People always take space as a matter of course. After all, it’s just emptiness – the capacity for everything else. Time also ticks continuously. But physicists are such people, they always need something to complicate things. Regularly trying to combine their theories, they found out that space and time merge in the system so complex that ordinary people do not understand.
Albert Einstein realized what awaits us, back in November 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 toss the ball into the air, it flies in an arc and returns to the ground, because the Earth warps space-time around itself, so the paths of the ball and the ground will cross again. In a letter to a friend, Einstein considered the problem of merging the general theory of relativity with his other child, the emerging theory of quantum mechanics. But his math skills simply did not suffice. “How I tormented myself with this!”, He wrote.
Einstein did not come anywhere in this respect. Even today the idea of creating a quantum theory of gravitation seems extremely remote. Disputes conceal an important truth: competitive approaches all say that space is born somewhere deeper – and this idea breaks the scientific and philosophical notion about it that has been established for 2500 years.
Down the black hole
The usual magnet on the refrigerator perfectly illustrates the problem that physicists have encountered. He can pin a piece of paper and resist the gravity of the whole Earth. Gravitation is weaker than magnetism or other electrical or nuclear force. No matter what quantum effects behind it, they will be weaker. The only tangible proof that these processes generally occur is a 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 quantum gravity. “This is the most suitable thing to find for experiments,” 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 that ideally work in the laboratory are taken and placed in the most extreme situations from conceivable ones? Will there be any subtle flaws?
The general theory relatively predicts that a substance falling into a black hole is infinitely compressed as it approaches the center – a mathematical dead end called the singularity. Theorists can not imagine the trajectory of an object beyond the singularity; all the 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 view this infinitesimal point of infinite density and understand what happens to the matter that enters it.
At the boundary of the black hole, matter is not yet compressed, gravity is weaker and, as far as we know, all the laws of physics should work. And the more discouraging is the fact that they do not work. The black hole is limited by the horizon of events, the point of no return: the substance overcoming the horizon of events will not return. 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 motion and recover all the particles that you had.
With a similar puzzle physics collided in the late 1800s, when they considered mathematics of the “blackbody” 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 the reservoir, which is maintained at a constant temperature,” explains Rafael Sorkin of the Perimeter Institute of Theoretical Physics in Ontario. From the thermal point of view, he will have a temperature of absolute zero. This conclusion contradicts the observations of real black bodies (such as an oven). Continuing his 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.
For almost half a century, theoretical physicists have tried to reach a similar solution for black holes. The late Steven Hawking of Cambridge University took an important step in the mid-1970s by applying the quantum theory to the radiation field around black holes and showing that they have a nonzero temperature. Consequently, they can not only absorb, but also emit energy. Although his analysis has 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 transfer information from the interior. This is random thermal energy. If you turn the process and feed this energy to a black hole, nothing will come up: you just get even more heat. And it is impossible to imagine that something remains in the black hole, it’s just trapped, because as the black hole emits radiation, it shrinks and, according to Hawking analysis, 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 restore. If the physics of black holes is really irreversible, something must make the information back, and our space-time concept may need to be changed to fit this 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 consist of parts – or, if in general, a microscopic structure. And since a black hole is just an empty space (according to GRT, 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 under the deceptive simplicity of a flat empty space there is a colossal complexity.
Even theories that were supposed to preserve the traditional notion of space-time, 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 that other forces of nature describe. And I found out that space-time is radically modified on its smallest scale.
Physicists initially visualized microscopic space as a mosaic of small pieces of space. If you increase them to the Planck scale, 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 chess space lines 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 – as in a glass prism that breaks light into color components. And although manifestations on a small scale will be very difficult to notice, violations of general relativity will be openly obvious.
Thermodynamics of black holes calls into question the 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. Reset the energy and look at the thermometer. If the column took off, the energy should spread to relatively few molecules. In fact, you measure the entropy of a system, which is its microscopic complexity.
If you do this with the usual 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, 1000 times more molecules will fit inside it. But if you increase the radius of a black hole by a factor of 10, the number of molecules in it will multiply by a factor of 100. The number of molecules from which it consists should be proportional not to its volume, but to the surface area. A black hole may appear three-dimensional, but behaves like a two-dimensional object.
This strange effect was called the holographic principle, because it resembles a hologram that we see as a three-dimensional object, and on 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 – what physicists allow, though not all – to create space it will not be enough to simply mate its smallest pieces.
In recent years, scientists have realized that in this everything should be implicated in quantum entanglement. 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 separating them.
Traditionally, when people talked about “quantum” gravity, they meant quantum discreteness, quantum fluctuations and all other quantum effects – but not quantum entanglement. Everything has changed, thanks to black holes. During the lifetime of a black hole, entangled particles get into it, but when the black hole evaporates completely, the partners outside the black hole remain entangled – with nothing. “Hawking was worth calling it a problem of entanglement,” says Samir Mathur of Ohio State University.
Even in a vacuum where there are no particles, the electromagnetic and other fields are internally tangled. If you measure the field in two different places, your readings will vary slightly, but will 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 that they have: the interface area. In 1995, Jacobson said 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 entanglement – gravity is weaker,” he said.
Some approaches to quantum gravity – above all, string theory – consider 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 the creation of 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 the three-dimensional space into a single and continuous whole.
In 2009, Mark Van Ramsundonck of the University of British Columbia provided an elegant explanation for this process. Suppose that the fields on the boundary are not confused – they form a pair of systems outside the correlation. They correspond to two separate universes, between which there is no way of communication. When the systems become entangled, a tunnel, a wormhole, forms between these universes and spacecraft can move between them. The higher the degree of entanglement, the less the length of the wormhole. Universes merge into one and more are not 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 the electromagnetic and other fields, they are the remainder of the clutch that binds the space together.
Many other features of space, in addition to its connectivity, can also reflect confusion. Van Raamsdonck and Brian Swingl, who works 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 the black holes, Leonard Susskind and Juan Maldasena believe that the entanglement between the black hole and the radiation emitted by it creates a wormhole – a black entrance to a black hole. Thus, information is stored and the physics of the black hole is irreversible.
Although these ideas of string theory work only for specific geometries and reconstruct only one dimension of space, some scientists try to explain the appearance of space from scratch.
In physics, and in general, in the natural sciences, space and time are the basis for all theories. But we never notice the space-time directly. Rather, we derive its existence from our everyday 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 is even deeper, the background of space, the reverse side of a smooth mirror. If they succeed, we will finish the revolution, which was started more than a century ago by Einstein.