Everything you wanted to know about... gravity

It's all around us, but how much do we actually know about gravity? Photo / 123RF

It's all around us, but how much do we actually know about gravity? Prominent UK science writer and broadcaster Marcus Chown, who summarises the story so far in his new book The Ascent of Gravity: The Quest to Understand the Force that Explains Everything, chats here with New Zealand's best-known cosmologist, University of Auckland head of physics Professor Richard Easther.

Richard Easther: What's next for gravitational waves?

Marcus Chown: On September 14, 2015, gravitational waves - ripples in space-time predicted by Einstein a century ago - were detected for the first time.

They came from the merger of two monster black holes - an event which briefly was 50 times more powerful than all the stars in the universe put together.

When the ripples arrived on Earth, weakened by their 1.3 billion year journey across space, they changed the length of two 4km rulers made of laser light - one in Washington state and one in Louisiana - by just one hundred-millionth the diameter of an atom.

Physicists now want to improve the sensitivity of the "laser interferometric gravitational wave observatory", or LIGO, so it can detect ripples from other events such as the merger of super-dense "neutron stars" and the stellar explosions of "supernovae".

They also plan to add more rulers around the world, so delays in the arrival time of gravitational waves will pin-point the direction of any source in the sky.

But, just as there is an electromagnetic spectrum, with rapidly oscillating light like X-rays and sluggishly oscillating light like radio waves, there is a gravitational wave spectrum.

The gravitational waves detected on September 14 oscillated almost 100 times a second.

But waves which oscillate on timescales of months or years are expected to come from the merger of "super-massive" black holes, which lurk in the heart of all galaxies, weighing up to 50 billion solar masses.

To detect such waves it is necessary to go into space.

Currently, there is a plan to launch a space-based gravitational wave observatory, known as LISA, in 2030.

If you had to nominate an observation most likely to change our ideas about gravity, what would it be?

The observation of gravitational waves from the Big Bang.

I'll explain... Einstein's theory of gravity - the general theory of relativity - contains within it the seeds of its own destruction.

UK science writer Marcus Chown. Photo / Supplied

It predicts that, at the heart of a black hole and at the moment the universe was born in the Big Bang, the density and temperature skyrocket to infinity.

Such a "singularity" is a sure sign a theory has been stretched beyond its domain of applicability and no longer has anything sensible to say.

Physicists are currently looking for a better, deeper theory of gravity, which contains no singularity and will tell them about the birth of the universe.

So far, we have observed no instance in which Einstein's theory breaks down.

But the hope is that gravitational waves from the extreme conditions of the Big Bang would differ from the predictions of Einstein's theory and have imprinted on them some clue to the elusive deeper theory of which general relativity is an approximation.

Alternatively, we may be lucky and, as giant gravitational wave detectors become more sensitive, they may catch a clue in a black hole merger over the next few years.

Gravity is everywhere but is there a phenomenon or situation where the impact of gravity came as a surprise to you?

I was amazed to discover that, when the tide at sea rises, the water in wells falls, and vice versa.

Incredibly, it has been known since 100BC but was explained only in 1940.

The effect was first noticed by the Greek philosopher Poseidonios, who lived between 135 and 51BC.

He observed that, when a spring at the Temple of Heracleium at modern-day Cadiz in Spain was low, the tide in the nearby Atlantic was high, and vice versa.

The explanation, due to American geophysicist Chaim Leib Pekeris, has to do with the gravitational pull of the moon, the principal cause of the tides.

The key thing is that the moon not only makes the ocean bulge upwards but the rock as well.

The water-logged rock in which a well sits is like a wet sponge.

It sucks water out of the well when the sponge is stretched upwards (high tide) and squeezes water back into the well when the sponge is released (low tide).

A modern manifestation of rock tides is apparent at the Large Hadron Collider, the giant atom-smasher near Geneva in Switzerland.

Twice a day, the rock in which the sub-atomic race track is bored is pulled upwards and released by the moon.

This changes the length of the 26.7km ring by 1mm.

Dark matter or modified gravity - do you think physics has made a rush to judgment?

Yes, I think physics has.

In the late 1960s and 1970s when American astronomers Vera Rubin and Kent Ford discovered that the stars in the outer suburbs of spiral galaxies were orbiting so ridiculously fast that, by rights, they should be flung off into intergalactic space, there were two possibilities.

Either the stars were in the grip of gravity stronger than Newton would have predicted or there was an awful lot of invisible, or "dark", matter, whose additional gravity was keeping hold of the stars.

Physics went for the latter - the hypothesis of least daring - basically because nobody wanted to mutilate Einstein's elegant theory of gravity, which had superseded Newton's.

Also, there were a lot of candidates for the dark stuff, from black holes formed in the Big Bang to sub-atomic particles predicted by speculative theories. None, however, has yet been found.

The best candidate for a modified gravity theory is modified Newtonian dynamics, or MOND, proposed by Israeli physicist Mordehai Milgrom in the 1980s.

In MOND, gravity switches from Newton's kind to a kind that weakens more slowly at a threshold acceleration - a ten billionth of a g.

With this single parameter, mond can explain the motion of stars in every spiral galaxy.

By contrast, the dark matter model requires a different amount of dark matter with a different distribution in each galaxy.

The Ascent of Gravity: The Quest to Understand the Force that Explains Everything. RRP: $37.99