Damascene Philosophy

QuestionsXII

Is time real?

Time feels like a river carrying us from past to future. Physics describes it more like a map, with no moving 'now' anywhere on it.

Reading time
5 min
Difficulty
Deep
Branch
Time
First posed
Parmenides and Heraclitus, c. 500 BCE; Augustine of Hippo, c. 400 CE; J. M. E. McTaggart, 1908

The question

Nothing seems more obvious than that time passes. The present is real; the past is gone; the future is not here yet. Events move from future to present to past, and we move with them.

Yet when you try to say what this passage is, it slips away. Augustine of Hippo admitted that he knew perfectly well what time was until someone asked him to explain it. How fast does time flow? One second per second, which is not much of an answer.

Physics makes things stranger. In Einstein's special relativity, observers moving relative to one another disagree about which distant events are happening 'now'. There is no single, universal present slicing through the cosmos. Many physicists and philosophers conclude that past, present and future are all equally real, laid out in a four-dimensional block universe, and that the flow we experience is a feature of our minds, not of the world.

“What then is time? If no one asks me, I know: if I wish to explain it to one that asketh, I know not.”

Augustine of Hippo, Confessions, Book XI (trans. E. B. Pusey)

Why it matters

Your view of time shapes how you think about death, regret and hope. If the past is still real in the block universe, those you have lost still exist, at earlier locations in time. Einstein, writing after the death of his friend Michele Besso, consoled Besso's family with the thought that the distinction between past, present and future is only an illusion, if a stubborn one.

It also bears on free will: if the future already exists in the block, is it open in any sense? And on time travel, which only makes sense if the past is somewhere to travel to.

Key terms

  • PresentismThe view that only the present exists; the past no longer exists and the future does not yet exist.
  • EternalismThe view that past, present and future events all equally exist. Opposed to presentism.
  • Block universeThe picture, suggested by relativity, of past, present and future as equally real parts of a single four-dimensional spacetime, with no moving present.
  • A-theory of timeThe view that time really passes: there is an objective present, and events genuinely move from future to present to past. Opposed to the B-theory of time.
  • B-theory of timeThe view that there is no objective present or flow of time. Events are simply earlier than, later than or simultaneous with one another, and 'now' is like 'here'. Usually paired with eternalism.
  • Relativity of simultaneityThe result from special relativity that whether two distant events happen at the same time depends on the observer's state of motion.

The positions

Position I

Presentism

Only the present exists. The past existed and the future will exist, but neither does.

Position II

Eternalism: the block universe

Past, present and future events all exist equally. 'Now' is like 'here': it marks your location, not a special status.

Position III

The growing block

The past and present exist, but the future does not yet. Reality grows as new moments come into being.

Position IV

Time is unreal

Time, at the deepest level, does not exist. It is an appearance or an emergent feature of something timeless.

Arguments and objections

Presentism

Best argument
This is common sense, and it fits our experience: we remember the past and anticipate the future, but we only ever live in the present. It explains why time seems to flow: what exists keeps changing. It also explains the asymmetry we feel between a past that is fixed and gone and a future that is open.
Strongest objection
Relativity says there is no absolute present. Which events are simultaneous depends on your state of motion. If only the present exists, existence itself becomes relative to the observer, which seems absurd. And if the past does not exist, what makes statements about it true?
The reply
Presentists answer that relativity describes how we measure time, not what exists, and that some versions of physics allow a preferred cosmic frame. Truths about the past can be grounded in present traces or in present-tense facts about what was the case.

Eternalism: the block universe

Best argument
Relativity strongly suggests that there is no universal present, so no objective fact about which slice of reality is the 'real' one. The natural conclusion is that the whole four-dimensional spacetime exists. The passage of time is how a being that remembers in one direction and not the other experiences its path through the block.
Strongest objection
It seems to make change an illusion and our experience of time's passage systematically misleading. If my future already exists, it is hard to see how my choices matter.
The reply
Eternalists answer that change is real in the block: it is simply variation along the time dimension, as a road changes along its length. And choices matter because they are part of what makes the future what it is.

The growing block

Best argument
This keeps what is plausible in both views: the past is fixed and real (so statements about it have something to be true of) while the future is open. It also gives time a direction and a real passage: the edge of the block moves.
Strongest objection
If the past exists just as the present does, how do we know we are at the growing edge? People in the past would have the same experiences and the same conviction that they were in the present.

Time is unreal

Best argument
J. M. E. McTaggart argued in 1908 that the idea of past, present and future is contradictory, since every event has all three properties, and that without them there is no real time. Some approaches to quantum gravity describe the universe with equations in which time does not appear.
Strongest objection
McTaggart's argument has been widely contested, and physics that lacks a time variable at the deepest level still needs to explain why time appears at ours. Calling it 'unreal' may only mean it is not fundamental.

Who argued what

  • Presentism Augustine of Hippo, A. N. Prior, Dean Zimmerman
  • Eternalism: the block universe Albert Einstein, Hermann Minkowski, J. J. C. Smart
  • The growing block C. D. Broad, Michael Tooley
  • Time is unreal J. M. E. McTaggart, Parmenides, Julian Barbour

Where you stand

Does time really pass?

Choose one to see how people answer.

Variations

The twin who aged less

One twin travels to a distant star at nearly the speed of light and returns. She finds her sibling has aged decades while she has aged only a few years. This is not a puzzle but a confirmed prediction of relativity, tested with atomic clocks on aircraft. Time is not the same for everyone.

Why don't we remember the future?

The laws of physics are, almost entirely, the same forward and backward. Yet we remember the past and not the future, and eggs break but do not unbreak. Most physicists trace this arrow of time to the very low entropy of the early universe. See Boltzmann brains.

Go deeper

McTaggart's A-series and B-series

McTaggart distinguished two ways of ordering events. The A-series orders them as past, present and future, properties that change as time passes. The B-series orders them as earlier than and later than, relations that never change: the signing of a treaty is always earlier than the war it ends. A-theorists hold that the A-series is real and fundamental. B-theorists hold that the B-series is all there is, and that 'now' is an indexical like 'here'.

Related ideas

Paradox

The grandfather paradox

You travel back in time and prevent your grandparents from ever meeting. Then you are never born, so you never travel back. So they do meet. So you are born...

Paradox

The bootstrap paradox

A time traveller gives Shakespeare a copy of Hamlet, which Shakespeare copies and publishes. So who wrote Hamlet?

Question

Is the future already fixed?

If the laws of physics plus the state of the universe settle everything that happens, then everything you will ever do was settled before you were born.

Question

Is free will an illusion?

You feel like the author of your choices. The question is whether that feeling tracks anything real.

Paradox

Zeno's paradoxes

Achilles can never catch a tortoise with a head start, because each time he reaches where it was, it has moved on. Motion itself, Zeno argued, is impossible.

Thought experiment

Are you a Boltzmann brain?

Given enough time, random fluctuations could assemble a single brain with your memories out of chaos. In some cosmologies, such brains vastly outnumber real observers.

Further reading and sources

Open questions

  • If the past still exists somewhere in spacetime, is anything ever truly lost?
  • What would it mean for time to flow, and could we detect it if it stopped?
  • Is the present special, or just the moment you happen to be in?

All questions · Take me somewhere · Print this page