Catching the Wave: LIGO Validates GR’s Last Big Prediction

“Ladies and gentlemen, we have detected gravitational waves. We did it!” – David Reitze, Executive Director of LIGO

[Updated February 14 and 21, 2016 to include additional links and references. Original posting on February 11, 2016.]

Last year, I wrote of the centenary of Einstein’s theory of general relativity. This morning, it was announced that the last remaining prediction of general relativity had been experimentally validated.

Courtesy Caltech/MIT/LIGO Laboratory

At a press conference this morning, the Laser Interferometer Gravitational-Wave Observatory (LIGO) Collaboration announced the detection of gravitational waves, the validating the last major prediction of general relativity. What’s more, the detected signal precisely matched theoretical predictions of the waves which would be produced by the merger of a binary black hole system. And, as if that were not enough, the waves are in the acoustic frequency range, making it possible to translate the signal into sound such that we can actually “hear” two black holes merging. (Okay, it is more of a “boop,” but it came from over a billion light years away.)

Rumors of this discovery had been circulating for a few months. Today’s press conference was timed to coincide with the publication of their results in a peer-reviewed paper in Physical Review Letters. The twin Advanced LIGO detectors (located at Hanford, WA and Livingston, LA) had officially come online on September 18, 2015, but this particular detection was made by both detectors (see graphic above) on September 14 during a “shakedown” run. Some of the early rumors regarding the detection had been called into question since it was known that the detectors had been designed to randomly inject false test signals into the experiment (in a manner to which the experimenters are blind) to evaluate their handling of the data. But, as it turned out, this signal was real. And now, after a whirlwind of rumors, the official announcement is out, along with the paper.

Here is a video of the press conference:

Background: What Are Gravitational Waves?

Einstein predicted the existence of gravitational waves as a direct result of general relativity in a paper published in 1916 (English translation here). Just as the acceleration of electrical charges causes the propagation of electromagnetic waves, Einstein predicted that the acceleration of mass would cause ripples in space-time. However, these predicted ripples would be so minute, even for powerful events, he despaired of them ever being detected.

As it turned out, this is one of a handful of things Einstein turned out to be wrong about. Gravitational waves were first indirectly detected back in 1974 by radio telescope observations of the binary pulsar PSR B1913+16 by Hulse and Tayler, work which netted the pair a Nobel Prize in 1993. (Sabine Hossenfelder has more of that story here.)

How LIGO Works

But that was an indirect observation, obtained by studying the decaying orbits of a binary pair of neutron stars. Direct observation is far more challenging, and is accomplished by means of laser interferometry.

Here is what happens in LIGO’s twin interferometers. Light from a laser is divided into two beams by a beam splitter, with the two beams going out at a 90 degree angle to one another. The beams travel along a four kilometer long path, strike mirrors at the end of the path, and return to the source location. There, the reflected light beams are re-combined into a single path, where constructive or destructive interference takes place depending upon the relative phases of the two beams, which in turn depends upon how far the beams have travelled. Subtle changes in this interference are used to measure minute changes in the distances that the two beams have travelled.

How minute?  In the case of the LIGO results being discussed, the difference in the travel path is a mere fraction of the diameter of a proton!

Of course, the biggest challenge for the LIGO team is to prevent ambient vibrations from adding noise to the data.  One of the most intriguing aspects of the experimental design of the LIGO detectors is the use of glass fiber quadruple-pendulums to suspend the optical elements, thus isolating them from local ambient vibrations.

It has been a long road since the first iteration of LIGO came online back in 2002, filled with hard work and innovation. Now, champagne corks are a-popping.

