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.

About Time...

I know that I haven't been posting as much as I usually do (I like to post once a week) but life gets in my way.  For example, I had a tooth break that needed fixed and both my husband and I have come down with the cold that has been making its way around the observatory.  Basically, between life and getting work done, I haven't had a lot of time.

But today is an important day since we will reach GPS time 1,000,000,000.  This time is measured in seconds from Sunday January 6, 1980 at midnight UTC (this is the official time of the planet measured at the Prime Meridian passing through Greenwich, England) without any leap second corrections to match the rotation of the Earth (astronomers use a similar time keeping method called Julian Date which is the number of days since January 1, 4713 BC without any corrections for outright changes to the calendar [like to the Gregorian calendar - which is the calendar we use today]).  Here at LIGO, this is important to us since this is how we measure the official time for everything and this time needs to be very accurate since we will never believe a potential gravitational wave detection unless it is measured at different observatories within the time it would take a it to travel between the sites - for the two LIGO observatories, the maximum time is 10 milliseconds.

Other than it being cool to watch the time roll over to one billion (like watching your car odometer roll over to 100,000 miles) this event can cause issues with the data analysis programs that we write to search for gravitational waves.  For example, I wrote a software package while I was in grad school that we still use to produce simulations to test the efficiency of data analysis software.  My baby is called GravEn (for GRAVitational-wave ENgine) and uses the GPS time to determine where the simulations will be added to the real data (this data with fake signals is never saved together so that we don't trick ourselves into thinking we saw something real).  GravEn has specifications in its programing to return the time of the simulation in whole GPS seconds in one column of the log file and the nanoseconds after that time in another column.  I have made it so that the whole-second time is returned with 9 digits and this is now an issue since the time will be 10 digits.  It is easy enough to fix, but it must be fixed!

This new 1,000,000,000 time is not going to be of any serious concern like people feared the Y2K bug to be.  Instead, all of us code monkeys (as computer programers are lovingly referred to) need to go back and make sure that we allow enough (10) digits in the parts of our programs that use GPS time.

So, GPS 1,000,000,000 will happen today (September 14) at 1:46:25 UTC (or September 13 at 9:46:25 PM in Eastern Daylight Time).



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My next blog post will be on Thursday and will answer the reader question on exactly what kind of gravitational waves Einstein@home seeks and how it looks for them.

Free Books by Einstein on Kindle and a Request by "The Big Bang Theory"

I will get back to answering reader questions with my next post, but I wanted to mention some other things...

Free Books by Einstein on Kindle

I love to read (preferably books with vampires or other creatures that go bump in the night) and my Kindle has become something I am rarely far away from.  I am also frugal so I like to browse pages that list free or reduced cost ebooks (my favorite is eReaderIQ).  Today, I noticed that there are a number of books by Einstein that are free (if you don't have a Kindle, you can still download these books and read them on the Kindle app for your computer/smart phone):

• The Theory of Relativity: and Other Essays
• Essays in Science
• The World As I See It
• Out of My Later Years: The Scientist, Philosopher, and Man Portrayed Through His Own Words
• Sidelights on Relativity
• Essays in Humanism
• Letters on Wave Mechanics: Correspondence with H. A. Lorentz, Max Planck, and Erwin Schrodinger
• Letters to Solovine: 1906-1955 

I've only read all of "Out of My Later Years" but I have also read bits and pieces of most of the other titles.  I've always been impressed by Einstein's thoughts on a wide range of topics like religion and politics, even when I didn't agree with him.  I hope that you check these out to get a unique view of who Einstein was.

A Request by "The Big Bang Theory"

I recently received an email letting me know that the producers of "The Big Bang Theory" requested permission to use the "Gravitational Waves" poster I worked on with the APS.  The request went to the APS (as the publisher), they granted it, and forwarded the legal paperwork through channels to me.  By and large, the release is what you would expect but regarding how, when, and where the producers can use the poster it states:

"in any and all media whether now known or hereafter devised, in perpetuity, throughout the universe by Producer or its assignee."

"... Throughout the universe ..."  I suppose it makes sense to cover all your bases these days.  Evolving technology has made the wording of copyright notices ever more complicated and this kind of generalization seems to take care of that.  That's great, but I am not sure that the Earth laws this release is tailored to will hold up in Martian court :)

So, take a close look at the backgrounds on this season's "Big Bang Theory" and look for the "Gravitational Waves" poster.  It will be quite a thrill to see something that I worked on glimpsed on television and my favorite show to boot!

