Merry Christmas, LIGO: Another Gravitational Wave!

WE DETECTED ANOTHER GRAVITATIONAL WAVE!

On the evening of Christmas day 2015, at 9:38 pm CST (3:38 am UTC) at the LIGO Livingston Observatory in Louisiana, another gravitational wave signal was recorded.  1.1 ms later, the LIGO Hanford Observatory in Washington state also picked up the same signal.  70 seconds later, the supercomputer that runs analyses on the near real-time data noticed that there was something special in the data and sent out emails and text messages that some of us affectionately call the "Bat Signal".  This goes out to scientists primarily to summon those who evaluate candidate gravitational wave events to determine if this event should be shared with traditional astronomers (i.e. ones with telescopes).  I am on the list because I am interested in keeping up on the latest results.  I remember exactly where I was: I was in my room at my mother's house outside of Pittsburgh changing clothes after getting back from visiting the in-laws (who live within a few miles of my family's home) for Christmas.  I looked at the event record and saw that this was an extraordinary candidate gravitational wave in that its statistical significance was high but the signal wasn't as obvious in graphs as the first detection in September was.

It was decided to send out the location of the possible detection to traditional astronomers and the emails started flying discussing the evidence that this was a true detection.  It was determined that the preliminary information on the signal warranted starting the detection checklist - the large-scale investigations that try to disprove that the signal is real.  Only after a candidate passes every test and has a high statistical significance is it accepted as a detection.  The same checklist that was applied to the first detection, labeled GW150914, was applied to this candidate as well.  Once this Christmas detection was verified, it was labeled GW151226 (the number reflects the UTC date that the gravitational wave was discovered) although we had nicknamed it the "Boxing Day Event" before the verification.

(Below I will often refer to GW150914 as the "first detection" and GW151226 as the "new detection".)

Read the paper on the detection here.


THE SIGNAL & THE SOURCE

The signal is similar to the first detected gravitational wave (GW150914).  We call this kind of signal a "chirp" because initially it has a low frequency which increases over time as does its amplitude.  You've heard signals like this before if you've ever hear a slide whistle increasing in tone.  The increase in tone reflects the increase in frequency and the loudness of the whistle represents the amplitude.  The signal we detected starts at about 35 Hz (close to the frequency of the sound made by the second black key from the left on the piano) and reaches its highest frequency at about 450 Hz (very close to the A above middle C if you convert this signal into sound).

Graph of the 1-second signal of GW151226.  The red line is the prediction of what a gravitational wave from a 14.2 and 7.5 solar mass black hole merger would look like and the grey area around it is the signal that LIGO recovered from its data.  The zoomed in portions allow you to get a better look at hour the prediction (in red) and the actual signal (in grey) compare.  At the end of this signal, the frequency and amplitude both go up.  The two black holes merge at the point where the amplitude of the signal is the highest (seen in the zoomed data to the far right).

The plot above shows what we detected in our data compared to the predictions of a pair of black holes orbiting each other and merging into one.  So this is similar to the last detection in that this is also a pair of stellar-mass black holes (formed from the death of extremely massive stars) but different because the masses of these new black holes are less than the first detection.  Here, our newly detected black holes are 14.2 and 7.5 solar masses where our last detection was 36.2 and 29.1 solar masses.  That makes this signal weaker than the last (the peak amplitude of this new signal is about 1/3 that of the first detection) but we are able to observe more orbits of the system here.  We see about 27 orbits of these new black holes (corresponding to the 55 cycles of the gravitational wave we see in the figure) where we only saw about 5 orbits (or 10 cycles) in the first detection.  It is interesting to note that lower mass black hole pairs will merge at higher frequencies than higher mass black holes.  This means that the signal will stay in LIGO's most sensitive frequencies longer and that is reflected in what we see here.  This new detection's signal is about 1 second long while the first detection is less than a half second long.

So, what did this new detection look and sound like?  As far as what it looked like, there was no light that we are aware of that was produced from this system.  But we can visualize the black holes as they orbit around each other and track the corresponding progression through the signal to the merger.  [Credit: SXS Collaboration/www.black-holes.org]:

We can also "listen" to gravitational waves by taking the signal, and converting it into sound through your speakers.  Below is a comparison of what the new detection "sounded" like compared to the first detection.  The actual "sounds" are quite low in tone so that they sound more like thumps.  We also have shifted the sounds up to a higher tone so that you can hear more of the detail in the signals.  That will play after the original lower tones.  The background graph shows how the frequency changes on the vertical axis (you will see that it increases for both signals) as time progresses on the horizontal axis:

The next question is where in the sky are these black holes?  We primarily determine this using the delay in detection time between the two detectors.  When the delay is large, there is a smaller area in the shape of a ring on the sky where the gravitational wave could have come from.  The detection delay for the new detection is much shorter than the first detection, so our uncertainty is going to be larger.  Below is an illustration of the areas on the sky where the new detection (the area to the left) and the first detection (the area to the right) are likely to have come from.  Note that for the new detection on the left, there is another similar area on the opposite side of the sky that cannot be seen in this image.

