Schrodinger’s Cheshire Cat

Is Schrodinger’s Cat old hat to you? Made peace with the idea that particles can be in two places at once? These ideas are so well established that Microsoft  is already advertising quantum computing, which is based on bits that can each be 1 and 0 simultaneously (although don’t go looking for qubits at Best Buy for the time being). Well, the Universe has plenty more weirdness where that came from. Get ready for the “Cheshire Cat” effect – the separation in space between objects and their properties. The name come from Alice in Wonderland, in which the smile of the Cheshire Cat remained even after the cat itself was gone. Imagine being able to throw a baseball so that it took a different trajectory than its “spin.” That’s like opening a whole new box of crazy.

But that’s what Nature allows, under the right conditions. In new research, exciting, not-completely understood concepts like “post-selection” and “weak measurement” have been used to watch neutrons fly down one path, while their magnetic moment goes along a different path.

Cheshire Cat
This is actually a figure from the paper


Quick digression: Quantum mechanics seems strange to us because the more we learn about the rules of physics that apply on small scales, the more we realize how vastly they differ from the rules that our brains are accustomed to, living at the human scale. By now, some quantum weirdness has already percolated within the cultural consciousness, with the most famous being the Schrodinger’s Cat thought experiment. The hypothetical situation illustrates the cognitive tension that pops up when we try to extend the idea of quantum superposition – which has been experimentally confirmed on at atomic level countless times experimentally – to the macroscopic world of people and cats.


Since the The Big Bang Theory is an extremely popular sitcom (with some of the cast holding out “Friends”-style for a $1 Million per episode each) quite a few people have been introduced to the concept by this episode.

Dr. Stanley Cohen, who was Einstein’s driver during the 1940s, said that the reason that he never came around to the idea of quantum entanglement – which he derided as “spooky action at a distance” – was that he was such an intuitive thinker that he had a hard time accepting as true such a counter-intuitive notion. But since the facts of QM weirdness are by now so well established, mostly people just spend time arguing about how to make sense of what happened, not the reality of what happens. Many go with the Copenhagen interpretation, which basically says “Don’t ask if the cat is alive or dead before you open the box – that’s not  a meaningful question,” while others are proponents of the many worlds interpretation, the idea that all possibilities happened somewhere in the multiverse. The problem is that if there is no practical difference between interpretations, there is no way to decide between them experimentally. What is exciting about these new results, is that the implications of weak measurements and post selection are not totally clear yet. This means that physicists can go back to what they love doing – testing theories based on empirical evidence and winnowing out the bad ones – instead of philosophizing about the right way to “frame” undisputed phenomena.

The idea of weak measurement is to circumvent the Heisenberg uncertainty principle, which places fundamental limits on how precisely one can know pairs of values (like position and momentum) at once. Every measurement disturbs the system being measured, but by making many weak measurements that don’t mess the system significantly, you can learn more about what is going on that would normally be allowed by Heisenberg. Some say that weak measurements don’t “collapse the wave-function” like a regular measurement would, although this assumes that we understand what wave-function collapse is, which we still don’t. Using weak measurements, scientists were able to follow the “Cheshire Cat” and its “smile” separately. Part of implementing weak measurements is the concept of “post-selection.” This is also a relatively newly appreciated part of quantum mechanics, and arises from the time-symmetry between choosing the initial quantum state and final state you want to measure. This starts to erode even our concept of causality, including our insistence that causes must occur earlier in time than their effects. In actuality, there are situations in which this does not appear to be true! So while our understanding of the Universe continues to improve, many mind-blowing discoveries keep coming up.

Author: lnemzer

Associate Professor Nova Southeastern University

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