Ernst Mach found atoms useful. But he didn’t believe they were real.
The 19th century physicist-philosopher gained his greatest fame as an atom denier. He believed that reality consisted of sounds and colors and pressures, features of nature susceptible to “sensuous contemplation.” Reality was what the senses perceived; things like atoms were mere “mental artifices” designed to represent nature for practical purposes.
Hypothesizing atoms permitted successful predictions about chemical reactions or properties of heat, for instance. It’s much like how hypotheses about circles and epicycles allowed ancient astronomers to plot the paths of planets and predict eclipses. Epicycles weren’t real, and Mach saw no reason that atoms should be, either.
“Atoms cannot be perceived by the senses,” he declared. “Like all substances, they are things of thought.”
No scientist today denies the reality of atoms. Mach did not foresee the technology that allows them to be imaged, split and smashed. But Mach’s conception of atoms and the question of their reality echoes today in a different debate on the physics frontier: the nature of the mathematical expression at the heart of quantum mechanics.
Quantum math describes how atoms behave, in much the way atomic theory helped explain the behavior of the observable world. But atoms, Mach insisted, would forever remain “a tool for representing phenomena, like the functions of mathematics,” and you shouldn’t mistake mathematical equations for the real phenomena they describe. Yet today some experts believe that the quantum formula describing atomic phenomena is not simply a mathematical tool, but is just as real as atoms are.
That formula is known as the state vector or (a bit loosely) the wave function. Atoms, or any physical system, can be considered to be in a “quantum state”; the wave function describing it permits predictions of the system’s future. But to the despair of many fans of traditional, or classical, physics, those predictions are not definite. In a classical state, the math describes the locations and motion of all the particles in the system, enabling a precise forecast of future states. Quantum math offers up only the odds for various possible futures.
Nature obeys those odds — in the long run, quantum probabilities accurately forecast the distribution of experimental results. But the explicit cause-and-effect certainty of classical Newtonian physics is rendered fuzzy. Hopes of restoring such certainty with “hidden variables” have been dashed by experiments based on work by John Bell a half a century ago.
Such experiments have persuaded many physicists to live comfortably with the wave function’s probabilities, happy to comply with an often-repeated quantum theorist creed: “Shut up and calculate.” But others insist that the wave function or quantum state has real physical existence. Whether it’s real or merely a tool for calculating probabilities is today “perhaps the most hotly debated issue in all of quantum foundations,” quantum physicist Matthew Leifer writes in a recent paper in the journal Quanta.
Dressing this debate in philosophical jargon, Leifer and others in the field label the two possibilities as “ontic” and “epistemic.” These are not words you should try to use at home. But when eavesdropping on quantum debates, you should know that “ontic” refers to something physically real; “epistemic” alludes to mere knowledge about something.
Physicist-philosopher Ernst Mach denied that atoms were more than a hypothetical tool, but later work confirmed their existence — as did IBM's 2013 stop-motion movie "A Boy and His Atom," made by precisely positioning individual atoms. These days, physicists debate whether quantum wave functions are as real as atoms or just tools for forecasting phenomena
An ontic state exists independently of anybody’s knowledge or awareness. “Ontic states are the things that would still exist if all intelligent beings were suddenly wiped out from the universe,” writes Leifer, of the Perimeter Institute for Theoretical Physics in Waterloo, Canada. “An epistemic state is … a description of what an observer currently knows about a physical system. It is something that exists in the mind of the observer rather than in the external physical world.”
Historically, quantum states have usually been regarded as epistemic. As the Danish quantum pioneer Niels Bohr famously said, “It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.”
Bohr’s attitude was incorporated into the view of quantum physics known as the Copenhagen interpretation, once the predominant approach but now widely challenged. Modern adherents of the Copenhagen view tend to favor the epistemic side of the ontic-epistemic debate.
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