Scientists know gravity exists. They just don’t know how it works

Amelia Urry

In 1888, astronomer Simon Newcomb proclaimed, “We are probably nearing the limit of all we can know.”

At the time, it was believed that the universe comprised some 6,000 stars – a vast expansion of the heavens previously charted by Galileo and Copernicus and Kepler, who had, in turn, radically overhauled the authority of Aristotle’s celestial projections.

As a man of his era, Newcomb had a point.

Having seen farther into the sky than previous generations ever could have imagined, and having settled on a way to explain what we saw there, how much more could we expect to learn?

A lot, of course. The struggle to see past what we think we already know gives Richard Panek the theme of his new book, The Trouble With Gravity.

“Nobody knows what gravity is, and almost nobody knows that nobody knows what gravity is,” he writes – that’s the trouble.

And though the subtitle is ‘Solving the Mystery Beneath Our Feet’, no new mysteries were solved in the making of this book.

Instead, by unfolding the succession of visions and revisions that led to our current cosmic understanding, Panek sets out to demonstrate how fundamental, and how fundamentally strange, gravity really is.

To get there, he first leaves the beaten track of science narrative for a detour through mythology.

In any case, this division between terrestrial and the celestial, Panek asserts, can be seen as an early attempt to delineate the force we would come to know as gravity.

There’s the world we can see and touch “down here,” and then there’s everything else “up there”.

Something keeps the two apart, after all. So what is it?

It can be hard to imagine just how radical the idea of universal gravity once was.

Isaac Newton’s theory that an invisible force might be acting equally on Earth’s tides and on the comets speeding through the solar system (not to mention on us) required a leap of imagination so dizzying it turned the known world upside down.

Theologians scoffed, philosophers balked. No one, not even Newton, claimed to know how it worked.

We’ve gotten used to the view since then. The decades spent grappling with the theory are now summed up by an apocryphal apple to the head, but the central puzzle remains. We can calculate what gravity does – whether to a cup elbowed off the edge of a table or the disk of light bent around a black hole – but we still don’t have a clue what causes it.

In other words, gravity is a “common unintelligibility”, a phrase coined by physicist Ernst Mach for a mystery that hides in plain sight. Just because we now accept the theory of universal gravitation – and its later elaborations by Einstein and others – doesn’t make the theory itself any more intelligible. We just forget how much we still don’t know.

Panek is no stranger to the incomprehensible.

A science writer and Guggenheim fellow, he has made a career out of explaining things that scientists themselves may barely understand.

In his 2011 book, The 4 Percent Universe, he set out to illuminate dark matter and dark energy, about which we know next to nothing other than that they make up 96 per cent of our universe.

Before the 1990s, humanity’s entire intellectual effort was concerned with what turns out to be just a sliver of all existence.

For a long time, this seemed to be our new horizon, the limit beyond which a modern Newcomb wouldn’t dare to dream. And yet. In a series of experiments, scientists are beginning to probe that void, too.

Similarly, the riddle of gravity shows signs of yielding – maybe not totally and maybe not in our lifetimes, but someday. In 2015, scientists detected the first “gravity wave”, confirming that space-time sloshes around like water.

That particular wave was the wake of two massive black holes colliding 1.3 billion light-years away, but its existence means we’re all making gravity waves, all the time.

That’s another radical legacy of Newton and Einstein: Light and time and space and matter operate by the same rules everywhere in our universe.

What happens out there must be happening here, too. Move your hand, and the universe vibrates with you. The implications are, once again, dizzying, and Panek takes evident pleasure in the whirl of new ideas. There are multiverses and quantum dynamics, pulsars and hypothetical particles, as Panek unearths the uncommon wonder hiding behind common unintelligibility.

Gravity is both inextricable from our daily lives and hard to reconcile with our sense of the world.

“We know that Earth orbits the Sun, but at the end of the day, we still say that the Sun goes down,” Panek writes. “When we think about gravity, what comes to mind isn’t black holes or the Big Bang but airplanes and apples and us.”

Today, we know that Newcomb’s 6,000 stars were just a small sample of the 300 billion that make up the Milky Way, which, in turn, is just one of 140 billion galaxies in our universe.

Edwin Hubble, the astronomer who first identified an “island universe” beyond our own galaxy, described astronomy as a history of receding horizons: The farther we look, the bigger the universe gets. But you have to know how to look to see the questions that persist much closer to home – so close, in fact, you might be forgiven for thinking we’ve already figured it all out.