It's been holding us back as well as holding us down. A previously overlooked effect of gravity on quantum systems could be messing up quantum experiments. If confirmed, it suggests that some quantum studies may be impossible to perform on Earth.
No matter how hard you try, you can't be in two places at once. But if you're an electron, popping up in multiple places is a way of life. The laws of quantum mechanics tell us that subatomic particles exist in this superposition of states until they are measured and found to be in just one – when their wave function collapses.
So why can't we do the same party trick as an electron? It seems that once something gets large enough, it loses its quantum properties, a process known as decoherence. That's mainly because larger objects interact with their environment, which forces them into one position or another. Erwin Schrödinger famously pointed out the absurdity of large-scale superposition with the example of a cat that is both dead and alive.
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But that hasn't stopped physicists from trying quantum experiments by isolating objects from external influences. In 2010, a team at the University of California, Santa Barbara, placed a strip of metal 60 micrometres long into a superposition for a few nanoseconds, cooling it to just above absolute zero to shield it from temperature fluctuations.
The hope is that more precise experiments could place larger objects, such as a virus, into a superposition, getting us closer to Schrödinger's mythical cat. But now it looks like there is a more fundamental obstacle: gravity.
General relativity, Albert Einstein's sweeping reassessment of gravity that celebrates its centenary this year, tends to be ignored by quantum physicists. "Usually people don't look much at it because gravity acts on very large scales," says Igor Pikovski of Harvard University. "They think there are probably not many effects that are relevant."
Now Pikovski and his colleagues have calculated what happens when you do quantum experiments in Earth's gravitational field. They say a quirk of relativity called time dilation could be making large systems lose their quantum nature.
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