Einstein’s prediction:

Indirect detection via Hulse-Taylor binary pulsars:

• Weisberg, J. M.; Taylor, J. H.; Fowler, L. A. (October 1981). “Gravitational waves from an orbiting pulsar”. Scientific American 245: 74–82. Bibcode: 1981SciAm.245…74W. doi: 10.1038/scientificamerican1081-74.
• Taylor, J. H.; Weisberg, J. M. (1982). “A new test of general relativity – Gravitational radiation and the binary pulsar PSR 1913+16”. Astrophysical Journal 253: 908–920. Bibcode: 1982ApJ…253..908T. doi: 10.1086/159690.
• Taylor, J. H.; Weisberg, J. M. (1989). “Further experimental tests of relativistic gravity using the binary pulsar PSR 1913 + 16”.Astrophysical Journal 345: 434–450. Bibcode: 1989ApJ…345..434T. doi: 10.1086/167917.
• Weisberg, J. M.; Nice, D. J.; Taylor, J. H. (2010). “Timing Measurements of the Relativistic Binary Pulsar PSR B1913+16”.Astrophysical Journal 722: 1030–1034. arXiv: 1011.0718v1. Bibcode: 2010ApJ…722.1030W. doi: 10.1088/0004-637X/722/2/1030.

Direct detection by LIGO/Virgo:

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Closing Out the UNESCO International Year of Light

Between being the centenary of general relativity, the 110th anniversary of special relativity, and the 150th anniversary of the introduction of Maxwell’s equations, it is not at all surprising that UNESCO designated 2015 as the International Year of Light and Light-Based Technologies. (For more, have a look at the IYL 2015 blog and the SPIE IYL website.) To mark the passing of this year, here is a collection of relevant articles.

The American Physical Society has highlighted a collection of groundbreaking articles on the subject from the pages of Physical Review.

PhysicsWorld has posted a list of its top 10 articles on light. That site also has an article and video about how to produce a single photon.

Here’s a quick article about the history of Maxwell’s equations as a first step on the path to unification. And another post at the wonderful Starts With a Bang blog treads the same ground.

PhysicsBuzz has a lovely article about visualizing the circular polarization of light.

Chad Orzel has an article about how the anti-bunching effect in light serves as evidence for the existence of photons, as well as a more general article on the body of evidence for the existence of photons. I came across both of these by way of Chad’s article at Forbes, “Physics: Complicating Everything Since The 1600’s.”

Brian Koberlein blogs about the latest experimental tests of the constancy of the speed of light in a vacuum, as does Phys.org. On that note, Nova has an article about the history of experimental efforts to pin down the speed of light. Also, this article discusses efforts to both measure the speed of light, as well as the experimental verification that light does not obey Galilean relativity.

In other news, the underlying premise of the quantum Hall effect has been extended from electrons to light.

The Bad Astronomer himself, Phil Plait, has a Crash Course Astronomy video on the topic of light.

A Century of General Relativity

On November 25 2015, Albert Einstein submitted a paper to the Prussian Academy of Sciences in Berlin presenting to the world, for the first time, the final form of his field equations relating the curvature of spacetime to the energy and momentum of matter, the final component of his general theory of relativity. (On a somewhat related note, this November also marks the 150th anniversary of the publication of Maxwell’s equations of electrodynamics.)

The Einstein Field Equations:

$R_{\mu\nu}\ -\ \frac{1}{2}\,R\,g_{\mu\nu}\ =\ 8\pi\,T_{\mu\nu}$

where $R_{\mu\nu}$ is the Ricci curvature tensor and $T_{\mu\nu}$ is the stress-energy tensor.
A. Einstein, “Die Feldgleichung der Gravitation”, Preussische Akademie der Wissenschaften, Sizungsberichte (1915), 844-847.

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A Nobel for the Study of Nature’s Poltergeists

My updates to this blog have been quite sporadic of late, as life has been having a tendency of getting in the way.  However, I could not let this week go by without noting this year’s Nobel Prize for Physics, particularly since it involves something near and dear to me: the ephemeral elementary particles known as neutrinos. (I’ve dropped hints in the past of a mega-post on the topic that has taken on a life of its own as I’ve continued to find new material to add. Much of this post will summarize material from that work.)

