Category: Physics

10 Years of GW Astronomy: Satisfaction Coupled with Concerns

Ten years ago today, 14 September 2015, in the early hours of an ordinary American morning, an exceptionally strong gravitational wave chirp flew through LIGO‘s Hanford and Louisiana detectors, after having traveled across the Universe at the speed of light for something like a billion years.

Nobody (myself included!) was expecting it, even though almost certainly similar chirps had passed through the two detectors before, given that the detectors had been taking science data on and off since 2002. But this time was different, because LIGO was just restarting after a five-year-long major sensitivity upgrade. It hadn’t been sensitive enough to catch the earlier ones, but it heard this one loud and clear. The arrival of GW150914 therefore marked the dawn of gravitational wave astronomy as a new and promising research field.

This first detection has been followed by well more than 200 further detections. Events are now being registered several times a week. Among the early detections was the profoundly important LIGO-Virgo observation of GW170817, in August 2017: two neutron stars spiraled together, merged, exploded, and taught us where our gold, silver, platinum, and uranium came from. This event led to a continuing collaboration of GW astronomers with other kinds of astronomers, something we call multimessenger astronomy. A few months later, the Nobel Committee confirmed the importance of this new field by awarding the 2017 Physics prize to Barry Barish, Kip Thorne, and Rai Weiss1.

During these ten years, a completely new way of detecting GWs has also made its debut: the pulsar-timing radio-telescope arrays have almost certainly detected a random cosmological background of ultra-low frequency GWs generated by the orbits of countless supermassive black hole binaries. This remarkable first decade of GW astronomy has brought us solid evidence, intertwined and in abundance, of the reality of all of Einstein’s theory’s most radical and unexpected predictions: waves of gravity, black holes, and Big Bang cosmology.

Naturally, the GW community is celebrating this anniversary. For my part, I am joining my good friend Clifford Will and many of our colleagues at a two-day meeting in Palma, Mallorca, which starts tomorrow. We will be looking backwards at what our community has achieved, and forwards at what we should be doing in order to keep this extraordinary momentum going. In this post, I want to muse about the future, because it has such promise and yet has to face possible pitfalls.

The LIGO and Virgo detectors are already significantly more sensitive than ten years ago. Further big upgrades are already being built and tested in labs around the world, in preparation for installation starting next year; the young Japanese KAGRA detector has an ambitious roadmap for improvement; India is building a third LIGO detector; and the space-based LISA gravitational wave mission is in Phase B of construction by the European Space Agency, having been handed over to the prime industrial contractor, on schedule for a 2035 launch. Into the 2040s we can hope for completely new third-generation ground-based detectors in Europe (Einstein Telescope) and the US (Cosmic Explorer), whose designs are already well advanced.

These upcoming instruments carry scientists’ high hopes for new science, unexpected or unpredicted discoveries. One remarkable and somewhat puzzling aspect of observations so far is that all the GWs detected so far originated from binary systems formed of black holes and neutron stars, whose brief death spiral emits a “chirp” of GWs and then falls silent, forever. Many other sources are expected, ranging from nearby asymmetrical spinning neutron stars emitting GWs continuously to distant exotic cosmic strings whose intense gravity emits a sharp burst of GWs when two of them happen to encounter one another. We apparently need even better sensitivity to detect the former; and we might just have to get lucky for the latter.

Future observations also should ultimately return the best-ever measurements of the expansion rate and acceleration of the Universe, values which are still a lot more uncertain than they should be. Their uncertainly lies in disagreements between values obtained by different methods, and I hope that the gravitational wave “standard siren” method will help resolve this, because it uses such different kinds of information than other methods rely on.

But all of this hope for the future is now hedged by uncertainties. One of the biggest is, of course, American political decisions about supporting science. We have seen devastating cuts in health research, including the halting of life-saving work mid-stream. The threats to space and ground-based science are also scary, although final decisions have not yet been made. There is talk of withdrawing NASA’s 20% contribution to LISA, as well as reducing LIGO to one detector, which would destroy its science return; LIGO itself has said it hopes that, if such a cut were made, then it could operate both detectors on a much reduced budget, presumably stopping the development of future upgrades and relying on university partners for keeping things going. We are hoping that Republican members of Congress will understand the importance of continuity in basic research and will vote that way, but the outlook is very uncertain. And that puzzles and distresses me, because besides being a Nobel-winning flagship for fundamental science — an endeavor that has captured large chunks of the public imagination –, LIGO and its partners have been wonderful training grounds for many hundreds of young scientists who eventually have taken their skills into industry. In the worldwide collaboration, they learned rigorous science, they learned how to push the frontiers of measurement time and again, and they learned how to work successfully and rewardingly in a huge international team. What could be better raw material for 21st century industry?

