In Germany 1865 physicist Rudolf Clausius He stated that heat cannot be transferred from a cold body to a hot body, if nothing has changed around them. Clausius came up with the concept he called “entropy” to measure this heat behavior – another way of saying that heat never flows from a cold body to a hot body is to say “entropy only increases and never decreases” (See the entropy framework and the rise of turbulence).

like Rovelli Confirmed in chronological orderThis is the Just The basic law of physics that can distinguish between the past and the future. A ball can roll down a hill or bounce to its top, but heat cannot 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 because it has some energy, then the energy turns into heat and heats my hand. Friction stops rebounding. Otherwise, if there is no heat, this will bounce back forever, and I will not distinguish the past from the future.”

So far, this is straightforward. That is, until you begin to think about what heat is at the 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, deflecting and colliding with each other quickly. The water molecules themselves are less agitated when they combine as condensation on the window glass.

Here’s the problem: when you zoom in to a level, say, one water molecule collides and bounces off the other, the time arrow disappears. If you watch a microscopic video of this collision and then rewind it, it won’t be clear which way forward and which way back. On the smaller scale, the phenomenon that produces heat – the collision of particles – is symmetrical with time.

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

“So the trend of time comes from the fact that we look at the big things, not the details,” says Rovelli. From this step, from the basic microscopic view of the world to the coarse-grained, approximate description of the macroscopic world – this is where the direction of time comes in.

“It’s not that the world is essentially oriented toward space and time,” says Rovelli. It’s when we look around we see a trend in which everyday, medium-sized objects have more entropy – a ripe apple fell from a tree, a jumbled pack of cards.

While entropy appears to be closely related to the arrow of time, it is somewhat surprising – and perhaps even confusing – that the only law of physics that has a strong direction of time built into it loses that direction when you look at very small things.