People always take space for granted. After all, it’s just a void - a container for everything else. Time also ticks by continuously. But physicists are such people, they always need to complicate something. Regularly trying to unify their theories, they found out that space and time merge in a system so complex that an ordinary person cannot understand.
Albert Einstein realized what awaited us back in November 1916. A year earlier, he formulated the general theory of relativity, according to which gravity is not a force that propagates in space, but a property of space-time itself. When you toss the ball into the air, it flies in an arc and returns to the ground, because the Earth bends space-time around it, so the paths of the ball and the ground will intersect again. In a letter to a friend, Einstein discussed the problem of merging general relativity with his other brainchild, the nascent theory of quantum mechanics. But his math skills were simply not enough. “How I tortured myself with this!” He wrote.
Einstein never made it anywhere in this regard. Even today, the idea of creating a quantum theory of gravity seems extremely distant. The disputes hide an important truth: competitive approaches all as one say that space is being born somewhere deeper - and this idea breaks the scientific and philosophical understanding of it that has been established for 2500 years.
Down the black hole
An ordinary fridge magnet perfectly illustrates the problem faced by physicists. He can pin a piece of paper and resist the gravity of the entire Earth. Gravity is weaker than magnetism or other electrical or nuclear force. Whatever quantum effects are behind it, they will be weaker. The only tangible evidence that these processes occur at all is the motley picture of matter in the earliest universe - which is believed to have been drawn by quantum fluctuations in the gravitational field.
Black holes are the best way to test quantum gravity. “This is the most appropriate thing to experiment with,” says Ted Jacobson of the University of Maryland, College Park. He and other theorists study black holes as theoretical pivots. What happens when you take equations that work perfectly in a laboratory setting and put them in the most extreme situations imaginable? Will there be some subtle flaws?
General theory relatively predicts that matter falling into a black hole will contract infinitely as it approaches its center - a mathematical dead end called a singularity. Theorists cannot imagine the trajectory of an object beyond the singularity; all lines converge on 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 the matter falling into it.
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At the edge of a black hole, matter is not yet so compressed, gravity is weaker and, as far as we know, all the laws of physics should work. And it's all the more discouraging that they don't work. The black hole is limited by the event horizon, the point of no return: matter that crosses the event horizon will not return. The descent is irreversible. This is a problem because all known laws of fundamental physics, including quantum mechanical ones, are reversible. At least in principle, in theory, you should be able to reverse motion and restore whatever particles you had.
Physicists faced a similar conundrum in the late 1800s when they looked at the mathematics of a "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 reservoir that is kept at a constant temperature,” explains Raphael Sorkin of the Perimeter Institute for Theoretical Physics in Ontario. From a thermal point of view, it will have a temperature of absolute zero. This finding contradicts the observations of true black bodies (such as a furnace). Continuing work on Max Planck's theory, Einstein showed that a black body can achieve thermal equilibrium,if the radiation energy will come in discrete units, or quanta.
For nearly half a century, theoretical physicists have tried to achieve a similar solution for black holes. The late Stephen Hawking of the University of Cambridge took an important step in the mid-70s by applying quantum theory to the radiation field around black holes and showing that they have nonzero temperatures. Therefore, they can not only absorb but also emit energy. Although his analysis screwed black holes into the realm of thermodynamics, he also exacerbated the problem of irreversibility. Outgoing radiation is emitted at the edge of the black hole and does not carry information from the interior. This is random heat 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 there is something left in the black hole, just trapped, because as the black hole emits radiation, it contracts and,according to Hawking's analysis, it eventually disappears.
This problem is called the information paradox, because a black hole destroys information about particles that have got into it, which you could try to recover. If the physics of black holes is truly irreversible, something has to carry information back out, and our concept of spacetime may have to be modified to accommodate that fact.
Atoms of space-time
Heat is the random movement of microscopic particles like gas molecules. Since black holes can heat up and cool down, it would be reasonable to assume that they are made up of parts - or, more generally, of microscopic structure. And since a black hole is just empty space (according to general relativity, matter falling into a black hole passes through the event horizon without stopping), parts of a black hole must be parts of space itself. And beneath the deceptive simplicity of flat, empty space, there is tremendous complexity.
Even theories that were supposed to preserve the traditional view of space-time have come to the conclusion that something is lurking under this smooth surface. For example, in the late 1970s, Steven Weinberg, now at the University of Texas at Austin, tried to describe gravity in the same way that other forces of nature describe it. And I found out that space-time has been radically modified in its smallest scale.
Physicists originally visualized microscopic space as a mosaic of small pieces of space. If you increase them to the Planck scale, immeasurably small in size of 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 light into its constituent colors. And although manifestations on small scales will be very difficult to notice, violations of general relativity will be frankly obvious.
The thermodynamics of black holes calls into question the picture of space as 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, the energy should be distributed to relatively few molecules. In fact, you are measuring the entropy of a system, which represents 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 a beach ball by 10 times, it will fit 1000 times more molecules inside it. But if you increase the radius of a black hole 10 times, the number of molecules in it will multiply 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 appear three-dimensional, but it behaves like a two-dimensional object.
This strange effect is called the holographic principle, because it resembles a hologram, which we see as a three-dimensional object, but upon closer inspection 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 - it will not be enough to create space by simply connecting the smallest pieces of it.
Tangled web
In recent years, scientists have realized that quantum entanglement must be involved. 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 become entangled, they will remain connected regardless of the distance separating them.
Traditionally, when people talked about "quantum" gravity, they meant quantum discreteness, quantum fluctuations, and all other quantum effects - not quantum entanglement. Everything has changed thanks to black holes. During the life of a black hole, entangled particles enter it, but when the black hole completely evaporates, the partners outside the black hole remain entangled - with nothing. "Hawking should have called it an entanglement problem," says Ohio State University's Samir Mathur.
Even in a vacuum, where there are no particles, electromagnetic and other fields are internally entangled. If you measure the field at two different locations, your readings will fluctuate slightly, but remain in coordination. If you divide the area into two parts, these parts will be in correlation, and the degree of correlation will depend on the geometric property 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 - which means it could explain the law of gravity. "More entanglement means less gravity," he said.
Some approaches to quantum gravity - most notably string theory - 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 tie the volumetric space into a single and continuous whole.
In 2009, Mark Van Raamsdonk of the University of British 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 systems become entangled, a kind of tunnel, a wormhole, is formed between these universes and spaceships can move between them. The higher the degree of entanglement, the shorter the length of the wormhole. The universes merge into one and are no longer two separate. "The advent of large spacetime directly links entanglement with these degrees of freedom of field theory," says Van Raamsdonck. When we see correlations in electromagnetic and other fields, they are the remnant of the cohesion that binds space together.
Many other features of space, in addition to being connected, can also reflect entanglement. Van Raamsdonk and Brian Swingle of the University of Maryland argue that the omnipresence of entanglement explains the universality of gravity - that it affects all objects and permeates everywhere. For black holes, Leonard Susskind and Juan Maldacena believe that the entanglement between the black hole and the radiation it emits creates a wormhole - the black entrance to the black hole. Thus, information is preserved and the physics of a black hole is irreversible.
Although these string theory ideas only work for specific geometries and reconstruct only one dimension of space, some scientists have tried to explain space from scratch.
In physics, and in general, in the natural sciences, space and time are the basis for all theories. But we never directly notice space-time. Rather, we deduce its existence from our everyday experience. We assume that the most logical explanation for 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 is even deeper, the ins and outs of space, the back of a smooth mirror. If they succeed, we will end the revolution that Einstein began over a century ago.
Ilya Khel