“The most incomprehensible thing about the universe is that it is comprehensible, ”Albert Einstein famously once said. These days, however, it is far from being a matter of consensus that the universe is comprehensible, or even that it is unique. Fundamental physics is facing a crisis, related to two popular concepts that are frequently invoked, summarized tellingly by the buzzwords “multiverse” and “uglyverse.”
Multiverse proponents advocate the idea that there may exist innumerable other universes, some of them with totally different physics and numbers of spatial dimensions; and that you, I and everything else may exist in countless copies. “The multiverse may be the most dangerous idea in physics” argues the South African cosmologist George Ellis.
Ever since the early days of science, finding an unlikely coincidence prompted an urge to explain, a motivation to search for the hidden reason behind it. One modern example: the laws of physics appear to be finely tuned to permit the existence of intelligent beings who can discover those laws—a coincidence that demands explanation.
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With the advent of the multiverse, this has changed: As unlikely as a coincidence may appear, in the zillions of universes that compose the multiverse, it will exist somewhere. And if the coincidence seems to favor the emergence of complex structures, life or consciousness, we shouldn’t even be surprised to find ourselves in a universe that allows us to exist in the first place. But this “anthropic reasoning” in turn implies that we can't predict anything anymore. There is no obvious guiding principle for the CERN physicists searching for new particles. And there is no fundamental law to be discovered behind the accidental properties of the universe.
Quite different but not less dangerous is the other challenge—the “uglyverse”: According to theoretical physicist Sabine Hossenfelder, modern physics has been led astray by its bias for “beauty,” giving rise to mathematically elegant, speculative fantasies without any contact to experiment. Physics has been “lost in math,” she argues. But then, what physicists call “beauty” are structures and symmetries. If we can’t rely on such concepts anymore, the difference between comprehension and a mere fit to experimental data will be blurred. 
Both challenges have some justification. “Why should the laws of nature care what I find beautiful?” Hossenfelder righteously asks, and the answer is: They shouldn’t. Of course, nature could be complicated, messy and incomprehensible—if it were classical. But nature isn’t. Nature is quantum mechanical. And while classical physics is the science of our daily life where objects are separable, individual things, quantum mechanics is different. The condition of your car for example is not related to the color of your wife’s dress. In quantum mechanics though, things that were in causal contact once remain correlated, described by Einstein as “spooky action at a distance.” Such correlations constitute structure, and structure is beauty. 
In contrast, the multiverse appears difficult to deny. Quantum mechanics in particular seems to be enamored with it. Firing individual electrons at a screen with two slits results in an interference pattern on a detector behind the screen. In each case, it appears that the electron went through both slits each time. 
Quantum physics is the science behind nuclear explosions, smart phones and particle collisions—and it is infamous for its weirdness such as Schrödinger’s cat existing in a limbo of being half dead and half alive. In quantum mechanics, different realities (such as “particle here” and “particle there” or “cat alive” and “cat dead”) can be superimposed such as waves on the surface of a lake. The particle can be in a “half here and half there” state. This is called a “superposition,” and for particles or waves it gives rise to interference patterns.
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Originally devised to describe the microscopic world, quantum mechanics in recent years has been shown to govern increasingly large objects—if they are sufficiently isolated from their environment. Somehow, however, our daily life seems to be protected from experiencing too much quantum weirdness.: Nobody has ever seen an undead cat, and whenever you measure the position of a particle you get a definite result.
A straightforward interpretation assumes that all possible options are realized, albeit in different, parallel realities or “Everett branches”—named after Hugh Everett, who first advocated this view known as the “many worlds Interpretation” of quantum mechanics. Everett’s “many worlds” are in fact one example of a Multiverse—one out of four, if you follow Max Tegmark's Scientific American feature from May 2003. Two of the others are not that interesting, since one is not really a multiverse but rather different regions in our own universe, and the other one is based on the highly speculative idea that matter is nothing but math. The remaining multiverse is the “string theory landscape” to which we will return later.
By appealing to quantum mechanics in order to justify the beauty of physics, it seems that we sacrificed the uniqueness of the universe. But this conclusion results from a superficial consideration. What is typically overlooked in this picture is that Everett’s multiverse is not fundamental. It is only apparent or “emergent,” as philosopher David Wallace at the University of Southern California insists. 
