Why we probably live in a quantum multiverseNEWS | 22 October 2026Quantum mechanics, which recently turned 100 years old, ranks as the most successful theory in the history of physics. Yet on a fundamental level, we still don’t understand what quantum mechanics is saying about how the world works.
Any standard textbook will tell you that the dynamics of quantum mechanics are based on two axioms—mathematical rules that allow us to theoretically compute the outcome of an experiment. Surprisingly, these two axioms are self-contradictory. They present one of nature’s deepest mysteries: the quantum measurement problem—the fact that the quantum rules describing an object to be measured don’t seem to apply to the object doing the measuring. This disturbing contradiction suggests that different physical laws apply to the observer and the observed.
One proposal to resolve the paradox is the Many Worlds interpretation of quantum mechanics. Many Worlds demands that our universe actually be a multiverse. The idea is often considered fringe, even silly. Lately, however, experimental progress has been changing the situation. Scientists have been able to measure quantum effects in larger and more complex systems and over great distances, blurring the line once thought to separate the scale of observer and observed. These signs that quantum rules might cross over into the macroscopic realm are making many physicists reconsider Many Worlds. But what does this notion really mean for the universe we call home?
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The first of the two contradictory axioms says quantum systems are described by a wave function called Ψ (or psi, pronounced “sigh”), a mathematical quantity that encodes all the information about a quantum system, such as the position, momentum and spin of all its particles. The wave function evolves over time in a way that’s dictated by Schrödinger’s equation, named for Austrian physicist Erwin Schrödinger. Like Isaac Newton’s law in classical mechanics or Albert Einstein’s field equations in general relativity, Schrödinger’s equation, which applies to only isolated systems, is the fundamental formula from which every calculation in quantum mechanics begins. In contrast to Newton’s and Einstein’s laws, however, this equation has a very peculiar property: it is a wave equation, and its solutions must obey the superposition principle.
Superposition is an uncanny aspect of quantum mechanics that allows particles to be in multiple states at once—such as literally here and there simultaneously. This property means that if Ψ1 and Ψ2 are both solutions of Schrödinger’s equation—they are two possible states of the quantum system—then the sum Ψ1 + Ψ2 is also a solution. A key aspect is that if you start with a state in a superposition, it remains in a superposition for all time: superpositions cannot be destroyed.
To appreciate the consequences of superposition, we must understand how waves are added and subtracted. Imagine you are standing in front of a pond with a still surface. Throw a stone into the pond, say, at position P1, and observe the resulting wave pattern. Wait until the surface is still again, then throw in a second stone at another position, say, P2, to watch the resulting wave pattern. Now imagine that you throw the stones simultaneously into the pond at positions P1 and P2. What will you see?
Matthew Twombly
If you watch carefully, you will find that the result is just the sum of the two patterns that each single stone created. If a wave crest from the first stone meets a wave crest from the second, they will combine to form a bigger wave crest, but if a crest and a trough meet, they will cancel each other out. These processes are called constructive and destructive interference, respectively. In this way, multiple solutions to a wave equation can be added together, and the result will still be a solution of the same equation. And it doesn’t stop with two solutions—if you want the wave pattern of three stones, you just combine the three patterns, and so on.
In most cases, wave equations describe an approximation of reality. Light that’s sufficiently bright and traveling through matter doesn’t obey the superposition principle. Close to shore, water waves break and violate the superposition principle. But Schrödinger’s equation obeys the superposition principle without exception.
The second axiom of quantum mechanics is called the measurement postulate. Imagine we want to measure the position of an atom. To make things simple, let’s assume that the atom can exist only at two different positions, P1 and P2, and that the wave functions corresponding to these positions are Ψ1 and Ψ2, respectively. If the atom is described by Ψ1, measuring the atom will reveal position P1, and if it is described by Ψ2, our measurement will reveal P2. So far, so good. But if the atom is described by the superposition Ψ1 + Ψ2, the measurement will randomly select one of the two possible positions as if the other part of the wave function did not exist. It would be as if all the waves created by the stones thrown into the pond suddenly disappeared except for the wave pattern of one particular stone. This phenomenon is called the collapse of the wave function, and the resulting physical picture is known as the Copenhagen interpretation of quantum mechanics.
You might wonder what is so disturbing about this interpretation—after all, it sounds like a coin-flip experiment, in which a result of either heads or tails is unknown until you look at the coin. But the quantum situation is much worse.
