Stephen Hawking: Einstein's dream

In the early twentieth century, two new theories completely changed our view of space, time and reality. Over the next 75 years, we always study problems related to those two theories to find ways to combine them into a unified theory to describe all that is in the universe.

Those two theories are general relativity and quantum mechanics. General theory of relativity examines their space, time and curvature on the macro scale caused by matter and energy in the universe. Quantum mechanics surveyed on the micro scale. Quantum mechanics has an uncertain principle. This principle confirms that it is impossible to determine the exact position and velocity of a particle simultaneously: the more accurate the position of the measurement, the more incorrect the measurement speed and vice versa. There is always an uncertain or random element and this basically affects the character of the material on the micro scale. Almost alone Einstein was responsible for the general theory of relativity and he played an important role in the development of quantum mechanics. He summarized his impression of quantum mechanics by saying: "God does not play dice." But every test has shown that God is a constant player, he does not miss the opportunity to throw a dice.

National laws are valid only in one country, but the laws of physics are the same in the United Kingdom, the United States and Japan. It is the same on Mars and in the Andromède galaxy. Moreover, they are the same for any of our movement velocities; with people traveling on express trains, traveling on jets or standing on the ground, the laws of physics are the same. Certainly a person standing still on the ground is moving at about 30km per second around the Sun; The Sun is moving around the Milky Way at hundreds of kilometers for a second and so on. But these movements do not change the laws of physics, they are the same for every observer.

Galilée discovered the independence of the system's velocity, he formulated the formulas that represent the laws that act on the movements of objects such as cannonballs or planets. But a problem arises when we apply to light. Since the eighteenth century, it has been discovered that light does not transmit instantly from the source to the observer: it transmits at a specified speed of about 300,000 kilometers in a second. But what is this speed compared to? It is thought that there must be an environment filled with the space that light passes through. That environment is called ether.

The idea of ​​watching TRANSMISSION WAVE at 300,000km per second compared to ether means that a observer standing still against the ether will measure the speed of light at 300,000km / s, while another observer The motion for the ether will measure the speed of light small or greater than 300,000 km / s. In particular, the speed of light must change when the Earth moves towards the ether in orbit around the Sun. But in 1887, by a very precise experiment, Michelson and Morley proved that the speed of light was always constant. The observers of different motions always measured the same speed of light as 300,000 km / s. What is the truth? Why do observers moving at different speeds measure the same speed of light? The answer is our usual thinking about space and time is no longer appropriate. In a famous work in 1905, Einstein noted that these observers could measure the same speed of light if they ignored the idea of ​​universal time. Each person has his or her own time measured by his own watch. The times measured by these different clocks will be nearly the same if they move slowly against each other, but the difference will be significant if they move at very high speeds. This phenomenon is actually observed by comparing two identical clocks: one standing on the ground, one on the plane moving at great speed. At normal speeds, the time difference of the clocks is negligible.

Einstein's theory of relativity was first presented by him in his famous work in 1905 and is today called "the theory of relativity". This theory describes the movement of objects in space and time; it proves that time is not a universal cosmic quantity, time-independent free space. The future and the past; High, right, left, front and back, are the dimensions of what we call space-time. One can only move in time toward the future according to a certain angle to the time axis. So time can go by different tempo.

Narrow theory of relativity has combined space and time, but they are still a fixed frame in which events occur. We can choose to follow different paths in space - time but everything we do cannot change the time-frame. From 1915 when Einstein stated general theory of relativity, everything changed. He had a revolutionary idea that gravity was not merely a force exerted in the frame - fixed time but the presence of matter and energy distorted space - time. Objects, cannon bullets or planets move in orbit straight in space-time but that trajectory has been bent because - time has curled rather than flattened. The Earth seeks to move in a straight trajectory in space-time but the Sun's mass bends space-time and forces Earth to move around the Sun.

Likewise, light tries to move in a straight line but the curvature of space - time near the Sun causes light emitted from distant stars to deflect transmission when approaching the Sun. Usually it is impossible to distinguish distant stars from the same direction as the Sun. But when there is an eclipse, most of the light from the Sun to the Earth is shielded by the Moon, when light can be observed from those stars. Einstein established general theory of relativity during World War I; At this time, it is not possible to conduct scientific observations. Immediately after the World War, an English survey team observed the solar eclipse of 1919 that confirmed the prediction of general relativity: no - time was not flat but curved in place with matter and energy. . This is Einstein's great victory. His invention completely changed the way we previously thought about space-time. No - time is not a passive frame in which events take place. We can no longer think that space, time goes on forever and will not change by what happens in the universe; on the contrary, they become motivational quantities interacting with events occurring in them.

