Further reading: "Black Holes and the Baby Universe", those who have time can take a look.

Falling into a black hole has become a terrifying spectacle in science fiction. Black holes are now in fact presented as a scientific reality, not a scientific fantasy. As I have described, there are strong reasons to predict the existence of black holes. Observational evidence strongly suggests that there are some black holes in our own galaxy and more in other galaxies.

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

Of course, what science fiction writers really do is when they describe what would happen if you did fall into a black hole. Many people believe that if a black hole is spinning, you can go through a small hole in space-time and go to another part of the universe. This apparently creates the possibility of space travel. If we were to think of other stars, not to mention other galaxies, travel would become a reality in the future, and that would indeed be something we could only dream of. Otherwise, the fact that nothing travels faster than light means that the journey back and forth to the nearest star would take at least eight years. That's how long it takes to spend the weekend in α half of the Sagittarius! On the other hand, if people can walk through a black hole, they can reappear anywhere in the universe. It's not clear how to choose your destination, but at first you may want to go on vacation in Virgo, only to end up in the Crab Nebula.

I'm sorry to tell future galaxy travelers that this scenario won't work. If you jump into a black hole, you'll be torn to shreds. However, in a sense, the particles that make up your body will continue to travel to another universe. I don't know if it would be a great comfort to someone who has been crushed into spaghetti in a black hole to learn that his particles might survive.

Although I have adopted a somewhat flippant tone here, this lecture is based on sound science. Much of what I have said here is now shared by other scientists working in this field, even though this is a recent occurrence. However, the final part of the presentation was based on recent work on which consensus had not yet been reached. It aroused great interest and excitement.

Although the concept of what we now call black holes dates back more than 200 years, the name "black hole" was not proposed until 1967 by the American physicist John Wheeler. This is truly a stroke of genius: the name itself guarantees the entry of black holes into the mystical realm of science fiction. Giving an exact name to something that was previously not satisfactory has also stimulated scientific research. The importance of a good name cannot be underestimated in science.

As far as I know, the first person to discuss black holes was a Cambridge man named John Michel, who wrote a paper on the subject in 1783. His idea is as follows: Suppose you light a cannonball upwards on the surface of the earth. As it ascends, its speed slows down due to gravitational effects. It will eventually stop rising and fall back to Earth. However, if its initial velocity is greater than a certain critical value, it will never stop rising and falling back, but will continue to move outward. This critical velocity is called escape velocity. The Earth's escape velocity is about seven miles per second, and the Sun's escape velocity is about one hundred miles per second. Both of these velocities are greater than the actual velocity of the shells, but they are too small for the speed of light, which is 186,000 miles per second. This suggests that gravity has little effect on light, and that light can escape from the Earth or the Sun without difficulty. However, Michelle reasoned, there might be a star that is massive enough and small enough to escape faster than the speed of light. Because the light emitted from the star's surface is pulled back by the star's gravitational field, it cannot reach us, so we cannot see the star. However, we can detect its presence based on the effect of its gravitational field acting on nearby objects.

It is inconsistent to treat light as a cannonball. 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 the effect of gravity on light. Nevertheless, it was not until the sixties of this century that people became widely aware of the implications of this theory for old stars and other heavy objects.

According to the general theory of relativity, space and time together are thought to form a four-dimensional space called space-time. This space is not flat, it is distorted or bent by the matter and energy within it. This curvature can be observed in the bending of light or radio waves near the Sun. In the case of the proximity of the light passing through the sun, the bend is very small. However, if the Sun is shrunk to a scale of only a few miles, the bending will be so severe that the light emitted from the Sun's surface cannot escape, and it is pulled back by the Sun's gravitational field. According to the theory of relativity, nothing can travel faster than light, so there is an area where nothing can escape. This region is called a black hole. Its boundaries are called event horizons. It is formed by light rays that just can't escape from a black hole and can only linger on the edges.

Assuming that solar energy shrinks to a scale of only a few miles sounds inconceivable. One might think that matter could not have been compressed to such an extent. But in reality it is possible.

Too xxxx has existing scales because it is hot. It is burning hydrogen into helium, like a controlled hydrogen bomb. The heat released during this process creates a pressure that allows the solar energy to resist its gravitational pull, which makes the solar smaller scale.

