A new thought experiment may have exposed a paradox within the theory of quantum mechanics, according to new research.
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 behavior 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.
But how is it possible to combine two theories that, despite both describing the physical processes in their domains very accurately, differ so greatly?
One possibility is to conduct quantum physics experiments with increasingly larger objects in the hope that discrepancies will eventually appear that point to possible solutions. But physicists must work within tight constraints. The famous double-slit experiment, for instance, which can be used to show that solid particles simultaneously behave like waves, can’t be performed with everyday objects.
Thought experiments, on the other hand, can transcend the boundaries of the macroscopic world. That’s exactly what Renato Renner, professor for theoretical physics at ETH Zurich, and his former doctoral student Daniela Frauchiger have now done in a paper in Nature Communications.
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.
One reviewer conceded that he made five attempts to find an error in the calculations—without success.
According to our current worldview, this indirect observation should yield the same result as direct observation, yet the pair’s calculations show that 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.
The thought experiment has already become a topic of discussion among experts.
The most common initial reaction of his colleagues in the field is to question the calculations, Renner says, but so far, no one has managed to disprove them. One reviewer conceded that he made five attempts to find an error in the calculations—without success.
Other colleagues presented concrete explanations as to how to resolve the paradox. Upon closer inspection, though, they always turned out to be ad hoc solutions that don’t actually fix the problem.
Revising what we know
Renner finds it remarkable that the issue evidently polarizes people. He was surprised to note that some of his colleagues reacted very emotionally to his findings—probably due to the fact that the two obvious conclusions from Renner’s and Frauchiger’s findings are equally perplexing.
The one explanation is that quantum mechanics is apparently not, as was previously thought, universally applicable and thus can’t be applied to large objects. But how is it possible for a theory to be inconsistent when experiments have so clearly and repeatedly been confirmed?
The other explanation is that it is evidently not only politics that suffers from a lack of clear facts, but also physics, and that there are other possibilities besides what we deem to be true.
Renner has difficulties with both of these interpretations. He rather believes that the paradox will be resolved in some other way: “When we look back at history, at moments like this, the solution often came from an unexpected direction,” he explains. The general theory of relativity, for instance, which solved contradictions in Newtonian physics, is based on the realization that the concept of time as it was commonly understood back then was wrong.
“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.”
Source: ETH Zurich