Imagine opening the weekend newspaper and looking for the pages of the Sudoku puzzle. You spend your morning working through this logical puzzle, only to realize the last few squares that there is no consistent way to finish it.
“I think I made a mistake,” you think. So try again, this time starting from the corner you could not finish and work in reverse. But the same thing happens again. You have reached the last few squares and found that there is no consistent solution.
Elaborating the basic nature of reality according to quantum mechanics is a bit like an impossible Sudoku. No matter where we start with quantum theory, we always come up with an enigma that forces us to rethink how the world works fundamentally. (This is what makes quantum mechanics so much fun.)
Let me take you on a short tour, through the eyes of a philosopher, of the world according to quantum mechanics.
1. Scary action at a distance
As far as we know, the speed of light (around 300 million meters per second) is the last speed limit of the universe. Albert Einstein mocked his famous prospect of physical systems influencing each other faster than a light signal could circulate between them.
In the 1940s, Einstein called this “frightening action at a distance.” When quantum mechanics appeared earlier to predict such frightening things, he argued that the theory had not yet been completed and a better theory would tell the true story.
We know today that such a better theory is very unlikely. And if we believe that the world is made up of well-defined pieces and independent of “things”, then our world must be one in which the frightening distance-action between these pieces of things is allowed.
2. Weakening control over reality
“What if the world is not made of well-defined pieces and independent of ‘stuff’?” I hear you say. “Then can we avoid this frightening action?”
Yes, we can. And many in the quantum physics community think so too. But this would not be a consolation to Einstein.
Einstein had a long debate with his friend Niels Bohr, a Danish physicist, about this very question. Bohr argued that we should really give up the idea that the things of the world are well defined so that we can avoid the frightening action at a distance. In Bohr’s view, the world has no definite properties unless we look 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 of it something whether we look at it or not, otherwise we could not talk to each other about the world, as well as science. But Einstein couldn’t have both a well-defined, independent world and a scary action at a distance … or not?
3. Back to the future
The Bohr-Einstein debate is a reasonably familiar price in the history of quantum mechanics. Less familiar is the foggy corner of this quantum logic puzzle, where we can save both a well-defined, independent world and a scary action. But we will have to become weird in other ways.
If doing an experiment to measure a quantum system in the lab could somehow affect what the system was like before the measurement, then Einstein could take his cake and eat it too. This hypothesis is called “retrocausality”, because the effects of performing the experiment should travel “back in time”.
If you think it’s weird, you’re not alone. This is not a very common view in the quantum physics community, but it has its proponents. If you are faced with the fact that you have to accept a frightening action at a distance or no world as we know it when we do not look, retrocausality does not seem such a strange option after all.
4. No view from Olympus
Imagine Zeus perched on Mount Olympus, exploring the world. Imagine being able to see everything that has happened and will happen, everywhere and from all times. Call this the “sight of God” upon the world. It is natural to believe that there must be a way in which the world is, even if it can only be known by an all-seeing God.
Recent research in quantum mechanics suggests that God’s view of the world is impossible, even in principle. In some strange quantum scenarios, different scientists can look closely at the systems in their labs and make detailed records of what they see – but they will not agree with what happened when they compared the notes. And there may be no absolute fact as to who is right – not even Zeus could know!
So, the next time you encounter an impossible Sudoku, be sure you’re in good company. The whole quantum physics community, and maybe even Zeus himself, knows exactly how you feel.
Peter Evans is an early career ARC researcher at the University of Queensland.
This article first appeared on The Conversation.