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Monday, July 11, 2016, 18:46

The view at the end of the universe

By Wang Yuke

The second recording of gravitational waves in US observatories might start a revolution in understanding the universe, as Tjonnie Li Guangfeng, the only HK scientist present at the event, tells Wang Yuke.

The view at the end of the universe

The second recorded discovery of gravitational waves on Dec 26 was almost as momentous as the first had been, three months earlier. For one thing, said Tjonnie Li, the only Hong Kong scientist who took part in the landmark event, it marked another step toward unlocking the mysteries of the universe. The distinctive chirp recorded at 0330 GMT, also proved beyond doubt that the first recorded event last September was no mistake, but a real event. Both events, each signaling the collision of two black holes, were recorded by the twin detectors of the Laser Interferometer Gravitational-wave Observatory, or LIGO, at Hanford, Washington, and Livingston, Louisiana.

“In the years to come, LIGO expects to observe more gravitational waves, not only from colliding black holes, but from clashing neutron stars. Then we can both ‘hear’ and ‘see’ the (gravitational) waves. Neutron stars emit light, while black holes do not,” Li said. As a physics professor at the Chinese University of Hong Kong (CUHK), Li has been attached to LIGO since 2009. He also leads his CUHK students on projects, helping LIGO in the search for gravitational waves. In the meantime, CUHK signed a memorandum of understanding in May, making it the first Hong Kong component of the LIGO Scientific Collaboration.

“As soon as LIGO detects a signal, we’ll tell all of our partners with telescopes to look where we observed the signal to see if there may be some light coming from the silhouette. If no light is seen, we can decide the colliding objects are nothing but black holes, with more confidence,” continued Li. “If there’s light emitted, we have to go back to our model and decide whether they are black holes or neutron stars or anything else that can radiate light.”

“But it is interesting that we always like to prove ourselves wrong,” Li laughed, implying that scientists are keen to challenge themselves.

“Students help remotely,” Li told China Daily. They analyze data arrays. One student tests general relativity, others look for gravitational waves in the region of light halos picked up by radio and optical telescopes. “This technique is the other way round, compared with the conventional approach by LIGO,” Li noted.

“The next big discovery, I think, will be gravitational waves produced by colliding neutron stars. And, I hope so,” predicts Li. “We will see light and gravitational waves together. That will be very exciting.”

What neutron stars tell

“Quo vadis, where are you going?” has become the eternal question. Will the universe expand forever, coming to frozen entropy, or will it prove, as Einstein predicted, to be an oscillating system contracting into another “Big Bang”? The answer is believed buried in the dark matter and dark energy that science believes comprise most of the physical mass of the universe.

As Li sees it, observing neutron stars will advance our understanding of the life cycle of stars within our own galaxy, the Milky Way. Scientists believe there may be hundreds of millions of old neutron stars in our region of space. Only about 1,000 have been identified so far.

A distinctive quality of neutron stars is that they give off light, unlike black holes, from which nothing can escape, not even light. “So, there will be light, when we see neutron stars collide. LIGO scientists already are on the hunt for gravitational waves emitted by neutron stars,” said Li.

Neutron stars are believed to be remnants of huge stars whose life cycles have ended in massive explosions. All that remains following the cataclysm are rapidly spinning cores, no more than 8 to 15 kilometers in diameter, but their inner matter is so dense that a single teaspoonful would weigh about 10 million tons. Most of the light emitted by neutron stars is from X-rays that can be detected by radio telescopes but are invisible to the naked eye.

Scientists believe that the neutron cores hold the clues about the nature of matter, energy and the basic components of the universe.

The view at the end of the universe
Tj onnie Li, a physics professor at the Chinese University of Hong Kong. (P hoto Provided to China Daily)

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