An important property of mass and energy is that they are always positive. That's why gravity between objects is always gravitational. Earth's gravity example pulls us towards Earth, wherever we are on the ground. The Sun's gravity holds its satellites in orbit around the Sun and prevents Earth from turning into the dark interstellar space. If the volume is negative, no - time will be bent in the opposite direction as if the surface of a saddle. The positive curvature of space-time, which expresses gravity as the gravitational force, poses a great problem for Einstein. Normally people think the universe is static, but if space and especially time are folded, why can the universe continue to exist forever in a state that is almost identical to the current state? its ?

Initially, the equations of general relativity predicted a universe that was expanding or shrinking. Einstein added a number of new terms - called cosmological constants - to the equations of mass and energy present in the universe with the curvature of space-time. This cosmological constant causes a gravitational gravitational effect. Thus the attraction of matter and energy can be balanced with the repulsive force of cosmological constant. In other words, the negative curvature of zero - time caused by cosmological constant can eliminate positive curvature caused by mass and energy in the universe. Such a universe model will exist forever in the same state. If Einstein kept his original equations, he would predict whether the universe was expanding, or shrinking. Before 1929 when Edwin Hubble discovered galaxies moving away from us, nobody thought that the universe changed over time. The universe is expanding. Einstein later declared that cosmological constant is "the biggest mistake in my life".

But with or without cosmological constants, the curvature of matter - time is still a general problem that is not well understood; Especially matter can bend a domain into a point separate from the rest of the universe, which becomes a "black hole". Things can fall into the black hole but cannot get out of there; To escape the black hole, the object must move at a speed greater than the speed of light, this theory does not allow. Thus matter will be imprisoned in a black hole and compressed by gravity to an unknown state with a very high density of matter. Einstein was confused by this fascinating collapse, he did not believe in the possibility of that collapse. But in 1939, Robert Oppenheimer proved that an older star twice the mass of the Sun once consumed its nuclear fuel would inevitably be gravely collapsed.

Then the Second World War broke out, Oppenheimer was attracted to the nuclear bomb program and he forgot about the gravitational collapse. Researchers who are much interested in physics can study Earth. They doubted the predictions of the universe's boundary because it seemed impossible to test with observations. But in the 1960s, the marvelous improvement of the long range and the quality of astronomical observations led to a new interest in gravitational collapse and the early universe. The exact predictions of general relativity in this situation are still not very bright until Roger Penrose and I prove some theorems. These theorems prove that time - must be bent and lead to singularities that are regions where no - time has a beginning or an end. The starting point is the Big Bang about 15 billion years ago and the end point will be in a victim star of a collapse or with all the objects falling into a black hole.

The prediction of the existence of singularities according to Einstein's general theory of relativity has been confirmed but has led to a crisis in physics. At a singularity, the equations of general relativity relate to the curvature of space - time with the distribution of mass and energy is unknown. This means that general relativity cannot predict what happens at a singularity. In particular, this theory does not have the ability to predict how the universe should be born in Big Bang. Thus general theory of relativity is not a complete theory. It needs an additional part to determine how the universe must be born and what must happen when matter collapses due to its own gravity. It seems that quantum mechanics is the necessary supplement. In 1905, the year Einstein wrote the theory of relativity, he also published the work on "photoelectric effect". He observed that when light shines on some metals, it will fire charged particles. The confusion is that if the intensity of the light is reduced, the number of charged particles will decrease but the velocity of the particles will not decrease. Einstein proved that this is only explained if the light that is coming is not small parts that are constantly changing like people then still conceive that light is only a definite bundle in one direction.

CONCEPT OF WATCHING THE PRESENT LIGHT ONLY in the form of bundles, called quantum, was launched a few years ago by German physicist Max Planck. Planck used the quantum concept to explain why a piece of red-heated metal did not emit an infinite amount of heat, but he viewed quantum just as a pure theoretical trick without a correspondence. in physical reality. In his work, Einstein proved that we can observe quantum directly. Each emitted particle corresponds to a light quantum that hits the metal. People realized this was a great contribution to quantum theory and this work gave Einstein the Nobel Prize in 1922 (He should have received the Nobel Prize for general relativity but the idea was not The time and time of bending at the time was considered too critical, so Einstein received the Nobel Prize for photovoltaic work, a work that also deserves to be awarded).