However, the sun will eventually run out of fuel. This will happen about five billion years later, so there's no need to rush to book a ticket to fly to other stars. However, stars with a greater mass than the Sun will run out of their fuel more quickly. When the fuel runs out, it begins to lose heat and shrinks. If they are less than twice the mass of the Sun, they will eventually stop shrinking and tend to a stable state. One of these states is called a white dwarf. They have a radius of several thousand miles and a density of hundreds of tons per cubic inch. Another such state is a neutron star. They have a radius of about ten miles and a density of several million tons per cubic inch.

A large number of white dwarfs are observed in the immediate vicinity of the Milky Way. However, it was not until 1967 that Joseline Bell and Anthony Helvesh made their first observations of neutron stars in Cambridge. That's when they discovered objects called pulsars that emit regular pulses of radio waves. At first, they were surprised if they had made contact with an extraterrestrial civilization. I do remember that the room where they were going to announce their discovery was decorated with a "little green man" pattern. However, they and all the others could only end up with the not-so-romantic conclusion that the objects turned out to be rotating neutron stars. This was bad news for writers writing about space westerners, but good news for those of us who believed in black holes at the time. If a star could shrink to a scale of 10 to 20 miles and become a neutron star, one can expect other stars to shrink further and become black holes.

A star with a mass greater than about twice the mass of the Sun cannot be stable as a white dwarf or neutron star. Under certain circumstances, the star can explode and throw enough mass to keep the rest below this limit. But there are always exceptions. Some stars become so small that their gravitational field bends the light to such an extent that it bends it back to the star itself. Neither light nor anything else can escape. The star has turned into a black hole.

The laws of physics are time-symmetrical. If there is something called a black hole that can fall into it but can't get out, then there should be other objects that can get out but can't. One 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 means of long-distance space travel mentioned earlier. All you need to do is look for a neighboring black hole.

This form of space travel seems possible at first glance. Einstein's theory of general relativity has such a solution, which allows one to fall into a black hole and then escape out of a white hole. However, later studies have shown that all of these solutions are very unstable: the slightest perturbation, such as the presence of a spacecraft, can wipe out the "wormhole", or the passage from the black hole to the white hole. The spaceship will be torn to shreds by infinitely powerful force. It's like ducking down from Niagara Falls by hiding in a vat.

Things seem to have gone hopeless. Black holes may be used to get rid of garbage or even some of people's friends. But they are "the land where the traveler has no place to go". However, everything I have said so far has been based on calculations made using Einstein's general theory of relativity. This theory agrees perfectly well with all of 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 says that particles cannot define both position and velocity well. The more accurately you measure the position of a particle, the less accurate it is to measure its velocity, and vice versa.

In 1973, I started researching how the uncertainty principle would change black holes. To my surprise and everyone else, I found out that it meant that the black hole was not completely black. They emit radiation and particles at a constant rate. When I announced these results at a conference near Oxford, everyone was not convinced. The president of the chapter said that these were meaningless, and he wrote a paper to reiterate them. However, when others repeated my calculations, they found the same effect. Thus, even the Chairman agreed that I was right.

How does radiation escape from the gravitational field of a black hole? There are several ways to understand this. Although they appear to be very different, they are in fact completely equivalent. One way is that the uncertainty principle allows particles to travel faster than light over short distances. This allows particles and radiant energy to escape from the black hole through the event horizon. However, what comes out of a black hole is not the same as what falls in. Only the energy is the same.

As a black hole releases particles and radiation, it will lose mass. This will make the black hole smaller and smaller and emit particles more quickly. It will eventually reach zero mass and disappear completely. What happens to objects that fall into a black hole, and possibly spacecraft? According to some of my latest research, the answer is that they set off into their own tiny baby universe. A small self-contained universe forks from our regions of the universe. This baby universe can reconnect to our areas of space-time. If this happens, it will appear to us as another black hole forming and subsequently evaporating. Particles that fall into one black hole appear as particles emitted from another black hole, and vice versa.

This sounds like exactly what is needed to allow space travel through black holes. You just have to pilot your spaceship into the appropriate black hole, preferably a fairly large black hole. Otherwise, gravity will tear you into spaghetti before you enter the black hole. You can hope to reappear outside of another black hole, although you can't choose where to be.

