Refers to a theory that has not been proved by physics. According to this theory, there are probably other universes besides our universe, and these universes are a reaction to the possible state of the universe. These universes may have the same or different basic physical constants as the universes we know. The term parallel universe was invented by American philosopher and psychologist william james in 1895.
Parallel universe level
With such a definition of "universe", people may think that this is just a metaphysical way. However, the difference between physics and metaphysics lies in whether a theory can be tested by experiments, not in whether it looks weird or contains something difficult to detect. Over the years, the frontier of physics has been constantly expanding, absorbing and integrating many abstract (even metaphysical) concepts, such as spherical earth, invisible electromagnetic field, high-speed and slow flow of time, quantum overlap, space bending, black holes and so on. In recent years, the concept of "multiverse" has also been added to the above list. It cooperates with some previously tested theories, such as relativity and quantum mechanics, and at least reaches the basic standard of an empirical scientific theory: making predictions. Of course, the conclusion may be wrong. So far, scientists have discussed as many as four types of independent parallel universes. What matters now is not the existence of multiple universes, but how many levels they have.
Edit the first level of this paragraph: invisible
All parallel universes constitute the first multiverse. -This is the least controversial layer. Everyone accepts the fact that although we can't see another self at the moment, we can observe it in another place, or simply wait in the same place for a long time. It's like observing a ship coming from above sea level-observing an object beyond the horizon is similar. With the flight of light, the radius of hubble volume expands by half a light-year every year. You just need to sit there and watch. Of course, you may not wait for the light from another universe to arrive here, but in theory, if the expansion theory of the universe holds, your descendants may see them with a super telescope.
How about the concept of the first layer of the multiverse? Isn't space infinite? Who can imagine that there is a sign somewhere that says "Space is over, be careful of the ditch below"? If so, people will instinctively question: What is "outside"? In fact, Einstein's theory of gravitational field has turned our intuition into a problem. Space may not be infinite, as long as it has a certain curvature or is not a topological structure in our intuition (that is, there is an interconnected structure).
A spherical, doughnut-shaped or trumpet-shaped universe may be limited in size, but it has no boundaries. Observations of cosmic microwave background radiation can be used to test these hypotheses. See also article, is the universe limited? Author Jean-Pierre Lumine, Glenn D. Stackman, Jeffrey R. Wilkes; Scientific American, April 1999 However, the observation results so far seem to contradict it. The model of the endless universe is consistent with the observed data, which has strong limitations.
Another possibility is that space itself is infinite, but all matter is confined to a limited area around us-the once popular "island universe" model. The difference of this model is that the distribution of matter will show fractal pattern on a large scale, and it will be dissipated continuously. In this case, almost every universe in the first multiverse will eventually become empty and fall into silence. However, recent observations on the distribution of three-dimensional galaxies and microwave background show that the organization of matter presents some fuzzy uniformity on a large scale, and no clear details can be observed on a scale larger than10.24 meters. Assuming this pattern continues, the space outside Hubble volume will also be full of planets, stars and galaxies.
There is data to support the theory that space extends beyond hubble volume. WMAP satellite recently measured the fluctuation of microwave background radiation (left). The strongest amplitude exceeds 0.5 kHz, suggesting that the space is very large, even infinite (middle picture). In addition, WMAP and 2dF galaxy redshift detectors found that matter was evenly distributed in space at a very large scale.
Observers living in different parallel universes of the first multiverse will perceive the same physical laws as us, but with different initial conditions. According to the current theory, matter was thrown at a certain randomness in the early stage of the Big Bang, and this process contains all the possibilities of material distribution, and each possibility is not zero. Cosmologists assume that our universe, with approximately uniform material distribution and initial wave state (one of the possibilities of 100000), is a fairly typical individual (at least in all parallel universes that produce observers). Then the nearest person exactly like you will be 10 (10 28) meters away; Only 10 (10 92) meters away will there be an area with a radius of 100 light years, and everything in it is exactly the same as the space where we live, that is to say, everything that happens in our world in the future 100 will be completely reproduced in this area; At least10 (1018) meters away, the area will increase to the size of Hubble, in other words, there will be a universe exactly like ours.
The above estimate is extremely conservative. It only lists all quantum states in a space with a Hubble volume and a temperature lower than 10 8 Kelvin. One of the calculation steps is this: at that temperature, how many protons can a Hubble volume hold at most? The answer is 10 1 18. Each proton may or may not exist, that is, there are * * * 2 (10118) possible states. Now all you need is a box that can hold two Hubble spaces (10 1 18), and all the possibilities will be exhausted. If the box is larger-for example, a box with a side length of10 (1kloc-0/18) meters-according to the pigeon hole principle, the arrangement of protons will inevitably repeat. Of course, the universe is not only a proton, but also more than two quantum states, but the total amount of information that the universe can hold can also be estimated in a similar way.
