What is the story of "Little Green Men" about?
In 1967, astronomers at the University of Cambridge in England built a radio telescope. The new telescope was designed and built to study radio sources using interplanetary scintillation phenomena. Interplanetary twinkle is somewhat similar to what we often call the "eye-punching" phenomenon of stars. When starlight passes through the Earth's atmosphere, due to the uneven fluctuations in the atmosphere, the light from the stars appears to us to be blinking, as if they are blinking. This flickering phenomenon is related to the angular size of the luminous object relative to us (the angle at which our line of sight opens when looking at the luminous object). The star is far away from us and is basically a point, so it is easy to flicker; while the planet is closer and looks like a circle in the telescope, so it is difficult to see the flicker. A similar phenomenon occurs in the radio band, but this flicker is not caused by the atmosphere, but by the solar wind that fills interplanetary space. Interplanetary scintillation is also related to the angular size of the radio source, and can even provide a method for calculating the angular diameter of the radio source. The new telescope at Cambridge University was built for this purpose. Because interplanetary scintillation is more significant in the low frequency band, the working band of the telescope is 81.5 MHz. And because the scintillation is very fast, the telescope should have a high time resolution: but in this way, the signal-to-noise ratio is It is smaller, so the receiving area of the telescope can only be increased. The completed telescope covers an area of nearly 20,000 square meters and is an antenna array composed of 16×128 dipole antennas.
The telescope began observing in July 1967, and produced seven or eight meters of recording paper every day. The recording paper could have been submitted to a computer for analysis, but because the instruments had just been put into operation, in order to check whether they were normal, it was decided to have a graduate student named Jocelyn Bell perform manual analysis. A few weeks later, she saw from the records that there was an unidentifiable record. It did not look like a flicker or other interference. The declination and right ascension it appeared were all 23°, which was about 19:20. . Although this record only occupied one or two centimeters on more than 100 meters of recording paper, it caught Bell's attention. She told her instructor, Hewish, and they decided to make a quick note of the phenomenon. After some setbacks, the first fast record was finally obtained at the end of November. The recording pen traced a series of pulses, the interval between each two pulses being equal to 11/3 seconds. Through further analysis, it was found that it passes through the antenna beam at the same speed and direction as other radio sources. It moves according to sidereal time - appearing 4 minutes earlier every day.
This is really a dilemma! Since it rises and sets with the star, it is probably the radio wave from the star. But stars cannot change on a time scale of seconds. Let’s say it is caused by human interference. Who sends out this signal regularly in a 23 hour and 56 minute cycle? I asked astronomy research groups across the UK and got a negative answer. Further records proved the signal. The pulse period remained very accurate and stable, and similar pulses were found at other frequencies. The signals at low frequencies arrive later than those at high frequencies. This is caused by the dispersion of the interstellar medium, which is well known to astronomers. From the amount of dispersion, they calculated that the distance of the pulse signal source from the earth is 212 light years, which is far outside the solar system, but within the Milky Way, it seems like a signal sent by "people" from outside the sky.
Sane "people" outside the earth have long been a common theme in fantasy novels. Some scientists imagine that because the gravitational pull on other planets may be very strong, or because their bodies have deteriorated due to the rapid development of civilization, the higher animals in other worlds may be very small, and they may be able to use them directly without using plants. The light energy of stars, so their skin color is green... As a result, "green men" have become the object of frequent discussion, but few people take this fantasy seriously. However, the telescope of Cambridge University has received this unexplainable signal. Maybe, the "little green men" are knocking on our door! What an exciting discovery this is!
But scientists are not people who want to impress people.
Hewish and his students continued to observe carefully. They thought that if the signal was really sent by the "little green men", they should live on a certain planet. The planet revolves around its "sun" and should cause the pulse interval time to change. change, but in reality there is no such change. By the end of January 1968, further records and detailed examination of past records convinced them that three other sources would also emit similar pulse signals, so the "little green men" hypothesis had to be put aside because it was impossible to imagine that there would be The "little green men" in four places so far apart have made an appointment to send signals to our earth on the same frequency band and within the same period of time.
