Of all our experiences of time, none is more vivid than its passage. The present feels alive and moving; the past feels fixed and gone; the future feels open and approaching. This sense of flow is so basic that questioning it seems perverse. Yet a long tradition in philosophy — and, arguably, modern physics — holds that the passage of time may not be a feature of reality at all, but a feature of us.
The dispute descends from McTaggart's distinction between tensed and tenseless time, and divides into two theories. The A-theory holds that the passage of time is real and objective: there is a genuine, moving present, and events really do change from future to present to past. On this view our experience of flow is veridical — it tracks something out there in the world. The B-theory denies all this. It holds that reality is a tenseless 'block' in which all times are equally real, and that no time is objectively present or moving. Events stand in fixed relations of earlier and later, and that is all. There is no flow in the world; what we call the passage of time is something our consciousness contributes as it experiences one moment after another.
Each side can point to powerful considerations. The A-theory has our overwhelming experience on its side — the flow of time is perhaps the most universal feature of human life, and a theory that calls it an illusion owes us an explanation of why it seems so real. The B-theory, in turn, presses a sharp question back: if time really flows, how fast does it flow? Any answer — 'one second per second' — sounds empty, suggesting that 'the rate of time's passage' may not be a coherent idea. And the B-theory sits far more comfortably with relativity, which recognises no universal, objective present of the kind the A-theory seems to need.
Present or blockTwo pictures of reality.
The two theories line up with two pictures of what exists. Presentism — that only the present is real — is naturally an A-theory: the flow of time just is the constant renewal of the ever-present 'now.' Eternalism — the block universe in which all times are equally real — is naturally a B-theory: with every moment equally existent, nothing is singled out as the moving present, and the felt passage must come from our perspective within the block. So the question of whether time flows is bound up with the question of what times exist, explored on the companion page.
Einstein himself, consoling the family of a friend who had died, wrote that the division between past, present, and future is a stubbornly persistent illusion — a remark often taken as an endorsement of the block universe and the B-theory. Yet the matter is far from closed. The sheer force of our experience of passage keeps the A-theory alive, and some philosophers argue that a physics-friendly account of real temporal becoming is still possible. Whether or not time flows, though, it clearly points one way rather than the other — and that asymmetry, the arrow of time, is the final question this wing takes up.