 

Q: What are gravitons?

duhoc asked the following question in a comment to my post calling for reader questions:

I love cosmology and physics but understand very little about it.  What are gravitons, in simple terms? 

Gravitons can be complicated.  To that end, I want to state that I am no expert on the subject...  But maybe that's just right for a simple answer.  If my answer here doesn't do the job, let me know and I will track down a friend who researches quantum gravity or string theory to do a better job.

Gravity is one of the four fundamental forces, along with electromagnetism, strong force (which holds the nucleus of atoms together) and weak force (which is responsible for radioactive decay).  Every other fundamental force has a particle associated with it that communicates the force.  Electromagnetism has the photon, the strong force has gluons an the weak force has W and Z bosons (I know the least about these).  The only exception here is that gravity does not have a fundamental particle associated with it that has been observed.  However, it is safe to work on a theory that establishes the graviton as its communicating particle.  [Constructing theories by extending observations from similar situations to a new one that is not completely explained has proven effective before - the neutrino was a theoretical particle assumed to exist to account for the missing energy carried away in some forms of radioactivity and it was later proved to be true!]

One of the reasons that it is so difficult to directly detect the graviton is that gravity is the weakest of the fundamental forces (even though it holds the Universe together - that's because there is no negative mass and we don't know why that is either).  The weaker the force, the more effort there is needed to detect it.  For the graviton, it isn't a lack of motivation on scientists' part but a limit on technology... Just the contamination shielding for a detector the size of Jupiter, which would let us observe a gravitation once every 10 years, would be so massive that it would collapse into a back hole.

So, instead of observing the graviton directly, we can observe the effects of gravitons and gain knowledge about the its properties.  One of the most promising ways is by observing gravitational waves (and I know a lot about this!).  You may (or may not) have hear that we expect gravitational waves to travel at the speed of light.  The reason we expect it to is that we expect the graviton to be massless and massless particles cruise through the Universe at the speed of light.  But, if we detect a gravitational wave and it takes significantly longer the the time light would take to travel between the LIGO detectors and her international partners, then that would imply that the graviton has mass.  Think about that for a second...  Gravity exerts forces between masses and if the particle that communicated this force has mass itself, that can really complicate things.  There are theoretical physicists who spend their careers thinking about what if some detail of general relativity that hasn't been thoroughly verified is different than we think; what would be the consequences of that?  They know and they have more complicated theories worked out and the results of a massive graviton has been pondered by them as well.  Again, I am not a theoretical physicist but I do appreciate the complications involved.  However, one property of the Universe that has been shown time and time again is that the simple solution is usually the correct one which is why we say the we expect gravitational waves to travel at the speed of light.

There are some really interesting theories in cosmology that focus on the graviton.  For example, in string theory it is theorized that our Universe lives on a brane inside a higher dimensional bulk (where there are more dimensions than length, width, depth and time).  Here, gravitons are able to travel to other branes (universes).  If this theory is correct, then this could explain the missing mass in the Universe we call dark matter.

I like to include pictures with each of my blog post to help keep things interesting.  But, since the graviton has never been observed I haven't been able to find any strictly scientific images for this post.  However, if you would like a graviton of your very own to cuddle with, the Particle Zoo (which sells stuffed versions of elementary particles) has one you will be attracted to (ha, ha...  get it?):

I'm not endorsing or advertizing this product, I just thought it was interesting.

I hope this answers your question.  If not or if you anyone has more question, feel free to leave a comment below or ask me on Twitter @livingligo.

Q: What Was/Will Be the Detection Range of Initial/Advanced LIGO?

REMINDER: I DO NOT speak for the LIGO Scientific Collaboration or the Virgo Collaboration.  Therefore, these answers are mine alone as are any mistakes.

@vicnice137 asked the following question:

What's the effective range for LIGO? and for adv LIGO?  Will it include Virgo Cluster?  

The answer to this includes calibrations (which are fundamental to any experiment), astronomy, and General Relativity (I promise, no math).

