Q: What's the Difference Between a "Gravitational Wave" and a "Gravity Wave"?

The things that LIGO looks for are called gravitational waves (which are discussed in depth here on my blog and on the LIGO website).  That can be a mouthful, especially when having a conversation about them.  People, including us professionals, realize this and often take the shortcut of calling them "gravity waves".  It sounds so similar that this must mean the same thing, right?  Well, no!


GRAVITY WAVES ARE NOT GRAVITATIONAL WAVES

The proper technical use of gravity wave refers to waves on the interface of two fluids, which can be liquid and/or gas.  Where this boundary is disturbed, gravity will pull it down and buoyancy will push it up.  This combination of opposite push and pull creates a wave that moves out over the surface.  You can make your own interface of two fluids by filling a glass with some water and oil:

A glass containing oil and water.  Oil settles at the top because it is less dense (more buoyant).  [Source: Wikipedia]

Water and oil will separate if left alone.  This separation creates a boundary between the oil and water with the oil on top since it is less dense.  Now imagine gently tapping on the side of the glass.  The vibration from your tap will transfer into the separated oil and water which will produce a gravity wave on their boundary.  If you actually do this carefully enough, you can produce a gravity wave ONLY on the oil/water boundary and not on the surface of the oil (though a surface wave on the oil is technically a gravity wave too since that is a liquid/gas fluid boundary).

While the oil and water example technically illustrates a gravity wave, the term is usually applied to gravity waves that occur in nature.  Examples include:

• The waves on water caused by wind from large ocean waves to ripples in a puddle; these are examples of gravity waves on an gas/liquid boundary.  
• Waves of different density waters under the oceans' surface (like warm/cool water, or fresh/salt water); these are examples of a liquid/liquid boundary.
• The rippling of clouds, like in the movie below; this is an example of a gravity wave on a gas/gas boundary.

CAN A GRAVITATIONAL WAVE DETECTOR DETECT GRAVITY WAVES TOO?

We've now established that a gravity wave is very different from the gravitational waves that LIGO is looking for.  But can LIGO detect them anyway?  Indirectly, yes!  Almost three-quarters of the Earth is covered by oceans.  These oceans are roiling with gravity waves both within the water and on top of it.  When these waves encounter solid earth, much of the wave is reflected but some of the energy is absorbed.  This absorbed energy can then create surface waves on the remaining part of the Earth's surface that is solid.  These ground vibrations are called microseism.

Since LIGO lives on the Earth's surface (many people think that LIGO is underground but it really is built above ground), these vibrations shake the detector and contribute to the measured detector noise.  So much so that, compared to the gravitational waves we seek, we don't expect to be able to detect low frequency (less than about 10 Hz or so) gravitational waves.  And it doesn't matter that both LIGO detectors are near shores since the microseism shakes the entire Earth - we could have built LIGO in the middle of Nebraska and the microseism would still negatively affect us. 

In order to detect low frequency gravitational waves, we need to get away from the microseism.  The proposed gravitational wave detector that can do this is the space-based eLISA satellites.  (I've also discussed eLISA and associated drama on this blog previously.)  eLISA would be exclusively sensitive to low frequency gravitational wave and would compliment LIGO well: there are many young systems producing low frequency gravitational waves all the time while there are few producing the high frequency death throes that LIGO can detect.  Together, LIGO and eLISA will provide a more complete gravitational-wave picture of the life cycle of some of the most energetic, violent objects in the Universe.


