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The Moon: A Treasure for Scientific Research

Gravitational waves are space-time ripples caused by massive celestial bodies perturbing the surrounding space-time. Just like dropping a stone in calm water, it will cause layers of ripples to spread around. It was the last missing piece of the puzzle in the experimental verification of Einstein’s general theory of relativity, and its existence was not detected for the first time until 2015. On October 16, 2017, scientists from many countries around the world held a simultaneous press conference, announcing the first direct detection of gravitational waves from the merger of double neutron stars, and at the same time “seeing” the electromagnetic signals emitted by this spectacular cosmic event. It was at 8:41 US Eastern Time on August 17 (20:41 Beijing Time) of the same year, the US “Laser Interferometer Gravitational Wave Observatory” (LIGO) and the European “Virgo” (Virgo) gravitational wave detector captured the This gravitational wave signal. Two seconds later, the Fermi Space Telescope in the United States observed a gamma-ray burst from the same source. This is the first time in human history that a gravitational wave observatory and an electromagnetic wave telescope have been used to observe the same astrophysical event at the same time, marking a new era of multi-messenger astronomy featuring multiple observation methods.

Proposed Cosmological Gravitational-Wave Lunar Observatory (GLOC) Layout

Before that, the LIGO gravitational wave detector had detected gravitational waves three times in September and December 2015 and January 2017, all of which were generated by the merger of two black holes. The high-frequency gravitational waves may have been emitted by one of the primordial black holes before the merger. And these primordial black holes have not been conclusively proven to exist. Primordial black holes are speculated to have been created within milliseconds of the Big Bang and could have been the “seeds” of supermassive black holes at the centers of galaxies. Scientists have since made dozens more detections, mostly by facilities like LIGO, which can detect gravitational waves at frequencies between 30 and 7,000 hertz.

Gravitational waves are often drowned out by background waves from earthquakes, traffic and other human activities, making them tricky to detect. To detect faint gravitational waves, we have to use extremely sensitive interferometers. However, the more sensitive the interferometer, the more easily it can be interfered with. To this end, we must arrange a vacuum environment for the gravitational wave detector to prevent the interference of air molecules. Even so, it can be disturbed by earthquakes. And because the interferometer is so sensitive, even small earthquakes that scientists don’t feel can cause them to mistakenly think they’ve received a gravitational wave signal. By studying the natural conditions on the moon, scientists believe that the most challenging spectrum of gravitational waves can be better detected on the lunar surface, which is nearly impossible on Earth or in space.

The lunar surface has some advantageous properties that are difficult to replicate on Earth. For one, there is currently no human activity on the moon, and even with future crewed missions, there’s still plenty of room to build a place away from the hustle and bustle. Second, moonquakes are much weaker and less frequent than earthquakes. Finally, the moon is in a high vacuum environment, which means that the probe doesn’t need to be placed in a vacuum tube like it is on Earth. Even the deep underground LIGO and Virgo gravitational-wave detectors, whose vacuums are far less pristine than the lunar surface, are still susceptible to noise and vibrations from their surroundings. These factors mean that the gravitational-wave infrastructure built on the lunar surface could be far more sensitive than any Earth-based detector, capable of detecting frequencies in a range that is barely detectable by Earth-based facilities.

To this end, a research team led by Vanderbilt University astrophysicist Karan Jani and Harvard science professor Avi Loeb has proposed for the first time a new, quieter location on the lunar surface to build a gravitational wave infrastructure. imagine. Called the Gravitational-Wave Lunar Observatory for Cosmology (GLOC), it aims to use the Moon’s environment and lunar orbit to analyze mergers of black holes, neutron stars and dark matter candidates within nearly 70 percent of the observable volume of the entire universe. This gravitational wave detector can capture 0.1-5 Hz gravitational waves, detect black hole and neutron star merger events, and search for dark matter candidates. The lower limit of gravitational wave frequencies that can be detected on the lunar surface is at least 2 orders of magnitude smaller than that of surface detectors.

