Silver and Gold: Clues to the History of Our Solar System

It's that time of year when many of us are buying gifts for our loved ones.  And there's a song from the old Rudolph the Red-Nosed Reindeer TV special that comes to mind (sung by the character Yukon Cornelius):  "Silver and gold, silver and gold.  How many wonders can one cavern hold?..."    But have you ever stopped to think about where your jewelery came from?  I don't mean how your silver and gold was mined, but how was it created?  It's much more interesting than anything you've probably imagined!

SILVER & GOLD

Gold crystal (image from Wikipedia)

First, let's go back to the birth of the Universe:  BANG!  (That was the Big Bang.)  Sometime between 10 seconds and 20 minutes from now, the first atoms will be formed (this is called Big Bang nucleosynthesis).  These first atoms were of the simplest elements in the periodic table; mostly hydrogen and some helium and lithium.  Even today, the most common elements in the Universe are hydrogen and helium.  A few hundred million years later, clouds of this material will collapse to form the first stars.  This birth is marked by the ignition of nuclear fusion: the smashing of lighter elements together to make heavier ones.  All stars start by smashing 2 hydrogen (H) atoms together to make a helium (He) atom.  Eventually, the star will start to run out of hydrogen and will start to fuse helium to make beryllium (Be) and carbon (C).  The largest stars will continue fusing atoms together until the product element is iron (Fe).  For elements lighter than iron, smashing two lighter elements together to make a heavier element will release some energy.  For elements heavier than iron, breaking an atom into two lighter atoms will release energy: this is called nuclear fission.  So, the heaviest element that any star can normally make during its life is iron.  Heavier elements like silver (Ag), gold (Au), and platinum (Pt) and many other elements can only be created in the most extreme environments such as a supernova explosion that marks the death of a star.  Therefore, the metal in our jewelery is truly a relic of a star that died before our Sun was born.

Our Sun and solar system was born from the supernova explosion of an older star that died and, in doing so, seeded the material that makes up our solar system with heavier elements (this is the nebular hypothesis).  For example, since the Earth is not a star it cannot perform nuclear fusion.  Yet the core of our planet is primarily iron and nickel and had to be made in a previous star.  This is what Carl Sagan meant in his famous quote, "We are all starstuff."


WHAT ABOUT DIAMONDS?

Diamond set into a gold ring (image from Wikipedia)

 While I'm on the topic of the exotic origins of jewelery (and many other things), let me talk a little about why our Sun may turn into a giant diamond.

First off, a diamond is made from carbon which our Sun is capable of making much later in its life (more on this in a little bit).  Carbon is everywhere around us - pretty much every molecule that isn't water in your body has at least one carbon atom in it (this is the definition of an organic compound).  What makes a diamond special from all of the other common forms of carbon is that it has a very specific crystal structure that is only formed under extreme pressure and heat.  (Any mineral that has the same chemical structure but a different crystalline organization is called a polymorph.)  So, the diamonds that are formed naturally on Earth started as a carbon deposit that transformed into a diamond under the extreme pressure and temperatures found beneath the surface of the Earth.

What does this have to do with our Sun?  Well, right now the Sun is busy turning hydrogen into helium.  It will eventually (in about 5 billion years) start making heavier elements, but only much more massive stars ever have enough oomph (enough mass to create the most extreme pressures) to fuse elements all the way up to iron.  Our Sun is not one of those stars (which is good since those massive stars live comparatively short lives).  The Sun will only be able to produce up to carbon and oxygen (O) before fusion stops and the outer layers of the Sun are blown into the interstellar medium to seed future stars and solar systems.  What is left behind is called a white dwarf composed of carbon that has been exposed to intense pressure and temperatures.  That means that the white dwarf our Sun leaves behind may very well be a diamond!

A white dwarf that could be a massive diamond has already been observed and nicknamed "Lucy" (after the Beatles song "Lucy in the Sky With Diamonds").  It is more massive than the white dwarf our Sun will leave behind, but seems to be created by the same processes we expect for our Sun.  All of this is very promising for a diamond to be the memorial for our Sun (and solar system).


WHAT DOES THIS HAVE TO DO WITH GRAVITATIONAL WAVES?

Directly, not much.  However, this past August it was confirmed that a short gamma-ray burst was the result of a kilonova, the collision of two neutron stars that releases a massive amount of energy (read more about this here and see my comments towards the end of the article).  Neutron stars are the remnants of dead stars that were much more massive than our Sun (about 10 to 40 times the mass of our Sun) that died in a supernova.  Those past supernovae would have created silver, gold, and everything heavier than iron and seeded the interstellar medium for future stars and solar systems.