On Tuesday, the Royal Swedish Academy of Sciences announced that the 2015 Nobel Prize in Physics was being awarded to Takaaki Kajita (of the Super-Kamiokande Collaboration in Japan) and Arthur B. McDonald (of the Sudbury Neutrino Observatory Collaboration in Canada) “for the discovery of neutrino oscillations, which shows that neutrinos have mass.”

This is all about neutrino oscillation, the changing of a neutrino from one “flavour” to another over time. This ability had for a few decades been considered a potential explanation for something called the solar neutrino problem, which I will describe shortly. However, in order for neutrino oscillation to take place, it would mean that neutrinos would have to have some mass, despite being treated as massless by the Standard Model of particle physics for decades. (In an Appendix at the bottom of this post, I work through the mathematics from which this requirement is derived.) The work of the teams led by Kajita and McDonald demonstrated that neutrino oscillation does indeed take place, thus neutrinos do have mass (albeit a VERY tiny mass).

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For Women’s History Month: The Heroines of STEM

This last Sunday, March 8, 2015, was International Women’s Day. As I watched posts fill my social media feeds highlighting the accomplishments of numerous women who have left their mark on our civilization, I couldn’t help but want to put together something highlighting some of my favorite women from the science, technology, engineering, and mathematics (STEM) disciplines.

That was last Sunday. In case you haven’t noticed, it is no longer Sunday, March 8. My list kept growing. I kept thinking of people to add, so this posting is a bit tardy. (Okay, I also spent a lot of time tinkering with CSS style tags.) I suppose I could have waited until Ada Lovelace Day, but I’m the impatient sort. Besides, this is Women’s History Month also.

So here, without further ado, and in no particular order, is a list of some amazing women who have overcome tremendous obstacles to contribute to the collective knowledge of humanity. Odds are that you won’t recognize most of the names on this list, but you should.

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50th Anniversary of the Beginning of the Higgs Revolution

Today marks the 50th anniversary of the publication of the first in a series of articles introducing the world to the Higgs/Englert/Brout/Guralnik/Hagen/Kibble mechanism (or what everyone tends to call the Higgs mechanism because, well, it is much shorter).  This mechanism is the theoretical framework by which certain particles acquire their mass (in whole or in part, depending upon the particle).

Note that Peter Higgs was but one of many individuals who arrived at this model concurrently. His was not even the first paper out of the gate. It is also worth noting that the second Higgs paper was initially rejected for publication. Higgs then added to the end of the paper a prediction that his hypothetical field could be excited to form a boson, which we now refer to as the Higgs boson, and re-submitted his paper for publication in PRL.  It was this boson whose discovery was announced on July 4, 2012 and for which Higgs and Englert were awarded the Nobel Prize in Physics in 2013. (Sadly, Robert Brout passed away in 2011.)

BICEP2 Redux: How the Sausage is Made

An ongoing problem with communicating science to the general public is the existence of widely-held misconceptions among the public regarding how science actually works. A case in point is the March 17 announcement by the BICEP2 Collaboration regarding the detection of B-Mode polarization in the Cosmic Microwave Background and the events which have unfolded since then.

All too often, news stories and blog posts will trumpet some announcement with sensational headlines like “Scientists Say Cheap, Efficient Solar Cells Just Around the Corner”, or “Scientists Close in on Cure for Cancer.” Many people take such announcements at face value and consider the case closed. The work has been done.  The reality of the situation, however, is that the initial announcement of a discovery or breakthrough is just the beginning of the hard work, breathlessly hyped headlines notwithstanding.

How Science Actually Works (or at least how it is supposed to work)

Once a researcher or a team of researchers do their initial work, they write up the results of their work, including any experimental details needed to reproduce the work, in a paper and submit it for publication in a scientific journal. It is at this point that the “peer  review” process kicks in. Peer review really is the “special sauce” of the scientific method, and one of the least generally understood aspects of science among the general public. It is one of the key self-correction mechanisms of science, a way to keep errors from creeping in.  When a journal receives a submission for publication, they pass along copies of the proposed article to subject matter experts in the specific area of research covered by the paper.  These are the peer reviewers. (Ostensibly, the reviewers are anonymous. However, in practice, the number of specialists in a given area of research tends to be so small that they all know each other, so it isn’t hard to tell who is doing what in terms of peer review.) The reviewers are then supposed to go over the proposed paper with a fine-toothed comb, looking for errors or inconsistencies.  They will either recommend that the paper not be published, recommend that it be published as-is, or offer recommendations for changes or corrections to make the paper suitable for publication.

