Posts Tagged ‘gravity’

The weekend is finally here! My friends have moved on to Paris, and today I am also packing. I will be spending the next week in Wales, attending the 2011 Edoardo Amaldi Conference on Gravitational Waves (sounds impressive, no?) I don’t know that I can describe how excited I am to have the opportunity to attend this conference. The Amaldi meeting happens every two years with the aim to cover all aspects of the science of gravitational waves. This year’s program has a large number of topics scheduled including: source modeling, ground based detectors, space based detectors, pulsar timing, multi messenger astronomy and current GW search results.

But before I get too far ahead of myself let’s get a little background information. I bet many of you are sitting there thinking what in the world is a gravitational wave? Good question.

We’ve all heard the anecdote of Isaac Newton’s inspirational apple (also an inspiration for my blog title) he saw an apple fall which made him start thinking about gravity leading him to write his Law of  Universal Gravitation, which was the first to suggest that gravity could reach so far as to be responsible for the moon’s orbit around the earth and the earth’s orbit around the sun. He also was the first to describe gravity as the “drawing power” in matter, famously stating, “the apple draws the Earth, as well as the Earth draws the apple.” Newton, however, was not the first, nor the last to ponder gravity. It’s still a topic we are actively researching today.

In 1916 Albert Einstein published his Theory of General Relativity which draws from his previous Theory of Special Relativity and the Law of Universal Gravitation to provide a unified theory of gravity as a geometric property of spacetime. There is only one prediction in this theory that has yet to be experimentally observed: the gravitational wave.

Gravitational waves are pretty much exactly what they sound like, gravity propagating as a wave through spacetime at the speed of light. You can imagine the result looking similar to ripples on the surface of a pond.

Gravitational waves are created by accelerating masses. We most commonly look at orbiting sources, such as a black hole binary or inspiraling neutron stars.

(left: an artist’s interpretation of gravity waves produced by neutron stars in their inspiral and collision. source: NASA)

We have yet to directly detect one of these GWs, however in 1993 the Nobel Prize in Physics was awarded for indirect proof of the existence of GWs through the measurements of the Hulse-Taylor binary system.

So if we know they exist why hasn’t anyone detected a gravitational wave? Well, we’re trying. There are numerous organizations currently attempting to detect GWs. But actually detecting a gravitational wave as it distorts matter requires a sensitivity on the hairy edge of science and technology.

There are three main types of detectors in development and use. Ground based detectors, space based detectors, and pulsar timing arrays.


Ground Based Detectors:

The most prevalent type of detector are ground based interferometers. A traditional interferometer is a device where we shoot a laser at a half transparent mirror, which splits the laser into two beams, reflecting half the light at a 90* angle and allowing the other half to continue on a straight path. After traveling some distance those beams are then reflected off mirrors and recombined at the half transparent mirror where they travel as a single beam to an observing screen.

Traditional Michelson Interferometer

If the separated beams of light have traveled the same distance they recombine and form constructive interference and a bright spot will appear on the screen, however if they have traveled different lengths while separated they recombine and form destructive interference and a dark or dim spot will appear on the screen.

What physicists have done is take this idea of an interferometer, made the path lengths kilometers in distance and hung the mirrors so they are “free particles.” That way if a gravitational wave were to propagate over the massive interferometer we would see a specific interference pattern as the mirrors are distorted.

In the US we have LIGO (Laser Interferometer Gravitational Wave Observatory) which has built two such interferometers, one in Livingston, Louisiana and another in Hanford, Washington, each with 4 km arms!

LIGO, Livingston

There are also multiple interferometers world wide: TAMA in Japan with 300 m arms, GEO in Germany with 600 m arms, and last but not least, with 3 km arms (the closest in arm length to LIGO) VIRGO in Italy. LIGO and VIRGO are currently in the process of revamping their interferometers and creating advanced LIGO and VIRGO to gain a greater sensitivity. This effort is to be completed by 2015.

Space Based Detectors:

There is currently an effort to send an interferometer into space. LISA the “Laser Interferometer Space Antenna” is an international corroboration planning on  building a space based interferometer with arm lenghts of 5 million km!


Unfortunately the U.S. stopped funding LISA in order to complete the James Webb Space Telescope, a very large and important project. Unfortunately it now appears as though congress is going to kill JWST as well. >:-(

Pulsar Timing:

(this is what I do!)

There is a group of physicists who are currently utilizing an array of millisecond pulsars to try to detect gravitational waves. Pulsars are spinning neutron stars that emit a beam of electromagnetic radiation. Their periods can be so well defined that the signal from a pulsar can be more accurate than an atomic clock. The idea is that if we observe these very accurate pulsars we should be able to observe a gravitational wave disturbance first as it distorts the earth and then the pulsars as a disruption in the signal responses. The pulsar people are also currently trying to improve their current sensitivity.

As I said I work with the pulsar people, though not directly in the actually detection process. I realize I’ve now written about 1,000 words about science in what’s been predominantly a food blog but really this is a huge topic and I’ve tried to stay as nontechnical and brief as I could.

Physics is fun!

~Any Questions?


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