The laws of physics work both forward and backward in time. Why then it seems that time moves only in one direction? One of the possible answers may also uncover the secrets of the missing mass. Some facts of our experience as obvious and widespread as the difference between the past and the future. We remember one thing but expect another. If you run the film backwards, it will not be realistic. We say “time arrow”, referring to the path from the past to the future.
One would assume that the existence of the arrow of time is built into the fundamental laws of physics. But the opposite is true. If you made a film about subatomic events, you would have found that it converted in time version looks quite reasonable. More precisely: the fundamental laws of physics — except for a tiny exotic exceptions, to which we will return — will work regardless of, the lever we turn time forward or backward. In light of the fundamental laws of physics, the arrow of time is reversible.
If you follow the logic, a transformation that changes the direction of time is supposed to change the fundamental laws. Common sense dictates that it should. But does not change. Physicists use a convenient shorthand to describe this fact. They call the transformation which reverses the time arrow, just T, “time reversal”. And include the fact that T does not change the fundamental laws to a “T-invariance”, or “T-symmetry”.
Everyday experience violates T-invariance, whereas the fundamental laws respect her. This glaring inconsistency raises complex issues. How real world fundamental laws which respect T-symmetry, manages to look so asymmetrical? Is it possible that one day we will find creatures living in the opposite rhythm of time — which become younger as we grow older? Can we, with the help of some physical process, to turn our own arrow of time?
These are interesting questions, and to him we will return. In this article Frank Wilczek, a theoretical physicist at mit, Nobel prize winner, decided to highlight another issue. It occurs if you start from the other end, as part of the overall experience. Mystery is this?
Why the fundamental laws are problematic and this strange property of T-invariance?
The answer that can be offered today, much deeper and more complex of what we could offer 50 years ago. Current understanding arose from the brilliant interaction of the experimental discovery and theoretical analysis, earned several Nobel prizes. But in our response lacks some elements. Their search may lead us to an unexpected reward: the determination of the cosmological “dark matter”.
The modern history of the T-invariance began in 1956. In that year T. D. Lee and C. N. Yang questioned another but related feature of the physical law, which before they took for granted. Lee and Yang were not concerned about the T, but its spatial analogue, the parity transformation P. while T includes watching movies, going back in time, P includes inhouse movies, reflected in the mirror. P-invariance is the hypothesis that the events that you see reflected in movies, are subject to the same laws as in the originals. Lee and Yang has defined indirect contradictions in this hypothesis and proposed an important experiment to validate them. A few months experiments have shown that P-invariance is violated in many cases. (P-invariance is preserved for the gravitational, electromagnetic and strong interactions, but in General violated for weak interactions).
These dramatic events around P-(non)invariance has forced physicists to think about the T-invariance, related to the assumption that also once was taken for granted. However, the hypothesis of T-invariance has undergone thorough testing in the next few years. And in 1964 the team under the leadership of James Cronin and Valentine Fitch found a peculiar, subtle effect in the decays of K-mesons, which violate T-invariance.
Wisdom understanding John Mitchell — “you don’t know what you have until it is gone” — has been proven subsequently.
If we like little children keep asking “why?” some time we will get more profound answers, but in the end will reach the bottom when we get to the truth, which can’t explain more simply. At this point we declare victory, “this is just how it is”. But if we later find exceptions to our supposed truths, this answer will not be satisfactory. We need to move on.
While T-invariance is a universal truth, it is unclear the extent to which our question posed at the beginning would be useful. Why the universe was T-invariant? Yes just like that. But after Cronin and Fitch the mystery of T-invariance simply cannot be ignored.
Many theoretical physicists are faced with the unpleasant problem of understanding how the T-invariance can be extremely accurate, but not quite. And here handy work of Makoto Kobayashi and Toshihide of Maskava. In 1973 they assumed that an approximate T-invariance is an accidental consequence of other, deeper principles.
The time has passed. Shortly before it drew the contours of the modern Standard model of elementary particle physics, and with them a new level of transparency of fundamental interactions. By 1973, was a powerful and empirically successful theoretical framework based on several “sacred principles”. This is relativity, quantum mechanics and the mathematical rule of uniformity called gauge symmetry.
But to make all these ideas work together has proved difficult. Together, they significantly limit the ability of the underlying interactions.
Kobayashi and Maskawa, in two short paragraphs, did two things. First, they showed that if you restrict the physics of the then known particles (for example, if there were only two families of quarks and leptons), then all interactions allowed by sacred principles that are also followed by T-invariance. If Cronin and Fitch never made his discovery, it would not be so. But they did, and Kobayashi with Mackaway went even further. They showed that if you introduce a special set of new particles (the third family), these particles will lead to new interactions which lead to violations of T-invariance. At first glance — direct what the doctor ordered.
In later years a brilliant example of detective work was fully justified. New particles, whose existence made Kobayashi and Maskawa were discovered, and their interaction turned out to be exactly as they should have been.
