Scientists are a bit like bird watchers. They set up their experiment and then watch quietly from their hide to see what nature reveals to them. Thousands of years' worth of science are built on this approach.

One of the theories that has emerged from such careful observation is quantum mechanics. It's been hugely successful and it dominates modern physics. But perhaps ironically, the theory seems to suggest that passive observation is impossible: the very act of looking at something can change what's being looked at. This apparent fact is the theory's greatest mystery.

Quantum mechanics was developed in the 1920s to describe the smallest building blocks of matter; particles such as electrons or protons. The theory worked so well, it had to be taken seriously, but it also seemed to suggest that at very small scales, reality becomes fuzzy. "Quantum mechanics, if you take it literally, seems to say that particles can be in two places at once, or go at two speeds at once," explains David Wallace, a philosopher of physics at the University of Oxford. "The mathematics of the theory also seems to say that when you make a measurement of a quantum system, then the macroscopic world also starts doing two things at once." That is, if you measure a particle that's in such a superposition state, such as being in two places at once, then, so the theory suggests, you yourself go into a superposition state of seeing it here and there. Developing this idea further, you can come up with thought experiments like that of Schrödinger's famous cat: because its life depends on a radioactive particle that can be simultaneously decayed and non-decayed, the cat is simultaneously dead and alive.

This seems like utter rubbish. We never find ourselves, or cats, in superposition. How can it be that such a crazy theory says anything useful about the world? The answer lies in probabilities. We all know that when we measure the location of a particle, we will always find it in exactly one place. The theory may appear to dispute that it really is in one place, but it also provides us with a set of numbers that form a probability distribution: for each possible location we could find the particle at, the theory gives the probability of finding it there. This really works. If the theory says the probability of finding the particle at location x is 1/3rd and the probability of finding it at location y is 2/3rds, then if you do a large number of identical particle-spotting experiments, you will find the particle at x 1/3rd of the time, and at y the other 2/3rds of the time. Things like MRI scanners, lasers and the internet heavily rely on the reliable predictions of quantum mechanics. But the theory also seems to suggest a deep distinction between what we observe, or measure, and what is really going on.

Erwin Schrödinger devised a famous thought experiment in which a cat in a box is in superposition of two states: dead and alive. Image: Dc987.

"This brings the observer into the game," says Adrian Kent, a theoretical physicist at the University of Cambridge. "You can't understand the theory without talking about measurements. So what is this thing, this process of measurement? Maybe it's to do with the [measurement] apparatuses, things with dials on, but that's a pretty funny thing to have right at the heart of what's supposed to be a fundamental physical theory. Maybe it's to do with us somehow. Physics is telling us about the probability of things we observe. That's also a pretty weird thing to say, because as far as we understand we are not really any more special than experimental apparatuses, or planets, or electrons. But somehow you have to break this chain — you have to say something about what's happening in physics that gives you a [definite] measurement outcome."

All this leaves us with essentially three options. Accept that the observer plays a fundamental role in physics, which poses the awkward task of defining what counts as an observer (a dog, a snail, an apparatus?) and explaining the mechanism by which an observer changes reality. The second option is to change something about the theory of quantum mechanics to bring it in line with our experience of the world. It's not a popular option with many physicists because the theory works so well. The third option is to take the mathematics literally: to admit that several things really can be happening at once without us noticing. This leads us into the strange realm of parallel worlds. We'll explore all three options in the next articles.

About this article

Adrian Kent is Professor of Quantum Physics in the Centre for Quantum Information and Foundations, DAMTP, University of Cambridge. He is currently working on a FQXi funded project developing a formulation of relativistic quantum theory in which simple additional mathematical postulates give us a description of a single real world consistent with cosmological observations and quantum experiments. He co-edited Many Worlds? Everett, Quantum Theory and Reality; one of his contributions to the volume was the question mark in the title.

David Wallace is a philosopher of physics at the University of Southern California, having previously received PhDs in physics and in philosophy at the University of Oxford. His 2012 book on the many-worlds interpretation of quantum mechanics, The Emergent Multiverse, was joint winner of the 2013 Lakatos Prize for philosophy of science.

Marianne Freiberger is Editor of Plus. She interviewed Kent and Wallace at the 5th FQXi International Conference in Canada in August 2016.

This article is part of our Who's watching? The physics of observers project, run in collaboration with FQXi. Click here to see more articles and videos about questions to do with observers in physics.