Recall that Schrödinger’s equation describes the dynamics of atoms and other microscopic particles such as electrons or molecules. Clearly, any tool we use to measure the atom’s position is also made up of atoms. We should be allowed to apply Schrödinger’s equation not only to the atom we want to measure but also to the detector we use to measure it (and everything else in the laboratory).
A substantial part, if not all, of the randomness in the world is of quantum origin.
Here’s the trouble. We know that superpositions can’t be destroyed because Schrödinger’s equation obeys the superposition principle. If Ψ1 + Ψ2 describes the initial state of the atom, then after the measurement the entire lab must be in a superposition corresponding to Ψ1 + Ψ2, thus describing the atom at position P1 or P2 and the detector in a state of having measured P1 or P2, respectively. There can be no collapse according to Schrödinger’s equation.
So if the measurement axiom is right—and it has been well confirmed empirically—we are forced to accept the following conclusion: the basic physical law, Schrödinger’s equation, that we use for the system we measure cannot be applied to the system with which we measure. It’s as if there exists a shifty split between the observer and the observed, even though both are made out of the same components of matter.
In 1957 Hugh Everett III, then a Ph.D. student in physics at Princeton University, proposed a radical solution to the quantum measurement problem: drop the measurement axiom as superfluous. What would that mean?
For one, there would no longer be any bizarre divide between the measured system and the measuring system: detectors would be just ordinary physical objects abiding by the superposition principle, just as atoms do. Schrödinger’s equation would be universally valid, and the wave function would never collapse.
The consequences of this effect, however, are enormous. Going back to our earlier example, when we measure an atom in a superposition of two positions Ψ1 + Ψ2, the final state after we make the measurement will be a lab that is in a superposition of the atom being in two different positions plus a detector in a superposition of having measured either of the two positions plus an experimenter in a superposition of having seen either of the measurement outcomes of the detector. In other words, there would suddenly be two experimenters in the same lab, each of them an exact copy of the experimenter before the measurement except for the tiny difference that they now know the position of the atom: for one of them the atom is at P1, and for the other it is at P2. Yikes!
The situation gets even more bizarre. Theoretical arguments suggest that a substantial part, if not all, of the randomness in the world is of quantum origin. Why? Well, although superpositions can’t be destroyed, they can spread from one quantum system to another almost like a virus. We saw this idea already when we noted that the original superposition of the atom resulted in a final superposition of the combined atom-plus-detector-plus-experimenter system.
To see how this spread creates randomness throughout the world, consider an ordinary coin-flip experiment again. There is nothing truly random about whether a coin comes up heads or tails. If we could flip the coin in the exact same way every time using a carefully crafted mechanical device, it would always produce the same outcome, which could be predicted according to Newton’s law. But if you try it yourself, you’ll quickly notice that it’s very hard to control the flight of the coin, and you get unpredictable outcomes. One reason is that a human hand can’t flip the coin in exactly the same way every time: tiny variations in the nerve pulses traveling from your brain to your thumb cause variations in your flip strength, which in turn produce different outcomes. But if you could trace things back to the origin of the tiny nerve-pulse variations, you would get to even smaller scales at which quantum effects become relevant: a superposition of different electron states could cause these slightly different nerve pulses. Thus, different coin-flip outcomes could be the result of amplified quantum fluctuations at tiny scales.
Chiara Morra
But if Everett’s proposal is true, then each time you flip a coin, you might be creating two copies of yourself: one who sees heads and one who sees tails. The result is a giant cosmic-scale superposition Ψ1 + Ψ2 + Ψ3 + ... that grows like branches on a tree. The universe becomes a superposition of different universes, one for each branch of the wave function. Put differently, the universe is actually a multiverse.
For that reason, Everett’s proposal is known as the Many Worlds interpretation of quantum mechanics. It’s confusing, however, to call it an “interpretation.” The Many Worlds interpretation and the Copenhagen interpretation are not really two different interpretations but two different theories. The former predicts the existence of a multiverse as an inevitable consequence of the superposition principle of Schrödinger’s equation. The latter predicts a single universe by denying the universal validity of Schrödinger’s equation.
Which theory is right? Is Schrödinger’s equation universally valid, or does it apply only to microscopic systems such as atoms?
We don’t know the answer yet, and for a long time it seemed that we might not need to care too much. Many of the pioneering quantum experiments in the early days could be explained with a modified classical theory. Light and matter can be “quantized” if we think of them as composed of small particles (photons or electrons, for instance) that can also behave like waves—a concept known as wave-particle duality. An explicit solution of Schrödinger’s equation was seldom required. Many well-known breakthroughs related to quantum mechanics—Max Planck’s computation of black-body radiation, Einstein’s explanation of the photoelectric effect, Niels Bohr’s idea to explain the spectral lines of the hydrogen atom, and the existence of the electron spin revealed by the work of Otto Stern and Walther Gerlach—preceded the discovery of Schrödinger’s equation.