The other concern I have is that much of the future science return from our network, even if LIGO remains well funded, depends on having three detectors of comparable sensitivity, so that the sky positions of sources can be triangulated. With only two detectors, as we had for GW150914, we have big errors in the estimates of the masses and distances of sources, because our detectors have different sensitivity in different directions. We currently observe mainly with three detectors, the two LIGOs and Virgo. But Virgo has not been able to match LIGO’s sensitivity, typically observing with only 1/3 of LIGO’s sensitivity. This affects the number of events we can reliably detect, but even more it gives us poor direction-finding, which makes all the other measurements (masses, distances, spins) more uncertain. With time and with new contributions from KAGRA and LIGO-India, we can hopefully relieve this problem, but in the near term it stretches out the time it will take the network to reach any particular measurement goal, and makes the science output even more vulnerable to political issues over funding in the future.

Virgo’s problems do not come from a lack of talented scientists, although they may be a bit understaffed. Rather, it shows how incredibly difficult it is to build these instruments, so much so that small differences can have large negative effects. These are the most precise measurements ever made by humans, and getting within one-third of the best attempts is no mean feat. One of Virgo’s disadvantages was that it started ten or more years later than the teams that eventually merged into LIGO, and because of this they decided to go straight from the drawing board to the 3-km detector without operating an intermediate prototype stage for training and learning. But in my view, which I have expressed publicly on other occasions, there was another fateful decision at the time Virgo was organizing itself: it formed up as a collaboration of a number of research institutes, with no strong central management structure. It is led by a ‘spokesperson’, as is typical of particle-physics experiments, rather than by a ‘director’, which LIGO has, as do other successful European international science projects, such as ESA, ESO and the SKA. In fact, despite the heritage of the Virgo organizational model in particle physics, CERN itself also has a director, steering a strong management structure that has ensured that accelerator after accelerator has reached its design goal, making CERN the most important accelerator lab in the world. The Virgo organization model inevitably takes longer to make hard decisions, and design decisions can be weighted by inter-institute political and budgetary considerations rather than just by what is technically and financially optimal.

If this were a problem only affecting Virgo, then we could hope that with time it would get resolved, or at least would become less important as KAGRA and LIGO-India near their design goals. But the problem might run on longer than that: the next-generation European Einstein Telescope may soon receive its initial funding, but the organizational structure of its current collaboration looks to me to be just as decentralized as that of Virgo. As we look past 2040, there may again only be the two detector projects mentioned above for the third generation, and if Europe does not learn from the difficulties that Virgo has had to deal with, then it might endanger a truly golden chance to probe the Universe more deeply and more fundamentally than can even be conceived of today.

So as we celebrate what we accomplished ten years ago, I hope we will also learn lessons from the succeeding ten years about how we can optimize our research future … if the politicians don’t succeed in taking away that future!

  1. I dedicate this post to Rai, who passed away less than three weeks ago. LIGO was his conception, and he worked tirelessly for over 40 years to realize it. His modesty, mentorship, and good humor have strongly influenced the character of the worldwide GW collaboration. It is still difficult to process that he is no longer with us. ↩︎

Eddington in Cardiff 100 years ago today: the first proposal that stars are powered by fusion

“… fulfilment [of] our dream of controlling this latent power for the well-being of the human race – or for its suicide.”

One hundred years ago today, on 24 August 1920, with over 1000 people gathered in Cardiff for the annual meeting of the British Association, Arthur Eddington gave his address as the incoming president of the physical and mathematical sciences section. He elected to speak on the subject of the “Internal Constitution of the Stars”. When I first came across the text of the address last year (published in Nature in 1920), I was amazed to find as early as this such an insightful proposal that stars are powered by the synthesis of helium from hydrogen. But what really brought me up short was this sentence:

If, indeed, the sub-atomic energy in the stars is being freely used to maintain their great furnaces, it seems to bring a little nearer to fulfilment our dream of controlling this latent power for the well-being of the human race – or for its suicide.

