## LIGO Does It Again!

Hot on the heels of the LIGO founders being awarded the 2017 Nobel Prize in Physics, the LIGO/VIRGO collaboration, in conjunction with the ESO, have struck gold again. After four detections of gravitational waves from black hole-black hole collisions, LIGO has detected what it was originally designed to detect: the merger of two neutron stars.  What’s more, as a bonus, the collision has also been observed in the EM spectrum by multiple astronomy teams around the world in the gamma, X-ray, UV, visible, IR, and radio portions of the spectrum.  Analysis of the data gathered by these observations indicates what was expected: the large-scale production of heavy nuclei.

We have well and truly entered the era of multi-messenger astronomy. Continue reading

## 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).

# Continue reading →

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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.)