How to detect the “memory” of gravitational waves

Beijing time on December 10, according tomedia reports, gravitational waves in the form of space-time ripples in the universe, and these space-time ripples are produced by the most intense events in the universe. Using devices such as the Laser Interference Gravitational Wave Observatory (LIGO) and virgo, astronomers can detect space-time ripples strong enough to “affect” the Earth.


As gravitational waves pass through space, they leave some kind of “memory”, a permanent bend of the instant space, and the detection of this phenomenon may soon be possible, which will further our understanding of gravity.

Gravitational waves

Although Einstein’s general theory of relativity has been around for more than a century, it is still the best theory we have in understanding how gravity works. The theory combines space and time into a unified framework, “space-time”. This space-time is not only a fixed stage, it also bends or twists with the presence of matter and energy.

The bending of space-time (or warping, distortion) determines how matter moves. In general relativity, everything from photons to high-speed bullets to exploding spaceships tries to fly in a straight line, but the space-time around them is distorted, forcing them all to follow a curved trajectory. It’s like you’re trying to walk straight through a mountain pass, but you’re going to follow the curved peaks and valley terrain.

What we call “gravity” is the result of all these space-time distortions, and moving objects have no choice but to follow the bends of the surrounding space-time and make corresponding movements. Like other bendable surfaces, space-time is not just bent or warped, it also produces vibrations.

Imagine you’re standing on a trampoline, and the trampoline will sink because of your weight. If someone jumps on a trampoline and tries to walk around you, they’ll feel your “gravity” and walk only a curved path. But if you’re far enough away, they won’t even notice your influence. And if you start jumping up and down on a trampoline, it will emit a wave vibration, and they will be involuntarily affected by your movements.

Remember the past.

Gravitational waves operate in a similar way, transmitting energy through the ripples of the space-time structure itself. The source of these ripples can be almost all types of motion, but because gravity is so weak (it is the weakest of the four basic interactions in nature), and the gravitational waves are weaker, only the most powerful movements of energy can create space-time ripples that can be detected by instruments on Earth.

So far, the Laser Interference Gravitational Wave Observatory (LIGO) and Virgo have observed dozens of gravitational wave events, ranging from large-mass black holes and merging between neutron stars. Gravitational waves from these events ripple through the universe and eventually reach Earth. As they pass the Earth, they move objects very slightly (perhaps smaller than the width of the atom).

Even we ourselves will be affected by this. At this moment, you are being gently squeezed and stretched by gravitational waves from a violent event billions of light-years away. You might think that once the gravitational waves are over, things are over, but gravity is a wonderful force, and the problem of gravitational waves is more complex.

Almost any movement produces gravitational waves, from colliding black holes to your waving hands, and even gravitational waves themselves. As gravitational waves travel in the air, they become new sources of gravitational waves and continue to do so. Each new generation of gravitational waves is weaker than the previous generation, and this effect accumulates, creating what scientists call the “memory” of space-time, a permanent space-time distortion left by gravitational waves.

In other words, when gravitational waves pass you, you are not only temporarily stretched and squeezed, but you will be permanently stretched as new gravitational waves are generated.

Looking to the future

Because gravitational waves are so weak, astronomers have not yet found any evidence of such space-time “memory”, but in theory it should exist, lurking in data obtained by LIGO and Virgo. What we should see is that the position of the probe will change over time long after the confirmed gravitational wave event has occurred.

Recently, a team of astronomers studied how they could finally see gravitational waves “memory”. Since the detection of each event left only very faint memory marks, it was not possible to observe this phenomenon continuously. Instead, we must combine multiple events to accumulate evidence of the existence of the phenomenon.

How many events do we need? The researchers predict that we will need to record about 2,000 separate black hole merger events to discover the permanent memory left by gravitational waves. Such a probe won’t happen any time soon, but the next generation of gravitational-wave observatories is expected to collect about 10 events a day, and perhaps within a year to find such evidence of such memory.

If the prediction of general relativity is correct, then this permanent memory of time and space should exist. If astronomers don’t find anything after years of searching, we’ll have to rethink our understanding of gravity to see if something is missing.

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