Welcome to the real mirror world

A universe that is exactly the same as our universe may be around us.

At first glance, everything looks so familiar. The clock on the wall is ringing; outside the window, the car is driving through; the “Technology” magazine in your hand, the cover is equally appealing. But there seems to be something wrong: the clock is going backwards; the car is driving to the left; the text you are reading is also upside down. Haha! It turns out that you are looking at the image in the mirror.

Almost everyone thinks that the world in the mirror is “fake” – although there is a book called “Alice in the Mirror”, which tells about all kinds of adventures in the mirror, but it is a fairy tale. But for some physicists, the world in which everything is flipped up and down (which may be called the “mirror world”) may be real, and may be hidden by us. In that world, there are mirror atoms, mirror molecules, mirror stars and planets, and even mirror life. It’s just that they hardly interact with our world and have never been discovered. But that doesn’t mean we can’t find it forever. Some particles may “switch” between our world and the mirror world – in the world for a while, and in the world for a while – thus exposing the existence of a mirrored world.

Today, physicists are arranging experiments to validate this hypothesis. If we really find such a mirrored universe, apart from our radical view of reality, we can also answer some questions about our own universe – they have been plaguing us for decades.

Discover a whole new world

It is certainly not an easy task to discover a whole new world and let the human eye open up. But in physics, this almost miraculous thing has not happened. In 1928, the British physicist Dirac predicted according to the theory of quantum mechanics that there is a new family of particles in the universe, in which the particles are identical to the known particles, except for the opposite charge. This is the “antimatter particles” that we often hang on our mouths; the world of these particles is called the “antimatter world.”

Not only that. In 1933, the Swiss astronomer Fritz Zwicky observed that the gravitation of the visible matter in the cluster of galaxies seemed to be unable to provide sufficient centripetal force for the rapid rotation of the cluster of galaxies. In other words, if there is no extra gravity, the galaxy group should have disintegrated and it does not exist.

Today, we believe that this extra gravity comes from “dark matter.” The ratio of the mass of dark matter to ordinary matter in the universe is about 5:1. Between dark matter and ordinary matter, there is no interaction other than gravity, especially the interaction of electromagnetic force, so it is difficult to find that the particles that make up dark matter have not been found so far. Despite this, the dark matter has been accepted by the mainstream scientific community.

The existence of antimatter and dark matter provides a new way of thinking for us to understand matter and understand the universe. For example, one of the problems that plagues modern physics is: Is there a world dominated by antimatter? Where it is, where is it (considering that the confrontation of positive and negative materials will be wiped out, the antimatter world must be very far away from us)? Also, there is no “dark” version of the world around us, where the material is “dark”, the force is “dark”, and even the “dark” version of the person?

With these precedents cheering for us, let us now set out to explore another new world – the mirror world!

Is the parity conservation or non-conservation?

In physics, a very useful concept is symmetry. Symmetry in physics refers to the fact that the laws of physics remain unchanged under certain transformations. For example, a ball moves from point A to point B. Although the spatial position changes (this is called “space translation transformation”), the physical laws it obeys (such as Newton’s three laws) remain unchanged. This is called spatial translation symmetry. For another example, just changing the color of a ball, its response to the Earth’s gravity is not affected. If you take a name, you can also call it “color conversion symmetry.”

A very important symmetry in particle physics is called “parity”, which is “left-right symmetry” or “mirror inversion.” “Protection of parity” means that even if all the positions and directions of the objects are turned over like in the mirror, the physical processes and laws remain unchanged.

For example, a small ball moves to the right, which follows Newton’s laws of motion. If we put a mirror and do this experiment in the mirror world – you may be curious: How do we go to the mirror world to do experiments? In fact, you don’t have to go to the mirror, just arrange the experimental device as shown in the image – the ball moves to the left. But like a small ball moving to the right, the ball moving to the left also follows Newton’s laws of motion.

For quite some time, physicists believe that in nature, parity is conserved. But this seemingly natural guess was later troubled.

In the early 1950s, scientists observed two new mesons (the particles whose mass is between protons and electrons) from cosmic rays: θ and τ. The spins, mass, lifetime, and charge of these two mesons are exactly the same, and many people think they are the same kind of particles. However, they have different decay modes. When θ decays, two π mesons are generated, and τ decays into three π mesons, which means that they follow different laws of motion.

If τ and θ are different particles, how do they have exactly the same quality and longevity? And if they admit that they are the same kind of particles, how can they have a completely different way of decay? In order to solve this problem, the physics community has proposed various ideas, but they have not succeeded.

