Physics seems to be one of the only domains of human life where truth is clear-cut. The laws of physics describe hard reality. They are grounded in mathematical rigor and experimental proof. They give answers, not endless muddle. There is not one physics for you and one physics for me but a single physics for everyone and everywhere. Physics often seems weird, but that's a good sign—it is not beholden to preconceptions. In a world that can seem claustrophobic, where the same debates go round in circles, physics injects some genuine novelty into life and jolts us out of the ruts we fall into.

Physics is also the bedrock of the broader search for truth. If you follow the chains of explanation in other sciences, you eventually wind up in physics. The success of physics and its role in grounding other sciences support a broadly naturalistic, or physicalist, worldview: that all phenomena have physical explanations and that notions such as élan vital or incorporeal souls have no place in serious thought anymore. Physics does not dictate how we run our lives or resolve pressing moral dilemmas, but it sets the backdrop against which we decide these questions.

Yet if physics strikes most people as truth seeking at its purest, it doesn't always seem that way to physicists themselves. They sometimes seem to be struck by 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 informal discussions, at conferences devoted to the broad direction of their subject, in renewed efforts to reach out to philosophers for help, and in books and blogs for the general public. These worries are most acute in fundamental physics, which is not the entire subject but does play an outsized role in it. Many fret that the Large Hadron Collider has yet to turn up any new phenomena, giving them nothing to work with to derive the next level of laws. They worry whether proposed unified theories, such as string theory, can ever be tested. Some deem their subject overly mathematical; others think it mathematically sloppy. Truth can be elusive even in the best-established theories. Quantum mechanics is as well tested a theory as can be, yet its interpretation remains inscrutable.

A bench scientist faces more concrete problems. Is a wire broken? Is the code buggy? Is the measurement a statistical fluke? Still, even these prosaic worries can be surprisingly subtle, and they are not entirely divorced from the overarching questions of physics. Everything must be judged within a broader framework of knowledge.

Many physicists take these troubles to mean that their field has gone astray and that their colleagues are too blinkered to notice. But another reading is that the elusiveness of truth is an important clue. Unlike other domains of human life, the difficulties with truth that physicists face come not from dissembling but from brutal honesty: from being completely frank about our limitations when we come face to face with reality. Only by confronting those limitations can we overcome them.

Misgivings about the progress of physics are hardly new. As long as there have been physicists, there have been physicists who worry their field has come up against an insuperable barrier. Research is always a muddle when you're in the thick of it. It seems remarkable that we humans could understand reality at all, so any roadblock could well be a sign our luck has finally run out.

Over the generations, physicists have oscillated between self-assurance and skepticism, periodically giving up on ever finding the deep structure of nature and downgrading physics to the search for scraps of useful knowledge. Pressed 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's views were complicated: Newton did, in fact, frame several hypotheses for gravity, and Bohr at other times said that quantum theory captured reality. On the whole, though, they made progress by setting aside grand questions of why the world is as it is.

Historically, physicists eventually do return to those questions. Newton failed to explain gravity, but later generations took up the challenge, culminating with Einstein's general theory of relativity. The interpretation of quantum mechanics came back onto the physics agenda in the 1960s and, though unsettled, has spun off practical ideas such as quantum cryptography. What reawakens physicists' curiosity is the sense that, as the late philosopher Hilary Putnam put it, the success of physics theories would be miraculous if they were not attuned to reality. Even more basically, how can we be doing experiments if there isn't something real to do them on? This position is known as realism. It holds that entities we do not directly observe but infer theoretically—such as atoms, particles, and space and time—really do exist. Theories are true because they reflect reality, albeit imperfectly. The cycling between realism and the opposing position, antirealism, will undoubtedly continue, as each evolves under pressure from the other.

This competition has been good for physics. Antirealist physicist-philosopher Ernst Mach inspired Einstein to rethink how we know what we know—or think we know. That set the course for all that followed in physics. When we accept we see the world through colored lenses, we can compensate. Some features of reality are relative to an observer, whereas others are common to all observers. Two people moving at different speeds may disagree on the distance between places, the duration of an event or, in some cases, which of two events came first. The dispute between them is unresolvable. But the arithmetic combination of distance and duration—the spatiotemporal distance—is a fact common to both, an “invariant.” Invariants define 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 things 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 think 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 causes are cloaked.

