Friday, March 25, 2016

The Superb LIGO Press Conference Announcing the Direct Detection of Gravitational Waves

The recent announcement that gravitational waves had been directly detected was one of the most exciting and important events in the history of astrophysics. I watched the press conference with dozens of other people at the Harvard-Smithsonian Center for Astrophysics (CfA), and we enjoyed sharing the excitement of the Laser Interferometer Gravitational-wave Observatory (LIGO) team. I was tweeting so quickly that my hands started to get sore.

For a long time I’d been skeptical that they would ever make this detection, as the technical challenges are incredibly formidable. It wasn’t until team member Daniel Holz visited CfA for a talk and discussed LIGO’s progress that I started to think they could actually do it. I chatted with Holz after his talk about how the announcement of a detection might be made, and since then I’d wondered: would the quality of the result’s communication be as strong as the result itself? On February 11th, 2016 I found out that the answer was a resounding yes.

I think the press conference to announce the LIGO detection was superb, perhaps the best that I’ve ever seen. I can give my opinion with some authority as I've helped organize many press conferences for the Chandra X-ray Observatory. David Reitze, Gabriela (Gaby) Gonzalez, Rainer Weiss and Kip Thorne all did excellent jobs. In full disclosure, I have previously worked with the press officer Whitney Clavin who organized the press conference, but I had already made most of the notes for this blog post before I found this out.  Congratulations to Whitney and her communications team, including graphics experts Robert Hurt and Tim Pyle, for doing such a good job, as I describe in detail below. Of course, this communication effort would not have occurred without the astonishing and extremely important achievement by the entire LIGO team. Experimental mastery, theoretical expertise and communications prowess have combined here in a powerful way.

Below I describe some of the many highlights of the press conference and then discuss the analogy of sound being used to explain gravitational waves. I finish with criticism of the Nature Physics editorial that made derogatory comments about the LIGO press conference.

Figure 1: The LIGO Hanford Observatory. Credit: Caltech/MIT/LIGO Observatory.

The LIGO press conference highlights
  • The press conference kept viewers in suspense for a few minutes at the start of the press conference, with France Cordova’s introduction and a short video including hints that they’d been successful. Then the official announcement was made. With our Chandra press conferences we usually jump to the announcement quickly, to avoid losing viewers, however this result was so important that it transcended typical practice.
  • The actual announcement of the detection by David Reitze was clear, simple and deliberate: “Ladies and Gentleman, we have detected gravitational waves. We did it.”

Figure 2: David Reitze, Executive Director of the LIGO project, making the announcement that they had detected gravitational waves - “We did it” - in a screen capture from the press conference. Credit: Caltech/MIT/LIGO Observatory.
  • There was a clear feeling of celebration amongst the panelists, including Reitze briefly throwing his arms upwards in victory just after making the announcement of a detection, and a lot of hugging between panelists. All of this was perfectly appropriate and humanizing.
  • Reitze quickly emphasized that a lot of checking was done before they announced the result, and he also briefly described the great future ahead for gravitational wave astronomy 
  • Simple language was used, especially early on, as emphasized in a Forbes article: “How The Epic Discovery Of Gravitational Waves Was Brilliantly Communicated”, by Carmine Gallo.
  • The structure of the whole press conference was excellent, with the punch-line early, followed by a good executive summary, followed by filling in details about the instrument and the history later on. This structure resembled a good press release or news article. 
  • They did the publicity after a paper describing the main result was accepted for publication, unlike some recent press conferences, such as the one for BICEP2. 
  • They emphasized the superlatives and there were a lot of them, like “first” and “proof” and “mind-boggling” and “scientific moon-shot”. 
  • They used excellent graphics and props, including a good mixture of helpful, intuitive animations and straightforward graphics, to explain what gravitational waves are, how they affect objects and what was oberved. I like how Reitze showed a view of the black hole merger as a nearby viewer would see them, followed by a version showing the gravitational waves. Later on the animation shown by Rainer Weiss to explain how LIGO detects gravitational waves was particularly good.
  • Gaby Gonzalez did an excellent job explaining what I think was the key scientific figure in the press conference (see Figure 3 for more details).