The location of both the new GW151226 detection (on the left) and the first detection, GW150914 (on the right).  These are pictured on a star map (you can see the center part of the Milky Way galaxy on the left and extending right).  There is another similar area for the new detection on the opposite side of the sky (not pictured here).  The outer purple area is where we are 90% confident where the sources are located.  The inner circles each have decreasing certainty.

We will be better able to determine the location of a gravitational wave source on the sky when we have more than two detectors in operation.  Fortunately, Advanced Virgo has completed their upgrades and is currently testing their new detector.  LIGO's next observing run is expected in the 4th quarter of this year and Advanced Virgo will likely join the search before the completion of that run.  When we detect more gravitational waves (which we expect since we will be even more sensitive than we were for the two detections we have already made and the run will be longer in duration) together with Virgo, we will know even more about what it is that we are seeing.

This is an exciting time to be a scientist!

Read the official LIGO "Science Summary" on this new detection, GW151226.

LIGO Makes the First Direct Detection of Gravitational Waves

On morning of 14 September 2015 at almost 4:51 am in Louisiana (09:50:45 UTC) the LIGO detectors in Livingston, LA and Hanford, WA detected a gravitational-wave signal we've labeled GW150914 (based on the date).  The online (near real-time) data analyses alerted scientists about 3 minutes later that there was something of substantial interest in the data.  While vetting this signal (that only lasted about a half of a second) took a substantial amount of time, it opened the new field of gravitational-wave astronomy.  We had not only made the first direct detection of gravitational waves but we also made the first direct detection of a black hole binary (pair) system and proved that these kinds of systems really do exist (it was contentious because the formation of one of the black holes was expected to have destroyed the star that would have made its partner).

At the time of the posting of this blog, the press conference making the announcement is going on and I am working the satellite event being held at the Livingston Observatory.  I will be sure to update this post with the link to the recording or the announcement later (update: see the bottom of this post).  There is too much to talk about in just this post, so I am going to keep this to the basics: what did we see and what does it mean?  I will be doing a series of posts about what we did to make sure that this is a real gravitational wave, the astrophysics of the source, how we detected it, the creation of black holes and why finding a pair like we did is important to astronomy.

Update: Read the Physical Review Letters journal article here.


THE SIGNAL

This gravitational-wave detection was seen as a common signal between the two LIGO sites:

This image shows the data (top row), signal (middle row), and what's left over after the signal is subtracted from the data (bottom row).  Detailed discussion on each image is provided below.

What you see here is a series of images (above and in detail below) that picks apart the signal that was detected.  In the left column is information focusing on the Hanford Observatory and on the right the Livingston Observatory.

TOP ROW:

The vertical (Y-axis) units are strain with a scale of 10^-21.

In the top row is the signal that was seen.  However, this is not the raw data as it was collected.  What you see here is data that has been filtered to 1) reduce noise and 2) to include only frequency components that are around the frequency range of the signal itself.  The red graph on the left is the signal as seen at Hanford and on the left the blue trace is as seen at Livingston.  For comparison, the light red line under the blue Livingston line is the Hanford signal that has been shifted in time to account for the travel time between detectors and flipped (multiplied by -1) to match the orientation of the arms (the arms of each site have a opposite orientation compared to each other so the positive signal in one detector will be negative in the other).  The common signal can be seen with the noise in this comparison.

MIDDLE ROW:

The vertical (Y-axis) units are strain with a scale of 10^-21.

These plots compare the signal predicted by numerical relativity (which are results of computer simulations where the predictions of general relativity cannot be solved by in explicit mathematical expressions) for a pair of black holes with one mass 36 times the mass of our Sun and the other 29 times.  (The red line in the left plot for Hanford and the blue line on the right for Livingston.)  Beneath each of these lines are grey shadowed areas that show the signal as detected from actual LIGO data with two different independent data analysis methods (wavelet and template).  Here again, we can see that the predictions and observations match well.

BOTTOM ROW:

The vertical (Y-axis) units are strain with a scale of 10^-21.

These are plots of residual signals which are the noise that this left behind when the gravitational-wave signal is removed.  Seeing that there is no pattern left in these plots supports that what was seen was a real common signal - a real gravitational wave (this is necessary for a gravitational wave detection but not sufficient - the extra investigations performed will be the subject of a future post).


THE SPECTROGRAM

A powerful tool in signal analysis is breaking up a signal into its frequency components in a graph called a spectrogram.  It allows us to see how much of a signal is made up different frequencies at different times.  If you can hear, then you do this everyday.  It is how you are able to pick apart the sound of a tuba from the sound of a flute when you listen to a symphony.  Both are playing at the same time, but you don't confuse their sounds as coming from anything else.