CONCLUSION

"Gravitational waves" and "gravity waves" are very different entities.  However, you may hear us refer to a gravitational wave as a "gravity wave".  This is a personal pet peeve of mine (can't you tell?).  While I work hard to use the term "gravitational wave" correctly, I am often hesitant to say anything to colleagues I hear using "gravity wave" instead.  Watch the NSF documentary Einstein's Messengers (also on the "Viewing Fun" page on this blog) and you will see some highly respected LIGO scientists refer to "gravity waves"; it makes me cringe a little every time but I'm not one to gainsay my betters.  Now that you've read this, you'll know what we really mean ;)


Read more:

• Another explanation of the difference between gravitational waves and gravity waves (this is correct but a little outdated on its discussion of gravitational waves)

SNEWS and LIGO: Neutrinos Tell of Possible Gravitational Wave

When I start off a tour at the LIGO Observatory, I usually start by talking about how gravitational waves will open a new window to view the Universe.  I've done this so many times that I have the talking point pretty much memorized:

"Up until recently, we've only been able to observe the Universe using light and its different forms.  Visible light, X-rays, and microwaves are just a few different kinds of light and every time we have looked at the Universe in a new way, we discovered something unexpected that revolutionized our understanding of the Universe.

Well, light has the inconvenient property of being fairly easily absorbed or reflected away from its path.  However, the Universe is transparent to gravitational waves; meaning that they can go through matter and come out the other side unchanged.  There is no such thing as a gravitational-wave shadow!"

Note that I start by saying that until recently all of astronomy has used light as its tool.  This is because there is another medium that has been used: neutrinos.  I've talked a bit about neutrinos previously here, namely when I discussed the debunking of the "faster-than-light neutrino" claim last year and how neutrinos are used in multi-messenger astronomy.  Quoting the important part from the multi-messenger astronomy post:

"Today, we can do astronomy with means other than light.  For example, neutrinos.  These are subatomic particles that have no electric charge, have nearly no mass, travel very near the speed of light and are able to pass through matter almost undisturbed.  However, these properties add up to make it very hard to detect neutrinos (did you know that there are billions of neutrinos from the Sun passing through your body every second?!).  Neutrinos are also emitted when a star dies in an explosion called a supernova.  That means we may observe the optical burst of light AND the neutrinos from a single supernova.  Any time that you can observe the same event in multiple ways, you almost always learn more than if you only observed it one way."

- 14 October 2010

What I didn't go into is that LIGO is working to detect gravitational waves from a supernova as well.  While we can do this without complementary detections from traditional and neutrino observatories, having that information from them will make it easier for us to find the signal buried under the detector noise that dominates what LIGO records.  This is done through the Supernova Early Warning System (SNEWS).

SNEWS

Yes, this is pronounced just like you pronounce "snooze".  A SNEWS alert is sent out shortly after neutrinos from a supernova (as opposed to neutrinos from the Sun) are detected.  Since neutrinos and travel through matter with very little disturbance just like gravitational waves, that means that if we saw neutrinos, there is also a good chance that we may see the gravitational waves from that event.  Even more compelling for us in the gravitational wave community is that SNEWS only really expects to see neutrinos from a supernova if it came from within the Milky Way galaxy (or the Magellanic Clouds that are two small galaxies that orbit the Milky Way).  As far as gravitational waves are concerned, that is in our own backyard so any accompanying gravitational waves would likely be large enough for us to detect!   (When searching for gravitational waves in general, we expect that almost all of our sources will come from galaxies outside of the Milky Way.)


EARLY WARNING SYSTEM?

The reason that the detection of neutrinos is considered an "early warning system" for a supernova is that the processes that produce these neutrinos happen hours to days before the optical explosion that traditional observatories would be able to see.  The supernova explosion occurs after the mass of the star collapses in on itself; this is called a core collapse.  Neutrinos are normally produced by the nuclear fusions inside the star (our Sun produces MANY all the time), but during the core collapse many more are produced (it is estimated that over 90% of the energy in the collapse is expended as neutrinos).  It is also during the core collapse, when so much of the star's mass is in motion, that gravitational waves are produced.  If there is a SNEWS alert, that means that there is a higher probability of a gravitational wave detection at that time.


WHAT HAPPENS AT LIGO DURING A SNEWS ALERT?

