Imagine opening the weekend page and looking through the puzzle pages for the Sudoku. You spend your morning working on this logical riddle, only to realize through the last few squares there is no consistent way to complete it.
“I should have made a mistake,” you think. So try again, this time from the corner you could not finish and work backwards the other way around. But the same thing happens again. You are up to the last few squares and find that there is no consistent solution.
Working out 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 end up in a mystery 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 tour through the eyes of a philosopher through the world according to quantum mechanics.
1. Ghostly action-at-a-distance
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 famously joked about the prospect that physical systems affect each other faster than a light signal can move between them.
In the 1940s, Einstein called it ‘ghostly action at a distance’. When quantum mechanics appeared earlier to predict such ghostly progress, he argued that the theory should not be finished yet, and a better theory would tell the true story.
We know it is very unlikely that such better theories exist. And if we think the world consists of well-defined, independent pieces of “good”, then our world must be one where ghostly action-at-a-distance between these pieces of things is allowed.
2. Loosen our grip on reality
“What if the world was not made of well-defined, independent pieces of ‘good’?” I hear you say. “Can we then avoid this haunting action?”
Yes we can. And many in the quantum physics community think so too. But that would not be a comfort to Einstein.
Einstein had a long debate with his friend Niels Bohr, a Danish physicist, on this question. Bohr argued that we must indeed define the idea of the good of the world so that we can avoid ghostly action at a distance. According to Bohr, the world does not have definite properties unless we look at them. If we do not look, Bohr thinks, the world as we know it is not really there.
But Einstein insisted that the world be made something whether we look at it or not, otherwise we would not be able to talk to each other about the world, and so would science. But Einstein could not have both a well-defined, independent world and no ghostly action at a distance … or would he?
3. Back to the future
The Bohr-Einstein debate is fairly well known in the history of quantum mechanics. Less well known is the misty corner of this quantum logic mystery where we can save a well-defined, independent world and no ghostly action. But we will have to get weird in other ways.
If an experiment to measure a quantum system in the laboratory could somehow affect how the system was before the measurement, Einstein would be able to eat his cake and eat as well. This hypothesis is called ‘retrocausality’, because the consequences of the experiment will have to move ‘backwards in time’.
If you think this is weird, you are not alone. This is not a very common view in the quantum physics community, but it has its supporters. If you have to be confronted with ghostly action at a distance, or no world as we know it if we do not look, retrocausality does not seem such a strange option after all.
4. No view from Olympus
Imagine Zeus sitting on top of Mount Olympus and exploring the world. Imagine being able to see everything that has happened and what will happen, everywhere and for all time. Call it the “God’s eye view” of the world. It is natural to think that there must be a way in the world, even if it can only be known by an almighty God.
Recent research on quantum mechanics suggests that God’s view of the world is impossible, even in principle. In certain strange quantum scenarios, different scientists can take a good look at the systems in their laboratories and make thorough recordings of what they see – but they will not agree on what happened when they compared the notes. And there can very well be no absolute fact about who is correct – not even Zeus could know that!
So be assured that you will be in good company the next time you encounter an impossible Sudoku. 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 on The Conversation.