Below is the spectrogram of this gravitational wave detection:

The horizontal (X-axis) is the progression of time (like above) and the vertical (Y-axis) is showing the contribution of each possible frequency.  The more yellow at a frequency, the stronger that frequency's contribution to the signal at that time.  Our gravitational wave starts at a low frequency (about 35 Hz) and increases to higher frequency (about 250 Hz) near the end of the signal.  This is similar to a signal a slide whistle increasing tone would produce.


WHAT WOULD THIS SOUND LIKE?

As I've mentioned in a previous post, the frequencies of gravitational waves that LIGO is sensitive to would be audible if they were sound waves (which they aren't).  Because of this, we can make them into sound waves by putting the signal through a speaker.  So we did!

Because the starting frequency of the gravitational wave is very low, it is difficult to hear.  The frequency is audible, but at that low of a frequency we tend to feel the sound vibration more than we hear it.  So unless you have a truly great subwoofer, you will probably only hear the end "whoop" of the signal.  In order to make the entire signal more audible, we shifted all of the frequencies up in the above sound up so you can hear the whole thing.  This is not unlike the false-color images made in astronomy for light that our eyes cannot see.

Now that you've heard the detected gravitational wave, you can see that when the tone of it becomes higher toward the end of the signal, the frequency in the spectrogram also goes up.


WHERE DID THE SIGNAL COME FROM?

Because the two LIGO detectors were the only detectors operating at the time of the event (Virgo in Italy is finishing their advanced detector upgrades and KAGRA in Japan is under construction with similar advanced instrumentation) it isn't easy to state precisely where the signal came from.  We can narrow it down to an area on the sky based on how long it took the gravitational wave to travel between the two LIGO detectors, and other factors like the strength of the signal in each detector (there is a slightly different response for each detector for different sky locations).  The most probable location is in the southern hemisphere around the constellations Volans and Carina:

The colored area on this map shows the most probable source of the detected gravitational wave where red is more likely than purple.  The location is shown against a map of the night sky centered on the Milky Way galaxy with constellations outlined.
[Credits: NASA Deep Star Maps (Visualization Credits, Ernie Wright (USRA): Lead Animator, Tom Bridgman (GST): Animator) by NASA/Goddard Space Flight Center Scientific Visualization Studio with constellation figures based on those developed for the IAU by Alan MacRobert of Sky and Telescope magazine (Roger Sinnott and Rick Fienberg), and the source location based on Gravoscope screen grabs (LIGO & Nick Risinger, skysurvey.org), all in galactic coordinates. Composition by University of Florida / S. Barke.]

 
WHAT MADE THIS GRAVITATIONAL WAVE?

Two different data analysis methods that look at the data in fundamentally different ways not only detected this event, but provided the same results for what the source of it was.  This gravitational wave was made by two stellar mass black holes (these are the remnants of extremely massive stars that have expended their fuel and collapsed under their own gravity).  As quoted above, their masses were about 29 and 36 times the mass of our Sun.  They orbited around each other for hundreds of thousands to millions of years before they come close enough together to start orbiting very quickly (much like an ice skater spins faster as they draw their arms into themselves).  LIGO was only sensitive to the very end of this process right before the two black holes merged into one black hole.  At the end, the stars had a relative velocity of about 1.8x10⁸ m/s, or 60% the speed of light (the universe's "speed limit").  Imagine that...  Two black holes that were each the size of cities but each about 30 times as massive as our Sun whirling around each other at more than half the speed of light!  The animation below shows what it may have looked like to see these black holes merge together.  Note that since they are black holes, no light come from them directly but they do bend the light that is coming from behind them in a process called gravitational lensing:

Based on how strong we know these gravitational waves were at their source as predicted by general relativity and how strong they were once they reached Earth, we estimate that this system is located about 1.3 billion light years (~410 Mpc) away.  That distance is about 10% of the way to the edge of the observable universe!  It also means that the gravitational waves we just detected have been traveling into the universe and toward us for 1.3 billion years.  When these gravitational wave were created the Earth was in the Proterozoic eon of Precambrian time, after when multicellular life developed but before animal life.

PRESS CONFERENCE RECORDING

Note:  Fast forward to 26:30.  It's just waiting before that. 

Next post: On the formation of stellar mass black hole and why this pair of them are interesting to astronomy...

First Science Data With Advanced LIGO is Near!

