Extended reading: "Black Holes and the Baby Universe", anyone with time can take a look.

Falling into black holes has become a horror scene in science fiction. Now black holes are actually called scientific reality, not scientific fantasy. As I would describe, we already have strong reasons to predict that black holes must exist. Observational evidence strongly shows that there are some black holes in our own galaxy, and more in other galaxies.

[17] Author's Note: This is a lecture at Hitchcock, University of California, Berkeley in April 1988.

Of course, what science fiction writers really do is to describe to you what will happen if you really fall into a black hole. Many people think that if the black hole is spinning, you can pass through a small hole in space and space to another area of ​​the universe. This obviously creates the possibility of space travel. If we want to think of other stars, and not to mention that other galaxies will become a reality in the future, this is indeed something we dream of. Otherwise, the fact that nothing can travel faster than light means that the journey to the nearest star will take at least eight years. This is the time to spend a weekend in Sagittarius! On the other hand, if people can pass through a black hole, they can reappear anywhere in the universe. It is not clear how to choose your destination. At first you might think of a Virgo vacation, but it ends up in the Crab Nebula.

I would like to tell future galaxy travelers with great regret that this scenario would not work. If you jump into a black hole, you will be torn to pieces. However, in a sense, the particles that make up your body will continue to run into another universe. I don't know whether someone who is crushed into spaghetti in a black hole will be of great comfort to him if he knows that his particles may survive.

Despite my slightly slight tone here, this lecture is based on reliable scientific basis. Most of what I speak here is now endorsed by other scientists who have studied in this field, although this has happened in recent times. However, the final part of this lecture is based on recent work that has not yet reached a consensus. It has aroused great interest and excitement.

Although the concept we now call black holes can be traced back more than two hundred years ago, the name "black hole" was proposed by American physicist John Wheeler until 1967. This is really a genius move: the name itself guarantees that the black hole enters the mysterious kingdom of science fiction. Providing an exact name for something that was originally not satisfied with the name also stimulates scientific research. The importance of a good name cannot be underestimated in science.

As far as I know, the first thing to discuss the black hole is a Cambridge man named John Michel, who wrote a paper in 1783. His thoughts are as follows: Suppose you ignite a shell upwards on the surface of the earth. During its ascent, its speed slows down due to gravitational effects. It will eventually stop rising and fall back on Earth. However, if its initial velocity is greater than a certain critical value, it will never stop rising and fall back, but continue to move outward. This critical velocity is called the escape speed. The escape speed of the earth is about seven miles per second, and the sun is about one hundred miles per second. Both of these velocities are

They are greater than the speed of actual shells, but they are too small than the speed of light, which is 186,000 miles per second. This shows that gravity has little effect on light, and light can escape from the earth or the sun without any difficulty. However, Michel inferred that perhaps there may be a star with a large mass and a small enough scale, so that its escape speed is greater than the speed of light. Because the light emitted from the surface of the star will be pulled back by the star's gravitational field, it cannot reach us, so we cannot see the star. However, we can detect its existence based on the effect of its gravitational field acting on nearby objects.

Treating light as a shell is inconsistent. According to an experiment conducted in 1897, light always travels at a constant speed. So how can gravity slow down light? It was not until Einstein proposed the general theory of relativity in 1915 that people had a self-consistent theory of gravity on light effects. Nevertheless, it was not until the 1960s that people widely realized the meaning of this theory to old stars and other heavy-mass objects.

According to general theory of relativity, space and time are considered to form a four-dimensional space called space-time. This space is not flat, it is distorted or bent by matter and energy in it. This curvature can be observed in the bending of light or radio waves coming to us near the sun. This bending is very small when the light passes through the sun's adjacent to the sun. However, if the sun is shrinking to a scale of only a few miles, this bending will be so powerful that the light emitted from the surface of the sun cannot escape and is pulled back by the sun's gravitational field. According to theory of relativity, nothing can travel faster than light, so that there is an area where nothing can escape. This area is called a black hole. Its boundary is called an event horizon. It is formed by light that just cannot escape from the black hole and can only stay on the edge.

It may seem incredible to assume that solar energy shrinks to a scale of only a few miles. One might think that matter cannot be compressed to this extent. But in reality it is possible.

