long enough for processing. This is also necessary in many quantum experiments. In such cases, extreme cold, RF shielding, and a vacuum are required.
All-in-all, the idea is that, effectively, the environment measures or observes the system, and this causes the system to have a definite (classical) state.
When it comes to large objects, I think they decohere on their own without needing any interaction with the surrounding environment. Further, this decoherence destroys their quantum nature. Effectively, large systems observe themselves.
An aspect I haven’t yet touched on is that measurement disturbs — “collapses” or “reduces” — a quantum system. The measurement conundrum is also called the question of wavefunction collapse.
That photon from a distant star exists as a spread-out wave until it interacts with some electron and localizes as a point-like “particle”. Likewise, electrons exist as waves until they interact with something that localizes them. This localization of the wavefunction to a point-like interaction collapses (or reduces) the wave from the cloud of probability to the certainty of the measured location. Part of the mystery is that the collapse is instant everywhere in seeming defiance of special relativity (which limits causality to the speed of light).
The classical reality we experience, as far as we can tell, is localized. Objects have definite positions (at least until you’re trying to find your keys). It should be noted that localization is just one form of measurement or wavefunction collapse. Many other types of measurement of quantum properties also do this.
Getting back to large objects and their apparent loss of quantum behavior, if it was just interaction with the environment that caused it, then presumably we could find such objects demonstrating quantum behavior if we could isolate them from the environment. Large objects in deep intergalactic space should then have quantum behavior.
Imagine we make a statue of a cat and isolate it from all environmental influence. If quantum mechanics applies to systems large and small, then that statue should act like a quantum object. (Obviously the extreme cold and vacuum prohibits using a living cat.)
In 2019 experimenters demonstrated quantum effects in objects comprised of 2000 atoms (about 25,000 atomic mass units). Until then, the record, from 2013, was 810 atoms (over 10,000 AMU).
The MAQRO Mission, a proposed ESA experiment, would use test objects about 100nm in size and, if it successfully demonstrates quantum behavior, will increase the record to 10,000,000,000 AMU. I hope I live long enough to see this mission fly. It will be a game-changer, especially if it fails. Even if it succeeds, we’re still a long way from cats, so success won’t be quite as compelling as failure (but will certainly be pause for thought).
My contention is that the self-interactions of the vast number of atoms in a large object are sufficient for the object to “observe” itself and for quantum behavior to vanish. Even considered as a quantum system, the wavefunction of each “particle” contributes a vanishingly small and separate fraction.
Imagine each “particle” as a singer with a song. Now imagine a vast collection of singers, on the order of 1,000,000,000,000,000,000,000,000,000 of them, each singing their own song. (In contrast, the world population is under 8,000,000,000.) The result is cacophony or white noise. No coherent music arises from such a collective even if they sang the same song. Any aspect of a song is destroyed.
Likewise, I contend that the quantum nature of large objects is destroyed merely in virtue of their size. Let alone contributions from the environment, but that does also contribute. The world at large is classical because it’s the song of an unimaginable number of singers.
Again, even if we consider large systems to be quantum systems, they are massively and completely, in their very nature, uncoherent systems that do not exhibit quantum behavior.
Simply put, reality measures itself. All the time. Constantly.
In future posts, I’ll dig deeper into some of the details, especially regarding decoherence and what are known as “objective collapse” theories. While it may be that large objects don’t show quantum behavior, mathematically explaining wavefunction collapse remains an issue.
As a teaser, I find myself leaning towards the Diósi-Penrose model, which suggests that gravity from the mass of large objects is responsible for wavefunction collapse. It may be that solving the quantum gravity problem will show this, or something similar, to be the case.
Stay measured, my friends! Go forth and spread beauty and light.