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from the depends-on-how-you-look-at-things dept.
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.
(Score: 3, Informative) by Immerman on Thursday September 20 2018, @06:33PM (6 children)
Thought experiments are *incredibly* valuable to physics, specifically to uncover both internal inconsistencies and contradictions with observed reality. Einstein developed Special and General relativity based entirely on them, and it wasn't until much later that we had the technology to test them - and to date we have found no flaws in his theory, and some deeply strange anomalies in our universe, such as (apparent) black holes, that were originally conceived of as thought experiments trying to find such flaws.
Any theoretical framework that allows you to formulate a scenario where its own predictions are self-contradictory *must* contain a logical flaw. It doesn't matter if the thought experiment could never be carried out due to outside factors - so long as a self-contradictory construct can be constructed within the limits of the framework, you *know* that the framework itself is self-contradictory, and thus fundamentally flawed
And that is a *good* thing! It gives us a hint at where to start looking to find the flaw in our theory, and thus improve our understanding of the universe. We know from various other sources, such as some fundamental incompatibilities between Relativity and QM, that one or both theories are flawed, and the Copenhagen Interpretation of QM (which is where this flaw was discovered) is especially unsatisfying, chock full of seemingly arbitrary constants and extremely non-explanatory as to *why* things operate the way they do.
Generally when these sorts of contradictions are uncovered, in science or mathematics, what happens is that experts get together and try to pick it apart, and eventually discover that the flaw isn't actually in the framework, but in the construct itself. Often it's a logical flaw, some subtle mistake that, once rectified, resolves the contradiction. Less often, but much more profound, the flaw is traced to some subtle common-sense assumption about the nature of reality that was taken as fact, at which point new, practical experiments can be created to test the legitimacy of the assumption - often the assumption proves false, exposing unexpected and counter-intuitive truths about the universe.
Least common, and most excitingly, sometimes there are no logical flaws, nor flawed assumptions - then we know we've found a legitimate crack in our theories, and joyously leap upon them to try to tear apart our understanding of the universe and build something more accurate in its place.
In this case it sounds like it's specifically the Copenhagen Interpretation which is being challenged, which even if true is unlikely to be too immediately exciting - it is in many ways a band-aid interpretation to resolve some very heated conflicts within the QM community at the time, until such time as we had accumulated more data (and the egos involved had passed on). Definitively proving it flawed would surprise very few, and offer hints as to which of the more well-developed theories might be more correct (and perhaps even more importantly, which are definitely *not*, and shouldn't have further time and resources wasted on them)
(Score: 0) by Anonymous Coward on Thursday September 20 2018, @06:45PM (5 children)
The Copenhagen interpretation, in my opinion, won out due to the stature of Bohr and the fact that Einstein couldn't come up with an alternate. Then when Bell did his work, most physicists moved on and left the philosophical implications to the philosophers and New Age thinkers. I don't know if it is society that moved on, or whether nobody can understand the string theorists, but there hasn't been a Tao of Physics written since theorists moved from 4 to 10 to 26 dimensions and strings and rings, etc..
(Score: 2) by Immerman on Thursday September 20 2018, @07:18PM (4 children)
I was not aware, and google offers no immediately compelling evidence, that Einstein was a significant player in the discussions. My impression is that Einstein was a fringe player when it came to QM, With Schrodinger, de Broglie, etc. being closer to the fray.
Einstein was opposed to the seeming randomness altogether, while alternate interpretations mostly embraced that aspect, but disagreed over many of the details. There were some few, such as de Broglie's Pilot Wave Theory , that offered a deterministic (if perhaps unknowable) interpretation - but that was abandoned by its creator after... Bohr? convinced him he was mistaken early on, and was not developed further until other scientists picked it up again decades later.
(Score: 0) by Anonymous Coward on Thursday September 20 2018, @08:47PM (1 child)
he was not a fringe player.
he got the nobel prize for the photoelectric effect explanation (i.e. proposing that photons are real indivisible objects).
when the full ramifications of his work were thought through and proper quantum mechanics was developed, he was unhappy with it, but he still understood the theory reasonably well (and he was able to at least publish the work with podolsky and rosen). it's just that he believed it was simply a first step towards a "proper" theory.
(Score: 2) by Immerman on Thursday September 20 2018, @09:00PM
Certainly. However, contributing key elements early on says nothing about his involvement in the later stages. I don't believe he even proposed a well-formulated overall interpretation of QM , unlike several others involved in the drama when the convention was called to try to "settle" things within the community.
(Score: 2) by Snotnose on Thursday September 20 2018, @11:12PM (1 child)
My understanding is that, when Quantum Mechanics (QM) was just finding it's feet 100 years ago, Einstein was good at looking into the dusty corners of the theory and asking "but what if...". He forced the architects of QM to flesh out the theory.
I find it interesting that the most counter-intuitive theory in history has the best track record of being proven correct.
My ducks are not in a row. I don't know where some of them are, and I'm pretty sure one of them is a turkey.
(Score: 0) by Anonymous Coward on Friday September 21 2018, @03:13AM
Probably because it's so damned counter-intuitive. If it were intuitive, many experiments probably would never have happened, because theoreticians wouldn't be groping blind in the dark. What I mean is that the way things fall to the ground on Earth is intuitive. If I drop a bowling ball and a feather at the same time, it is intuitive to me that the feather takes much longer to reach the ground. If it weren't intuitive, I'd drop a bowling ball, then a feather, and having observed quite a difference, I would probably need extensive testing--test a brick, now test a leaf, now test a hunk of meta, now test a sheet of paper. Then somebody would point out that crumpling the paper makes it behave like a brick instead of like a feather and I'm fearsome confused as to how the same object could fall in two very different ways. I might even come up with an explanation for crumpled/flattened paper duality that would be completely and utterly wrong. It could be a while before I test enough objects to find a counterexample to show me that my theory is wrong.