Search
  • M. Josh Roberts

Direct detection of Gravity Waves: When do headlines do a disservice?

Black holes have long endured a reputation as an “unknowable mystery” that has been highly distorted and warped in the funhouse mirror of pop culture and science fiction. They have been depicted as everything from sources of infinite energy to doorways to other realms. Given the hysteria that surrounds them, their actual natures seem mundane by comparison - no small feat when their true nature is the physics bending result of a star’s highly energetic death!



Black hole animation credit: NASA

Black holes have recently come back into the public eye with the gravitational wave discovery known as GW150914. This result has been a hundred years in the making and confirmation of some of the most exciting ideas in physics. This discovery has not only opened up a new possibility for observing and understanding the Universe, but allowed us to perceive realms and events previously thought of as unobservable.

Most of the smaller (non-supermassive) black holes in the Universe are thought to have begun their existences as stars. The mass of a star at the beginning of its life determines much of the star’s eventual fate. While modest red dwarf stars start with very little hydrogen to power their internal furnaces, they use it slowly and efficiently with very little going un-utilized. Some of these stars are thought to take between ten billion and a hundred billion years or so to end their lives. Stars like our own Sun start with more material but lead faster paced lives - our Sun has been shining for five or so billion years and could shine for five billion more but when it does go out, there will be an envelope of unused hydrogen around it as it becomes a planetary nebula surrounding a glowing white dwarf. For the largest stars, like proverbial rock stars, they live fast and die young, leaving behind “attractive” corpses (gravitationally attractive, that is…). While their ten million year lifespan might seem ancient to us bipeds, it is shockingly brief for stars. It is jarring to imagine that, for stars like this, 6.6 generations have come and gone since the demise of the dinosaurs! These high mass stars also leave behind neutron stars or black holes as their remnant, depending on their initial mass.

The stars that gave their lives to produce the binary black holes that merged to cause the recent gravitational wave discovery are somewhat mysterious - characterizing a pair of stars from 130 million light-years away would be an astounding feat. What we do know from the result is the mass of the black holes that remained at the end of the star’s lives - . Based on the respective masses of 29 times the mass of the Sun and a whopping 36. We have stars firmly within the largest class of stars since much of their original mass would have been expended into energy over their brief lives, thought to be between 40-100 times the mass of the Sun at their formation ( Belczynski, 2016) - they would be considered very high mass stars. When the stars went supernova, (type II supernova, most likely) they left behind very massive black holes as a result of their dramatic ends; yet this is where our story really begins.

These two star-crossed stellar remnants met their mutual end 130 million years ago, but due to their tremendous distance from us here on Earth, we only learned of their merger in 2014. The two had likely been orbiting each other for a long time, slowly losing gravitational energy. As their binary orbit decayed, they moved in closer and closer to one another, orbiting faster and faster. The final stage of their merger is what created the distinctive rising “tone” that was detected clearly by the LIGO team; this was the ever accelerating ringdown phase where the rapidly orbiting stars entered a mutual Schwarzschild radius and became one. The signal of this merger spread outwards across the Universe, eventually reaching Earth and triggered both apparati. As the waves propagated outward, the ripples in spacetime became more diffuse and therefore more challenging to detect so when they reached and passed through LIGO’s detection facilities, the contraction of space that was observed was minute. The change in distance for the mirrors within the cavity was on the order of 1/1000 of the diameter of a proton which is quite a challenging distance to wrap one’s mind around. Several of the analysis compared the measurement to observing a change in distance equivalent to the thickness of a human hair over 4.24 light-years, the distance to Alpha Centauri. These nearly imperceptible fluctuations in spacetime could have come from any number of sources, meaning that the LIGO team had to painstakingly compare the received signal to a variety of scenarios to determine which one was the closest match and eventually (after months of calculation and deliberation) determined that the origin was a black hole merger.

What was actually observed? A laser ever so slightly ceased interfering with itself in a laboratory in Louisiana tiny fractions of a second after a duplicate apparatus experienced a similar ceasing of interference across the United States in Washington. The two facilities had set up long tube-like optical cavities, well shielded from the pervasive vibrations and noise of the outside world that were set at right angles to one another. Lasers were passed through a partially coated mirror called a beam-splitter and sent down these chambers to reflect off a mirror 4 kilometers away. The laser is then recombined and calibrated so that it interferes with itself, cancelling itself out when the system is “at rest” but when a gravity wave passes through the apparatus, it would contract or extend the distance the light travels through on one axis-arm, allowing the light to avoid canceling out whereupon it would be detected by a photometer sensitive to the laser’s wavelength.

The first recorded detection of gravity waves is a world-changing event, akin to the discovery of radio waves from space by Jansky in the 1930’s but perhaps even more fundamental and potentially more revealing. It was greeted warmly by the press but quickly overtaken by the next “big discovery” that made a social media splash and this is a crying shame. In attempting to share nearly every published result to garner support, scientific institution are creating a good deal of publication “noise” that can diminish the “signal” of truly groundbreaking discoveries. It falls to those who report on science and share it with the public to emphasize the good and contextualize the rest.