It has been a very exciting time for Advanced LIGO recently.  A few weeks ago we completed a test run of the instrument to identify any remaining bugs in the instrument or other stability issues.  The commissioners (instrumental scientists who work on making LIGO more sensitive) have been busy adjusting various settings in a multitude of subsystems to increase our sensitivity to gravitational waves.  We are continuously learning more about how all of these subsystems react to one another and to the environment.  And learning is never without its own pains.  Some bugs have been bigger than others. We've had to actually touch the new instrumentation - meaning we had to seal off the chamber the part was in, let the air back in (since almost all of the instrument is in a vacuum), fix it, close up the chamber, and pump the air back out.  This is rare but it has happened.  Once the instrument was performing well, that's when we decided to stop tinkering with it and use it like we would if we were looking for gravitational waves.  More subtle issues in stability and other bugs will make themselves apparent only after you use it the way it's meant to be used - all the time.

Installing one of Advanced LIGO's seismic isolation platforms at the Hanford observatory in 2013.

ENGINEERING RUNS

These test runs are called engineering runs.  We abbreviate them ER followed by the number of the run.  The last one was called ER8.  I've already talked about the first one (ER1) back when almost everything was being simulated since the installation of the instruments was just getting off the ground.  The purpose of those early engineering runs was to test out the ability of our data analysis systems to handle the large amount of data we will collect.  As parts of aLIGO were installed, we replaced the simulated data from that component with real data.  ER8 was our first test of all of the instrument without anything being simulated.  While the purpose of this data is to test the stability of the whole system and to find other small bugs, we are still running all of our data analysis methods over the collected data.  We don't expect to find a gravitational wave in this data, but if we have compelling reason to believe that we really did see something we will certainly pursue it as a real detection.  Don't get too excited, though, since there are no indications that we collected a gravitational wave.

OBSERVING RUNS

What is really exciting is that we are preparing to make that first detection.  We don't really expect to detect a gravitational wave with our first science data (which will be called O1 - observation 1) with aLIGO but it is not as improbable as it was with Initial LIGO.  We are talking about what we learned from the blind injection in our last iLIGO data set (otherwise known as the "Big Dog" event) and what our detection validation should entail.  We are talking about writing the paper that we will publish announcing the first detection and its details.  We are even talking about how we will engage the public with this announcement.  Don't misunderstand me - we have not seen anything yet, but we are preparing ourselves for the possibility of detection.

DETECTION IS POSSIBLE...

You really don't have any idea how exciting this is especially for those of us who have been around a while (I have been working on LIGO since starting grad school in 1999 and I'm a youngster).  I have been working on this project that is so much bigger than myself since before we took our first data with Initial LIGO.  I remember when the collaboration was a couple hundred scientists (there are now almost 1000 of us).  I remember when we analyzed our first data and debated how to interpret our detection candidates when we almost sure that everything we had was noise (i.e. garbage).  Now we are talking about confidently making a detection, and doing astronomy with it.  This is the dawn of a new age in astronomy and I'm proud to be here to see it.

Distance in parsecs (1 pc = 3.26 light years) Initial LIGO was able to detect its standard source of 2 neutron stars orbiting each other just before they merge into one body.  (Read more here.)
aLIGO wil be able to "see" up to 200 Mpc (about 650 million light years).

Remember, we don't expect a detection, but it is possible.  To give you an idea of how possible, once we have aLIGO working at the sensitivity it was designed to work at, it will observe as much of the universe in several hours as Initial LIGO did in an entire year.  We won't be at design sensitivity for O1, but we can already detect our standard source 4 times farther away than we could on our best days with Initial LIGO.

An image of light that was filtered out of the laser before entering the LIGO detector.  Bend your neck to the right and you should be able to see a smiley face.  This is just a chance configuration and has no significance, but we thought it was cool.

Gravitational Waves Seen in the Polarization of Light From the Big Bang

THE COSMIC MICROWAVE BACKGROUND

The oldest light we can see in the Universe is called the cosmic microwave background (CMB) and it is the relic light from the Big Bang.  While this light is old, it isn't quite as old as our Universe.  Before an event called recombination, the Universe was not transparent to light, so the light couldn't propagate very far before being absorbed.  Recombination happened about 380,000 years after the Big Bang and the light from this time is what we observe in the CMB.

Everywhere we look on the sky, the frequency of this microwave light is very nearly the same.  Since heat can be transmitted through radiation (such as microwaves), we can characterize this light to have a temperature of about 3 K (or about 3 ^oC or 5.4 ^oF above absolute zero - the coldest anything in the Universe can be).  Why this temperature is the same everywhere on the sky doesn't immediately make sense since the heat hasn't had enough time to be transferred across the Universe.

The slight variations in the CMB temperature from opposite sides of the sky as measured by 9 years of data from the WMAP mission.  The fluctuation in the CMB temperature is measured to be ± 0.0002 ^oC (0.00036 ^oF).  [Source: Wikipedia]


The CMB is almost 14 billion light years away from us.  This is approximately the age of the Universe.  But there is no way for light to transfer heat from one side of the Universe (14 billion light years from us) and reach the other side (14 billion light years away from us in the opposite direction) since this would take about 28 billion years of travel time or twice the age of the Universe!  So there is no way that these widely separated parts of the Universe should have the same temperature if the Universe has expanded in a continuous way since the Big Bang.