Figure 3: The figure shown by Gaby Gonzalez demonstrating that they had really detected gravitational waves, because a very similar signal was seen at both observatories, separated in time by ~7 ms, roughly equal to the light travel time between the two observatories. As Gonzalez said about this figure, another screen capture from the press conference, “This is it. That’s how we know we have gravitational waves”. At CfA this figure drew gasps and cries of “whoa” from the audience. Credit: Caltech/MIT/LIGO Observatory.
  • They had a server that didn’t, as fair as I know, crash during the webcast. It was smart that they were prepared for a big following, despite the leaked rumors on Twitter by Lawrence Krauss (here and here) and in Science Magazine. They encouraged group watches of the press conference, presumably to help limit the risk of server crashes. It was also more fun to watch that way too. At CfA it felt a bit like watching an exciting sports event, with a bunch of parochial supporters. The atmosphere was electric and appropriately so.
  • They discussed some “gee whiz” aspects, including the huge amount of power produced when three solar masses is suddenly converted into gravitational waves: during the collision the total power output in the gravitational waves was 50 times greater, for that brief period, than all of the power produced by all of the stars in the universe put together. I also liked the analogy Reitze gave for explaining the sensitivity of their measurement:
“..if we were trying to measure the distance between the Sun and the nearest star, which is about 3 and a quarter light years away, LIGO is capable of measuring that, if it could do that, to the level of about the width of a human hair." 
That’s extraordinary, though it would have been even better for a general audience to give the distance in miles or kilometers – amounting to 30 trillion kilometers, or 30,000 billion kilometers – rather than light years.
  •  They didn’t use cliches like “Holy Grail” or “smoking gun”, even though it must have been tempting to do so, as such phrases were much more appropriate here than for any other science press conference I can recall.
  • There was a sense of humor at times, like Weiss’s remarks that Einstein could have built LIGO back in 1916 if the technology had been available: “He was smart enough and he knew enough physics. He wasn’t just a theorist”. This got a good laugh from the other panelists, then the audience. Weiss then immediately introduced Kip Thorne “who is a theorist, but really also an experimenter”. Again, laughter.
  • Gaby Gonzalez carefully and graciously acknowledged the large number of people who worked on LIGO.
  • There was a very clear explanation who the founders of LIGO are, Ronald Drever, Kip Thorne and Rainer Weiss, who will be hot favorites to receive the Nobel Prize in Physics where the limit, probably not coincidentally, is three Laureates in a given year. If I was on the Nobel Committee I would be pushing to award the prize to them later this year, or as soon as possible.
  •  The speakers provided clear comments on how their results would improve in the future, including the much better positions for events that will come from adding extra observatories similar to the two that already exist, but at different locations. This is crucial for follow-up searches for counterparts using observatories like NASA’s Chandra X-ray Observatory, ESA’s XMM-Newton Observatory or the Australia Telescope Compact Array, as suggested in searching for the possible counterpart to the newly discovered merger of two black holes.

The gravitational waves as sound analogy

Many of the words and graphics used by the panelists in this press conference can be adopted by speakers in scientific and especially in public talks about gravitational waves. One exception to this recommendation is that I think the panelists could have been clearer in explaining that it is an analogy to describe gravitational waves as sound. This analogy is vivid and has a couple of key strengths, as explained to me by LIGO team member and former CfA colleague Vicky Kalogera:
1. "The frequencies LIGO is sensitive to are in the same range human ears are sensitive to, so there is an “easy” conversion of LIGO signals into sounds one can make." 
2. "More importantly, “regular” (ie most electromagnetic) telescopes are pointing instruments, like our eye. In contrast the LIGO detectors work like human ears: LIGO can detect signals from above, below, back, and front." 
Despite these key strengths this is still only an analogy: gravitational waves are very different from sound waves, e.g. the former are transverse waves, or ripples in space-time and the latter are longitudinal waves traveling through a medium, such as air or water; the former can travel through the near-perfect vacuum of space and through dense objects and the latter cannot. Also, in a terrestrial setting the speed of propagation for these waves in very different, by a factor of almost a million.

