For the better part of a century, the world’s greatest minds have struggled with the mathematical certainty that objects can be in multiple positions at the same time before something causes them to snap into place.
A number of physicists have wondered if good old gravity is responsible for forcing the particle equivalent of a roulette ball to settle into its metaphorical pocket. That’s looking a little less likely in the wake of a new experiment.
Researchers from across Europe recently tested a potential explanation of the apparent collapse of a waveform, determined not by observations or weirdly branching multiverses, but by the geometry of spacetime.
In fact, it was Diósi who teamed up with a handful of scientists to determine if we could blame gravity for one of quantum physics’ most brain-numbing paradoxes.
“For 30 years, I had been always criticised in my country that I speculated on something which was totally untestable,” Diósi told Science Magazine’s George Musser.
New technology has finally made the untestable a possibility. But to understand how it works, we need to take a brief dive into quantum insanity.
Back in the early 20th century, theorists modelled particles as if they were waves in order to reconcile what they were learning about atomics and light.
These particles weren’t quite like waves rippling across the surface of a pond, though. Think of the curving line you might draw on a graph to describe your chances of winning a bet in a dice game.
To some physicists, this whole gambling analogy was just a convenient fudge-factor, to be later resolved when we worked out more about the fundamental nature of quantum physics.
Others were adamant quantum physics is as complete as it gets. Meaning it really is a muddy mess of maybes down in the depths of physics.
Explaining how we get from a rolled dice to a clearly defined number describing things like particle spin, position, or momentum is the part that has had everybody stumped.
The famous Swiss physicist Erwin Schrödinger was firmly on team ‘fudge factor’.
He came up with that outrageous thought experiment involving a hidden cat that was alive and dead at the same time (until you looked at it), just to show just how nuts the whole ‘undecided reality’ thing was.
And yet here we are, a century on, and still superposition – the idea of objects like electrons (or bigger) occupying multiple states and positions at once until you measure them – is a core feature of modern physics.
To avoid needing to invoke half-baked notions of consciousness or infinite co-existing versions of reality to explain why many possibilities become one when we look at a particle, something less whimsical is needed for quantum probability to collapse into.
For physicists like Penrose and Diósi, gravity might be that very thing.
Einstein’s explanation of this force rests upon a curving fabric of three-dimensional space woven with time’s single dimension. Frustratingly, a quantum description of this ‘spacetime’ continues to elude theorists.
Yet this firm discrepancy between the two fields makes for a good backbone to pull waves of possibility into line.
Penrose’s version of this idea rests on the assertion that it takes different amounts of energy for particles to persist in different states.
If we follow Einstein’s old E=mc^2 rule, that energy difference manifests as a difference in mass; which, in turn, influences the shape of spacetime in what we observe as gravity.
Given enough of a contrast in all possible states, spacetime’s immutable shape will ensure there’s a substantial cost to pay, effectively choosing a single low-energy version of a particle’s properties to yank into place.
It’s an alluring idea, and luckily one with a potentially testable component. For all purposes, that snap should affect a particle’s position.
“It is as if you gave a kick to a particle,” Frankfurt Institute for Advanced Studies physicist Sandro Donadi told Science Magazine.
Kick an electron enough and you’ll force it to cry photons of light. Logically, all that’s left is to create a kind of Schrödinger’s cat experiment by locking the right kind of material inside a lead box, buried far from the confounding effects of radiation, and listen for its cries. That material, in this case, is germanium.
If Penrose’s sums are right, a crystal of germanium should generate tens of thousands of photon flashes over several months as its superpositioned particles settle into measured states.
But Diósi and his team didn’t observe tens of thousands of photons.
Over a two month period when they conducted the experiment underground five years ago at INFN Gran Sasso National Laboratory, they measured barely several hundred – just what you’d expect from the radiation that managed to leak through.
Penrose isn’t too worried. If gravity were to cause particles to emit radiation on collapse, it might run against the Universe’s tightly controlled laws of thermodynamics, anyway.
Of course, this isn’t the end of the story. In future experiments, gravity might yet be shown to be responsible for flattening quantum waves. Right now, anything seems possible.
This research was published in Nature Physics.