General, Research, Technology

The greatest mysteries: what is space-time?

People always take space for themselvesfor granted. In the end, it's just emptiness - a container for everything else. Time is ticking continuously. But physicists are such people, they always need to complicate something. Regularly trying to combine their theories, they found that space and time merge in a system so complex that an ordinary person can not understand.

Visualization of space and time.

Albert Einstein understood what awaits us, back inNovember 1916 A year earlier, he formulated the general theory of relativity, according to which gravity is not a force that spreads in space, but a property of space-time itself. When you throw the ball into the air, it flies in an arc and returns to the ground, because the Earth bends the space-time around itself, so the paths of the ball and the earth intersect again. In a letter to a friend, Einstein considered the task of merging the general theory of relativity with his other brainchild, the nascent theory of quantum mechanics. But his mathematical skills were simply not enough. “How did I torment myself with this!” He wrote.

Einstein never came anywhere in this regard. Even today, the idea of ​​creating a quantum theory of gravity seems extremely distant. Disputes hide an important truth: competitive approaches all speak as one that space is born somewhere deeper - and this idea breaks the scientific and philosophical view of it that has been established for 2500 years.

Down the black hole

An ordinary fridge magnet is greatillustrates the problem that physicists have encountered. He can pin a piece of paper and resist the gravity of the entire Earth. Gravity is weaker than magnetism or another electric or nuclear force. Whatever quantum effects are behind it, they will be weaker. The only tangible proof that these processes generally take place is the motley picture of matter in the earliest Universe - which is believed to have been drawn by quantum fluctuations of the gravitational field.

Black holes are the best way to test quantumgravity. “This is the best fit for experimentation,” says Ted Jacobson of the University of Maryland College Park. He and other theorists study black holes as theoretical points of support. What happens when equations are taken that work perfectly in the laboratory and fit into the most extreme situations imaginable? Will there be any subtle flaws?

The general theory relatively predicts thata substance falling into a black hole contracts infinitely as it approaches the center - a mathematical dead end called a singularity. Theorists cannot imagine the trajectory of an object beyond the limits of singularity; all lines converge in it. Even talking about it as a place is problematic, because the space-time itself, which determines the location of the singularity, ceases to exist. Scientists hope that quantum theory can provide us with a microscope that will allow us to examine this infinitesimal point of infinite density and understand what happens to matter falling into it.

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At the border of the black hole, the substance is not sosqueezed, gravity is weaker and, as far as we know, all the laws of physics should work. And the fact that they do not work is even more discouraging. The black hole is limited by the event horizon, the point of no return: the substance that overcomes the event horizon will not return. The descent is irreversible. This is a problem because all the known laws of fundamental physics, including quantum mechanical, are reversible. At least, in principle, in theory, you should be able to reverse the movement and restore all the particles that you had.

Physicists faced a similar puzzle at the end1800s, when they considered the mathematics of the "black body", idealized as a cavity filled with electromagnetic radiation. The theory of electromagnetism by James Clerk Maxwell predicted that such an object would absorb all the radiation that falls on it and never come into equilibrium with the surrounding matter. “It can absorb an infinite amount of heat from a tank that is maintained at a constant temperature,” explains Rafael Sorkin of the Ontario Institute of Theoretical Physics of Perimeter. From a thermal point of view, it will have a temperature of absolute zero. This conclusion contradicts the observations of real black bodies (such as a furnace). Continuing work on the Max Planck theory, Einstein showed that a black body can achieve thermal equilibrium if the radiation energy comes in discrete units, or quanta.

Theoretical physicists have been trying to achieve almost half a centurya similar solution for black holes. The late Stephen Hawking of Cambridge University took an important step in the mid-70s, applying quantum theory to the radiation field around black holes and showing that they had a non-zero temperature. Therefore, they can not only absorb, but also radiate energy. Although his analysis screwed black holes into the field of thermodynamics, he also exacerbated the problem of irreversibility. Outgoing radiation is emitted at the boundary of a black hole and does not carry information from the bowels. This is random thermal energy. If you reverse the process and feed this energy to a black hole, nothing pops up: you just get even more heat. And it is impossible to imagine that something was left in the black hole, just trapped, because as the black hole emits radiation, it contracts and, according to Hawking’s analysis, eventually disappears.

This problem is called informational.paradox, because a black hole destroys information about particles that got into it, which you could try to recover. If the physics of black holes is truly irreversible, something needs to carry the information back, and our concept of space-time may need to be changed to fit this fact.

Space-time atoms

Heat is the random movement of microscopicparticles, like gas molecules. Since black holes can heat up and cool down, it would be reasonable to assume that they are composed of parts - or, if in general, of a microscopic structure. And since a black hole is simply empty space (according to GR, matter falling into a black hole passes through the event horizon without stopping), parts of the black hole must be parts of the space itself. And beneath the deceptive simplicity of a flat empty space is a colossal complexity.