So, once an article passes muster in the peer-review process, it gets published. In the case at hand, the actual paper describing the BICEP2 results announced in March has now been published in Physical Review Letters:

P. A. R. Ade et al. (BICEP2 Collaboration), “Detection of B-Mode Polarization at Degree Angular Scales by BICEP2“, Phys. Rev. Lett. 112, 241101 – Published 19 June 2014 DOI: http://dx.doi.org/10.1103/PhysRevLett.112.241101

This is where the next step in the process kicks in: replication of results. It isn’t enough for a paper to say, “Hey, we’ve observed Whatsits decaying to a new particle, which we’ve dubbed Plaitons.”  Someone else has to try to reproduce the results (putting their own papers through the peer review process). Another team might give it a try and say, “Nah, we’re not seeing it.  Did you calibrate your framistasis properly?” Or, perhaps, “Yeah, we see something like that as well. Plaitons may be a real thing.” And just one or two follow-up papers like this don’t necessarily settle the issue. It can take years, or decades even, for the evidence to be strong enough or unambiguous enough for a consensus to build among the community of subject matter experts. (It took decades for particle physicists to accept the idea that the deficit of solar electron anti-neutrinos detected in the late sixties at the Homestake Gold Mine was due to neutrinos having a tiny amount of mass, allowing them to transform into different neutrino types. But that is a story for another day.) Again, like peer review, this is part and parcel of the self-correcting mechanism of the scientific method. It is all about keeping us from fooling ourselves.

For replication or refutation of the BICEP2 results, the scientific world eagerly awaits results from either the Planck Collaboration (which may come this fall), or from SPTpol (an instrument at the other end of the building that houses the BICEP2 instrument). It is only through validation from independent teams that the BICEP2 results can go from “Ooh, that is interesting” to physicists nodding sagely and saying “Yeah, we’ve got this” and placing serious wagers on Nobel Prizes.

I should note here that it is not essential that the replication phase come sequentially after publication of initial results. Sometimes, independent teams working in parallel come to the same conclusions at the same time.  This was the case with the Higgs boson discovery, in which the Atlas and CMS teams were working independently with separate detectors, not sharing data with one another until after the fact (in order to keep from contaminating each other’s work with errors or mistaken assumptions). Ditto for the discovery of the acceleration of cosmological expansion due to dark energy.

I should also point out that press conferences and press releases aren’t really part of this process.  They happen, but they aren’t part of the process.  When a researcher or team makes a big breakthrough, it isn’t uncommon for the associated institution to hold a press conference or issue a press release (with the latter often botching the story as badly as the press tends to mangle science stories, but that is a rant for another time). However, these things have little to do with advancing the process, and tend to be done for some combination of the following purposes:

• Promoting the prestige of the institution or institutions under which the research is performed. (“Hey, see what cool stuff our faculty are doing. Send your kids and money to our university, and they can do cool stuff like this too.”) Sometimes, this is tied to announcing a result before a competing team announces similar results.
• Public outreach.  Research institutions have a legitimate interest in communicating what they do to the public, even if the message gets mangled by the media.  (Sorry, I should still save that rant.)
• Funding/politics. (“Hey, Congress, we are getting results. Please don’t cut our funding.”)  BICEP2, by the way, is funded by a grant from the National Science Foundation.
• Rapid communication to the scientific community. Remember the big LHC meeting where the Higgs announcement was made on July 4, 2012?  It was pretty technical in content and aimed at scientific peers.  They had embargoed their results for about as long as they could and didn’t want to wait for the next big science conference or for their papers to get published, and wanted to get the actual results out before the rumor mill took over. However, due to the hype that had been built up around it in the press and the blogosphere, LOTS of lay-people watched the live-stream, even going so far as to endure a PowerPoint presentation using Comic Sans. (Really, Fabiola? Oh, well.)