It was only fully understood the photoelectric effect when 1924 Werner Heisenberg noted that the effect was meant to be impossible to accurately measure the position of a particle. In order to identify a particle, light should be projected onto the particle. But Einstein proved that we cannot use an arbitrary small amount of light but need to use at least one bundle or one quantum. Bundling this light to the particle disturbs the particle and transmits the particle to a certain speed. If the more we want to accurately measure the position of the particle, the greater the energy of the bundle of light coming into it and the stronger the disturbance. Regardless of how the particle is measured, the uncertainty about the position multiplied by the uncertainty about the particle's velocity is always greater than a small amount. Heisenberg's uncertainty principle proves that it is impossible to accurately measure the state of a system and cannot accurately predict the nature of the future system. The best we can do is predict the probability of different possibilities happening. This is a random element that disturbed Einstein. He did not believe that the laws of physics were incapable of making a definite prediction; there is no possibility of double ambiguity. But every test has proved that quantum phenomena and uncertainty principle are inevitable and that we encounter it in every physics.

Thus Einstein's general theory of relativity is a classical theory, it has not been attached to the uncertainty principle. It is necessary to find a new theory which incorporates the theory of general relativity with the uncertainty principle. In most cases, the difference between the new theory and the classical general theory of relativity is extremely small because the uncertainty principle predicts quantum effects only play a significant role in small, internal scales. when the general theory of relativity examines the structure of zero - time is very large. Roger Penrose's bizarre theorems and I proved that space-time can only bend very strongly on very small scales. The consequences of the uncertainty principle will become very important, the results are expected to be significant.

Part of the difficulty that Einstein encountered with quantum mechanics and the uncertainty principle derived from common sense, consistent with conscience, was a system with a definite history. A county must be in one place or another; it can't be half here, half there. Also, the event of bringing the astronaut to the Moon is either happening or not; It is impossible for each possibility to happen in half. A person cannot be dead a little or in a womb. Either he exists or does not exist. But if a system has a unique history and determination, the uncertainty principle leads to all kinds of paradoxes such as the simultaneous presence of a particle or a space pilot half on the Moon. .

American physicist Richard Feynman subtly proposed to avoid the paradoxes that bothered Einstein. Feynman became famous in 1948 by his work on light quantum theory. He was awarded the Nobel Prize in 1965 along with Julian Schwinger of the United States and Japanese Shinchiro Tomonaga. Einstein did not like solemnity and frivolous arguments; He resigned from his position at the National Academy of Sciences because he found it took too much time to discuss the admission of new members. Feynman is famous for his contributions to theoretical physics. He died in 1988. In his contributions, the graphs named after him - Feynman graphs - are the basis for most calculations in elementary particle physics. But more importantly, his concept of taking total on history.

THE IDEA IS A SYSTEM WITH NO ONE history in space - time as was normally acknowledged in non-quantum theory that has all its possible history. For example at a point A at a time of verification there is one particle. Normally we consider that particle moves in a straight line starting from A. But in the way of taking the total on the history, the particle can follow any line derived from A. This happens similarly when we to drop a drop of ink onto an absorbent paper. Ink particles spread out on a sheet of paper that permeates every possible path. For each road or every history of the county will link some sugar-dependent dependencies. We will obtain the probability of particles going from A to B by adding up all the numbers associated with the paths from A to B. For most roads, the number associated with one line will cancel with the number of links of the adjacent roads and they only contribute very little to the probability of the particle going from A to B. But with the straight line, its number of links to the number of links of the proximal lines will strengthen each other. Thus, the main contribution is from straight and straight lines. That is why a primary particle when moving in a bubble chamber will leave a straight line. But if on the path of the particle, we place a shield on it with a narrow gap, the orbits of the particle will spread out behind the slit. Not to mention on the straight line passing through the slot, in other places the ability to find particles will be more.

In 1973 I began to study the effect of uncertainty principle on particles in space - time bent near a black hole. I found it very surprising that the black hole was not completely black. Uncertainty principle allows particles and radiation to often leave the black hole. This result surprised me and everyone and it was well received with skepticism. But looking at the past, it is almost obvious. A black hole is a spatial domain where everything cannot leave it unless they move at speeds greater than the speed of light. But the summing of all the histories of Feynman says that particles can follow every possible path in space - time. Thus a particle has the ability to move faster than light. The probability of particles moving in orbit at speeds greater than the speed of light is very small, but enough for the particle to reach the distance allowed to escape the attraction of the black hole. Thus the uncertainty principle allows the particles to leave the black hole - the most secure prison in the prison - The probability of a particle leaving a black hole of mass equal to the mass of the Sun will be very small because so the particle must move at a speed greater than the speed of light on a multi-kilometer road.

However, there may be many smaller black holes; they were created from the beginning of the universe. These primitive black holes are smaller than the size of atomic nuclei but they have a mass of up to a billion tons like Mount Fuji. These black holes can emit energy with the energy of a giant power plant. Just need to find one of those black holes we will master in terms of energy! Unfortunately there don't seem to be many of those black holes in the universe. The prediction of emitting black holes is the first result of a combination of general relativity theory and quantum mechanics. It proves that gravitational collapse is not an absolute bottom of the bag as we thought. The beads of a black hole have yet to fully complete its history at a singularity. They may leave the black hole outside to continue their history.

