Imagine opening your weekend diary and searching through the pages of the Sudoku puzzle. You spend the morning working through this logic puzzle, only to realize from the last few squares that there is no consistent way to finish it.
“You must have made a mistake,” you believe. So try again, this time starting from the corner you couldn’t finish and work back the other way. But the same thing happens again. You have reached the last squares and find that there is no consistent solution.
Finding out the basic nature of reality according to quantum mechanics is a bit like an impossible Sudoku. Regardless of where we start with quantum theory, we always end up in an enigma that forces us to rethink how the world fundamentally works. (This is what makes quantum mechanics so much fun.)
Let me give you a brief tour, through the eyes of a philosopher, of the world according to quantum mechanics.
1. Fantastic action at a distance
As far as we know, the speed of light (about 300 million meters per second) is the maximum speed limit in the universe. Albert Einstein famously mocked the possibility that physical systems influence each other faster than a light signal could travel between them.
As early as the 1940s, Einstein called this “spooky remote action.” When quantum mechanics previously seemed to predict these spooky events, he argued that the theory had yet to be finished, and some theory would better tell the real story.
We know that today there is very unlikely to be such a better theory. And if we believe that the world is made up of definite and independent pieces of “things”, then our world must be a place where a fantastic remote action between these pieces is allowed.
2. Loosen control of reality
“What if the world is not made up of well-defined and independent pieces of things”? “I hear you say, ‘So can we avoid this spooky action?’
If we can. And many in the quantum physics community think so too. But that would be no consolation to Einstein.
Einstein had a long debate with his friend Niels Bohr, a Danish physicist, on this same issue. Bohr argued that, in fact, we should give up the idea that things in the world were well defined, so that we can avoid spooky action at a distance. According to Bohr, the world has no definite properties unless we are looking at it. When we don’t look, Bohr thought, the world as we know it is not really there.
But Einstein insisted that the world must be made something whether we look at it or not, otherwise we could not talk to each other about the world, and science as well. But Einstein couldn’t have an independent, well-defined world and no spooky action at a distance … or not?
3. Back to the future
The Bohr-Einstein debate is a reasonably familiar fare in the history of quantum mechanics. Less familiar is the foggy corner of this quantum logic puzzle where we can rescue an independent and well-defined world and no spooky action. But we will have to be weird in other ways.
If doing an experiment to measure a quantum system in the lab could somehow affect how the system was before the measurement, then Einstein could take his cake and eat it too. This hypothesis is called “retrocausality,” because the effects of doing the experiment would have to travel “back in time.”
If you think this is weird, you’re not alone. This is not a very common view in the quantum physics community, but it has its supporters. If you have to accept a spooky action at a distance or any world as we know it when we don’t look at it, retrocausality doesn’t seem like such a weird option after all.
4. No view from Olympus
Imagine Zeus perched on top of Olympus, exploring the world. Imagine being able to see everything that has happened and will happen, everywhere and forever. Call this the “vision of God” of the world. It is natural to think that there must be some way in the world, even if it can only be known by a God who sees everything.
Recent research in quantum mechanics suggests that God’s view of the world is impossible, even in principle. In certain strange quantum scenarios, different scientists may look closely at their lab systems and make exhaustive recordings of what they see, but they will not agree on what happened when they came to compare notes. And it is possible that there is no absolute fact about who is right, not even Zeus knew it.
So the next time you come across an impossible Sudoku, rest assured that you are in good company. The whole quantum physics community, and maybe even Zeus himself, knows exactly how you feel.
Peter Evans is an ARC Discovery Early Career Research Fellow at the University of Queensland.
This article first appeared in The Conversation.