Einstein's gravitational waves: what are they, and what does their discovery mean?

By Bernie Hobbs

Scientists have finally found direct evidence of gravitational waves — a feat Albert Einstein never thought we would manage. But what does this discovery actually mean?

For starters, it opens a new field of astronomy — gravitational wave astronomy — that will let us see everything from the heart of a black hole, to the moments after the Big Bang.

What are gravitational waves?

Einstein’s general theory of relativity tells us that gravity is the curvature of space and time.

The stronger the gravity an object has, the greater the deformation of space and time it causes.

Gravitational waves are caused when objects with strong gravity accelerate. As they accelerate, ripples of space travel away from them at the speed of light.

They are not like light waves travelling through space, they are actual waves in space: rhythmic stretching and squeezing of space.

All objects sitting in the path of gravitational waves rhythmically move further apart and closer together as the space they exist in is stretched and squeezed.

The strongest gravitational waves — the only ones we have a hope of detecting — are formed when objects with enormous gravity undergo dramatic acceleration. Like when two black holes merge to form another.

If scientists have indeed detected the to and fro movement caused by passing gravitational waves it will be a monumental achievement.

These ripples are so small — only a fraction the size of an atom — that Einstein thought they had to be beyond our technology.

What does the discovery mean?

Being able to detect and measure gravitational waves opens up an entire new field of astronomy.

Gravitational wave astronomy would allow us to look further back in time and deeper inside the most extreme objects in the sky — to the earliest instant after the Big Bang.

All of our existing knowledge of the universe comes from telescopes, and all telescopes (optical, radio, X-ray etc) rely on light coming from distant objects.

Telescopes tell us a lot, but the light they detect has been absorbed and scattered by lots of gas and dust between the source and the telescope. At best, we are getting blurry images.

Like light waves, gravitational waves are imprinted with information about their source — but it is information that light could never provide.

Gravitational wave astronomy will reveal the insides of distant objects because it will let us “see” their mass.

The pattern of movement as black holes coalesce, the changes inside a supernova, the mechanisms of a gamma ray burst will all become visible to us.

And because gravitational waves only interact with gas and dust to a tiny extent, their signal is much cleaner than those from light.

Our picture of the universe will come into much sharper focus.

Why was it so hard to find gravitational waves?

While Einstein’s general theory of relativity predicted gravitational waves, he thought that if they did exist, they would be far too small to ever be detected.

And that was certainly true with the technology used in previous detectors.

Although they are produced by some of the most massive accelerations in the universe — like black holes colliding, or a supernova explosion — gravitational waves are incredibly tiny wiggles in space.

The biggest gravitational waves would only cause the equivalent of stretching and shrinking of the Australian continent by 10 millionths of the width of an atom.

And detecting movement on that scale is no mean feat, hence Einstein’s scepticism, and the failure of previous detectors built over the last 50 years to pick up a signal — their technology was simply too blunt.

There were high hopes for the discovery of gravitational waves at the upgraded LIGO (Laser Interferometer Gravitational-wave Observatory) facility.

Called Advanced LIGO, it is three times more sensitive than the original LIGO detector and was designed to detect vibrations in the range expected.

The advances in the technology led Australian member of the LIGO collaboration, Professor David Blair, to speculate early last year that he “would be most surprised if they didn’t detect the elusive waves soon after it was switched on”.

We have actually had indirect evidence of gravitational waves since 1974, from observations of the behaviour of pulsars — fast rotating neutron stars that give off a beam of light that appears as a regular pulse to detectors on Earth.

These pulsar observations show energy loss from gravitational waves, but they do not allow us to measure — or learn from — the waves themselves.

How were they found?

The technique used at LIGO and other observatories hunting for gravitational waves is a highly refined version of a method that has been around since the 1880s.

Called laser interferometry, it uses a split laser beam to measure extremely small distances with incredible accuracy.

The Advanced LIGO experiment comprised twin detectors located 3,000 kilometres apart, one in Washington and one in Louisiana.

Each detector used laser beams to constantly measure the lengths of two perpendicular pipes with stunning accuracy. Any change in length indicated a passing gravitational wave.

A single laser beam was split in two, with each beam travelling down one arm of the interferometer.

Mirrors at either end of the arms bounce the beam back and forth, and it is then recombined.

Because the two arms are identical in length (4 kilometres), the recombined laser beams perfectly cancel each other out.

When a gravitational wave from a distant cataclysmic event reaches the detectors, the rhythmic stretching and squeezing of space and time make the pipes longer and shorter in turns, and the recombined beams no longer cancel out perfectly. Instead, a telltale pulsing signal is detected.

Such a signal gives direct evidence for gravitational waves.

Haven’t we heard this news before?

In March 2014, a discovery was claimed based on a signal from the BICEP2 telescope at the South Pole.

It was later retracted when analysis confirmed the signal was coming from galactic dust.

The LIGO discovery has been rigorously checked by the collaborating scientists and — likely — accepted for publication in a peer-reviewed journal.

This is not the first time the team has been through this arduous process — there have been several dry runs, called “blind injections”.

In these cases a signal is planted in the data, but scientists are not told it is artificial until after they have gone through the entire checking process and prepared a paper for publication.

Where to from here?

Gravitational wave astronomy is in the business of serious data collection.

Detecting gravitational waves tells us more about the types of events that create them, and allows astronomers to point telescopes in the directions of the source to complete the picture.

And the regularity with which gravitational waves are detected indicates just how common these massively energetic collisions and explosions are in our universe.

As well as Earth-based observatories like the Advanced LIGO experiment, a huge space-based detector is due to go live in 2028.

The 1 million-kilometre-wide space antenna eLISA relies on three spacecraft orbiting the sun in a triangular formation.

Although vastly different in scale, eLISA works on the same interferometer principle as Advanced LIGO, without the need for pipes and minus the background noise on Earth.

But instead of detecting waves that we can hear (the kind of signal expected from LIGO could be played on a cello) the space detectors will listen for waves with a frequency of about one cycle per hour created by much bigger black holes.

The information we gather from these and other detectors will reveal aspects of the universe that would otherwise remain invisible.

Thanks to Prof David Blair, Director, Australian International Gravitational Research Centre, University of Western Australia.

This post originally appeared on the ABC.

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