In 1956, after studying various factors in detail, Li Zhengdao and Yang Zhenning boldly asserted that τ and θ are exactly the same kind of particles (later called K meson), but in the decay involving weak nuclear forces. The parity is not conserved, causing it to have no fixed decay mode, either decaying into two π mesons or decaying into three π mesons.

The views of Li Zhengdao and Yang Zhenning shocked the physics community at the time. Shortly thereafter, Wu Jianxiong verified that “the parity is not conserved” with a clever experiment. Since then, “the parity is not conserved” is truly recognized.

Is the mirror world real?

But little known is that Li and Yang have also proposed another rather crazy explanation. They think that the reason why the parity does not seem to be conserved is because we only look at the situation in our universe; if there is another hidden universe, where the parity is not conserved in the opposite direction, then two The universe is considered together, and the parity is generally conserved.

For example. There is a bowl, and there are several irregularly distributed notches on the bowl. As far as the bowl itself is concerned, the left-right symmetry is “broken”. However, if you let it face a mirror and treat the bowl and its image in the mirror as a system, the left-right symmetry remains. The premise of this interpretation is that the mirror world is as real as our world.

The idea of ​​a real mirror world was not popular at the time, so Li and Yang quickly gave up. But today, particle physics faces many challenges, and some people are beginning to regain this view. They said that in fact we may have seen the signs of the existence of the mirror world from the behavior of neutrons.

Neutrons are one of the basic particles that make up the nucleus. It is stable in the nucleus, but outside the nucleus, the so-called free neutron, it is unstable and will decay into electrons and protons (this is called beta decay). For decades, physicists have struggled to figure out how long free neutrons can exist before decay, but the results are contradictory.

There are two main methods for measuring the lifetime of free neutrons: one is to capture with a “magnetic trap”, and the other is to use a neutron beam. The trap capture method is quite simple: you use a weak magnetic field to gather a bunch of neutrons into a “trap”, let them decay inside, and then count the remaining neutrons. According to this method, the average lifetime of neutrons is 14 minutes and 39 seconds.

The second method is to extract the neutron beam from the nuclear reactor and measure the number of protons it produces. Protons are charged and easier to measure. A neutron decay produces a proton. Measured by this method, the resulting neutron lifetime was 14 minutes and 48 seconds.

The difference in life measured by the two methods is 9 seconds. At first, physicists attributed it to experimental error. However, with the improvement of measurement technology, the measurement error is getting smaller and smaller, but these two values ​​are not consistent. Neutrons seem to have two lifetimes.

Some physicists believe that if the mirror world exists, it may be the culprit of the above problems. They proposed a conjecture: neutrons may oscillate back and forth between the two worlds. The neutrons only spend part of their lives in our universe, and the rest of the time is spent in the mirror world of “parallel” (because it is around us), where any protons they emit are not discovered by us.

They calculated: If one of the 100 neutrons “switches” into the mirror world before decaying into protons, it can explain why the neutron lifetime measured by the “magnetic trap” method is shorter. Because this neutron is hidden behind the mirror world, we can’t detect it. That is to say, when we measured only 50 neutrons, it actually only has 49 neutrons decaying, which is naturally shorter than the time required for 50 neutron decays.

Dark matter hiding in the mirror world?

The mirror universe can even provide a hiding place for dark matter and explain why dark matter is so hard to find. This guess seems to be more attractive when you know how much dark matter might be hidden in the mirror world.

In keeping with the model of early cosmic evolution, it is necessary to assume that the part of the mirrored world is much colder than our own universe. If we imagine our universe and mirror universe as hot water and cold water, then this temperature difference will make it easier for particles in our world to cross into the mirror universe and disappear from our universe. The material in our universe is decreasing, and the material in the mirror world is increasing. There is a mirror world model that predicts that every ordinary particle in our world corresponds to five particles in the mirror world—this is exactly the ratio of the normal matter to the measured dark matter 1:5.

In our world, the remaining particles form stars, planets, and eventually humans. We seem to have reason to expect this evolution to occur in the mirror world. There are mirrored stars, mirrored planets, and even mirrored humans. Because we say dark matter is “dark”, but they are “cold” with the material relations in our world (except for gravity, there is no other interaction). As for themselves, the relationship is “hot”, and maybe there is also mirror electromagnetic force, mirroring Nuclear power, etc. The ordinary matter in our world, in the mirror world, has become a “dark matter.” Who knows, maybe even mirroring humans are trying to figure out why the ratio of “substance” to “invisible matter” in their universe is 5:1.

Hope for the neutron experiment

This is a bold guess, but finding hard evidence is not easy. The difficulty lies in the fact that the mirror world and our world, apart from gravity, do not interact with each other. The electromagnetic force, the strong and weak nuclear forces are not there, and the gravity is too weak to conduct experiments.

