Physics appears to be one of the only domains of human life where truth is unambiguous. The laws of physics define hard reality. They are based on mathematical rigor and experimental proof. They give answers, not endless jumble. There is a single physics for everyone and applicable everywhere. Physics frequently seems weird, but that’s a good indication that it is not beholden to preconceptions. In a world that can seem claustrophobic, where the same arguments go around in circles, physics injects some genuine originality into life and jolts us out of the channels we fall into.
Physics is also the foundation of the broader search for truth. If you track the chains of explanation in other sciences, you eventually end up in physics. The success of physics and its part in grounding other sciences support a largely naturalistic, or physicalist, worldview: that each phenomenon has a physical explanation and that notions such as élan vital or intangible souls have no place in serious thought anymore. Physics does not direct how we run our lives or resolve persistent moral dilemmas, but it sets the backdrop against which we resolve these questions.
Hitherto if physics strikes most people as truth-seeking at its core, it doesn’t always seem like that to physicists themselves. They sometimes get stuck in a collective imposter syndrome. Although they may presume that the truth is out there and they are capable of finding it—they have to, or what would be the point?—they have their doubts, which surface in casual discussions, at conferences dedicated to the broad direction of their subject, in renewed efforts to consult philosophers for help, and in books and blogs for the general public. These uncertainties are most acute in fundamental physics, which is not the complete subject but does play a massive role in it. Many agonize that the Large Hadron Collider has yet to produce any new phenomena, giving them little to work with to derive the next level of laws. They worry whether proposed unified theories like the string theory can ever be tested. Some consider their subject overly mathematical; others think it mathematically disordered. Truth can be vague even in the best-established theories. Quantum mechanics is a well-tested theory but yet its interpretation remains inscrutable. Everything needs to be judged within a broader framework of knowledge.
Unlike other spheres of human life, the difficulties with the truth that physicists face originate from brutal honesty and not dissembling, from being completely frank about our limitations when we face reality. Only by confronting such limitations can we overcome them.
Misgivings about the progress of physics are scarcely new. Ever since there have been physicists, there have been physicists who worry that their field has hit an insuperable barrier. Across generations, physicists have vacillated between self-assurance and skepticism, intermittently giving up on ever finding the deep structure of nature and downgrading physics to the search for scraps of beneficial knowledge.
Pushed by his contemporaries to explain how gravity works, Isaac Newton responded, “I frame no hypotheses.”
Niels Bohr, commenting on quantum mechanics, wrote, “Our task is not to penetrate into the essence of things, the meaning of which we don’t know anyway, but rather to develop concepts which allow us to talk in a productive way about phenomena in nature.”
Both men held complicated views as Newton did frame several hypotheses for gravity, and Bohr himself said that quantum theory captured reality. Overall, though, they both made progress by overlooking grand questions of why the world is as it is.
Historically, physicists sooner or later do return to those questions. Newton failed to completely describe gravity, but future generations took up the challenge, concluding with Einstein’s general theory of relativity. The interpretation of quantum mechanics returned to the physics agenda in the 1960s and, though unsettled, has incited practical ideas such as quantum cryptography. What revives physicists’ curiosity is the wisdom that, as the late philosopher Hilary Putnam articulated, the success of physics theories would be astonishing if they were not in agreement with reality. Even more fundamentally, how can we conduct experiments if there isn’t something physical to do them on? This approach called realism holds that entities we do not directly detect but infer theoretically like atoms, particles, space and time, really do exist. Theories are right because they reflect reality, even if imperfectly. The cycle between realism and the opposing position of antirealism will unquestionably continue, as each evolves under pressure from the other.
This rivalry has been good for physics. Antirealist physicist-philosopher Ernst Mach stirred Einstein to reconsider how we know what we know or think we know. This contemplation set the course for all that followed in physics. When we accept what we see in the world through colored lenses, we can compensate. Certain features of reality are relative to an observer, whereas others are mutual for all observers. Two people traveling at different speeds may disagree on the distance between places, the duration of an occurrence or, in some cases, which of two events arose first. The disagreement between them is unresolvable. But the mathematical combination of distance and duration i.e. the spatiotemporal distance is a fact that is common to both, an “invariant.” Invariants explain objective truth.