Twelve years before Chadwick discovered the neutron, twenty-five years before Hiroshima, thirty-seven years before the nations of the world agreed to cooperate to make peaceful fusion power a reality, Eddington basically saw the whole package. 

What Eddington had learned in 1920 — and it was enough to open the whole vista to him — was the rest-mass deficit:

The nucleus of the helium atom, for example, consists of four hydrogen atoms bound with two electrons. But Aston has further shown conclusively that the mass of the helium atom is less than the sum of the masses of the four hydrogen atoms which enter into it; and in this, at any rate, the chemists agree with him. There is a loss of mass in the synthesis amounting to about 1 part in 120, the atomic weight of hydrogen being and that of 1·008 and that of helium just 4.

Francis W. Aston in fact presented this (Nobel-Prize winning) result at the Cardiff meeting the following day, as recorded in the meeting’s archives. So Eddington the relativist had what he needed: energy would necessarily be released if four hydrogen atoms could be persuaded to combine to form a helium nucleus. How they might decide to do this, and then how the helium nucleus might contrive to stay together against the electrostatic repulsion that the four protons exerted on one another, offset by only two electrons: these were not things that Eddington even talked about. Eddington knew that answering these questions wasn’t necessary at this point. All that mattered was that the mass deficit, converted into energy, was ample to power the stars:

If 5 per cent. of a star’s mass consists initially of hydrogen atoms, which are gradually being combined to form more complex elements, the total heat liberated will more than suffice for our demands, and we need look no further for the source of a star’s energy.

Notice that Eddington is not stopping just with helium. He expects this process will synthesise even heavier elements, although he observes that the energy payoff is not so dramatic as for the conversion of hydrogen into helium.

Apparently most people in 1920 still seemed to believe in Kelvin’s hypothesis that stellar contraction and the liberation of gravitational energy should power the stars. To prepare for his nuclear fusion hypothesis, Eddington earlier in the article ruthlessly destroys this idea, particularly pointing out that the short lifetime of stars implied by Kelvin would already have produced observable changes in the pulsation periods of some Cephied variable stars. Anyway, he says, most scientists no longer take Kelvin’s idea seriously, even though they have nothing to replace it with:

Lord Kelvin’s date of the creation of the sun is treated with no more respect than Archbishop Ussher’s.

Mindful that, even so, his audience may not be prepared to switch to the nuclear hypothesis so readily, he acknowledges

I should not be surprised if it is whispered that this address has at times verged on being a little bit speculative … 

and then he goes on to defend the role of speculation in theoretical physics, including pointing out that even wrong speculations can help advance a field if they motivate experiments that clarify a subject.

Eddington’s address covers much more than just how stars shine. He starts by trying to establish the track in the Hertzsprung-Russel diagram that stars follow as they evolve. This is not what we understand today, although it was a hugely important step in his time: he thinks giant stars are young stars that evolve into a Sun-like stage and then become dwarfs. It would take a good number of years before the theory of nucleosynthesis in stars would lead to the more complicated evolutionary tracks we are familiar with today. And speaking of dwarfs, the story is well-known of how, fifteen years later, Eddington arrogantly rejected the perfectly sound calculations of a young Chandrasekhar, who had had the temerity to suggest that dwarfs had a maximum mass, so that evolutionary tracks of very massive stars had to lead elsewhere — Eddington was not having anything to do with what we now call black holes. Maybe speculation had lost its lustre in those fifteen years!

Eddington of course published a book with the same name as the lecture, the first edition appearing in 1926 and covering much the same material, but more quantitatively and extensively. So his Cardiff presidential address was just the taster. But what a taster! 

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Eddington’s address was printed as a news item in Nature a week later: vol 106, p. 14. It is freely available from Nature here. The book of the same name was published by Cambridge University Press, which now has an online edition available, with an introduction by none other than Chandrasekhar. Eddington in his book also credits J. Perrin for independently coming to the same conclusion about the fusion of hydrogen to helium, also in 1920: Revue du Mois 21, 113.

