• In a vacuum all photons travel at the same speed. They slow down when travelling through air or water or glass. Photons of different energies are slowed down at different rates. If Tolstoy had known this, would he have recognised the terrible untruth at the beginning of Anna Karenina? 'All happy families are alike; every unhappy family is unhappy in its own particular way.' In fact it's the other way around. Happiness is a specific. Misery is a generalisation. People usually know exactly why they are happy. They very rarely know why they are miserable.