Time

The central drive in physics recently is to fuse the two main theories, quantum theory and relativity. Both are well tested, but among the things they disagree about is time. That means two different notions of time must now somehow be reconciled - it has brought to the surface long-standing contradictions about time, and some now see time as the main obstacle in attempts to reach quantum gravity. So a puzzle that for decades was set to one side has shifted from the edge to the centre, and become a key issue. One interesting part of the puzzle is the contradiction between the two main theories about whether the future exists yet. Quantum theory tells us the future is unformed and undecided, because of a deep-seated randomness about events at the particle scale. But special relativity has led to the idea that the entire future already exists, and is unalterable. Both theories are well tested - the mathematics of both is certainly right. But it seems our understanding of at least one of