Scientists Can’t Stop Arguing About This Particle Mystery
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For more than a decade, physicists have argued (and counter-argued) that a subatomic particle can be separated from certain quantum properties, such as its spin—a phenomenon known as the Cheshire cat paradox.
A new preprint paper suggests that, theoretically, a quantum particle should be separable from its moment as well.
However, a recent study also argues that the Cheshire cat paradox doesn’t exist due to the ways in which scientists measure the paradox in the first place.
Cats are very clearly crucial to the very fabric of our reality. In 1935, Austrian physicist Edwin Schrödinger demonstrated the idea of quantum superposition using the analogy of a cat that’s both dead and alive—at least, until it’s measured. However, the famous experiment known as Schrödinger’s cat isn’t the only feline-themed property of quantum mechanics.
Meet the Cheshire cat paradox.
Named after the troublesome cat in Alice in Wonderland who’s known for his disturbing smile and frequent disappearing acts, the Cheshire cat paradox pertains to the strange (and controversial) quantum quirk that properties of a subatomic particle (in this case, a neutron) can be separated from the particle’s mass—similar to how the Cheshire cat disappears and leaves only its unsettling grin behind.
Scientists first demonstrated this paradox more than a decade ago by seemingly separating the spin of a neutron from the mass of the neutron itself using a complicated experimental set-up involving interferometers and what’s known as “weak measurements,” which essentially measures a particle without it collapsing into a single value (for example, if you checked on Schrödinger’s cat with this method, you wouldn’t know if the cat is actually dead). However, scientists from Chapman University in California argue in a new paper, published on the preprint site arXiv and yet to be peer reviewed, that it should be possible to separate a particle’s momentum from its mass right along with its spin.
“The Quantum Cheshire Cat experiment showed… that a neutron is measured to be in one place without its spin, and its spin is measured to be in another place without the neutron,” the paper reads. “A generalization of this effect is presented with a massive particle whose mass is measured to be in one place with no momentum, while the momentum is measured to be in another place without the mass.”
Testing this theory would require a more complicated experimental set-up than its spin-separating forebear. According to New Scientist, instead of one particle, this experiment requires two—each the opposite of the other in every way. When the particles would annihilate, they’d cancel each other out with a reading of no mass. But leftover momentum should still be measured due to their differing speeds and distances.
However, some physicists don’t think the Cheshire cat paradox is actually a paradox at all, but rather an error in the way that “weak measurements” capture data in the first place.
The history of trying to debunk the Cheshire cat paradox is almost as long as the paradox itself. In 2015, some experts determined that the paradox could be explained away by standard quantum mechanics, and research as recent as November of 2023 says the paradox actually portrays another quantum property known as “contextuality,” where quantum systems change depending on the order in which things are measured.
“For example, measuring a particle’s location and then its speed will yield different results than measuring its speed first and then its location,” a January 2024 press release states. “This contextuality allows quantum systems to be measured as having properties that would typically be mutually incompatible.”
With these sly, counterintuitive results sparking continuous debate, the Cheshire cat paradox may be more true than we realize.
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