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Skywatching

A new sort of astronomy

As we sit on the threshold of a new decade, it is interesting to pick what was the most significant astronomical development of the last one.

Considering what has happened in astronomical science in those years, we have a wide range of things to choose from, so here I will take the opportunity to push forward my personal choice.

Until a few years ago, everything we had observed about the universe had been done via electromagnetic waves, such as light and radio waves, high-energy particles, or, in the case of some bodies in the Solar System, actually going there. 

It was in this decade that we first got to see the universe in a completely new way, through a completely new window: gravity waves.

In the early 20th Century, Albert Einstein came up with the concept of the "fabric of space-time,” which is stretchable and twistable.

Large masses, such as planets and stars, stretch the fabric rather like the way a bowling ball sinks into the middle of a trampoline. Gravity is how we experience this deformation.

 If we have a compressible or stretchable medium, there is the possibility of waves propagating through it, like the ripples that run across the trampoline if we drop a bowling ball on it from some height.

In the same way, bodies moving through space-time will have a bow wave and leave a wake.

Events such as collisions between neutron stars or black holes will launch gravity wave ripples across the universe like the ripples radiating out from where a thrown stone landed in a pond.

Unfortunately, Einstein's calculations also showed these waves would be tiny, and he thought they might be too weak to detect. However he had no idea of how technology would improve over the following decades.

In principle, if you hold a ruler in the direction of an approaching gravity wave, as the wave passes the ruler will get very slightly shorter and longer, just as the gas molecules in air get closer together and further apart as a sound wave passes by.

However, detecting the really tiny changes caused by gravity waves requires major engineering.

Since the gravity wave shortens the ruler by a very small percentage, the longer the "ruler" the better. Even for a ruler a few kilometres long, a gravity wave would change its length by an amount smaller than a proton — the nucleus of a hydrogen atom.

A conventional ruler kilometres long would be impractical, if not impossible to make. Instead a beam of laser light is used.

The beam is straighter than any possible ruler, and we can bounce the light off a distant mirror, return it to where it started and see if its length is changing.

That's when we encounter the next problem, the air. Wind and turbulence bend the beam and dust can scatter it, changing its length slightly. So we send the beam of light down a long tube from which the air has been removed.

Next problem: how can we measure a length change that tiny? 

The easiest way to do this is to split the light beam and send part of light down another tube, set up at right angles to the first tube, to another mirror.

Then, we take the two reflected light beams and compare them. As the wave passes by, it is most likely to cross the two tubes at different angles, so they change length by different amounts.

Amazingly, it works. Gravity waves have been detected and measured, and observations of black hole interactions, neutron stars and other otherwise invisible events taking place across the universe are now piling up.

Not only has Einstein been proven right (again), but also we are now established in a new science — Gravity Wave Astronomy — and an exciting new window through which we can study the universe we live in.

  • Venus lies low in the southwest after sunset.
  • Mars rises in the early hours.
  • The Moon will reach First Quarter on Jan. 2.

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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