Physics has no use for the present moment. The equations that describe the world are, with a single disputed exception, indifferent to the direction of time and contain nothing corresponding to now. A complete description of the universe specifies what occurs at every moment; it does not mark one of them as the moment that is occurring. On the resulting picture, sometimes called the block view, the difference between 1815 and next Tuesday is of the same kind as the difference between here and Vienna: a matter of where one is situated, not of what exists. Against this stands the most insistent testimony available to anyone. It seems evident that the past has gone and the future has not arrived, that the present is not merely the place where I happen to be but the only time there is. The philosophical difficulty is that this conviction is very hard to state in a way that does not immediately consume itself. To say that only the present exists is to say something that, if true, was equally true at every moment that has ever been called the present, which suggests that what is being asserted is not a fact about a moment but a feature of every moment, and therefore not a distinction between them at all. Attempts to preserve the flow of time by adding it to the block have not fared well. The proposal that a moving spotlight of presentness travels along the sequence invites the question of how fast it travels, and the only available answer — one second per second — is not a rate but a tautology dressed as a measurement. Nothing has been explained; a metaphor has been made to look like a mechanism. It is tempting to conclude that the sense of passage is an illusion generated by the way memory accumulates, and this is now the majority view among physicists who consider the matter. But the conclusion is less tidy than it appears. An illusion is a misrepresentation of how things are, and specifying what is being misrepresented here has proved unexpectedly difficult. The claim cannot be that we wrongly believe the present is objectively distinguished, since on the block view every moment is present relative to itself, and our belief that this one is present would then be correct. To call the experience illusory is to owe an account of the error, and no such account has been supplied. What remains is a stalemate whose shape is worth noticing, because it recurs elsewhere. A physical description that is complete by its own standards leaves out something that appears indispensable to the describer, and the two parties then dispute whether the omission reveals a gap in the description or a confusion in the describer. Each side can accommodate all the evidence, at a cost the other regards as prohibitive. Grief supplies the sharpest instance. The bereaved do not doubt that the dead exist at earlier times.
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