On a quiet autumn afternoon in 1935, in a set of rented rooms in Oxford, an Austrian physicist named Erwin Schrödinger sat down at his desk and invented a cat.

He had been driven out of Berlin two years earlier. He had taken a fellowship at Magdalen College and accepted a Nobel Prize. He was carrying on a long, exasperated correspondence with Albert Einstein. And he was, by his own admission, increasingly horrified by the direction physics had taken under his name.

Quantum mechanics, as developed at Copenhagen by Niels Bohr and at Göttingen by Werner Heisenberg, held that a particle like an electron did not have a definite position before you measured it. Until the moment of observation, the electron was in a state called superposition: it was, in some real and not merely metaphorical sense, in many places at once. Only when an instrument interacted with the particle did the various possibilities collapse into a single outcome. Schrödinger, in 1926, had written the equation that described the mathematics of this. By 1935, he had decided the philosophy attached to the mathematics was nonsense.

In a long paper published in the German journal Naturwissenschaften that November, he set out to demonstrate why. He took the Copenhagen position seriously. He extended it to a domestic animal.

Imagine, he wrote, that you put a cat in a sealed steel box together with a flask of hydrocyanic acid and a tiny radioactive sample. Wire the apparatus so that if a single atom of the sample decays, a hammer falls and breaks the flask. The decay of a single atom in a given hour is exactly the kind of quantum event that the Copenhagen interpretation treats as a superposition: decayed and not decayed, both at once, until observed. By the same logic, after one hour the cat in the box must be in a state of being both alive and dead, both at once, until somebody opens the lid.

The case, Schrödinger wrote, was burlesque. Ridiculous. So obviously ridiculous, in fact, that it constituted a proof by contradiction. If the Copenhagen interpretation produced cats that were both alive and dead, the Copenhagen interpretation had to be wrong.

The paper was reductio ad absurdum. Schrödinger meant to bury the theory. He died in 1961 still believing he had.

What he did not anticipate was that physicists would treat the cat not as a knockdown argument but as a project specification.

The technical problem with Schrödinger's cat, as opposed to his single radioactive atom, is that anything large and warm interacts very strongly with the environment around it. Air molecules collide with it. Thermal photons strike it. Each of these interactions is a kind of involuntary measurement, and each measurement collapses the superposition. A cat sitting in a steel box is in continuous contact with several quadrillion air molecules and an infrared bath emitted by its own skin. It cannot stay in superposition for the time it would take to detect it. This is called decoherence. It is the reason quantum effects are usually invisible at human scales.

For seventy years, physicists chased that decoherence threshold. In 1927, Clinton Davisson and Lester Germer at Bell Labs showed that single electrons behaved as waves, scattering off a nickel crystal in exactly the pattern that wave physics predicted. In 1974, Helmut Rauch and his colleagues at the Atominstitut in Vienna demonstrated the same wave behaviour with single neutrons. In 1999, Markus Arndt and Anton Zeilinger's group at the University of Vienna pulled it off with C60, a single molecule consisting of sixty carbon atoms arranged in a soccer-ball shape: a buckyball. In 2013, the same Vienna group reached molecules of around eight hundred atoms. Every advance required colder conditions, more isolated apparatus, more clever shielding from the environment, and the willingness of an experimentalist to spend a career trying to build, in laboratory steel, the thing Schrödinger considered a logical absurdity.

In November 2025, the Arndt group published in the journal Nature (volume 649, issue 8098) the result of their latest attempt. The lead author was Sebastian Pedalino, a doctoral student in Vienna. The theoretical analysis came from Klaus Hornberger's group at the University of Duisburg-Essen. The apparatus shot ultracold clusters of metallic sodium, each cluster containing somewhere between five and ten thousand atoms and weighing more than 170,000 atomic mass units, through three diffraction gratings made of ultraviolet light. The clusters were eight nanometres across, comparable to the components of a modern transistor. Each cluster, in the course of about one hundredth of a second, was placed into a quantum state spread over a region dozens of times larger than the cluster itself. The clusters were, in the most literal sense the physics allows, in several places at once.

The figure of merit physicists use to compare experiments like this is called macroscopicity. It is roughly a measure of how seriously the experiment tests the Copenhagen prediction at large scales. Vienna's macroscopicity score was 15.5. The previous record was approximately one-tenth of that. To match the same level of test using a single electron, the experimenter would need to keep that electron in superposition for nearly a hundred million years. The Vienna team did it in a hundredth of a second.

This matters, the technologists will tell you, because matter-wave interferometry, the practical name for the apparatus that splits a particle's path and brings the paths back together to interfere, is also how atomic clocks work, how the inertial-navigation systems on nuclear submarines work, how the satellite gravity gradiometers that map the world's aquifers work, and how the laser interferometers at LIGO and Virgo detect the ripples in spacetime produced by colliding black holes. Schrödinger's burlesque has become the foundational instrument of a quarter of modern measurement physics.

It also matters, the philosophers will tell you, because the experimental gap between an electron and a cat is now mostly a question of more atoms. A cat is roughly ten trillion atoms heavier than a Vienna sodium cluster. The Vienna group's stated next benchmark is a cluster of ten million atoms. Each generation of apparatus has roughly tripled the previous best.

Schrödinger died in 1961, ninety-one years before this paper was published. He believed to the end that the Copenhagen interpretation was, at best, an incomplete description of reality. He was perhaps right about that. He was also wrong about the cat. The cat was supposed to be the part of the argument that no laboratory could ever build. The Vienna apparatus is the cat, only smaller and metallic and a hundred and twenty-five years too late for Schrödinger to be told.

The trade is still measured, of course, on the precision of the rangefinder, the warmth of the chamber, the count of atoms. Some things in physics do not change.