This is back to the previous topic, the answer may be to make better measurements of neutron lifetime. In 2012, a paper claimed that because of the gravity of the Earth, the Earth’s rotation will drag a small amount of mirror material; the movement of mirror-loaded particles (such as mirror electrons) will produce a mirror image; neutrons in a common magnetic field (such as “magnetic” The neutrons in the trap are affected by the mirrored magnetic field, and the probability of switching to the mirror world increases.

This idea has aroused the interest of experimenters. They used a more sensitive instrument to test whether the mirrored magnetic field would affect the neutron lifetime in a “magnetic trap.” The experiment also included applying magnetic fields of different intensities to the “magnetic traps” to see if they affect the lifetime of the neutrons. The experiment is currently complete and the data is being analyzed.

Another experiment is being prepared intensively at the Oak Ridge National Laboratory in the United States. The idea behind it is quite simple: launch a bunch of neutrons on a wall, the walls are thick enough, and the neutrons are absorbed by the special material on the wall and cannot penetrate. Therefore, in general, the neutron detector on the other side of the wall is unable to detect neutrons. But if the neutron is “switching” to the mirror world during the march, it becomes a mirror neutron – for mirror neutrons, the wall does not exist. Then, after crossing the wall, it “switches” back to our world and becomes a normal neutron, so the neutron detector on the other side of the wall may detect neutrons.

In the unlikely event that these experiments did not find clues to the existence of the mirror world? It is estimated that theorists will not give up this conjecture easily. After all, the “mirror” is too important, they have put a lot of bets on it.

In addition to dark matter, what other bets? Please see the extended reading.


Particle lifetime

In the microscopic world, the lifetime of particles is also uncertain due to quantum uncertainty. The particle lifetime we are talking about is only the result of statistics (that is, not measuring a single neutron, but measuring a group of neutrons). Physically, the lifetime of a particle is equated to its half-life. The so-called half-life is the time required for a group of identical particles to decay to only half of them.

For example, to measure the lifetime of a neutron, first collect n neutrons. If t time is left, only n/2 neutrons are left. We say that the lifetime (or half-life) of the neutron is t. It is not difficult to see that the faster the neutron is reduced, the shorter the measured life.

Expand reading

What physics problems can the mirror solve?

1. Why is the universe “having” rather than “nothing”?

At the beginning of the birth of the universe, there should be as much material and antimatter as it should be. However, if the same amount of positive and negative matter annihilate each other, the universe will return to nothing, and there will be no us today. It can be seen that in our universe, matter is more than antimatter.

Why is this happening? We have mentioned in the text a kind of K meson that destroys the parity conservation. Some scientists believe that in the early universe, the oscillation of K meson between our universe and the mirror universe may be the key to solving this problem.

K-mesons and its anti-particles – anti-K mesons – are as much as they are, but since K-mesons play back and forth between our universe and the mirror universe, “the game is switched,” the anti-K meson is also true, so in our In the universe, the two are not the same at all times. For example, in our universe, there were originally 100 K mesons and 100 anti-K mesons. At some point, there are 2 K mesons and 4 anti-K mesons “switched” to the mirror world, so in our universe, there are only 98 K mesons and 96 anti-K mesons. K meson is two more than the inverse K meson. This phenomenon of more and more is called fluctuations.

If the conditions remain unchanged, the number of positive and negative K mesons varies, which is easy to eliminate. For example, maybe the next moment, the positive and negative K mesons who ran to the mirror universe all ran back, so in our universe, the number of positive and negative K mesons was flat.

However, this condition has not been maintained for a long time. As we all know, our universe has been inflated and the temperature has dropped sharply, which has caused the temperature difference between our universe and the mirror universe to shrink. In the past, we used to compare the two to hot water and cold water. The narrowing of the temperature difference means that there is less “convection” between the two universes. In this way, the positive and negative K mesons that run into the mirror universe are not easy to run back, resulting in more substances than antimatter in our universe.

2. Why is lithium-7 so much?

Physicists have long noticed that the content of this lithium isotope in the real world does not match the theoretically predicted content of the universe in the first few minutes. why? According to a French physicist, mirror neutrons entering our world can make 铍-7 unstable, while 铍-7 decay produces lithium-7, resulting in a higher content of lithium-7 than theoretically expected.

3. Where did the ultra-high energy cosmic rays come from?

Our telescopes are detecting particles from outside the Milky Way – although they are high-energy particles at the start, after a long journey, the energy of these particles should be too small to be detectable.

However, if these particles are “switching” back and forth between the mirror world and our world during the flight, they can hold a large portion of the energy because the motion in the mirror world consumes almost no energy. So, when we detect them, the energy is still high.