To explain how the sound analogy was introduced in the press conference, here’s a quote from Reitze’s presentation:

“What LIGO does is that it actually takes these vibrations in space-time, these ripples in space-time and it records them on a photo-detector and you can actually hear them. So, what LIGO has done, it’s the first time that the universe has spoken to us through gravitational waves. And this is remarkable, up to now we’ve been deaf to gravitational waves, but today we are able to hear them. That’s just amazing to me. I think this is big, again because what’s coming now is we’re going to be able to hear more of these things. And no doubt we’ll hear things that we expected to hear, like binary black holes or binary neutron stars colliding, but we will also hear things we never expected.”

He correctly describes the vibrations as being in space-time, but then proceeds to describe gravitational waves as sound without being clear that this is an analogy. The people who may not know this is an analogy probably overlap a lot with those who don’t know what space-time or a photo-detector is, so his introduction might not have helped much. Later on Gaby Gonzalez played the “chirp” from the merger, without a clear explanation that the gravitational waves were converted into sound, which didn’t help.

Was this a problem for a general audience, who would generally hear about the result through the press, where further simplifications often occur? I didn’t make a systematic analysis of the press articles, but for an example, this is how Dennis Overbye started his page one New York Times article about the discovery:

“A team of scientists announced on Thursday that they had heard and recorded the sound of two black holes colliding a billion light-years away, a fleeting chirp that fulfilled the last prediction of Einstein’s general theory of relativity.”

This is misleading for non-experts. Also, the title for the video by Overbye is “LIGO Hears Gravitational Waves Einstein Predicted”. 

Figure 4: Page one of the New York Times on Friday, February 12th, 2016. Credit: New York Times and the Newseum.

For another example, the second paragraph of the page one Boston Globe story said:

“At 5:51 a.m. on Sept 14, a new, highly advanced observatory in Louisiana heard the explosion as it passed the Earth in ripples known as gravitational waves, a high-pitched chirp in the steady hum of the universe. Seven milliseconds later, the distinct sound was registered at a sister observatory in Washington.”

Only minor changes are needed to explain that gravitational waves are not the same as sound waves. For example watch Brian Greene’s appearance on “Late Night with Stephen Colbert”. First he does a great job, comparable in standard to the LIGO panelists, at explaining how gravitational waves distort objects as they pass through them. Then he introduces the sound analogy in the following way:

Brian Greene: “In fact you can actually in some sense hear the gravitational waves. They vibrate at a frequency that if you turn it into sound the human ear can hear.

Stephen Colbert: “So literally these waves can be turned into sound.”

Brian Greene: “They can”.

Of course, there’s no guarantee that writers will explain the analogy correctly because of this subtle change, but it gives them a better chance. Also, it’s not vital that non-experts understand the difference between gravitational waves and sound waves, however it certainly doesn’t hurt.

Another, related issue − that I never would have thought of myself − is that talking about “hearing” the universe and formerly being deaf to it, is ableist language. See this interesting discussion on Twitter led by Chanda Prescod-Weinstein.

In defense of the press conference panelists, I’ll note that the analogy has been used by scientists before. For example Janna Levin used it extensively in her TED talk “The sound the universe makes”. This talk included some brilliant explanations, such as a description of a journey into a black hole, but I didn't feel completely comfortable with heavy use of the sound analogy, placing it on my personal radar.

In science communication I think it’s useful to discuss the care that should be taken with using analogies, especially a cool example like this one, and how these can end up being distorted. A second, loosely-used analogy is saying that astronomers listen to cosmic phenomena using radio telescopes. Besides being potentially misleading, another problem for these two analogies with sound is that there are real examples of sound in space. To name three, sound exists in oscillating stars, in galaxy clusters and it existed in the early universe. I do have a bias here as I used to do research in oscillating stars and I helped organize a press conference publicizing the incredibly deep note in the Perseus galaxy cluster, however my point remains. Again, only small changes are needed.