Even theories that should have been keptthe traditional notion of space-time, we came to the conclusion that something is hiding under this smooth surface. For example, in the late 1970s, Stephen Weinberg, now working at the University of Texas at Austin, tried to describe gravity in the same way as other forces of nature. And he found out that space-time is radically modified on its smallest scale.

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Physicists initially visualizedmicroscopic space as a mosaic of small pieces of space. If you increase them to Planck scales, immeasurably small in size 10-35 meters, scientists believe that you can see something like a chessboard. Or maybe not. On the one hand, such a network of lines of chess space will prefer one direction to another, creating asymmetries that contradict the special theory of relativity. For example, light of different colors will move at different speeds - like in a glass prism that breaks the light into its constituent colors. Although manifestations on a small scale will be very difficult to notice, violations of general relativity will be frankly obvious.

Black hole thermodynamics cast doubt ona picture of space in the form of a simple mosaic. By measuring the thermal behavior of any system, you can count its parts, at least in principle. Release energy and look at the thermometer. If the column has taken off, energy should extend to relatively few molecules. In fact, you measure the entropy of a system, which is its microscopic complexity.

If you do this with an ordinary substance,the number of molecules increases with the volume of the material. So, in any case, it should be: if you increase the radius of the beach ball by 10 times, inside it will fit 1000 times more molecules. But if you increase the radius of a black hole by 10 times, the number of molecules in it multiplies by only 100 times. The number of molecules of which it consists should be proportional not to its volume, but to the surface area. A black hole may seem three-dimensional, but it behaves like a two-dimensional object.

This strange effect is calledof the holographic principle, because it resembles a hologram, which we see as a three-dimensional object, and upon closer examination it turns out to be an image produced by a two-dimensional film. If the holographic principle takes into account the microscopic components of space and its contents - which physicists admit, though not all - to create space, it will not be enough to simply pair its smallest pieces.

Intricate Networks

In recent years, scientists have realized that this is allquantum entanglement must be implicated. This deep property of quantum mechanics, an extremely powerful type of connection, seems much more primitive than space. For example, experimenters can create two particles flying in opposite directions. If they are confused, they will remain connected regardless of the distance that separates them.

Time is relative, do not forget about it

Traditionally, when people talked about "quantum"gravitations, they had in mind quantum discreteness, quantum fluctuations, and all other quantum effects - but not quantum entanglement. Everything has changed thanks to black holes. During the life of the black hole, entangled particles fall into it, but when the black hole completely evaporates, partners outside the black hole remain entangled - with nothing. “The hawking should call it a problem of entanglement,” says Samir Matur of Ohio State University.

Even in a vacuum where there are no particles,electromagnetic and other fields are internally confused. If you measure the field in two different places, your readings will fluctuate slightly, but remain in coordination. If you divide the region into two parts, these parts will be in correlation, and the degree of correlation will depend on the geometric property that they have: the interface area. In 1995, Jacobson stated that entanglement provides a link between the presence of matter and the geometry of space-time - and therefore could explain the law of gravity. “More confusion - gravity is weaker,” he said.

Some approaches to quantum gravity - beforeIn all, string theory - I see entanglement as an important cornerstone. String theory applies the holographic principle not only to black holes, but to the universe as a whole, providing a recipe for creating space - or at least some of it. The original two-dimensional space will serve as the boundary of a larger volumetric space. And entanglement will connect volumetric space into a single and continuous whole.

In 2009, Mark Van Raamsdonck of the UniversityBritish Columbia provided an elegant explanation for this process. Suppose the fields at the boundary are not entangled - they form a pair of systems out of correlation. They correspond to two separate universes, between which there is no way of communication. When the systems become entangled, a tunnel, a wormhole is formed, as if between these universes and spaceships can move between them. The higher the degree of entanglement, the shorter the length of the wormhole. Universes merge into one and are no longer two separate. “The advent of large space-time directly connects entanglement with these degrees of freedom of field theory,” says Van Raamsdonk. When we observe correlations in electromagnetic and other fields, they are the remainder of the cohesion that ties space together.

Many other features of space besidesits connectedness may also reflect confusion. Van Raamsdonck and Brian Swingle, working at the University of Maryland, argue that the ubiquity of entanglement explains the universality of gravity - that it affects all objects and penetrates everywhere. As for black holes, Leonard Sasskind and Juan Maldacena believe that the entanglement between a black hole and the radiation emitted by it creates a wormhole - a black entrance to a black hole. In this way, information is saved and the physics of the black hole is irreversible.

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Although these ideas of string theory work only for specific geometries and reconstruct only one dimension of space, some scientists are trying to explain the appearance of space from scratch.

In physics, and indeed in the natural sciences,space and time are the basis for all theories. But we never notice space-time directly. Rather, we deduce its existence from our daily experience. We assume that the most logical explanation of the phenomena that we see will be some mechanism that functions in space-time. But quantum gravity tells us that not all phenomena fit perfectly into such a picture of the world. Physicists need to understand what’s even deeper, the ins and outs of space, the back of a smooth mirror. If they succeed, we will end the revolution that began more than a century ago by Einstein.