Do parallel universes exist? What physicists say

What physicists mean by parallel universes

"Parallel universes" is not one scientific idea but a popular label stretched over several unrelated ones. Physicist Max Tegmark sorted them into four levels in a 2003 paper and a companion piece for Scientific American. Level I is simply more of the same universe, too far away to see: an infinite space implies distant regions with our own laws of physics but different histories, including, by Tegmark's estimate, an identical copy of every observer 10^(10^29) meters away. Level II regions, produced by inflation, can have different physical constants. Level III is the branching many-worlds of quantum mechanics, and Level IV covers universes built on entirely different mathematics.

Whether parallel universes exist is not something physics has settled. Three specific versions of the idea are taken seriously by working physicists today: quantum mechanics' many-worlds interpretation, cosmology's inflationary multiverse, and string theory's landscape of possible universes. Every one of them currently lacks direct observational evidence, and no experiment has ever detected another universe.

The popular claim

In popular use, a parallel universe usually means a fully formed alternate world, one where a different version of you made different choices, existing somewhere reachable in principle. That picture got a boost in May 2020, when headlines claimed NASA had detected a parallel universe where time runs backward. The real trigger was unusual behavior in neutrinos picked up by an experiment called ANITA, the Antarctic Impulsive Transient Antenna. Newsweek later reported that the claims were "not based on new scientific findings but are instead a distorted interpretation of older research," and that NASA was not behind the interpretation.

Quantum many-worlds: a universe that never collapses

The oldest of the three ideas comes from quantum mechanics. In 1957, physicist Hugh Everett proposed that when a quantum measurement has several possible outcomes, the universe does not pick one and discard the rest. According to the Stanford Encyclopedia of Philosophy's entry on the many-worlds interpretation, written by Lev Vaidman, every outcome happens, each in its own branch of reality that splits off at the moment of measurement. The appeal, the entry explains, is that this removes the need for an unexplained "collapse" of the wave function, a random event that standard quantum theory otherwise has to assume.

The interpretation also carries open problems. If every outcome happens somewhere, the encyclopedia notes, it is not obvious what "probability" even means, since nothing is actually more or less likely to occur than anything else. Supporters answer this with an added rule about how much a branch should count, which critics see as reintroducing what many-worlds was supposed to remove. Testing the idea directly would require detecting interference between separate branches, which the entry says is not achievable with any experiment available today. Physicists remain split on whether many-worlds is the most reasonable interpretation of quantum theory or an unnecessary multiplication of universes.

The inflationary multiverse: bubbles beyond bubbles

A second version comes from cosmology. Inflation theory holds that the newborn universe expanded, in NASA's description, "by more than a trillion trillion fold in less than a trillionth of a trillionth of a second," stretching tiny quantum fluctuations into the seeds of every galaxy. Physicist Brian Greene, quoted on NASA's overview of the Wilkinson Microwave Anisotropy Probe, or WMAP, said the resulting data "support the notion that galaxies are nothing but quantum mechanics writ large across the sky." The base theory is well tested: Physics World reported that three years of WMAP data confirmed the universe's geometry is flat "to within 2%," matching one of inflation's central predictions.

The multiverse comes from pushing that same theory further. In many models, inflation never stops everywhere at once; it keeps going in some regions forever while ending in isolated pockets, sometimes called bubble universes, each becoming a separate universe of its own, corresponding to Tegmark's Level II. Unlike the base theory, this "eternal" extension has not been confirmed by any measurement. It is this extra step, not inflation itself, that produces the popular idea of a multiverse.

The string landscape: too many possible universes

A third version comes from string theory, physicists' leading attempt to unify quantum mechanics and gravity. Rather than settling on one set of physical constants, the mathematics allows an enormous number of internally consistent variations, often called the string landscape. As Quanta Magazine has reported, many string theorists had hoped their equations would point to a single true universe; when that hope faded, some physicists, including Eva Silverstein, argued instead that a landscape of many different universes makes sense, because only universes with the right conditions would ever produce anyone able to notice them. Other physicists regard the whole framework as untestable rather than explanatory.

What is still debated

None of the three proposals has been observed, and one physicist has argued publicly that none of them can be. Cosmologist George Ellis wrote in Scientific American in 2011 that "I do not believe the existence of those other universes has been proved—or ever could be," because "all the parallel universes lie outside our horizon and remain beyond our capacity to see, now or ever." His sharper complaint was about flexibility. As he put it, "Because theories involving a multiverse can explain almost anything whatsoever, any observation can be accommodated by some multiverse variant," which he argued keeps it from working like an ordinary theory that could be shown wrong.

Physicists have still tried to find indirect evidence. If our universe is one bubble among many, a collision with a neighboring bubble early on might have left a faint circular mark in the cosmic microwave background. A team searched WMAP data for exactly that pattern in 2011; researcher Daniel Mortlock said the group took "great care to assess how likely it was that the possible bubble collision signatures we found could have arisen by chance." They confirmed no collision, only publishing the first upper limit on how many such marks could be hiding in the data.

Questions people ask

Verdict

Real or Legend has filed this one as disputed. Cosmic inflation itself is well tested, the many-worlds framework is internally consistent, and string theory is mathematically productive. But the specific claim that other universes actually exist, whether as quantum branches, inflating bubbles, or points in a string landscape, has no confirmed observation behind it, and the one direct search for evidence, for bubble collisions in the cosmic microwave background, came back empty. Serious physicists disagree about whether that will ever change. A page that tells you flatly that parallel universes are proven, or that they are nonsense, is going beyond what the evidence shows.