QuestionsDoes time really exist?Rotating universes: if one can travel into the past, the passage of time is not objective
Does time really exist?Passage is an illusion

Rotating universes: if one can travel into the past, the passage of time is not objective

The Gödel universe; closed timelike curves

Kurt Gödel 1906 · 1978A logician who gave a formal version of the ontological proof.
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What it claims

The passage of time is not an objective fact, even in our world: time passing is an appearance tied to our mode of perception, as Parmenides, Kant and the idealists held.

Why

In worlds the laws of nature allow, one can return to one's own past: then nothing justifies believing in an objective lapse of time.

The same idea through history
Albert Einstein1905Gödel extends Einstein's argument (the relativity of simultaneity removes the single 'now') by adding a solution of Einstein's equations in which travel into the past is possible. In Einstein's argument, Gödel appears only as a defender at one node of the debate.
Kurt Gödel1949For Einstein's 70th birthday, Gödel offers a model of a rotating universe containing closed paths in time, then draws a philosophical lesson about time from it.
In short

"Rotating universes: if one can travel into the past, the passage of time is not objective" is an argument by Kurt Gödel ('A Remark about the Relationship between Relativity Theory and Idealistic Philosophy' (1949); Rev. Mod. Phys. 21 (1949), 1949) on the question "Does time really exist?" (answer: passage is an illusion). Its claim: The passage of time is not an objective fact, even in our world: time passing is an appearance tied to our mode of perception, as Parmenides, Kant and the idealists held. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P5). AskPhilosophy presents 3 objections, 1 of them with a documented reply, and the original text behind each premise, checked word for word.

'A Remark about the Relationship between Relativity Theory and Idealistic Philosophy' (1949); Rev. Mod. Phys. 21 (1949)
Valid with an implicit premise

Einstein had shown that 'now' depends on the observer. Gödel goes further. In 1949 he finds a solution of Einstein's equations: a rotating universe in which, by travelling in a rocket on a sufficiently wide curve, one can reach any region of the past, present and future. A time that flows objectively would be a time in which the past never returns. Yet here the past can be visited. Gödel concludes that no reason can be given to assume an objective lapse of time in these worlds, and that this throws light on our own world too: our experience of time passing may be an appearance, as Parmenides and Kant thought.

P1Einstein's equations admit rotating universes in which, for events P and Q, there is a possible path linking P to Q although Q precedes P: one can return to one's own past.
P2In these worlds, however one may assume time to be lapsing, there will always exist possible observers to whose experienced lapse of time no objective lapse corresponds.
P3If the experience of the lapse of time can exist without an objective lapse of time, no reason can be given why an objective lapse of time should be assumed at all.
P4That such worlds are compatible with the laws of nature throws light on the meaning of time also in worlds where an absolute time can be defined: to say that this time is lapsing is to make the existence of time depend on the distribution of matter, a 'bizarre materialism'.
P5 · implicit premiseIf an objective lapse of time is unjustified in possible worlds, it is unjustified in ours too, since its reality cannot depend on the distribution of matter.
Therefore the passage of time is not an objective fact, even in our world: time passing is an appearance tied to our mode of perception, as Parmenides, Kant and the idealists held.

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.
Valid with P5. P1 to P3 concern possible worlds; P4 explains why this reaches our world; P5, hidden, makes the leap from possible worlds to ours. This is the most contested point: our universe does not rotate, and observations show no closed timelike curves. Gödel knows this: he replies that there might be models with closed curves observationally indistinguishable from ours. The argument differs from Einstein's: it does not rest on the relativity of simultaneity but on the possibility of travel into the past.
Analogy. A spiral staircase that loops back on itself: one always climbs, yet one finds the starting floor again. If one can climb endlessly without ever ceasing to 'go up', the word 'higher' no longer has an absolute meaning. Likewise, if one can go back to one's past, 'later' no longer has an absolute meaning.

Objections

3

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

C
Cosmologists (observations) 1949 · attacks premise 5

Our universe does not rotate like Gödel's model: it is expanding, and observations show no closed timelike curves in it. An argument drawn from a world that is not ours therefore says nothing about real time.

a defender replies
Kurt Gödel defender of the argument · 1949
Gödel anticipates the objection: our universe differs from the model, but there might be models with closed timelike curves observationally indistinguishable from ours. It is therefore possible that our universe is one in which objective time is an illusion.
Where the debate stands (our summary)Objection: our universe is not Gödel's. Gödel: neighbouring models of ours admit such curves. Core disagreement: does a mere physical possibility suffice to undermine the reality of time passing?
Stephen Hawking 1992 · attacks premise 1

Hawking proposes the 'chronology protection conjecture': the laws of physics (quantum effects in particular) would prevent closed timelike curves from forming. If true, Gödel's universe is a mathematical curiosity physics rules out, and the conclusion cannot be drawn from it.

no reply
No reply possible: objection raised in 1992, after the author's death (1978).
The debate goes on (our summary) The conjecture is not proven. And even if it were, it would concern physically realisable universes, while Gödel speaks of 'worlds compatible with the laws of nature' (classical general relativity). The disagreement is over the place of quantum physics in defining those laws (our reading).
Where the debate stands (our summary)Hawking: physics probably rules out these curves. Gödel: they are compatible with the known equations. Core disagreement: what is a possibility worth if a more complete theory excludes it?
Arthur Prior (A-theory of time) 1959 · attacks premise 3

Objection reconstructed from Prior, who does not target Gödel: when we say 'thank goodness that's over' after an ordeal, we are glad that it is past, not about a relation of dates between two events. Our attitudes therefore presuppose a real past and a real present. For defenders of passage, this experience is a datum a theory of time must explain, which a physical model cannot simply declare illusory.

no reply
see the argument
We have not yet found a direct reply by Gödel in the texts.
Where the debate stands (our summary)Prior: our attitudes presuppose a real present. Gödel: the experience of lapse can exist without an objective lapse. Core disagreement: does the experience of time prove its real passage?