Into Unscientific

"Pulsar?"

Yang Zhenning was slightly stunned when he heard this word spoken by Xu Yun thousands of miles away.

Pulsar?

What the hell is this?

One thing to say, when it comes to the concept of pulse alone, Yang Zhenning is not very unfamiliar.

The literal interpretation of this concept was proposed as early as the last century. The proposer was Xu Yun's male ketone essay subject Xiaomai, who is Maxwell's classmate.

The so-called pulse, as it sounds, is somewhat similar to the pulse of the human body.

Every ups and downs of the human body's pulse can be regarded as a wave, and these periodic waves are called pulses.

As for the first practical technology that applied the pulse concept, it was the pulse body. It was invented by the German Marx in 1924 - not that Marx, but E. Marx.

The so-called pulse body is the famous Marx generator.

This thing mainly generates high-voltage pulses through a low-voltage DC power supply, and is a high-voltage device that charges capacitors in parallel and then discharges them in series.

It can simulate processes such as lightning and operating overvoltage, so it is often used in high-energy physical tests such as insulation impact withstand voltage, dielectric impact breakdown, and discharge.

With the development of human science and technology to this period, pulse has been used in more than 30 technical applications.

but

Among these more than 30 technologies, none of them have anything remotely close to the word pulsar.

To know.

As a top theoretical physicist who has been living across the sea for a long time and won the Nobel Prize, Yang Zhenning can basically be said to be at the forefront of human technology.

Whether it is some theories, scientific and technological achievements or projects, even some of the most secret research across the sea, he can at least hear some rumors about it.

But he had never heard of the word pulsar before.

However, Xu Yun’s previous work on dark matter and industrial software had shocked Yang Zhenning, so the boss did not rush to question Xu Yun, but asked patiently:

"Xiao Xu, forgive me for being ignorant, may I ask what this pulsar is?"

Xu Yun, who was on the other end of the phone, thought for a while and said:

"Mr. Yang, have you heard of the neutron star concept proposed by Zwicky and Bader?"

"Neutron star?"

Yang Zhenning gave a quick reply this time:

"Is it the kind of compact object proposed by Zwicky? The neutron model later summarized by Oppenheimer and Volkov?"

As everyone knows.

In the universe, compact celestial bodies can be divided into three types:

The first type of compact star with a smaller mass is called a white dwarf, which has a mass similar to or less than the mass of the sun.

Theoretically, it is believed that the mass of a white dwarf is less than 1.4 solar masses, its radius is about one percent of the solar radius, and it is a terminal celestial body.

For example, our old neighbor the sun will eventually become a white dwarf after a series of changes such as a red giant star.

If the sun is compared to a normal person, a white dwarf star is as small as your toes, but weighs about the same as you, which shows its high density.

The second compact object is a black hole. The mass of a black hole can span a large range, from several solar masses to hundreds of millions of solar masses.

According to the mass of the black hole, astronomers divide black holes into stellar-mass black holes, intermediate-mass black holes and super-massive black holes.

As for the third compact celestial body?

It's a neutron star.

It is slightly more massive than a white dwarf, with its mass ranging from 1.4 solar masses to 3.2 solar masses.

But its radius is only about one hundred thousandth of the sun's radius, which is about 10km.

If we use the previous example, it would be equivalent to one cell weighing the same as a normal person.

At the same time, a very special point is.

The proposal of the concept of neutron stars is relatively complicated and involves the grievances of Oppenheimer and several other people:

Neutrons were discovered by Chadwick in 1932, and in 1933, the bear physicist Landau proposed that there is a type of star that can be composed entirely of neutrons.

Landau therefore became the scholar who first proposed the concept of neutron stars.

However, there are big problems with the neutron star model proposed by Landau. It can be said that except for the name and neutron structure, it is very different from the actual neutron star.

His model is more inclined to speculate that after the discovery of neutrons, they can form celestial bodies - of course, the real situation is definitely not so random. The derivation of this model is mainly related to the degeneracy theory.

Then after Landau.

Zwicky, the man who proposed the concept of dark matter, also proposed a neutron star model.

The accuracy of Zwicky's neutron star model is very high, and it is ahead of Landau in the essential framework of the model. He even directly proposed that neutron stars are the products and energy sources of supernova explosions.

If we simply stop here, then the attribution of the concept of neutron stars is actually relatively clear:

Landau was the first to come up with the concept of words, and Zwicky came up with the correct framework. This kind of thing is very common in the world of physics.