Black Lives Matter to Physics

A racist attack on a black person is an attack on the core of America. White Americans need to understand that it is an attack on them too.

Today I am joining the Strike for Black Lives at the urging of the particle physics movement Particles for Justice. Here I want not just to show my support publicly, but to share my concern that this issue, which I thought way back in 1965 we were well on the way to eliminating, continues to poison American society. Let me summarise where I am going by saying that thinking about this struggle as just an issue of failed justice, as the particle physicists’ webpage name suggests, is not going to be enough to save black lives in the future.

We all need to understand that what happens to black people happens to America. Killing black people harms the whole society. I am afraid that the demonstrations of the past days won’t accomplish their aim if the struggle is framed as one of protecting and bringing justice to a minority. The frame must be bigger. Americans need to begin to see black people as a cross section of America. Simply, black people embody America.

Before explaining my point of view, I want to start with the situation in physics. I started my PhD in 1967, and there were no black Americans in my cohort. There was also only one woman. Today there are significant numbers of women physicists (although in the US it is far short of 50%), but many fewer black Americans. There are also plenty of immigrants, many with dark skin from South Asia. Now, physics may be esoteric to many but it carries high prestige. When physicists are interviewed on TV or give advice to government, what they say is important, but so is the metadata: these white men, these women, these immigrants are making important contributions to society. The paucity of black Americans in these forums certainly does nothing to change the minds of people who think that black lives don’t much matter. Physics is failing here. And this doesn’t even mention how we are failing the young blacks who – just as we ourselves were – are turned on by the subject, but who – unlike us – won’t get the chance to go for it.

The disparity in the progress of women and black Americans in physics between 1967 and today contains a bitter irony. The women’s movement for equal treatment took huge inspiration from the black civil rights movement of the 1960s. The Civil Rights Act of 1968 prohibited discrimination on the basis of race, religion or national origin. It wasn’t until 1974 that it was amended to include sex in that list. Protection against discrimination for people with disabilities and for gay people followed even later.

This takes us to the core of my concern. In the words of Pulitzer Prize winner Nikole Hannah-Jones in last year’s podcast series 1619 (which I highly recommend) on American slavery, “It is black people who have been the perfectors of democracy in this country.” They haven’t taken democracy for granted they way most of us whites have (and now realize that we shouldn’t have). If there were two things that America has, at least until recently, been admired abroad for, they were its dedication to democracy and its creative, ebullient musical culture. America owes both to its black population.

Black Americans have roots in their country that go much, much further back in time than do those of most white Americans. The ancestors of any black American, taken together, probably did more to build America than my ancestors did, more than the ancestors of almost any other American who claims German or Irish or Scandinavian or Italian or Polish heritage. If any group could be said to own America, black Americans have the second-best claim, after native Americans. But of course neither group runs the place.

And then there is the question of what we mean when we talk about black Americans. There is no well-defined distinction between black Americans and the rest of us. Their ancestors’ heritage in Africa was just as culturally diverse as that of the later European immigrants. And as geneticists have learned, their ancestors in Africa were genetically more diverse from one another than any European is from any other non-African, let alone from other Europeans. And then of course most black Americans have European genes as well, from a long history of rape. Uncomfortable as that may be to whites, what it means is that black Americans are the best example America has of the melting pot. Slaves were brought to America and thrown together with greater differences than our European ancestors had in languages, customs, skin colors, stature, lip shapes, nose shapes — you name it.

From all this variety, black Americans built much of America, gave it its music, led the defence of its democracy. We white Americans must understand, no – we must have respect for, how central to the core of America our fellow black citizens are. There is a political battle going on in America right now over who defines the country, and very clearly democracy and the rule of law are under threat. Science is also clearly under attack by the same forces, already partly forced to toe the political line. No people in America has a greater interest in preserving America’s core values than the black population. It is not enough for us whites or us physicists just to want to help people who are beleaguered. A racist attack on a black person is an attack on the core of America. White Americans need to understand that it is an attack on them too.

Black people embody America. Unless we whites understand that, I fear that there won’t be enough momentum to change once and for all the deeply racist institutions that over the years have kept their power, that have kept denying the important fact that black lives matter. BLM matters to democracy, to science, to everyone.

(Image of Washington DC and the White House courtesy of Planet.)