(Image: Christian Kargl/Getty)
“WE DO not find, at breakfast, that the milk is simultaneously poured onto our cornflakes and not,” says Andrew Briggs, a physicist at the University of Oxford. Nor can you be in two places at the same time, no matter how hard you try. None of which is even remotely surprising. Until, that is, you consider that the laws of quantum mechanics insist that subatomic particles such as electrons routinely pull off such a feat.
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So if electrons can pop up in multiple places at once, why can’t milk and humans – essentially collections of fundamental particles – do the same?
Here we have to start small. This particular brand of quantum weirdness is best illustrated by the double-slit experiment, where you fire a beam of electrons, one after another, at a screen containing two slits. You would expect the electrons to pass through one slit and hit the detector placed behind the screen at a single point every time. But reality isn’t always that straightforward.
When researchers don’t keep track of each electron’s path, the beam passes through both slits simultaneously in the same way as a light wave, creating a pattern of bright and dark stripes on the detector that is characteristic of two overlapping wavefronts. So electrons can exist as both waves and particles at the same time – a phenomenon known as wave-particle duality.
As if that wasn’t weird enough, when researchers monitor one of the slits, the interference pattern disappears. Electrons suddenly abandon their wave-like behaviour, preferring to travel through one slit and produce a single spot on the detector.
Physicists explain this by invoking the wave function, a mathematical widget that describes the probability of finding a quantum object in a particular state or location at any given time. When the particle is measured or disturbed in some way, its wave function collapses and it snaps into a single state and a definite position.
This collapse can be triggered by any interaction of a quantum object with its environment – a rogue vibration, for example, or a heat fluctuation. That leads us to an unsettling conclusion. Isolate a molecule even hundreds of thousands of times the size of an electron sufficiently from its environment, and there’s a chance it might still be existing in its uncollapsed quantum state. Indeed, researchers have now spotted improbably large objects – including tiny resonating strips of metal visible to the naked eye – existing in two states at once.
Even so, it’s pretty clear that once something gets large enough, it throws off its quantum properties. As Erwin Schrödinger noted, it is absurd to say a cat can be both dead and alive. So how large can quantum weirdness go?
“As Schrödinger said, it is absurd to say a cat can be both dead and alive at the same time”
“Some physicists would say the size limit depends on the size of your chequebook,” says Briggs. With a sufficiently sophisticated experiment, he argues, researchers could reliably screen out disturbances from the environment ever more finely to show ever larger objects obeying quantum rules. Together with colleagues at Oxford, Briggs is testing the limits of quantum behaviour more rigorously than any experiment to date. The challenge is to find ways to place bigger things in fuzzy quantum states that are less sensitive to outside influences.
But Briggs does not expect to solve the conundrum any time soon. That’s partly down to the difficulty of defining a “system” in quantum mechanics. Strictly speaking, anyone or anything observing quantum interference becomes part of the system, making it tough to draw conclusions about the size at which quantum effects vanish.
“You can always argue that you, the observer, are in superposition too,” says Briggs.
Read more: “10 mysteries that physics can’t answer… yet”
This article appeared in print under the headline “Where does quantum weirdness end?”