First let's talk about calibration.  Any time you do an experiment, you use an instrument to measure something and before you can make statements with any confidence about your result, you need to know exactly how your instrument responds or works.  For LIGO, that means we need to understand how the light at the output of LIGO (where the interference pattern between the arms and gravitational waves are observed) changes when the mirror moves a known amount.  This is a bit more complicated than it sounds since there are feedback systems in LIGO that serve to keep the mirrors still (mostly to cancel vibrations of the mirror from our environment).  There are measurements that are made at the beginning and the end of each science data run that are used as reference characterizations of these feedback systems and there are measurements of small vibrations with a specific frequency that are purposefully and continually applied to the mirrors to characterize the calibration of LIGO at a given time.  (These calibration vibrations only affect our ability to detect gravitational waves at that specific frequency.)  Together, this information allows us to convert the intensity of the interference pattern LIGO produces into the change in length of LIGO's arms (which is what a passing gravitational wave will induce).

Now that we understand how LIGO responds to passing gravitational waves, we need to establish a reference gravitational wave that we will use to talk about how far out into space we would be able to detect that specific gravitational wave.  This reference source is known in traditional astronomy as a standard candle.  There are four main kinds of gravitational waves:

• binary inspiral mergers (like 2 dense stars, 2 black holes or a dense star/black hole pair that orbit each other rapidly and then merge to become one), 
• continuous gravitational waves (like from deformations on otherwise spherical rotating stars),  
• stochastic gravitational waves (weak gravitational waves from many sources at once; perhaps the relic gravitational waves from the Big Bang), and 
• burst gravitational waves (these are short gravitational waves from previously unknown sources or sources that are not well modeled - like what happens inside a star as it collapses right before the bright burst of light from a supernova).  

Since there is more than one kind of gravitational wave to choose from, we need to establish a priority and LIGO's priority is that:

1. The gravitational wave should be from a source we theoretically know very much about.  That is, a simple system that produces a well characterized gravitational wave.
2. The gravitational wave source should be abundant in the near Universe so that we can reasonably expect to detect it as one of the first direct gravitational wave measurements.

From these priorities the binary inspiral mergers stand out since bursts are inherently unknown, stochastic gravitational waves are inherently weak and the population of sources that produce continuous gravitational wave is not well established.

Now that we have settled on a source, we want to use a pair that is representative of the source but not exceptional in strength (otherwise, we would be overestimating how far out into space we can realistically expect to detect these gravitational waves).  LIGO chose a 1.4 solar mass neutron star paired with another 1.4 solar mass neutron star (FYI: 1.4 solar masses is on the lower limit of the expected mass of a neutron star so LIGO picked a very conservative measure for our detection distance measure).  Since General Relativity tells us how strong a gravitational wave this source will produce with respect to how far away the source is, we can combine this with our calibration (or current sensitivity) of LIGO to establish how far into the Universe we can expect to detect this representative gravitational wave.

This is such an important measure to us, that we constantly measure this value and project it on the wall at each of the control rooms in LIGO (there are 2 LIGOs: the one I work at in Louisiana and the other in Washington state).  By glancing at this and other figures-of-merit, we can quickly assess how the detector is working at that time.  Below is an example figure-of-merit that shows how far away we could detect two 1.4 solar mass neutron stars inspiraling:

The green line represents the detection range for the LIGO here in Livingston, Louisiana (known to us as L1) and the red line is other range of the LIGO detector in Hanford, Washington (known to us as H1).  The horizontal axis shows what time the measurement was taken (going back in time moving from right to left; the far right [Time = 0] was when this plot was produced) and the vertical axis measures the distance into space in Mpc (megaparsecs).  Parsecs are a somewhat odd unit of distance to anyone who doesn't study astronomy but it is equal to about 3.26 light years.

During the last data run with LIGO before the advanced LIGO upgrades began, you can see from the plot above that we were able to detect our neutron star pair out to about 20 Mpc or 65 million light years.  This did indeed encompass the Virgo Cluster of galaxies (which are between 53.5 and 54.1 million light years away).  However, LIGO was not able to maintain this detection distance all the time (you can see on the plot above that starting at about -1.5 hours the L1 range dropped from about 20 Mpc to about 15 Mpc - this was due to increased seismic activity in the region that was beyond our control).  Since the Virgo cluster is located at about 16.5 Mpc, you can see that it was not always within our detection range.

Advanced LIGO will increase our sensitivity, and therefore our detection range, by 10 times.  Once advanced LIGO reaches its design sensitivity, we will be able to detect our standard candle gravitational waves out to about 200-300 Mpc (or 650-978 million light years).  This will enable us to see 1000 times more of the Universe (since volume is proportional to the radius [distance] cubed and 10x10x10 = 1000):

Each dot in this illustration is an entire galaxy, not just a star!

