The Null Result: NOT Finding What You Were Looking For

I've written many times on this blog about why we are looking for gravitational waves and what we hope to learn from them.  I also get to tell real people about this all the time with the work I do at the observatory.  As they get excited, they want to know how many gravitational waves we've detected.  My standard response (as of this date, of course) is, "None, and we didn't expect to either."  And I say this with a smile on my face.  Cue the confused and disappointed expressions...

With Initial LIGO, we were hoping to maybe see a gravitational wave from a pair of dense stars merging together about every ten years or so (based on astronomers' estimates on how many of these system there are out there and what fraction of them will be in this merger stage).  While LIGO has been taking data on and off (we take breaks to improve the detector) since 2002, we only have about 2 years of data when both LIGO detectors (in Louisiana and Washington state) were collecting data at the same time (things like earthquakes, weather, etc. can put one or both offline temporarily).  This gives us about a 20% chance of detecting a gravitational wave with LIGO - not the best odds but better than humans have ever done before!  When the Advanced LIGO upgrades are done and the detectors are running at their design sensitivity, we expect to find one of these "merger" gravitational waves about once a month!

An artist's impression of two stars orbiting each other and progressing (from left to right) to merger with resulting gravitational waves. [Image: NASA]

So, what have we been doing with all this data that has no gravitational waves in it?  Some may think it is worthless but that is far from the truth!  Most people think that when you do an experiment and don't find the thing you are looking for (a null result), the experiment was a failure.  While that can be true if your experiment is inherently flawed, it can also tell you valuable information about the world and Universe around you; now you know how something doesn't work or how rare something is.

Since we know that gravitational waves exist from the observations of the Taylor-Hulse pulsar (1993 Nobel Prize in Physics), we know gravitational waves are real.  So we are left with knowledge on how rare gravitational waves are.  We estimate this by carefully studying how sensitive our detectors are and how well our data analysis methods perform.  Once you know these things, you can combine that with how long you looked for gravitational waves (how much data was taken) and estimate the highest rates that the different kinds of detectable gravitational waves pass by Earth.


THE UPPER-LIMIT FOR BURST GRAVITATIONAL WAVES

I am going to use the example of burst gravitational waves which are short duration gravitational waves from unanticipated sources or from known sources where we can't be sure what the gravitational waves will look like, e.g. supernovae, etc.  (The science summary for this work is here.)  I chose this example because this is the kind of gravitational waves I specialize in.

In order to determine the sensitivity of our data analysis method, we need to put fake signals into real data and measure how many of each kind and strength are found by our data analysis method.  This gives us our efficiency.  When combined with the amount of data collected (giving us a measure of how long we were sensitive to gravitational waves) we can estimate the maximum rate that we would expect a kind of gravitational wave to pass by Earth.  We used many different signals to test the data analysis method, but my favorite is called a sine-Gaussian and looks like this:

Sine-Gaussian signal: a sine-wave (trigonometry) multiplied by a bell-curve (Gaussian function).  The horizontal axis measures time in seconds and the vertical axis measures the strength of the signal.  The spacing between the waves provides a measure of frequency.

Using different strengths and frequencies of this signal, we were able to set an upper-limit on the rate of these kinds of gravitational-wave bursts:

Upper limits on the rate of gravitational wave bursts, determined using the LIGO and Virgo data. The different curves represent signals with different characteristic frequencies; the vertical position of each curve shows what rate should have given us at least one detectable burst with 90% probability, for different assumptions about the strength of the burst (horizontal axis). Since no signal was detected, higher rates are ruled out with good confidence.  [Source: S6 Burst Science Summary]

The current results for the burst upper-limits (from the plot above) are that we expect up to about 1.3 sine-Gaussian-like signals a year in LIGO's most sensitive frequency range (64-1600 Hz) with 90% confidence.  


RESTRICTING PARAMETERS ON OBSERVATIONS

Another thing you can do with data that doesn't contain gravitational waves is restrict the possibilities on how observed events happened.  In 2007, there was a strong gamma-ray burst (GRB) observed by the InterPlanetary Network (IPN).  The area that the source could have originated from also overlapped with an arm of the Andromeda Galaxy:

Bottom left corner box: possible area where GRB 070201 could have originated.  Remaining part of image: close-up of the part of the candidate source area that overlaps the Andromeda Galaxy.  An dwarf spheroidal galaxy, called M110, is also labeled.

This was very exciting to all of us at LIGO because one of the possible causes of gamma-ray bursts is the merger of two neutron stars (like in the first image in this post).  With the Andromeda Galaxy being only about 2.5 million light-years away (LIGO was sensitive to these merger gravitational waves all the way out to several hundred million light-years away), if it really was a merger that caused this GRB then we should have seen it!

