Encyclopedia of popular science knowledge: "Time Knowledge"

Chapter 15 Time's Fun Story (8)
Of course, this is just an idea.With the continued improvement in the quality of atomic clocks and the improved understanding of the laws of motion of satellites, we believe that the day of such verifications will not be long.

What does it mean that time has a beginning and an end?Is the universe infinite or finite?What do you mean by universe?With these problems resolved, the answer to the third question becomes clear.

Time is a form of motion and change in matter.Asking whether time has a beginning and an end is equivalent to asking whether the movement and change of matter have a beginning and an end.Indeed, the question has fascinated people for centuries, and has been addressed by everyone from Augustine to Kant.In modern times, with the development of mathematics, physics and astronomy, some natural scientists have given these problems a new and strange form.For convenience, let's start with "The End of Time."

A long time ago, Boltzmann, who confidently solved the mystery of the unidirectionality of time, proceeded from the principle of thermodynamics and conceived the end of the universe (and thus the end of time) as the stage when the entire universe reached the maximum entropy.He speculated that at some uncertain time in the future, nothing in the universe will be colder or hotter than others, the slowest radioactive elements will all decay into stable elements, and the stars will radiate out their energy; Part by part the frigid interstellar space warms up.

The rotation of the earth and its sister planet, the moon, will be slowed down by the friction of cosmic dust, and eventually they will fall out of their orbits and fall towards the sun, with the destruction of human beings themselves.In a "burned out" universe there is no longer the motion of matter upon which time can be observed and measured.At this time, time will come to an end, which is the so-called "heat death theory".

So, when will "this time" come?

玻尔兹曼本人没有给出具体期限。后来，支持他的学术观点的物理学家计算了铅”的衰变周期，得到“时间终结”到来的大概日期是：1.4×120年。他们说；铅204的半衰期是1.4×1017年，如果这个数字是正确的，假定它又是经过1000次这样衰变后成为稳定元素，那么就有1.4×1017×1000=1.4×1020年。

This is a conclusion based on the premise that lead does not change into other substances during decay.This is of course an extraordinary proposition.Extraordinary claims must have extraordinary evidence, otherwise they are unbelievable.Unfortunately, these physicists couldn't produce any evidence that lead would not transform into other substances.Therefore, rather than saying that this kind of calculation is science, it is better to say that it is a kind of fantasy.

In the 20th century, due to the advancement of astronomical observation technology, high-energy astrophysics flourished, and people learned more about the "mysterious island in the universe"-black holes.Therefore, some cosmologists re-propose the problem of the end of time based on the research results of black holes.The end of time, they say, lies in a black hole!

Now when it comes to black holes, maybe everyone no longer thinks it is new.This is not to say that its problems have been completely solved. On the contrary, there are more and more problems about black holes. Of course, the old problems have not been completely solved, and new problems still need to be further studied by scientists.

In general, the study of black holes is still in its infancy.

Black holes are in the universe, just like the long-rumored Bermuda Triangle on the earth, which is mysterious and terrifying in the eyes of ordinary people.From some brief reports, we know that it is a small and special celestial body in the universe, which evolved from some massive celestial bodies.It has a closed horizon (or border), and it keeps devouring the matter around it, even light radiation is hard to escape, but any matter within the horizon cannot escape to the outside.Scientists speculate that when a huge star dries up and collapses, the surface gravity increases, the surrounding space-time is greatly distorted, and finally it becomes a point, the so-called "cosmic singularity" - a black hole.Like other substances, time also has a singularity, and when it reaches this singularity, it will come to an end.This singularity is nothing but a black hole.Black holes represent the end of time or the frontier of time.Beyond this boundary, the concept of time in our universe cannot be applied.

If time has an end, it is natural to ask does it have a beginning?Where is the starting point?

The simple answer of scientists is: Yes, this is the Big Bang.

These scientists believe that our universe is currently expanding.After the expansion reaches the maximum value, it will explode and fall into destruction at the singularity of the universe, leaving only some invisible black holes.If the plot is reversed, the beginning of time is a huge explosion-the Big Bang of the universe.They say that the big bang is not a blast in the void, but a real "creation" in which time, space and all matter depend, that is, the source of time.

The Big Bang has been inferred to be the initial drive for the expansion of the universe.Many scientists like this theory, thinking that it better explains many known facts about the change and structure of matter in the universe.

However, there is a so-called childish problem here.

A child naively asked a priest, "You say God created the world, so who created God?"

