Why, then, does time seem to move in only one direction?
One of the possible answers may also reveal the secrets of the missing mass. Some of the facts of our experience are as obvious and widespread as the difference between the past and the future. We remember one thing, but we expect another. If you run the movie in the opposite direction, it will not be realistic. We say "arrow of time", meaning the path from the past to the future.
One might assume that the existence of the arrow of time is built into the fundamental laws of physics. But the opposite is also true. If you made a film about subatomic events, you would find that its time-reversed version looks quite reasonable. More precisely, the fundamental laws of physics - with the exception of tiny exotic exceptions, to which we'll come back - will work regardless of whether we turn the time lever forward or backward. Against the background of the fundamental laws of physics, the arrow of time is reversible.
Logically, a transformation that reverses the direction of time must also change fundamental laws. Common sense dictates what should. But it doesn't change. Physicists use a convenient acronym to describe this fact. They call the transformation that reverses the arrow of time, simply T, from time reversal. And the fact that T does not change fundamental laws is referred to as "T-invariance" or "T-symmetry".
Everyday experience violates T-invariance, while fundamental laws respect it. This glaring discrepancy raises difficult questions. How does the real world, whose fundamental laws respect T-symmetry, manage to look so asymmetrical? Is it possible that one day we will find beings living in the opposite rhythm of time - who get younger as we get older? Can we, through some physical process, reverse our own arrow of time?
These are interesting questions, and we will come back to them later. In this article, Frank Wilczek, a theoretical physicist at the Massachusetts Institute of Technology and a Nobel Prize laureate, decided to cover another issue. It arises when you start at the other end, within the framework of a shared experience. The riddle is this?
Why do fundamental laws have this problematic and strange property, T-invariance?
The answer that can be offered today is incomparably deeper and more complex than what we could offer 50 years ago. Today's understanding has emerged from the brilliant interplay of experimental discovery and theoretical analysis, which have won several Nobel Prizes. But our answer is missing some elements. Searching for them may lead us to an unexpected reward: the definition of cosmological "dark matter".
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The modern history of T-invariance began in 1956. That year, T. D. Lee and C. N. Young questioned another but related feature of physical law that had previously been taken for granted. Lee and Young were not bothered by T itself, but by its spatial counterpart, the parity transformation of P. While T involves viewing movies that go back in time, P includes viewing movies reflected in a mirror. P-invariance is the hypothesis that the events you see in reflected films obey the same laws as in the originals. Lee and Young identified indirect inconsistencies in this hypothesis and proposed an important experiment to test them. Experiments over several months have shown that P-invariance is violated in many cases. (P-invariance is conserved for gravitational, electromagnetic and strong interactions,but generally violated for weak interactions).
These dramatic events around P- (non) invariance have led physicists to think about T-invariance, a related assumption that was also once taken for granted. However, the T-invariance hypothesis has undergone rigorous testing for several years. It was only in 1964 that a group led by James Cronin and Valentina Fitch discovered a peculiar, subtle effect in the decays of K-mesons, which violates T-invariance.
The wisdom of John Mitchell's understanding - that "you don't know what you have until it is gone" - has been proven afterwards.
If we, like little children, keep asking “why?” We will get deeper answers for a while, but eventually we will hit rock bottom when we come to a truth that we cannot explain more simply. At this moment we declare victory: "Everything is as it is." But if we later find exceptions to our supposed truth, this answer will no longer satisfy us. We must move on.
As long as T-invariance is a universal truth, it is not clear how useful our question at the beginning will be. Why was the universe T-invariant? Just because. But after Cronin and Fitch, the T-invariance puzzle simply cannot be ignored.
Many theoretical physicists have faced the vexing problem of understanding how T-invariance can be extremely accurate, but not quite. And here the work of Makoto Kobayashi and Toshihide Maskawa came in handy. In 1973, they suggested that the approximate T-invariance is an accidental consequence of other, deeper principles.
Time has passed. Not long before that, the contours of the modern Standard Model of elementary particle physics were drawn, and with them a new level of transparency of fundamental interactions. By 1973, there was a powerful - and empirically successful - theoretical framework based on several "sacred principles." These are relativity, quantum mechanics, and a mathematical rule of uniformity called gauge symmetry.
But getting all these ideas to work together proved difficult. Together they significantly limit the possibilities for basic interactions.
Kobayashi and Maskawa, in two short paragraphs, did two things. First, they showed that if we restrict physics to the then known particles (for example, if there were only two families of quarks and leptons), then all interactions allowed by sacred principles also follow T-invariance. If Cronin and Fitch had never made their discovery, this would not be the case. But they did, and Kobayashi and Maskawa went even further. They showed that if we introduce a special set of new particles (the third family), these particles will lead to new interactions, leading to violations of T-invariance. At first glance, exactly what the doctor ordered.
In the years that followed, their brilliant example of detective work was fully justified. The new particles that Kobayashi and Maskawa admitted to exist were discovered, and their interactions turned out to be exactly what they should have been.