First off, let me say that there has not been a SNEWS alert yet since these supernovae in our galaxy are rare (they happen about every 50 years or so that we are aware of).  But if a SNEWS alert comes through while LIGO is looking for gravitational waves, the protocol is quite simple: don't do anything that would cause the quality of the data to be degraded.  More specifically, don't create vibrations.  Don't walk close to the detector (your footsteps appear to be little earthquakes to the detector), don't leave the site in an automobile (any acceleration by that a car or larger vehicle will create a little wave in the ground that will affect the detector).  This has brought up the question of what to do with the FedEx guy if he is on site making a delivery...  While we cannot hold anyone against their will, I am sure that he would be asked to stick around for a while. 

This may sound a little harsh, but considering the rarity of these events and what is to be gained, sitting around isn't all that bad!


WHAT IS TO BE GAINED?

First, to the traditional astronomy community (telescopes detecting light), it is exceedingly rare to see a supernova from its beginning and doing so can tell astronomers more about what kind of supernova it is (see this Wikipedia page for more information about the different types of supernovae).

Also, if the gravitational waves from the core collapse of a star were to be detected, this will allow us to "see" what went on inside the star - something that can never be done with traditional astronomy.  Knowing what goes on inside the star will allow us to use the dying star as a nuclear reactor unlike any we could ever create on Earth.  This may be able to tell us more about nuclear physics which could have implications for technology in the future (I have no idea what those may be).


NEUTRINOS AND SUPERNOVA IN THE PAST

So far, there are only two detected sources of neutrinos other than those produced by nuclear reactions on Earth: those from the Sun and those from the supernova known as SN 1987A.

NASA image of 1987A supernova remnant near the center.  Inset: a close up of the supernova  [Source: Wikipedia]

SN 1987A happened on 23 February 1987 (hence the name) and was located in the (relatively) nearby Large Magellanic Cloud and could be seen from the Southern Hemisphere.  About 2-3 hours before the star exploded (as seen from Earth), neutrinos were detected at 3 different neutrino detectors.  This detection not only was the birth of neutrino astronomy, but also allowed for the early observation of the light from the supernova.

Also, this supernova is thought by some to be the instigator of the LIGO concept.  This was when Joseph Weber made his claims of the first detection of gravitational waves (which was debunked - but that is a discussion for another blog post).  Weber used a method of looking for gravitational waves called a resonant bar gravitational-wave detector (a.k.a. Weber Bar).  Even though there wasn't a gravitational-wave detection, his claims and SN 1987A made scientists begin to consider other way to look for gravitational waves and that the technology needed was within reach.  So, that February day in 1987 was also the birth of LIGO in a way!

Happy New Year!

I can't believe how long it's been since I've last blogged - I've had so many ideas of stories to post, but I've also had some life issues that have kept me away.  Not to worry!  My most important resolution for 2013 is to write blog posts a few weeks ahead of time so that I can still post weekly even when life gets in the way.  I will be back in full force in 2013!  Expect posts on Thursdays, unless there is something timely I want to share before then.  I will make sure to post on Twitter when I a new post is available so if you don't follow me already, please follow @livingligo.

This is a smiley face the deicing crew at the Pittsburgh International Airport made in the snow.  As seen through the deicing fluid on the window of my plane on the evening of 29 December 2012.

2012

This year has been a year of many changes for me.  My days as a postdoc have come to an end and I now hold a dual position with Caltech as a scientist at the LIGO Livingston Observatory and as a physics instructor at LSU.  It is great being back in the classroom but that is also something that has kept me from posting as much as I would like.  It takes a lot of time to create interesting lectures for a class of 150 students and handle all of the class administration myself (office hours, grading, etc.).  This semester I am teaching the second semester of physical science (astronomy, chemistry, earth science) and will only have a 30 students.  I am very excited about the more personal instruction I will be able to do!

There have also been many changes at LIGO.  When I first started working at the Livingston observatory in 2007, there were about 25-30 people who worked there on a daily basis.  Starting with the Advanced LIGO preparations in 2010, we nearly doubled the number of daily staff.  Since the installation is well underway, we no longer need to have so many people on site (having too many people on site while we are looking for gravitational waves will cause ground vibrations that will decrease our sensitivity).  The parking lots are noticeably less full and it is starting to feel a little lonely even though we still have more people working on site than when I started.

