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What is time, really?
· Open question
Open questionNobody knows the answer.
The open question of what time actually is, why it flows one way, and whether it is fundamental at all.
See it on the Science map →How it works
Physics treats time strangely. Einstein's relativity shows there is no single universal "now": clocks run at different rates with speed and gravity. Most basic laws work equally well backward, so the one-way arrow of time seems to come from entropy, starting from a very orderly Big Bang. And the 1967 Wheeler-DeWitt equation, an attempt to describe the whole universe quantum mechanically, contains no time at all. Some physicists suspect time emerges from entanglement.
Why it matters
Time underlies every law and every measurement. Understanding it is likely essential to joining quantum theory and gravity.
How we know
Time dilation is measured constantly, including corrections in GPS satellites. The entropy arrow is well supported. A 2013 lab experiment (Moreva and colleagues) showed a toy version of time emerging from entanglement.
Still a mystery
Why the early universe was so orderly, whether "now" means anything physical, and whether time is basic or emergent are all unanswered.
Connections (3)
▸Space and time may be emergent reframes What is time, really?Speculative
The relationship
If space can emerge from entanglement, time might also be derived rather than basic. That reframes the question: not what time is made of, but what it emerges from. One proposal, from Page and Wootters (1983), says time appears as correlations between a clock and the rest of a static universe.
How the link was found
Don Page and William Wootters (1983); Carlo Rovelli's thermal-time idea (1990s); recent holography research extending space emergence toward time.
Is it proven?
Speculative. A 2013 lab experiment by Moreva and colleagues demonstrated the Page-Wootters mechanism with entangled photons, but only as a toy model.
Open questions and mysteries
Time behaves differently from space in all known physics. Emerging it is far harder, and nobody has a convincing account.
▸What is time, really? asks about The arrow of timeOpen question
The relationship
Asking what time is includes asking why it has a direction. The arrow of time is the most concrete piece of that larger question: physics equations barely distinguish past from future, yet everything we observe does. Explaining the arrow, through entropy or cosmology, is one step toward explaining time itself.
How the link was found
Eddington framed the arrow in 1927. Thinkers from Augustine to modern physicists such as Julian Barbour and Carlo Rovelli have tied it to the nature of time.
Is it proven?
This is a question, not a claim. The entropy explanation of the arrow is well supported; deeper theories of time remain untested.
Open questions and mysteries
Is time fundamental, or does it emerge from something deeper? Does it 'flow' at all, or is flow a feature of minds?
▸The no-boundary proposal (Hartle–Hawking) makes time emerge, reframing What is time, really?Speculative
The relationship
In the no-boundary picture, time is not fundamental. At the very start all four dimensions behave like space; ordinary time, with a past and a future, appears only as the universe expands. That fits the Wheeler–DeWitt equation, which has no time in it: time becomes something that emerges inside the universe, tracked by its own changing size.
How the link was found
Hartle and Hawking (1983). Hawking later used it on the arrow of time, first claiming disorder would decrease if the universe recollapsed; Don Page and Raymond Laflamme convinced him otherwise, and he called it a mistake.
Is it proven?
Unproven. It is mathematically consistent, but no measurement reaches the epoch where time would emerge.
Open questions and mysteries
If time emerges, what does 'the universe began' mean? Why does emergent time have a direction, from the smooth early universe toward disorder?