Copenhagen treats collapse as a practical step in calculation. Many worlds and pilot wave theory remove it altogether. A third group takes the opposite path: collapse is real, it is a physical process like any other, and it happens whether or not anyone is watching. The price is that quantum mechanics itself must be changed. The reward is a theory that experiments can put to the test.

The GRW theory

In 1986 three Italian physicists, Giancarlo Ghirardi, Alberto Rimini and Tullio Weber, proposed the first detailed objective collapse theory, now known by their initials as GRW. They added one new law to quantum mechanics. Every particle has a tiny chance, at every moment, of spontaneously "localizing": its wavefunction suddenly narrows around one spot, as if it had been measured by nature itself.

The chance is extremely small. In the original proposal, a single isolated particle would undergo such a collapse on average only once in hundreds of millions of years. That is why atoms and molecules in the lab show interference undisturbed: the new law almost never touches them.

The clever part is what happens in large objects. A cat, a detector or a pointer contains something like a trillion trillion particles, all entangled with one another. If any one of them localizes, it drags the whole entangled object with it. With so many particles, one of them is almost certain to collapse in a tiny fraction of a second. So large objects collapse almost instantly, because a hit on any one particle collapses the whole entangled object, while small systems keep their superpositions for ages.

A smoother version, called continuous spontaneous localization, was developed by Philip Pearle, working with Ghirardi and Rimini, around 1990. It replaces sudden jumps with a gentle, continuous drift toward definite states.

Penrose and gravity

The British mathematician Roger Penrose proposed in the 1990s that gravity is the cause of collapse. In general relativity, mass shapes the geometry of spacetime. A superposition of a lump of matter in two places would then be a superposition of two different geometries, which Penrose argues is unstable. Penrose proposed that superpositions of different arrangements of mass collapse on their own, and faster the larger the mass involved. The Hungarian physicist Lajos Diósi had developed a closely related model in the 1980s, so the idea is often called the Diósi and Penrose model.

The case for collapse theories

Defenders point out that these theories give a single world, no special role for observers, no hidden particles and a precise physical answer to when collapse happens. But their greatest strength is scientific: unlike most interpretations, collapse theories predict results that differ from standard quantum mechanics, so they can be tested. Strictly speaking they are rival theories rather than interpretations, which is exactly why many experimental physicists find them attractive.

How the tests work

There are two main strategies. The first is to put larger and larger objects into superposition and see whether interference survives. If collapse is real, it should fade beyond some size. Groups such as Markus Arndt's in Vienna have shown interference with molecules made of about two thousand atoms, and every such success narrows the room left for collapse.

The second strategy looks for side effects. Each spontaneous collapse gives a particle a tiny random kick, so collapse theories predict a faint random jiggling that should very slightly heat matter and make charged particles give off weak radiation. In a study published in 2020, a team working deep underground at the Gran Sasso laboratory in Italy looked for this radiation from a block of germanium. They found none at the predicted level, and the experiment ruled out the simplest version of the Diósi and Penrose model. More flexible versions survive, and GRW style models remain open within limits set by many experiments.

Fair criticism

Critics raise several points. The most common is that the new constants, such as the collapse rate, are chosen to fit what we observe rather than derived from deeper principles. The theories also break exact energy conservation by a tiny amount, through the heating just described. There is the "tails" problem: after a collapse, the wavefunction is sharply peaked but never exactly zero elsewhere, so a faint trace of the other outcomes remains, and it is debated whether that undermines the claim of a single definite result.

Worked example: a race between theory and experiment

Picture a research team that wants to test GRW. Their plan is to send ever heavier molecules through a grating and look for the interference pattern on the far side. Standard quantum mechanics says the pattern should appear for any mass, provided the molecules are kept isolated. GRW says that beyond some mass and travel time, spontaneous collapse should begin to wash it out.

Each successful run with a heavier molecule does not refute GRW outright, since the collapse rate is a free setting. Instead it rules out a band of possible rates. Meanwhile other teams squeeze from the opposite side, using the absence of heating in cold, isolated systems. Year by year, the space where collapse could hide gets smaller. Either the two squeezes meet and collapse theories are ruled out, or someone finds a deviation and physics changes. This is what makes collapse theories valuable even to physicists who doubt them: they turn a philosophical question into a measurable one.