Why Doesn't Time Run Backwards? — Probability, Not Law, Forbids It

The director's question

The universe we live in moves towards more and more disorder as time passes; science measures that disorder as entropy. So I imagined that on the balloon's other side there might be a universe running the other way, towards order, towards negentropy. Not through a black hole as a tunnel, since I know it does not swallow things and send them out the far side, but somewhere beyond it.

In a place like that, might the old grow back into babies, and a burnt, scattered book gather itself out of the smoke into one volume again, if every particle of smoke retraced its path exactly backwards? I know it turns cause and effect around, and that today's physics finds it hard to accept. Still, I could not easily set down the thought that the two universes might once have been one balloon.

Science's answer — the laws do not care which way time runs

Play a film of two billiard balls colliding backwards. Nothing looks wrong. Newton's laws of motion, the laws of electricity and magnetism, the basic equation of quantum mechanics all hold just as well with time reversed. The laws do not forbid smoke particles moving exactly backwards into a book.

So what forbids it? Probability. Shake a finished jigsaw in its box and it falls apart; shake a scattered one all you like and it will not assemble itself, because there are uncountably more scattered arrangements than assembled ones. Almost every shake lands somewhere scattered. This scatteredness is called entropy, and in things that happen on their own it increases: the second law of thermodynamics.

Tool The Rewind Board — Can a Scattered Book Come Back Together? Shake forty-eight golden grains apart and rewind. Played backwards they come home; nudge just one by a hair and they never do; grains that escaped as light never return. See with your hands why time runs one way when the laws do not.

Information that never disappears but cannot be read

That is why physics holds that information is never lost. In principle the burnt book's contents remain, scrambled into smoke, ash and heat. If the photos and memories of part 1 were information we can read but that fades, this is information that never fades but cannot be read. Science in principle; impossible in practice.

To actually rewind you would have to clear three walls. One: the heat of the burning has already flown into space as light. Two: the uncertainty principle means no particle's position and speed can both be known exactly. Three: nudge a single particle slightly and the error multiplies with every collision. The three boards of the Rewind Board are these three walls.

Small rewinds still happen every day. In a hospital MRI, the tiny magnets of atoms in the body drift out of step; a radio pulse flips them, and they gather back together. That brief rewind is caught as a signal and turned into an image. This is magnetic resonance.

A little further in

How would someone on the other side feel?

Many physicists think our very sense that time moves forward is the direction in which entropy grows. We see cups break and feel time pass; memories pile up only that way. If so, people in a universe where entropy shrinks would not feel they were living backwards; for them, that way is forward. Old people turning into babies is only how it looks from our side.

That is also why Fitzgerald's short story The Curious Case of Benjamin Button feels strange: the world goes forward while one man's body goes back. Time's direction is set by the whole world together, not by one person. This is called the arrow of time.

Those who asked first

The Austrian physicist Boltzmann showed that entropy counts the number of scattered arrangements, explaining time's direction by probability. His formula is carved on his gravestone.

Some physicists have had ideas like this balloon. Julian Barber in Britain sees the universe stretching out both ways from a point of greatest simplicity and calls it the Janus point, after the two-faced Roman god. Sean Carroll and Jennifer Chen in the US proposed a model in which two universes with opposite arrows of time grow from one point. In these pictures both sides feel they are moving forward: a balloon swelling at both ends rather than one side shrinking.

The American physicist Lawrence Schulman calculated whether two systems with opposite arrows of time could share one space. They can if they barely affect each other; strongly linked, one arrow collapses.

Lee Smolin in the US proposed that every black hole gives birth to a new universe inside it. The white hole, a black hole run backwards in time, is also allowed by Einstein's equations but has never been observed. Stephen Hawking argued for years with other physicists over whether information falling into a black hole is truly lost.

None of this is proven; these are physicists' grand imaginings, on the same level as the balloon above. And the real universe does not expand because one side is squeezed; space stretches evenly everywhere.

What we do not know yet

If the laws allow both directions, why does our world run only one way? Probably because the universe began in a highly ordered state of very low entropy. Why it began that way, no one knows. What happens to information that falls into a black hole is also still unsettled.

In the film

David's device gathers what has scattered back into one: a machine to reassemble a burnt encyclopaedia, a machine against entropy. So when he says it is still unstable, science agrees: as board 2 of the Rewind Board shows, going against entropy collapses at a hair's breadth of error. And he draws the line himself: this doesn't bring anything back, it only shows where it went.

The question that remainsIf what scatters never disappears, is what we call lost really gone, or only unreadable?