These kilonova sources should produce copious amounts of gravitational waves, however, there has never been a short gamma-ray burst (like the one I mentioned above) detected close enough for LIGO to see it.  Once Advanced LIGO is complete (soon!) the closest short gamma-ray burst will be just on the edge of the distance we expect to be able to detect these kinds of gravitational waves.  But the great thing about what we do is that we really don't know how strong the gravitational waves from the actual explosion will be so they could very well be detectable in the future.

Fingers crossed!

Living LIGO's Belated 3rd Anniversary

This past Saturday, 5 October, was the 3rd anniversary of the Living LIGO blog (you can see the first ever post here).  I remember that because it also happens to be my birthday - I started this out of a desire to do something I always wanted to so what a better day to start than your own birthday.  (If you must know, I've just celebrated the 6th anniversary of turning 29.)

I know it has been a long time since I've posted.  I've been teaching at LSU and doing research at LIGO.  On paper my life looks great but the reality is that there are many details, both personal and professional, that have added up to me not being in a great place for a while.  I've been getting my jobs done but after that I've been pretty exhausted, at least mentally.  This has happened to me before, so I thought I would direct you to my thoughts on what it's like being down but getting up again anyway here.  (Also see my last section below: "A WORD OF ADVICE...".)

But there is one thought that has come up many times in the last few months:  "I'd like to write a blog post on that."  There are many different things, like continuing the series of posts I've started about methods of looking for gravitational waves or telling the story of where silver and gold come from (as in, how did it come to be on Earth).  So, I am going to dig myself out of my slump and get back on my metaphorical horse - starting now!


WHAT I DID THIS SUMMER

Let me tell you a little about what I've been doing since I've last posted.  I got to go to a large meeting called the GR20/Amaldi10 Meeting in Warsaw, Poland (where almost 850 gravity theorists and experimentalists gathered for this joint meeting) and gave 2 invited talks.  The first was a formal talk on outreach skills and media (featuring this blog) and the other was less formal and was on the benefits searching for gravitational waves can bring mankind focusing on spin-off technology (I've written about this before here).

This is a picture of the gates of the Uniwersytet Warszawski where GR20/Amaldi10 took place.

A view through the gates at my colleagues on a coffee break in the distance.

One of the best parts of meetings like this is that the meeting dinner is usually somewhere a normal person couldn't go.  Our dinner was at the in the Royal Castle in Warsaw.  And this is my husband and I on the lawn beforehand:

My husband, Derek, and I on the lawn behind the Royal Castle in Warsaw, Poland.

Just before my trip to Warsaw, I took a short holiday to Paris, France.  There was a debacle with lost luggage and then wrong luggage being delivered to us, but outside of wearing the same clothes for a few days (this is why I always pack extra underwear in my carry-on luggage), my husband and I had a great time just relaxing and wondering around.  My new phone takes panoramic pictures and this is a good one I got of the Louvre:

Panoramic view of the Louvre (Paris, France)
Click on the picture for a larger view.

Once I returned home to the United States, I had tons of work to do.  I've talked before about how LIGO has been testing its data infrastructure to prepare for Advanced LIGO here.  We are then had our 4th software engineering run; that means that I had to improve upon the gravitational wave simulation software I wrote to perform better, faster, and incorporate more features.  It was a bit stressful come the deadline, but it all got done and turned out well.


... AND THEN SCHOOL STARTED AGAIN

Then school started again.  I am teaching (at LSU) the first semester of physical science (sometimes referred to as "Physics for Poets" since it is more conceptually based than mathematically focused).  Most of my students in this class (this year my lecture has only about 100 students) are elementary education majors.  Some might think that teaching an "easy" class like this would be, well... easy.  But it is far from it.  The less math you can rely on to teach the subject matter, the better you have to be as a teacher in communicating what the math means.  Since I love challenges like this, this is one of my favorite classes to teach.  I am also team teaching a junior/senior level class on Science Methods for secondary education pre-service teachers majoring in science or math.  This class shows them how science is done by doing experiments using the scientific method, analyzing their data and reporting it in both papers and presentation (since these are the two main means that scientists communicate with each other).