To explain why the CMB is essentially in thermal equilibrium in every part of the Universe, something extraordinary needed to happen...


INFLATION

Almost immediately after the Big Bang, it is believed that the Universe entered a period of extremely rapid expansion called inflation.  This began at about 10^-35 seconds after the Big Bang and the Universe proceeded to expand its volume by about 80 orders of magnitude (that's a 1 followed by 80 zeroes) in a fraction of a second.  During this time, gravitational waves would have originally been produced on a quantum mechanical scale and then blown up to cosmological scales during inflation.  The gravitational waves from the Big Bang are exactly these fluctuations in space-time that are still vibrating from the period of inflation.  (The wavelength now is its original wavelength, i.e. about 1% of the size of the Universe as it was then, stretched by the amount the Universe has expanded since then.)


EVIDENCE OF GRAVITATIONAL WAVES IN THE CMB

Since gravitational waves were able to propagate through the early Universe long before light was, it is expected that there is evidence of these gravitational waves contained within the CMB.  We expect to see this in a special kind of polarization of the CMB (where polarization refers to the rotational orientation of the light waves).  There should be 2 kinds of polarization in the CMB, E-mode and B-mode.

A graphical history of the Universe showing when gravitational waves would have been created and how they affect matter along with density waves and their affect.  The effects that gravitational waves have on mattert cause B-mode polarization in the CMB while density waves are the primary contributors of E-mode polarization.  [Source: Wikipedia]

E-mode polarization means that the orientation of the polarization should not change as you move in a straight line.  B-mode polarization means that the rotation of the polarization changes or "curls" around itself.  The E and B in these mode names refer to how electric (E)  and magnetic (B) fields behave: a single charge will have an electric field pointing radially away from a single change while a magnet always have 2 poles causing the magnetic field to always curl back to the magnet.  The E-mode polarization in the CMB provides information about the fluctuation of density in the early Universe.  Because gravitational waves alternate, compressing space in one direction and expanding it in the orthogonal (at a right angle) direction, they caused the "curling" B-mode polarization.

Graphical illustration of the polarization patterns for E-modes and B-modes.  Note that B-mode patterns can be characterized by "rotating" clockwise or counter-clockwise while the E-modes cannot.  [Source: Press conference screen grab]

An experiment called BICEP2 (Background Imaging of Cosmic Extragalactic Polarization) announced to the world a few days before this post (17 March 2014) that they did indeed detect the B-mode polarization in the CMB.  The results are cataloged here.

The above image is the polarization of different points in the sky they observed from the South Pole.  The red colored areas are where the B-modes can be classified as clockwise and the blue colored areas are where they can be classified as counter-clockwise.

THIS IS THE IMPRINT OF GRAVITATIONAL WAVES FROM THE PERIOD OF INFLATION!

Any time scientists think they found something that they wanted to find, we immediately set to trying to disprove what we found.  (This is discussed on this blog in regard to LIGO with the blind injections known as "The Big Dog".)  After thorough vetting and analysis of this work, it has been determined that the chance of this B-mode signal has a chance of 1 in 3.5 million of being a false detection. 


WHAT THIS MEANS FOR LIGO AND SIMILAR DETECTORS

This discovery of the imprint of gravitational waves on the CMB further hints at the promise that gravitational-wave astronomy with detectors like LIGO will have in the future.  Their discovery in no way diminishes the potential of LIGO and gravitational-wave astronomy - instead it increases its promise.

LIGO seeks to work like a gravitational-wave radio and record the gravitational-wave signals directly.  (This analogy is discussed in more depth on this blog here.)  For this analogy, the information about what made the gravitational wave is the music being carried on the radio wave (the gravitational wave in this analogy).  In this sense, LIGO will be making a distinctly different kind of detection than BICEP2 did.  We will be directly recording a gravitational wave as it passes by Earth and BICEP2 detected the imprint of gravitational waves on the CMB.

Also, LIGO looks for a wider range of gravitational waves.  While we also look for the relic gravitational waves from the Big Bang which we call stochastic gravitational waves, we search for three other kinds: continuous, inspiral, and burst.  (These are described in more detail on this blog here.)  This broad range of gravitational waves that detectors like LIGO will be able to "see" will allow gravitational waves to tell their own story of how they were made; perhaps from the collapse of a star into a black hole or the merging of two stars into one, or the echoes of the birth of the Universe.  We will not be seeing the evidence of gravitational waves that is imprinted onto light, but collecting information from the gravitational waves themselves.

As a side note:  Kip Thorne, a physicist who has pioneered work in general relativity and gravitational waves, made a prediction in 2006 of what detections will be made with gravitational waves in the next 50 years:

"Over the next 50 years, gravitational waves from the big bang will be detected, first indirectly by the imprint they leave on the cosmic microwave radiation and then directly, by space-based gravitational wave observatories."