It is possible that quantum mechanics also means that it avoids history having a beginning in time, a point of creation in Big Bang. This is a more difficult question to answer because it is necessary to apply quantum mechanics to the structure of time and space, not only for the orbit of particles in a particular region of space-time. What to look for is how to take history by history not only for particles but also for the whole background of space and time. We do not know how to calculate it properly but there are some instructions about what to do. First, the above calculation will be easiest if we examine the history in virtual time and not in real time. Virtual time is an elusive concept and it must have raised the most questions for readers of the book: "A Brief History of Time " *).

CONCEPT OF MY FANTASY TIME Has received strong criticism from some philosophers. Why does virtual time have a connection with the real universe? I think these philosophers have not learned the history lesson. People used to see the Earth as flat and the Sun orbiting the Earth is obvious. Since Copernic and Galilée, we have to get used to the idea that the Earth is round and that it revolves around the Sun. People also saw the same time-lapse with all observers as obvious; but since Einstein's theory of relativity, we have to accept different time lapse for different observers. Also, the universe is considered to have a unique history but when there is quantum mechanics, we need to examine the universe with all its possible histories. For me, it seems that virtual time is something that needs to be accepted. This is a level of intellectual leap as the Earth is considered round. I think that virtual time today appears naturally as the roundness of the previous Earth. There is no more advanced person who agrees with the idea of ​​Flat Earth. We can represent real time in a straight line from left to right, but we can examine another direction of time from low to high. It is virtual time, it is perpendicular to real time.

What are the benefits of bringing the concept of virtual time into? Why not stop at normal real time as you know? The reason as seen above is matter and energy that bends space - time. The direction of real time will inevitably lead to singularities, leading to the regions in which there is no - the end time. In a bizarre, physical equations are no longer defined, we cannot predict what will happen. But virtual time is perpendicular to real time. This means that virtual time behaves in a three-dimensional way of moving in space. Như thế độ cong của không - thời gian gây bởi vật chất trong vũ trụ có thể dẫn tới ba chiều của không gian và một chiều của thời gian ảo tạo thành một vòng. Chúng cũng tạo thành một mặt kín như bề mặt Trái Ðất. Các chiều của không gian và thời gian ảo sẽ tạo thành một không - thời gian đóng kín không có biên, không có bờ, không có điểm có thể gọi là bắt đầu hoặc kết thúc, giống như trên mặt Trái Ðất không có điểm bắt đầu hoặc điểm kết thúc.

Năm 1983, Jim Hartle và tôi đã gợi ý là phép lấy tổng trên các lịch sử của vũ trụ không thể thực hiện trong thời gian thực nhưng lại thực hiện được trong thời gian ảo; các lịch sử được đóng kín lại vào chính nó như mặt của Trái Ðất. Các lịch sử này không có kỳ dị, không có bắt đầu hoặc kết thúc; những gì sẽ xảy ra được xác định hoàn toàn bởi các định luật vật lý. Như thế là những gì sẽ xảy ra trong thời gian ảo có thể tính toán được; và nếu biết lịch sử của vũ trụ trong thời gian ảo thì có thể tính toán được sự diễn biến của nó trong thời gian thực.

Như vậy ta có thể hy vọng đi tới một lý thuyết thống nhất hoàn toàn, một lý thuyết cho phép tiên đoán mọi cái trong vũ trụ. Einstein đã đi tìm một lý thuyết như vậy trong những năm cuối đời nhưng ông không tìm được vì ông nghi ngờ cơ học lượng tử. Ông đã không sẵn sàng chấp nhận vũ trụ có thể có nhiều lịch sử xen kẽ như trong phép tổng trên các lịch sử. Chúng ta không phải lúc nào cũng biết cách thực hiện tổng trên các lịch sử của vũ trụ, nhưng gần như chắc chắn tổng đó sẽ đi qua thời gian ảo và một không - thời gian khép kín. Tôi nghĩ rằng những khái niệm này sẽ đi vào thế hệ tương lai cũng tự nhiên như ý tưởng Trái Ðất là tròn. Thời gian ảo đã là một nơi chung cho khoa học viễn tưởng. Nhưng đâu còn là khoa học viễn tưởng hoặc một thủ thuật toán học; thời gian ảo cấu tạo nên vũ trụ mà ta đang sống.

(Stephen Hawking trích trong "Trous noirs et bébés univers", NXB Odile - Jacob, 1994)
Người dịch : Nguyễn Ðình Ðiện