At LIGO, I am working on a paper with a group of other LIGO scientists who are looking for gravitational waves from supernovae that may have occurred while LIGO and/or other gravitational wave observatories were in operation (before the advanced detector upgrade began).  And, as always, I continue to refine my gravitational wave simulation software.


A QUICK WORD OF ADVICE...

I'll be writing again soon (probably next week).  When I first started this blog, I promised you a peek into the life of a working scientist.  Lately I've been answering lots of questions about gravity and how to look for gravitational waves.  But since there has been something major going on in my life, and it kept me from writing my blog as I would have liked, I wanted to share that with you.

I am lucky since even though I know I have times when depression can get the better of me, I have wonderful support from my husband, friends, and family.  I'm not sure why, but they all seem to love me even when I can't stand to be around myself. 

For anyone reading this who has issues with depression and/or anxiety:  Don't fool yourself that everything you are feeling inside is not affecting you because you may be able to keep it together and have others think you are happy.  This will eat at you and everything you are feeling will come out sometime (and usually at the least opportune time).  If you are sad or anxious for long periods of time, even if it's on-and-off, find some help.  It is not weak to seek help (I've been told that before and it's usually by people who need help for themselves and are too afraid to get it).  It takes an inner strength to admit when you are hurting and need a hand up, an ear to listen to you, or a shoulder to cry on.  If that's not enough, talk to your doctor.  Not once has a doctor been anything but 100% supportive of me when I've gone to them seeking medical help.  NOT ONCE did they look down on me, or suggest that my feelings will pass, or that I need to "buck up".  With support and help, I've always clawed my way back to feeling like a normal person.  You can too!

Methods of Detecting Gravitational Waves I: Resonant-Mass Detectors

As the name of this blog suggests, I use LIGO and similar gravitational-wave detectors (like Virgo and GEO).  These detectors are all interferometric detectors meaning that they use the interference of light to measure gravitational waves.  But interferometers were not the first means used to look for gravitational waves...

WEBER BARS

In 1966, Joseph Weber of the University of Maryland constructed a gravitational-wave detector that consisted of a very precisely machined cylinder of aluminum 2 meters long and 1 meter in diameter.  The idea was that when a gravitational wave passed over the bar at a specific frequency, the bar would start to ring like a bell.  This "ringing" frequency, also called the resonant frequency, for Weber's bars was 1660 Hz (cycles per second).

Weber working on one of his bars at the University of Maryland, c. 1965.

The way these bars are be used to detect gravitational waves involves the phenomenon of sympathetic resonance.  This is when the vibration of something external to an object matches its resonant or "ringing" frequency and causes it to begin vibrating.  Even after the external vibration stops, the now vibrating object will continue to ring like a bell (and eventually stop ringing just like a bell as well).

Now, when a gravitational wave at or very near to 1660 Hz passes by one of Weber's bars, it will stretch space in one direction and compress it in the perpendicular direction, much like illustrated in the animation below:

This stretching and compressing is the vibration that makes the bar 'ring'.

One property of gravitational waves is that even the strongest of them that we can hope to detect on Earth are exceedingly weak.  Because of that, any ringing of the bar will be too small to hear or even to detect using normal ways of measuring vibration.  Instead, crystals that produce an electric voltage when stretched or compressed (called a piezoelectric crystal) were mounted around the bar.  Measuring the voltage from these crystals is also measuring the motions of the bar that could be from it ringing.

But just like there are many things other than gravitational waves that can cause noise in the data that LIGO collects, there were many other things that could cause the motion of Weber's bars.  Even the vibration of the aluminum atoms in the bar due to their temperature (Weber's bars were kept in a vacuum at room temperature) created significant noise and limited how small of a gravitational wave they could detect.  Ultimately, they were limited to a strain (which is defined to be the change in length divided by the original length of an object) of about 10^-16.  To give this scale some reference, a "large" gravitational wave to LIGO would produce a strain of about 10^-21 and we think we can expect a gravitational wave this large about once every 10 years!  But, it is still possible that Weber's bars could have detected gravitational waves...

By 1969 Weber thought that he may have detected gravitational waves with his bar detectors and continued to make several claims over the years but none were regarded as significant enough to declare that the first elusive gravitational wave had truly been directly observed.  These claims were ultimately not accepted for many reasons including that other groups were not able to reproduce his rate of detections which Weber was claiming to be up to several a day.  Weber lost the financial support of the National Science Foundation (which now funds LIGO) after a disputed claim of the detection of gravitational waves from a supernova observed in February 1987 (SN1987A).

