The counterintuitive predictions of quantum theory have now been experimentally confirmed with unprecedented rigor. Yet, the question of how to interpret the meaning of these predictions remains controversial. A Wikipedia table summarizing different interpretations of quantum mechanics included no less than fourteen entries at the time of this writing. New interpretations regularly appear.
The problem is that quantum theory contradicts our intuitive understanding of what “real” means. According to the theory, if two real particles A and B are prepared in a special way, what Alice sees when she observes particle A depends on how Bob concurrently observes particle B, even if the particles—as well as Alice and Bob—are separated by an arbitrary distance. This “spooky action at a distance,” as Einstein called it, contradicts either local causation or the very notion that particles A and B are “real,” in the sense of existing independently of observation. As it turns out, certain statistical properties of the observations, which have been experimentally confirmed, indicate the latter: that the particles do not exist independently of observation. And since observation ultimately consists of what is apprehended on the mental screen of perception, the implication may be that “the Universe is entirely mental,” as put by Richard Conn Henry in his 2005 Nature essay.
The problem, of course, is that the hypothesis of a universe whose very existence depends on our minds contradicts mainstream scientific intuitions. So physicists scramble to interpret quantum theory in a way that makes room for a mind-independent reality. A popular way to do this entails postulating imagined, empirically unverifiable, theoretical entities defined as observer-independent. Naturally, this goes beyond mere interpretation; it adds redundant baggage to quantum theory, in the sense that the theory needs none of this stuff to successfully predict what it predicts.
Some cringe at such attempts to modify quantum mechanics to make it fit one’s worldview, as opposed to adapting one’s worldview to make it consistent with quantum mechanics. So the question that naturally arises is: If we stick to plain quantum theory, what does it tell us about reality? Physicist Carlo Rovelli tried to answer this question rigorously and the result is now known as relational quantum mechanics (RQM).
According to RQM, there are no absolute—that is, observer-independent—physical quantities. Instead, all physical quantities—the entire physical world—are relative to the observer, in a way analogous to motion. This is motivated by the fact that, according to quantum theory, different observers can account differently for the same sequence of events. Consequently, each observer is inferred to “inhabit” its own physical world, as defined by the context of its own observations.
The price of this uncompromising honesty in acknowledging the implications of quantum mechanics is a number of philosophical qualms. First, the idea that the physical world one inhabits is a product of one’s private observations seems to imply solipsism, an anathema in philosophy. Second, RQM entails that “a complete description of the world is exhausted by the relevant [Shannon] information that systems have about each other.” However, according to Shannon, information isn’t a thing unto itself. Instead, it is constituted by the discernible configurations of a substrate.
Yet, if there is no absolute physical substrate, what then constitutes information? Third—and perhaps most problematic of all—the RQM tenet that all physical quantities are relative raises an obvious question: relative to what? We only see meaning in a relative quantity such as motion because we assume there to be absolute physical bodies that move with respect to one another. But RQM denies all physical absolutes that could ground the meaning of relative quantities.
Notice that the root of all these philosophical qualms is the assumption that only physical quantities exist. If physical quantities arise from personal observation and they are all there is, then solipsism is indeed implied. If physical quantities are grounded in information and they are all there is, then information indeed lacks a substrate. If physical quantities are relative and they are all there is, then there are indeed no absolutes to ground their meaning. I shall return to this insight shortly.
For now, however, it would seem that biting the bullet of plain quantum theory, without decorating it with imagined bells and whistles, forces us into unresolvable philosophical qualms. Yet, this conclusion is false. To see how we can get out of this quagmire we need only to be rigorous about the epistemic scope of physics.
Stanford physicist Andrei Linde, of cosmic inflation fame, provided an important clue when he observed that “our knowledge of the world begins not with matter but with perceptions.... Later we find out that our perceptions obey some laws, which can be most conveniently formulated if we assume that there is some underlying reality beyond our perceptions.... This assumption is almost as natural (and maybe as false) as our previous assumption that space is only a mathematical tool for the description of matter.” Hence, in the absence of an absolute, observer-independent substrate, the physical world of RQM can only be the contents of perception. There is nothing else for it to be.
Now recall that the philosophical qualms of RQM rest on the assumption that only physical quantities—that is, contents of perception—exist. However—and here is the key point—next to the contents of perception there are, of course, also non-perceptual mental categories such as thoughts. Many physicists posit that thoughts should be explainable in terms of physical quantities and, as such, become part of the physical world by reduction. But this is a philosophical assumption that does not change the scientific fact that quantum mechanics does not predict thoughts; it only predicts the unfolding of perception, even when what is predicted—and later perceived—is the output of instrumentation.
So the possibility that presents itself to us is that thoughts are the absolutes that ground the meaning of the relative physical quantities of RQM. In other words, all physical quantities on the screen of perception may arise as relationships between thoughts. Moreover, since both thoughts and perceptions are mental in essence, this line of reasoning points to mind as the primary substrate of nature, the discernible states of which constitute information.
The hypothesis here, which I have elaborated upon in detail elsewhere, is that thought—whose characteristic ambiguities may in fact be what quantum superposition states ultimately represent—underlies all nature and isn’t restricted to living organisms. The physical world of an observing organism may arise from an interaction—an interference pattern—between the organism’s thoughts and the thoughts underlying the inanimate universe that surrounds it. Although each organism—in accordance with RQM—may indeed inhabit its own private world of perceptions, all organisms may be surrounded by a common environment of thoughts, which avoids solipsism at least in spirit.
Conn Henry’s courageous assertion that “the Universe is entirely mental” isn’t only a seeming implication of recent experimental observations, it may also point the way to an elegant philosophical underpinning for what is perhaps the most rigorous and parsimonious interpretation of quantum mechanics. Mind, it seems, may offer a path out of the quantum quagmire in more ways than one.
Note: This essay is based on the paper “Making Sense of the Mental Universe,” published in Philosophy and Cosmology, Vol. 19, pages 33-49.