 You can read the rest of his prediction on NewScientist.com.

Read LIGO's official congratulatory statement on the BICEP2 results to the ligo.org web page.

WHAT THIS MEANS FOR COSMOLOGY

The BICEP2 results do much more than suggest or support that inflation happened: it gives us some information about what happened during inflation.  The strength of the signals observed here informs us on the energy involved in inflation.  The ratio of the strength of the E-modes to the B-modes (a value referred to as r and measured here to be r = 0.2) is proportional to the energy density of the Universe at the time of inflation and this is consistent with energies needed in some of the grand unified theories (GUTs) (this is where the strong, weak, and electromagnetic forces become indistinguishable).

The BICEP2 results also serve to constrain the theories of what happened during inflation.  Several of these have been ruled out (e.g. large field inflation models are now highly unlikely).

Ultimately, these results need to be reproduced and refined by coming experiments.  This doesn't mean that the scientific community isn't confident in BICEP2's results, but science needs to be reproducible.  And in reproducing results, they are often refined and expanded upon.

This truly is an exciting time to be a scientist!

See Also:

• Archive of the BICEP2 press conference can be found here (it is large so give it a little while to load).
• BICEP2 results page
• New York Time's article on BICEP2's results with an excellent description of how inflation explains why the CMB has nearly the same temperature everywhere.
• New Scientist's article on BICEP2 results

What Is a Higgs Boson?, What Did CERN See?, and Why It's a Big Deal!

This is my disclaimer - I AM NOT A PARTICLE PHYSICIST!  Therefore, this subject does not fall into my realm of expertise.  However, I do have a very basic training in the physics behind all of this so I would like to share with you a little bit about why all of us physicists have been so excited of late...


WHAT IS A HIGGS BOSON?

The Higgs boson (regular Wikipedia entry, Simple English Wikipedia entry) is the elementary particle that gives matter mass.  Many of us have probably heard about Einstein's famous equation:

This image is taken from Wikipedia.

While this equation is famous, the true meaning behind what it means is often not fully appreciated (I used to see it and simply think "Einstein!").  It means that mass can be converted to energy and energy can be converted to mass, also known as the mass-energy equivalence.  The energy, E, from an amount of mass, m, is equal to the mass multiplied by the speed of light, c, squared (c² = c*c, c is about 670,616,629 miles/hour).  If you were to convert 1 oz of matter into energy, you would have about 2,500,000,000,000,000 Joules of energy and this would keep a 100 W light bulb lit for over 807,389 years!  So, a little bit of matter can be converted into an immense amount of energy!  It is this conversion of mass to energy that makes nuclear weapons so destructive.

But, if we convert energy into mass, where does this mass come from?  The Standard Model, which is the working description of how the fundamental particles interact (I talked about this in my post discussing gravitons), says it comes from the Higgs field.  This is a field similar to the electric field and the magnetic field (more on fields in general here).  At the instant after the Big Bang, all particles moved at the speed of light (c from above) since none of them had mass because there was no Higgs field.  In the next instant (about a trillionth of a second later), the Higgs field came into existence and produced a resistance to particles based on what they were made of.  This resistance manifests itself as mass: the slower a particle moves through the Higgs field, the more massive it is.  The Standard Model also allows fields to manifest themselves as particles (the photon is the particle associated with the electric and magnetic fields).  Therefore, this Higgs field should also manifest itself as a particle and we call it the Higgs boson.

The only particle from the Standard Model that has not been detected is this Higgs boson.  (Note that gravity is not described by the Standard Model.)  This is because the Higgs boson is very massive for a fundamental particle at about 133 times the mass of a proton.  The amount of energy conversion needed to produce this mass is much larger than we have been able to create in the past.  That is, until CERN built the Large Hadron Collider (LHC)...


WHAT DID CERN SEE?

View of the CMS experiment [note the person near the center] (© CERN)

CERN is the home to several experiments including CMS and ATLAS.  Both of these experiments smash together protons with very large energies.  The protons and their energy can change into a Higgs boson (if there is enough energy).  The Higgs boson decays (changes into something else) almost immediately, so the LHC experiments look for the Higgs boson's signature in the resulting particles.  There are 5 pairs of particles that result from the decay of a Higgs boson, including 2 photons.

Both CMS and ATLAS saw the Higgs signature in 2 of the 5 different resulting decay pairs.  Only CMS had sufficient data to look for all 5 kinds of decay pairs and saw with high certainty signatures in 4 of the pairs.  More research is needed for the signature CMS didn't see.

Diagram of the ATLAS experiment [note the people on the bottom left] (© CERN)

WHY IT'S A BIG DEAL!

