Will quantum mechanics absorb reality?

The competition between gravity and quantum physics takes a new turn

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

Today, physicists have two sets of rules in their hands explaining how nature works. There is a general theory of relativity that perfectly describes gravity and all that 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 copes well with the explanation of what happens when a uranium atom decays or when individual particles of light and a photocell collide. This is a small scale.

And now the problem: relativity and quantum mechanics are fundamentally different theories, formulated in different ways. And this is not a matter of scientific terminology, this is a clash of truly incompatible descriptions of reality.

The conflict between the two halves of physics has matured for more than 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 quantum - but recently it entered a very interesting and unpredictable phase. Two prominent physicists have identified extreme positions, each in his own camp, and are conducting experiments that can once and for all 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 “grainy”. In general relativity, events are continuous and deterministic, that is, each action causes a certain local effect. In quantum mechanics, events that occur due to interactions of subatomic particles occur by leaps (yeah, quantum leaps), with probabilistic rather than deterministic results. Quantum rules make it possible to establish connections forbidden by classical physics. This has recently been shown in an often-discussed experiment in which Dutch researchers challenged the effect of locality. They showed that two particles - in their case, electrons - can instantly influence each other, although they were spaced one and a half kilometers away.

THAT gives meaningless answers if you try to reduce the scale to quantum dimensions, and drops to infinite values ​​when describing gravity. Conversely, quantum mechanics faces serious difficulties, being inflated to a cosmic scale. Quantum fields carry a certain amount of energy, even in seemingly empty space, and the larger the field, the more energy becomes. According to Einstein, energy is equivalent to mass (E = mc ² ), so to accumulate energy is the same as to accumulate mass. If you accumulate it quite a lot, then the amount of energy in the quantum field creates a black hole, due to which the entire Universe collapses. Oh.

Craig Hogan , theoretical astrophysicist at the University of Chicago and director of the Center for Particle Astrophysics at Fermilab, reinterprets the quantum aspect of physics with the help of a new theory in which the quantum units of space can be large enough to be studied. Lee Smolin , founder of the Institute of Theoretical Physics Perimeter at the University of Waterloo, is trying to get physics to return 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 GTR to the world, she not only replaced Isaac Newton’s theory of gravity, but also discovered a new way of looking at physics that led to modern concepts of the Big Bang and black holes, not to mention the atomic bombs and the time adjustment necessary to The phone worked GPS. In the same way, quantum mechanics did not simply reformulate Maxwell's equations describing electricity, magnetism, and light. She provided the tools necessary to create the Large Hadron Collider, photocells and all modern microelectronics.

As a result of these disputes, there will be no more, no less - the third revolution in modern physics, which will lead to stunning consequences. We can find out where the laws of nature came from, and whether the cosmos is built on the basis of uncertainty, or whether determinism is based on it, when a specific reason is associated with each event.


Craig Hogan

Granular cosmos

Instead of wandering in the dark, Hogan, the leader of the quantum worldview, prefers to act according to a joke, and search 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 does at extremely short distances - so he decided to take a closer look at what was happening there. “I’m sure that it’s possible to conduct an experiment that will allow us to see what is happening, how this interface works, which we don’t understand yet,” he says.

The simplest assumption in Einstein's physics - and traces of its origin still 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 this approach. Just as your screen has the smallest unit - a pixel, and light has the smallest unit - a photon, so the distance, according to him, should have an indivisible smallest unit - a quantum of space.

According to Hogan, it would be pointless to ask how gravity behaves at distances shorter than a unit of space. At such scales, gravity will not be able to work, since such scales do not exist. In other words, GTR will have to make peace 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 to those of his colleagues who advocate a quantum interpretation. Since the late 1960s, a group of physicists and mathematicians have been developing a platform called “string theory” to reconcile GR with quantum mechanics. Over the years, it has become a basic theory, although it has not been able to fulfill its early promises. Like a solution with granular space, string theory assumes that 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 gravitational catastrophe by introducing a finite minimum unit of space, although the size of these strings is much smaller than the spatial structures sought by Hogan.

Granular space does not fit 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,” Hogan says lightly. “But there is no standard theory, is there?”

If he turns out to be right, then many formulations of string theory will be out of work, and his theory will inspire a fresh approach to rewriting GR in quantum terms. New ways of understanding the internal 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 takes the “pixels” metaphor seriously - just as a picture on a TV can create the illusion of depth from flat pixels, so space, he said, can arise from a set of elements behaving as if they are in two-dimensional space.

Like many ideas that are far beyond the boundaries of modern theoretical physics, Hogan’s reasoning may sound like evening philosophical conversations from freshmen. They differ in that the physicist plans to test them in an experiment. Right now.

