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Albert Einstein first contemplated the existence of gravitational waves a century ago. (Source: Getty images)

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A brief history of gravitational waves

Tuesday 15th March 2016 3:33 pm

It took a century of speculation and experimentation to discover gravitational waves, as Dr Karl explains.

Previously, I’d written about how a pair of orbiting black holes smashed into each other at around half the speed of light - and as they coalesced into a heavier single black hole, they emitted gravitational waves, with a power output about 50 times greater than all the stars in the universe combined.

But that enormous event happened a very long distance away, about 1.4 billion light years. So by the time those gravitational waves had reached us, about 1.4 billion years after the event actually happened, their energy had been diluted over a huge volume of deep space.

So before we dive in deeper, let me give you a little history on gravitational waves. We’ve had a century of speculation, four decades of doubt as to whether they could be detected, and a quarter century of building successively more sensitive machinery until we actually measured them.

Back in 1916, Albert Einstein formulated gravitational waves as part of his theory of general relativity. But then he went deeper into the mathematics, and as a result he thought gravitational waves might not exist. And for the next two decades he worried back-and-forth about it, until he finally decided they should exist.

By the mid-1950s, there was still doubt as to whether they could be theoretically detected by any instruments we could build. Finally, Richard Feynman and Hermann Bondi developed the so-called “sticky bead” thought experiment.

Let me give you the ‘simple’ version. Suppose you have some beads threaded onto a sticky rod. Suppose a gravitational wave comes along and accelerates the beads. These beads would move and (thanks to friction) would transfer some heat to the rod. This is ‘proof’ that gravitational waves must carry energy, and that theoretically, they are detectable.

However, another worker in the field, Joseph Webber, came up with a very reasonable objection. He said that the existing technology of the day was too insensitive to detect these gravitational waves, by factors of thousands and millions.

Two decades later, in the mid-1970s two important events happened.

The first one was an indirect detection of gravitational waves.

Two physicists, Joseph Taylor and Russell Hulse, had used the largest radio telescope in the world to find 40 pulsars.

(A pulsar is a rotating neutron star, with a powerful magnetic field, and which emits regular pulses — hence the name ‘pulsar’. And a ‘neutron star’? Just a star that after a few billion years of burning has shrunk down to a ball of neutrons, about 20 kilometres across, and with a mass of up to about two-and-a-half times that of our Sun).

Anyhow, one ‘object’ they discovered was actually a pulsar and a neutron star orbiting each other every eight hours. It turned out that their orbit was shrinking at about 3.5 metres a year.

Why?

Almost certainly, the orbit was shrinking because the neutron stars were emitting energy in the form of gravitational waves.

The observed rate of the decay of the orbit agreed with Einstein’s general relativity predictions to within 0. 2 per cent. The power emitted was about 2 per cent of the Sun’s total power output. (By the way, this pair — the neutron star and the pulsar — are expected to collide within 300 million years.)

But this test did not actually detect gravitational waves — it just indirectly detected their effect, which was the orbit shrinking.

The second thing that happened in the mid-1970s was that two physicists, Kip Thorne and Rainer Weiss, ended up sharing a hotel room. They spent the whole night doing what physicists do — excitedly talking about gravitational waves, and the best way to actually detect them. They then teamed up with a gifted experimentalist, Ronald Drever.

They settled on a novel detection method. Split a laser beam into two separate beams. Then, run them through two arms at right angles to each other. These arms are four-kilometres long. (Just as aside, way back in the year 1917, Einstein laid down the theoretical framework that made lasers possible.)

At the end of the long arms are mirrors that bounce the laser beams back to where they first split. Then, recombine the beams and look at their interference pattern. (You can see interference patterns in rivers, when the bow waves from passing boats ‘interfere’ with each other.) Once the system is up-and-running, nothing should happen to the interference pattern.

But suppose a gravitational wave comes rippling through our solar system. It ripples the fabric of space-time itself. The arms should change in length, and the interference pattern from the two laser beams should change.

It took about 20 years, but in the mid-1990s, construction of LIGO began. LIGO stands for Laser Interferometer Gravitational Wave Observatory, and in September 2015, its significance stormed across the world stage. So I’ll write about that, next time …

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This blog first appeared on Dr Karl's Great Moments in Science

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