It is hard to shake the intuition that there's a real and objective physical world out there. If I see an umbrella on top of a shelf, I assume you do too. And if I don't look at the umbrella, I expect it to remain there as long as nobody steals it. But the theory of quantum mechanics, which governs the micro-world of atoms and particles, threatens this commonsense view.
The fourth episode of our podcast Great Mysteries of Physics - hosted by me, Miriam Frankel, science editor at The Conversation, and supported by FQxI, the Foundational Questions Institute - is all about the strange world of quantum mechanics.
According to quantum theory, each system, such as a particle, can be described by a wave function, which evolves over time. The wave function allows particles to hold multiple contradictory features, such as being in several different places at once - this is called a superposition. But oddly, this is only the case when nobody's looking.
Although each potential location in a superposition has a certain probability of appearing, the second you observe it, the particle randomly picks one - breaking the superposition. Physicists often refer to this as the wave function collapsing. But why should nature behave differently depending on whether we are looking or not? And why should it be random?
Not everyone is worried. "If you want to explain everything we can observe in our experiments without randomness, you have to go through some really weird and long-winded explanations that I am much more uncomfortable with," argues Marcus Huber, a professor of quantum information at the Technical University of Vienna. And indeed, you can get rid of randomness if you accept that the future can influence the past, that there's more than one outcome to every measurement or that everything in the universe is predetermined since the dawn of time.
Another problem is that quantum mechanics seems to give rise to contradictory facts. Imagine a scientist, Lisa, inside a lab measuring the location of a particle. Before her colleague, Nikhil, knocks on the lab door and asks what outcome she saw, he would measure Lisa as being in a superposition of both branches - one where she sees the particle here and one where she sees the particle there. But at the same time, Lisa herself may be convinced that that she has a definite answer as to where the particle is.
That means that these two people will say that the state of reality is different - they'd have different facts about where the particle is.
There are may other oddities about quantum mechanics, too. Particles can be entangled in a way that enables them to somehow share information instantaneously even if they're light years apart, for example. This challenges another common intution: that objects need a physical mediator to interact.
Physicists have therefore long debated how to interpret quantum mechanics. Is it a true and objective description of reality? If so, what happens to all the possible outcomes that we don't measure? The many worlds interpretation argues they do happen - but in parallel universes.
Another set of interpretations, collectively known as the Copenhagen interpretation, suggests quantum mechanics is to some extent a user's manual rather than a perfect description of reality. "The Copenhagen interpretations what they share is at least a partial step back from the full-blown descriptive aim of physics," explains Chris Timpson, a philosopher of physics at the University of Oxford. "So the quantum state, this thing which describes these lovely superpositions, that's just a tool for making predictions about the behaviour of macroscopic measurement scenarios."
But why don't we see quantum effect on the scale of humans? Chiara Marletto, a quantum physicist at the University of Oxford, has developed a meta-theory called constructor theory which aims to encompass all of physics based solely on simple principles about which physical transformations in the universe are ultimately possible, which are impossible, and why.
She hopes it can help us understand why we don't see quantum effects on the macroscopic scale of humans. "There's nothing [in the laws of physics] that says it's impossible to have quantum effects at the scale of a human being," she says. "So either we discover a new principle that says that they really are impossible - which would be interesting - or in the absence of that, it is more a question of trying harder to create conditions in the laboratory to bring these effects about."
Another problem with quantum mechanics is that it isn't compatible with general relativity, which describes nature on the largest of scales. Marletto is using constructor theory to try to find ways to combine the two. She has also come up with some experiments which could test such models - and rule out certain interpretations of quantum mechanics.
You can listen to Great Mysteries of Physics via any of the apps listed above, our RSS feed, or find out how else to listen here. You can also read a transcript of the episode here.
Authors: Miriam Frankel - Podcast host, The Conversation | Chiara Marletto - Research Fellow of Physics, University of Oxford | Christopher Timpson - Professor of Philosophy of Physics, University of Oxford | Marcus Huber - Professor of Physics, TU Wien