“Only atoms and the void are real,” said the ancient Greek philosopher Democritus. “Well, actually, it’s a bit more complicated than that. Atoms can also be waves, the void is made up of fields and everyone is going to need to start using the word ‘quantum’ all the time.” OK, he said only the first bit. Democritus pioneered atomism – the idea that everything in the universe can be divided into atoms, which can then be divided no further. But as we mark 100 years since the development of quantum mechanics in 1925, I have been wondering whether, somewhere in a corner of the multiverse, he and other ancient philosophers could have come up with a version of the theory millennia earlier. If so, what would that world look like? This article is part of a special series celebrating the 100th anniversary of the birth of quantum theory. Read more here. “The story could have started 2400 years ago,” says Bob Coecke, chief scientist at quantum computing firm Quantinuum. For him, the point of divergence came when Parmenides, another ancient Greek philosopher, declared that the universe is singular and unchanging. He even believed that motion was impossible – something his contemporary Heraclitus reportedly attempted to disprove by waving his arm in front of his face. Parmenides was having none of it: just because the arm was in one place, then another, it doesn’t mean we can say it moved, he argued. Parmenides inspired Democritus, who – quite reasonably – rejected the wacky view that motion was impossible, but embraced the idea of an unchanging reality. His atomism squared these…