Is the universe infinite? Titus Lucretius Carus (c. 99 BC – c. 55 BC), Roman poet and philosopher, proposed the following wonderful thought experiment: «One of the most beautiful early examples of thought experimenting (in Lucretius, De Rerum Natura, or the Nature of Things) attempts to show that space is infinite: if there is a purported boundary to the universe, we can toss a spear at it. If the spear flies through, it isn’t a boundary after all; if the spear bounces back, then there must be something beyond the supposed edge of space, a cosmic wall that stopped the spear, a wall that is itself in space. Either way, there is no edge of the universe; space is infinite.»