Encyclopedia of popular science knowledge: "Time Knowledge"

Chapter 13 Time's Fun Story (6)
The focus of people's thinking is on these three basic questions: what is the essence of time, where does the arrow of time point, and whether it has a beginning and an end?

Among these questions, some have already been understood by people, and some have not yet been satisfactorily answered.Perhaps no such answer exists at all.However, the raising of such questions and the exploration of their answers are not without meaning, it can give us the opportunity to reveal more about the characteristics of time and the universe in which human beings live.

Proof that time does not go backwards We are all familiar with such phrases as "time flies like an arrow" and "time waits for me".It seems to imply that time is passing at a certain speed.We also often have the feeling that the day "passes by in a hurry".However, on closer inspection, this concept has no practical significance, because how does time flow, how fast does it flow, is it a fraction of a second?

Movement and marching require some sort of fixed time frame in which to measure their progress.For motion through space we frame time, but when it comes to motion through time, what should we frame?
We only know that the macroscopic motion process in nature (a large-scale motion process) is unidirectional, and it is impossible for people to watch the image of the TV station before it broadcasts the program.

In daily life, most of the processes we see are also one-way, from young to old, and from new to old houses.Mountains are split and eroded, stars slowly dissipate energy, and the universe continues to expand... We have never seen the opposite process, for example, houses automatically gather together, and rivers flow uphill.

These facts show that no matter on the earth or on the scale of space, there is a time direction in the universe: it goes forward in one direction and never goes backward.

But in ancient times, people used to fantasize about going back in time.They hope that there will be a kind of panacea, after people take it, they can live forever, or even rejuvenate.This is not uncommon in ancient Chinese myths and legends.There are similar descriptions in foreign literature.

However, common sense tells us that time does not turn back!Reversing time is as much an illusion as making time stand still.

The kettle boiled, only after we put it on the stove to heat it, never before.Apples can only ripen after the fruit tree has bloomed.People will grow old with the passage of time, there is no such thing as living younger.These are common sense that everyone knows.

However, common sense is often not all truth.How do you prove that common sense is reliable?
The evidence is there, heat conversion is one example.

If we put a piece of ice in a glass of water, the ice cube will absorb the heat of the water and the glass and dissolve, and the water and the glass will become colder by giving up some of their heat.If we make a movie of this process and replay it in the opposite direction, we will see that as one part of the water heats up, the other part freezes.This means that heat can only flow from a warmer object to a cooler object, not the other way around.

More than a century ago, physicist Clausius summed up this phenomenon as the second law of thermodynamics.It shows that the flow of energy, especially heat energy, always goes in one direction.Energy can be stored, but it has a tendency to dissipate.The second law of thermodynamics deals with entropy, one of the most fundamental concepts in physics.While Clausius himself did not apply the principle of entropy to time, many subsequent physicists did.They found that the flow of thermal energy and the flow of time occurred simultaneously, thus demonstrating the unidirectionality of the arrow of time.

However, why does time only flow in one direction?
Some physicists believe that the unidirectionality of time is a tendency to disrupt order, with the universe becoming slightly more disordered every day.

At the end of the last century, the Austrian physicist Boltzmann used simple experiments to study the evolution of disorder.The experiment requires only 3 jars, 40 numbered cards and 40 numbered balls.

At the beginning of the experiment, all the cards were placed in the first bottle, all the balls were placed in the second bottle, and the third bottle was empty.Draw a card at random from the first bottle, transfer its numbered ball from the second bottle to the third bottle, and put the card back in the first bottle.Do it repeatedly.Each time, either the ball was transferred from the second bottle to the third bottle, or vice versa, the ball was transferred from the third bottle to the second bottle.After about 25 times, the number of balls in the two bottles is nearly equal.Boltzmann pointed out that as long as the cards are drawn at random, order must give way to disorder.From this, he thought he had solved the mystery of the unidirectionality of time.

But we know that the so-called order-disorder trend is not a strict law, it is just a matter of probability and statistics.The crux of the problem is not the process of order and disorder itself, but how to achieve the order state in the first place.Why does the universe tend to change from order to disorder, while matter and energy have a high degree of order?
Boltzmann responded somewhat playfully to this.The present order, he said, was due to a rare and gigantic fluctuation in the universe which lifted it out of its most probable state of chaos—pure luck!
Boltzmann attributes his conclusions to "chance," which is tantamount to declaring that he has not solved the mystery of the unidirectionality of time.