The official conclusion is that CERN has indeed observed a Higgs-like particle.  "Higgs-like" does not mean that they are unsure if they found what they were looking for.  Instead, it appears that the Higgs boson they observed has more properties to it than the Standard Model predicted.  The LHC has found what it is looking for as well as hints that there is new physics to be discovered.

So this is the big deal: the population of the Standard Model is complete but the model itself doesn't appear to describe everything.  That is something we knew before, but now we are seeing it with the apparent complication in what was and wasn't observed in the discovery of the Higgs boson.

I am a physicist because every time you discover something new, not only do you understand the Universe better, but you have even more exciting questions to answer!  Soon enough, it is going to be gravitational waves stirring up excitement like this and I am going to be in the midst of it all!


Side Note:
IT'S NOT THE 'GOD PARTICLE'

The Higgs boson is sometimes referred to as the "God particle," especially by the media.  However, it has nothing more to do with God than any other particle in the Universe.  The origin of this misnomer is a book by Nobel Laureate Leon Lederman titled The God Particle: If the Universe Is the Answer, What Is the Question?.  Likening the Higgs boson as a God particle refers to it being the origin of all matter's mass.  Physicists (like me as well as Lederman himself) generally dislike this nickname since it places much more import on this particle than it deserves.  But above that, it is offensive to people of faith and makes us look like we are trying to replace God.  We aren't. 


Want More? ...
Watch the announcement seminar
Read the official CERN press release

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!

More on LISA/NGO and What Would You Ask a Scientist?

LISA/NGO Update

On May 2^nd, the ESA officially decided to accept their Science Programme Committee's recommendation to choose the JUICE (JUpiter ICy moons Explorer) mission for the 2020ish launch opportunity.  While this is unexpected, it is still a disappointment.  A thoughtful article on possible factors contributing to why LISA/NGO was passed over can be found here.

While this is disappointing, all isn't lost.  The scuttlebutt is that LISA may have a good shot at being the next mission chosen when the next mission selection takes place in 2015. (I know...  That's still a long while off.)  The science case for LISA/NGO has always been strong and by then the results of the LISA Pathfinder mission will be in (it appears that it will fly in 2014, not in June like I cited from the NASA website on my last LISA post).  Assuming success, many of the technical concerns should be resolved.

As LISA/NGO detects lower frequency gravitational waves than LIGO ever can (since seismic vibrations create too much noise for LIGO to be sensitive there), it is a beautifully complimentary observatory.  Who wouldn't love a detector where your main noise source is detecting too many gravitational waves at the same time (which is exactly the case for LISA/NGO)?

I'm looking forward to it!

What Would You Ask A Scientist (or Engineer, etc.)?

One of the reasons that I started this blog is that I wanted to show people that scientists are real people.  I've been told too many times to count that I look too young, normal, etc. to be a "real" scientist.  I know that they are trying to compliment me, but the subtext of this is that they thought I wouldn't be human like them.  So, since not all of you can come to visit me, I figured I would share some of my experience and life with you.

Well, it turns out that I am not as interesting as I thought I was (actually, I never thought that - I like vampire books, I have migraines, and I love my job at LIGO - that is me in a nutshell).  But the LIGO Scientific Collaboration is made up of over 800 scientists from across the country and around the world and I am guessing that you may like to find out more about them.

Here is what I am thinking...  I have a few basic questions I would like anyone profiled here to answer:

• What is it that you do for a living?
• What motivated you to choose this career?
• What kind of education do you have/need to do your work?
• Where are you from?
• Please tell us something unique about yourself.

I also have a list of questions that I would like the person being profiled to pick and choose from (not all are appropriate for everyone or would produce interesting answers).  Of these, I would love to know that you think is most interesting.  This may help guide the person being profiled in choosing what they wish to answer (or I can ask that your favorites be answered).

So, what would you like to ask someone who works on LIGO?  I have a survey you can click below and there is also a link just below the blog banner called "Survey" where you can respond.  Please note that the last option is where you can put your pressing question that isn't already listed.  Also, feel free to let me know if there is anyone in particular from LIGO that you would like me to profile!  Hope to hear from you!!!

No "Faster Than Light" Neutrinos

SCIENCE AS A PROCESS

Most people see science as purporting itself to be infallible and they can twist this perception for many reasons (e.g. "See, they didn't see what they thought they saw so science cannot be trusted.").  The truth is that science is a process.  It must be reproducible by others.  Sometimes, an experiment comes around that seems to defy the current understanding of science and people are quick to jump and accuse science of being unreliable.  Really, when results like this come to light, it is the duty of other scientists to scrutinize the results: to try to reproduce them and, if they cannot, try to find where the errors in the original experiment occurred.  Most of the time, radical findings are disproved.  When they are not, this is an exciting time for science to learn more about the world around us!  We scientists often spend as much time trying to disprove things as we spend trying to prove them.  Truly revolutionary results often exploit a subtlety in a theory (which in science means a highly tested and verified description of how something works and NOT a hypothesis or guess as it is sometimes used in everyday language) or law that opens the way to a deeper understanding.  Science is not created or invented by scientists - the Universe has its properties and we simply pursue the discovery of them so we can understand better how it works.

