Why were the gravitational waves predicted by Einstein detected only a hundred years later?

The dream of archaeologists is to record the sound of Li Bai chanting, the dream of paleontologists is to record the sound of dinosaurs roaring. Their dreams are too difficult. Only astronomers are lucky. They "hear" the "sound" of supermassive stars and even early cosmic activities billions of years ago - nanohertz gravitational waves.

Gravitational waves are fluctuations in space itself . Our world consists of two parts: matter and the space in which the matter resides (including time, which physicists also call "space-time").

Space seems to be very stable. If you take out a ruler, no matter whether you are in Beijing or New York, or whether you look left or right, front or back, or up or down, one meter is just that long and there is no change.

But the theory of relativity tells us that space will bend. Not only that, Einstein and mathematician Poincare also predicted that in many cases, the curvature of space can spread out like ripples: the entire universe is like a lake, and the (certain) movements of stars cause all kinds of ripples on the lake - gravitational waves.

Scientists recorded the gravitational wave pattern and played it as a sound wave, and then they "heard" the sound of the universe. This may be the oldest sound that humans can imagine, hundreds of millions or even tens of billions of years ago.

Gravitational waves have different frequencies. Low-frequency gravitational waves generally come from the rotation and precession of supermassive black holes, or even from the chaotic early universe. They are a very precious window for people to explore the mysteries of supermassive black holes and even study the activities of the early universe.

It is very difficult to detect the fluctuations in space because they are too weak. The gravitational waves predicted by Einstein a hundred years ago were not detected by humans until 2015.

The original method was to use a laser device to form an "array", which acts as a "ruler" to measure the length of space. If the ruler suddenly becomes longer or shorter, it means that a ripple of space curvature has passed by, and gravitational waves have been detected.

Rulers of different sizes can find gravitational waves of different wavelengths. The larger the ruler, the longer the wavelength of the gravitational waves found, the lower its frequency, and the more likely it is to provide information about the rotation and precession of supermassive black holes, as well as clues left behind by the early universe.

The first ruler used by humans to detect gravitational waves was several kilometers long, and it detected spatial fluctuations of tens to hundreds of hertz.

It is difficult to make it bigger on the ground, so scientists build rulers in space. Including my country's "Tianqin Project", many research groups rely on satellites to build huge laser rulers in the vast outer space to measure larger-scale space fluctuations. They found gravitational waves with a frequency of several hertz.

This is already amazing, but there is no end to human exploration. A few hundred hertz is not enough, a few hertz is not enough, we need nanohertz (one billionth of hertz).

How can we make the ruler bigger? To search for gravitational waves with nanohertz frequencies, we must use the entire universe as a laboratory and use a ruler as long as the distance between galaxies to detect them. Is this possible?

Not only is it possible, it has been done.

Scientists directly use pulsars in the universe as their experimental equipment and make an extremely huge "ruler" on a cosmic scale.

Pulsars are a special type of celestial body that continuously emits high-energy rays while also rotating steadily and rapidly, just like a person holding a high-brightness flashlight and spinning it back and forth in the dark universe. If his flashlight could sweep across the earth, humans would observe an intermittent "flashlight" signal with very stable time intervals.

So, if this stable "flashlight" signal suddenly shortens or delays, what does it mean? It is very likely that the space in which it is propagating is distorted. In other words, there may be gravitational waves sweeping through this area. If multiple pulsars are observed at the same time, the signal of gravitational waves sweeping through will be more obvious and more accurate.

Scientists regard the many pulsars known so far as an "array". By observing which pulsar signals in the array appear and when and how they change, they can infer the information of gravitational waves. This array arranged in the universe is more academically called a pulsar timing array (PTA). The Chinese team and the related project in China are the CPTA mentioned in the title.

Recently, China used a huge radio telescope (FAST, the "Chinese Sky Eye") located in the mountains of Guizhou to observe an array of 57 millisecond-period pulsars, and obtained evidence of the existence of gravitational waves at the nanohertz level with very high precision (a false alarm rate of about one in five hundred thousand).

Such low-frequency gravitational waves have a wavelength of several light years and a frequency of several years, but the distortion of space is negligible. The Chinese team has achieved outstanding precision. This is the result of our scientists' long-term and meticulous polishing using cosmic-scale experimental equipment.

At the beginning of the exploration of nahtz gravitational waves, we cannot say that the signals we have captured correspond to a certain astronomical event. It is more like the background of the entire universe rippling at nahtz frequencies; just like the lake water is still slowly rising and falling besides the wind waves, fish patterns and raindrops. These signals are the huge, shallow patterns drawn in the vast space by the supermassive black holes and even the universe itself over tens of billions of years; just as a Chinese poet once wrote:

"When the pen draws the horizon

You were awakened by the Oriental Gong

Blooming in the echo is

The Rose of Time".