A Nature Physics editorial criticizing LIGO’s communication

As noted above I was very impressed with the LIGO press conference. I was much less impressed with an editorial – here is a publicly available copy – that recently appeared in Nature Physics about LIGO’s communication efforts. (This contrasts strongly with the excellent coverage of the gravitational waves result itself by the Nature News team, including Davide Castelvecchi.) Here’s one excerpt from the Nature Physics editorial:

“Of course, the announcement, made on 11 February 2016, immediately hit the headlines. “We did it”, stated David H. Reitze, executive director of LIGO. The excitement was palpable. Some of us cried. But the public’s response was largely summed up by the satirical news source, The Daily Mash, with their headline: “Scientists completely fail to explain ‘gravitational waves’”.”

I disagree strongly with the final quote in this statement and it’s telling to note that a satirical news source was used. The editorial goes on to say “And it isn’t difficult to explain” by giving an explanation that is much less clear, for non-experts, than the one that the LIGO panelists gave. For example, nowhere does the Nature Physics article mention how the gravitational waves stretch and shrink objects as these ripples in space-time travel outwards from their source. The editorial also discusses fields and charges. Again, not very clear for a general audience.

The editorial then says:

“But what is truly mind-blowing is that not one of the telescopes operating at electromagnetic wavelengths has detected a counterpart event.”

This statement isn’t completely correct because a possible counterpart has been found with the Fermi Gamma-ray Burst Monitor, one that is plausible enough to motivate quick papers by smart theorists like Avi Loeb and Stan Woosley to explain this unexpected phenomenon, if true (the papers are here and here, respectively).

The editorial finishes by mentioning the prospects for detecting gravitational waves from just after the Big Bang, and concludes with:

“In the meantime, we should learn to explain the physics of these spectacular events to non-physicists.”

No, because we already have.

Friday, February 12, 2016

Exciting News: Direct Detection of Gravitational Radiation

(Note: this blog post was first published at the Chandra X-ray Observatory blog and was mostly written before the LIGO press conference on Feb 11th.)

It's a fitting coincidence. Just a few months after celebrating the 100th anniversary of Einstein's theory of General Relativity (GR), we have just heard that gravitational waves, a key prediction of GR, have been directly detected for the first time. The February 11th, 2016 announcement by the Laser Interferometry Gravitational-Wave Observatory (LIGO) team is one of the most important moments in the history of astrophysics. Here, I discuss how observations with NASA’s Chandra X-ray Observatory and other traditional observatories help complement the detection and study of gravitational waves.

Figure 1: The LIGO Hanford Observatory. Credit: Caltech/MIT/LIGO Observatory

Gravitational waves are produced by violent events, such as the collisions and mergers of neutron star or black hole pairs, or the collapse and explosion of massive stars in supernovas.  As a September 2015 news release by LIGO eloquently explains,

“These events are so cataclysmic that when they occur, they cause the very fabric of space itself to vibrate like a drum. The waves of rippling space-time emanate in every direction, traveling at the speed of light, physically distorting everything in their paths.

Indirect evidence for gravitational waves had been found before. The most famous example earned a Nobel Prize for Russell Hulse and Joseph Taylor, who discovered a pair of neutron stars in close orbit. The shrinking separation between these two stars is precisely explained by energy lost as gravitational waves are emitted. An example observed with Chandra involves the shrinking separation between a pair of white dwarf stars.

