A puzzling discovery made by an observatory at the end of the Earth has scientists scratching their heads as to how a series of high-energy particles managed to pass southward through the entire Earth to emerge through the ice in Antarctica, to be detected by an observatory there. As each possible conventional explanation either fails to explain the odd phenomenon or becomes too convoluted to be plausible, one theory comes to the forefront: the mystery particles might be from a mirror universe that overlaps our own.
Suspended from a helium-filled balloon floating at an altitude of more than 120 thousand feet, the Antarctic Impulsive Transient Antenna, or “ANITA” experiment is an odd observatory, in that it looks down toward the ground to make its discoveries. It has this odd orientation to look for the radio waves emitted by the Antarctic ice sheet that are generated when ultra-high-energy (UHE) cosmic neutrinos interact with the glacier’s ice. While the vast majority of the neutrino signatures detected come from the sky above the sensors, during ANITA’s first flight in 2006 it detected one coming from below the ice sheet, implying that one of these high-energy neutrinos had passed through the entirety of the Earth.
Ordinary neutrinos are electrically neutral particles—hence their name— that have next to no mass, causing them to only have extremely weak interactions with normal matter, a property that would allow the vast majority of them to pass through the entire mass of the Earth unimpeded. But ultra-high-energy neutrinos, a form of cosmic ray theorized to be generated by the intense magnetic fields of neutron stars, should be stopped by any matter that gets in their way, acting much “like bulls in a china shop—they become much more likely to interact with particles in Earth,” as physicist Alex Pizzuto of the University of Wisconsin–Madison puts it.
This means that the particle detected by ANITA might have been a once-in-a-lifetime find, as a single UHE neutrino could conceivably manage to accidentally to pass through the Earth without being intercepted by one of the countless atoms that make up the planet.
Except that this inconceivably rare event happened twice.
ANITA detected a second UHE during a 2014 flight, calling into question the probability that the first detection was mere happenstance. Possible errors in both the equipment and data were eventually ruled out, and three separate studies of data collected by the IceCube Neutrino Observatory, a similar observatory buried under the ice at the Amundsen–Scott South Pole Station, failed to uncover a potential source for such high-energy particles; being closer to the Earth than the ANITA array, IceCube should have seen such high-energy particles before its airborne counterpart, but detected no such phenomena.
“Our analysis ruled out the only remaining Standard Model astrophysical explanation of the anomalous ANITA events,” according to Pizzuto. “So now, if these events are real and not just due to oddities in the detector, then they could be pointing to physics beyond the Standard Model.”
Other theories fitting the Standard Model that would account for this nigh-impossible phenomenon required a series of contrived events that amounted to jumping through proverbial particle physics hoops, such as the idea that a high-energy tau neutrino spontaneously converted into a tau lepton, traveled through the Earth, and re-converted into a tau neutrino on the other side. The only simple explanation left, as posited by Neil Turok, a physicist with Canada’s Perimeter Institute for Theoretical Physics, is that the mysterious particles came from a parallel universe where time travels in the opposite direction from ours.
Turok’s field of study, CPT symmetry (CPT standing for Charge, Parity, and Time reversal), posits that our universe has a mirror copy, one that was created at the time of the Big Bang but is made up of antimatter, and that the laws of physics there, despite being the same as ours, are reversed symmetrically from the ones we’re used to. While objects in that universe and the events involving them might seem bass ackward to someone in our dimension, ours would seem equally reversed for an inhabitant from this proposed mirror universe.
Part of Turok’s work involves trying to discover what happened to our universe’s “right-handed” neutrinos: all known neutrinos have a left-handed spin—hence their label—but much like the dearth of antimatter in our universe, right-handed neutrinos are conspicuously absent; these alt-neutrinos must exist somewhere, so they might reside in the anti-verse. And perhaps a series of these extra-dimensional right-handed UHE neutrinos slipped into our universe, and reverted to the direction of time our particles follow, to be detected by the ANITA array.
According to Turok, this other-worldly theory would explain the presence of these otherwise impossible UHE neutrinos, without the need to tie physics as we know it up into complicated knots. And perhaps some of us can visit such a parallel universe, like the one Whitley describes in his visionary 2020 book, A New World.