No, we haven’t finally found evidence for a parallel Universe

For some of us, the idea of parallel Universes sparks our wildest dreams. If there are other Universes where certain events had different outcomes — where just one crucial decision went a different way — perhaps there could be some way to access them. Perhaps particles, fields, or even people could be transported from one Universe to another, enabling us to live in a Universe that’s better, in some ways, than our own. These ideas have a foothold not only in science fiction, but in theoretical physics as well, from the infinity of possible outcomes from quantum mechanics to ideas related to the Multiverse.

But do these ideas have anything to do with observable, measurable reality? Recently, a claim has surfaced asserting that we’ve found evidence for parallel Universes from the ANtarctic Impulsive Transient Antenna: ANITA. It’s true: the experiment found evidence for cosmic ray particles that’s quite difficult to explain using only conventional physics. But to leap to the most fantastical, outlandish, revolutionary explanation is absolutely premature. In all fields of science, we have to take tremendous care not to fool ourselves. We must endeavor to knock down any new, wild hypotheses, and instead make sure that known laws of nature can’t conceivably explain what we’re seeing.

Science is about being appropriately skeptical, and when we take that approach, we see that the evidence for a parallel Universe all but evaporates.

An illustration of multiple, independent universes, causally disconnected from one another in an ever-expanding cosmic ocean, is one depiction of the multiverse idea. (Credit: Ozytive/Public domain)

From a physics point of view, parallel Universes are one of those intriguing ideas that captures our imaginations, and compels us to consider their existence, but at the same time, it’s an idea that’s very difficult to test. Parallel Universes first arose in the context of quantum physics, which is notorious for having unpredictable outcomes even if you know everything possible about how you set up your system. If you take a single electron and shoot it through a double slit, you can only know the probabilities of where it will land; you cannot predict exactly where it will show up.

One remarkable idea — known as the many-worlds interpretation of quantum mechanics — postulates that all the outcomes that can possibly occur actually do happen, but only one outcome can happen in each Universe. It takes an infinite number of parallel Universes to account for all the possibilities, but this interpretation is just as valid as any other. There are no experiments or observations that rule it out.

The Many Worlds Interpretation of quantum mechanics holds that there are an infinite number of parallel Universes that exist, holding all possible outcomes of a quantum mechanical system, and that making an observation simply chooses one path. This interpretation is philosophically interesting, but may add nothing-of-value when it comes to actual physics. (Credit: Christian Schirm/Wikimedia Commons)

A second place where parallel Universes arise in physics is from the idea of the Multiverse. Our observable Universe began 13.8 billion years ago with the hot Big Bang, but the Big Bang itself wasn’t the very beginning. A very different phase of the Universe occurred previously to set up and give rise to the Big Bang: cosmological inflation. When and where inflation ends, a Big Bang occurs.

But inflation doesn’t end everywhere at once, and the places where inflation doesn’t end continue to inflate, giving rise to more space and more potential Big Bangs. Once inflation begins, in fact, it’s virtually impossible to stop inflation from occurring in perpetuity at least somewhere. As time goes on, more Big Bangs — all disconnected from one another — occur, giving rise to an uncountably large number of independent Universes: a Multiverse.

While many independent Universes are predicted to be created in an inflating spacetime, inflation never ends everywhere at once, but rather only in distinct, independent areas separated by space that continues to inflate. This is where the scientific motivation for a Multiverse comes from, and why no two Universes will ever collide. There simply aren’t enough Universes created by inflation to hold every possible quantum outcome owing to the interactions of particles within an individual Universe. (Credit: MUSTAFABULENT / Adobe Stock)

The big problem for both of these ideas is that there’s no way to test or constrain the prediction of these parallel Universes. After all, if we’re stuck in our own Universe, how can we ever hope to access another one? We have our own laws of physics, but they come with a whole host of quantities that are always conserved.

Particles don’t simply appear, disappear, or transform; they can only interact with other quanta of matter and energy, and the outcomes of those interactions are similarly governed by the laws of physics.

In all the experiments we’ve ever performed, all the observations we’ve ever recorded, and all the measurements ever made, we’ve never yet discovered an interaction that demands the existence of something beyond our own, isolated Universe to explain.

The Standard Model of particle physics accounts for three of the four forces (excepting gravity), the full suite of discovered particles, and all of their interactions. Whether there are additional particles and/or interactions that are discoverable with colliders we can build on Earth is a debatable subject, but there are still many puzzles that remain unanswered, such as the observed absence of strong CP violation, with the Standard Model in its current form. (Credit: Contemporary Physics Education Project/CPEP, DOE/NSF/LBNL)

But according to the various reports regarding the ANITA experiment’s unexpected findings, you may have read that scientists in Antarctica have discovered evidence for the existence of parallel Universes. If this were true, it would be absolutely revolutionary. It’s a grandiose claim that would show us that the Universe as we currently conceive of it is inadequate, and there’s much more out there to learn about and discover than we ever thought possible.

Not only would these other Universes be out there, but matter and energy from them would have the capability to cross over and interact with matter and energy in our own Universe. Perhaps, if this claim were correct, some of our wildest science fiction dreams would be possible. Perhaps you could travel to a Universe:

• Where you chose the job overseas instead of the one that kept you in your country?
• Where you stood up to the bully instead of letting yourself be taken advantage of?
• Where you kissed the one-who-got-away at the end of the night, instead of letting them go?
• Or where the life-or-death event that you or your loved one faced at some point in the past had a different outcome?