But on February 15, 1939, Oppenheimer suddenly got involved.

At that time, Oppenheimer and Volkov published a paper on "Massive Neutron Cores" in "Physical Reviews", which was also the recognized mathematical framework of the neutron star model.

This paper quoted some of Zwicky's results, but Oppenheimer did not mention Zwicky in the paper because of his poor personal relationship with Zwicky. Instead, he mentioned Landau.

Although Zwicky later published a paper "Observations and Theory of Highly Collapsed Stars" by himself, his reputation was far different from that of Oppenheimer.

In addition, Landau was indeed the first person to propose the concept of neutron stars, so the water was muddied - many people thought that it was Landau who proposed the correct concept of neutron stars.

And the most complicated thing about this is that Zwicky has not actually been robbed of the title of proposing the "correct neutron star model", but if you want to know this, you either have to major in astrophysics, or you have to search a lot of information in depth Only then will you know the truth.

If you just search for the proposer of neutron stars, you will basically get the result Landau.

His eyes returned to reality.

Although there is a lot of nonsense about the proposal of neutron stars, and no one has actually discovered neutron stars in this era, the concept has finally become popular - at least for Chen Ning Yang.

It's okay that Xu Yun didn't mention neutron stars. But now that Xu Yun mentioned it, Yang Zhenning's doubts became even more intense:

"Xiao Xu, if I remember correctly, according to the model proposed by Zwicky, the so-called neutron star should be an ultra-high-density celestial body."

“Because its mass is too large, but not large enough to collapse into a black hole, which is the Oppenheimer limit, the extranuclear electrons of general elements will eventually combine with the protons in the nucleus under the action of gravity to become neutrons. Together with the original neutrons in the core, it forms a star with neutrons sitting in rows.”

"Not to mention that this kind of star has not been discovered yet, even if it does exist, what does it have to do with pulsars?"

Seeing that Yang Zhenning was able to describe the concept of neutron stars relatively completely, Xu Yun finally breathed a sigh of relief for what he was going to say next:

"Mr. Yang, you don't know something. The so-called pulsars are actually neutron stars that have been rotating at high speed."

Yang Zhenning was stunned for a moment.

Pulsars are rapidly rotating neutron stars?

This was the first time he had heard of this concept.

However, he was not in a hurry to ask the reason. He knew that Xu Yun would definitely explain further.

as expected.

Xu Yun's voice soon came from the other end of the phone:

"Mr. Yang, you should know that according to Zwicky's theory, neutron stars are not stars simply made of neutrons."

"Due to the pressure difference between the inside and outside of neutron stars, it is actually not as simple as one next to another."

"For example, the core part of a neutron star has greater pressure and is actually a hyperon, while the middle layer is the real free neutron."

"The outer layer is composed of neutrons undergoing beta decay into electrons, protons, and neutrinos - this involves the category of degeneracy pressure."

Yang Zhenning nodded slightly.

Degeneracy.

This is a concept that has a profound impact on modern physics.

It was precisely because of the discovery of degeneracy pressure that astrophysics, quantum mechanics and even the special theory of relativity were developed.

Students who have read "Handbook of Conquest of Different Worlds" should all know this.

For most stars, the end point of fusion is iron.

However, as long as the star is large enough, the iron will continue to compress in the future. This process is a degeneracy reaction.

in degenerate reactions.

The nuclei and electrons will be separated, and the nuclei will be stacked closely together. At this time, the star is not called a star, but a white dwarf.

White dwarfs rely on electron degeneracy pressure to fight against gravity to prevent the star from shrinking, while neutron stars rely on neutron degeneracy pressure to fight against collapse pressure.

Once the tension generated by the internal degenerate neutron gas cannot resist the collapse pressure, the star will further collapse into a black hole.

Then Xu Yun paused and continued:

"Mr. Yang, according to the results of our metahadron model, the fact that neutrons are uncharged only means that the neutron as a whole is electrically neutral. It does not mean that any part of the neutron is uncharged."

"Just as the iron atom is also electrically neutral, the iron atom as a whole is also uncharged, but this does not exclude that one part of the iron atom is positively charged and another part is negatively charged."

"In addition, neutrons have a magnetic moment, so neutron stars theoretically also have a magnetic field."

"The high-speed rotating neutron star is like the rotor of a high-speed generator cutting magnetic lines of force, so the rotating neutron star will inevitably emit electromagnetic pulse signals."

