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A new version of a strange experiment in which a particle鈥檚 properties are separated from its body or mass, called the quantum Cheshire Cat, has physicists arguing over whether these disembodied properties are quite what they seem and how reality works in the quantum world.

In 2014, physicists experimentally demonstrated this strange effect 鈥� compared to the Cheshire Cat from Lewis Carroll鈥檚 Alice鈥檚 Adventures in Wonderland, which can disappear leaving only its grin 鈥� by apparently measuring a subatomic particle known as a neutron and one of its quantum properties, called spin, divorced from one another and in separate places.

To do this, they forced a neutron to choose one of two paths along an instrument called an interferometer, measuring its properties at certain points along each path. Further measurements were recorded once the paths had merged again.

These measurements weren鈥檛 made in the standard way that they are in most quantum experiments, which involves a particle collapsing from a cloud of possible values for each of its properties, like its position or its momentum, to each having a single value. Instead, the researchers made what are called weak measurements, which very faintly measure a particle without it collapsing.

Separated spin

A single weak measurement isn鈥檛 enough to learn about a particle or its properties, but by repeating the experiment and measurements many times, and also measuring after the interferometer paths have merged again, you can theoretically glean information about what the particle was doing through running a statistical calculation.

It is with this method that scientists first measured a neutron鈥檚 spin on one side of an interferometer while the particle itself was measured on the other side. But in the decade since that experiment was first done, about what weak measurements are really measuring and how they can be used.

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In the meantime, physicists have developed more complicated forms of the experiment that don鈥檛 rely on specific systems like a neutron鈥檚 spin.

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Now, , and , all at Chapman University in California, have proposed a more general version of the Cheshire Cat experiment, in which a particle鈥檚 mass is separated from its momentum.

The experiment would be difficult to perform, but should be possible for almost any particle as long as it has mass, says Waegell. It works using similar equipment as that widely used in other experiments, he says, with the major difference being in how the particle鈥檚 mass is detected, which involves having an extremely sensitive gravitational sensor. 鈥淚t looks like we鈥檝e separated the mass from the momentum of the particle,鈥� says Waegell. 鈥淗ow you interpret what鈥檚 really going on, case by case, I think is still open to debate. But the effect is interesting, nevertheless.鈥�

Waegell is 鈥渞elatively agnostic鈥� as to how to interpret such a counterintuitive outcome as a particle being separated from its momentum. But Aharonov, who was part of the team that originally came up with the quantum version of the Cheshire Cat, has a more unusual theory that he calls the counter-particle view.

Particle annihilation

Rather than just one particle going down one of the paths, Aharonov suggests that a pair of particles, one the opposite of the other in every respect, can be conjured out of the quantum vacuum to travel down the paths, similar to virtual particles that are allowed by the laws of quantum mechanics and are used to explain a wide range of quantum phenomena.

These particles annihilate in one of the arms, cancelling each other out and providing a reading of no mass, but because they travel different distances, at different speeds, some momentum is left over to be measured.

This would be one possible explanation for such counterintuitive results, says at Hiroshima University in Japan, but it is also possible that the weak values gleaned from these experiments can be interpreted in other ways.

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Hance and his colleagues have described the original quantum Cheshire Cat experiment using something called quantum contextuality, which is a hypothesis that argues that the order and context of what you鈥檙e measuring in a quantum system matters. 鈥淭he measurements that you鈥檙e doing in each of these cases have a kind of an incompatibility, which is allowing you to get these paradoxical results,鈥� says Hance.

鈥淚t only looks like it鈥檚 separated because you鈥檙e measuring one of the properties in one place and measuring the other property in the other place, but that doesn鈥檛 mean that the properties are in one place and the other place, that means that the actual measuring itself is affecting it in such a way that it looks like it鈥檚 in one place and the other place,鈥� says Hance.

Regardless of what might be behind the quantum Cheshire Cat effect, testing out the mass and momentum version of the experiment could help us to learn about how gravity affects quantum mechanics, because it is a testable experiment that incorporates both concepts, says Hance.