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martyb writes:

Theoretical physicists at ETH (Eidgenössische Technische Hochschule) Zurich have come up with a real puzzler in Searching for Errors in the Quantum World:

The theory of quantum mechanics is well supported by experiments. Now, however, a thought experiment by ETH physicists yields unexpected contradictions. These findings raise some fundamental questions – and they’re polarising experts.

There is likely no other scientific theory that is as well supported as quantum mechanics. For nearly 100 years now, it has repeatedly been confirmed with highly precise experiments, yet physicists still aren't entirely happy. Although quantum mechanics describes events at the microscopic level very accurately, it comes up against its limits with larger objects -- especially objects for which the force of gravity plays a role. Quantum mechanics can't describe the behaviour of planets, for instance, which remains the domain of the general theory of relativity. This theory, in turn, can't correctly describe small-scale processes. Many physicists therefore dream of combining quantum mechanics with the theory of relativity to form a coherent worldview.

[...] Thought experiments... can be used to transcend the boundaries of the macroscopic world. That’s exactly what Renato Renner, Professor for Theoretical Physics, and his former doctoral student Daniela Frauchiger have now done in a publication that appears in Nature Communications magazine today. Roughly speaking, in their thought experiment, the two consider a hypothetical physicist examining a quantum mechanical object and then use quantum mechanics to calculate what that physicist will observe. According to our current worldview, this indirect observation should yield the same result as direct observation, yet the pair’s calculations show that precisely this is not the case. The prediction as to what the physicist will observe is exactly the opposite of what would be measured directly, creating a paradoxical situation.

[...] "Our job now is to examine whether our thought experiment assumes things that shouldn't be assumed in that form," Renner says, "and who knows, perhaps we will even have to revise our concept of space and time once again." For Renner, that would definitely be an appealing option: "It's only when we fundamentally rethink existing theories that we gain deeper insights into how nature really works."

Journal Reference:
Daniela Frauchiger, Renato Renner. Quantum theory cannot consistently describe the use of itself. Nature Communications, 2018; 9 (1) DOI: 10.1038/s41467-018-05739-8

See also: Ars Technica Quantum observers with knowledge of quantum mechanics break reality.

Original Submission