As a result, the effort to find answers turned from intellectual life back to nature. What kind of celestial body can emit such a fast and stable pulse signal? First, the scale of the celestial body should be very small, because otherwise, the pulses emitted by various points on the celestial body that are not far away from us will overlap with each other and we will not be able to detect them. Distinguish individual pulses. Based on the observed pulse width of 16 milliseconds, it can be determined that the size of the celestial body's emission area should be less than 3,000 kilometers. At such a small scale, it seems that it can only be a white dwarf or a neutron star. Second, the pulse period is 1 to 3372275 seconds, and it is highly stable, with an accuracy of 10-8 seconds. There are three types of periodic processes on celestial bodies: orbital motion, pulsation and rotation. Orbital motion was ruled out when considering "little green men" as a possibility, and later precision measurements ruled out the possibility of pulsations, so the observed periodic pulses could only be caused by rotation. Third, the pulse period found is on the order of one second, and since it is caused by rotation, it means that the star needs to rotate once in a little more than one second. Calculations show that the white dwarf star has long been torn to pieces at such a crazy rotation, so there is only one conclusion - this celestial body known as a "pulsar" is a rapidly rotating neutron star!
By 1978, more than 300 pulsars had been found, with periods as short as 0.033 seconds and as long as 3.7 seconds. New observations further prove that they are all spinning neutron stars. For this major discovery, Hewish received the 1974 Nobel Prize in Physics. A purely theoretical prediction that was unnoticed and even ridiculed in the 1930s was finally confirmed in the 1960s. It seems that if we pursue the existing theories to the end, we will get some "weird theories" that are difficult for the world to accept, but they may contain truths that will be revealed for future scientific development. What happened to the neutron star prophecies illustrates this point.
So, how do you know that a pulsar is a neutron star? What played a major role in answering this question was not the first pulsar discovered by Bell, but the Crab-shaped star discovered in October 1968. That pulsar in the nebula. There is a long and interesting story about this star. About 900 years ago, during the Song Dynasty of my country, a very famous supernova was recorded. It suddenly appeared one morning in January 1054 AD. It was so bright that it could be seen during the day. It took 23 days before it gradually dimmed. 700 years later, that is, in the 18th century, an Englishman used a telescope to see a cloud-like thing where the bright star once appeared. It looked a bit like a crab, so he named it crab-like. nebula. Later, it was discovered that this nebula is still expanding. According to the expansion speed and size of the nebula, we can calculate the time when it began to expand, which happened to be the time when the supernova observed in our country 900 years ago appeared. From this It is concluded that the Crab Nebula is the remnant of this supernova explosion.
We know that previously discovered pulsars cannot be seen with optical telescopes, but the aforementioned pulsar discovered in October 1968 can be seen with optical telescopes. Further Observational studies have shown that this star can emit very short period and very stable pulse signals in all magnetic wave bands.
How are such regular pulses generated? Scientists carefully analyzed and studied various possibilities, and finally believed that this can only be due to the rotation of the star. A pulsar is like a rotating lighthouse.
Once every time the lighthouse turns around, we can see the light coming out of its window. The lighthouse keeps rotating. From a distance, the light of the lighthouse keeps turning on and off. The same goes for a pulsar. Every time it rotates, we receive the electromagnetic wave it radiates once, thus forming an intermittent pulse. The pulse period of the Crab Nebula pulsar is 1/30 seconds, that is to say It only takes 1/30 of a second to rotate once. For such a fast-rotating celestial body, its size must be very small, but the luminosity of this star is very large, about 100 times the luminosity of the sun. It means that its mass is quite large. Its mass is very large and its volume is very small, which means that its density is very high. Only a neutron star composed of very tightly packed neutrons can have such a high density. Rotating rapidly without disintegrating, it can be determined that the Crab Nebula pulsar is a neutron star that rotates at high speed.
Finally, let’s list some data to describe the various strange properties of the pulsar. Pulsars are very small, and one earth can accommodate 10 million of these wonderful celestial bodies. However, despite their small size, they are very strong, and their density is as high as hundreds of millions of tons or even billions of tons per cubic centimeter. . It would take tens of thousands of 10,000-ton ships to move a walnut-sized object in a pulsar. The pulsar is also an ultra-high-temperature world with a surface temperature of up to 10 million degrees and a core temperature of up to 6 billion degrees. In an ultra-high-pressure world, the central pressure is about 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000000000000 times the energy radiation of a pulsar’s celestial body with the strongest magnetic field is known. It can be seen that pulsars possess various unattainable "extreme physical conditions" such as ultra-high temperature, ultra-high pressure, ultra-high density, ultra-strong magnetic field, ultra-strong radiation, etc. This provides a natural basis for studying the state of matter under extreme conditions. It is an ideal laboratory and has greatly promoted the research of various extreme technologies.
Precisely because the discovery of pulsars has contributed to the testing and development of stellar evolution theory, the research of extreme technologies, and the enrichment of human understanding of the universe. It was so important that it was listed as one of the four major discoveries in astronomy in the 1960s, and the discoverer of pulsars won the 1974 Nobel Prize in Physics.