From Sky and Telescope: “About The LIGO Gravitational-Wave Rumor. . .” The Best Article on this Subject

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Sky & Telescope

January 13, 2016
Shannon Hall

The physics and astronomy world is agossip with a rumor: has LIGO heard its first black-hole merger?

Caltech Ligo
MIT/Caltech Advanced aLIGO

Rumors are swarming on social media that the newly upgraded LIGO, the Advanced Laser Interferometer Gravitational-Wave Observatory or aLIGO, has finally seen the gravitational-wave signature of two stellar-mass black holes spiraling together and merging. Maybe even two such events since September. Or not.

Such an observation would not only confirm one of the most elusive predictions of [Albert] Einstein’s general theory of relativity, it would open a new field of cosmic observation: gravitational-wave astronomy.

Temp 1
Artist’s concept of gravitational waves produced by closely orbiting black holes in a 2-dimensional sheet. K. Thorne (Caltech)/ T. Carnahan (NASA GSFC)

First, the background: According to general relativity, any accelerating mass should produce weak ripples in the fabric of spacetime itself. But it would take enormous, dense masses accelerating extremely fast to emit a significant amount of them. Neutron stars or black holes spiraling together and merging would qualify, and LIGO was built with those events particularly in mind.

Simulation of gravitational lensing by a black hole, which distorts the image of a galaxy in the background.

Radiation from the pulsar PSR B1509-58, a rapidly spinning neutron star, makes nearby gas glow in X-rays (gold, from [NASA]Chandra) and illuminates the rest of the nebula, here seen in infrared (blue and red, from [NASA] WISE)

NASA Chandra Telescope

NASA Wise Telescope

As gravitational waves pass by, they stress and compress time and distance. But after travelling millions of light-years across the universe, they would be extremely weak. The typical expected signal strength would stretch and squeeze the distance from the Earth to the Sun, for instance, by the width of a hydrogen atom. Yet even that weak an effect could be detected by the laser beams bouncing back and forth along LIGO’s 4-kilometer vacuum pipes. It would be the first direct detection of gravitational radiation. (We already know it exists by its indirect effect of draining orbital energy away from close neutron-star binaries.) A Nobel Prize probably awaits the first direct observation. If it ever happens.

The tunnel for one of the LIGO arms in Livingston, Louisiana. Having two units nearly 2,000 miles apart provides essential error checking and would help triangulate to find the incoming direction of any gravitational waves. A third detector in Italy, named VIRGO, is scheduled to join the network.

Advanced Virgo

Such a feat “will open up a new window into the way we see the universe,” says astronomer Tanaka Takamitsu (Stonybrook University). Take gamma-ray bursts, for instance. These are quick, incredibly powerful explosions that are presumed to come, in some cases, from a pair of neutron stars spiraling together and merging, and in other cases from the fraction-of-a-second disruption of a dying star’s neutron-star-like core. Both kinds of cataclysm should be violent enough to send detectable gravitational waves far across the universe. “If we could see such events from gravitational-wave and conventional telescopes [both], then we can learn a lot more about the physics and what’s really going on with those events,” says Takamitsu.

Still, the rumors remain just rumors. And they’re really bothering the LIGO people.

Gravitational Whispers

The gossip started spreading in physics circles just a week after the upgraded aLIGO began running in September. The rumors escaped from physics circles when cosmologist Lawrence Krauss (Arizona State University) tweeted about them on September 25th: “Rumor of a gravitational wave detection at LIGO detector. Amazing if true. Will post details if it survives.” More recently he commented that he’s 60% sure the story will pan out. Yesterday he noted the caveat that he is not one of the 900-plus members of the LIGO scientific collaboration, nor does he represent anyone there.

Steinn Sigurdsson (Pennsylvania State University), who has also speculated on the rumors via social media, says “I have absolutely no inside information on what is going on. I hear stories, I can make inferences, I can see patterns in activity. And there has been a consistent whisper for several months now that [aLIGO] saw something as soon as they turned it on.”

Researchers work on a LIGO detector in Livingston in 2014. Michael Fyffe/LIGO

Those whispers grew to a lively babble after further tantalizing clues. First, Sigurdsson points to a flurry of papers that have appeared this week on the arXiv preprint server that were curiously specific. Astronomers, says Sigurdsson, “posted somewhat different scenarios for ways in which you could have black hole binaries form, all of which coincidentally predicted almost the exact same final configuration, and said ‘Gosh our model predicted that this very specific sort of thing will be the most likely thing that LIGO sees.’ ” And Sigurdsson isn’t the only one who has noticed. Derek Fox (Pennsylvania State University) pointed to one paper, for example, tweeting “this seems a rather specific GW [gravitational wave] scenario to pull out of thin air?”

Temp 1
The meeting of the arms. The light pipes and the equipment in their ends (seen here) are kept in an ultrahigh vacuum.