Figure 2: Artist's concept of gravitational wave propagation from the close orbit of two compact stars. Credit: R. Hurt: Caltech/JPL

Although the observation of gravitational waves has opened up a completely new field of astrophysics, separate from the studies of electromagnetic radiation that we’re so familiar with, the technical challenges for direct detection of gravitational waves are formidable. As the same LIGO new release states about gravitational waves:

“… the farther they travel from their source, the smaller and smaller the ripples become until, by the time they reach the Earth, the spatial distortion caused by the waves is on the order of a billionth the diameter of an atom! This unimaginably small movement is what LIGO’s detectors are designed to sense.”

Clearly, the direct detection of gravitational waves is a remarkable achievement and easily deserving of a Nobel Prize. However, it’s important to point out that the combination of gravitational waves and data from electromagnetic radiation (a.k.a., light) will provide the most powerful astrophysics in the future.

Observations with traditional observatories have been crucial in identifying the types and abundance of objects that should produce gravitational waves. As mentioned earlier, key targets for detecting these waves are collisions and mergers between two black holes. LIGO is sensitive to mergers between stellar-mass black holes, which have been observed to weigh between about five and twenty five times the mass of the Sun but, as LIGO has shown, they can be heavier. Until LIGO's recent observation, a black hole merger had never been observed. However, mergers between two neutron stars or between a black hole and a neutron star have likely been observed before with the detection of so-called “short gamma-ray bursts” using, e.g. NASA’s Swift satellite and following up with Chandra and other observatories. These events, along with longer lasting gamma-ray bursts from the collapse of massive stars, have generally been considered to form a black hole.

Recent work suggests that the phenomenon of gamma-ray bursts may be more complicated than previously thought. Observations with Chandra and other X-ray observatories suggest that a significant fraction of gamma-ray bursts might be caused by the formation of neutron stars with very strong magnetic fields, known as magnetars. This would mean that the contribution of magnetars to gravitational wave signals would be larger than previously thought.

Double stars are not the only objects expected to produce gravitational waves. Observations of the Vela pulsar suggest that this neutron star is precessing as it spins. This may imply that the neutron star is slightly distorted, making it a persistent source of gravitational waves and a prime target for future detectors of these space-time ripples.

As explained by the LIGO team, the astrophysics enabled by their instrument will address science questions such as “how abundant are stellar-mass black holes?”, “what is the central engine driving gamma-ray bursts?” and “what happens when a massive star collapses?” Just as significantly, they will provide unprecedented tests of GR. However, LIGO will not be sensitive to slower, but even more powerful events such as mergers between two supermassive black holes. For these events, projects such as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) and the Parkes Pulsar Timing Array, are needed. These exciting projects are both in operation, but have yet to make a detection of gravitational waves.

As context for this work, Chandra has detected several pairs of supermassive black holes in single or merging galaxies, including work released in 2002, 2006 and 2010. These black hole pairs may merge and produce copious amounts of gravitational radiation in millions or billions of years. These events won’t be observable by us, but give astronomers an idea of how common supermassive black hole pairs are.

Going further, astronomers have studied one exotic, possible outcome of supermassive black hole mergers. Using Chandra, the Hubble Space Telescope, and ground-based telescopes, astronomers have found evidence for a supermassive black hole that is being ejected from its host galaxy. The black hole may have collided and merged with another black hole and then received a powerful recoil kick because more gravitational waves were emitted in one direction than another.  If this mechanism is indeed the correct explanation then, as theorist Avi Loeb explained in a blog post for us, this black hole provided the “first observational validation of Einstein's equations in the unexplored regime of dynamical strong gravity, which is responsible for gravitational wave kicks.”

Figure 3: Subrahmanyan Chandrasekhar. Credit: AIP

Beyond the science, there is one special, historical connection between the Chandra X-ray Observatory and gravitational waves. The observatory was named after Nobel Laureate Subrahmanyan Chandrasekhar, shown in Figure 3, who did important theoretical work on gravitational waves, beginning in 1970 and extending right up until the year of his death in 1995. It’s apt that the observatory named after him has gone on to observe so many objects that are producing – or will produce – gravitational waves. Theorists like Chandrasekhar have laid the foundation, now it’s time for the observers to surge forward with more groundbreaking science.