Imagine a physicist observing a quantum system whose behavior is akin to a coin toss: it could come up heads or tails. They perform the quantum coin toss and see heads. Could they be certain that their result was an objective, absolute and indisputable fact about the world? If the coin was simply the kind we see in our everyday experience, then the outcome of the toss would be the same for everyone: heads all around! But as with most things in quantum physics, the result of a quantum coin toss would be a much more complicated “It depends.” There are theoretically plausible scenarios in which another observer might find that the result of our physicist’s coin toss was tails.

At the heart of this bizarreness is what’s called the measurement problem. Standard quantum mechanics accounts for what happens when you measure a quantum system: essentially, the measurement causes the system’s multiple possible states to randomly “collapse” into one definite state. But this accounting doesn’t define what constitutes a measurement—hence, the measurement problem.

Attempts to avoid the measurement problem—for example, by envisaging a reality in which quantum states don’t collapse at all—have led physicists into strange terrain where measurement outcomes can be subjective. “One major aspect of the measurement problem is this idea ... that observed events are not absolute,” says Nicholas Ormrod of the University of Oxford. This, in short, is why our imagined quantum coin toss could conceivably be heads from one perspective and tails from another.

But is such an apparently problematic scenario physically plausible or merely an artifact of our incomplete understanding of the quantum world? Grappling with such questions requires a better understanding of theories in which the measurement problem can arise—which is exactly what Ormrod, along with Vilasini Venkatesh of the Swiss Federal Institute of Technology in Zurich and Jonathan Barrett of Oxford, have now achieved. In a recent preprint, the trio proved a theorem that shows why certain theories—such as quantum mechanics—have a measurement problem in the first place and how one might develop alternative theories to sidestep it, thus preserving the “absoluteness” of any observed event. Such theories would, for instance, banish the possibility of a coin toss coming up heads to one observer and tails to another.

But their work also shows that preserving such absoluteness comes at a cost many physicists would deem prohibitive. “It’s a demonstration that there is no pain-free solution to this problem,” Ormrod says. “If we ever can recover absoluteness, then we’re going to have to give up on some physical principle that we really care about.”

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