As far as my personal life is concerned, I'm glad that 2012 is over.  It has been full of drama and uncertainty and it is one of the things that have been getting in the way of keeping up with this blog and my career in general.  But I wouldn't change a moment of it since I have so many great people around me, at home and at work, who care for me. 


2013

This coming year will prove to be exciting!  The installation of Advanced LIGO should be completed and the first commissioning (use of the detector to fine tune it to its best sensitivity) started.  This is always an interesting time when you get to use the detector for the first time and solve novel problems.  I will be sure to tell you all about them here! 

I will also continue teaching at LSU.  As I mentioned above, I will be teaching the second semester of physical science with about 30 students.  I also expect to teach a masters degree class on inquiry learning for in-service teachers this summer (I've done this class twice before with LSU).  

Of course, the most exciting events are usually the unexpected.  I look forward to sharing the professional and personal excitement with you here.

Thank you to all of my readers, followers on Twitter, and those who found me through a search engine!  Keep coming back for more!

What are you looking forward to this year?

Gravity - The Love Story II: Starstruck!

So, where was I when I last posted...  Ahh... The great corny love story between two objects bound together by gravity.  I started that post asking what would happen to the Earth if the Sun were to suddenly become a black hole.  Many people think that the Earth would be sucked in because they assume that a black hole will suck everything into it like water going down a drain.  But, from careful examination of the universal law of gravitation and the story it tells, we see that isn't the case and the Earth will stay in the same orbit that is it now - no closer and no farther away.

But what about an object flying by a black hole (or any other massive object) instead of being in a nice stable orbit (like the Earth is in the previous example)?  This makes things a little more complicated, so I am going to let go of telling a love story.  That being said, there will be more equations here, but like the previous love story post the equations will only serve to help tell the story and we will not be using any numbers.


THE FATE OF A COSMIC WANDERER

Instead of looking at the universal gravitation law, we are going to look at how a passing object comes to be in orbit, or not, around another object (this governed by Kepler's laws of planetary motion).  To keep things simple, let's assume that the moving object has much, much less mass than the object it's passing (this is so that we can ignore the motion of the big object due to its gravitational attraction to the passing object).  Basically, picture something small whizzing through space (I'll call this the small object) that passes by a star or black hole (I'll call this the big object).  It is now safe to assume that any motion caused by gravity is going to be seen in the small object.


IT'S ALL ABOUT THE SPEED

The one factor that completely determines the fate of the small object is its speed.  If this speed is great enough, then the small object will be able to escape the big object, though its speed and direction will have changed.  The minimum speed at which the small object will not be captured into an orbit is called the escape velocity:

Here, we see that the escape velocity, v_e, changes as the square root of 1/distance (1/r) between the objects' centers.  That means that the closer the small object is to the big object, the more speed it must have in order not to get caught by it; the farther away, the less speed it needs to escape.  2GM is a constant value and never changes; G being the universal gravitational constant and M being the mass of our big object.


ORBITS AND ELLIPSES

Any speed less than the escape speed and the small object will be captured by the big object and will likely start orbiting the big object (or collide with it, we'll get to that later).  Let's say that we are traveling at a speed less than the escape velocity.  Kepler's laws of planetary motion (which are a consequence of gravitation) provide that the shape of the orbit is an ellipse (an oval shape).  Instead of having one center like a circle does, an ellipse has 2 each called a focus.  A classic way to draw an ellipse for yourself is to put two pins into a piece of paper, put a loop of string around the pins, place a pen in the loop and pull the loop taut.  The shape that you draw doing this is an ellipse:

[Image from: Wikipedia]

Here, each pin is a focus.  This is what having 2 "centers" means - if you were to draw a shape using this same method but using only one pin, then you would draw a circle (the pin being the true center).  When talking about an orbit of a very massive object and much smaller object (like we have in this example, or like the Earth orbiting the Sun), the more massive object will be located at a focus and there isn't anything at the other focus.