In addition to the generic concerns that physicists of the past shared, physicists today have come up against many specific and unexpected limits to knowledge. Almost no matter which interpretation of quantum mechanics you choose, some effects about the quantum world are beyond us. For instance, if you shoot a photon at a half-silvered mirror, it might pass through, or it might reflect off, and there’s no way you can tell what it will do. The outcome is decided randomly. Some deliberate the photon does what it does for no reason at all; the randomness is intrinsic. Others think there is some hidden reason. Still, others think the photon both passes through and reflects, but we are able to see only one of these outcomes. Whichever it is, the underlying roots are cloaked.
Particles are easy to manipulate, which is why quantum physics is generally described in terms of particles. But a majority of physicists think the same rules apply to all things, including living things. Thus, it is not evident when the photon makes its choice to pass through or reflect if indeed it ever selects. When it hits the mirror, the joint system of the two enters a state of indecision. When a measuring device registers the path, it, too, is trapped between the possibilities. Physicists have as yet not found any threshold of size or complexity of a system that forces the outcome. For now, they are aware of only one place where the ambiguity is resolute: in our own conscious perception. We never encounter photons doing two mutually contradictory things at once. Therefore, physicists are stuck with an unwanted element of subjectivity in their theory.
According to Christopher A. Fuchs of the University of Massachusetts Boston, the message is that observers are active participants in nature, helping to construct what they perceive, and a fully third-person perspective is impossible. The mathematics of quantum theory mixes together subjective and objective elements.
Philosopher Richard Healey of the University of Arizona has an associated “pragmatist” view that quantum theory is a representation of the interface between the world and a human or another agent and not of the world itself. We can use it to judge the probabilities of events that might happen, similar to a technical stock trader who buys and sells based on market trends rather than economic fundamentals. This trader can become rich without having a clue about what the companies are doing. Unlike Fuchs, Healey doesn’t think that a description of physical reality is locked inside the quantum theory. He thinks this will require entirely new theory.
In contrast, if you do take quantum theory to be a representation of the world, you are directed to think of it as a theory of co-existing alternative realities. Such multiple worlds or parallel universes also appear to be a consequence of cosmological theories: the same processes that gave birth to our universe should produce others as well. Other parallel universes could exist in higher dimensions of space beyond our view. Those universes are populated with variations on our own universe. There is not a single definite reality.
Even though theories that predict a multiverse are entirely objective, no observers or observer-dependent quantities appear in the basic equations, they do not eliminate the observer’s role but merely relocate it. They say that our opinion of reality is heavily filtered, and we have to consider that when applying the theory. If we do not see a photon do two contradictory things simultaneously, it does not mean that the photon is not doing both. It just means we get to see only one of them. Likewise, in cosmology, our simple existence creates a bias in our observations. We essentially live in a universe that can support human life, so our measurements of the cosmos might not be wholly representative.
Parallel universes do not vary the truth that we experience. If you agonize in this universe, it is little comfort that similar duplicates of you thrive elsewhere. But these other worlds are corrosive to the pursuit of a broader truth. Because the other universes are generally not observable, they represent an insuperable limit to our direct knowledge. If those universes are utterly unlike our own, our empirical knowledge is not merely limited but deceived. The laws of physics risk plunging into anarchy: they do not say that one thing occurs rather than another, because both transpire, and which we perceive is blind luck. The distinction between fact and fiction is simply a matter of location.
Even certain aspects of fundamental physics that seem firmly established are amazingly subtle. Physicists regularly speak of particles and fields: localized bits of matter and continuous, fluidlike entities like the electric or magnetic field. Yet their theories show that no such things can exist. The combination of quantum mechanics with relativity theory dismisses particles: according to numerous mathematical theorems, nothing can be localized in the system that the traditional concept of a particle implies. The number of particles that observers will witness depends on their own state of motion; it is not invariant and consequently does not qualify as an objective fact. Groups of particles can possess collective properties above and beyond the properties of the individuals.
Fields, too, are not really what they appear to be. Modern quantum theories years ago did away with electric and magnetic fields as concrete structures and swapped them with a hard-to-interpret mathematical abstraction. Among its countless odd features, the abstraction is highly redundant; it is more complex than the actual phenomena it is meant to represent. Physicists have pursued alternative structures that align with reality, but those structures are no longer truly fields. For now, they continue to define the world in terms of particles and fields, aware that the full story still evades them.
