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By now, you've probably seen some news articles announcing the discovery of a parallel universe where time runs backwards. I'm sorry to say we have discovered no such thing. But let's look at what the research behind that announcement was really all about, and how the story got so twisted.

The whole thing started a few years ago, when a radio telescope known as ANITA made two really weird detections. ANITA sits in a balloon over Antarctica, and its job is to detect high-energy particles called cosmic rays that rain down from space. It does that by picking up radio pulses that get emitted when cosmic rays interact with Antarctic ice or the atmosphere.

But between 2016 and 2018, ANITA detected something strange. It picked up two signals that looked like they'd been triggered by neutrinos, which are a type of cosmic ray, but weirdly, they were coming up from the ground, instead of down from space. Now, because neutrinos are so small and have no charge, they can travel straight through a lot of matter without interacting with it.

So the idea that they'd originally come from space and traveled all the way through the Earth wasn't inconceivable, but there was one problem. By working back from the detections, the scientists working on the project found that the neutrinos that produced those signals must have had a ton of energy. And according to the Standard Model, which is basically all the math behind our current understanding of particle physics, neutrinos with higher energies are more likely to interact with matter.

So the Earth should have absorbed these neutrinos before they reached the surface. If the detections were real, that could overturn the Standard Model and everything we thought we knew about the subatomic world. Because even though neutrino interactions are just one part of the Standard Model, it's kind of like a Jenga tower: Pull out one piece of math, or one piece of evidence, and the whole thing comes crashing down.

But the Standard Model has held up to a lot of tests, so the authors weren't so quick to suggest it was wrong. As another possibility, they suggested that maybe a bunch of neutrinos had come from a really strong, focused source, like a massive supernova. Because even though no single neutrino with that much energy is likely to pass through Earth, if you had enough of them, then the odds are a little better that at least a few might make it all the way through.

Still kind of a long shot, but if the idea was right, we wouldn't have to completely redo all of particle physics. But before the team even had a chance to investigate that possibility, another paper, published in 2018, suggested that something else could explain those weird detections, and it was also a long shot: a thing called the CPT-symmetric universe. CPT stands for “charge, parity, and time,” and CPT symmetry means that the laws of physics should hold if all the particles in the universe flipped their charges and spacetime were a mirror-image of itself.

But it's purely hypothetical. All it says is that, if all you consider is the math, it's hypothetically possible to have a universe that's full of antimatter where time moves backward. And the scientists suggesting that these cosmic rays might have come from a.

CPT-symmetric universe weren't saying that it was a sure thing or even a likely thing. They were just saying that the math checks out, and that if a CPT-symmetric universe exists, it would have particles that behave the opposite of particles we're familiar with. And they suggest that one of those particles could potentially, potentially, explain these upward-moving cosmic rays.

So, at this point, scientists had a weird observation and three possible explanations that all seemed unlikely:. Either our understanding of particle physics is fundamentally flawed, or this super-energetic supernova event sent some cosmic rays all the way through the Earth, or there's a parallel universe that's a mirror image of ours. Earlier this year, scientists ruled out the supernova explanation, because data from a second cosmic ray detector in Antarctica failed to find evidence backing up that scenario.

They suggested that maybe the observations had just been an error, or maybe they were a sign of some weird physics. But the explanation could also be something no one has thought of yet. Unfortunately, when reporters covered that paper, some of them highlighted the CPT explanation, and then other outlets went running with that angle.

In reality, there's still no answer to the mystery, but there's also no evidence for a parallel, mirror universe. Thanks for watching this episode of SciShow Space News! Even though so far there's no evidence of a parallel universe where time runs backwards, if you want to learn more about the strange nature of time, you could check out the book “The Order of Time” on Blinkist.

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