91av

AT FIRST glance, everything looks familiar. The clock ticks placidly on the wall, cars motor along outside your window, the story you are reading has the same eye-catching pictures. But something is wrong. The clocks are running backwards. Cars are driving on the wrong side of the road. The article you are reading is written back to front. Suddenly, it clicks. You are looking at your own reflection.

The uncanny world on the other side of the mirror may not seem real to you. But Leah Broussard thinks parallel universes where everything is flipped might be very real indeed. Along with her colleagues at Oak Ridge National Laboratory in Tennessee, she is on the hunt for a universe that is identical to our own, but flipped so that it contains mirror atoms, mirror molecules, mirror stars and planets, and even mirror life. If it exists, it would form a bubble of reality nestling within the fabric of space and time alongside our own familiar universe, with some particles capable of switching between the two.

After decades of tantalising hints about its existence, the first experiments aiming to go through the looking glass are about to get under way. Finding such a mirror universe would not only transform our view of reality, but could also answer questions about our own universe that have puzzled scientists for decades. “The implications would be astounding,” says Broussard.

Physicists have found new worlds before. In 1928, Paul Dirac realised that the equations of quantum mechanics allowed for the existence of particles with properties beyond those anyone had seen before. He predicted that a whole new family of them was lurking in the universe, made up of particles identical to those we knew but with opposite electrical charges. This hidden world of antimatter doubles the number of fundamental particles known in the universe.

That’s not all. In 1933, Swiss astronomer Fritz Zwicky observed that the rotations of galaxy clusters suggested that they were experiencing a stronger gravitational pull than could be coming from nearby visible matter.

In the 1970s, US astronomer Vera Rubin observed this same effect across a range of galaxies and clusters. Today we think the “dark” matter causing this extra pull outnumbers regular matter 5:1. But we have never found this missing stuff, despite decades of dedicated direct and indirect searches.

Antimatter and dark matter have entered the scientific mainstream. But perhaps the most ambitious new world has spent 60 years in the shadows. In 1956, Chinese physicists Tsung Dao Lee and Chen Ning Yang made a remarkable prediction about the way physics works. Until then, it had been assumed that all physical processes must obey certain fundamental symmetries, meaning they remain the same when other things around them change. The way a ball responds to Earth’s gravity, for example, is unaffected by its colour.

A key symmetry in particle physics was parity, which mandated that everything should stay the same even if all positions and orientations were flipped as though in a mirror. Lee and Yang proposed an experimental test for parity violations. When Chinese-American physicist Chien-Shiung Wu built and ran the experiment, she found that parity could occasionally be violated. This was so significant a discovery that Lee and Yang (though not Wu) were awarded the Nobel prize in physics the very next year.

Lee and Yang also came up with a rather off-the-wall explanation. They suggested that parity was in fact conserved, and only appeared to be violated because we were looking at half the picture. “They suggested parity is broken in our universe only because there is another sector where parity is broken in the opposite direction,” says Zurab Berezhiani at the University of L’Aquila in Italy. “So it’s retained overall.”

This concept of a “mirror matter” world didn’t find favour at the time, but faced with a number of intractable problems in fundamental particle physics, researchers such as Broussard and Berezhiani have begun to embrace it again. In fact, says Berezhiani, we may already have seen signs of its existence.

Most clearly, they believe, its fingerprints can be glimpsed in the behaviour of the neutron, one of the three particles atoms break down into. Over time, neutrons outside an atomic nucleus decay into the other two – electrons and protons – in the process of beta decay. For decades, we have been trying to work out exactly how long these so-called free neutrons live before they decay, and we have been getting strangely conflicting results.

There are broadly two ways of measuring the lifetime of a free neutron – by bottle and by beam. The bottle experiment is fairly straightforward. You use a weak magnetic field to herd neutrons into what is called a bottle trap. Then you wait a certain amount of time before counting how many neutrons are left. According to this method, the neutron lives for an average of 14 minutes and 39 seconds.

The beam experiment, by contrast, counts the number of protons that emerge from a beam of neutrons channelled out of a nuclear reactor. Each proton can only appear as a result of a decaying neutron. Using calculations based on the beam intensity, this method sets the neutron lifetime at 14 minutes and 48 seconds. And there is the problem. “These two measurements should be the same,” says Berezhiani.

“We may already have seen signs of its existence in the behaviour of the neutron”

At first, physicists thought these extra 9 seconds could be put down to experimental error. But as we have improved our technical abilities and narrowed down the errors in the measurements, our certainty about both results has only grown. There are, it seems, two different neutron lifetimes.

The mirror world could be the culprit, if it exists. A key feature of these models, says Berezhiani, is that neutrons oscillate back and forth between the two worlds. “When passing through a magnetic field, the oscillation probability increases,” says Berezhiani. The jaw-dropping suggestion is that neutrons are only a part-time resident of our universe. The rest of their time is spent in a parallel plane of reality, where any protons they emit would go undetected.

If one in 100 neutrons swapped into the mirror world before emitting a proton, that would explain the longer measured neutron lifetime in the magnetic fields of beam experiments. “It’s a very natural explanation,” says Berezhiani.