After careful analysis of our data before, during, and after this GRB, we did not find any evidence of a gravitational wave.  While it would have been very exciting to see something, this null result tells us that:

1. if the GRB was located in the Andromeda Galaxy, it was not caused by a neutron star pair merger, or
2. if the GRB was created by a neutron star pair merger, it was located far behind the Andromeda Galaxy. 

A LOT FROM SEEMINGLY NOTHING...

As of today (14 June 2012), LIGO has submitted 64 papers for publication.  You can also read the public science summaries for the latest published papers here.  This a quite a lot of papers based on data without any gravitational waves!

Crawfish Boils and On the Road Again...

Last week's post came to you from the LIGO-Virgo Meeting in Boston, MA.  This week, it is coming to you from Atlanta, GA.  I just arrived here for the APS April Meeting.  My room is a beautiful corner room with two great views!  So I've chosen the better view from one of my windows:

Actually, I am here a day early so that I can attend the Professional Skills Development Workshop which is designed to improve the communication and negotiation skills of women physicists.  I'm looking forward to this as, while I would love to improve my communication skills, I really feel that I need to develop negotiation skills.  I am currently working to transition my current postdoc position (which is temporary by definition) into a more permanent position, not only because I love my work and the opportunities I have here, but my husband also works at the LIGO Livingston Observatory as an engineer.  The fact that I am this early in my career and living under the same roof with my husband, who is also happily employed in his field, is almost unheard of.  This is known as the two-body problem - when two academic professionals are challenged to find a way to find jobs together; I plan to write a blog post on this later.  That being said, I almost want to jump and any offer that can be scrapped together for me - the last thing I want to do is ruin the good thing I have going.  This is exactly one of the reasons that women tend to make less than men, even in physics - we undersell ourselves.  While I have no intention of trying to wring every penny I can out of a new position, I want to make sure that I am at least being compensated properly for my work.

As for the APS April Meeting, I will be giving a talk on the latest burst gravitational wave all-sky search results.  The information for my talk is here and the "plain English" science summary of the paper is here.  Once I give my talk, the presentation will be publicly available on the LIGO Document Control Center (DCC).  I will also be attending the APS Forum on Education Executive Committee Meeting and this will be the last of my term.  I have more than enjoyed the others I was privileged to serve with and it has been wonderful to get a chance to spread my wings a little more in physics education.  Of course, I have days and days of interesting talks and other activities to look forward to.  I will make sure to Tweet points of interest so make sure to follow me @livingligo.  You can also follow others' Tweets from the meeting using the hashtag #APSapril.

Between my trips to Boston last week and this one to Atlanta, I did get to be home in Baton Rouge for a few days.  Yesterday, the observatory staff was updated on the large scale status of the Advanced LIGO upgrade by the program leader, David Shoemaker.  While that was very informative (all is going well), the best part of the day of the crawfish boil we had outside afterwards.  For those of you who don't live in the American South (specifically the deep south), crawfish/crawdads/mudbugs/crayfish (but don't call them the latter around the natives lest you truly out yourself as not one of them) are essentially small freshwater lobsters that yield about the same amount of meat in their tail as a shrimp.  The meal takes "family style" to a new level: everything is served in heaps and you get a tray instead of a plate and it is heaped with the crawfish, sausage, corn-on-the-cob, and potatoes.  Oh yeah, and you don't get utensils.  This is a get-your-hands-dirty kind of meal.  Here is what my lunch looked like (before I started tearing the little critters apart - the communal aftermath from everyone at the table wasn't nearly as pretty):

This is considered a "dainty" portion.  A few other tips on how to fit in as a local:

• Don't sit while you eat crawfish - you stand so that the juice that can sometimes explode out of the body when you separate the tail from the head doesn't get all over you.
• Suck the heads!  Once your remove the tail, don't through away the top part of the body - that's where all the best flavor is.  As I have never done this myself, I am not sure if they are referring to the seasoned boil that remains inside or if they actually suck the "stuff" out.  To me, it just looks like the poor thing is trying to escape!
• Again, don't call them crayfish.

And a point of common sense - take your watch off!  Mine still smells like crawfish!

Q: What does LIGO look like?

Today's question comes to you from the site statistics search terms used to find this blog:

What does LIGO look like?

