Block Universe

← Encyclopedia

A question hides inside the most ordinary fact about time: that this moment is happening and last year is not. Common sense treats the present as the thin lit edge of reality, the past gone and the future not yet made. Three rival accounts of what exists divide along exactly that line. Presentism says only the present is real — the past has been but is no longer, the future will be but is not yet. The growing-block holds that past and present are real and the future is not, so reality enlarges as each moment is laid down — the homely picture of a fixed past and an open future. And eternalism, the block universe, says that past, present, and future are all equally real: every event, from the first instant to the last, is as actual as this one, and the difference between now and then is no deeper than that between here and there. The dispute is ontological: not how time is best described but what time is made of, and modern physics has given the question a sharp new edge.

The edge is the relativity of simultaneity, among the first consequences of Einstein’s 1905 special relativity. Whether two separated events happen “at the same time” turns out not to be absolute; it depends on the observer’s state of motion. The result follows from two premises experiment has confirmed without exception: that the speed of light is the same in every inertial frame, and that the laws of physics are the same in all of them. The familiar illustration is a train. Lightning strikes both ends at once as measured from the platform; an observer riding the train, moving toward one flash and away from the other, measures the strikes at different times — and neither verdict is mistaken, since each frame has synchronized its clocks correctly by light. Observers in relative motion simply disagree about the order of events too far apart to send a signal between, and no fact above the frames settles which happened first. That much is settled physics, confirmed in laboratories for a century.

Hermann Minkowski drew the geometry out three years later. In a lecture at Cologne in September 1908 he recast Einstein’s theory as a claim about a single four-dimensional manifold, and opened with the line that has shadowed the debate ever since: “Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.” Space and time become axes of one spacetime; each event is a point in it, each object a worldline through it. The cost to the present is structural. Because each inertial frame slices spacetime into its own set of simultaneous events, there is no single “now” running through the whole of reality — there is, instead, an infinity of planes of simultaneity through any given point, one for each state of motion. The privileged universal present that presentism requires has no place to sit.

From here the bridge to metaphysics was built, and built twice over. Independently, the physicist C. W. Rietdijk in 1966 and the philosopher Hilary Putnam in 1967 argued that relativity favors the block. The reasoning is brief. Each observer’s plane of simultaneity defines that observer’s present, a three-dimensional sheet of events taken to be real. Observers in relative motion have differently tilted planes, hence different presents; and if each present is equally real, the only thing that can ground them all at once is a single four-dimensional reality in which past and future events are as definite as present ones — a result Putnam reinforced by holding that statements about distant events are already true or false wherever the speaker stands. Roger Penrose gave the argument its vivid form. Two people pass on a sidewalk, walking opposite ways; because their planes of simultaneity tilt by a hair, an Andromedan fleet has, on one person’s now, already launched its invasion, while on the other’s the decision to launch has not been taken. Same street, same instant, two incompatible verdicts about a settled or unsettled future. The conclusion carries a forbidding name: chronogeometric fatalism, the thought that if anything is definite then all of spacetime must be.

The philosophical groundwork was older than the physics. In 1908, Minkowski’s year, J. M. E. McTaggart published “The Unreality of Time” and gave the field a vocabulary it still uses. He distinguished two ways of ordering events. The A-series ranks them as past, present, and future, relative to a moving now; the B-series ranks them only as earlier-than and later-than, relations that never change. His argument ran in two strokes. First, time requires real change, and only the A-series supplies it, since on the B-series the relations are permanent — if one event is ever earlier than another it always was and always will be. Second, the A-series is contradictory: past, present, and future are incompatible, yet every event has all three in turn, and the escape that it has them successively only sets off a regress. Time needs the A-series, the A-series cannot be had, so time is unreal. Almost no one accepts the conclusion, and the distinction outlived the proof. What the defender of the block takes from McTaggart is the first stroke turned against him: deny that change requires the A-series, analyze it in B-series terms instead, and time survives with the moving now subtracted. That is the block’s real shape — not the abolition of time, but the keeping of the earlier-and-later order while letting the privileged present go.