THE "FASTER THAN LIGHT" NEUTRINOS

While at the APS April Meeting this past week, there was a lot of excitement (see the talk abstracts in this session) about the "faster than the speed of light" neutrinos that the OPERA collaboration claimed to have observed.  There was extra excitement since there was a final resolution at the beginning of the meeting along with a little drama.  There were even talks on how to use this new coverage as a great outreach opportunity to illustrate science as a process (don't think of the scientific method that you were taught in school - science almost never follows that prescription but it is a good starting point).  I've had many people bring this up to me when I talk about how gravitational waves are expected to travel at the speed of light but could travel slower - never faster.  Then there is usually someone who asks about the new neutrino results and this is when I get to talk about how science is a process.  So, I've decided that I would dedicate today's blog post to the subject matter.  Spoiler alert: there are NO "faster than light" neutrinos!  If you are interested in a very good discussion of these results, disproof, and aftermath, read more about it here.

***  What is a neutrino?  ***

A neutrino is a virtually massless particle that interacts so weakly with matter that it can travel right through any matter with only a few (of billions and billions) interacting with matter.  The neutrino has never been directly detected but we know when one interacts with matter because it produces other subatomic particles or radiation.  Every second, about 10,000,000,000,000 (that's 10 trillion) neutrinos from our Sun pass through every square foot when the Sun is directly overhead.  Those neutrinos pass right through you and, since they so rarely interact with anything, you don't notice a thing. 

Because neutrinos are virtually massless (I say virtually because there is evidence they they do indeed have mass, but it is so small that it hasn't been accurately measured) they can travel at or so near the speed of light that we haven't measured evidence of them traveling slower.  This agrees with special relativity: only massless particles can travel the speed of light and massive particles can only travel slower (there are theoretical particles called tachyons that can only travel as slow as the speed of light and travel faster otherwise - these have never been observed).

***  What is the OPERA experiment?  ***

The OPERA experiment used a beam of neutrinos created at CERN on the Franco-Swiss border to send to the OPERA detector in Gran Sasso, Italy.  That's right, the beam of neutrinos was shot right through the intervening earth between these 2 sites.  Since the distance is known to high precision, the time it takes the neutrinos to arrive at OPERA is directly related to their speed.  It appeared that they were measuring their arrival about 60 nanoseconds (0.00000006 seconds) before they should have if they traveled at the speed of light. 

***  What did we know about the speed of neutrinos before OPERA?  ***

There have been many experiments that have observed neutrinos traveling at the speed of light.  These experiments have been both Earth-sourced (where we create and then detect the resulting neutrinos) and Universe-sourced.  A spectacular example of using neutrinos from space was the detection of neutrinos that preceded the supernova 1987a.  They arrived 3 hours before the light from the stellar explosion did.  This is what is expected because neutrinos are created when the matter in the star collapses before the supernova explosion.  If neutrinos traveled as fast as the OPERA collaboration claimed to have observed them traveling, then after traveling the more than 160,000 light years to Earth they would have arrived 4 years before the accompanying light we observed.

***  Should OPERA have published their result?  ***

So, was the OPERA collaboration wrong to publish their observations?  Absolutely not (in my opinion at least)!  Nowhere in their paper did they claim that they have found a fault with the current understanding of the physics - they simply couldn't disprove their own observations so they opened their experiment up to the scrutiny of the scientific community.  They even recognize the controversial results and their desire for scrutiny of their experiment in their paper (which can be read in full here):

"Despite the large significance of the measurement reported here and the stability of the analysis, the potentially great impact of the result motivates the continuation of our studies in order to investigate possible still unknown systematic effects that could explain the observed anomaly. We deliberately do not attempt any theoretical or phenomenological interpretation of the results. "

THE RESOLUTION TO THE CONTROVERSY AND THE FALLOUT

In the end, it was found that a loose fiber optic cable and an error in their timing produced the superluminal (fancy way of saying 'faster than the speed of light') observations.  THERE IS NO EVIDENCE TO SUPPORT THAT NEUTRINOS CAN TRAVEL FASTER THAN THE SPEED OF LIGHT.  Also, the ICARUS experiment (located in Gran Sasso with OPERA) independently reproduced the experiment and found no faster than light neutrinos.

The heads of the collaboration resigned their post on March 30 (just a few days ago) after a vote of no confidence.  There were scientists in the collaboration who felt the publication of the results was premature, and that not everything that was done was good experimental procedure.  It seems that the resignations were the result of their rush to publish the paper, more than what they published.