Thursday, September 22, 2011

Q: How Does Einstein@Home Search for Gravitational Waves?

@umbonfo asked the following question:
What about the Citizen Science project Einstein@Home? I'm running it but I don't know which GW data it's analyzing.
This is a great question and there is so much I want to tell you all about that I am going to break this down into smaller questions:

What is Einstein@Home?

If you aren't familiar with Einstein@Home (read more, sign up), it is a screensaver that looks for gravitational waves in data collected by LIGO and other detectors like it.  Basically, users allow Einstein@Home to become part of a large supercomputer seeking gravitational waves but ONLY when the users are not using their computers.  How many times have you gone to bed at night and left your computer on?  If it doesn't have anything to do, it just sits there.  Einstein@Home gives it something productive to do that comes at no additional cost to the user.  Below is a screenshot of Einstein@Home as it is running on my computer.  It shows you where the two LIGO detectors are (the green and blue 'L' on the starsphere), where the GEO detector in Germany is (the red 'L') and where on the sky this program is looking for gravitaional waves right now (the orange cross-hairs).  To learn more about what you see on the screensaver, click here.

This is a screen shot of the Einstein@Home screensaver from my laptop just moments ago.
How does Einstein@Home get data?

Once you install the Einstein@Home screensaver, the central data servers (at the University of Wisconsin at Milwaukee) send a small portion of data to your computer for it to analyze.  Once your computer is done looking at that data, it sends a message back to the central computers telling them if there was a candidate gravitational wave in it.  Regardless of the result, 1 to 2 other computers also process the same data to make sure that they all get the same results (and we are sure that there isn't someone out tampering with the software to report false results).  If the same results are found and there is a candidate gravitational wave in the data, then it is looked at more closely by physicists who specialize in data analysis (like me - although I look for different kinds of gravitational waves than Einstein@Home looks for).

What kind of gravitational waves does Einstein@Home look for?

Einstein@Home looks for a very specific kind of gravitational wave call a continuous gravitational wave.  The are expected to be emitted by rapidly spinning, dense objects like neutron stars.  If there is even a small imperfection in the spherical shape of these stars, they will be constantly emitting a gravitational wave (if you were to put the signal of the gravitational wave through speakers, it would sound like a single tone).  We look for this kind of gravitational wave by breaking down the data collected from the detector into its different wave components.  Think of the data as the sum of a collection of many different waves each with a constant frequency.  We can then take a chunk of data and break it up into its component waves.  (This is called a Fourier transform.)  Since we know what a continuous wave should look like, Einstein@Home then inspects each of the component waves to see if it could be a gravitational wave.

Does Einstein@Home do anything else?

Why, that's insightful of you to ask!  :)  Einstein@Home also processes data from the Arecibo radio telescope looking for pulsars - a special kind of neutron star that emits radio waves from their magnetic poles.  Every time that the star spins its jet of radio waves across the Earth, radio telescopes can detect it.  The data analysis in not quite the same as when Einstein@Home looks for gravitational waves, but the basic process of breaking down the data into its component waves is the same.

Why is Enstein@Home interested in discovering pulsars?

Knowing more about where pulsars are in our universe lets us know better where to look for them in our gravitational wave data (notice that the screensaver has crosshairs that show you specifically were Einstein@Home is looking for gravitational waves) and, since pulsars are a special kind of neutron star, we can get a better sense of how many of them are out there in the Universe which give us more accurate measures of how often we should expect to detect gravitational waves from them.

What has Einstein@Home found?

Well, since there has been no direct detection of gravitational waves yet, it is obvious that Einstein@Home has not produced a real gravitational wave yet.  However, it has found over 10 previously unknown pulsars including the fastest known spinning pulsar!

All of this would not be possible without users like you!  When all of the computing power of Einstein@Home is combined, it is within the top 20 or so supercomputers in the world!   

I hope I answered at least most of your questions about Einstein@Home.  As always, feel free to ask me questions by leaving a comment on this blog or tweet me @livingligo.