Where is the line between quantum and classical?

1. The textbook story (and why it's wrong)

For most of the twentieth century the answer most physics students were given went something like this: quantum mechanics describes the very small — electrons, atoms, photons, perhaps small molecules. Classical mechanics describes the everyday — planets, projectiles, fluid flow, the room you are sitting in. Somewhere between these domains, around the size of large molecules or microscopic dust grains, the quantum strangeness fades and ordinary physics takes over. There was even a name for this fade: the correspondence principle, due to Niels Bohr — in the limit of large quantum numbers, quantum mechanics should reproduce classical results.

That story is incomplete to the point of being misleading. The correspondence principle is correct as a mathematical statement: quantum mechanics does reproduce classical mechanics in the appropriate limit. But the textbook gloss — that there is a scale above which quantum effects vanish — turns out to be empirically false. Three things have been steadily clarifying since the 1980s:

• The disappearance of quantum effects in everyday objects has a specific physical cause — decoherence — that is now well understood and can be measured directly.
• Decoherence depends on environmental coupling, not on size as such. Larger objects decohere faster only because they tend to couple more strongly to their environment. Isolate them and the size limit recedes.
• Numerous laboratory systems already show macroscopic quantum behaviour — superconductors, superfluids, Bose-Einstein condensates, lasers — and experimental molecules with tens of thousands of atomic mass units have been put into superposition.

The line, in other words, has been moving. As of 2024 it sits somewhere above 10⁵ atomic mass units in matter-wave interferometry, well into the regime of small viruses. Whether there is any in-principle ceiling above which quantum mechanics breaks down is itself one of the active research questions of foundational physics. The current best guess is: no.

2. What "classical" actually means

Before asking where the line is, it helps to be precise about what we mean by classical. A system behaves classically when:

• Its properties have definite values at all times — not probability distributions of possible values, but actual single values.
• The properties of widely separated parts of the system can be assigned independently — no spooky correlations between distant pieces.
• Observation of the system does not change its state — you can look at it without disturbing it (in any fundamental sense).

A system behaves quantum-mechanically when these properties fail — when it shows interference patterns characteristic of being in two places at once, when it shows the Bell-violating correlations the Bell page describes, when measurement actively shapes the outcomes rather than simply revealing what was already there.

So the question is not really "where does the size threshold sit" but rather: at what point do these properties stop holding? And it turns out the answer depends almost entirely on how isolated the system is from its environment.

3. Decoherence — the actual mechanism

The cleanest insight of the last forty years of foundations work is that the disappearance of quantum effects in everyday objects is not a switch from one theory to another. It is the result of a specific physical process called decoherence, worked out in detail by Zurek, Joos, Zeh, Schlosshauer and others.

Here is the picture. A quantum system in superposition — say an electron in two places at once — remains in clean superposition only as long as it is perfectly isolated. The moment it interacts with anything else — a stray photon, an air molecule, a thermal vibration of the apparatus — the superposition spreads. The electron is no longer just in two places at once; the electron-plus-photon system is. Then the photon flies off and bounces off other things, and now the electron-plus-photon-plus-everything-it-touched is in superposition. The interference pattern that would have been visible if you'd looked at just the electron disappears, because to see it you would have had to also track the position of every photon and air molecule it had subsequently entangled with — and you cannot.

Crucially, the superposition has not been destroyed. It has been distributed. The information about which branch the electron is on is now spread across an environment so large and so warm and so chaotic that no laboratory can recover it. For all practical purposes the electron now behaves classically — you measure it in one place or the other, and you cannot make the alternative branches interfere again. But the mathematical structure of quantum mechanics has not changed. The system is still quantum; it just can't be made to act quantum any more.

This is the decoherence picture, and it has been confirmed in detail in many systems. It explains why larger objects appear classical: they couple more strongly to their environments, decohere faster, and are practically impossible to isolate. It does not explain that they are fundamentally classical — because in this picture they are not. The world is quantum all the way up; the classical appearance is what quantum mechanics looks like when it has been smeared into an environment you can't track.

4. Macroscopic quantum effects already known

It is sometimes underappreciated how many macroscopic phenomena already require quantum mechanics to explain. None of these is exotic — they are in undergraduate textbooks. All of them violate the textbook intuition that quantum is confined to atomic scales.