The average distance from another universe exactly like ours may not be as far as theoretically calculated, but it may be much closer. Because the organization of matter is also restricted by other physical laws. In view of the formation process of planets and chemical equations, astronomers suspect that at least 10 20 planets are inhabited in our Hubble volume alone. Some of them may be very similar to the earth.
The framework of the first multiverse is usually used to evaluate the theory of modern cosmology, although this process is rarely clearly expressed. For example, let's look at how our cosmologists try to draw the cosmic geometric map of "spherical space" through the microwave background. With the different curvature radius of space, the sizes of those "hot zones" and "cold zones" on the cosmic microwave background map will show some characteristics; The observation area shows that the curvature is too small to form a spherical closed space. However, it is very important to keep the statistics rigorous. The average size of these regions in each Hubble space is completely random. So it is possible that the universe is fooling us-it is not that the curvature of space is not enough to form a closed sphere, which makes the observed area very small, but only because the average area of our universe is naturally smaller than others. So when cosmologists swear that their spherical space model is 99.9% reliable, what they really mean is that our universe is so unsociable that only one Hubble volume in 1000 will be like that.
The point of this lesson is: Even if we can't observe other universes, the multiverse theory can still be verified by practice. The key is to predict the * * * of each parallel universe in the first multiverse and point out its probability distribution-what mathematicians call "measurement". Our universe should be one of those "most likely universes". Otherwise-unfortunately, we live in an unlikely universe-then the previously assumed theory is in big trouble. As we will discuss next, how to solve this measurement problem will become quite challenging.
Edit the second level of this paragraph: the bubble left after inflation.
If the concept of the first-level multiverse is not easy to digest, we can try to imagine the structure of the next infinite group of the first-level multiverse: the groups are independent of each other, and even have different space-time dimensions and physical constants. These groups constitute the second multiverse-predicted by modern theory as "disorderly and continuous expansion".
As an inevitable extension of the Big Bang theory, "inflation" is closely related to many other inferences of the theory. For example, why is our universe so big and so regular, smooth and flat? The answer is that "space has undergone a rapid stretching process", which can not only explain the above problems, but also explain many other properties of the universe. See Allen H. Goose and Paul J. Steinhardt's The Expanding Universe; Scientific American, May1984; Andrei linde, cosmologist of self-propagating expansion, 1 1 month 1994 "expansion" theory is not only expressed by many elementary particle theories, but also confirmed by many observations. "Disordered persistence" refers to the largest-scale behavior. Space as a whole is being stretched and will last forever. However, some specific areas stop stretching, resulting in independent "bubbles", just like the bubbles inside the expanded toast. There are countless such bubbles. Each of them is the first multiverse: infinite in size, full of matter precipitated by fluctuations in the energy field.
For the earth, another bubble is infinitely far away, so far that you can never reach it even if you travel at the speed of light. Because the space between the earth and the "other bubble" extends much faster than your travel speed. If there is another you in another bubble, even your descendants will never want to observe him. For the same reason, that is, the space is expanding at an accelerated rate, the observation results are frustrating: even the other one in the first multi-space can't see himself.
The second layer of the multiverse is very different from the first layer. Bubbles not only have different initial conditions, but also different appearances. The mainstream view of physics now holds that the dimensions of time and space, the characteristics of elementary particles and many so-called physical constants are not part of the basic laws of physics, but only the result of a process called "symmetry destruction". For example, theoretical physicists believe that our universe once consisted of nine equal dimensions. In the early history of the universe, only three dimensions participated in the space pull, forming the three-dimensional universe we are observing now. The other six dimensions can't be observed now, because they are curled up in a very small scale, and all substances are distributed on these three fully stretched dimensional "surfaces" (for nine dimensions, three dimensions are just a surface, or a "film").
It is not particularly surprising for us that we live in a space-time of 3+ 1 dimension. When the partial differential equation describing nature is elliptic or hyper-hyperbolic equation, that is, one of space or time is 0-dimensional or multi-dimensional, it is impossible for observers to predict the universe (purple and green parts). In other cases (hyperbolic equation), if n >；; 3. Atoms cannot exist stably, n
From this, we say that the symmetry of space has been destroyed. The uncertainty of quantum wave will cause different bubbles to destroy the balance in different ways during the expansion process. And the result will be very strange. Some of them may extend to four-dimensional space; Others may only form two generations of quarks instead of the three generations we know; There are also some universes whose basic physical constants may be larger than ours.