Later it became clear that all these different effects follow from Schrödinger’s equation. Experimental evidence for the equation, however, remained restricted to the subatomic domain for many decades. Some wondered whether Schrödinger’s equation might be an approximation that works extremely well at the microscopic scale but breaks down at the macroscopic scale, which would mean the measurement postulate of the Copenhagen interpretation is right.
But experimental progress over the past few decades shows that Schrödinger’s equation is not valid only for minuscule particles. The double-slit experiment, for example, is a classic setup where scientists send particles through two gaps in a screen and observe a resulting interference pattern, proving that the particles act as waves. In 1999 a group led by Austrian physicist Anton Zeilinger confirmed that fullerenes, large molecules consisting of 60 carbon atoms, behave just the same. The current record, from 2019, proves molecules with 2,000 atoms show this same quantum property. Meanwhile, a different kind of experiment has demonstrated that clouds of thousands of trapped atoms can be cooled down to low-enough temperatures that exotic quantum mechanical phases of matter become visible, such as a state called a Bose-Einstein condensate. The wave nature of quantum mechanics as predicted by Schrödinger’s equation has even been shown at centimeter scales in superconducting devices.
Although the multiverse sounds like science fiction, it is a real scientific prediction.
And in a very recent experiment, quantum optical signals in a superposition of different states of light were communicated between South Africa and China over a distance of more than 12,900 kilometers. Because light obeys a superposition principle even under classical physics, this experimental breakthrough doesn’t imply that Schrödinger’s equation necessarily applies to objects 12,900 kilometers wide. Nevertheless, taking all these results together, researchers who still believe the wave function must collapse at macroscopic scales appear to be walking on thin ice. Schrödinger’s equation might be here to stay.
If we assume that Schrödinger’s equation applies universally, have we then solved the quantum measurement problem? Unfortunately not, because another issue is lurking around the corner. If everything is described by the wave equation, why do so many things behave like classical particles? Why can’t we interact with other branches of the multiverse? If something as simple as a coin flip can create a macroscopic superposition, why don’t we see any interference between heads and tails?
The answer is somewhat counterintuitive at first: we actually do see interference effects all the time but only from destructive interference. This is what physicists call decoherence—the inability of many waves to show strong constructive interference.
Note that decoherence is not a new, fundamental law that we add to Schrödinger’s equation to obtain classical behavior. It is a consequence of Schrödinger’s equation that arises naturally once there are many interfering waves. In fact, decoherence is a universal phenomenon that we’re already familiar with. Think of rain falling on a pond. Even though it is theoretically possible for all the droplets to generate a giant wave crest through constructive interference, in reality we see small ripples everywhere.
One way to understand why decoherence happens is to compare the number of possibilities for constructive and destructive interference. Suppose you have four identical waves meeting at some point of the pond. There is one possibility that only wave crests will meet at this point, and the resulting wave will be four times bigger than the individual ones (constructive interference). There are four possible ways for three crests and one trough to meet, producing a wave that’s twice the size of the originals (we could call this medium interference). And there are six possible ways for two crests and two troughs to meet, causing the waves to completely cancel out and produce a flat surface (destructive interference). Repeating this argument for more than four waves reveals that there are many more possibilities for destructive than for constructive interference.
Matthew Twombly
Let’s look at the numbers. Water waves caused by, say, 50 stones thrown into a pond have a wavelength of a few centimeters and oscillate with a frequency of around 10 hertz (Hz). In comparison, a quantum mechanical object that is visible to the naked eye contains around 1023 particles, each of which has a wavelength less than one billionth of that of a water wave and a frequency a few billion times larger than a water wave’s.
Now recall how slow and coarse our human senses are: movies show us only 24 images per second (that is, movies have an image frequency of 24 Hz), and we perceive them as continuous motion. We barely notice that the 0.1-millimeter-wide pixels on our computer displays are discrete. If there are 1023 particles interfering with one another on such tiny and fast quantum scales, no human has a chance to see constructive quantum interference. In fact, numerical simulations of Schrödinger’s equation have recently shown that with each additional particle, decoherence becomes roughly twice as strong. This means experiments with only 50 atoms need detectors sensitive to relative deviations of about 0.0000000000001 percent to distinguish quantum from classical effects. That’s like trying to measure a 100-meter Olympic race on a femtosecond timescale.
