Science › Classical physics
The arrow of time
1927· Open question
Open questionNobody knows the answer.
The puzzle of why time has a direction, past to future, when the basic laws of physics work equally well run backward.
See it on the Science map →How it works
Film a billiard-ball collision and play it backward: it still looks legal. Film an egg breaking and the reverse looks absurd. The difference is entropy: the past had lower entropy than the future. Arthur Eddington named the 'arrow of time' in 1927. Most physicists trace it to the Big Bang starting in a highly ordered state, which shifts the question to why that start was so special.
Why it matters
It links everyday experience (memory, cause and effect, ageing) to cosmology, and is a key test for any deep theory of time.
How we know
The entropy explanation is well supported by lab physics. A low-entropy Big Bang fits the very smooth cosmic microwave background. Why the start was so ordered has no tested answer yet.
Still a mystery
Why the early universe had such low entropy, and whether memory and causation reduce fully to entropy, are unresolved.
Connections (3)
▸Entropy: disorder always grows explains The arrow of timeWell supported
The relationship
The fundamental laws work the same forward and backward, but entropy rises in only one direction. That rise distinguishes past from future: eggs break, heat spreads, records form. If entropy began very low at the Big Bang and has climbed ever since, the direction of time we experience follows from that slope.
How the link was found
Boltzmann argued this in the 1870s–1890s. Arthur Eddington coined 'arrow of time' in his 1927 lectures, published in 1928.
Is it proven?
Well supported: every irreversible process tracks rising entropy. Not fully proven, because the explanation depends on an unexplained low-entropy beginning.
Open questions and mysteries
Why the Big Bang was so ordered, and whether our sense of time's flow, memory and causation all reduce to entropy.
▸The arrow of time traces back to the low-entropy start of The Big BangWell supported
The relationship
Time has a direction because entropy, roughly disorder, keeps rising: eggs break, heat spreads. That only works if entropy started very low. Tracing back, the low point is the Big Bang, which was remarkably smooth and orderly in the gravitational sense. Every arrow we experience, memory, aging, cause before effect, inherits that one starting condition.
How the link was found
Ludwig Boltzmann linked entropy to probability in the 1870s-90s. Roger Penrose emphasized from 1979 that the Big Bang's low entropy is the real puzzle. Philosopher David Albert later called it the 'Past Hypothesis'.
Is it proven?
The microwave background shows the early universe was smooth to 1 part in 100,000, a very low-entropy state for gravity. This link is well supported reasoning rather than a direct measurement.
Open questions and mysteries
Why did the universe start in such an improbable, low-entropy state? Inflation, a multiverse or a deeper law are candidate answers; none is established.
▸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?