Since 2007, Hogan has been thinking about how to build a device that can measure the extremely fine grain size of space. His colleagues had many ideas on this subject, based on technology developed for the search for gravitational waves. In two years, Hogan developed a proposal and worked with colleagues from Fermilab, the University of Chicago, and other institutes to build a machine to search for grit, which he calls a " holometer". This is an esoteric pun that makes reference both to the 17th-century measuring instrument and to the theory that two-dimensional space may seem three-dimensional, which resembles storing an image in a hologram.

Under the layering of conceptual complexity in the holometer, there is such a technologically uncomplicated device as a laser, a translucent mirror that branches the laser beam into two perpendicular ones, and two more mirrors that reflect the rays back into the 40-meter tunnel. The rays are calibrated to record the exact location of the mirrors. If the space is grainy, 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 reuniting the rays, they will turn out to be slightly out of sync, and the magnitude of the mismatch will show the scale of the graininess of the space.

For the scale that Hogan expects, he needs to measure distances with an accuracy of 10 ^-18meters, that is, 100 million times smaller than the 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 feasible. “We were able to do without serious costs thanks 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 would not have been carried out if it had not been inexpensive.” The holometer is buzzing for itself now, and collecting data with the necessary accuracy. By the end of the year, preliminary results are expected.

Hogan was faced with criticism of violent skeptics, many of whom belong to the community of theoretical physicists. The topic of debate is easy to understand: the success of the 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 GTR will have to submit to quantum mechanics as a result. The other three laws of physics [apparently, they mean fundamental interactions - approx. transl.] are subject to quantum rules, so it makes sense for gravity to behave this way.

For most modern theorists, the belief in the advantage of quantum mechanics extends even further. On philosophical and epistemologicallevel, they believe that the large-scale reality of classical physics is an illusion, an approximation arising from the more “true” aspects of the quantum world working on small scales. And the granular space is consistent with this view of the world.

Hogan compares his project with the landmark experiment of the 19th century by Michelson-Morley, who were looking for ether - a hypothetical substance that, according to the theory that was the leader at that time, conducts light waves in a vacuum. The experiments did not find anything - and this perplexing lack of result inspired Einstein in the SRT, from which GTR grew, eventually turning the whole world upside down. Complementing the connection of time, the Michelson-Morley experiment measured the structure of space using mirrors and a divided beam of light - which is very similar to the Hogan 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 anything that supports the theory, ”says Hogan. “By the way your theoretical colleagues relate to the experiment, you can judge their nature.” Our theories favor a mathematical style of thinking. I hope for such results that will force people to conduct theoretical research in a different direction. "

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


Only in black holes does quantum physics collide with GR in a way that cannot be ignored

Extremely Great Performance

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


Lee Smolin

And this is only the beginning of more extensive criticism. Smolin believes that the approach to physics from a small scale is incomplete in its essence. Current versions of quantum field theory well explain how individual particles or small systems of particles behave, but completely do not take into account what is needed to build a reasonable theory of the whole cosmos. They do not explain why THAT is exactly what it is. As Smolin says, quantum mechanics is simply a “theory of 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, GTR does not contain the possibility of having an external observer or an external clock - there is simply no “outside”. Instead, reality is described through the interaction of objects and various regions of space. Even about such a basic thing as the inertia of an object (the resistance of your car to trying to start moving until it is forced to do it by the engine, and its tendency to move after you take your foot off the gas pedal), you can argue as a connection with all other particles universe through a gravitational field.

The last statement is so strange that it is worth considering in more detail. Let's conduct a thought experiment, closely related to the fact that Einstein led 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 at rest. The first one feels dizzy from rotation. But which of them is spinning? From the point of view of either of the two, he is not spinning, but the other astronaut. 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 GRT, inertia exists only because you can measure it in relation 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 part of it is inextricably linked with all the others. If you ever wanted to be 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.

“GRT is not a description of subsystems. This is a description of the entire 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 Einstein's footsteps and think in the largest categories.

Smolin is well aware that he is opposed to 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 findings and wait for the dust to settle,” he says good-naturedly. “I hope that people will argue with arguments, and that verifiable predictions can be made as a result.”

At first glance, Smolin’s ideas are inconvenient for organizing real experiments. According to him, in addition to the fact that all parts of the universe are connected to each other through space, they can also be connected through time. His reasoning led him to the hypothesis that the laws of physics evolve along with the development of the universe. Over the years, he has developed two detailed assumptions about how this can happen. His theory of cosmological natural selection, developed by him in the 1990s, considers black holes as cosmic eggs from which new universes hatch. Later, he developed a provocative hypothesis about the emergence of the laws of quantum mechanics called the "principle of precedence" - and now, apparently, it can already be tested.

