A Galaxy-Sized Observatory For Gravitational Waves

KentuckyFC writes "Gravitational waves squash and stretch space as they travel through the universe. Current attempts to spot them involve monitoring a region of space several kilometers across on Earth for the telltale signs of this squeezing. These experiments have so far seen nothing. But by monitoring an array of pulsars throughout the galaxy, astronomers should be able to see the effects of gravitational waves passing by. They say such an array of pulsars should effectively shimmer as the gravitational waves wash over it, like a grid of buoys bobbing on the ocean. That'll create an observatory that is effectively the size of the entire galaxy. These observations should be capable of monitoring how galaxies and supermassive black holes evolve together, and shed light on the physics of the early universe. Best of all, the next generation of radio-telescope arrays should be capable of making these observations at a cost of around $66 million over ten years. That's a small fraction of the hundreds of millions that Earth-based observatories have already cost."

A complementary approach

[beanyk]

Just wanted to point out that the pulsar timing array approach will cover a completely different frequency range (~ 10^-9 to 10^-7 Hz) to existing ground-based detectors (LIGO, Virgo and friends), which operate in the 10^1 to 10^4 Hz range. In between are projects like LISA (http://lisa.jpl.nasa.gov/).

The different frequency ranges mean different astrophysical sources of gravitational waves; generally speaking, the more massive the system, the lower the GW frequency. LISA, for instance, would see the radiation produced by the supermassive black holes at the centres of galaxies, while the other detectors would be targetting much smaller systems.

Re:I don't get it.

[icebike]

> We spend billions on observatories, but what's the point?

Calm down. There is a depression going on.

All that money enters the economy employing everyone from astronomer to shoe-shine guys. In the mean time some science gets done.

If your tag line is to be believed, you will forgive us if we wait till you are actually OUT of your mom's basement before we task you with prioritizing our national science budget.

Re:A complementary approach

[TropicalCoder]

Thank you for your interpretation of the meaning behind today's article. It was a revelation for me to consider gravity waves as an analogue of electromagnetic waves. On the The North American Nanohertz Observatory for Gravitational Waves web site [arxiv.org] there is more information. They say "The timing precision of today's best measured pulsars is less than 100 ns. With improved instrumentation and signal-to-noise it is widely believed that the next decade could see a pulsar timing network of 100 pulsars each with better than 100 ns timing precision." I thought it interesting that they only get 100. Then if you did a long term integration of these signals, you may get down to pico-second timing. Such a timing base may be used to correct atomic clocks in GPS satellites and have many other uses. This is all just pure speculation by a non-physicist, so take it with a grain of salt.

Re:So what happens

[Ralph Spoilsport]

I think that is an excellent question. It's the classic divide between Einstein and Bohr. For Einstein, gravity is geometric, for Bohr et al, it is a product of Stuff and Stuff exists as particles, waves, and/or both.

If they don't find gravity waves in this attempt, I would suspect the following to happen:

A: One bunch, the Einsteins of the lot will say "Well, I toldja so..."

B: The Quantum types will simple demand more money for an even bigger test that will look at clusters of Galaxies or some such conglomeration of Stuff.

When that test fails, go back to step A. Rinse. Repeat.

RS

Re:A complementary approach

[electrostatic]

If the pulsars under observation are, say, 100-1000 light years apart, then the time necessary to notice a gravitational wave perturbation would seem to be on the order of 100-1000 years, respectively.

IOW, because gravitational waves travel at light speed (general theory of relativity), then a "stretch and squeeze" at one pulsar would reach the more distant pulsar many years later. The observed delay is of course a function of the distance between the pulsars, the angle of the wave and the angle of them to earth.

OTOH, a gravitational wave train with a wavelength much shorter the the distance between the pulsars could also be observed if a lot of pulsars were involved -- and if the observation period was at least one cycle. The 10^-9 frequency mentioned equates to a 31.7 year period.

Just because a theory cant be tested

[voss]

Doesnt mean its not true...Democritus 2400 years ago proposed the existence of the atom.

He had no way testing this, he simply used logic to deduce it.

Re:So what happens

[dido]

If gravity waves didn't exist, you'd have to find some other explanation for PSR B1913+16 [wikipedia.org], which is a pulsar in orbit around another star. The pulsar and its companion are spiraling in together, losing energy in exact agreement with the phenomenon of gravitational radiation predicted by General Relativity. This binary pulsar system has been hailed as sufficiently convincing indirect evidence for the existence of gravity waves that Russell Alan Hulse and Joseph Hooton Taylor Jr. were awarded the 1993 Nobel Prize in Physics for its discovery.

No, it doesn't seem that the existence of gravitational waves is in any question here. The only thing is that there might be much yet we don't understand about gravity that is stifling our ability to observe them directly. It's obvious that General Relativity is far from being the final word on gravitation.