• Superconductivity. In a superconductor, billions of electrons pair up (Cooper pairs) and form a single coherent quantum wavefunction extending across the entire material. Currents flow without resistance because the wavefunction is too coherent to scatter. SQUID magnetometers exploit this directly: a single superconducting loop, millimetres in size, can be put into a superposition of carrying clockwise and counterclockwise current at the same time. This was first demonstrated by Friedman et al. in 2000 and is now routine.
• Superfluidity. Liquid helium below ~2 K loses all viscosity and flows in coherent quantum modes. Liquid helium-4 is a Bose-condensed fluid; helium-3 is a fermionic superfluid analogous to a superconductor. Both involve macroscopic quantum coherence in everyday-visible amounts of matter.
• Bose-Einstein condensates. Atomic clouds cooled to nanokelvin temperatures collapse into a single shared quantum state — thousands to millions of atoms acting as one wavefunction. BECs are now routine lab apparatus and have been used to demonstrate many-body entanglement (see Entanglement at every scale).
• Lasers. A laser beam is a coherent state of trillions of photons in a single quantum mode. The intensity is macroscopic. The coherence is essentially that of a single photon.
• Quantum Hall effect. A two-dimensional electron gas in a strong magnetic field shows conductivity quantised in exact integer or fractional multiples of e²/h — macroscopic measurement, atomic-scale precision.

These are not engineered exceptions. They are everyday phenomena in everyday laboratories that prove the quantum description does not stop working at large scales when the relevant coherence can be maintained.

5. Pushing the boundary — the mass record

The most direct test of "how big can quantum be" is to put progressively heavier objects into the classic two-slit interference setup. If the object shows interference fringes when both paths are open, it was genuinely in superposition. If the fringes wash out, decoherence has won. The record has been climbing steadily for thirty years.

• 1999 — C60 (buckminsterfullerene). Zeilinger's group in Vienna showed that buckyball molecules — 60 carbon atoms arranged as a soccer ball, mass 720 amu — produce a clear two-slit interference pattern. A 60-atom molecule was demonstrably in two places at once.
• 2013 — phthalocyanines and beyond. Hornberger, Arndt and colleagues extended the technique to organic molecules up to ~10,000 amu, with interference visible despite each molecule containing hundreds of atoms.
• 2019–2020 — Arndt's heavy-molecule frontier. The Vienna group reported interference of "oligo-tetraphenylporphyrin" molecules at ~25,000 amu — molecules containing roughly two thousand atoms, comparable in mass to a small protein. Interference confirmed. Quantum behaviour intact.
• 2021–2024 — mechanical resonators. Aspelmeyer, Lehnert, and others have entangled and put into squeezed states the vibrational modes of mechanical drums containing 10¹⁴ atoms — trillions of particles — visible under an electron microscope.
• Proposed — nanocrystals and beyond. The Bouwmeester / Aspelmeyer / Marletto proposals push toward picogram-scale superpositions of mirrors and crystals, with the explicit goal of testing whether quantum mechanics holds in regimes where gravity is also non-negligible.

The pattern is unambiguous. Every time the experimental technique improves, the mass record goes up. Nothing in the data so far suggests there is a fundamental ceiling — only that the engineering is hard. Interference has been seen at every scale where it has been carefully looked for.

6. Is there a fundamental ceiling? — collapse models

Standard quantum mechanics, taken at face value, predicts no ceiling. Any object, however large, can in principle be put into superposition if you can shield it from decoherence well enough. This is what the experiments above have been steadily confirming.

A minority view holds that standard quantum mechanics is incomplete — that something must intervene at some scale to collapse macroscopic superpositions and produce the definite outcomes we experience. These are the spontaneous collapse models, and the two most studied are:

• Continuous Spontaneous Localization (CSL), developed by Ghirardi, Rimini, Weber and refined by Pearle. Particles undergo random spontaneous localizations at a rate that scales with mass — negligible for atoms, fast enough to collapse macroscopic superpositions almost instantly.
• Diósi-Penrose gravitational collapse. Penrose proposed that mass-energy distributions cannot be in superposition for long because the gravitational fields they would produce are inconsistent — superposition collapses on a timescale of ℏ over the gravitational self-energy difference of the branches. Picks out a mass scale near the Planck mass (~22 micrograms) where collapse becomes fast.