The speed of the object determines the shape of orbit:

Here v is the speed of the orbiting object (which is less than the escape velocity), μ is a constant (G times the mass of the big object), r is the distance from the objects' centers when the velocity is measured, and a is the semi-major axis of the ellipse (the distance between the midpoint of the foci and the farthest point of the ellipse). 

[Image from: Wikipedia]

STARSTRUCK!

Under what conditions does the small object collide with the big object?  So far it sounds like the small object is either going to escape the gravitational pull of the big object or start orbiting it.  Can a black hole (assuming it's our big object) ever "swallow" anything?  Yes, indeed, but only under certain conditions...

To determine the conditions for an object to be swallowed by a black hole or collide with a star, we need to realize that neither of these objects is a nice point as we have been treating them (well, the singularity inside the black hole is a nice point, but more on that below).  Instead, objects occupy a volume and the points we were considering were really the center of mass of the object (approximately the actual center for a spherical object).  So, the small object will collide with the big object if the radius of the big object is more than the distance of closest approach of the small object's orbit.  This distance is called periapsis and is the distance along the red line (the semi-major axis) between the ellipse (orbit) and the focus (the big object) in the previous figure.  If a star has a radius of this or more, then the small object will slam into it.

NOTE:  This scenario for collision (and the one for merger with a black hole below) assumes that the objects only interact through gravity.  That means that there is no consideration here for other forces like the interactions of the objects' magnetic fields (if they have them) or resistance from the matter and radiation that stars tend to spew out.


HUNGRY, HUNGRY, BLACK HOLES

But what about the specific case of a black hole?  I mention that there is a point-like singularity in the black hole where all the mass is located.  How do we determine the shape of the whole black hole?  First, consider why a black hole is called "black"; because the gravity inside of it is so strong that the speed of light is less than the escape velocity (now you can think of our small object as a photon of light).  Since nothing can travel faster than the speed of light, nothing can escape a black hole.  So we define the edges of the black hole to be the radius at which the escape velocity equals the speed of light.  This radius is called the event horizon.  Therefore, an object (even a photon) will merge with a black hole when the distance of closest approach of its orbit is equal to or less than the event horizon.

Now that's what I call "starstruck" lovers!  Get it?  The small object strikes the big object which could be a star...  Okay, I know it's lame, but that's why I'm a physicist and not a comedian (though I do try!).

---

 

♥  Speaking of love, happy anniversary to my husband, Derek, who is always nice enough to proofread these posts.  We've been together for 16 years, married for 9 and looking forward to many more!  ♥

Happy Halloween!

Wow!  I can't believe how long it has been since I've posted.  I've been horribly busy keeping up with teaching at LSU (and trying my best to make my lectures interesting), getting my LIGO work done (we are preparing for the 3rd software engineering run for Advanced LIGO [read about the first one here]), and some personal life complications that we all deal with from time to time.  I understand better why the blessing, "May you live in interesting times," is more of a curse.

So, to tide you over until my next full post (tomorrow), here is the feature presentation of the Science Education Center's monthly Science Saturday - Halloween Edition (2011):

Here, William Katzman (Science Education Center Lead) plays a laid back fellow with some paranormal explanations of "spooky phenomena".  I play a scientist who explains all of the phenomena in terms of science.  Before the day of the presentation, we decided what spooky phenomena we were going to use, but we never rehearsed the show - I'm surprised it turned out so well (if I say so myself)!

Gravity - The Love Story I: Black Holes Are Not 'Universal' Drains

This semester I am teaching a conceptual physics class at LSU that uses minimal mathematics to understand how the Universe works.  Yesterday, we covered the chapter on gravity and my closing question to my students was, "What would happen to the Earth's orbit if the Sun were to become a black hole instantly?"  Assume that it simply changes in size from what it is now to how big a black hole with the same mass would be and the center of mass never changes.

