Did NASA really find a parallel universe where time runs backward? It's not impossible! But we're still a long way from proving it.
A flurry of stories recently hit my timeline claiming that NASA had discovered a parallel universe where time runs backward. I was immediately suspicious because none of those stories actually came from NASA. And as the famous astronomer Carl Sagan once said, “extraordinary claims require extraordinary evidence.” So what is the evidence?
What is a parallel universe?
First, let’s look at what we mean by a parallel universe. The term can take different meanings in science fiction stories versus scientific theory.
Theory tells us that in an infinitely big universe, there are only so many ways all the stuff inside it can be arranged before those patterns have to start repeating themselves. To put this in more relatable terms, a common analogy is to take a deck of cards and lay out all of the cards in the order you find them. Now shuffle that deck and lay the cards out again. Repeat this exercise enough times and you're destined to repeat the same order eventually.
A parallel universe would be a universe much like our own, with the same laws of physics and makeup, but with a different history or evolutionary path.
So a parallel universe would be a universe much like our own, with the same laws of physics and makeup, but with a different history or evolutionary path. The multiverse, this universe of universes, can also take other forms besides a series of parallel universes. Other realities could be folded away in different dimensions or even exist as disconnected bubble universes where the laws of physics are completely different.
These ideas tend to be highly abstract. Their existence comes out of patterns in the mathematical framework that describes our physical universe. Or they are invoked as physicists try to explain why two of our biggest physical theories, quantum mechanics and general relativity, don’t play well together.
But could we actually find a parallel universe?
Can we find a parallel universe?
The story of a parallel universe where time runs backward starts with an experiment in the Antarctic called ANITA, or the Antarctic Impulsive Transient Antenna. ANITA is a set of radio antennas onboard a helium balloon flying approximately 37,000 meters above Antarctica, or about four times as high as a commercial jet flies. These radio dishes are looking for evidence of neutrinos—tiny subatomic particles produced in high energy astronomical events like supernova explosions. Neutrinos are nearly massless, with no electric charge, and they don’t interact much with matter. That makes them almost impossible to detect. Almost.
And while having limited interactions with matter makes them hard to detect, it also means they can travel to us from the far reaches of space without losing energy. And there are so many of them that detections are possible even if only a small fraction end up interacting with our detectors. In fact, physicists behind the IceCube neutrino experiment estimate that 100 trillion neutrinos pass through your body every second!
ANITA specifically looks for radio waves that are emitted when neutrinos speeding in from space interact with Antarctic ice. The ice is radio transparent, so the radio waves produced by interactions with the high energy neutrinos can escape back up to the detector. There’s lots of ice, too. And the more ice, the more opportunities for the neutrinos to interact. There are also not a lot of other radio signals in Antarctica relative to more populated spots on the planet, so it is easier to pick out the very faint signals from these interactions.
The detectors picked up radio waves as expected after the neutrino interactions, but the angle of the incoming radiation suggested the neutrinos had come not from space but from the Earth.
Two years ago—that’s right, these results are not even new—ANITA detected what the researchers called “anomalous events,” meaning their origins couldn’t be explained. The detectors picked up radio waves as expected after the neutrino interactions, but the angle of the incoming radiation suggested the neutrinos had come not from space but from the Earth.
Could the neutrinos still have originated in space, just from the other side of the Earth? Neutrinos can pass through the Earth, but only if those neutrinos are relatively lower energy. The thing is, a neutrino with lower energy has lower chances of interacting with the matter that makes up the Earth. The odds that higher-energy neutrinos, like those capable of producing the radio emissions observed by ANITA, could travel unscathed through an entire rocky planet are very, very low.
So what’s going on here? Well, scientists don’t know yet, but a few explanations have been proposed. For starters, we know the odds of these high-energy neutrinos passing through the Earth are incredibly low, but if the odds are nonzero, and enough of these neutrinos are produced, some of them will make it through. That would be the most straightforward explanation.
But if the answer was that a few neutrinos passed through Earth despite the incredible odds against that happening, we would have expected another neutrino detector IceCube to have made similar detections. It did not.
A glitch in the detector would be an easier explanation. But if there's something internal to the detector causing the anomalies, the researchers have not been able to find it.
Another possible straightforward explanation—systematics. An inconsistency or glitch in the detector would be an easier explanation than having to invoke a parallel universe. But if there is something internal to the detector causing the unexpected neutrino angles, the researchers have not been able to find it.
Another theory focuses on the way the detector classifies events. The detector relies on something called a phase inversion to determine the direction of the incoming emission (and thus whether that emission comes from space or from the Earth). A very cool collaboration of particle physicists, glaciologists, and theoretical physicists has suggested that reflections of the radio signals beneath the surface of the ice could affect this phase inversion in the signal and thus make the signals appear as if they came from the opposite direction.
And of course, if these theories ultimately fail to explain the anomalous events, the remaining possibility is always an exclusion to the Standard Model, meaning an explanation outside of the ideas we have about how things work. Although it makes the most sense to test the more straightforward theories first, scientists, especially astronomers, don’t rule out an idea just because it's unlikely. (Just look at dark matter and dark energy!) But sometimes a less likely solution makes a more exciting headline, which means it ends up getting much more attention than the straightforward theories. Remember our interplanetary visitor Oumumua? Some astronomers noted that it could be an alien probe, which is true—and wouldn’t that be cool?—but unlikely.
Sometimes a less likely solution makes a more exciting headline.
In the case of the ANITA results, theorists have proposed that such upward-moving events could have been created by the decay of a certain theoretical dark matter particle that exists in a CPT symmetric universe, a hypothesized parallel universe made up of antimatter rather than matter, and that runs backward from the Big Bang.
If it sounds like I took a lot of theoretical steps to go from a few detections in Antarctica to a parallel universe of antimatter where time runs backward, I did. That doesn’t mean the solution is wrong, just that it’ll be a harder journey to prove or support with firmer evidence.
So where did the story go wrong?
We’ve discussed here before about how science stories can gain traction in the media even when they're not entirely accurate, like the numerous pieces from a few years ago on how science had supposedly discovered the best time to drink your coffee for maximal effect.
Who doesn’t wonder how life may have turned out if they had made that one choice differently?
In this case, the problem appears to be rooted in authors either not taking the time to trace results back to their original source or perhaps extrapolating their own ending to an original New Scientist article that's behind a paywall.
There is something appealing about an alternate, parallel universe. Maybe there, we can correct our failures or fix our mistakes. And who doesn’t wonder how life may have turned out if they had made that one choice differently? But at least for now, we’re stuck with this universe and righting our wrongs without the help of the cosmos.