These models are testable and have been progressively cornered by experiment. Vinante et al. (2017, 2020) used a microcantilever cooled to millikelvin to bound CSL parameters; Donadi et al. (2021) used measurements of spontaneous X-ray emission from germanium nuclei (a side-effect predicted by collapse models) to rule out the original GRW values. The simplest versions of both CSL and Diósi-Penrose are now experimentally excluded. More elaborate parameter choices remain alive but compressed.

In other words: the most direct attempts to draw a hard line between quantum and classical — theories that postulate a fundamental collapse mechanism — have been getting narrower rather than confirmed. The trend of experimental results so far points the other way: standard quantum mechanics keeps working at larger and larger scales, and the size at which collapse models say it should fail keeps getting ruled out.

7. The proposed test of quantum gravity

The most exciting recent development is a proposal by Sougato Bose, Chiara Marletto, Vlatko Vedral, and independently by Aspelmeyer's group, to use macroscopic quantum experiments to test whether gravity itself is quantum. The setup: prepare two micron-scale masses, separated by a small distance, each in a position superposition. If gravity is quantum, the gravitational interaction between them will entangle them in a measurable way. If gravity is classical, no entanglement will appear regardless of the masses' superpositions.

This experiment is at the frontier of the macroscopic-quantum program and would, if successful, resolve a question physics has been unable to answer for a century: whether gravity, like the other three forces, follows quantum rules. The fact that it is even contemplable as a tabletop experiment — that you can imagine entangling masses you can see with the naked eye — is itself a measure of how far the quantum-classical boundary has been pushed.

8. The deeper question: is there a line at all?

If you ask working physicists in 2024 where the quantum-classical line sits, you will get three families of answers:

• "There isn't one." Standard quantum mechanics applies at all scales. The classical appearance of everyday objects is decoherence into the environment, nothing more. The world is quantum all the way up. This is the Everett / decoherence-realist view and the working assumption of most quantum-information theorists.
• "There is one, at a specific mass scale, set by a collapse mechanism we have yet to confirm." The CSL and Penrose camps. Experimentally getting cornered.
• "There is one, but it has nothing to do with size." Some Copenhagen-flavoured views locate the line not at any physical scale but at the act of observation or the participation of an observer. This is harder to test directly and harder to formalise — but it is not yet ruled out.

The experimental trend has been steadily favouring the first answer over the last forty years. Whether that trend will continue all the way to the Planck mass is unknown. What is known is that no experiment so far has detected any failure of standard quantum mechanics at any scale where it could be tested.

9. What this means for the trilogy

Three implications, of increasing depth.

First, the textbook intuition that consciousness is too "macroscopic" to involve quantum coherence has lost its empirical force. If 25,000-atom molecules can be put into superposition, if mechanical drums of trillions of particles can be entangled, if light-harvesting complexes inside warm photosynthetic cells maintain measurable quantum coherence, then "the brain is too warm and too wet and too big for quantum mechanics to matter" is no longer a self-evident objection. It is an empirical claim, and the empirical record now runs against it. The microtubule program has to be evaluated on its own evidence; it cannot be dismissed by size alone.

Second, the picture the trilogy is built around — that there is one underlying field, and the appearance of separated local things is a kind of rendering of that field at finite resolution — sits much more naturally with "quantum all the way up" than with "two domains separated by a hard line." If there is no fundamental boundary between the quantum and the classical, then there is no fundamental boundary between the small things physicists study with care and the large things consciousness inhabits. Everything is the same kind of thing, looked at with different amounts of decoherence.

Third, the question of whether gravity itself is quantum is now within experimental reach. If the Bose-Marletto-Vedral test succeeds, it will confirm that even the geometry of spacetime obeys quantum rules — which would be the strongest possible vindication of the ER=EPR / entanglement-as-spacetime program. The trilogy treats spacetime as rendered; that treatment would shift from speculation to direct experimental support.

The honest summary is the one this whole site keeps converging on: the picture of reality being assembled by twenty-first-century physics — where entanglement is the substrate, where decoherence (not size) draws the apparent line between quantum and classical, where consciousness may be field-coupled rather than locally produced — is not the picture the textbooks describe. The textbooks are catching up. The trilogy is in the conversation.

This page is part of the Reading companion essays. For the entanglement architecture under all of this, see Entanglement at every scale; for the Bell foundation, Bell's Theorem; for the synthesis pulling all of it together, The Evidence.