The macroscopic one-way flow of time (also known as irreversibility) is a fact that human beings have observed.How to explain this phenomenon has not been concluded so far.It stands to reason that one can and should continue to study it, taking advantage of the expanded horizon and the latest scientific results.However, in recent decades, some people started from the physical phenomenon of space-time curvature in general relativity, and drew the conclusion that time can be cycled.This is tantamount to saying that time can turn back!They believe that the theory of curved space-time harbors a new possibility that the line of time will close into a circle or some other more complicated curve.In this way, time can flow along this closed curve, from today to tomorrow, or back to yesterday.They said that as long as the shape of the universe is subject to certain constraints, such as constantly "winding up" itself, it is possible to return to the past state, all one-way processes can be reversed, and time can also be reversed.

So in what way will the universe keep "winding up" itself?Only by relying on God and the gods1 In this way, the time cycle theory will return to the medieval creationist philosophical point of view.

Aristotle's Paradox of Time and Motion The first person in the world who tried to determine the relationship between time and motion from the perspective of physics was probably the ancient Greek thinker Aristotle.

Aristotle lived in the 4th century BC, which is equivalent to the Warring States Period in my country.He wrote a book called Metaphysics.In this book, he declares: "We can only comprehend time when we have grasped motion."

But he added: "We measure not only movement by time, but also time by movement, because they define each other."

If Aristotle came close to describing the partial relationship between time and motion correctly, his views became paradoxical when it came to explaining the nature and causes of motion.

Starting from the superficial "facts" in nature, Aristotle believed that any moving object has a natural tendency to tend to rest.A thrown stone will quickly change from rolling to stationary, and the cart will stop if the horse does not pull it.Aristotle thus led to his own theory: the speed of motion is directly proportional to the force that produces it.A cart drawn by two horses is "naturally" twice as fast as a cart drawn by one horse; a stone weighing 10 kg is "naturally" dropped faster than a stone weighing 5 kg. It's twice as fast.

This is how Aristotle described the nature of motion.

However, how did the movement arise?Aristotle believed that nothing in nature can move by itself, and the movement of an object must have another object to push it.He said: "If one motion is caused by another body, the latter's motion must also be caused by some other motion. If the reasoning goes on infinitely, it is impossible to arrive at the result. The original motion of each motion must be attributed to In a divine body moving in the sky."

Thus, Aristotle was the first to introduce the gods into physics as the first movers not moved by other things, and thus also ascribed the role of the gods to time, because time and motion "are mutually defined" .

After the collapse of religious theology in the Middle Ages, Aristotle's erroneous views on motion, time, and many other issues still dominated science for hundreds of years.

It was not until about the 13th century that scientists defined speed more accurately.The motion of an object, they said, was the simple change in its position in space, and the velocity was how much the object changed its position in a given period of time.To this day, we still use this way of expressing speed in meters per second or kilometers per hour.

However, it was Galileo who really broke the Aristotle paradox.

We refer to Galileo more than once in this book.He is a great astronomer, the second to invent the telescope and expand people's horizon, and he is also a famous physicist.The records of his troubled life have been made into a movie and put on the screen to meet millions of viewers.

Galileo pointed out sharply: "The heavier the object, the faster the fall" is logically contradictory.If a heavy object and a light object fall at the same time, the times are t1 and t2 respectively, and the two objects are tied together, what is the time for them to fall?According to Aristotle, there will be two answers:

(1) The heavy object drives the light object to fall quickly, and the light object affects the heavy object to fall slowly, so t1<t<t2; (2) Two objects tied together must be heavier than a single object, and the falling time must be t<t1< t2.

These two results contradict each other.Therefore, Galileo believed that Aristotle's assertion could not be established.

According to legend, Galileo also conducted a falling experiment on the Leaning Tower of Pisa to prove the error of Aristotle's theory.Whether or not this story is true (and historians of science have long disputed it), the important point is that this genius Italian actually timed moving objects.He rolled metal balls down slopes of different lengths, and at the same time collected the water dripping from the leak in a cup, weighed the water, and determined the time it took for the balls to roll down different slopes.

Based on these experiments, Galileo further pointed out that velocity alone—the change of position with time—is not enough to define motion, and the change of velocity with time must also be considered, which is acceleration.

Here, Galileo just raised questions, but made no theoretical discoveries.The establishment of the concept of acceleration was completed later by Newton.

Newton's "absolute time" concept Newton is one step further than Galileo.Newton believed that, contrary to Aristotle's "theory," a body in motion could never stand still if nothing else prevented it.The reason why a falling stone falls to the ground and does not move is because it is stopped by the earth; the reason why a chariot cannot stop is because of the friction between the wheels and the road surface.On a smooth, level road, a carriage with frictionless bearings will keep rolling.Therefore, Newton pointed out that the effect of force on an object is only to change its speed of motion with time.This amount of change is called acceleration, and it is proportional to the magnitude of the applied force.