Proposed unified theories of physics present new complications. String theory, in particular, has been contentious. It focuses on parallel universes, with all their weird consequences for truth. It also relies profoundly on purported dualities: different mathematical expressions that make the same predictions for observations, demonstrating they are alternative ways to describe the same condition. These dualities are powerful because they permit lateral thinking. If an equation is too rigid to solve, you can employ a duality to translate it into a simpler one. But if multiple mathematical formulations are equivalent, how do we know which, if any, agrees with reality?
Many critics of string theory contend that no known instrument can test it since it involves such minuscule effects. But that criticism applies similarly to its competitors. This is the curse of success. There are not a lot of cracks in prevailing theories that could let us see through to an unseen level. Lacking experimental direction, physicists have had to develop these theories mathematically. Quantum mechanics and relativity theory are so firmly constraining that they are virtually enough on their own to dictate the system of the unified theory. Nevertheless, all the proposed theories depend heavily on judgment calls about beauty and elegance that might prove wrong.
A bizarre tendency is built into the entire project of unification. The deeper physicists go into reality, the more reality seems to evaporate. If distinct entities are manifestations of the same underlying stuff, their distinctness must be a product of how they behave instead of their intrinsic nature.
Some philosophers conclude that the whole category of “thing” is misguided. According to an approach called structural realism, relations are the primary ingredient of nature, and what we observe as things are hubs of relations. This view has its peculiarities, however. What distinguishes physical from mathematical objects or a simulation from the original system? Both comprise the same sets of relations, so there seems to be little to tell them apart.
It is not just the problems in physics that make physicists ponder whether they are on the right track. Many have gotten absorbed in consciousness, drawn by the supposed hard problem of consciousness. The methods of science seem integrally incapable of describing subjective experience. Our inner mental life is concealed from external observation and does not seem reducible to mathematical description. It strikes various researchers as an unnecessary add-on with no place in the physical outline of things. By this argument, some researchers believe understanding the mind could demand certain new principle of science or new ways of thinking. Physicists are fascinated that their basic picture of the world could be missing something so significant.
That is not the sole reason why physicists have been giving thought to the mind. The multiverse is one case of how we may perceive a filtered form of reality, and once you start down this path of questioning how truth might be skewed, you might consider possibilities that make the multiverse appear tame. Immanuel Kant contended that the structure of our minds conditions what we perceive. In that practice, physicist Markus Müller of the Institute for Quantum Optics and Quantum Information in Vienna and cognitive scientist Donald Hoffman of the University of California, Irvine, and others, have reasoned that we perceive the world as divided into objects positioned within space and time, not necessarily because it has this structure but because that is the lone way we could perceive it.
Our brains do navigate the world successfully but that does not mean they capture its structure authentically. In machine learning, researchers have discovered that computer systems are often better at making predictions or directing equipment when they eschew direct representations of the world. Similarly, reality might be completely different from what our minds or our theories present to us. Scholars like philosopher Colin McGinn and Harvard University psychologist Steven Pinker have recommended that our particular style of reasoning is why we find consciousness so tough. Perhaps one day we will develop artificial minds that see right through the problems that stub us, although they might get hung up on things, we think are easy.
No matter what, truth is within our grasp, it is that we can divide and conquer. Though “real” is sometimes equated with “fundamental,” each of the multiple levels of explanation in science has an equal claim to be measured as real. Therefore, even if things vanish at the origins of nature, we are perfectly permitted to think of things in daily life. Even if quantum mechanics is baffling, we can build a solid understanding of the world on it. And even if we are apprehensive that we aren’t experiencing the fundamental reality, we are still experiencing our reality, and there’s enough to study there.
If we realize that our theories are clutching at straws, that’s not a bad thing. It’s a reminder to be humble. Physicists can sometimes be full of themselves, but the most experienced and accomplished among them are typically circumspect. They incline to be the first person to point out the problems with their own ideas if only to evade the embarrassment of someone else doing it for them. No one ever claimed that finding the truth would be easy.