"As for the period of these signals and the strength of the magnetic field, Mr. Yang, you can calculate it now by combining it with our meta-hadrons. It should be very simple."

When Yang Zhenning heard this, he couldn't help but frowned slightly.

Xu Yun's explanation was not difficult to understand, but now he was asked to calculate the strength of the magnetic field and the signal period. This was somewhat confusing to him.

Do these two data make sense?

However, as Xu Yun said, the calculation of these two parameters is not complicated, so Yang Zhenning hesitated for a moment and started calculating.

As everyone knows.

As long as you believe that general relativity has no problems with stars, then the structure of stars can be given by the TOV equation:

M(r)=∫0r4πr′2ρ(r′)dr.

Once you give another initial condition ρ(0) and the equation of state p(ρ), you can solve the above differential equation to give the density, pressure and so on inside the entire star.

From the center of the star outward, at a certain R, ρ(r) drops to 0. You can interpret this R as the radius of the star with the central density ρ(0).

Although this equation is not very clear about the physical state of extremely dense celestial bodies, and in a sense it is even one of the major physical problems to be solved, it is not difficult to calculate the approximate range.

For example, there is a formula in later generations that calculates height based on foot length. This formula may not be accurate, but the calculated height range is more or less in line with the definition of [human being] - at least it will not give you a height of three meters. giant.

In addition, Xu Yun and the others also added a semi-empirical formula for nuclear binding energy into the metahadron model, so Yang Zhenning quickly derived the approximate data.

However, when he was about to write down the final score, Yang Zhenning's pen tip suddenly stopped, and he let out a light sigh:

"Well?"

I saw him pulling the calculation paper to the beginning again, and then started calculating again.

ten minutes later.

Yang Zhenning's brows furrowed even more tightly. He picked up the phone again and asked:

"Xiao Xu, based on the rotational inertia and the conservation of angular momentum, the period of a high-speed rotating pulsar is only about 6 seconds?"

Xu Yun hummed:

"That's right."

Bada——

There was a clear sound of something falling to the ground from the opposite side of the microphone. If nothing else, it should be the ballpoint pen in Yang Zhenning's hand.

at the same time.

Yang Zhenning, who was on the opposite side of the microphone, also fell into a long silence.

See this situation.

Xu Yun sighed with understanding.

Although Oppenheimer and Volkov worked out the TOV limit back then, they estimated that the upper limit of neutron star mass was only about 0.7 times that of the sun.

In fact, according to later observation results, the state equation they used was not ideal for neutron stars, and the deviation was very large.

because

The structure of a neutron star is far from simple, and is even many times more complicated than what Xu Yun introduced to Yang Zhenning.

Just like there is an atmosphere outside the earth, the outermost layer of a neutron star also has a very thin "atmosphere".

It is mainly composed of some light nuclei, such as hydrogen nuclei, helium nuclei, and carbon nuclei.

Then going inward is the outer shell of the neutron star. Their density spans seven orders of magnitude and is mainly composed of protons, neutrons and electrons in chemical equilibrium (note that electrons are beginning to appear and will provide a huge Fermi pressure, which will determine Changes in the composition of neutron stars as density increases) composition.

More precisely.

The top of the outer shell is still composed of atomic nuclei and electrons, but as the depth increases, the density continues to increase, and the electron Fermi energy continues to increase, so the nuclei with larger charges also continue to increase.

From the iron-56 core on the surface to the end of the periodic table of elements - the iron core is the core with the largest binding energy per nucleon on the nuclide diagram, but as the density increases, it is not enough to provide sufficient Coulomb energy to bind electrons

Ultimately, the nuclear symmetry energy competes with the electron's Fermi energy.

Further inside is the inner shell of the neutron star. An excessive proportion of neutrons in the nucleus will cause nuclear instability.

They pair up with each other to form a superfluid phase of neutron gas in an attempt to reduce the energy.

Next is the outer core of the neutron star, which is the area covered by most of the mass source and radius of the neutron star. The symmetry energy in nuclear physics determines the possible components here.

The density of this shell reaches the density of nuclear matter, forming a compact and uniform neutron system - perhaps this is the shell that best fits the public's understanding of neutron stars.

At this time, the shell is composed of more muons, because the Fermi energy of the electron continues to increase, even reaching the rest mass of the muon.

Then there is the core. The physics community expects that hyperons with s quarks will appear (similar to the reason for the emergence of muons). There is the famous hyperon problem.

