“More than one reality exists” screams the headline. Cue sighs of tired dread from physicists everywhere as they wonder what otherwise bland result has been spun out of control.

In this case, though, it turns out that the paper and the underlying theory are much more interesting than that takeaway. Essentially, modern physics tells us that two observers of the same event may never agree on the result, even if they have all possible knowledge. This is already accepted as part of special relativity, but now we have experimental proof that it applies to quantum mechanics as well.

## What Galileo and Einstein tell us

Let’s start with the simplest possible example of how we typically resolve conflicting measurements. I am standing on a platform and measure the speed of an approaching train to be 180km/hr. You are on the train and measure the speed of the train to be 0km/hr. We can resolve the difference by making an additional measurement on our relative speeds. Afterward, we both know that we’ve measured the speed correctly relative to our own motion.