However, there is an unexpected obstacle in this planning for intergalactic transport. The baby universe, which takes away particles that fall into a black hole, takes place in so-called imaginary time. In real time, the end of an astronaut who fell into a black hole was tragic. The difference in gravitational force on his head and feet will tear him apart. Not even the particles that make up his body are immune. Their history in real time ends at a singularity. However, the history of particles in imaginary time will continue. They will enter and pass through the baby universe and reappear as particles emitted from another black hole. In this way, in a sense, it can be said that the astronaut is transported to another region of the universe. However, the particles that appear do not resemble astronauts. When he enters the singularity in real time, he will not be comforted by the knowledge that his particles will survive in virtual time. The motto for anyone who falls into a black hole is: "Think of the imaginary".

What determines where particles reproduce? The number of particles in the baby universe is equal to the number of particles that fall into the black hole plus the number of particles emitted when it evaporates. This suggests that particles that fall into one black hole will emerge from another black hole with 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 into which the particles fall. However, the black hole will equally emit any other collection of particles with equal total energy. Even if the black hole does emit particles of the opposite species, it is still not possible to tell if they are the same particles that fell into another black hole. The particles do not carry an ID card. All particles of a given species appear to be very similar.

All of this suggests that crossing black holes is not a popular and reliable method of space travel. First of all, you have to travel in imaginary time to get there, ignoring the tragic end of your history in real time. Secondly. You can't choose your own day and place. It's like traveling on a route I can't name names.

Although the baby universe is of little use for space travel; But it is significant for our attempt to find a complete unified theory that can describe everything in the universe. Our existing theories include quantities, such as the magnitude of the charge of a particle. Our theories cannot predict these quantities. Rather, they must be selected to match the observations. However, many scientists believe that there is a fundamental unified theory that predicts all these quantities.

It is quite possible that such a basic theory exists. The so-called special-shaped superstring is currently the most promising candidate. The idea is that time and space are filled with many small circles that resemble a string. What we think of as elementary particles is actually these little circles that vibrate in different ways. This theory does not contain any numbers whose values can be adjusted. It was then expected that such a unified theory would be able to predict the value of all these quantities, such as the charge of a particle, which was a quantity left over from the uncertainty of existing theories. While we can't predict any of these quantities from superstring theory yet, many believe that we will eventually be able to do so.

However, if the image of the baby universe is correct, our ability to predict these quantities is reduced. This is because we cannot observe how many infant universes exist there, waiting to connect with our regions of the universe. Some baby universes contain only a few particles. These infant universes are so tiny that people are unaware of their connections and bifurcations. However, when they are connected, they change the apparent value of the amount of charge that a particle has. In this way, because we don't know how many baby universes are waiting there, we can't predict the apparent values of these quantities. There could also be a population explosion of the baby universe. Unlike humans, however, there do not seem to be constraints such as food availability and standing space. The infant universe exists in their own reality. It's a bit like asking the question of how many angels can fit on the tip of a needle to dance.

The infant universe seems to introduce a certain, if not a little, degree of uncertainty into most quantities of prophecy. However, they can provide an explanation for a very important quantity, the observed value of the so-called cosmological constant. This is one of the general relativity equations that give space-time an intrinsic tendency to expand or contract. Albert Einstein proposed a very small cosmological constant, which was intended to balance the tendency of matter to shrink the universe. This motivation ceased to exist after it was discovered that the universe was expanding. But getting rid of the cosmological constant is never easy. One can expect that the fluctuations implicit in quantum mechanics give very large cosmological constants. However, we were able to observe how the expansion of the universe changes over time, and thus determine that the cosmological constant is very small. To date, no good explanation has been found as to why the observations must be so small. However, the bifurcation of the baby universe and the connection back affect the apparent value of the cosmological constant. Because we don't know how many baby universes there are, the cosmological constant may have different apparent values. However, a value of almost zero, is most likely. This is fortunate because it is only when the cosmological constant is very small that the universe is suitable for habitation by creatures like us.

To sum it up: it seems that particles are able to fall into a black hole, which then evaporates and disappears from our region of the universe. These particles enter the baby universe. These baby universes forked out of our universe. These baby universes can be connected back to somewhere else. They are of little use for space travel, but their presence means that we are less capable of predicting than we would like, even if we do find a complete unified theory. On the other hand, we may now be able to provide explanations for certain measurements of quantities like the cosmological constant. In the past few years, many people have started to study the baby universe. I don't think anyone has made a fortune patenting them as a method of space travel, but they have become a very exciting area of research.