Will quantum mechanics absorb reality?

The competition between gravity and quantum physics takes a new turn

It was the biggest of problems, it was the slightest of problems.



Today physicists have two sets of rules explaining how nature works. There is a general theory of relativity that perfectly describes gravity and everything it rules over: planets moving in orbit, colliding galaxies, the dynamics of an expanding universe. This is a large scale. And there is quantum mechanics working with three other interactions - electromagnetism and two nuclear forces. Quantum theory does an excellent job of explaining what happens when a uranium atom decays or when individual particles of light and a photocell collide. This is a small scale.

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And now the problem: relativity and quantum mechanics are fundamentally different theories that are formulated differently. And this is not a question of scientific terminology, it is a collision of truly incompatible descriptions of reality.



The conflict between the two halves of physics has matured for over a hundred years - it began with a pair of Einstein's works from 1905, one of which described relativity, and the other introduced the concept of a quantum - but recently it entered a very interesting and unpredictable phase. Two eminent physicists have identified extreme positions, each in their own camp, and conduct experiments that will be able once and for all to determine the advantage of one of the approaches.



The difference between relativity and quantum systems can be thought of as the difference between “smooth” and “granular”. In GR, events are continuous and deterministic, that is, each action causes a certain local effect. In quantum mechanics, events occurring due to interactions of subatomic particles occur by leaps (aha, quantum leaps), with probabilistic, rather than deterministic, results. Quantum rules allow one to establish connections forbidden by classical physics. This was recently shown in an often-discussed experiment in which Dutch researchers challenged the effect of locality. They showed that the two particles - in their case, the electrons - can instantly influence each other, although they were separated by one and a half kilometers. If you try to interpret smooth relativistic laws in a granular quantum style, or vice versa, everything flies to hell.



THAT produces meaningless answers if you try to scale down to quantum sizes, and it drops to infinite values ​​when describing gravity. Conversely, quantum mechanics is faced with serious difficulties, being bloated to a cosmic scale. Quantum fields carry a certain amount of energy, even in seemingly empty space, and the more fields there are, the more energy becomes. According to Einstein, energy is equivalent to mass (E = mc ² ), so accumulating energy is the same as accumulating mass. If you accumulate it a lot, then the amount of energy in a quantum field creates a black hole, because of which the whole Universe collapses. Oh.



Craig Hogan , an astrophysicist at the University of Chicago and director of the Center for Astrophysics of Particles at Fermilab , rethinks the quantum aspect of physics with the help of a new theory in which quantum units of space can be large enough to be studied. Lee Smolin , founder of the Perimeter Institute for Theoretical Physics at the University of Waterloo, is trying to get physics back to the philosophical Einstein roots, and then stretch these roots in an interesting direction.



To understand what is at stake, take a look at previous theories. When Einstein discovered the world of general relativity, she not only replaced Isaac Newton’s theory of gravity, but also opened up a new way of considering physics, leading to modern concepts of the Big Bang and black holes, not to mention atomic bombs and adjusting the time required for your The phone worked GPS. Similarly, quantum mechanics did not simply reformulate Maxwell’s equations describing electricity, magnetism, and light. It provided the tools needed to create the Large Hadron Collider, photovoltaic cells and all modern microelectronics.



According to the results of these disputes, no less than a few will happen - the third revolution in modern physics, which will lead to stunning consequences. We can find out where the laws of nature come from, and whether space is built on the basis of uncertainty, or it is based on determinism, when a specific reason is associated with each event.





Craig Hogan

Granular cosmos

Hogan, the leader of the quantum world view, prefers, instead of wandering in the dark, to act according to an anecdote, and look for a place where it is brighter, where the light is brighter, and where there is a higher probability of seeing something interesting. This is a basic principle in his current research. According to him, the collision between reality and quantum mechanics occurs when you try to understand what gravity is doing at extremely small distances - so he decided to look at what is happening there. “I am sure that it is possible to conduct an experiment that will allow us to see what is happening, how this interface works, which we still do not understand,” he says.



The simplest assumption in Einstein's physics — and the traces of its origin lead to Aristotle — is that space is continuous and infinitely divisible, and any distance can be divided into even smaller distances. But Hogan raises the question of the truth of such an approach. Just as your screen has the smallest unit - a pixel, and light has the smallest unit - a photon, so the distance, he said, must have an indivisible smallest unit - a quantum of space.



According to Hogan, it will be meaningless to ask how gravity behaves at distances smaller than a unit of space. On such scales, gravity cannot work, because such scales do not exist. In other words, GR will be required to reconcile with quantum physics, since the space in which the effects of relativity are measured will be divided into indivisible quanta. The reality theater, where gravity plays, will perform on the quantum stage.