The principle of precedence arises as an answer to the question of why physical phenomena are reproducible. If you are conducting an experiment that you have previously performed, you expect the result to be the same as in the past. Light a match and it will light up. Light another match in the same way - well, you get the point. Reproducibility is a part of life so familiar to us that we don’t even think about it. We simply attribute consistent results to the work of a natural “law” that works unchanged. 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.

One of the possible ways to catch the moment of appearance is to conduct an experiment that no one has previously conducted so that it does not have previous versions (use cases) that could be copied. Such an experiment can create a high-complexity quantum system containing many components that exist in a new entangled state. If the principle of precedence is true, then the initial reaction of the system will be random. When the experiment is repeated, the precedence will accumulate, and the reaction of the system should become predictable - in theory. “It will be difficult to distinguish the system by which the Universe sets precedents from the random noise of experimental practice,” Smolin says, “but it is possible.”

Although precedents may be involved in what is happening on an atomic scale, their influence will extend to the whole cosmos. This is due to Smolin's idea that reductionist, small-scale thinking is the wrong approach to solving big problems. But it’s not enough to get the two classes of physical theories to work together, although this is important. He, like all of us, wants to know why the Universe is what it is. Why is time moving forward rather than backward? How did we end up in such a universe, with such laws, and not others?

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 with 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 having the opportunity 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 lesson of GR is that relativism wins ,” says Smolin. The most likely way to get great answers is to look at the Universe as a whole.

And the winner is ...

If you need a judge in this dispute, big and small, it’s hard to find a better candidate than Sean Carroll , an expert in cosmology, field theory and gravitational physics at Caltech. He understands relativity, quantum mechanics, and has a sense of the absurd: he called his blog " Absurd Universe ."

And right away, Carroll almost completely sided with quantum mechanics. “Most of us believe that quantum mechanics is more fundamental than general relativity,” he says. This view has prevailed since the 1920s, when Einstein tried and could not find flaws in the counterintuitive predictions of quantum theory. A recent Dutch experiment, demonstrating an instantaneous quantum connection between two strongly separated particles - what Einstein called "frightening long-range action" - only emphasizes the power of evidence.

More broadly, the real problem is not GTR versus quantum field theory, as Carroll says, but classical dynamics versus quantum dynamics. Relativity, in spite of its strangeness, is classical in the sense that it relates to cause and effect; quantum mechanics definitely not. Einstein was sure that some deep discoveries would reveal the classical, deterministic reality hiding under quantum mechanics, but so far no such order has been discovered. The demonstrated reality with frightening long-range indicates that this order does not exist.

“People clearly underestimate how strongly quantum mechanics refute our concepts of space and locality (the idea that a physical phenomenon can only affect its immediate environment). There are simply no such things in quantum mechanics, ”Carroll says. Large-scale consequences can arise from small-scale phenomena, such as Hogan’s reasoning about three-dimensional reality, arising from two-dimensional units of space.

But despite the apparent support, Carroll believes that the Hogan holometer has few chances, although he admits that this is not his area of ​​research. On the other hand, he does not consider Smolin’s attempt to start from space as a fundamental thing to be something special. He believes this is as absurd as trying to prove that air is more fundamental than atoms. Regarding the question of which of the quantum systems can take physics to the next level, Carroll is optimistic about the string theory, which, according to him, “seems like a natural extension of quantum field theory”. In any case, he stands for the conventional thinking in modern physics based on quanta.

And yet, Carroll’s opinion, which is almost entirely inclined toward quanta, does not fully support small-scale thinking. There are still huge gaps in the explanations of quantum theory. “Our inability to choose the right version of quantum mechanics is a shame,” he says. - And our current way of representing quantum mechanics is a complete failure, if we argue from the point of view of the cosmology of the entire Universe. We don’t even know what time is. ” Hogan and Smolin support this statement, although they do not agree on what to do with it. Carroll stands for the inverted explanations in which time arises from quantum-level interactions, but claims to be skeptic about Smolin's rival assumption that time is more universal and fundamental. So nothing has been decided on the issue of time.

No matter what theories come to, the large scale 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, on average, smooth out to the state of reality that we face daily. Even if it is wrong, we have a whole cosmos, with its own properties that need to be explained - and so far, quantum physics cannot do this.

Expanding the boundaries of understanding, Hogan and Smolin help physics to build such connections. They push it not only toward the reconciliation of quantum mechanics and general relativity, but also toward the reconciliation of ideas and perceptions. The next great theory of physics will no doubt lead to excellent mathematics and unimaginable technologies. But the best she can do is create a deeper meaning that leads back to us, observers who define themselves as the fundamental scale of the universe.