1850 – today
Is time real? The arrow of time, entropy and physics
Physicists agree that time is real in the sense that clocks measure it and it shapes every experiment. What's debated is whether it's fundamental. Most laws of physics work the same forwards and backwards; the direction of time seems to come from entropy always increasing. Some theories of quantum gravity have no time variable at all, which suggests time may emerge from something deeper.
Key facts
| Second law of thermodynamics | Entropy of an isolated system never decreases (Clausius, 1850s–1865) |
|---|---|
| Statistical explanation | Ludwig Boltzmann, 1870s |
| 'Arrow of time' coined | Arthur Eddington, 1927 lectures (published 1928) |
| A-series and B-series | J. M. E. McTaggart, 1908 |
| Wheeler–DeWitt equation | 1967, no time variable |
| Planck time | about 5.4 × 10⁻⁴⁴ seconds |
| Age of the universe | about 13.8 billion years |
Time feels like the most obvious thing in the world. Physics makes it strange.
Laws that don't care about direction
Film two billiard balls colliding and play it backwards; nothing looks wrong. Newton's laws, Maxwell's equations and quantum mechanics all work the same in reverse. Yet our world plainly runs one way. Eggs break and don't unbreak; we remember the past, not the future.
Entropy and the arrow of time
The exception is the second law of thermodynamics: in an isolated system, entropy, roughly the amount of disorder, never decreases. In the 1870s Ludwig Boltzmann explained why. There are vastly more disordered arrangements of molecules than ordered ones, so systems drift toward disorder simply because it's overwhelmingly more likely. In 1927 the astronomer Arthur Eddington called this one-way direction the "arrow of time."
That raises a deeper question: why was entropy so low to begin with? The usual answer is that the universe started in an extraordinarily ordered state at the Big Bang, about 13.8 billion years ago. Physicists call this the "past hypothesis." In that sense, every broken egg is a consequence of the Big Bang.
Relativity removes "now"
Einstein's relativity shows that two observers moving differently can disagree about whether two distant events happen at the same time. There's no single universal "now." Many physicists take this to support the block universe view, in which past, present and future all exist equally in four-dimensional spacetime and the flow of time is how it looks from the inside. Einstein himself wrote, late in life, that the distinction between past, present and future is a "stubbornly persistent illusion."
The problem of time
General relativity treats time as part of a flexible spacetime; quantum mechanics treats it as a fixed background. Combining them is the central unsolved problem in physics. When John Wheeler and Bryce DeWitt wrote down an equation for the quantum state of the whole universe in 1967, time disappeared from it entirely. This is known as the problem of time.
Some physicists think time is emergent. Carlo Rovelli argues that time is relational, a feature of how parts of the universe change relative to each other. Julian Barbour has argued that the universe is a collection of timeless "nows." Others, like Lee Smolin, argue the opposite: that time is the one thing that's truly fundamental. At the scale of the Planck time, about 10−43 seconds, our current theories stop working, and nobody knows what time looks like there.
What philosophers argue
In 1908 the philosopher J. M. E. McTaggart split ways of describing time into the A-series (past, present, future, which keep changing) and the B-series (earlier than and later than, which never change). Presentists say only the present exists; eternalists say all times are equally real. The debate is still open.
So is time real?
Time is real enough to be measured to 18 decimal places and to steer every GPS receiver on Earth. Whether it's a basic ingredient of reality or something that emerges from deeper physics is one of the biggest open questions in science.
Questions people ask
Why does time only go forward?
Because entropy, the measure of disorder, almost always increases. Disordered states are so much more numerous than ordered ones that the universe moves toward them, and that gives time a direction.
Did Einstein say time is an illusion?
In a 1955 letter after the death of his friend Michele Besso, he wrote that the distinction between past, present and future is only a stubbornly persistent illusion. He was talking about the block view of spacetime.
Can time go backwards?
The basic laws of motion allow it, but the second law of thermodynamics makes it so improbable that it never happens for anything larger than a few particles.
Sources
Last updated October 18, 2026. Found an error? Tell us.