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For the last few episodes (check here), we have been living in the world of the very small and the very precise. In the last episode (read here), we have seen how carefully you can measure things when you use light. We have learnt detecting changes so tiny they seem to belong to no practical purpose at all.

You might have been wondering, somewhere in the back of your mind:

This episode is the answer. It's the story of the single most spectacular thing that human precision measurement has ever accomplished. And it happens at a intersection point between the two most alien corners of physics, the world of the quantum, so small it defies imagination, and the world of gravity, so vast it bends space itself.

Let me tell you how those two worlds met.

Let us begin with a brief story about space!

We tend to think of space as the empty stage on which things happen. Stars move through it, light travels across it, but space itself just sits there, an inert backdrop.

Einstein's great insight (100 years ago!), was that this is wrong. Space is not a backdrop. It's more like a fabric, something that can bend, stretch, and flex. In his picture, gravity isn't something that pulls on you. Instead, it's the curvature of the fabric of space and time around a massive object.

There is a deep consequence if Einstein is right in saying that space is like an elastic fabric that can bend:

Massive objects bend the fabric of space. But when those objects accelerate—especially when enormous masses collide, that curvature doesn't remain fixed. The changing distortion travels outward through space as a ripple: a gravitational wave.

Those ripples race outward as waves in space itself, alternately stretching space in one direction while squeezing it in the other. As each wave passes, the pattern reverses again and again, carrying the disturbance across the universe.

So the trillion-dollar question: Is space really a physical entity, as Einstein envisioned? We have already encountered one place where Einstein's ideas turned out to be incomplete (check Episode #27). Could this be another?

To find out, we need a way to disturb spacetime itself and see whether it responds. Fortunately, nature provides exactly the right experiment. For this we need the most heaviest object in the universe; a black hole.

A black hole is both simpler and stranger than almost anything else in the universe. It is an object so massive and so compact that it curves spacetime into an abyss. The curvature is so extreme that once anything crosses a certain boundary—the event horizon—not even light can escape. That's why it's called black: no light ever returns from it, that is why it is called a “black“ hole.

Now imagine not one black hole, but two.

Locked in orbit around each other, they circle faster and faster as they lose energy through gravitational waves. Their orbit shrinks, their speed increases, and eventually they collide, merging into a single black hole.

During the final fraction of a second, the merger releases an astonishing amount of energy as gravitational waves. If spacetime is a real physical entity, these waves are ripples traveling through it, stretching and squeezing space itself as they race across the universe.

That is the experiment.

If we can detect those ripples, then spacetime is doing far more than serving as a mathematical backdrop. It behaves like something physically real.

But there is a catch.

Those gravitational waves begin their journey unimaginably powerful. Yet they must cross billions of light-years before reaching us. As they spread through the universe, they grow weaker and weaker, their effect diluted across an ever-expanding sphere of space.

By the time one finally sweeps across the Earth, the distortion it produces is almost beyond comprehension. Space itself stretches and squeezes, but by such a tiny amount that it seems impossible any instrument could ever notice.

That raises an obvious question.

How could anyone hope to detect something so absurdly small? By the time the wave reaches Earth, it changes distances by only a tiny fraction of the width of a single proton—one of the particles inside an atom. How do you measure something that small? How do you measure a ripple in space when the ripple itself is smaller than almost any length you can imagine?

At first glance, the answer seems simple: you can't. And yet, somehow, we can, using quantum technologies!

The instrument built to do this is called the Laser Interferometer Gravitational-Wave Observatory, or LIGO.

At first glance, it doesn't look like the sort of machine that should detect ripples in spacetime. It consists of two long vacuum tubes arranged in the shape of a giant L, each four kilometers long. A laser beam is split into two identical beams. One travels down each arm, reflects from a mirror at the end, and returns to where it started (figure below).

If both arms are exactly the same length, the returning light recombines in a way that produces almost complete darkness at the detector.

But if a gravitational wave passes through, something remarkable happens. Space stretches in one direction while squeezing in the other. One arm becomes ever so slightly longer, while the other becomes ever so slightly shorter. The two light beams now travel different distances, arriving just a little out of step. Instead of perfect darkness, a tiny flicker of light appears.

That flicker is the signal.

We have seen this idea in the last episode (read here). Rather than measuring an absolute distance, an interferometer measures a change in distance. And it uses the wavelength of light itself as the ruler.

The challenge, of course, is the scale. The difference in the two paths is far smaller than the size of a proton. To measure something so incredibly small, every part of the instrument must be pushed to its limits.

On the 14th of September, 2015, it moved the needle.

Two detectors, one in Louisiana and one in Washington State, thousands of kilometers apart, each caught the same faint pattern within a hundredth of a second of each other: a signal that rose in pitch and cut off—the unmistakable shape of two black holes spiraling together and merging, more than a billion years ago. The arms had stretched by their ten-thousandth of a proton. The light had flickered by exactly the predicted amount. Somebody, at last, had heard the fabric of space ring.

It won the Nobel Prize in Physics two years later.

But the prize almost undersells it. Think about what the achievement really is. We took the most patient, delicate skill humans have ever developed, the art of quantum-scale precision measurement, the very thing these episodes have been about, and we pointed it at the sky. And the sky answered with a sound from before there were eyes to see or ears to hear.

That is what all the precision was for.

This is where quantum measurement and gravitational physics meet: the tiny fluctuations of light become just sufficient to detect some of the most energetic yet subtle signals in the universe. In this regime, Einstein’s description of gravity passes one of its most demanding tests.

Here are a couple of videos that explain the LIGO detector and the whole experiment of detecting gravitational waves!