But again, Lawrence, Sigurdsson, and Takamitsu claim to have no privileged information. “It’s the equivalent of watching for pizza deliveries at the Pentagon,” says Sigurdsson. He’s referring to the open-source intelligence technique that Washington reporters reportedly used to spot when big events were about to emerge based on the number of late-night pizzas delivered to the White House. “You can play the same game with physicists,” he says. (Unfortunately there have been no reports of LIGO ordering an overabundance of Dominos.)

Second, it’s a small community. So when a few collaborators — who all happen to be members of LIGO — duck out of a future conference due to new overlapping commitments, it doesn’t go unnoticed. A similar pattern played out right before physicists announced the discovery of the Higgs boson.

Higgs Boson Event
Possible Higgs event.

Based on dates cancelled, Sigurdsson speculates that an announcement will come from the team on February 11th. Takamitsu, however, speculates that it will take months.

Details of the supposed detection, however, were not publicly bandied about until Monday, when theoretical physicist Luboš Motl posted on his blog the latest version of the rumor: that aLIGO has picked up waves produced by two colliding black holes each with 10 or more solar masses. He also said he’s been told that two events have been detected.

Reason for Silence

There’s a good reason why LIGO’s people refuse to confirm or deny that something is going on. Scientists really want to get things right before they announce a major finding to the world, whether positive or negative. LIGO’s data-analysis task alone is vast and full of potential gotchas, and the most likely gravitational-wave detections would be buried deep in the noise. The experiment is looking for changes in the distance between mirrored blocks of metal 4 km apart as slight as 10–22 meter, about a millionth the diameter of a proton. In other words, changes in measurement of 1 part in 1025. What could possibly go wrong?

Fresh on the minds of everyone in astronomy and physics is an announcement fiasco that blew up spectacularly in 2014. The astronomers of the Harvard-based BICEP2 collaboration announced to the world’s media, at a packed press conference, that they had very likely discovered primordial gravitational waves from the earliest instant of the Big Bang.

BICEP images

BICEP 2 interior
BICEP at the South Pole, exterior and interior

The signal was unexpectedly strong. It would have been the much-sought, crowning evidence for the inflationary-universe theory of how the Big Bang happened. Not until later did their work go through full peer review. The discovery literally turned to dust — leaving a very public mess and a lot of criticism. Many dread a repeat.

The current excitement could easily be a false alarm. Even if LIGO has a promising signal, it may be a false test signal planted as a drill. It’s been done before, in 2010 near the end of LIGO’s last pre-upgrade run. Three members of the LIGO team are empowered to move the mirrored blocks by just the right traces in just the right way. Only they know the truth, and the test protocol is that they not reveal a planted signal until the collaboration has finished analyzing it and is ready to publish a paper and hold a press conference. “Blind tests” like this are the gold standard in all branches of science.

So we’ll just have to cool our heels. But maybe not for long. If the detection is real, it’s likely to be announced in February or March according to various reports. If it was just a test, this will presumably be announced in a similar time frame.

“Essential to the Process”

A premature “discovery” getting loose, and then being denied or retracted, could diminish the public’s trust in scientists — and the scientific process — in general. “We live in a crazy time when it comes to science and the public, as the ongoing ‘debate’ about climate change shows us again and again,” wrote astronomer Adam Frank (University of Rochester) in his NPR blog on the BICEP2 fiasco in 2014. “I wish they’d have let the usual scientific process run its course before they made such a grand announcement. If they had, odds are, it would have been clear that no such announcement was warranted — at least not yet — and we’d all be better off.”

Sigurdsson, however, disagrees. When the BICEP2 team announced their results, he used it as an example in his cosmology 101 class, encouraging students to view it as an uncertain result in mid-discovery phase. “I think most of the public appreciates the fact that you can make mistakes for the right reasons and that’s part of the process,” says Sigurdsson. “We proceed by falsification. We make conjectures, we test them, and some of the time we find that things were wrong and we throw them out. But that’s still essential to the process. We need to get that across.”

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Sky & Telescope magazine, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

Sky & Telescope magazine, now in its eighth decade, came about because of some happy accidents. Its earliest known ancestor was a four-page bulletin called The Amateur Astronomer, which was begun in 1929 by the Amateur Astronomers Association in New York City. Then, in 1935, the American Museum of Natural History opened its Hayden Planetarium and began to issue a monthly bulletin that became a full-size magazine called The Sky within a year. Under the editorship of Hans Christian Adamson, The Sky featured large illustrations and articles from astronomers all over the globe. It immediately absorbed The Amateur Astronomer.

Despite initial success, by 1939 the planetarium found itself unable to continue financial support of The Sky. Charles A. Federer, who would become the dominant force behind Sky & Telescope, was then working as a lecturer at the planetarium. He was asked to take over publishing The Sky. Federer agreed and started an independent publishing corporation in New York.

“Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”