This is Newton's second law of kinematics.Expressed in a well-known formula is:
F=ma where F is the force, m and a are the mass and acceleration of the object under force respectively.

Newton's laws of motion, together with the law of universal gravitation he deduced in 1684, laid the foundation of classical physics and had a great influence on the development of natural science at that time and later. It is still widely used today and continues to play a huge role .

It should be noted, however, that Newton's laws are based on the concept that the time used to measure motion is an "absolute time" that passes uniformly.

What is absolute time?Newton gave the following definition in his 1687 book, The Mathematical Principles of Natural Philosophy:
"Absolute, true mathematical time, by itself and by its very nature, flows uniformly forever, independent of anything external."

This view of Newton distorts the relationship between time and motion, and is also contradictory within his own theoretical system.Because he has already admitted that movement is not absolute.That being the case, how do you measure or perceive absolute time?
Newton argued that he was able to do this.He said that he could prove the existence of absolute time by means of other forms of motion, namely rotational motion.In his view, rotational motion is absolute.Newton gave such an example at the beginning: If a bucket is hung on a curly rope and it is rotated in the direction where the rope is untied, the water surface will rise along the edge of the bucket and form a concave shape.The faster the spin, the higher the water rises.This is the famous "bucket experiment".Newton said that the rise of the water surface is an absolute motion, which proves the existence of absolute time in principle and provides a method for measuring absolute time.

Newton's defense is obviously untenable.Because the bucket rotates in space, it must be relative to some other object in the universe, so it is not absolute.But Newton said again, "If it rotates in a vacuum; it will still give the same result." However, Newton did not, and subsequent physicists did not produce any experimental evidence to prove that the rotation of the bucket in the universe is absolute .In fact, such evidence will never be found!
Still, Newton stuck to his point.He believes that, in principle, there should be an ideal time scale—absolute time, which can exist independently of any specific events and processes.

This view of Newton was opposed by his contemporary mathematician Leibniz.Leibniz argued that events are more fundamental than time, and that it is absurd to think that time without events can exist.In his view, time is derived from events, and all simultaneous events constitute a phase of the universe, and these phases follow one another like yesterday, today, and tomorrow.Leibniz's theory of relative time seems to be more acceptable than Newton's theory today, because it is more in line with the development of modern physics.

However, Newton's views remained dominant in the 18th and 19th centuries.Because it is supported by the church.Newton himself said this in a letter to the church: "Using these principles may make thoughtful people believe in the existence of God." Therefore, Newton's absolute time theory was praised as the absolute truth of the entire universe at that time.Until the beginning of this century, it was generally believed that there was a unique, universally applicable system of time that did not depend on anything else.Because of this, physics suffered an earthquake when in 1905 Einstein discovered a hole in the theory of time that no one had ever suspected, thereby overturning these assumptions and the entire philosophy of time based on them.

This loophole is the relativity theory of time revealed by the special theory of relativity.

The relativity of time When he was still a student, Einstein pondered such a puzzling question: If he traveled at the speed of light through the ether, what would he see?According to the principle of relativity of motion, the light beam should be equivalent to the oscillating electromagnetic field in the static space at this time, but this view is inconsistent with Maxwell's theory.Einstein then began to conjecture that the laws of mechanics, and other laws of physics, including the propagation of light, must have the same form for observers moving at different velocities.He believes that the principle of relativity can be applied not only to mechanical phenomena, but also to optical and electromagnetic phenomena.The speed of light is the same not only for relatively stationary observers, but also for those in relatively uniform motion.The zero result of the Michelson-Morley experiment is "correct" because: first, there is no ether; second, the speed of light remains unchanged.

Einstein then took these two conclusions as the premise, promoted Galileo's principle of relativity, and established his own, more general new theory - special relativity.The so-called "narrow sense" means that it is limited to the occasion of uniform motion.

The special theory of relativity points out that whether it is a mechanical phenomenon, or an optical and electromagnetic phenomenon, the laws they follow have nothing to do with the motion state of the inertial system.

In this way, Einstein perfectly resolved the contradiction between Maxwell's electromagnetic wave theory and other parts of physics based on Newton's laws of mechanics, thus creating a new era of physics.

The special theory of relativity was published in 1905, the title of the paper was "On the Electrodynamics of Moving Bodies".From this article, we can see that Einstein solved the problem by analyzing the concept of time, and also made a breakthrough on the problem of "relativity of simultaneity".He realized that time was suspicious, believed that time could not be absolutely defined, and pointed out that the measurement of the elbow depends on people's understanding of "simultaneity".That is, the measurement of time intervals necessarily involves judgments of simultaneity, the coincidence of one event with another in time.In his article "On the Electrodynamics of Moving Bodies", he has a wonderful statement on this point:

(End of this chapter)