A representation of the different parallel “worlds” that might exist in other pockets of the multiverse. (Credit: Pixabay/public domain)

So what was the remarkable evidence that demonstrates the existence of a parallel Universe? What observation or measurement was made that brought us to this remarkable and unexpected conclusion?

The ANITA (ANtarctic Impulsive Transient Antenna) experiment — a balloon-borne experiment that’s sensitive to radio waves — detected radio waves of a particular set of energies and directions coming from beneath the Antarctic ice.

This is good; it’s what the experiment was designed to do! In both theory and in practice, we have all sorts of cosmic particles traveling through space, including the ghostly neutrino. While many of the neutrinos that pass through us come from the Sun, stars, or the Big Bang, some of them come from colossally energetic astrophysical sources like pulsars, black holes, supernovae, or even mysterious, unidentified objects.

Researchers prepare to launch the Antarctic Impulsive Transient Antenna (ANITA) experiment, which picked up signals of impossible-seeming particles as it dangled from its balloon over Antarctica. (Credit: NASA)

These neutrinos also come in a variety of energies, with the most energetic ones (unsurprisingly) being the rarest and, to many physicists, the most interesting. Neutrinos are mostly invisible to normal matter — you’d have to pass a typical astrophysical neutrino through about a light-year’s worth of lead to have a 50/50 shot of stopping one — so they can realistically come from any direction.

However, most of the high-energy neutrinos that we see aren’t produced from far away, but are produced when other cosmic particles (also of extremely high energies) strike the upper atmosphere, producing cascades of particles that also result in neutrinos. Some of these neutrinos will pass through the Earth almost completely, only interacting with the final layers of Earth’s crust (or ice), where they can produce a signal that our detectors are sensitive to.

While cosmic ray showers are common from high-energy particles, it’s mostly the muons which make it down to Earth’s surface, where they are detectable with the right setup. Neutrinos are also produced, some of which can pass through the Earth, but this explanation for the ANITA signal is not as compelling as some of the more mundane possibilities. (Credit: Alberto Izquierdo; Francisco Barradas Solas)

The rare events that ANITA saw were consistent with a neutrino coming up through the Earth and producing radio waves, but at energies that should be so high that passing through the Earth uninhibited should not be possible. So now, we have to put our skeptical goggles on, and ask some important questions concerning how seriously we should take these observations.

1. How many events like this did they see? They saw a total of three such events. Just three.
2. Did they have to have come through the Earth? No. The first two could have been normal air-shower tau neutrinos (one of the three types of neutrino allowed) instead of neutrinos that passed through the entire Earth, while the third may have merely been part of the experimental background.
3. Do we have confirmation from IceCube, the other, larger neutrino detector in Antarctica? This would be the critical question, since a more sensitive experiment should have seen an even more compelling signal than ANITA did. As it turns out, IceCube’s existence stands as an extraordinary piece of evidence that disfavors the notion that these neutrinos came through the Earth. If high-energy tau neutrinos are regularly passing through the Earth (and the Antarctic ice), IceCube would have definitively seen a signal. And, quite unambiguously, they have not.

When a neutrino interacts in the clear Antarctic ice, it produces secondary particles that leave a trace of blue light as they travel through the IceCube detector. The lack of high-energy tau neutrinos seen by IceCube effectively rules out the astrophysical origin of whatever created the radio waves that ANITA saw. (Credit: Nicolle Rager Fuller/NSF/IceCube)

Scientifically, this means that:

• ANITA saw radio signals that it could not explain,
• their leading hypothesis was that high-energy tau neutrinos are traveling upwards through the Earth,
• and that hypothesis was refuted by IceCube observations,
• teaching us there is no astrophysical point source out there that is creating the particles that ANITA is indirectly seeing.

So where, in all of this, do the parallel Universes come in?

Because there were only three explanations for what ANITA saw.

• Either there was an astrophysical source for these particles,
• or there’s a flaw in their detector or their interpretation of the detector data,
• or something very exotic, remarkable, and beyond the Standard Model (known as CPT violation) is happening.

Some very good science ruled out the first option (back in January of 2020), which means it’s almost certainly the second option. The third? Well, if our Universe cannot violate CPT, maybe this comes from a parallel Universe where CPT is reversed: an explanation that’s as unlikely as it is poorly reasoned.

This diagram shows the different ways signals can reach ANITA as the device floats over Antarctica hanging from its balloon. Simply, a reflected cosmic ray could easily be confused with a cosmic neutrino that traveled through the Earth and decayed just before it could be detected by ANITA. (Credit: ANITA Collaboration/NASA)

In fact, back in April of 2020, physicist Ian Shoemaker came up with a spectacular but mundane explanation for what ANITA saw: ultra-high-energy cosmic rays could have simply reflected off of certain types of ice at or near the Antarctic surface, creating the illusion that these particles traveled through the Earth from the perspective of ANITA. Remember: in science, we must always rule out all the conventional explanations that don’t involve new physics before we resort to a game-breaking explanation. Over the past decade, a number of remarkable claims have been made that have disintegrated upon further investigation. Neutrinos don’t travel faster-than-light; we haven’t found dark matter or sterile neutrinos; cold fusion isn’t real; the impossible “reactionless engine” was a failure.

There’s a remarkable story here that’s all about good science. An experiment (ANITA) saw something unexpected, and published their results. A much better experiment (IceCube) followed it up, and ruled out their leading interpretation. It strongly suggested something was amiss with the first experiment, and completely mundane explanations involving no new physics at all could wholly account for the full suite of what we’ve seen. As always, conducting more science will help us uncover what’s truly occurring. For now, based on the scientific evidence we have, parallel Universes will have to remain a dream solely confined to the realm of speculation and science fiction.