FROM THE OUTSIDE:

LIGO is BIG!  Instead of being looking like a traditional observatory with a dome and telescope, LIGO looks for gravitational waves (small changes in the gravity of the Universe as they pass by Earth like a ripple on a pond) by comparing the lengths of two arms, each 4 km (or almost 2.5 miles) long.  A gravitational wave will compress space in one direction (say, East-West), expand space in the other (North-South), and alternate back and forth between compression and expansion in these directions.  Below is an animation (from Wikipedia) on how a ring of matter can be compressed and expanded in orthogonal (at 90^o) directions:

To best observe this stretching, LIGO is an "L" shaped detector (although many of the students that visit tell me it is shaped more like a "corner" since each side is the same length).  Because the facility is so large, it is hard to get it all in a single image.  Below is an aerial image of the LIGO Hanford Observatory in Washington state (which is in the eastern desert part of the state):

... and the aerial view of the LIGO Livingston Observatory in Louisiana (where I work):

Google has wonderful satellite images of the sites (click here to view the Livingston site).  A noticeable feature of the Livingston site is that there are large rectangular bodies of water along the arms:

During construction, the arms needed to have an elevation above the 500 year flood line (meaning that the probability of there being a flood reaching that elevation is once every 500 years).  To do that, soil needed to be excavated from the sides to build up where the arms would be built.  In Louisiana, the water table is so near the surface that whenever you dig a hole, even a shallow one, it will most likely fill up with water.  (The locals here aren't impressed about that, but being a Northerner who loved to yell down her family's water well as a child to hear my echo, the thought of gardening and hitting water is unique!)  Another reason that the arms needed to be built up above the surface is to correct for the Earth's curvature.  Assuming the Earth is a perfect sphere (which it isn't), the Earth curves down away from the corner of LIGO a little over 4 feet over its 2.5 mile long arms.  Since the beam tubes were we shine the laser in are only about 4 feet in diameter, if we didn't ensure that the ground under the arms was perfectly flat (not flat with the curvature of the Earth), the light would never reach the ends!

ON THE INSIDE - The Control Room

So, that's what LIGO looks like from the outside.  Now, let's look at what it looks like from the inside.  The heart of the observatory is its control room where almost anything inside of the detector can be changed with the press of a button (and I've learned the hard way that you can also "break" LIGO with a press of a button as well - oops!).  Below is one of my favorite pictures (not just because I am in it) since you can see almost all of the control room in this image:

I liked this image so much that I purchased a print and it is proudly framed in my office. [Image by Roger Zettler of The Advocate newspaper (Baton Rouge, LA); published on 25 February 2010.]

Just behind me (I'm the woman standing in the front of the image) is the on-duty scientist (left - Peter Saulson of Syracuse University) and the on-duty operator (right - Danny Sellers).  The operator maintains the instrument in working order and responds promptly to any malfunctions while the scientist ensures the quality of the data.  Together they make sure that there is as much good quality data as possible.  You can also see that there are projections of graphs and videos on the back wall (top of image) and the side walls.  These let those working in the control room know at a glance how the instrument is behaving.  Currently, this control room is being renovated in preparation for the completion of Advanced LIGO.

FROM THE INSIDE: The Interferometer

Finally, there is the detector itself (also known as an interferometer - the "I" in LIGO).  Below is an image of the corner of LIGO where laser light is split between the 2 arms:

The following is a large video (~33 MB - so it may take a little while to load) that shows views of the interferometer from the inside and the outside.  I made this in July 2008 when the upgrades for Enhanced LIGO were finishing up.  There is no audio to accompany this video, as I use it when I give talks about LIGO and narrate it live.  An index is located below the movie describing what you are seeing and when.

Video Index:

0:00-0:12 - This is the input portion of LIGO.  In the far back of the image, in the white room, contains our main laser source.  The laser is infrared (wavelength = 1064 nm) so it can't be seen by the eye but it is powerful enough to permanently blind you.  In front of this white room is a pink draped room.  This is a portable clean room and inside the air is filtered so that less than 100 6-micron particles are floating around every cubic foot.  Basically, you will never see any dust motes in there.  This is needed to help ensure that the vacuum inside of LIGO (which is the largest sustained ultra-high vacuum in the world - to the best of my knowledge) isn't contaminated.  (When the camera moves to the next view, you will see the top of another portable clean room and the HEPA filters that push air down into it.)

0:20-0:27 - This is the corner of LIGO.  In the center chamber is an instrument called the beam splitter that does just what it says: it lets half of the light pass through it and go into one arm while the other half of the light is reflected off of it into the other arm.

0:30-0:34 - This is the X-arm (the arms are named after the X and Y axis of a standard [Cartesian] graph) of the interferometer where the light that is transmitted through the beam splitter goes.  This metal tube continues through the wall and outside.

