For the first time, mankind is afraid of the "dark forest", female graduate students found a mysterious signal, should we reply?

After reading "Three Bodies", one of the laws of the universe is deeply shocked, and even chills down the back, which is the law of the dark forest. In the dark forest of the universe where all kinds of "hunters" lurk, every civilization will be careful to hide as much as possible, otherwise, it may attract other more advanced civilizations' devastating blow. Each civilization wants to preserve itself and can only nip external threats in the bud.

The Law of the Dark Forest is an explanation of the Fermi paradox. Humans have not existed on Earth for more than 3 million years, but we have developed astronautical technology with the ability to fly away from Earth and travel to other planets.

On the other hand, the galaxy was formed in an estimated 13.5 billion years. In the long past, if only alien intelligent species had appeared hundreds of millions of years before humans, even millions of years, they would now have the ability to develop into galactic civilizations and operate throughout the galaxy.

However, the universe seems to be silent, no extraterrestrial civilization has ever visited or contacted Earth, and our signals to the universe have never been answered. So, physicist Fermi asked the question: Where are all the alien civilizations?

As early as 1967, mankind has been afraid of the universe's "dark forest" for the first time.

In addition to visible light, many radio waves in the universe are invisible to the naked eye, which need radio teleneedses to receive (China's eye in the sky is also a radio telescope). In the 1960s, radio astronomy had already made several significant discoveries, such as the cosmic microwave background radiation, the afterglow left by the Big Bang, and quasars, active supermassive black holes at the centers of distant galaxies.

At that time, Jocelyn Bell Burnell, a female graduate student at Cambridge University, was studying quasars through a radio telescope when she accidentally discovered a very regular pulsating signal with an extremely stable period, beating every 1.337 seconds.

This bizarre signal appears 3 minutes and 56 seconds earlier every day, which coincides with the time it takes for the Earth to make one revolution on its axis. In life, a day lasts exactly 24 hours, which is the amount of time it takes for the sun to pass the same meridian one day apart. However, since the Earth is still in orbit around the Sun, after 1 solar day, the Earth's rotation angle is more than 360 degrees. In other words, the time for the Earth to rotate 360 degrees around its axis is less than 24 hours, a difference of 3 minutes and 56 seconds.

Thus, Jocelyn determined that this particular signal must have come from somewhere in the universe. Whenever the Earth turns in that direction, it just happens to pick up the radio waves from that signal source, which is why there is such a time difference.

Jocelyn could not explain the impulse signal she found, and the idea suddenly flashed through her that it might be an extraterrestrial signal. Human civilization's communication is based on radio waves, which is the inevitable result of the development of a technological civilization. If there are also aliens in the universe, it is entirely possible that they also use radio waves to communicate.

Jocelyn did not dare to say anything, but she told her mentor, Professor Anthony Hewish, about the newly discovered "alien signal". After much research and discussion, Professor Hewish agreed that the signal was unusual and that its narrow-band frequency suggested that it was not of natural origin.

They did not dare to make this discovery public for fear of causing trouble or even bringing danger to the Earth. Because if this is an alien signal and someone on Earth transmits it to reply, it will potentially attract an alien civilization with a technology level far beyond that of humans, which will put humanity in danger, just like the one depicted in "Three Bodies".

In the following months, they found multiple regular pulse signals one after another, only with different periods. Later, after repeated studies, these signals were believed not to be sent by aliens, but from a very special kind of object - a pulsar, a neutron star predicted by physicist Landau more than 30 years ago.

When a massive star with 8 to 20 times the mass of the Sun runs out of fuel, the core region will collapse violently, and the strong gravitational force will compress the electrons into the nucleus and neutralize them with protons to form neutrons, which will form a dense object made of neutrons - a neutron star. Theoretically, a neutron star is about twice as massive as the Sun but has a radius of fewer than 20 km.

Since the magnetic and rotational axes of neutron stars do not overlap, when they rotate rapidly, radio waves emitted in the direction of the magnetic axis sweep through the universe with them. If the Earth is in the direction of the magnetic axis, we can receive a steady pulse signal, the signal period is the rotation period of the neutron star.

Since then the truth is clear, although what Jocelyn discovered is not an extraterrestrial signal, the discovery of pulsars is still extremely important, which strongly confirms the correctness of the theory of stellar evolution, and can also be used to test Einstein's general theory of relativity, and can also provide coordinates for future interstellar navigation. As a result, the discovery has been hailed as one of the greatest astronomical discoveries of the twentieth century. Hewish won the Nobel Prize in Physics for this discovery, but unfortunately, the original discoverer, Jocelyn, was not eligible for the prize.