Attention, question. Are these sacred principles really sacred? Of course not. If experiments lead scientists to complement these principles, they will certainly complement. At the moment, sacred principles look pretty damn good. And they were fruitful enough to take them seriously.
So far, it has been a story of triumph. The question that we posed at the beginning, one of the most difficult puzzles about how the world works, received a partial answer: deep, beautiful, fruitful.
A few years after the work of Kobayashi and Maskawa, Gerard t'Hooft discovered a loophole in their explanation of T-invariance. Sacred principles allow an additional kind of interaction. The possible new interaction is quite subtle, and t'Hooft's discovery came as a surprise to most theoretical physicists.
The new interaction, if present with significant strength, would violate T-invariance to a much more obvious degree than the effect discovered by Cronin, Fitch and their colleagues. In particular, it would allow the rotation of the neutron to generate an electric field, in addition to the magnetic field it can induce. (The magnetic field of a spinning neutron is analogous to what our spinning Earth produces, albeit on a completely different scale.) Experimenters have been searching hard for such electric fields, but their search has yielded no results.
It is as if nature does not want to use t'Hooft's loophole. Of course, this is her right, but this right again raises our question: why does nature follow T-invariance so carefully?
Several explanations have been offered, but only one has stood the test of time. The central idea belongs to Roberto Pezzie and Helen Quinn. Their proposal, like that of Kobayashi and Maskawa, involves extending the Standard Model in a special way. For example, through a neutralizing field, the behavior of which is especially sensitive to the new t'Hooft interaction. If a new interaction is present, the neutralizing field adjusts its own magnitude to compensate for the influence of this interaction. (This tuning process is generally similar to how negatively charged electrons in solids gather around positively charged impurities and shield their influence.) Such a neutralizing field, it turns out, closes our loophole.
Pezzie and Quinn have forgotten the important testable implications of their idea. The particles produced by their neutralizing field - its quanta - must have remarkable properties. Since they forgot about their particles, they did not name them either. This allowed me to fulfill my childhood dream.
A few years earlier, I had seen a brightly colored box in a supermarket called Axion. It seemed to me that the "axion" sounds like a particle and, it seems, is. So when I discovered a new particle that "cleans up" the problem with an "axial" flow, I felt like I had a chance. (I soon learned that Steven Weinberg also discovered this particle, independently. He called it the Higglet. Fortunately, he agreed to drop that name.) Thus began the epic, the conclusion of which only remains to be written.
In the Chronicles of the Particle Data Group, you'll find several pages covering dozens of experiments describing unsuccessful searches for the axion. But there are still reasons for optimism.
Axion theory predicts, in general terms, that axions should be very light, very long-lived particles that interact weakly with ordinary matter. But to compare theory and experiment, you need to rely on numbers. And here we are faced with ambiguity, since the existing theory does not fix the value of the axion mass. If we knew the mass of the axion, we would predict the rest of its properties. But the mass itself can be in a wide range of values. (The same problem was with the charmed quark, Higgs particle, top quark, and several others. Prior to the discovery of each of these particles, the theory predicted all of their properties, except for the mass value). It turned out that the force of interaction of the axion is proportional to its mass. Therefore, as the value of the mass of the axion decreases, it becomes more and more elusive.
In the past, physicists have focused on models in which the axion is closely related to the Higgs particle. It was assumed that the mass of the axion should be of the order of 10 keV - one-fifty of the mass of an electron. Most of the experiments that we talked about earlier looked for an axion of just such a plan. At the present time, we can be sure that such axions do not exist.
Dark matter
And therefore, attention was drawn to much smaller values of the axion masses, which were not excluded experimentally. Axions of this kind appear quite naturally in models that combine interactions in the Standard Model. They also appear in string theory.
We calculated that axions should have been produced in abundance during the early moments of the Big Bang. If axions exist at all, then the axion fluid fills the Universe. The origin of axion fluid roughly resembles the origin of the famous cosmic microwave background, but there are three major differences between the two. First, the microwave background is observed, and the axion fluid remains purely hypothetical. Second, because axions have mass, their fluid affects the overall mass density of the universe. Basically, we calculated that their mass should roughly correspond to the mass that astronomers have determined behind dark matter! Third, because axions interact so weakly, they should be more difficult to observe than CMB photons.
The experimental search for axions continues on several fronts. Two of the most promising experiments are aimed at finding axion fluid. One of them, ADMX (Axion Dark Matter eXperiment), uses special super-sensitive antennas to convert background axions into electromagnetic pulses. Another, CASPEr (Cosmic Axion Spin Precession Experiment), looks for tiny fluctuations in the motion of nuclear spins that could be caused by axion fluid. In addition, these sophisticated experiments promise to cover almost the entire range of possible axion masses.
Do axions exist? We don't know yet. Their existence would bring a dramatic and satisfying conclusion to the history of the reversible arrow of time, and perhaps also solve the mystery of dark matter in the bargain. The game started.
Frank Wilczek, based on Quanta Magazine
