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Twisted clock face. Time concept
Twisted clock face. Time concept

The Central Mystery of the Universe Is Time

Physicists assume everything is reversible in principle — yet time isn’t. Why not?

Physicist Paul Davies unpacks it, so far as anyone can:

Physicists first got to grips with the problem of the arrow of time in the middle of the nineteenth century by considering the behavior of gas molecules rushing around and colliding. Imagine a box of gas with a barrier down the middle. Suppose the gas on the left is hotter than on the right. If the barrier is removed, the faster-moving molecules on the left collide with the slower ones on the right, redistributing the energy. Soon the gas reaches a uniform temperature, a condition known as thermodynamic equilibrium. This process is irreversible. You never see the opposite happening. Without external interference, heat always flows from hot to cold. It’s an example of the so-called second law of thermodynamics.

Although this result seemed straightforward, there was a troubling paradox lurking. The laws of physics governing the motion of molecules are time-symmetric – they make no distinction between past and future. If by some magic all the molecules in a box of gas had their velocities simultaneously reversed, the box of gas would heat up at one end and cool at the other. There is nothing in the laws of motion themselves to prevent heat flowing from cold to hot, in violation of the second law of thermodynamics. The reason we don’t see such bizarre sequences of events is because of the initial conditions. The gas in the above example started out in an ordered state, with heat energy distributed unevenly. To explain the arrow of time in the box of gas we need to explain how it attained its initial state.

Paul Davies, “We know how time will end, but not how it started” at IAI TV (December 6, 2021)

All the laws of nature flow in both directions except for the arrow of time. Is the arrow of time a way of creating causality in our universe?

Here are some time-related conundrums imagined as short science fiction films:

What if a loved one aged much faster than you? – Sci-fi Saturday It’s one of the implications of faster-than-light travel (8:19 min). Should youthful Cpt. Bernhard take her now very old husband to the new Earth, Gaia? That’s the emotional and ethical dilemma in ARK.


In a Future Market, Time To Live Is Bought, Sold (10:57 min) An employee wants to rebel against the greed and injustice but then she would run out of time … “The Bargain” raises some issues — as a thought experiment — that appear in real life in the illegal organ trade.

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The Central Mystery of the Universe Is Time