By Michael Brooks
AT FIRST glance, everything looks familiar. The clock ticks placidly on the wall, cars motor along outside your window, the magazine in your hand has the same eye-catching cover. 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 antimatterdoubles 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.
Patryk Hardziej
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.
Read More: We've seen signs of a mirror-image universe that is touching our own
