Gravity Rules

Along with a group of people from Rose City Astronomers, I visited the LIGO (Laser Interferometry Gravitational-Wave Observatory) facility in Hanford, WA last weekend.  The impressive looking structure in the above photo is actually just a water tank (mainly in case of a fire), but has the best LIGO signage on the site – the LIGO logo, if you like.

I’ve written about LIGO and gravitational waves here before, but in case you missed it, here’s a quick summary:  Gravitational waves are distortions of time-space caused by the movement of mass through space.  Their existence was predicted by Albert Einstein about 100 years ago, but he also predicted that we would never be able to detect them because they are so weak.  Various people have been trying since at least the 1950s to detect these waves, and in 2015 LIGO finally succeeded, simultaneously proving Einstein right about their existence and wrong about being able to detect them.  To be clear, gravitational waves actually cause a distortion in physical dimensions as they pass through an object.  The object becomes longer or shorter, for example, although at an incredibly small scale.  In fact, LIGO is the most sensitive instrument man has ever created.  It can detect dimensional distortions smaller than the diameter of a proton.

Several more detections have occurred since 2015, and most of them were similar to the first in that the waves were caused by the merger of 2 black holes.  Such a merger releases more energy than can be imagined, and all in the form of a gravitational wave – no visible light or other electromagnetic radiation.  This is key to the importance of LIGO because it allows us to “see” and measure things in space that simply cannot be detected any other way, so it is a powerful new tool in the study of astronomy.  Essentially, this is equivalent to the invention of the telescope over 400 years ago.  Although the scientists at LIGO spent months carefully analyzing the data collected from each event, the fact that there are only 2 working observatories detecting these waves (the other is the LIGO facility in Livingston, Louisiana) and that the weakness of the signal makes for very limited accuracy in the detection, causes some skepticism about the veracity of detection reports.  Such concerns were largely abated when LIGO detected waves from the collision of 2 neutron stars because this event (unlike the black hole mergers) also created visible light and a gamma ray burst, both of which were detected with time and location correlation through conventional means.

Earlier attempts to detect GWs used a variety of methods.  Above is a “gravity antenna” designed for this purpose.  I don’t know how it was supposed to work, but it seems likely that it was not even close to working.

LIGO uses a laser beam that is simultaneously sent down 2 long tunnels that are orthogonal to each other.  Each tunnel is 4 kilometers long, and each laser beam is actually bounced back and forth many times before they are finally recombined.  The detection happens because the waves are arranged such that they are normally 180 degrees out of phase, so when they are added together the result is zero.  But when a GW causes a distortion in the length of the path (and this distortion is different between the 2 paths) there is a differential phase shift, and so the signal is no longer zero.  This is an extremely simplified explanation of how it works, but captures the essential facts.

This is one segment of a tunnel.  The concrete structure to the right of it encloses the entire length of the tunnel to protect it from the elements.  The tunnel had nearly all of the air in it pumped out back in 1998, and it has remained sealed since then.  I didn’t get the number for the amount of air pumped out of the tunnels, but heard that it is equivalent to the volume of 1.8 million soccer balls.

Here we see the 2 tunnels emerge from the main building, which contains the laser and the actual detection equipment.  One tunnel is obvious in the foreground, the other is a little harder to see, but is just below the horizon to the right of the building.

Looking down the length of one tunnel, you can just barely see a small building at what appears to be the end.  It’s actually at the half way point, so the tunnel goes that far again.

We didn’t get to see the most sensitive parts of the system, which includes an incredibly complex suspension for the mirrors to suppress any vibrations that could be coupled into it through the ground.  Even so, the operators told us they can easily detect the crashing of waves on the coast, roughly 250 miles away.  The “control room” is filled with monitors showing not only the final processed signal, but a huge variety of system parameters that must be monitored to ensure accurate GW detection.

The most important thing I learned from this trip came from asking a question.  Our tour guide did not know the answer, but directed it to one of the senior scientists, who provided a fairly long and complicated answer, which I think I understood correctly, but the following should be taken as just my understanding, not necessarily fact:  The question had to do with how the distance of the event causing a GW can be determined.  When we observe electromagnetic waves from distant objects we can (usually) determine their distance indirectly based on their speed, as cosmological expansion of the universe means that everything is effectively moving away from us.  This motion causes a “red shift” that we can measure, giving us the speed, and applying a constant for expansion, we get the distance.  GWs also experience red shift, but it is much more difficult to measure.  Specifically, we can easily measure red shift in electromagnetic signals because we can recognize spectral lines for hydrogen and other elements that are prevalent throughout the universe.  Once the pattern of spacing between spectral lines is recognized, we can pick a particular one (such as hydrogen-alpha) and compare its wavelength to the same line here on Earth.  But GWs have no spectral lines, so what reference can be used to determine the red shift?  The essential answer is that the rapid spinning of 2 objects around each other before they merge produces additional red shift, both positive and negative, on opposite ends of the plane of rotation.  Unfortunately, this information is buried in a plethora of factors that distort the GW waveform.  To sort out this and other information from the raw signals, LIGO uses templates created by a computer program simulating an enormous range of possible events (100s of thousands of templates are used).  A GW signal is compared against all templates to identify the best fit, and the parameters applied to create that template are then deemed to be the best explanation of what actually created the GW.

I’d like to see and learn more about LIGO.  It looks like it would be a great place to work, but I’d probably have to get a PHD in some relevant science to get a job there.  Well, I’m also too old to get a job almost anywhere!


About Greg Marshall

I am a retired electronics engineer and after a few months of enjoying my leisure I began to miss doing product development. My astronomy hobby always needed new solutions to unique problems, so I decided that whenever I came up with a good solution I would try to make it available to others.

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