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This blog is all about science and technology (with occasional math thrown in for fun). The goal of this blog is to try and pass on the sense of excitement and wonder I feel when I read about these topics. I hope you enjoy the posts.

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Indirect Evidence for Gravitational Waves

Posted December 29, 2015 9:13 AM by Bayes

General Relativity Prediction

One of the more interesting predictions of General Relativity is the existence of gravitational waves. Gravitational waves are basically ripples in the curvature of spacetime that propagate as waves. They are essentially a form of gravitational radiation and there are several ongoing and planned experiments that are trying to measure them directly. So far without success.

There are, however, indirect means of detecting gravitational waves; by studying their dissipative effect on certain systems like binary pulsars. I recently came across an interesting article in which scientists had observed the dissipative effects due to gravitational waves predicted by general relativity. See below:

Pulsar pair ripples spacetime

A dancing duo of cosmic beacons has provided scientists with the most precise measurement, albeit an indirect one, of ripples in spacetime called gravitational waves. The measurement comes from analyzing the only known pair of gravitationally bound pulsars, dense cores of dead stars that emit intense beams of radio waves with the regularity of a nearly perfect clock. Michael Kramer, an astrophysicist at the Max Planck Institute for Radio Astronomy in Bonn, Germany, and colleagues precisely tracked the deterioration of the pulsars' orbits, presumably due to loss of energy in the form of gravitational waves. The rate of orbital wane matches perfectly with the predictions of general relativity, Kramer reported December 16 at the Texas Symposium on Relativistic Astrophysics.

The double pulsar system J0737-3039A, discovered in 2003, is an astrophysicist's dream. By analyzing the radio beams, researchers can probe the wild things that happen when the small but massive celestial objects circle each other at roughly a million kilometers an hour. Under the rules of general relativity, the pulsars should plow through spacetime and generate ripples that carry away energy, leading the pulsars to gradually fall toward each other. Using observations from several telescopes over more than a decade, Kramer and his team determined that the pulsars are approaching each other by 7.152 millimeters a day, give or take a micrometer. That's exactly what theory predicts based on the mass and acceleration of the pulsars.

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Re: Indirect Evidence for Gravitational Waves

12/29/2015 2:06 PM

It's possible an event has already been detected by LIGO, or maybe it was a training exercise:

http://www.nature.com/news/has-giant-ligo-experiment-seen-gravitational-waves-1.18449

I'm thinking that the power of gravitational waves, being quadrupole radiation, drops of as the fourth power of distance, which means they are extremely weak at a large distance from the source.

Here is a good description:

https://en.wikipedia.org/wiki/Gravitational_wave#Difficulties_in_detection

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Re: Indirect Evidence for Gravitational Waves

01/01/2016 10:00 AM

"I'm thinking that the power of gravitational waves, being quadrupole radiation, drops of as the fourth power of distance, which means they are extremely weak at a large distance from the source."

AFAIK, g.w. energy changes the same as for em waves, with 1/d2. The problem is the very low frequency of typical events (milli-Hz order), making them extremely low power to start with. Fortunately, LIGO detects amplitude, not power, so it is a 1/d situation, but at very long wavelengths. Hence the very long arms needed. Unless something big happens close-by soon, we may have to wait for the LISA space detector for direct detection.

-J

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Re: Indirect Evidence for Gravitational Waves

01/01/2016 5:07 PM

Thanks, Jorrie. I thought I read somewhere 1/r^4, but thinking about it, I'm sure you are right. Any kind of radiation has to fall off as the inverse square.

The proof is simple...the surface area of a sphere is proportional to the square of the radius, so if energy is conserved, the energy per square meter passing through any sphere centered on a source has to fall off as the square of the distance.

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