margine.

Una lettera di Giovanni Perani

We were taught to picture the past as a match: religion on one side, science on the other. Then science supposedly won, and testable explanations took the place of faith. It is a compelling story, but too simple: historians find real conflicts, of course, as well as overlap and coexistence. The history is more complicated.

Science is not an alternative faith: it is a method that asks explanations of the natural world to face the evidence. And yet the more physics I read, the more some answers sound like magic. Quantum physics unsettles many everyday intuitions. You don’t have to believe it; you have to see whether the experiments hold up.

One example.

PHYSICS NOTEBOOK · NO. 01

One question. One atom. One surprise.
Curiosity is our little particle accelerator.

We are made of stars. Not just a line from a chocolate wrapper. Carbon, oxygen and many other elements in our bodies were forged in earlier generations of stars and scattered through space. Hydrogen is older still: it dates back to the Big Bang. NASA explains.

But we are also far less full than we look. In an atom, almost all the mass is concentrated in the nucleus, tiny compared with the atom itself.

Imagine an atom enlarged to the size of a football stadium: its nucleus would be about the size of a pinhead at midfield. Electrons are not little balls sitting in the stands; quantum physics describes them with a probability distribution, a kind of cloud. The stadium helps us picture the scale; it is not a photograph.

AN ATOM, SCALED TO A STADIUM

Illustration, not to scale. The nucleus and dots are enlarged to be visible; electrons do not have fixed positions.

That is what our bodies are like. So are the sofa, the coffee and the person asleep beside us.

So why can’t we walk through walls? Atoms are not empty little boxes that can simply slot into one another. Electromagnetic interactions and a quantum rule, the Pauli exclusion principle, make matter resist compression. Pauli says no two electrons can occupy the same quantum state—not merely the same place. What we call “solidity” is, in effect, a “you can’t come through here.”

Now for a thought experiment—not a weekend project. Imagine compressing all 8 billion human beings to the density of neutron-star matter. This is not simply a matter of removing the empty space inside atoms: it takes stellar pressures, and matter changes its nature. We would not remain people.

How much room, then, would the mass of all humanity take up?

About half a sugar cube.

The arithmetic, assuming an average of 60 kilograms per person: 8 billion people weigh about 480 billion kilograms. NASA gives a sugar-cube-sized amount of neutron-star matter a mass of roughly one trillion kilograms. At that density, our total mass would occupy about 0.48 cubic centimetres—almost half a sugar cube. Ordinary matter cannot simply be compressed this way: this is a comparison of densities, not a transformation we could carry out here. NASA’s comparison.

HUMANITY AT NEUTRON-STAR DENSITY

8,000,000,000people · assuming 60 kg each≈ 480,000,000,000 kg≈ 0.48 cm³ · almost half a sugar cube

A thought experiment and an order-of-magnitude estimate, not a realizable object. The mass does not disappear.

Think of that next time you put sugar in your coffee.

For anyone who finds physics boring

Last Thursday was my wife’s birthday. Blu, who is three, suggested a cake. In the end, Grandma made it; the rest of us just ate it. With dedication, I should add: not a crumb survived. Our experimental trial was short, but the results were conclusive.

It was a meringue cake with Smarties, from Clementina’s recipe notebook. That, too, is mostly air—or so it seemed, until it disappeared without a trace.

Here it is, copied from the notebook:

SWEETS · FROM CLEMENTINA’S NOTEBOOK

Meringue cake (Cristiana’s)

Ingredients · serves 5

butter

185 g

sugar

185 g

pasteurized egg yolks

dark chocolate

150 g

milk

1 small espresso cup

mini meringues

300 g circa

Smarties(our addition)

as many as you like

Method

Beat the butter and sugar very well. Add the yolks one at a time.

Melt the chocolate with the milk; once lukewarm, stir it into the mixture.

Line a cake tin with baking paper.Put the Smarties on the bottom.

Layer the mini meringues and the chocolate mixture until they are used up.

For a more chocolatey cake, finish with the mixture; otherwise, finish with the meringues.

Refrigerate.

When ready to serve, turn the cake upside down: it looks prettier, and the Smarties end up on top.

A note from outside the notebook: this cake is not baked. Use pasteurized egg yolks for an uncooked recipe.

Giovanni

Have a good Sunday from A margine: a small curiosity to carry with you.

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We’re more hollow than we look.

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