England, born 1937

Julian Barbour

Time does not exist. There are only Nows: complete configurations of the universe, each one static. Change is the difference between them, and our sense of the past is the fact that some Nows contain what look like records of others.

The view

Barbour is a British physicist who has worked for most of his career outside universities. His book The End of Time (1999) made his case to a general readership. He began from Ernst Mach's idea that motion should be defined only relative to other matter, never relative to an absolute space or time, and with Bruno Bertotti in the early 1980s developed a way of doing mechanics on that basis.

Push the idea to its limit and time drops out. What is real is the set of all possible relative configurations of the universe. Barbour calls this timeless landscape Platonia, and each point in it a Now. A history is a path through Platonia. But nothing travels along the path.

The argument

Only relative arrangements are observable. We never see where the universe is in absolute space, or when it is in absolute time. We see how its parts are arranged relative to one another. A theory should be built from those arrangements alone.

Duration can be read off from change. Barbour shows that in classical mechanics, the time that clocks measure can be derived from how much the configuration changes, so that time needs no independent existence. Astronomers had already done something like this in defining "ephemeris time" from the motions of the planets.

Quantum gravity is timeless. The central equation of one canonical approach to quantum gravity, the Wheeler–DeWitt equation, contains no time variable at all. Barbour takes this at face value: the quantum state of the universe is a static assignment of probabilities to the Nows of Platonia.

Time capsules. Why, then, do we have a past? Because the Nows with high probability, Barbour conjectures, are special: their contents are arranged as if they recorded a history. A rock with fossils, a brain with memories, a page of print. Each is a feature of one static Now. Our sense of the past is the experience of being in such a Now.

Each Now holds all there is; its records are its past. after Barbour, The End of Time (1999)

In later work, including The Janus Point (2020), Barbour argues that the arrow of time comes from growing complexity, which increases in both directions away from a point of minimum complexity in the history of the universe.

Where he meets Deutsch

Barbour is Deutsch's nearest ally among the thinkers on this site. Both say that reality consists of snapshots, and that nothing flows between them. Deutsch's thought experiment of slicing spacetime into snapshots and shuffling them could have come from The End of Time. The inhabitants cannot tell the difference, because every record and memory sits inside a snapshot. Each snapshot carries its own "time stamp" in its contents, and the laws of physics are the only glue.

Both draw the same moral from quantum gravity. Deutsch credits Bryce DeWitt's work in the 1960s with the discovery that other times are a special case of other universes. Barbour builds his whole picture on DeWitt's equation. For both, there is no time outside the collection of snapshots against which they could be ordered.

And both explain the feeling of passage in the same way. For Deutsch, we experience differences between present perceptions and present memories. For Barbour, we experience being in a Now that contains records.

Where they part

Does time exist? Barbour says no. Deutsch says time is real but is not what we thought: it is a quantum concept, a structure within the multiverse, and the sequence of moments is a good approximation in our region of it. The disagreement is partly about words, but not entirely.

Causation. Deutsch insists that cause and effect make perfect sense in the multiverse: an event causes another when, across the variants, the second depends on the first. Barbour's static probabilities on Platonia leave less room for causes, and he is readier to say that the order we find is a matter of which Nows are probable.

Many universes, or many Nows? Barbour draws on Everett's many-worlds idea, but his worlds are the Nows themselves: there is no separate stack of universes for each moment. Deutsch's multiverse also blurs other times into other universes, but he describes it in terms of deterministic laws relating groups of snapshots, and he does not lean on any one theory of quantum gravity.

See every view compared →

Further reading

• Julian Barbour, The End of Time (London, 1999).
• Julian Barbour, The Janus Point (London, 2020).
• Julian Barbour, The Discovery of Dynamics (Oxford, 2001): the history behind his relational view.
• Absolute and Relational Space and Motion, Stanford Encyclopedia of Philosophy.