One of Weber's original bars on display at the LIGO Hanford Observatory in August 2004.

There are many more interesting details surrounding Weber's career that I will write a post on later.  But while the controversy surrounding his claims of detection do not seem to cast him in a favorable light, it was through his work that others became inspired to look for gravitational waves in different ways, including using an interferometer like LIGO does.  Joseph Weber is truly the father of the search for gravitational waves!


MODERN BARS

Since Weber, there have been many advancements in using resonant-mass bars to detect gravitational waves.  Most bars today are made from new aluminum alloys, are cryogenically cooled to reduce the noise from the bar's thermal vibrations, have mechanical means to amplify the vibration, and piezoelectric crystals have been replaced with even more sensitive motion sensors.  Different shapes (like spheres) have also been used to increase sensitivity to gravitational waves coming from different directions because bars are most sensitive to gravitational wave directly above or below the bar.


COMPARISON TO DETECTORS LIKE LIGO

Resonant-mass gravitational-wave detectors like Weber's bars and interferometric detectors like LIGO look for the same effect: the stretching and compressing of space caused by the gravitational wave.  Bars are far less expensive than detectors like LIGO but are only sensitive to narrow ranges of gravitational wave frequencies.  Bars are also only sensitive to small portions of the sky at once where detectors like LIGO are sensitive to most of the sky at once (including the sky on the other side of the planet).  But because of how each of these detectors look for gravitational waves, resonant-mass detectors are likely to only be sensitive to the strongest gravitational waves.


THE DETECTORS RIGHT NOW

If you are interested in the operational state of gravitational-wave detectors (including resonant-mass detectors) click here.  AURIGA and NAUTILUS are both bar detectors while GEO 600, LIGO, and Virgo are interferometers.

Here's Your Chance to Help LISA Happen!

A depiction of a LISA satellite with its lasers.

I've written about LISA several times on this blog and most of those times the news hasn't been good.  Today I have a bit of encouraging news and a way for you to help!

For readers who've recently found Living LIGO, LISA is a space-based gravitational wave detector that will be sensitive to lower frequency gravitational waves than any Earth-based counterpart ever will be.  Gathering gravitational-wave information from low frequencies will help complete the picture that gravitational-wave astronomy can paint; Earth-based detectors are really only sensitive to the "death throes" of violent astrophysical interactions while LISA will be sensitive to these same sources in their youth.  This youth stage is so long that the predominant noise source for LISA is continually measuring the gravitational waves from these young sources coming from all over the sky at the same time.  This is called the "confusion limit" and it is like trying to listen to a conversation on the other side of the room at a busy party.  This can be overcome, but what a wonderful problem to have! The noise you measure is really just measuring so many gravitational waves at the same time that they mix together!

You can follow the unfolding of the LISA drama through these Living LIGO posts:

• News on LISA  [25 May 2012]
• More on LISA/NGO  [3 May 2012]
• The Likely End to a Space-Based GW Detector  [19 April 2012]  (The title turned out to be over dramatic - I'm glad!)

In the "News on LISA" (25 May 2012) post, the statement from the eLISA Consortium was that they were going to push for the next launch opportunity.  The European Space Agency has initiated the process to choose candidate missions for the next launch.  The eLISA mission team is looking for your support! 


HOW YOU CAN HELP...

Please go to the eLISA - Make History page and sign your name as a supporter.  Anyone from around the would can lend their support.  If you are a scientist, you can also opt to have your name and institution listed in the eLISA white paper.  (You can see the ESA's call for white papers here.)

If you support gravitational-wave science at any level, please consider putting your name on the list!  Let's show the world that there is true support for this science!

HURRY!  There are only 2 days (or less) to add your name to the list of supporters! 
eLISA - Make History

NEW LISA YOUTUBE CHANNEL

The LISA Mission has also created their own YouTube channel and is starting to post some truly exceptional animated educational videos.  Right now, two are available:

Gravity Ink. - Einstein's Gravity (Episode 1)

Gravity Ink. - The Future of Astronomy (Episode 2)

Consider subscribing to their YouTube channel to keep up on new installments (I will most likely feature them here too!). 

Black Holes 101

Most of what I discuss on this blog has to do directly with gravitational waves.  This time I'd like to talk about one of their most talked about exotic sources: black holes.  Black holes are an exemplary source because they are highly concentrated mass.  Just add a touch of accelerated motion and gravitational waves are emitted in abundance (well, it's not quite that simple and "abundance" is a relative term, but you get the idea).  But what are the fundamental concepts that add up to the existence of black holes?  That's what we are focusing on now.


