What is time? Is our perception of time passing an illusion which hides a deeper, timeless reality? Or is it real, indeed, the most real aspect of our experience of the world? Einstein said that "the distinction between past, present, and future is only a stubbornly persistent illusion," and many contemporary theorists agree that time emerges from a more fundamental timeless quantum universe. But, in recent cosmological speculation, this timeless picture of nature seems to have reached a dead end, populated by infinite numbers of imagined unobservable universes.