In addition, the collective excitation of pi mesons and k mesons will destroy space parity, and meson condensation may also occur, etc.

Later generations also had the so-called lighthouse model regarding high-speed rotating neutron stars, that is, pulsars, but this model currently seems to be at risk of being deduced all over again.

At that time, Xu Yun also wrote the beginning of a new book based on certain properties of pulsars, thinking about releasing the next book.

As a result, I didn’t expect that theories that were used less than a year would be almost obsolete. I can only say that the update speed of modern theoretical results is indeed a bit fast.

all in all.

Later generations knew very little about neutron stars, let alone physicists of this period.

Even a big shot like Yang Zhenning seems a little powerless in the face of these concepts.

Therefore, Xu Yun kept much of what he said in conversation with Yang Zhenning, such as various parameters of pulsars.

The FAST eyes of later generations of rabbits in Guizhou Province have detected more than 800, sometimes several a day, sometimes one every few days. (Here we recommend the official website of FAST)

The slowest pulsar observed so far has a period of about one rotation every 10 seconds. The fastest known pulsar rotates at 716 revolutions per second, and its surface linear speed reaches one-quarter of the speed of light, numbered PSRJ1748-2446ad.

Without revealing your identity.

Xu Yun dared to tell Yang Zhenning this number. This big boss didn't think that Xu Yun had a mental illness, which meant he was in a good state of mind.

It must have been three or four minutes.

Yang Zhenning just picked up the phone again and asked Xu Yun:

"Xiao Xu, even if the pulsar you mentioned really exists, what does it have to do with gravitational wave detection?"

When Xu Yun heard this, he secretly praised that he was indeed a big boss. In this situation, he was able to grasp the key to the problem - Xu Yun's purpose of eliciting the pulsar was for the original gravitational wave.

If a pulsar has nothing to do with primordial gravitational waves, then it makes no sense no matter how fast it spins.

So Xu Yun organized some words and continued:

"Mr. Yang, you should know that according to the neutron star model summarized by Oppenheimer, pulsars will emit strong bipolar radiation."

"Suppose - I mean suppose, suppose that the pulsar's rotation axis and magnetic axis have a certain deflection angle, what would happen?"

"Declination?"

Yang Zhenning blinked and said thoughtfully:

"If there is a declination between the rotation axis and the magnetic axis, then when the pulsar's magnetic axis sweeps across the Earth, we will receive a pulse signal."

"And the interval between two pulse signals is equal to the rotation period, wait!"

I saw Yang Zhenning’s voice suddenly rose a little higher:

"Xiao Xu, is that what you mean?"

"If we can find a pulsar with a rotation period of milliseconds, can we detect primary gravitational waves based on changes in the rotation period?"

Snapped!

Hearing this, Xu Yun snapped his fingers in the air, with a bright expression on his face:

"That's right!"

Mentioned earlier.

If you rely solely on technological equipment, if you want to detect primordial gravitational waves, you will need to set up a detector larger than the Kuiper Belt.

This is obviously impossible at the current level of human technology, but later generations of physicists found a natural gravitational wave detector in the universe.

That's pulsars.

In addition to high rotational speed, pulsars also have high magnetic field strength.

The unit of measurement for the magnetic field is called "Gauss", and the letter is Gs.

The Earth's magnetic field is 0.7Gs, which is enough to withstand the solar wind;

Jupiter's magnetic field reaches 14Gs, 20 times that of Earth;

The general magnetic field in the polar regions of the solar magnetic field is very low, only 1Gs. However, the solar magnetic field is very active, reaching 1000Gs when erupting at the poles. The magnetic node magnetic field strength in the quiet area of ​​the sun also reaches thousands of Gs, and the magnetic field of sunspot bursts can reach 4000Gs.

These seemingly strong magnetic fields are nothing compared to the magnetic fields of neutron stars:

The magnetic field intensity of neutron stars is at least hundreds of billions of Gs, and the magnetic field intensity of the polar regions on the surface of most pulsars is higher than 1,000 billion Gs, or even as high as 20 trillion Gs.

Ultra-high-strength magnetic fields can power beams of radiation that blast out in all directions from magnetic poles — which are not always aligned with the pulsar's axis of rotation, just like Earth's north and south magnetic poles are not aligned with our planet's. The axis of rotation is aligned the same.

in this case.