Hogan admits that this concept sounds rather strange, even for those of his colleagues who advocate a quantum interpretation. Since the late 1960s, a group of physicists and mathematicians developed a platform called "string theory" to reconcile GTR with quantum mechanics. Over the years, it has become a basic theory, although it failed to fulfill the early promises. Like the solution with a granular space, string theory assumes that the space has a fundamental structure, but then the two theories diverge. String theory claims that every object in the universe consists of vibrating energy strings. Like granular space, string theory avoids a gravitational catastrophe by introducing a finite minimal unit of space, although the size of these strings is much smaller than that of spatial structures sought by Hogan.



The grain space does not fit in with the ideas of string theory, or with any other proposed physical model. "This is a new idea, it is not in the textbooks, it does not follow from any standard theory," says Hogan carelessly. “But there’s no standard theory, is there?”



If he turns out to be right, then many formulations of string theory will be irrelevant, and his theory will inspire a fresh approach to rewriting GR in quantum terms. New ways of understanding the inner nature of space and time will appear. And, most surprisingly, the theory will support the fashionable idea that our three-dimensional reality consists of simpler two-dimensional units. Hogan is serious about the “pixel” metaphor - just as a picture on a TV can create the illusion of depth from flat pixels, so space, he says, can arise from a set of elements that act as if they are in two-dimensional space.



Like many ideas that are located on the far borders of modern theoretical physics, Hogan's reasoning may sound like the evening philosophical conversations of freshmen. They differ in that the physicist plans to test them in the experiment. Right now.



Since 2007, Hogan has been pondering how to build a device that can measure extremely fine grain sizes. His colleagues had a lot of ideas on this subject, based on technology developed to search for gravitational waves. For two years, Hogan developed a proposal and worked with colleagues from Fermilab, the University of Chicago and other institutions to build a machine for searching for grain, which he calls a " holometer ". This is an esoteric pun, referring simultaneously to the measuring device of the XVII century and to the theory by which two-dimensional space may seem three-dimensional, which resembles the storage of an image in a hologram.



Under the layers of conceptual complexity in the holometer there is such a technologically simple device as a laser, a translucent mirror that splits the laser beam into two perpendicular, and two more mirrors reflecting the rays back in a 40-meter tunnel. Rays are calibrated to record the exact location of the mirrors. If the space is granular, then the position of the mirrors will constantly change (more precisely, the space itself will change), which will create constant and random changes in the distance between them. After the reunion of the rays, they will be slightly out of sync, and the magnitude of the discrepancy will show the scale of the grain size of the space.



For the scale that Hogan expects, he needs to measure distances with an accuracy of 10 ^-18 meters, that is, 100 million times less than a hydrogen atom, and collect data at a speed of 100 million measurements per second. Surprisingly, such an experiment is not only theoretically possible, but also practically realizable. “We were able to do without serious costs due to the achievements of photonics , the use of many ready-made components, fast electronics and other things,” says Hogan. “This is a rather bold experiment, so they wouldn’t conduct it if it hadn’t been inexpensive.” Golometer now buzzing itself, and collects data with the desired accuracy. Preliminary results are expected by the end of the year.



Hogan faced criticism from furious skeptics, many of whom belong to the community of theoretical physicists. The topic of controversy is easy to understand: the success of a holometer will mean the failure of a large amount of work on string theory. But despite these disputes, Hogan and most of his colleagues are convinced that, as a result, GR will have to submit to quantum mechanics. The remaining three laws of physics [apparently, refers to the fundamental interaction - approx. transl.] obey the quantum rules, so it makes sense that gravity behaves this way.



For most modern theorists, belief in the advantage of quantum mechanics extends even further. At the philosophical and epistemological level, they believe that the large-scale reality of classical physics is a kind of illusion, an approximation that arises from the more "true" aspects of the quantum world, operating on a small scale. And the granular space is consistent with this view of the world.



Hogan compares his project with a landmark 19th century Michelson-Morley experiment, searching for ether - a hypothetical substance that, according to the theory that was in the lead at the time, conducts light waves in a vacuum. Experiments did not find anything - and this perplexing lack of results inspired Einstein to the service station, from which UTO grew, turning the whole world upside down. Complementing the connection of times, the Michelson-Morley experiment measured the structure of space using mirrors and a divided beam of light - which is very similar to Hogan's experiment.



“We make our holometer with the same mood. Whether we see something or not - in any case, the result will be interesting. The experiment is being done to see if we can find something that supports the theory, says Hogan. - By the way your theoretical colleagues relate to the experiment, one can judge their nature. Our theories have a mathematical style of thinking. I hope for such results that will force people to conduct theoretical research in another direction. ”



Hogan will find the quantum structure of space or not, but he is confident that the holometer will help physicists to come closer to the problem of large and small. He will show the right (or close the wrong) way to understanding the quantum structure of space and how it affects the relativistic laws of gravity that permeates it.





Only in black holes does quantum physics collide with GR in a way that is impossible to ignore.

