QuestionsDoes time really exist?There is no 'now' for the whole universe: past, present and future form a block
Does time really exist?Passage is an illusion

There is no 'now' for the whole universe: past, present and future form a block

The block universe; the relativity of simultaneity; an admittedly tenacious illusion

Albert Einstein 1879 · 1955The physicist of relativity: simultaneity depends on the observer, and there is no 'now' valid for the whole universe.
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What it claims

The distinction between past, present and future is only an illusion, however persistent: the world is a block of space-time in which all events exist.

Why

Events simultaneous for one observer are not for another: no single "now" holds for the whole universe.

The same idea through history
Albert Einstein1905Einstein shows that two events simultaneous for one observer are not so for another who is moving. This is the starting point of the whole argument.
Hermann Minkowski1908Minkowski joins space and time into a four-dimensional space-time: 'space by itself and time by itself are doomed to fade into mere shadows', only their union keeps independence.
Albert Einstein1955In a letter of condolence, Einstein writes that for physicists the distinction between past, present and future is only an illusion, however persistent.
Hilary Putnam1967Putnam turns this idea into an argument: if what is real does not depend on the observer, all events, past, present and future, are equally real.
In short

"There is no 'now' for the whole universe: past, present and future form a block" is an argument by Albert Einstein (Special relativity (1905); Minkowski, 'Raum und Zeit' (1908); letter of condolence to the Besso family (1955), 1905) on the question "Does time really exist?" (answer: passage is an illusion). Its claim: The distinction between past, present and future is only an illusion, however persistent: the world is a block of space-time in which all events exist. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P3, P5). AskPhilosophy presents 3 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

Special relativity (1905); Minkowski, 'Raum und Zeit' (1908); letter of condolence to the Besso family (1955)
Valid with an implicit premise

The sentence is famous: 'For people like us who believe in physics, the separation between past, present and future has only the importance of an admittedly tenacious illusion.' A caution: it comes from a letter of condolence Einstein wrote to the family of his friend Michele Besso, on 21 March 1955, a few weeks before his own death. It is a private consolation, not a result of physics. The argument that supports it is something else: it is built by Minkowski, Putnam and others from special relativity. If two observers in motion disagree about what is simultaneous, no 'now' holds for the whole universe. Then past, present and future would all exist equally, like the points of one block. This reading is a philosophical interpretation of relativity, disputed by other philosophers and physicists.

P1Simultaneity is relative: two events simultaneous for one observer are not so for another who moves relative to the first, and no experiment can tell which one is right.
P2So there is no 'now' valid for the whole universe. Time and space do not each subsist on their own: only their union, four-dimensional space-time, has a reality independent of the observer.
P3 · implicit premiseWhat exists does not depend on an observer's state of motion.
P4Then all events are equally real. An event that is future for me is simultaneous, for a moving observer, with what is present for me: so it is real for him, and therefore simply real. The same goes for the past.
P5 · implicit premiseIf past, present and future all exist equally, nothing in the world 'passes': the passing of time is a way we live events, not a fact about the world.
So the distinction between past, present and future is only an illusion, however persistent: the world is a block of space-time in which all events exist.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P3, the computer finds a case where simultaneity is relative and yet only some events are real: for example if what is real for an event is only its light-cone past. Without P5, it finds a case where all events exist and the passage of time remains a fact about the world.
Valid with P3 and P5, two hidden premises. P3 is the principle that reality does not depend on the observer; P5 is the step from 'everything exists equally' to 'passage is an illusion'. Opponents mostly dispute P3 and P5: P3 because relativity itself does not force one to define 'real' that way, P5 because they think becoming is an ultimate fact.
Analogy. A reel of film contains all the frames at once. The screen's 'now' is only the place where the projector shines. For defenders of the block, the world is the whole reel, and our 'now' is the place where we are on it.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

Henri Bergson 1922 · attacks premise 2

Bergson does not dispute Einstein's calculations but their reading. The times of different observers are measured times, conventions of physics. They all presuppose a lived, shared time, duration, which physics forgets. Saying there is no 'now' for the whole universe confuses the measurement of time with time itself.

a defender replies
see the argument
Einstein did reply, but that text is not yet in our corpus.
Kurt Gödel defender of the argument · 1949
Gödel argues that relativity, by removing the single 'now', supports the thesis of idealist philosophers, Kant first among them, for whom the passing of time is not an objective fact.
The debate goes on (our summary) On 6 April 1922, before the Société française de philosophie in Paris, Einstein replied to Bergson: in his view, there is no 'philosopher's time' beside the physicist's time and psychological time. We have not collected the minutes of that session (our summary, to be checked).
Where the debate stands (our summary)Our summary: the opposition is about what comes first, lived time (Bergson) or measured time (Einstein). Both accept the results of relativity; they part over what these mean for reality. Core disagreement.
Arthur Prior 1959 · attacks the conclusion

When we say 'Thank goodness that's over', what relieves us is not that the pain precedes this utterance, it is that it is over now. A world described without a 'now' does not say what we think when we want something to be behind us. Facts of past, present and future are therefore real, and the block leaves them out.

a defender replies
see the argument
No reply possible: objection raised in 1959, after the author's death (1955).
D
D. H. Mellor defender of the argument · 1981
Mellor answers that 'it is over' is true when the pain precedes the utterance itself. The relief comes from our belief that the pain is over, and this belief can be explained without any fact of 'now' in the world.
Where the debate stands (our summary)Our summary: everyone agrees that we are relieved when the pain is over. The question is whether this requires a fact of 'now' in the world, or only a belief on our part. An open point.
H
Howard Stein 1968 · attacks premise 4

Stein shows that Putnam's argument is not forced by relativity. One can define, for each event, what has already happened for it (its light-cone past), and keep a becoming without a universal 'now'. Events in my 'elsewhere' are not already real for me.

no reply
No reply possible: objection raised in 1968, after the author's death (1955).
Where the debate stands (our summary)Our summary: the two camps agree on the facts of relativity and part over what 'real' means. For Putnam, what is real for someone is simply real; for Stein, a becoming can be relative to each event. An open point.
What today's voices say
Defend it
Sabine HossenfelderShe thinks the block universe is how nature really works: a special present moment is just a false perception.