1.  Escape velocity

You've probably noticed that the harder you throw an ball straight up in the air, the higher it goes.  We also know that the farther away the ball gets from the Earth, the lower the gravitational attraction is between the ball and the Earth.  When you connect these two concepts, you can imagine that there is a speed at which you can throw the ball up and it will never come back down.  This is called the escape velocity:

We have discussed escape velocity before on this blog (specifically when discussing the conditions an object must have to be 'eaten' by a black hole).  In this equation, G is the gravitational constant, M is the mass of the 'thing' you are trying to escape, and r is the distance you are from the center of the 'thing'.  The bigger the mass of the 'thing', M, is, the faster the object must be thrown to escape it, v_e.

Going back to throwing a ball up in the air from the Earth's surface, you would need to throw that ball about 25,000 mph so that the ball would not come back to the Earth (good luck with that)!


2.  The speed of light is the universal 'speed limit'

Nothing can travel faster than the speed of light (in a vacuum), represented by c.  No matter, energy, or information about the Universe can travel faster than that.  That is pretty much the long and the short of this concept.

[Source: Knight Science Journalism at MIT blog]

This 'speed limit' comes from Einstein's special relativity and the effect of simultaneity.  Perhaps I will write a longer post about this in the future, but all that is important now is to recognize that if something needed to travel faster than the speed of light to communicate information to you, then you are never going to know about it.


3.  Light is affected by gravity

Light is both a particle and a wave.  As a particle, it has no mass.  Since gravity acts between two masses, it may be surprising that light can be affected by gravity at all!  But it does.  This effect is called gravitational lensing and I've written about this previously here. 


CONCLUSION:  How these concepts form a black hole

The simplest black hole is called a Schwarzschild black hole.  This is a black hole that has no electrical charge and is not rotating - it's just "there" meaning that there won't be anything to complicate our black hole situation.  For now, let us think of our black hole as having mass but no volume.  You can think of this as being the ultimate implosion.  Since this mass has no volume, there isn't any surface to it.  Eventually, as we get closer and closer to where the mass is centered, the escape velocity will become so large that the escape velocity will be greater than the speed of light.  And since light is indeed affected by gravity, that means that nothing will be able to escape the black hole.  We can even figure out what this distance is from the equation for escape velocity by setting v_e to the speed of light, c, and solving for r (the distance away from the mass where the escape velocity equals the speed of light):

This distance from a black hole where light will not be able to emerge from a black hole is also called the event horizon and this sphere around the black hole is what is being referred to when we talk about the size of the back hole.  For this simple Schwarzschild black hole, it is also known as the Schwarzschild radius.  You can also think of this radius as how small a mass would have to be to become a black hole.

So, how big would the Schwarzschild radius be for some things we are familiar with?  Well, for a black hole with the mass of our Sun it would be just about 3 km or about 1.9 miles.  For a black hole with the mass of the Earth it would be 8.87 mm or a little under 11/32" (0.349 in).

Lessons From My Childhood on How to Teach

One of the best parts of my job is getting to do outreach.  This is going out and teaching the public about the research that I do.  Since I love what I do and those that I encounter are usually interested in what I have to say or they wouldn't be there (like you wouldn't be reading this if you didn't want to), it is almost always a rewarding experience all around.  However, I had some childhood experiences with outreach that were, well, a little traumatic.  However, they have taught me lessons that I use every time I teach whether in the classroom, engaging the public at LIGO, or writing for you.

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"HAIR-RAISING" TRAUMA

Ever since I was a young child, I've always known that where I am now is where I wanted to be.  That is, I've always known that I wanted to be a physicist or an astronomer.  Of course, that's not what I said; I wanted to be an astronaut since that is the hero job for the physical sciences.  My family has also been supportive of me and one of my favorite things to do was go to the planetarium.  At the time, I lived outside of Pittsburgh, PA and we would go to the Buhl Planetarium (before it became part of the newer Carnegie Science Center - the building is now part of the Children's Museum of Pittsburgh).

Front entrance of the Buhl Planetarium in Pittsburgh, PA. [Source: Wikipedia]

On the fateful trip in question, I was no more than 7 or 8 years old and I was watching a demonstration in between planetarium shows with my father.  The presenter asked for a volunteer from the crowd, preferably with long fine hair.  The next thing I felt is my father's hand on my back pushing me forward.  I wasn't interested in being the center of attention, but the presenter thought that I would be perfect for the role.

