What Is in Empty Space? — Even Emptiness Has a Field

The director's question

The explanation was easy and fun. Yet it also left me puzzled. Why had I never heard something so basic before? Searching my school memories, I could not find it. So what came first was not delight but doubt. The universe filled with an electromagnetic field, like water in a swimming pool: could that really be so?

Science's answer — even emptiness has a field

Yes, it is true. But the swimming-pool comparison is only half right.

Think of a weather map. Temperature has a value in Seoul, in Daejeon, and at every point in between; nowhere lacks a temperature. In physics, something with a value at every point in space is called a field. The electromagnetic field has a value at every point in the universe. In having no gaps, it is like water. But nothing is filling space the way water fills a pool; every point of space simply has this property. A pool can be empty of water; space is never empty of fields.

Even in the emptiest vacuum the electromagnetic field does not vanish. It trembles, minutely and without end. The energy in that trembling is zero-point energy. Experiments have even confirmed that two metal plates set very close together in empty space are pulled slightly towards each other because of it.

The emptiest place in the universe

Even the emptiest reaches, between galaxies, are not completely empty. Every sugar-cube-sized volume holds about four hundred particles of light: the glow of the newborn universe, cooled over 13.8 billion years into the cosmic microwave background. A little of it was in the hiss of old television sets between channels.

Light is a ripple in this field: an electromagnetic wave. Around you now, countless ripples overlap: sunlight, lamplight, Wi-Fi and phone signals, the infrared heat from your body. Visible light is a very narrow slice of them.

Human bodies stir this field too. Nerves run on electrical signals, and a thinking brain makes weak electromagnetic ripples. But they are so faint they can be picked up only by very sensitive instruments right by the head in a room shielded from outside signals. A hand's breadth away they are lost among other ripples.

A little further in

Those who asked first

About a hundred and fifty years ago the Scot James Clerk Maxwell, working out the equations of electricity and magnetism generating each other in ripples, noticed that the ripples travel at exactly the speed of light. Light is a ripple in the electromagnetic field.

Over a century ago, physicists pictured the same thing: as sound travels through air, light must travel through something. They called the invisible stuff filling space the aether. The Americans Michelson and Morley built an exquisitely precise experiment to catch the wind of Earth moving through it, and caught nothing. There was no water, only the field. That is why the pool comparison is half right.

The Korean physicist Kim Sang-wook's book Tremble and Resonance explains light, fields and atoms through the idea that everything in the world is vibrating. A good next step for anyone curious after this piece.

What we do not know yet

Calculate the energy in the vacuum's trembling from theory and the result is absurdly far from what we observe in the real universe: off by a figure with more than a hundred zeros. It is called the worst prediction in the history of physics, and it is tangled up with dark energy, which makes the universe expand ever faster. No one knows the answer yet.

In the film

David's device follows and gathers waves. That the universe is all fields and everything is a ripple on them is science. Picking out the ripples one person left and reading them two thousand years later is the film's imagination, because a person's ripples are lost a hand's breadth away. The film imagines past that hand's breadth, where science stops.

The question that remainsIf nowhere is empty, what exactly was missing from the places we used to call empty?