Another way to produce the second multiverse is to experience the complete cycle of the universe from creation to destruction. In the history of science, this theory was put forward by a physicist named Richard C in the 1930s. Recently, two scientists, Paul J. Steinhardt of Princeton University and Neil Turok of Cambridge University, elaborated on this in detail. Steinhardt and Turok put forward a model of "secondary three-dimensional membrane", which is quite close to our space, but has some translation in higher dimensions. See "I've been there, I've done it," George Mercer; News Scan Scientific American March 2002 The parallel universe is not really an independent universe, but the whole universe-past, present and future-has formed a multiverse, which can prove that the diversity it contains is just like a disorderly expanding universe. In addition, Lee Smolin, a physicist in Watlu, put forward another theory with similar diversity to the second multiverse, that is, the universe was created and mutated through black holes instead of film physics.
Although we can't interact with other things in the second multiverse, cosmologists can indirectly point out their existence. Because their existence can be used to explain the contingency of our universe. For example, suppose you walk into a hotel and find that the house number of a room is 1967, which is the year of your birth. What a coincidence! You were shocked at that moment. But it's no coincidence that you reacted immediately. There are hundreds of rooms in the whole hotel, and one room is as normal as your birthday. However, if you see another number that has nothing to do with you, it will not cause the above thinking. What does this mean? Even if you don't know anything about hotels, you can use the above methods to explain many accidental phenomena.
Another pertinent example: examining the mass of the sun. The mass of the sun determines its luminosity (that is, the total radiation). Through basic physical calculation, we know that only when the mass of the sun is within a narrow range of1.6x10 30 ~ 2.4x10 30 kg, the earth is likely to be suitable for living. Otherwise, the earth will be hotter than Venus or colder than Mars. And the mass of the sun is exactly 2.0x10 3030kg. At first glance, the quality of the sun is an amazing luck and coincidence. The mass of most stars is randomly distributed in the huge range of 10 29 ~ 10 32kg, so if the mass of the sun is randomly determined at birth, the probability of falling within the appropriate range will be very small. However, with the experience of the hotel, we understand that this superficial accident is actually the inevitable result of large-scale systems (here refers to many solar systems) (because we are here, the quality of the sun has to be like this). This choice, which is closely related to the observer, is called "anthropic principle". Although it is conceivable how much controversy it has caused, physicists have widely accepted the fact that this selection effect cannot be ignored when verifying the basic theory.
The principles that apply to hotel rooms also apply to parallel universes. Interestingly, when the symmetry of our universe is broken, all (at least most) properties are "adjusted" just right. If we make even the slightest change in these characteristics, the whole universe will be unrecognizable-no living thing can survive in it. If the mass of protons increases by 0.2%, they will immediately decay into neutrons, and atoms will not exist stably. If the electromagnetic force is reduced by 4%, there will be no hydrogen and no stars. If the weak interaction is weaker, hydrogen cannot be formed. On the contrary, if they are stronger, those supernovae will not be able to spread heavy element ions to stars. If the cosmological constant is large, it will blow itself apart before the galaxy is formed.
Although "how well the universe is adjusted" is still inconclusive, every example mentioned above implies that there are many parallel universes that contain every possible adjustment state. See martin rees's Exploring Our Universe and Others; Scientific American1February 1999 The second multiverse shows that it is impossible for physicists to determine the theoretical values of those constants. After considering the selection effect, they can only calculate the probability distribution of expected values.
Edit the third level of this paragraph: quantum parallel world
The parallel worlds predicted by the first and second multiverses are so far apart that astronomers cannot reach them. But the next multiverse is around you and me. It comes directly from the famous and controversial explanation of quantum mechanics-any random quantum process will cause the universe to split into many parts, and each part represents a possibility.
Quantum parallel universe. When you roll the dice, it seems to get a specific result at random. But quantum mechanics points out that you actually throw out every state at that moment, and the dice stop at different points in different universes. In one universe, you voted 1, and in another universe, you voted 2. But we can only see a small part of the whole truth-one of the universes.
At the beginning of the 20th century, the success of quantum mechanics theory in explaining atomic phenomena set off a revolution in physics. In the field of atoms, the motion of matter no longer obeys the classical laws of Newtonian mechanics. Although quantum theory explained their extraordinary success, it triggered an explosive and heated debate. What exactly does this mean? Quantum theory points out that the universe is not as described in classical theory. What determines the state of the universe is the position and velocity of all particles, not a mathematical object called wave function. According to Schrodinger equation, states evolve with time in a way called "unification" by mathematicians, which means that wave functions evolve in an infinite dimensional space called "Hilbert space". Although quantum mechanics is described as random and uncertain most of the time, the evolution mode of wave function itself is completely determined and has no randomness at all.