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The Independent

February 24, 2018

Bankhead Talk: Seeing the Universe Through Gravitational Waves

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Posted: Thursday, January 11, 2018 12:00 am

A new diagnostic tool that is beginning to help scientists explain some of the most exotic phenomena in the universe will be discussed next Thursday evening at Livermore’s Bankhead Theater.

The tool is the use of gravitational waves long thought to be generated by black hole collisions, exploding stars and other powerful astronomical events.

Although the existence of gravitational waves was proposed more than a century ago, the technology for detecting them did not exist until recently. They were first detected in 2015, with four more observations since.

Their discovery was considered so central to understanding the nature of the cosmos that three pioneers in the field were awarded the Nobel Prize late last year.

The Bankhead presentation will be made by David Shoemaker, a prominent MIT astrophysicist who has studied gravitational waves for three decades. His talk, part of the Rae Dorough Speaker Series, is scheduled to begin at 7:30 p.m.

Shoemaker was recently named spokesperson for a 15-nation international collaboration that is searching for gravitational waves reaching Earth and studying what they reveal about astronomical events.

In international scientific collaborations, “spokesperson” is a formal position, the leading scientist chosen to represent the effort’s public face all over the world.

‘A New Sense’

The potential value of gravitational wave research was summarized by a physicist and astronomer from Dartmouth, Robert Caldwell, at the time the 2017 Nobel prizes were announced.

“There is a ‘dark world’ out there of black holes and other exotic objects, which we can access only through gravitational waves,” Caldwell said. “This is truly exciting – like gaining a new sense.”

Astrophysicist Allan Adams, one of Shoemaker’s colleagues at MIT, noted that gravitational waves pass easily though the opaque outer layers of stars, offering a window into star cores.

“I’m positive that there are things out there that we have never seen and…haven’t even imagined” that may be analyzed now that gravitational waves can be detected, he said.

Like the more familiar electromagnetic waves – X rays, radio waves and visible light, for example – gravitational waves travel at the speed of light. In a concept that is easy to state but mind-stretching to contemplate, they cause ripples in both space and time.

Einstein is commonly credited with predicting the existence of gravitational waves in his General Theory of Relativity, published in 1915, but French physicist Henri Poincaré had actually proposed them 10 years earlier.

Poincaré was building on the work of Dutch physicist Hendrik Lorentz, who had shown that time slows down and objects become more massive as they approach the speed of light. These also are concepts for which Einstein often gets credit, because they became part of the foundation of his famous Special Theory of Relativity.

Einstein himself had doubts about the existence of gravitational waves, but he offered mathematical descriptions of three possible varieties of them when he published his General Theory in 2015.

Other physicists soon found fault with his mathematics, however, and the existence of gravitational waves remained a matter of skepticism and debate.

In any case, technology did not offer a means of detecting them – of confirming that they exist -- until very recently.

Detectors 1,865 Miles Apart

The first detection of gravitational waves took place in September 2015 on an American system called LIGO, for Laser Interferometer Gravitational-wave Observatory. Over the past 40 years, much of the impetus to search for gravitational waves has come from MIT, where Shoemaker is a professor, and from CalTech.

LIGO is the largest project ever undertaken by the National Science Foundation. Its two detector systems are located 1,865 miles apart in Hanford, Wash., and Livingston, La.

The detectors look for the extraordinarily slight distortion of space that occurs as gravity waves pass.

Each detector has two perpendicular tunnels 4 kilometers (2.4 miles) long, configured in a giant letter L.

A laser beam is split by a half-mirror into two beams that travel the perpendicular arms of the detector and bounce back to a light detector. If no gravitational waves are present to distort the paths, the two beams arrive at the detector at exactly the same time because they traveled exactly the same distance.

When that happens, the laser beams cancel each other out – “interfere” with each other, as the name interferometer indicates -- so the light detector shows a blank pattern.

On the other hand, a gravitational wave rippling through space could shift the positions of the detectors very slightly. The wave is likely to be extremely weak, having traveled millions or even billions of light years from its source.

To be able to sense subtle shifts in the detectors, the length of the laser pathway has to be known to an accuracy of less than one-thousandth the diameter of an atomic nucleus, according to MIT’s Adams.

As it turned out, the September 2015 gravitational wave came from about 1.3 billion light years away. It distorted the length of the detector arms enough to stagger the arrival times of the laser beams and trigger a signal in the light detector.

Light detector signals can be read directly and converted into audible sound. Adams said that the “chirp” sound of the 2015 gravitational waves helped scientists identify two black holes, each about 30 times the mass of our sun, spiraling in on each other at an astounding rate of about 100 revolutions per second, some 1.3 billion light years from Earth.

The first version of LIGO, which operated from 2002 through 2010, failed to detect any gravitational waves. The system was shut down for extensive upgrades, resuming operations as Advanced LIGO in the summer of 2015.

It soon picked up signals of colliding black holes, producing intense excitement in the astrophysics world.

LIGO has recorded four more gravitational wave emissions since then, three from black hole collisions and one – the most recent, occurring last August – from the collision of neutron stars.

The neutron star waves were also recorded by an Italy-based detector called Virgo, which had just been switched on.

Linking the two LIGO detectors with Virgo dramatically narrows the search for the exact source of gravitational waves, according to a National Science Foundation statement. This is important because it allows other institutions to join observations, looking for X-rays, visible light and other signs of some event.

Shoemaker’s talk is scheduled to begin at 7:30 next Thursday, January 18. Tickets may be purchased online at https://lvpac.org/event/david-shoemaker/ or by contacting the box office at (925) 373-6800.

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