The existing scale is because it is hot. It is burning hydrogen into helium like a controlled hydrogen bomb. The heat released in this process creates pressure, which makes the solar energy resist the attraction of its own gravity, which makes the solar scale smaller.

However, the sun will eventually run out of its fuel. This will happen after about five billion years, so there is no need to worry about booking tickets to fly to other stars. However, stars with a larger mass than the sun will deplete their fuel more quickly. After the fuel is exhausted, they start to lose heat and shrink. If their mass is smaller than about twice the mass of the sun, they will eventually stop shrinking and tend to a stable state. One of these states is called white dwarfs. They have a radius of thousands of miles and a density of hundreds of tons per cubic inch. Another such state is neutron stars. They have a radius of about ten miles and a density of millions of tons per cubic inch.

A large number of white dwarfs were observed in the area close to our Milky Way. However, it was not until 1967 that Joseline Bell and Anthony Hervesh first observed neutron stars in Cambridge. At that time, they discovered objects called pulsars that emit regular pulses of radio waves. Initially, they were surprised to have contact with alien civilizations. I do remember that the "little green man" was decorated in the room they were about to announce their discovery. However, they and all others finally came to a less romantic conclusion that these objects turned out to be rotating neutron stars. This was bad news for writers who wrote about the West of Space, but for the few who believed in black holes at the time, it was good news for those of us. If the stars could shrink to a scale of ten to twenty miles and become neutron stars, one could predict that other stars could further shrink and turn into black holes.

A star with a mass larger than about twice the mass of the Sun cannot be stable to a white dwarf or neutron star. In some cases, the star can explode and throw enough mass to make the remaining mass below this limit. But there are always exceptions. Some stars will become so small that their gravitational field will bend the light to this extent, causing it to fold back to the star itself. Whether it is light or anything else, it cannot escape. The star has become a black hole.

The laws of physics are symmetrical in time. If there is an object called a black hole that can fall into something but cannot run out, there should be other objects that can come out into something but cannot fall into it. People can call these objects white holes. One can guess that a person can jump into a black hole in one place and run out of a white hole in another. This should be the ideal method for long-distance space travel mentioned earlier. All you need to do is to find a nearby black hole.

This form of space travel seems possible at first glance. There are solutions in Einstein's general theory of relativity, which allow people to fall into a black hole and then run out of a white hole. However, later research shows that all these solutions are very unstable: the slightest disturbances, such as the existence of a spacecraft, will destroy this "wormhole", or the channel from the black hole to the white hole. The spacecraft will be torn to pieces by infinitely powerful forces. This is like hiding in a big bucket and drifting down from Niagara Falls.

Things seem to be desperate. Black holes may be used to get rid of garbage and even some of people's friends. But they are "the country where travelers have gone and no return". However, everything I have said so far is based on calculations made using Einstein's general theory of relativity. This theory fits very well with all our observations so far. However, since it cannot be combined with the uncertainty principle of quantum mechanics, we know that it cannot be completely correct. The uncertainty principle is that particles cannot define both position and velocity at the same time. The more precise you measure the position of a particle, the less accurate it will measure its velocity, and vice versa.

In 1973 I began to study how the principle of uncertainty would change black holes. To my surprise and everyone else, I found that it meant that black holes were not completely black. They emit radiation and particles at a constant rate. When I announced these results at a conference near Oxford, no one believed it. The chapter chairman said that these were meaningless and he also wrote a paper to reiterate. However, when others repeated my calculations, they found the same effect. In this way, even the chairman agreed that I was right.

How does radiation escape from the gravitational field of a black hole? We have several ways to understand it. Although they seem very different, they are actually completely equivalent. One way is that the uncertainty principle allows particles to travel faster than light over a short distance. This allows particles and radiation to escape from the black hole through the event horizon. However, what comes out of the black hole is different from what falls into it. Only the energy is the same.

As black holes release particles and radiation, it will lose mass. This will make the black hole smaller and emit particles more quickly. It will eventually reach zero mass and disappear completely. What happens to objects that fall into the black hole, possibly spacecraft? According to some of my latest research, the answer is that they will set out into their own tiny baby universe. A small self-sufficient universe forks from our area in the universe. This baby universe can reconnect to our area in space-time. If this happens, it seems to us that another black hole forms and then evaporates. Particles falling into one black hole will appear as particles emitted from another black hole, and vice versa.

