How science separates "reasonably scientific" hypotheses from "unscientific"
The idea of other universes is deeply rooted in science fiction. But even outside of fiction, one can find reasoning about multiverse and many parallel worlds, so Attic decided to figure out how close these ideas are to real physics.
The multiverse, about which Sean Carroll, an expert in cosmology and the author of the recently published in Russian popular book “Eternity. In Search of the Ultimate Theory of Time”, is a hypothesis about the structure of our Universe beyond the limits of the region accessible to our observation.
What does it mean? The speed of light is limited, and the Universe is expanding in all directions - while we can see only a certain part of space. And it is far from the fact that the world outside its borders is arranged in the same way as in the vicinity of the Earth. Hypothetically, outside the sphere accessible for observation, there may be, for example, a completely different ratio of ordinary and dark matter. Or at all - some other physical principles work, up to an increase in the number of dimensions.
Illustration: Anatoly Lapushko / Chrdk.
Common sense, of course, tells us that the properties of the universe should be the same everywhere. However, "common sense" is not a very good thing for cosmology, the science of space-time on a very large scale. The assumption that the substance we know in the Universe is ten times smaller than some mysterious dark matter was also completely contrary to common sense, however, it is in such a world, consisting mainly of dark matter, that we live today. The problem with the idea that the universe is changing dramatically where we can no longer see it is not unusual, but that such an idea cannot be tested.
A universe with hypothetically different physical laws is called the cosmological multiverse. Such a Universe is geometrically one - in the sense that a continuous line can be drawn between any two of its points without the construction of any portals and other exotic things. And this cosmological multiverse should not be confused, for example, with a multiple universe in the many-worlds interpretation of quantum mechanics.
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Many-Worlds Quantum Mechanics
At the other end of the "scale grid of the universe" there is a microcosm, events in which are described by quantum mechanics. We already know that elementary particles: electrons, quarks, gluons and their other cousins behave in accordance with rules that are not followed in the world we are used to. So, every particle in quantum mechanics can be considered as a wave - and seemingly "solid" atoms, which in the school chemistry course are represented as balls, when colliding with an obstacle, they will scatter like waves. Each quantum object is mathematically described not as a ball or a point limited in space, but as a wave function - existing simultaneously at all points of its trajectory through space. We can only calculate the probability of its being found in one place or another. Quantities such as the momentum of a particle,its energy and more exotic characteristics like spin are also calculated from the wave function: we can say that this mathematical object covering all space is the fundamental basis of quantum mechanics and all physics of the 20th century.
Calculations made on the basis of wave functions and operators (operators make it possible to obtain specific quantities from the wave function) are in excellent agreement with reality. Quantum electrodynamics, for example, today is the most accurate physical model in the history of mankind, and among quantum technologies there are lasers, all modern microelectronics, the fast Internet we are used to and even a number of drugs: the search for promising substances for medicine is also carried out by modeling the interactions of molecules. with a friend. From an applied point of view, quantum models are very good, but on the conceptual level, a problem arises.
Wave functions corresponding to an electron in a hydrogen atom at different energy levels. Light areas correspond to the maximum of the wave function and in these places the particle is most likely to be detected; the probability of finding the same electron in the next room, though negligibly small, is not zero.
The essence of this problem is that quantum objects can be destroyed: for example, when a photon (quantum of light) hits the camera matrix or simply collides with an opaque surface. Up to this point, the photon was perfectly described by the wave function, and after a moment, the wave extended in space disappears: it turns out that a certain change affected the entire Universe and happened faster than the speed of light (how can this even be?). This is problematic even in the case of a single photon, but what about the wave function of two photons emitted from one source in two opposite directions? If, for example, such two photons were born near the surface of a distant star and one of them was caught on Earth by a telescope, what about the second, which is many light years away? Formally, it forms a single system with the first,but it is difficult to imagine a scenario where a change in one part of the system is instantly communicated to all other parts. Another example of a quantum system, for which the disappearance of the wave function leads to conceptual problems, is the famous Schrödinger's cat, which is inside a closed box with a device that, based on a probabilistic quantum process, either breaks an ampoule with poison or leaves it intact. Before opening the box, Schrödinger's cat is simultaneously alive and dead: its state reflects the wave function of a quantum system inside a mechanism with poison.which is inside a closed box with a device that, based on a probabilistic quantum process, either breaks an ampoule with poison, or leaves it intact. Before opening the box, Schrödinger's cat is simultaneously alive and dead: its state reflects the wave function of a quantum system inside a mechanism with poison.which is inside a closed box with a device that, based on a probabilistic quantum process, either breaks an ampoule with poison, or leaves it intact. Before opening the box, Schrödinger's cat is simultaneously alive and dead: its state reflects the wave function of a quantum system inside a mechanism with poison.
The most common interpretation of quantum mechanics, Copenhagen, suggests simply accepting the paradox of the world - and admitting that yes, in spite of everything, the wave / particle disappears instantly. The alternative to it is the many-worlds interpretation. According to her, our Universe is a collection of non-interacting worlds, each of which represents one quantum state: when you open a box with a cat, two worlds appear - in one of them the cat is alive, and in the other it is dead. When a photon passes through a semitransparent mirror, the world is also divided into two: in one, a quantum of light is reflected from the surface, and in the other it is not. And so, each quantum process leads to the emergence of more and more branching worlds.
In theory, some of these branches can be very different from ours. One atom that flew in the wrong direction soon after the Big Bang could well lead to a different distribution of hot gas, the birth of stars in completely different places and, as a result, to the fact that the Earth, in principle, did not arise. But this picture cannot be called a problem of many worlds interpretation. The real problem is the impossibility of verifying the correctness of this understanding of quantum mechanics in practice: the individual components of the multiple Universe do not interact with each other by definition.
The idea of time travel and alternate universes has worn down a lot since the days of classic fiction. In addition to the notorious term "hitman" among fans of the genre (a hero from our days finds himself, for example, in the times of Ivan the Terrible), one can recall the parody film Kung Fury, from where this screenshot was taken.
Somewhere, perhaps, there is Earth inhabited by intelligent dinosaurs, somewhere the Great Mongol Empire landed on the moons of Jupiter in 1564, but there are no portals between these worlds - they diverged as a result of quantum processes in the distant past. A theory that would suggest the possibility of getting into one of these worlds, from the point of view of the philosophy of science, would be no less, but more scientific, since one could try to test it.