Of course, sometimes science by press conference backfires, as in the case of the claimed discovery by Fleischmann and Pons of “cold fusion” back in 1988. That “breakthrough” turned out to be a bunch of hot air. Well, hot water, anyway, not to mention poor calorimetry. I recall eagerly following the ensuing debates on USENET, hoping that there was something to it, but, alas, it was not meant to be. Failure to replicate put a nail into that coffin.  The self-correcting mechanism of science asserted itself.  Evidence is the final arbitrator of reality, trumping any degree of wishful thinking we might hold.

Crowd-Sourcing Peer Review

While the scientific process generally follows the form outlined above, the BICEP2 case has included an added element that is somewhat novel. With the availability of a preprint on Cornell’s ArXiv preprint server since the initial announcement of the results, the preliminary peer review process has effectively extended well beyond the official reviewers. Analysis and criticism of the work has come in from numerous quarters, to such an extent that the final published form of the paper incorporates feedback beyond that provided by the official review process.  In this case, the peer review process was essentially crowd-sourced.

It isn’t as if such a phenomenon couldn’t have happened before. The ArXiv preprint service has been available in one form or another since the early nineties. However, a tipping point seems to have been reached in this instance, perhaps due to the hype and media attention which accompanied the original announcement. Researchers well beyond the circle of scientists who would typically be involved in the formal process have taken an interest, and they have dug in, looking for, and potentially finding, flaws in the original work. The possible flaws that have been identified (which I’ll go into shortly) have been insufficient to invalidate the conclusions, but they do weaken the confidence in the results somewhat, rendering the results of replication efforts something even more eagerly awaited. The important point, though, is that more eyeballs than ever have been scrutinizing the results, something which can only bode well for the process.

Piling On

It did not take long after the initial announcement for critiques to start appearing.  One of the earliest was regarding the possibility that some of the observed B-mode polarization observed might be a foreground effect due to something called galactic radio loops , a phenomenon caused by magnetized dust interacting with our own galaxy’s magnetic field.

Then on May 12, physicist and blogger Adam Falkowski (a.k.a. “Jester”) revealed on his Résonaances blog a rumor of an even bigger potential problem with the BICEP2 analysis.  One of the biggest challenges for the BICEP2 team has been to filter out B-mode polarization effects due to foreground dust. One component of the procedure that was employed was to use data on foreground contributions collected by the Planck team. However, Planck hasn’t yet released the raw data for this, and the BICEP2 team had to effectively “scrape” the data from a graph on page 6 of this presentation PDF. (This is not an ideal scenario, and efforts are reportedly underway to make the raw Planck data available to the BICEP2 team to improve their analysis.)

The rumor revealed by Jester is that the BICEP2 team had misinterpreted the content of this graph, thinking that it represents polarization contribution only from dust, when in fact it represents ALL foreground contributions. (Note where the slide says “Not CIB subtracted.”) If this is the case, some or all of the effect reported might not be present.

The net result of these (and other) criticisms can best be summed up by the following lines added to the abstract of the final paper:

However, these models are not sufficiently constrained to exclude the possibility of dust emission bright enough to explain the entire excess signal…  Accounting for the contribution of foreground dust will shift this value downward by an amount which will be better constrained with upcoming data sets.

Ultimately, more data and observations by independent teams will be needed to settle this once and for all. And if the critiques sometimes seem harsh, don’t worry about it.  That is how the process works. Science is to some degree an adversarial process, although the slings and arrows aren’t really directed at the researchers themselves, but their results. And it is the results that survive such harsh scrutiny that move us forward.

As an aside, I note that this entire kerfuffle has been transpiring amidst the 50th anniversary of the initial discovery of the Cosmic Microwave Background.

For some useful commentary on these latest results and where we go from here, have a look at these articles:

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