That being said, those who wrote on the discovery did an excellent job - one example of surprisingly good science journalism in an unexpected place was the Huffington Post, a publication best know for reviews of Starbuck beverages and celebrity gossip. The author of their article, an astrophysicist by the name of Jeffrey Bennett writes: “this discovery is that direct detection opens up an entirely new way of studying the universe.” (Bennett, 2016) Emphasizing that while the subjects observed (large and merging black holes) are certainly sensational, the lion’s share of the discovery is an entirely new way to study the Universe and directly observe phenomena that had been outside our abilities, historically.

In order to better understand the astronomical implications, more studious and academic sources might be required. Yet once again, things are not as they seem. The New York Times published a verbose piece (Overbye, 2016) that is constantly and confusingly expanding the analogy between the observation and audible sound. They do, however, share a graphic and a fair amount of column inches to the black holes themselves, illustrating the method by which they merged and how the merger expended a huge amount of energy, thereby explaining the missing mass/loss of energy as predicted by Einstein nearly a century ago.

Unsurprisingly, the Caltech/LIGO release (LIGO lab, 2016) is full of excellent detail and emphasizes the teammates who contributed to the discovery as well as detailing information and the instrumentation, but I feel there is something of a lack in reporting about the black holes themselves. They do reference a “mathematically accurate model” as being a major factor in confirming their detection but on the release itself there is not a lot of detail about the astronomy. While this is somewhat understandable given their institutional proximity to the apparatus and the science team, it does not seem to be the strongest response.

Massachusetts Institute of Technology (MIT) offered a factual review (Chu, 2016) of the celestial event as well as the detection - highlighting that this was detected shortly after LIGO was technically upgraded into Advanced Laser Interferometer Gravity-wave Observatory (or ALIGO) and that soon, it will be improved so that it will be able to achieve three times the current sensitivity and be able to detect farther and fainter signals. Given the nearly implausible precision of the system today, this is both hard to comprehend and really exciting!

Public Radio International, like MIT, gave due credence to both the discovery teams and their instrument, but was a bit scant on the nature of the black holes. What made their article noteworthy was their exploration of the principles of Einsteinian gravity. They had a very good explanation of the principles behind the gravity waves and how this discovery is simultaneously a confirmation of hundred year old physics and a wonderful “open window” into new astronomical realms. Using the Sun as a familiar “space-time distorter” and including a fun video on the subject is a great example of scaffolding the content for a layperson.

Science and research focused publications seem to be an obvious choice for a trustworthy and detailed write up and the well esteemed journal Nature did just that. Their examination of the result is measured, accurate, well sourced, and well explained. They share the nature and mechanism by which the energy is expended in the generation of the gravity waves and how the energy loss led to the orbital decay and increase in orbital frequency until the black holes themselves were orbiting at nearly half the speed of light for their final five rotations before merging into the single static black hole. Davide Castelvecchi (2016), the author of the Nature article also describes a plausible explanation for the oversize mass of the participant black holes, in that both are a little past the 30 solar mass limit for type II supernovae but that migrating through a rich cloud of material would have given the massive objects a chance to scoop up more material and become more massive. Overall, I would say that the Nature piece has had the best mixture of style and content.

Composing content that is both factual and engaging is a huge challenge, a balancing act of sorts. When this challenge is compounded by a monumental discovery, an amazing team of scientists, further vindication for one of our best loved theoreticians, and a whole new kind of astronomy that enables us to study black holes, this is like a balancing act while carrying the Queen Elizabeth 2’s anchor. It makes me glad to know that there are science communicators out there capable of rising to the challenge and sharing these wonderful new ideas with the world.


References:

Bennett, Jeffrey. "What Are Gravitational Waves, and Why Should You Care?" The Huffington Post. February 18, 2016. Accessed April 23, 2017. http://www.huffingtonpost.com/jeffrey-bennett/what-are-gravitational-waves_b_9253680.html.

Castelvecchi, Davide. "The black-hole collision that reshaped physics." Nature 531, no. 7595 (2016): 428-31. doi:10.1038/531428a.

Daniel, Ari. "Listen to the collision of two black holes. Einstein was right." Public Radio International. February 11, 2016. Accessed April 27, 2017. https://www.pri.org/stories/2016-02-11/listen-collision-two-black-holes-einstein-was-right.

"Gravitational Waves Detected 100 Years After Einstein's Prediction." LIGO Lab | Caltech. Accessed April 24, 2017. https://www.ligo.caltech.edu/news/ligo20160211.

Jennifer Chu | MIT News Office. "Scientists make first direct detection of gravitational waves." MIT News. February 11, 2016. Accessed April 26, 2017. https://news.mit.edu/2016/ligo-first-detection-gravitational-waves-0211.

Belczynski, Krzysztof, Holz, Daniel E., Bulik, Tomasz, O’Shaughnessy, Richard “The first gravitational-wave source from the isolated evolution of two stars in the 40–100 solar mass range”

Nature 534 no. 7608 (2016) 512-515 doi:10.1038/nature18322

Overbye, Dennis. "Gravitational Waves Detected, Confirming Einstein’s Theory." The New York Times. February 11, 2016. Accessed April 23, 2017. https://www.nytimes.com/2016/02/12/science/ligo-gravitational-waves-black-holes-einstein.html.

1 view0 comments

Recent Posts

See All