To be fair, these estimates are true for most, but not all, quantum many-particle systems and only if we focus on properties that are easy to measure, such as the coarse and slow stuff humans can perceive. If we precisely tuned a quantum system, we would see that the quantum effects remained. But doing that requires an enormous amount of control and precision—like trying to juggle 50 balls with your eyes closed during a hurricane. This is why it’s so incredibly hard to build a working quantum computer.
In our example of the pond, simply comparing the number of possibilities assumes that each stone emits a wave independently. In this case, strong constructive interference could be generated by a precisely tuned pattern of stones. For instance, identical stones thrown in a perfect circle onto the pond would create strong constructive interference when all the waves met in the middle of the circle. But natural systems lack such a high degree of regularity. Real stones will have different masses and shapes, each generating a slightly different wave. Physicists call such systems nonintegrable or chaotic. In these systems, the absence of regularities makes it extremely unlikely that all waves will interfere constructively.
You might wonder how anything interesting can arise from quantum mechanics at all if destructive interference is so prevalent. The answer is that there are some cases where quantum waves interfere constructively and give rise to a pattern familiar from classical physics. Here the analogy with water waves isn’t quite right, but another analogy familiar from everyday life helps. For many years people believed that light was made out of particles flying in straight rays. In most cases, this description is a good approximation because the wave nature of light is revealed only when it interacts with objects whose size is comparable to the wavelength of light (around 500 nanometers).
Under typical circumstances, therefore, light seems to follow a straight line. This effect arises from constructive interference in the direction of the ray and destructive interference outside it, where the light is “dark.” You can picture the quantum multiverse as a collection of light rays flying through a vast, dark space. Each ray describes a distinct macroscopic world. The vast darkness between rays doesn’t correspond to the absence of waves—it arises from destructive interference among many waves.
Although the multiverse sounds like science fiction, it is a real scientific prediction if we assume that Schrödinger’s equation, the fundamental law governing our microscopic world, also applies to the macroscopic world. Right now we can’t say for sure, and we have no perfect test of the question. It’s not clear whether this assumption is true, nor is it clear how to test it experimentally. But there are indirect consequences of the Many Worlds hypothesis that could lend the idea credibility.
For instance, the cosmic microwave background astronomers see throughout space is currently well explained by the idea of inflation, which posits that the early universe was dominated by a quantum field called the inflaton that rapidly stretched space. If physicists can assume that quantum mechanics was applicable to the entire cosmos at early times, shouldn’t it be applicable to the full universe today as well? Or are there other mechanisms coming into play?
Another clue is the fact that the universe appears highly fine-tuned: a slight variation in its fundamental parameters, such as the precise charge of an electron, the mass of a proton or the strength of the gravitational constant, would make life as we know it impossible. How can we explain the fact that these values seem almost chosen for their convenience for life? A popular answer is known as the anthropic principle: the fact that we are alive means we must find ourselves in a version of the universe compatible with life. We have essentially selected this universe from the array of all possible universes, which may be mostly hostile to life. But what does “select” mean? From which “selection of universes” are we choosing? Here the answer strongly depends on whether there is only one unique universe or a vast collection of them.
Many practical questions ultimately depend on which scales Schrödinger’s equation is valid at. Quantum computers’ power, for instance, relies on their ability to put large quantum systems composed of many particles into well-controlled superpositions. If that is impossible for systems much larger than atoms, many of humanity’s current technological efforts would be wasted.
The question also has consequences for the quest to unify quantum mechanics with general relativity. Finding the right unifying theory strongly depends on which “interpretation” of quantum mechanics is right. Do we need to quantize gravity, or can it be described classically at macroscopic scales?
The Many Worlds interpretation is gaining momentum as a candidate to solve the quantum measurement problem. Despite its seemingly weird prediction that our world is just one part of a multiverse, because of destructive and constructive interference, it is perfectly compatible with our sensation of living in one classical universe. Moreover, it opens possibilities for addressing fundamentally and technologically relevant questions.
Still, many challenging open questions must be answered before the Many Worlds interpretation can be accepted as our best explanation for the measurement problem. One such puzzle concerns the origin of uncertainty, chance and probability, which are so central to quantum mechanics. What does it mean that an event appears with a certain probability if all possible events are always realized simultaneously? And how do the specific quantum probabilities that we observe in experiments arise in our branch of the multiverse?
Currently not even the strongest proponents of the Many Worlds interpretation agree on how to answer these questions. For now the nature of our universe—or universes—and the quantum rules that guide it are as undecided as the toss of a coin.Author: Clara Moskowitz. Philipp Strasberg. Source
