Quantum theory is our most successful theory of physics. There is not one shred of experimental evidence that doesn't fit with its predictions. So why, if it ain't broke, is a growing number of researchers expressing a desire to fix it?
"Everything depends on whether you believe quantum mechanics is going to go on describing the physical world perfectly to whatever level you push it," says Nobel laureate Anthony Leggett, who studies the quantum world at the University of Illinois at Urbana-Champaign.
Leggett thinks it won't, that there are too many issues with quantum theory to think it anything more than an approximation of reality. "I'm inclined to put my money on the idea that if we push quantum mechanics hard enough it will break down and something else will take over - something we can't envisage at the moment," he says.
The question is, how hard can we push it? Experiments have never had the sensitivity to pinpoint a weak spot in quantum mechanics. But thanks to a breakthrough earlier this year, that might be about to change. A new swathe of experiments is coming onto the scene that should be up to the job. Welcome to the dawn of the quantum machines.
Such machines are promising to patch a gaping hole in every experiment that has ever been used to back up our view of the quantum world. Take the simple process of measuring a photon's spin. Thanks to the strange nature of the quantum world, it can actually be spinning in two directions at once, a phenomenon known as superposition. When we use a detector to measure the spin, however, the superposition disappears and we register a spin occurring in one direction or the other.
Quantum theory does not explain why this happens. "We don't really understand the measurement process," admits Stephen Adler at the Institute for Advanced Study in Princeton, New Jersey.
If you want to know how little we know, ask a roomful of physicists what goes on when we measure a particle's properties. All will be able to calculate the result of the measurement, but the explanation they give will differ wildly. Some will tell you that new parallel universes necessarily sprang into being. Others will say that, before a measurement is performed, talk of particles having real properties is meaningless. Still others will say that hidden properties come into play.
Another group will tell you that they deal with physics, not philosophy, and dismiss the question without giving you an answer. It has been thus for more than 80 years. "These conceptual challenges are still not understood at all," says Markus Aspelmeyer at the University of Vienna in Austria. "We're still right at the beginning."
Experiments investigating the quantum world have traditionally focused on what are known as interferometers. Researchers fire a single quantum particle, such as a photon, towards two apertures in a screen. Common sense says the photon has to go through one aperture or the other. However, as long as you don't measure which aperture it went through, something remarkable happens.
At a screen on the far side of the twin slits, an interference pattern forms. This can only occur if the photon goes through both slits at the same time and interferes with itself. In other words, as long as nobody is watching, the photon exists in two different places at once.
A measurement changes everything, however. If you set up the experiment so you can see which slit the photon goes through, the interference pattern disappears; the photon will have gone through one slit or the other, but not both.
The situation is analogous to one of the most famous thought experiments in physics, that of Schrödinger's cat. Here, an unfortunate feline is sealed in a box with a vial of poison and a lump of radioactive metal. When the metal emits a radioactive particle, it triggers a mechanism which will break the vial, killing the cat. But because the box is sealed, there is no measurement, and the particle remains in a superposition of emitted and not emitted. According to quantum logic, the cat is therefore alive and dead at the same time.
Schrödinger came up with this bizarre scenario to show that there was something wrong with quantum theory. There's no way, he said, that something as non-quantum as a cat can be in a superposition of alive and dead - whether it is being observed or not.
Others beg to differ. Markus Arndt at the University of Vienna has demonstrated that carbon-70 molecules can go through two slits at once, too. Though these ball-shaped molecules aren't quite as substantial as cats, they can nonetheless be seen through a microscope