I'm not going to make you wait...  Nothing would happen to the Earth's orbit!

This is one of the most dramatic examples of simply using an equation to tell a story that I have come across.  I suspect that much of the drama comes from the misconception that black holes WILL consume EVERYTHING, turning most people's mental picture of a black hole into a universal drain.

(I know the following analogy is a bit corny, but it makes the point that equations can tell stories and aren't just recipes to combine numbers into new numbers...)


EQUATIONS TELL A STORY

In order to resolve this misconception, consider Newton's law of universal gravitation:

  

Now, don't worry overly that this is an equation because we will be making no calculations.  Instead, we are going to use it as a script for a play.  This play just so happens to be a love story...  ♥


THE CAST OF CHARACTERS

On the left side of "=" we don't have a character, but the ending of our story: F.  (This is the gravitational force that will be felt between two masses.)  We can also think of F as the attraction between our characters.  Therefore, the larger the attraction F, the better the 'Happily ever after...' ending.

The story is told by our characters on the right side of "=": G, m₁, m₂, and r:

• G is a VERY small constant that is fundamental to the Universe.  That is, there is no way to derive its value from any theory, we simply determined this value from measurements.  Since G doesn't change, it is more of a background prop than a character; we don't need to worry about it since the moral of our story will be the same with or without it.
• Next we have our two lovers: masses m₁ and m₂.  I call them lovers because they are attracted to each other (literally since gravity tends to pull mass together).  
• Finally, we have our villain, r, who keeps our lovers apart.  (This is the distance between each of our lovers' center of mass.)  

That is the complete cast of characters in this story!  There are no extras milling around in the background.


THE PLOT

When you multiply G, m₁, and m₂ together and then divide by r² (which is equivalent to r*r), you are able to determine the ending to our story which is the attraction (F) between our lovers.  Now we are able to establish some plot points:

• The more massive either of our lovers (m₁ or m₂) are, the more they will be attracted to each other.
• The farther apart (r) they are, the less they will be attracted to each other; the bigger the number you divide by, the smaller your result.  (The square on r only serves to make the reduction in attraction between our lovers less even faster.  For example, if you double the distance between the lovers, you quarter their attraction.)

THE SUBPLOT

Now let's take a look at some of the more subtle plot points, specifically the properties that determine the attraction of our lovers (m₁ and m₂):

• No unrequited love:  m₁ and m₂ are always equally attracted to each other.  It doesn't matter if one is more massive than the other.  
• Love is blind:  There is nothing in our script which describes the size or shape of our lovers.  Assuming m₁ and m₂ stay the same distance apart and their masses don't change, they will always be equally attracted to each other.  m₁ will love m₂ the same regardless of whether its mass is made up of dense muscle or voluminous blubber. 

"ACTION!"

Now that we have the script to our play, let's see how the ending turns out when we cast the Sun as m₁ and the Earth as m₂.  The scene opens the with Earth orbiting the Sun a fixed distance r away (this is called an astronomical unit, AU, and it is about 93 million miles).  We sit and watch the Sun and the Earth be attracted to each other, but the villain of distance keeps them apart.  In an attempt to overcome our villain, the Sun decides to implode on itself, sucking all of its mass into a ball less than about 3.72 miles across.  Now it is a black hole but, according to our script, the Earth felt no change since its love it blind!  The mass of the Sun didn't change and its center is still in the same place.  Drat, the Sun didn't succeed in increasing its attraction with the Earth!

~ FIN ~

♥  Stay tuned for the next installment of "Gravity - The Love Story"!  We will find out what properties our lovers need to have to come together (that is, what properties a mass needs to have to actually get "eaten" by a black hole).  ♥

Q: If Light is Stretched/Compressed by a GW, Why Use Light Inside LIGO?