What goes with the present is the felt passage of time, and here the block exacts its steepest price. If eternalism holds, the sense that the moment is flowing, the future rushing toward us and the past receding, has to be accounted for as something other than an objective feature of the world — a perspective local to each worldline, or a habit of mind, the way McTaggart called the rush of time a constant illusion of our minds. The “now,” on the deflationary view, is merely indexical, true for the speaker and weightless in the world, as no place is objectively “here.” Against that stands the idea of absolute becoming, the sheer coming-to-pass of an event distinct from any change within it, which some argue physics has not ruled out — and Carnap recorded that Einstein felt the unease, that the experience of the now means something to a person which physics neither captures nor can capture.

It is Einstein’s most quoted sentence that fixed the block in the public mind. In March 1955, writing to console the family of his friend Michele Besso — the only person thanked by name in the 1905 paper — and weeks from his own death, he wrote that for those who believe in physics the distinction between past, present, and future is only a stubbornly persistent illusion. It comes from a private letter of condolence, not a published claim, and the philosopher Tim Maudlin argues there is nothing in relativity that strictly entails it — noting that a man who truly thought his theory dissolved time would hardly have confined the conclusion to a note of grief. Maudlin’s deeper point is that temporal structure differs fundamentally from spatial structure, so the union of space and time does not license treating time as just another direction one might face.

The contest is the heart of the matter, and the block does not win it by default. Eternalism is a serious view, perhaps the majority one among philosophers of physics — Dean Rickles reports a consensus that relativity is incompatible with presentism — but it is genuinely disputed, and the bridge from the geometry to the metaphysics is where the disputes live. Howard Stein answered Rietdijk and Putnam by showing that their argument smuggles in an un-relativistic assumption: that what is real for an observer must be a global plane of simultaneity. Relativity offers no such plane, only the light cone. From spacetime’s own causal structure Stein built a “has already become” relation, transitive and well-defined, that does not hold between every pair of points — a formal refutation of the fatalist inference. Its cost is severe, shrinking each event’s present to the event itself, and critics have pressed on whether so thin a present can ground becoming at all; the dialectic stays open. Other replies crowd in beside it. Reichenbach held that distant simultaneity rests on a conventional choice of synchronization, undercutting any appeal to objective timing; neo-Lorentzian presentists retain a hidden preferred frame indistinguishable from special relativity, which cosmology’s cosmic time may even supply; and some, a later Putnam among them, suspect the quarrel turns on a slippery word and dissolves once the senses of “exists” are sorted out. The eternalist case is strong; it is not closed, and “the now is an illusion” is a position in a live argument, not a result.

The block sits nearer than any physical doctrine to an old metaphysical intuition — that all moments stand at once before an eternal view, that nothing real is ever lost. The kinship is worth bounding, for the block is narrower than the visions it resembles. It does not, by itself, abolish freedom: that an event tenselessly exists, or that a statement about it is already true, is not yet a claim that anything causes it, and the gap between fixity of being and fixity of cause is where free will is fought. Nor does it say that time repeats; the eternal return concerns time’s content recurring, the block the standing of all times alike, and a block need never cycle. Nor does it open a window onto things to come — the future’s already existing is an ontological claim, not an epistemic one, supplying no channel by which the present could be informed of it, and lending precognition only a metaphor. And it is not the many-worlds branching of the quantum theorists, which multiplies outcomes within a moment rather than affirming the reality of moments across time; a single, unbranching spacetime can be a perfect block. What the geometry has actually shown is narrower and stranger than the intuition it flatters — that there is no fact, above all the frames, about which distant event is happening now. Its elevation into the eternity of all things is a step physics gestures toward and has never compelled.

→ Related: Free Will · Retrocausality · Eternal Recurrence · Many Worlds Interpretation · Precognition