Extremely large presentation

If you wish to look in a completely different direction, then you need Smolin from the Institute of Theoretical Physics. If Hogan carefully picks up the seeds, Smolin can be called an absolute dissident: “When I was a graduate student, Richard Feynman told me something. It sounded like this: "If all your colleagues tried to show that something is true, and they did not succeed - perhaps it happened because it is something that really is not true." So string theory has been stretching for 40-50 years without visible progress. ”





Lee Smolin



And this is only the beginning of more extensive criticism. Smolin believes that an approach to physics from a small scale is incomplete in its essence. Current versions of quantum field theory explain well how individual particles or small systems of particles behave, but they do not completely take into account what is necessary to build a reasonable theory of the entire cosmos. They do not explain why THAT is exactly what it is. As Smolin says, quantum mechanics is simply "the theory of the subsystems of the universe."



According to him, a more productive approach would be to consider the universe as one giant system, and build a new theory that applies to everything at once. And we already have a theory that provides a platform for this approach: GR. Unlike the quantum platform, GRT does not contain the possibility of having an external observer or an external clock — no “outside” simply exists. Instead, reality is described through the interaction of objects and various areas of space. Even about such a basic thing as the inertia of an object (the resistance of your car trying to start moving until it is forced to do by the engine, and its tendency to move after you take your foot off the gas pedal), can be thought of as a connection with all other particles the universe through a gravitational field.



The last statement is so strange that it should be considered in more detail. We will conduct a mental experiment, closely related to what led Einstein to this theory in 1907. Suppose the Universe is completely empty, with the exception of two astronauts. One of them is spinning, the second is resting. The first one feels dizzy from spinning. But which of them is spinning? From the point of view of any of the two, it is not he who is spinning, but another cosmonaut. And without external reference points, according to Einstein, there is no way to say which of them is right, and there is no reason why one of them should feel something that the other does not feel.



And the difference between the two astronauts appears only if you return the rest of the universe back. Therefore, in the classical interpretation of GR, inertia exists only because you can measure it with respect to the cosmic gravitational field. What is true in this thought experiment is also true for all objects of the real world: the behavior of each of its parts is inextricably linked with all the others. If you ever wanted to be a part of something bigger, then this physics is for you. And according to Smolin, this is also a promising method of obtaining answers to questions about the functioning of nature on all scales.



“GTR is not a description of subsystems. This is a description of the whole universe as a closed system, ”he says. When physicists try to get rid of the discrepancy between TO and quantum mechanics, it seems reasonable for them to follow in the footsteps of Einstein and think in the largest categories.



Smolin understands perfectly well that he is going against universal attachment to thinking on a small, quantum scale. “I'm not going to stir up the water, it just happens. I want to carefully reflect on these complex topics, publish my conclusions and wait for the dust to settle, he says good-naturedly. “I hope that people will argue with the arguments, and that as a result, it will be possible to derive testable predictions.”



At first glance, Smolin's ideas are inconvenient for organizing real experiments. As he claims, besides the fact that all parts of the universe are connected to each other through space, they can be connected through time. , . , . , 1990-, , . « » – , , .



The principle of precedence arises as an answer to the question of why physical phenomena are reproducible. If you are conducting an experiment that has already been done before, you expect the result to be the same as in the past. Light a match and it will catch fire. Light one more match in the same way - well, you understand. Reproducibility is so familiar to us a part of life that we don’t even think about it. We simply attribute the consistent results to the work of the natural "law" that works consistently. Smolin suggests that such laws may appear over time due to the fact that quantum systems copy the behavior of similar systems, observed in the past.



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Although precedents may be involved in atomic-scale events, their influence will extend to the entire cosmos. This is due to the idea of ​​Smolin that reductionist, small-scale thinking is the wrong approach to solving large problems. But it is not enough to make the two classes of physical theories work together, although this is important. He, like all of us, wants to know why the Universe is the way it is. Why does time move forward and not backward? How did we find ourselves in such a universe, with such, and not other, laws?



The lack of meaningful answers to these questions suggests that “with our understanding of quantum field theory, something is wrong at a deep level,” says Smolin. Like Hogan, he is not so concerned about the result of any experiment, as the general scheme of the program of searches for fundamental truths. For him, this means having the opportunity to tell a complete, coherent story of the universe. This means being able not only to predict experiments, but also to explain the unique properties that led to the appearance of atoms, planets, rainbows and people. And here he is also inspired by Einstein.



“The GRT lesson is that relativism triumphs ,” says Smolin. The most likely way to get big answers is to view the universe as a whole.

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No matter what theories will come to, large scales cannot be ignored, since it is in this world that we live and observe it. In fact, the whole universe as a whole is the answer, and the task of physicists is to make it appear from the equations. Even if Hogan is right, his granular cosmos should smooth out on average to the state of reality that we face every day. Even if he is wrong, we have a whole cosmos, with its own properties that need to be explained - and this is not what quantum physics can do for now.



Expanding the boundaries of understanding, Hogan and Smolin help physics to build such connections. They push it not only to reconcile quantum mechanics and general relativity, but also to reconcile ideas and perceptions. The next great theory of physics will undoubtedly lead to excellent mathematics and unimaginable technologies. But the best thing she can do is create a deep meaning that leads back to us, the observers, who define themselves as the fundamental scale of the universe.