Today, there are also many scientists who are not naive but seriously ask: "Who created the big bang? What was it like before the big bang?"

The answers to these questions may provide us with a more magnificent picture of the evolution of the universe.

Going back to the first 3 minutes of time, what was the scene like in the first few minutes of the birth of the universe?
It's a question that astronomers and physicists have hotly debated for years.Nobel laureate Steven Weinberg wrote a thick book for this - "The First 3 Minutes", which described in detail the interesting scene within only 180 seconds after the big bang.

Comparing 1 second with the 150 billion-year history of the universe is like comparing a mosquito with the diameter of the Milky Way!

1 second, fleeting.How have scientists been able to recreate the phenomena that took place in that brief moment 150 billion years ago?They are still based on "now".

The present is the key to the past.The same laws that can predict the future should also be able to reproduce the past.

Relying on the modern physical laws of cosmic matter, scientists not only revealed everything that happened when the life of the universe was less than 1 second, but also amazingly, they were able to go back tens of billions of years and reproduce the 10-43-[-] years when the universe was formed. Various phenomena at [-] seconds!
At 1 second from the Big Bang, protons and neutrons in the universe combined to form atoms.First, 2 protons and 2 neutrons combine to form a helium nucleus, and later to synthesize the nucleus of a slightly heavier element like lithium.This period is very short, the temperature drops rapidly, and there is no time to synthesize heavier elements.

The time is slightly pushed back, and in less than 1 second, the temperature of the universe is as high as 100 billion degrees, and the energy of light radiation is extremely large, which is called the radiation era of the universe.

When the cosmic time is 10-2 seconds (1/100), the main particles participating in the interaction are not photons, but electrons, U particles and neutrinos.These particles are all leptons, so this period is called the lepton age.

In the lepton era, the dominant force in the universe is the weak interaction, and a large number of electrons and positrons meet and annihilate, turning into photons.

Pushing back, at 10-6 seconds, which is one millionth of a second, the temperature of the universe is 10 trillion degrees, some particles with a mass larger than electrons are very active, there are a large number of protons and neutrons in the universe, and there are many particle.The role between them is strong interaction, so it is called hadron.This era is also called the Hadron era.

In the hadron era, the density of matter is very high, and it is in an ultra-dense state.Ultra-dense substances undergo phase transitions as the temperature drops, releasing a large amount of heat.The whole universe is like a small burning fireball.

The farther back in time, the hotter the universe gets.At cosmic time 10-36 seconds, the temperature reaches as high as 1028 degrees Kelvin.At this time, positive particles and antiparticles annihilate each other to form photons.

At 10-43 seconds, the temperature of the universe was 1032 degrees Kelvin, and the leptons known today and the quarks that make up hadrons were probably created at this time.

After 10-43 seconds, the temperature of the universe is higher, and the intense radiation destroys the atoms, causing the nuclei to decay.Under such conditions, matter as we know it today would not be able to exist, and elementary particles would themselves break down into more fundamental building blocks.

Going further back, such as the case where the cosmic time is 10-44 seconds, we don't know anything about it.

The above are the explanations of scientists for various phenomena that have occurred from the first moment of the Big Bang to the present.They relied on the principle of thermodynamics (that every time the radius of the universe doubles, its absolute temperature drops to 1/2 of its original value), and by monitoring the microwave radiation thrown off by the formation of the universe.The picture they paint seems to reveal to us an irresistible course of human evolution.

The Big Bang produced the gas that formed the galaxies, and the stars in the galaxies scattered fragments rich in various elements, then formed planets, and then began the evolutionary history of our species.

What was it like before the Big Bang?To be honest, scientists have not yet been able to answer this question.

Of course, there are many speculations about it, and there are different opinions.

Among them is the tantalizing idea that the universe is currently expanding outwards, but that the interactions of receding galaxies will slow the expansion down, possibly stopping it at some point, and then starting the opposite process - contraction. .If the contraction and extension continue, the universe will be condensed into one point, another big bang will appear, and everything will start again.

This means that the explosion that started our universe is the explosion after the previous explosion, and the end of our universe will also be the beginning of another universe.The universe, it seems, is like the Longevity Island in the fable, which periodically burns and destroys, and at the same time is born in its own ruins.So the end of time is also the beginning, and the universe itself is a fundamental clock, ticking its own magnificent cycle of expansion and contraction.

How long is this cycle of the universe?This is again an unimaginable and difficult to calculate problem.

(End of this chapter)