She called me forward and had me stand on a plastic milk crate beside a metal dome that was bigger than my head.  She told me that I was going to have to hold on the the metal dome with one hand but I was not to do a list of things or I would get hurt.  Then I was worried.  She had me put one hand on the dome and turned the machine on.  It made a lot noise and I feel an odd tingling over my skin.  Then I was scared.  The presenter was very happy about everything and told me to shake my head.  I did so timidly.  Then she encouraged me to shake my head with more vigor.  I shook the heck out of my head so she would leave me alone and I could be done with all of this.  Then EVERYONE who is watching this demonstration WAS LAUGHING AT ME.  Then they applauded as the machine was turned off and I was helped down from my perch and left to think I was being laughed at.

The machine with the big metal dome attached to the top.  I later discovered that this is a Van de Graaff generator.  [Source: UMN Physics department]

It wasn't until I was in middle school that I figured out why everyone was laughing at me.  That machine was a Van de Graaff generator and it deposited static electricity on me.  The warnings that worried me were to prevent me from getting "zapped" and everyone was laughing at me because my hair was standing on end.  The harder I shook my head, the more the static electricity made my hair stand out.  A lot like this:

WHAT WENT WRONG?:  The presenter didn't show me what I looked like in a mirror (as is featured in the clip above) or tell me what I looked like.  I had no idea why everyone was laughing at me or what the point of the "hair raising" demonstration was.  Without this knowledge, I walked away from the experience thinking that everyone was really laughing at ME and not the effects of static electricity.

LESSON LEARNED:  If you use a volunteer in a demonstration, make sure that they understand what is happening.

I don't have many occasions where I need a volunteer for a demonstration, but when I do I make the volunteer the focus of the demonstration so that, at the very least, they walk away understanding what happened. 

Read more about how Van de Graaff generators work.

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SWINGING FOR A "BREAKTHROUGH"

When I was too young for school, I wanted to be a big girl and play school.  Even then I loved science.  One day I convinced my father to play school with me.  Using the sliding green chalk board doors on my toy box, my father taught me about the layers of the Earth.

A toy box much like the one my father used to play school with me.  [Source: It's Still Life blog]

The Earth's layers can be generalized into 4 main layers: the crust at the surface where we live, then the mantle, and finally the outer and inner cores.

Diagram showing the layers of the Earth.  [Source: About Earth blog]

I was told that the crust was very thin and the mantle is hot molten rock (magma) [note: only the mantle near the outer core is molten but the mantle under the crust is about 1000^oF so I equated that to "molten" too as a child].  I'd seen documentaries about volcanoes on television and knew what "molten rock" meant.  This completely changed the way I saw the swing set in my back yard.  Why?  Well, have you noticed the divot under the swings where you drag your feet to slow the swing to a stop?  I saw that as eating away at the crust and I was afraid that I would break through to the mantle and sink my feet into molten rock!  I know that it really isn't logical since I'd seen deeper holes before and there was nothing but dirt at the bottom, but I was a little kid and didn't think like that.  Anyway, I then was afraid of breaking through the crust if I dragged my feet and I was too chicken to jump off.  That left me sitting on the swing waiting for it to slow down on its own.  The wait took a lot of the fun out of swinging!

WHAT WENT WRONG?:  The scale of "thick" and "thin" was not established.  When I heard that the crust was thin, I defined for myself what "thin" was.  I assumed it was only as deep as I could dig through it.  What "thin" really meant is compared to the size (radius) of the Earth.

For the record, the radius of the Earth is almost 4,000 miles and the crust is up to about 22 miles.  Since 22 miles is much, much less than 4,000 miles, the crust is indeed "thin" compared to the size of the Earth!

LESSON LEARNED:  When you tell someone that something is "big" or "small", make sure you establish what you are comparing that something to, i.e. make sure you set the scale for your comparison.

I sometimes tell this story after I admonish people to always ask a scientist how big or small they think "big" or "small" are.  I tell them that I think a "big" gravitational wave, one that we would only expect to see every 10 years or so, would change the length of LIGO's 4 km (2.5 mile) long arms less than 1/1000^th the diameter of a proton (10^-18 m).   That may be "big" to me now, but to a 5-year-old me something smaller than an atom would be most certainly be considered "small". 

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!