Why Does Time Go Forwards, Not Backwards?


The arrow of time began its journey at the Big Bang, and when the Universe eventually dies there will be no more future and no past. In the meantime, what is it that drives time ever onward?

When Isaac Newton published his famous Principia in 1687, his three elegant laws of motion solved a lot of problems. Without them, we couldn't have landed people on the Moon 282 years later. But these laws brought to physics a new problem, which wasn't fully appreciated until centuries after Newton and still nags at cosmologists today.

The issue is that Newton's laws work about twice as well as we might expect them to. They describe the world we move through every day – the world of people, the hands that move around a clock and even the apocryphal fall of certain apples – but they also account perfectly well for a world in which people walk backwards, clocks tick back afternoon to morning, and fruit soars up from the ground to its tree-branch.

"The interesting feature of Newton's laws, which wasn't appreciated till much later, is that they don't distinguish between the past and the future," says the theoretical physicist and philosopher Sean Carroll, who discusses the nature of time in his latest book The Biggest Ideas in the Universe. "But the directionality to time is its most obvious feature, right? I have photographs of the past, I don't have any photographs of the future."

The problem is not confined to the centuries-old theories of Newton. Virtually all of the cornerstone theories of physics since then have worked just as well going forward in time as they do backwards, says physicist Carlo Rovelli of the Centre for Theoretical Physics in Marseille, France, and the author of books including The Order of Time.

"Starting from Newton, and then Maxwell's theory of electromagnetism, then Einstein's work, and then quantum mechanics, quantum field theory, general relativity, and even quantum gravity – there is no distinction between past and future," Rovelli says. "Which came as a surprise, because the distinction is so evident to all of us. If you make a movie, it's obvious which way is the future and which one is the past."

How does a clear direction of time emerge from these descriptions of the Universe, which all lack their own arrow of time? As Marina Cortês, an astrophysicist at the University of Lisbon, puts it: "There's a lot of implications that start with taking seriously the question, 'Why does time pass?'"

Part of the answer lies at the Big Bang nearly 14 billion years ago. Another insight comes from the opposite extreme, in the Universe's eventual death.

But before embarking on this epic journey back and forth along the timeline of the Universe, it's worth stopping off in 1865, just as the first truly time-directional law of physics came hurtling down the tracks of the Industrial Revolution.

Gathering Steam

In the 19th Century, when coal was shovelled into furnaces to generate steam power, scientists and engineers hoping to develop better engines embraced a set of principles that described the relationship between heat, energy and motion. They became known as the laws of thermodynamics.

In Germany, 1865, the physicist Rudolf Clausius stated that heat cannot pass from a cold body to a hot one, if nothing else around them changes. Clausius came up with the concept he called "entropy" to measure this behaviour of heat – another way of saying heat never flows from a cold body to a hot one is to say "entropy only ever increases, never decreases" (see box Entropy and the Rise of Disorder).

As Rovelli stresses in The Order of Time, this is the only basic law of physics that can tell apart the past from the future. A ball can roll down a hill or be kicked back to its summit, but heat can't flow from cold to hot.

To illustrate, Rovelli picks up his pen and drops it from one hand to the other. "The reason this stops in my hand is that it has some energy, and then the energy is turned into heat and it warms up my hand. And the friction stops the bouncing. Otherwise, if there was no heat, this would bounce forever, and I would not distinguish the past from the future."

So far, so straightforward. That is, until you start to consider what heat is on a molecular level. The difference between hot things and cold things is how agitated their molecules are – in a hot steam engine, water molecules are very excited, careening around and colliding into each other rapidly. The very same water molecules are less agitated when they coalesce as condensation on a windowpane.

Here's the problem: when you zoom in to the level of, say, one water molecule colliding and bouncing off another, the arrow of time disappears. If you watched a microscopic video of that collision and then you rewound it, it wouldn’t be obvious which way was forwards and which backwards. At the very smallest scale, the phenomenon that produces heat – collisions of molecules – is time-symmetric.

This means that the arrow of time from past to future only emerges when you take a step back from the microscopic world to the macroscopic – something first appreciated by the Austrian physicist-philosopher Ludwig Boltzmann.

"So the direction of time comes from the fact that we look at big things, we don't look at the details," says Rovelli. "From this step, from the fundamental microscopic vision of the world to the coarse-grained, the approximate description of the macroscopic world – this is where the direction of time comes in.

"It's not that the world is fundamentally oriented in space and time," Rovelli says. It's that when we look around, we see a direction in which medium-sized, everyday things have more entropy – the ripened apple fallen from the tree, the shuffled pack of cards.

While entropy does seem to be inextricably bound up with the arrow of time, it feels a bit surprising – perhaps even disconcerting – that the one law of physics that has a strong directionality of time built into it loses this directionality when you look at very small things.

"What is entropy?" Rovelli says. "Entropy is simply how much we're forgetting about the microphysics, how much we are forgetting about the molecules."

The publishing continues. The World Students Society thanks author Martha Henriques, BBC.


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