To appreciate this point one needs to understand the principle behind both quantum measurements and “spooky action at a distance.” Instrumental for both phenomena is a concept known as “entanglement,” pointed out in 1935 by Einstein, Boris Podolsky and Nathaniel Rosen: In quantum mechanics, a system of two entangled spins adding up to zero can be composed of a superposition of pairs of spins with opposite directions while it is absolutely undetermined in which direction the individual spin points. Entanglement is nature‘s way of integrating parts into a whole; individual properties of constituents cease to exist for the benefit of a strongly correlated total system. 
Whenever a quantum system is measured or coupled to its environment, entanglement plays a crucial role: Quantum system, observer and the rest of the universe become interwoven with each other. From the perspective of the local observer, information is dispersed into the unknown environment and a process called “decoherence”—first discovered by H. Dieter Zeh in 1970—sets in. Decoherence is the agent of classicality: It describes the loss of quantum properties when a quantum system interacts with its surroundings. Decoherence acts if it would open a zipper between quantum physics’ parallel realities. From the observer’s perspective, the universe and she herself seem to “split” into separated Everett branches. The observer observes a live cat or a dead cat but nothing in between. The world looks classical for her, while from a global perspective it is still quantum mechanical. In fact, in this view the entire universe is a quantum object.
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This is where “quantum monism,” as championed by Rutgers University philosopher Jonathan Schaffer, enters the stage. Schaffer has mused over the question what the universe is made of. According to quantum monism, the fundamental layer of reality is not made of particles or strings but the universe itself—understood not as the sum of things making it up but rather as a single, entangled quantum state.
Similar thoughts have been expressed earlier, for example by the physicist and philosopher Carl Friedrich von Weizsäcker: Taking quantum mechanics seriously predicts a unique, single quantum reality underlying the multiverse. The homogeneity and the tiny temperature fluctuations of the cosmic microwave background, which indicate that our observable universe can be traced back to a single quantum state, usually identified with the quantum field that fuels primordial inflation, support this view.  
Moreover, this conclusion extends to other multiverse concepts such as different laws of physics in the various valleys of the “string theory landscape” or other “baby universes” popping up in eternal cosmological inflation. Since entanglement is universal, it doesn’t stop at the boundary of our cosmic patch. Whatever multiverse you have, when you adopt quantum monism they are all part of an integrated whole: There always is a more fundamental layer of reality underlying the many universes within the multiverse, and that layer is unique.
Both quantum monism and Everett’s many worlds are predictions of quantum mechanics taken seriously. What distinguishes these views is only the perspective: What looks like "many worlds" from the perspective of a local observer is indeed a single, unique universe from a global perspective (such as that of someone who would be able to look from outside onto the entire universe).
In other words: many worlds is how quantum monism looks like for an observer who has only limited information about the universe. In fact, Everett’s original motivation was to develop a quantum description of the entire universe in terms of a "universal wave function.” It is as if you look out through a muntin window: Nature looks divided into separate pieces but this is an artifact of your perspective.
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Both monism and many worlds can be avoided, but only when one either changes the formalism of quantum mechanics—typically in ways that are in conflict with Einstein’s theory of special relativity—or if one understands quantum mechanics not as a theory about nature but as a theory about knowledge: a humanities concept rather than science.
As it stands, quantum monism should be considered as a key concept in modern physics: It explains why “beauty,” understood as structure, correlation and symmetry among apparently independent realms of nature, isn’t an "ill-conceived aesthetic ideal” but a consequence of nature descending from a single quantum state. In addition, quantum monism also removes the thorn of the multiverse as it predicts correlations realized not only in a specific baby universe but in any single branch of the multiverse—such as the opposite directions of entangled spins in the Einstein-Podolsky-Rosen state.
Finally, quantum monism soothes the crisis in experimental fundamental physics relying on increasingly large colliders to study smaller and smaller constituents of nature, simply since the smallest constituents are not the fundamental layer of reality. Studying the foundations of quantum mechanics, new realms in quantum field theory or the largest structures in cosmology may turn out to be equally useful.
This doesn’t mean that every observed coincidence points to the foundations of physics or that any notion of beauty should be realized in nature—but it tells us we shouldn’t stop seeking. As such, quantum monism has the potential to save the soul of science: the conviction that there is a unique, comprehensible and fundamental reality.