Wow!  It's been a while since I've posted...  After the start of a new semester (I have 150 students in the class I am teaching at LSU) and Hurricane Isaac (which shut LIGO Livingston down for almost a week, LSU for 3 days, and left me without power for a while), I am just getting my life back to a somewhat normal routine.  I love even the hectic parts of my life, but I've missed writing about gravitational waves here on Living LIGO!


Q: IF LIGHT IS STRETCHED/COMPRESSED BY A GRAVITATIONAL WAVE, WHY USE LIGHT INSIDE LIGO?

Today I am addressing a question that many professional physicists fully don't understand!  I wrote a little while ago about how light and gravitational waves will stretch out as the Universe expands (this is called redshift).  If an object is coming towards us, its light is compressed (and this is called blueshift).  Basically, if objects are moving, light and gravitational waves will experience a Doppler effect.  I have also written about how a passing gravitational wave will stretch and compress space in perpendicular directions.  When you put these two facts together, you come to the conclusion that the light inside the arms of LIGO is also be stretched and compressed by a gravitational wave.  So, how can we use this light to measure gravitational waves when the light itself is affected by the gravitational wave?

Like I suggested earlier, this is not obvious upon first inspection.  The apparent paradox arises from thinking of laser light as a ruler.  When you think of light, you usually think of it as a wave (which it is, but light is also a particle - however that isn't relevant to this discussion).  Waves have a wavelength -- the distance between each successive wave:

Illustration of wavelength (represented by λ) measured from various parts of a wave. [Source: Wikipedia]

A passing gravitational wave will expand and compress space-time and the wavelength of the light we are using to measure gravitational waves is itself affected by the gravitational wave.  Since LIGO and detectors like it effectively measure the length of its arms and compares them to each other,  how can we rely on light to measure any length changes from a passing gravitational wave?

The solution begins to become clear when you start thinking of the laser light as a clock instead of a ruler.  When the light comes out of the laser, there is a fixed time between each crest of the wave (this is called the period of the wave).  Let's label each crest as 'tick' (like a clock).  Our laser (labeled 'Laser' in the image below) is very stable in that it produces a very consistent wavelength of 1064 nm (near-infrared light).  Because the speed of light is constant no matter how you measure it, that means that there are almost 282 trillion (2.817 x 10¹⁴) 'ticks' every second.  This light is then split into two equal parts (at the 'Beam Splitter' in the image below), one for each arm.

Basic diagram of the LIGO detectors.

Since different things can happen to the light once it is in the arms, let's reference the beam splitter for making length measurements (i.e., let the beam splitter stay in the same place while the gravitational wave alternates squishing and stretching the arms).  A real gravitational wave will cause one arm to shorten and the other to lengthen.  This will also cause the laser wavelength in the shortened arm to decrease (blueshift) and the wavelength in the lengthened arm to increase (redshift).  But there is nothing in the detector that measures wavelength.  What it really measures is the shift in the arrival time of each 'tick' of the wavelength crests.  If the arms stay the same length (no gravitational wave), then the 'ticks' of the laser light come back to the beam splitter at the same time and produces destructive interference where we measure the light (labeled 'Photodetector' in the image above).  If a gravitational wave causes the length of the arms to change and shifts where the 'ticks' of the laser light occur, the two light beams will no longer return to the beam splitter at the same time.  It is this "out of sync" arrival time of the crests of the laser light that produces the interference patter we utilize to detect gravitational waves - we couldn't care less about the actual wavelength of the light (other than it was consistent going into the detector).


READ MORE FROM OTHER LIGO SCIENTISTS:

A wonderful, concise summary on why light can be used in gravitational wave detectors like LIGO has been published in American Scientist here.  The author, Peter Shawhan, is an associate professor at the University of Maryland, College Park.

There is also an article in the American Journal of Physics (vol. 65, issue 6, pp. 501-505) titled "If light waves are stretched by gravitational waves, how can we use light as a ruler to detect gravitational waves?"  This is a more technical article by Peter Saulson who is a professor at Syracuse University.