Black mirrors

And that isn’t all the mirror sector can do. “Many other puzzles can be naturally explained using the same model with the same parameters,” says Wanpeng Tan at the University of Notre Dame in Indiana (see “They do it with mirrors“). The alternative universe could even provide a hiding place for dark matter and explain why it is so difficult to find. “The mirror neutron seems like a good dark matter candidate,” says Rabindra Mohapatra, a theorist at the University of Maryland. “It’s very compelling.”

It is even more compelling when you learn about the amount of mirror matter that should exist. In order to be consistent with our models of early universe evolution, the mirror sector must have been much cooler than our own. Too much heat, and some mirror matter would have leaked across the divide, increasing the gravitational attraction between particles in our own universe and taking it down a different historical path. That temperature difference, in turn, would have made it much easier for particles to cross over into the mirror universe, oscillating out of our own world for good. The best-developed mirror models suggest five mirror particles for every regular particle: exactly the prescription given by our cosmological measurements of the ratio of dark to “normal” matter.

What’s more, since the particles left behind went on to form stars, planets and, eventually, people, it seems reasonable to expect that there is also a mirror version of life – and significantly more of it than we can see. “In the mirror universe, it would happen five times more frequently,” says Berezhiani. Who knows, there might even be a race of mirror humans trying to work out why their dark matter is five times less abundant than their normal matter.

Great theories, but finding the clinching proof is far from easy. A mirror sector embedded in our own universe will have zero interplay with three of the four fundamental forces of nature: the electromagnetic, strong and weak force. “It won’t interact with us except by gravity, and gravity is too weak to experiment with,” says Yuri Kamyshkov, who researches mirror matter at the University of Tennessee in Knoxville.

The answer might lie in better neutron lifetime experiments. In 2012, Berezhiani published a paper claiming that previous experiments that held a bottle of neutrons in a varying magnetic field had spotted a signal consistent with mirror neutrons. His suggestion is that a small amount of mirror matter is dragged through our world by the rotation of Earth. The motion of mirror particles that carry charges – mirror electrons, say – would create mirror magnetic fields, and these could increase the chances of neutrons oscillating out of our universe in certain ordinary magnetic fields.

That idea intrigued Klaus Kirch and his colleagues at the Paul Scherrer Institute in Villigen, Switzerland. They used a more sensitive apparatus with the potential to test the possibility that mirror magnetic fields affect the neutron lifetime in a bottle trap as suggested by the claimed signal.

Kirch thought the claim far-fetched, but interesting enough to investigate. “The experimentalist’s view is, if it doesn’t look completely crazy, can it be tested?” he says. “I don’t really believe the signals are there, and we have designed an experiment that can disprove them, and we’ll see what comes out of it.” The exercise involved applying magnetic fields of varying strength to the apparatus to see whether they affect the abundance of neutrons in the trap. It is now complete, says Kirch, but the team is still analysing the data.

Broussard is watching with interest. Along with her colleagues at Oak Ridge, she is getting ready to test Berezhiani’s predictions about the magnetic fields that cause neutron oscillations in a purpose-designed experiment that should give more detail and control than the apparatus in Switzerland.

“It seems reasonable to expect that there is also a mirror version of life”

The idea behind it is fairly simple: fire a beam of neutrons at a thick wall that they can’t penetrate. If a neutron detector behind the wall detects any neutrons, it could be because they have oscillated into mirror neutrons en route, failed to see the wall because it exists in a different sector of the universe, and then oscillated back before hitting the detector. “Only the ones that can oscillate and then come back into our universe can be detected,” says Broussard.

By varying the magnetic fields on both sides of the wall, Broussard wants to see if she can find a field strength and shape that increases the number of neutrons passing through the wall. “If my numbers are right, they should see something,” says Berezhiani.

Further reflections

The apparatus is built and ready to go. Broussard is currently negotiating with the neutron beam operators at Oak Ridge to find a time when they can install the experiment in the beam path and perform the tests. Although excited, she isn’t expecting a breakthrough on the first run – no one knows what magnetic fields might sufficiently enhance the likelihood of oscillations. “I fully expect to measure zero,” she says. Instead, for her, it is all about narrowing down the possible range in size of the effects.

But if Kirch’s team sees a signal in its data that could be consistent with the existence of mirror neutrons, Broussard and her team could search the corresponding magnetic field with an independent approach. If the neutron count changes with the presence or absence of the magnetic field, that would suggest the existence of a mirror universe.

Kamyshkov, who is collaborating with Broussard, thinks we are reaching an important milestone. “The probability of finding anything is low, but it’s a simple and inexpensive experiment,” he says. “When a positive result would usher in a revolution in physics, we have to try.”

Even if these experiments do find mirror neutrons, Broussard says a lot of work is still required to make them a fit for dark matter – and to populate the rest of the mirror sector. “I would say it’s a good first step, but I think there are still challenges to work out,” she says.

And if we don’t find mirror neutrons? One thing Broussard is sure of is that the mirror universe won’t die. “Theorists are very good at evading the traps that experimentalists leave for them,” she says. “You’ll always find someone who’s happy to keep the idea alive.” But with the number of problems physicists have failed to solve with their current theories, you can excuse them looking in the mirror.