On April 7, 2020, NASA’s Jet Propulsion Laboratory (JPL) submitted a new plan with a sense of technology and the future. They intend to build a “lunar crater radio” with a diameter of 1 km in the crater on the far side of the moon. Telescope” (LCRT), which will use craters on the lunar surface as natural radio parabolic antennas to amplify the signal. The starting point of the scientists’ conception is to make full use of the huge advantage of the position of the back of the moon to collect signals that cannot be received on Earth due to interference. These signals exist in ultra-long wavelengths of the radio spectrum that are largely unobservable by humans exploring the universe from Earth.

The LCRT can completely avoid radio interference from the earth, and it can also shield the radio interference from the sun after the moon is dark, so that the observation effect of the telescope is optimal. In addition, because the gravitational force of the moon is only one-sixth of that of the earth, it is completely possible to build the LCRT into a super telescope with a large aperture.

On Earth, there are many factors that interfere with the observations of radio telescopes. One is that there is an ionosphere above the surface. When the wavelength of the signal is longer, the ionosphere will block the signal. Second, there are too many man-made signals from mobile phone communications, radars, satellites, etc. on the earth, and they are millions of times stronger than the signals from the universe, causing interference to telescope observations. If the bands used by these interference sources are avoided, the purpose of reducing interference can indeed be achieved, but at the same time, it also means that there are blind spots in the observation of the telescope, which cannot cover signals in certain bands, so that certain signals from the universe cannot be captured.

In contrast, LCRT can detect weak signals on the back of the moon without an atmosphere, which facilitates the study of ultra-long waves. Ultra-long waves have wavelengths in the 10 to 50 meter band. This wavelength corresponds to the dark ages of the early universe, and is what the universe looked like at the beginning of its birth. At that time, the cosmic signal was very far away from us, and it was quite weak when it was transmitted to the earth. To conduct scientific exploration in the ultra-long wave band, we can only hope in the open space or the quiet back of the moon.

According to NASA’s vision, the diameter of the LCRT will be twice as long as that of the 500-meter Aperture Spherical Radio Telescope (FAST), and the receiving area of ​​the antenna will be increased by 3 times, so that early cosmic radiation can be observed. It will help scientists further reveal the structure of the Milky Way and galaxy clusters, the acceleration and propagation of cosmic rays, understand the formation of large-scale structures in the universe, and explore the origin of the universe.

Researchers have been searching for extraterrestrial signals for years, and the LCRT built on the far side of the moon could help expand the search to find those signals buried by Earth’s interference. The LCRT can listen to information from the depths of the universe without worrying about confusion with the surrounding electromagnetic environment, and there will be no lamentable fallacy of treating microwave oven signals as cosmic signals. If alien civilizations do exist in the universe, and aliens also send radio waves, then the LCRT is likely to receive alien signals. Some people think that if LCRT is successfully installed on the moon in the future, it will become a milestone event for mankind to search for alien civilizations.

Physicists dream of building a ‘lunar particle collider’ on the moon

To uncover the mysteries of the tiniest subatomic particles, physicists must keep particle accelerators and detectors as cold as possible and remove excess air to obtain reliable results. Some physicists have proposed that the moon is actually a very suitable place for high-energy physics research, and proposed the idea of ​​building a “lunar particle collider”.

This is because the lunar surface is very cold, and there is no atmosphere, no medium to transport the heat of sunlight from one place to another. On a moonlit night, when the sun falls below the lunar horizon, the temperature drops to -73°C, well within the typical low temperature experimental setup on Earth. During the lunar day, the temperature of the moon will rise a little, reaching above 38°C, but it is still cold in places without direct sunlight. Water ice is hidden in the shadows of lunar craters, for example. In particle accelerators, the role of superconducting magnets is to throw particles into the accelerator at nearly the speed of light, and the low temperature ensures that the superconducting magnets do not melt on their own. And the hotter the detector, the more noise it has to deal with in sifting out tiny signals from subatomic particles, because more heat equals more molecular vibrations, which equals more noise.

In addition to its low temperature, the absence of an atmosphere on the moon is also an important advantage. On Earth, physicists have to pump all the air out of particle accelerators and detectors. The moon’s vacuum is more than 10 times better than any vacuum conditions physicists have created in the lab, and it’s all natural and requires no effort.

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