This sounds like it is exactly what is needed to allow space travel through black holes. You just need to fly your spacecraft into the proper black hole, preferably a rather huge black hole. Otherwise, gravity will tear you into spaghetti before you enter the black hole. You can expect to reappear outside another black hole, although you can't choose where to go.

However, there is an unexpected obstacle in this intergalactic transport planning. The baby universe that takes away particles falling into the black hole occurs in the so-called virtual time. In real time, the ending of an astronaut who falls into the black hole is tragic. The difference in gravity acting on his head and feet will tear him apart. Even the particles that make up his body are not spared. Their history in real time will end at a singularity. However, the history of particles in virtual time will continue

Continue. They will enter and pass through the baby universe and reproduce as particles emitted from another black hole. In this way, in a sense, astronauts are transported to another area of ​​the universe. However, the particles appearing have no similarities with the astronauts. When he enters the singularity in real time, he will not receive any comfort by knowing that his particles will survive in virtual time. The motto for anyone who falls into a black hole is: "Think about virtual".

What determines where the particles reappear? The number of particles in the baby universe is equal to the number of particles falling into the black hole plus the number of particles emitted when it evaporates. This indicates that particles falling into a black hole will come out of another black hole of roughly equal mass. In this way, one can choose where the particles come out by creating a black hole of the same mass as the black hole in which the particles fall into. However, the black hole will equally likely emit any other set of particles with equal total energy. Even if the black hole does emit particles of the opposite species, one cannot tell whether they are those particles falling into the other black hole. The particles do not carry ID cards. All particles of a given species appear very similar.

All this shows that traveling through black holes is not a popular and reliable way to travel in space. First, you have to travel in virtual time to get there, ignoring your history to achieve a tragic ending in real time. Second, you cannot choose your own place of day at will. It's like traveling on a route that I can't tell you.

Although the baby universe is of no use for space travel, it is of great significance to our attempt to find a complete unified theory that can describe all things in the universe. Our existing theories include quantities, such as the magnitude of the charge a particle holds. Our theories cannot predict these quantities. Instead, they must be selected to match observations. However, many scientists believe that there is a basic unified theory that can predict all these quantities.

There is likely to be a basic theory like this. The so-called superstring is the most promising candidate at present. The idea is that space-time is full of many small circles like a string. What we think is that elementary particles are actually these small circles that vibrate in different ways. This theory does not contain any number whose values ​​can be adjusted. So people expect that this unified theory should predict the values ​​of all these quantities, such as the charge carried by a particle, which is a quantity left behind by the existing theory that cannot be determined by the existing theory. Although we cannot predict any of these quantities from the superstring theory, many people believe that we can finally do this.

However, if the image of the baby universe is correct, our ability to predict these quantities will be reduced. This is because we cannot observe how many baby universes exist there, waiting to connect to our region in the universe. Some baby universes only contain some particles. These baby universes are so small that people cannot perceive their connections and forks. However, when they are connected, they change the apparent value of the amounts such as the charge of a particle. In this way, because we do not know how many baby universes are waiting there, we cannot predict the apparent value of these quantities. There may also be a population explosion in the baby universe. However, unlike humans, there seems to be no limiting factors such as food supply and standing space. The baby universe exists in their own reality. It is a bit like asking how many angels can accommodate dancing on the needle tip.

The baby universe seems to introduce a certain even small uncertainty for the prophetic values ​​of most quantities. However, they can provide an explanation for a very important quantity, the so-called observations of the cosmic constant. This is an example of the general relativity equation that gives space-time an inherent tendency to expand or contract. Einstein proposed a very small cosmic constant, which originally intended to balance matter's tendency to shrink the universe. This motivation after people discovered that the universe expanded, would no longer exist. However, it is not easy to get rid of cosmic constants. One can expect that the implicit ups and downs of quantum mechanics would give

Very large cosmic constants. However, we can observe how the expansion of the universe changes over time, thus determining that the cosmic constants are very small. So far, no good explanation has been found for why the observation must be so small. However, the fork out and connection of the baby universe will affect the apparent value of the cosmic constant. Because we do not know how many baby universes there are, the cosmic constants may have different apparent values. However, a value that is almost zero is the most likely. This is thankful, because the universe is suitable for living creatures like us only when the cosmic constant is very small.
Chapter completed!