0:34-0:59 - This is the X-arm as seen from the roof.  The video zooms out to the end of the arm where there is a building that houses the tip of LIGO and the mirror that reflects the light back.  The concrete barrier that protects the metal beam tube (as seen in the previous view) is also seen.  This concrete barrier protects the tube from the environment (like lightning strikes).

1:03-1:11 - The Y-arm where the light reflected off of the beam splitter travels.

1:11-1:35 - The view of the Y-arm from the roof.

1:40 - The light from both of the arms combined again at the beam splitter (seen here).

1:40-2:03 - The combined light from both of the arms goes to the output where the interference pattern (the brightness of the light) is measured to determine if the lengths of the arms changed compared to each other.

King Cakes, Elevators, and More Questions!

On King Cake...

One of the great things about living in Louisiana between between the Epiphany and Mardi Gras is a wonderful sweet called King Cake.  Before I moved here, I'd never heard of it (although I did see something about it on the Food Network shortly after I moved here).  If you've never heard of it, let me tell you a little about it.

King Cake (from Wikipedia)

First, King Cake isn't a cake at all.  It is more like a very large cinnamon roll.  Instead of cutting individual rolls from a log of dough swirled with cinnamon, the log is turned back onto itself to make a circle (anywhere from about a foot in diameter to as large as the baker can handle).  Once the "cake" is baked, it is covered with icing (anything from a doughnut glaze type of icing to a cream cheese icing) and then covered in colored sugar, usually in the Mardi Gras colors of purple, yellow, and green.  Inside (or underneath) the cake somewhere is a trinket, usually a plastic baby.  Whoever gets the baby gets some sort of priviledge (in Mardi Gras tradition, the person who got the baby literally was the king for whatever their Mardi Gras celebration was).

Here at LIGO, King Cakes can usually be found in our kitchens (there is a full kitchen in the main observatory building and a kitchenette in the building across the street that houses the Science Education Center and other things - like my office).  If you get the baby, you get to buy the next King Cake.  But I've been noticing that there has been a shortage of King Cake in the kitchenette in the building were my office is!  Where has all the King Cake gone?

AMBER'S DIABOLICAL PLAN FOR MORE KING CAKE...

I have come up with a plan to increase the King Cake near my office.  Over the years, I have collected my own little orphanage of plastic babies.  This one has been living in my desk for a long while now (I almost decapitated this little fella when I cut into his cake):

 

I plan on seeding the kitchenette with a King Cake and stuffing it full of babies!  Mwa-ha-ha!!!  Today, 1 King Cake...  Tomorrow, MANY King Cakes!!!


Well, since I am writing about it in by blog I'm not really going to do this, but it was a thought. ;)

On Elevators...

One of the questions I get from students visiting LIGO is, "Wow!  You must be a genius!"  Not hardly.  I wasn't always a good student and even once I was, I always had a struggle to be the good student I wanted to be.  My elevator story below proves my "not a genius" claim.

I got on the elevator this morning (they are painting the exterior stairs so they are temporarily off limits).  I have been in this elevator many times before, but today I stop and look at the control panel:

 

This elevator only goes between 2 floors and yet there is a button for each floor.  Wouldn't it make sense just to have a single button that took you to the "other" floor?  Really, if I am on the first floor and just entered the elevator, where else would I want to go.  While I am having these deep thoughts, the door closes and after about a minute or so I am standing there wondering why the elevator isn't moving.  It turns out that pushing any button is a good start but that was a mental leap I simply didn't make.  (Of course, I did eventually push 2 and it delivered me there directly.)  At least I was alone in a elevator so no one knows the depths my brilliance can plunge (except for you and I am sure you won't tell anyone, right?). 

More Questions Please! 

So, I am starting to run out of questions from you, my wonderful readers!  Please send me more!!!  (In the comments below or on Twitter to @livingligo.)  However, I have figured out a "sneaky" way to start digging up those questions you are wondering about but never asked.  You tend to Google them and you may end up on my site.  When this happens, the terms you Googled show up in my statistics for how people found my blog.  Here are some of the questions I have found this way:

• Is there anything like LIGO outside of the USA?  Answered on 17 February 2012
• Has LIGO seen anything yet?
• What does LIGO look like?  Answered on 15 March 2012

Starting next week, I will begin answering these questions (and the few that I have left over from the last time I made a call for questions).  Please ask me more!  If you are a teacher and you have students who may have questions, please have them ask (or you can for them).

I look forward to hearing from you!


UPDATE:

@Astroguyz asked:

• Where do you see gravity wave/astronomy & physics a decade from now? 
• Are there really Vegas odds on GW detection?  Answered on 23 February 2012