Astronomy's biggest event: Huge gravitational waves! Captured!

Rumors of a major discovery in astrophysics had been circulating for days, and on June 30, the truth finally came out: a multinational scientific team, including Chinese scientists, simultaneously published a series of papers announcing that we had found evidence of cosmic background gravitational waves. A new path to the secrets of the universe has been opened.

Kip Thorne, a famous American theoretical physicist, was at the press conference.

01 What are gravitational waves?

Just as a rock thrown into water creates waves, and accelerated electrons create electromagnetic waves, so too do gravitational waves : waves created when an object with mass and gravity accelerates . Unfortunately, gravity is so small that the gravitational pull of the entire Earth is no match for a few of your muscles—just lift your phone off a table and you’re there. And gravitational waves are so tiny that they have no impact on our daily lives.

It is not difficult to understand why scientists in the 19th century predicted the existence of gravitational waves, but people were unable to detect them for more than a hundred years. It was not until 1974 that we indirectly observed gravitational waves, and it was not until the 21st century that we directly confirmed the existence of gravitational waves .

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02 Aren’t gravitational waves old news?

In 2015, humans directly observed the first “gravitational wave” . This discovery won the 2017 Nobel Prize in Physics. Since then, scientists have detected several more gravitational waves, which have also made the news. However, there is one difference between the previous ones and this one: those were “small” waves, produced by the collision and fusion of small and medium-sized black holes. This time it’s a big one!

Actually, it is not entirely fair to say that it was "small" before. The two black holes observed in 2015 were 36 times the size of the sun and 29 times the size of the sun, which are already too big to be imagined by human scale. However, even with these two large celestial bodies, the gravitational waves they produced could only expand and contract the 4-kilometer-long detector by a thousandth of a proton.

But the collision of small and medium-sized black holes has one advantage, that is, the gravitational waves it emits are high-frequency and relatively "sharp", and only a "small" detector of a few thousand meters can be detected. If it is a larger-scale gravitational change, such as a super-giant black hole, the reverberation of the Big Bang, etc., the gravitational waves emitted are low-frequency and relatively "deep", which requires a very long detector - the kind that the entire Earth or even the entire solar system cannot accommodate .

The entire Earth cannot contain it, so how can humans possibly build it? Actually, it’s okay. Physicists and astronomers often encounter this kind of problem. They are very good at turning super-large objects that already exist in nature into parts of detectors. This time, they used the stars in the sky .

03 The Ruler of Stars Across the Void

In 1967, British astrophysicist Jocelyn Bell was still a graduate student. But that year, she discovered a strange celestial body that emitted regular electromagnetic waves very steadily. Some scientists joked that this might be a signal sent by aliens to the earth. So, she and her colleagues nicknamed this celestial body " Little Green Man 1 ". Subsequent research soon proved that this was a purely natural phenomenon, without the participation of intelligent life.

This electromagnetic wave signal appears and disappears at regular intervals, like a pulse. Radio tradition calls such discontinuous signals " pulses ", so the celestial body that emits this signal is called a pulsar . The discovery of pulsars later won the Nobel Prize. (But Bell herself did not win the prize, which is one of the famous cases of injustice to female scientists in the history of the Nobel Prize) Gravitational waves burst in pulsars.

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Now we know that pulsars are a type of neutron star , in other words, "the corpse of a dead star." The electromagnetic waves emitted by a pulsar are continuous rather than pulsed, but their electromagnetic waves are only emitted to their own poles, and they rotate at high speed, so we can only see them when they sweep across the earth . Looking at it from the earth is like looking at the light of a lighthouse on the sea, which flashes once every period of time. In theory, the interval between flashes corresponds to the interval between its rotations, and should be completely regular - in fact, its electromagnetic waves will be subject to various small interferences on the way to the earth, so tiny irregularities can be observed.

What scientists are targeting is the irregularities hidden in this law.

Gravitational waves are waves transmitted by space itself. When they change the length of the detector, it is not the detector itself that is changed, but the space to which the detector belongs. If the pulsar signal encounters gravitational waves on the way to the earth, its path itself will become longer or shorter due to the gravitational waves, and the time it takes will also change . By observing this change, we are equivalent to turning the entire space between us and the pulsar into a gravitational wave detector.

Of course, this is easier said than done. In addition to gravitational waves, there are many other factors that can interfere with pulsar signals, which need to be eliminated one by one. Moreover, the impact of gravitational waves is extremely small, and the periodic changes caused may only be 1 microsecond, which requires very precise instruments. Therefore, scientists from all over the world have worked together for more than 20 years to obtain this result.

04 The Swaying Ocean of Gravitational Waves

This discovery is not just a single gravitational wave event, but a culmination of data from around the world, with researchers from North America, Europe, Australia, India, Japan, and of course China reporting observations from countless telescopes. This result supports long-standing speculation: the universe is immersed in a background of gravitational waves, with long-standing low-frequency gravitational waves oscillating everywhere .

To use an analogy, the Earth is like a small boat, drifting in the vast ocean of cosmic gravitational waves. In the past few years, we have detected the vibration of the hull, proving that there are waves in the ocean. But now we can finally look out to the ocean and see the waves all over the surface with our own eyes.

Space landscape of nebulae and pulsars. Copyright image. Reproduction may cause copyright disputes.

But this also brings up a lot of new questions. For example, we are not sure what event these low-frequency gravitational waves come from. There are too many strange things in the universe. The collision of two supermassive black holes seems to be the most likely cause, but there are many other possible causes. Maybe it is a ripple as the universe expands? Maybe it is a trace of a phase transition in the early universe? Maybe it is a network of defects in space? There is still a lot we don’t know about the large-scale structure of the universe. But this is also what makes observing the low-frequency gravitational wave background so exciting: it opens the door to many unknown cosmic events .

Moreover, the quality of these results does not all meet the “gold standard” of physics. Convention in modern physics requires that a result must reach “5σ” — that is, the probability that a new phenomenon is purely coincidental is less than one in a million — before it can be considered a “solid hammer.” Not all the results published this time meet this requirement.

In the strictest sense, it should be said that this discovery "found evidence consistent with the gravitational wave background" rather than "detected the gravitational wave background." However, more detection and data analysis are still underway, and most people believe that this method will soon be able to obtain irrefutable 5σ level evidence.

Compared with the universe, the range that human senses can cover is too small. What a person saw when he looked up at the stars millions of years ago may have surpassed all human experience, but in the entire material world, what he saw was not even a drop in the ocean. Later, humans invented telescopes, radios, and spectrometers, and the sides of the universe were revealed one by one, but there are always more secrets hidden in the darkness that humans cannot perceive. The detection of gravitational waves is the latest of countless new doors opened over millions of years, and the gravitational wave background is its latest milestone.

The road continues .

Author: Fang Gang, PhD student in evolutionary biology

Reviewer: Yu Heng, Associate Professor, Department of Astronomy, Beijing Normal University

Planning Editor: Xu Lai

Editor in charge: Xu Lai