Imagine opening the weekend newspaper and searching the puzzle pages for the Sudoku. You spend your morning working through this logic puzzle, only to realize by the last few squares that there is no consistent way to finish it.
“I must have made a mistake,” you think. So you try again, this time starting from the corner you couldn’t finish and work the other way. But the same thing happens again. You are at the last few squares and discover that there is no consistent solution.
Working out the fundamental nature of reality according to quantum mechanics is a bit like an impossible Sudoku. Wherever we start with quantum theory, we always end up with a riddle that forces us to rethink the way the world works fundamentally. (This is what makes quantum mechanics so much fun.)
Let me take you on a short journey, through the eyes of a philosopher, through the world according to quantum mechanics.
1. Spooky remote action
As far as we know, the speed of light (about 300 million meters per second) is the ultimate speed limit of the universe. Albert Einstein scoffed at the prospect that physical systems would influence each other faster than a light signal could travel between them.
In the 1940s, Einstein called this “spooky remote action.” When quantum mechanics previously seemed to predict such eerie events, he argued that the theory was unfinished and that a better theory would tell the true story.
We know today that it is very unlikely that there is such a better theory. And if we think the world is made up of well-defined, independent pieces of “stuff,” our world must be one where spooky remote action is allowed between these stuff.
2. Losing our grip on reality
“What if the world isn’t made up of well-defined, independent pieces of ‘stuff’?” I hear you say. “Can we avoid this creepy action?”
Yes we can. And many in the quantum physics community think that too. But this wouldn’t be any comfort to Einstein.
Einstein had a long-running debate with his friend Niels Bohr, a Danish physicist, about this question. Bohr argued that we should indeed give up the idea that the things of the world are well defined so that we can avoid creepy remote action. According to Bohr, the world has no definitive properties unless we look at it. If we don’t look, Bohr thought, the world as we know it isn’t really there.
But Einstein insisted that the world must be made something whether we look at it or not, otherwise we couldn’t talk to each other about the world, and that includes science. But Einstein couldn’t have both a well-defined, independent world and no spooky action at a distance … or could he?
3. Back to the future
The Bohr-Einstein debate is fairly well known in the history of quantum mechanics. Less known is the foggy corner of this quantum logic puzzle where we can save a well-defined, independent world as well as no spooky action. But we’ll have to get weird in other ways.
If doing an experiment to measure a quantum system in the lab could somehow affect how the system looked before the measurement, Einstein could have his cake and eat it too. This hypothesis is called “retrocausality” because the effects of conducting the experiment should travel “back in time”.
If this is strange to you, you’re not alone. This isn’t very common in the quantum physics community, but it has its supporters. If you’re faced with accepting spooky remote action, or no world-like-we-who-it-when we’re not watching, retrocausality doesn’t seem like such a weird option after all.
4. No view from Olympus
Imagine Zeus sitting on top of Mount Olympus and surveying the world. Imagine if he could see everything that has happened and will happen, everywhere and forever. Call this the “God’s eye” of the world. It is normal to think that the world is somehow or other, even though it can only be known by an all-seeing God.
Recent research in quantum mechanics suggests that a view of the world from God is impossible, even in principle. In certain strange quantum scenarios, different scientists can carefully look at the systems in their labs and take thorough recordings of what they see – but they will disagree about what happened if they start comparing notes. And there may not be an absolute fact about who is right – not even Zeus could know!
So the next time you come across an impossible Sudoku, rest assured that you will be in good company. The entire 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.