It is also important to consider the limitations of this detection distance since we are limiting ourselves to thinking about only one particular kind of gravitational wave.  For example, I specialize in searching for burst gravitational waves.  These were not chosen as our standard candle because we don't know much about them (we like to call them the gravitational waves that go bump in the night).  But to apply the detection distance measured in our figure-of-merit based on a different class of gravitational waves would be short-sighted since bursts could very well be stronger (gamma ray bursts are some of the most energetic light signals astronomers have observed and, not only do we not know what causes them, many of them are VERY far away).  When we use a catalog of galaxies to target our burst searches (after all, a gravitational wave is much more likely to come from an area on the sky containing a galaxy unless we lucked out and had a nearby gravitational wave from our own galaxy), we include all known galaxies up to about 2.5 times the standard candle detection range (about 50 Mpc, or about 163 million light years).

I hope this answered this questions satisfactorily!  I know this is a bit long, but I wanted to be thorough and explain as much jargon as possible.

If you have questions about this or anything else, feel free to ask in a comment below or send it to me on Twitter @livingligo!

Questions to be answered...

Wow... I can't believe the response that I got to my open call for questions!  I am so happy to see the interest in LIGO (and science in general) and I'm eager to start answering questions.  Here is the list of those that I have so far and who asked them (if you are unfamiliar with Twitter, the screen names listed after the @ sign are people who asked me questions through Twitter; otherwise the screen name is what was used to post a comment to my blog post):

duhoc - I love cosmology and physics but understand very little about it.  What are gravitons, in simple terms?  Answered on 25 August 2011

@AstroGuyz - You know the question on every science bloggers' mind is the Big One; "When will LIGO discover gravity waves?"  Are the prospects for gravitational wave detection good before AdLIGO goes online?  Think we'll nab it before the Higgs? Answered on 4 November 2011

@umbonfo - What about the Citizen Science project Einstein@Home? I'm running it but I don't know which GW data it's analyzing.  Answered on 22 September 2011

@HughScot - What do you hope to discover about gravitational waves that will help mankind in the future?  Answered on 26 January 2012

David Dickinson - Am curious if any "spin-off" discoveries are expected from the discovery of gravity waves... (i.e. exotic objects, new cosmological theories etc) 

@EclipseMaps - What are consequences for theory of gravity/relativity if null results for gravitational waves after extended observations?  Answered on 11 November 2011

@vicnice137 - What's the effective range for LIGO? and for adv LIGO?  Will it include Virgo Cluster?    Answered on 12 August 2011

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ADDED 12 August 2011:

BDR - On the twitter feed it says LIGO/you look for space-time ripples; is that something that occurs naturally? If there was some kind of intentionally produced ripple effect (for time travel, maybe, or maybe just galaxy/universe disruption), would it be possible to tell that it was created by another being as opposed to a natural occurrence? Would it be possible to disrupt the universe with a big or chronic enough space-time ripple? I must know these things; tia.

***

I promise that I will address each of these questions in the coming weeks (and I will be sure to contact the inquirer when I answer their question so that they don't miss it).  As always, feel free to send me a question anytime and I will be happy to try and answer it on this blog!

Come back tomorrow and I will answer one of these questions (I haven't decided which one yet).  I may not answer them in order since I like to make sure I am giving you the clearest most accurate answer I can which may include consulting with colleagues.  

Today's picture - Advanced LIGO input beam tubes arriving on site at Livingston (yesterday):

These tubes replace ones that were a smaller diameter in Initial LIGO.  We now need bigger tubes to accommodate the new upgrades near the input of the laser into the detector and near the output.

This picture is from the new LIGO - Livingston Facebook page.  If you are interested in knowing more about what is happening, like us and you will see updates in your News Feed.

What Do You Want to Know About?

In all of my posts so far, I have talked about my day-to-day life as a LIGO scientist (which is the point of the blog after all).  That means I tell you about the things I think are interesting (otherwise I wouldn't be writing about them).

This time, I would love you hear from you!  What do you want to know about?  I can tell you more about becoming a scientist, answer questions about my research or general questions about LIGO and gravitational waves - whatever.  Let me know what you want know!

Post what your interested in as a comment below or tell me on Twitter @livingligo.

Below is a picture of me in my office not two seconds ago as seen from my laptop's webcam:

Bye!