A millisecond pulsar is like a space beacon with a stable period. When it sweeps across the Earth, we detect a pulse in the radio band.

We can accurately record the arrival time of pulses. The intervals between the arrival times of such pulses are theoretically constant, but in fact these intervals will vary slightly.

There are many factors that cause these changes, including the movement of the earth, the gravitational red shift caused by solar system objects, changes in the interstellar medium, etc.

Physicists include these factors into our models to fit the observed pulse arrival times. The difference between the model predictions and the actual observations is called the timing residual.

Timing residuals contain physical phenomena that are not included in the model, such as primordial gravitational waves.

The change in pulse arrival time caused by gravitational waves has two significant characteristics: one is coherence, and the other is fourth-order property.

The so-called coherence means that gravitational waves will affect all pulsars in all arrays synchronously, while some effects - such as pulsar starquakes will only affect the timing of a single pulsar, and starquakes between different pulsars are There is no connection whatsoever.

Quadrupole means that the effect of the force wave is the same in the direction of rotation of 180°, and opposite in the direction of rotation of 90° and 270°.

Qualitatively speaking.

For two pulsars, if their angle relative to the Earth is 0° or 180°, their timing residuals should be positively correlated. On the contrary, if their angle relative to the Earth is 90°, their timing residuals should be is anti-correlated.

Through careful calculation, the change of correlation with the angle can be obtained, which is the famous Heiling-Down curve.

Other factors that can cause coherence are difficult to have quadrupole. Therefore, if the correlation between different pulsar timing residuals can be found to satisfy the Heiling-Down curve, it can mean that the gravitational wave background in the universe has been detected.

This type of [pulsar detector] in later generations also has a name, called pulsar timing array.

The Rabbits’ Sky Eye, FAST, discovered evidence of the existence of nanohertz gravitational waves by relying on the pulsar timing array.

By the way.

At present, the most cutting-edge result in the field of gravitational waves is the discovery of scalar transversely polarized gravitational waves, which is somewhat deviated from the broad phase - Einstein's general theory of relativity predicts that gravitational waves only have tensor polarization mode.

Of course.

It would be a bit premature to say that the universal phase is wrong or that gravitons exist, but it is still very exciting in this regard.

His eyes returned to reality.

"Pulsar"

Then Yang Zhenning thought carefully about the idea mentioned by Xu Yun for a while and found that it could indeed solve a major problem he faced.

True.

If we only rely on pulsar timing arrays, the frequencies of gravitational waves that can be detected are relatively limited.

but

If the pulsar timing array can be combined with the space interferometer he designed, with one receiving on the ground and the other detecting at high altitude, the frequency of detectable gravitational waves can be reduced a lot.

Because gravitational waves are a physical phenomenon that can be modeled according to a power law, for some systems with relatively precise measurements, the rate of change of the orbital period can even be directly calculated through the wide phase.

In later generations, China had two gravitational wave projects, called Tai Chi and Tian Qin.

Among them, Tai Chi is a direct cooperation with LISA, while Tian Qin is purely domestic.

One of the principles of these two space gravitational wave detectors is to link with the original gravitational wave detection station on the domestic ground.

In December 2020, the Rabbits also launched two satellites and successfully put them into orbit, codenamed "Jimu" and "Xiaomou", the full name is "Gravitational Wave Burst High Energy Electromagnetic Counterpart All-Sky Monitor Satellite", which is actually Tianqin The youth version of the plan.

The ground unit they are linked to is the "Ali Project" implemented by the High Energy Institute of the Chinese Academy of Sciences, which is also one of the three major gravitational wave detection plans of the Rabbits.

Although Yang Zhenning does not know these things in the future, it is not difficult to judge the feasibility of this plan based on his academic ability.

In other words.

The issue he has to consider now is

"Xiao Xu, do you have any ideas about detecting pulsars?"

I heard Yang Zhenning’s question.

Xu Yun was silent for a while, and his tone became slightly more subtle:

"Mr. Yang, to tell you the truth, I do have some plans for this part, but the specific projects may be different from what you think."

"In and out?"

Yang Zhenning blinked and asked unclearly:

"what do you mean?"

Xu Yun quickly replied:

"I want to build a large-scale cosmic research base, and pulsars are just a small part of the research project."

Yang Zhenning was suddenly startled:

"base?"

After a while.

Xu Yun's voice came from the other side of the microphone:

"That's right, a large-scale cosmic observation and experiment base, the name is"

"Red Bank."