Attention, question. Are these sacred principles sacred? Of course, no. If the experiments lead to the fact that scientists should complement these principles, they certainly complement it. At the moment the sacred principles look pretty damn good. And was quite productive to take them seriously.
Still it was a story of triumph. The question we put at the beginning, one of the most difficult puzzles about how the world works, got a partial answer: deep, beautiful, fruitful.
But this Apple has a worm.
A few years after the work of Kobayashi and Maskawa, Gerard t Hooft found a loophole in their explanation of T-invariance. Sacred principles allow an additional kind of interaction. Possible new interaction is pretty thin, and the opening t of Hooft was a surprise to most theoretical physicists.
New interaction, in the presence of substantial force, would violate T-invariance in a much more obvious extent than the effect, discovered by Cronin, Fitch and their colleagues. In particular, it would allow the rotation of a neutron to produce an electric field, in addition to the magnetic field that it can cause. (The magnetic field of the rotating neutron — analogue that produces our rotating Earth, although in a completely different scale). Experimenters strenuously searched for such electric field, but their search did not bring results.
Nature did not seem to want to use a loophole t Hooft. Of course, it’s her right, but this right raises again our question: why is nature so closely follows the T-invariance?
It was suggested several explanations, but only one has passed the test of time. The Central idea belongs to Roberto Peccei and Helen Quinn. Their proposal, like Kobayashi and Maskawa, includes an extension of the Standard model in a special way. For example, using a neutralizing field, the behavior of which is especially sensitive to the new interaction t Hooft. If there is a new interaction, neutralizing box adjusts its size to offset the impact of this interaction. (This process of adjustment in General, similar to how the negatively charged electrons in solids gather around the positively charged impurities and their impact shield). This neutralizing field, it turns out, closes our door.
Peccei and Quinn forgot about the important testable implications of their ideas. Particles produced by neutralizing their field, its quanta — must have remarkable properties. Because they forgot about their particles, they also did not name. This allowed me to realize a childhood dream.
A few years before I saw in the supermarket, brightly painted box with the name “axion” (Axion). I thought “axion” sounds like a particle and, like, such is. So when I discovered a new particle, which “cleans” the problem with the help of the axis (axial) of the thread I thought had a chance. (Soon I learned that Steven Weinberg also found this piece, regardless. He called it “highly”. Fortunately, he agreed to renounce this title). So began the epic, of which only the conclusion left to write.
In the Chronicles of Particle Data Group you will find several pages covering dozens of experiments describing unsuccessful search of Acciona. But the reasons for optimism is still there.
For axions the theory predicts, in General terms, the axions must be very light, very long-lived particles which interact only weakly with ordinary matter. But to compare theory and experiment, we need to look at the numbers. And here we are faced with ambiguity, because the existing theory does not fix the value of the mass of axion. If we knew the mass of axion, we would have predicted and other properties. But the mass may be in a wide interval of values. (The same problem was with the charmed quark, the Higgs, top-quark and a few others. To detect each of these particles, the theory predicted all their properties other than mass). It turned out that the force of interaction of axion proportional to its mass. Therefore, decreasing the mass of axion, it becomes all the more elusive.
Previously, physicists have focused on models in which the axion is closely linked to the Higgs. Assumed that the mass of axion should be of the order of 10 Kev — one fiftieth the mass of the electron. Most of the experiments, which we said earlier, the axion was looking for exactly such a plan. Now we can be sure that such does not exist for axions.
And so the attention turned to much smaller values of the masses of Acciona, which have not been excluded experimentally. The axions of this kind naturally arise in models unifying the interaction in the Standard model. They also arise in string theory.
We have calculated that the axions were abundantly produced during the first moments of the Big Bang. If axions exist at all, Acciona liquid fills the Universe. The origin aksionau fluid roughly resembles the origin of the famous cosmic microwave background, but there are three major differences between the two concepts. First, a background is observed, and Acciona liquid remains purely hypothetical. Second, since axions have a mass, their fluid affects the overall mass density of the Universe. In fact, we calculated that their mass should roughly correspond to the mass that astronomers have identified over dark matter! Third, since the axions are so weakly interacting, they should be more difficult to observe than the photons of the background radiation.
Experimental search for axions is continuing on several fronts. Two of the most promising experiments aimed at finding aksionau fluid. One of them, ADMX (Axion Dark Matter eXperiment), uses a special ultra-sensitive antenna for converting a background for axions into electromagnetic pulses. The other, CASPEr (Cosmic Axion Spin Precession Experiment), looking for tiny variations in the motion of nuclear spins, which can be caused aksionau liquid. In addition, these complex experiments promise to cover almost the entire range of possible masses of axion.
Axions are there? We don’t know yet. Their existence will bring the history of the reversible arrow of time is a dramatic and satisfying conclusion, and possibly solve the mystery of dark matter to boot. The game has begun.
Frank Wilczek, based on Quanta Magazine