Does the past still exist?

Two observers in relative motion slice spacetime into moments differently. Events that one calls simultaneous, the other calls separated in time, and at great distances the gap is large even at walking pace: for two people walking past each other, the moments they would call now on the Andromeda galaxy can be days apart.

This is not an artefact of signal delays, which can be corrected for. It is a feature of the geometry, and special relativity, which entails it, has been borne out by more than a century of experiment.

In 1967 Hilary Putnam argued, as Wim Rietdijk independently had the year before: if all events in my present are real, and reality is not something that depends on who is asking, then events in a passing observer’s present must be real too. Their present includes events that are in my future or my past.

Repeating the step reaches the conclusion that all events, past and future, are equally real, which is commonly taken to mean that the passing of time is not a feature of the world.

The argument is short, the physics in it is uncontroversial, and the conclusion is enormous, which is why it has been examined for sixty years.

The step that does the work is the assumption that reality is a single global notion that cannot be relative to a frame. Kit Fine relativises existence to a frame and so blocks the conclusion, at a price Christian Wüthrich calls immense: what exists would depend on how you move.

Jonathan Lowe rejects the transitivity of the relation instead. Others, Putnam himself by 2008, have come to think the dispute is verbal, and that whether the past and future are real is a pseudo-question. The literature is substantial and there is no resolution.

In 1968 Howard Stein answered the argument on its own terms. Relativity does provide a notion of what is definite relative to an event: everything in its past light cone, the region from which a signal could have arrived.

That relation is not the one the argument uses. Stein requires it to be reflexive and transitive, and it is not symmetric, which is what stops it sweeping the whole of spacetime into the present.

The cost is steep. On his relation, in Stein’s words, “an event’s present is constituted by itself alone”, which some philosophers, Callender among them, take to be too thin to count as becoming at all.

So relativity can be read as offering a well-defined and local notion of what has happened, without a global now and without the block. Which reading is correct is still argued, and this is the reason the question sits in the uncertain section rather than the known one.

If the block is real, the commonest answer is that the sense of time passing belongs to what it is like to be a system embedded in it, rather than to the world.

That is a claim about minds inside a physics, and the accounts of it are much less developed than the geometry they lean on.