Particles are easy to manipulate, which is why quantum physics is commonly described in terms of particles. But most physicists think the same rules apply to all things, even living things. Thus, it is not clear when the photon makes its choice to pass through or reflect, if indeed it ever chooses. When it hits the mirror, the combined system of the two enters a state of indecision. When a measuring device registers the path, it, too, is caught between the possibilities. If you send your friend to see what has happened, to you that person sees both eventualities. Physicists have yet to find any threshold of size or complexity of a system that forces the outcome. (Size and complexity are important in defining what the options are, but not in the final selection.) For now they know of only one place where the ambiguity is resolved: in our own conscious perception. We never experience photons doing two mutually contradictory things at once. Therefore, physicists are left with an unwanted element of subjectivity in their theory.

To Christopher A. Fuchs of the University of Massachusetts Boston, the lesson is that observers are active participants in nature, helping to construct what they observe, and a fully third-person perspective is impossible. The mathematics of quantum theory jumbles together subjective and objective elements. His “QBist” interpretation tries to strip away the subjective elements and reveal the real structure that lies within, much as Einstein did with relativity theory.

Philosopher Richard Healey of the University of Arizona has a related “pragmatist” view that quantum theory is a representation not of the world but of the interface between the world and a human or another agent. We can use it to judge the probabilities of things that might happen, just as a technical stock trader buys and sells based on market trends rather than economic fundamentals. Such a trader can become rich without a clue what the companies are doing. Unlike Fuchs, Healey doesn't think that a description of physical reality is tucked inside quantum theory. That, he thinks, will require an entirely new theory.

At the opposite pole, if you do take quantum theory to be a representation of the world, you are led to think of it as a theory of co-existing alternative realities. Such multiple worlds or parallel universes also seem to be a consequence of cosmological theories: the same processes that gave rise to our universe should beget others as well. Additional 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.

Although 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 view of reality is heavily filtered, and we have to take that into account when applying the theory. If we do not see a photon do two contradictory things at once, it does not mean the photon is not doing both. It might just mean we get to see only one of them. Likewise, in cosmology, our mere existence creates a bias in our observations. We necessarily live in a universe that can support human life, so our measurements of the cosmos might not be fully representative.

Parallel universes do not alter the truth that we experience. If you suffer in this universe, it is little comfort that near duplicates of you thrive elsewhere. But these other worlds are corrosive to the pursuit of 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 descending into anarchy: they do not say that one thing happens rather than another, because both happen, and which we see is blind luck. The distinction between fact and fiction is just a matter of location.

Even some aspects of fundamental physics that seem firmly established are surprisingly subtle. Physicists routinely speak of particles and fields: localized motes of matter and continuous, fluidlike entities such as the electric or magnetic field. Yet their theories indicate that no such things can exist. The combination of quantum mechanics with relativity theory rules out particles: according to several mathematical theorems, nothing can be localized in the way that the traditional concept of a particle implies. The number of particles that observers will see depends on their own state of motion; it is not invariant and therefore does not qualify as an objective fact. Groups of particles can have collective properties above and beyond the properties of the individuals.

Fields, too, are not what they appear to be. Modern quantum theories long ago did away with electric and magnetic fields as concrete structures and replaced them with a hard-to-interpret mathematical abstraction. Among its many odd features, the abstraction is highly redundant; it is more complex than the real phenomena it is meant to represent. Physicists have sought alternative structures that align with reality, but those structures are no longer really fields. For now they continue to describe the world in terms of particles and fields, aware that the full story still eludes them.

Proposed unified theories of physics introduce new complications. String theory, in particular, has been controversial. It goes all in on parallel universes, with all their strange consequences for truth. It also relies heavily on so-called dualities: different mathematical expressions that make the same predictions for observations, indicating they are alternative ways to describe the same situation. These dualities are powerful because they allow for lateral thinking. If an equation is too hard to solve, you can use a duality to translate it into a simpler one. But if multiple mathematical formulations are equivalent, how do we know which, if any, corresponds to reality?

Many critics of string theory complain that no known instrument can test it because it involves such minuscule effects. But that criticism applies equally to its competitors. This is the curse of success. There are not a lot of cracks in existing theories that could let us see through to a deeper level. Lacking experimental guidance, physicists have had to develop these theories mathematically. Quantum mechanics and relativity theory are so tightly constraining that they are almost enough on their own to dictate the form of the unified theory. Nevertheless, all the proposed theories rely heavily on judgment calls about beauty and elegance that might turn out to be wrong.

A strange tendency is built into the entire project of unification. The deeper physicists dive into reality, the more reality seems to evaporate. If distinct things are manifestations of the same underlying stuff, their distinctness must be a product of how they behave rather than their intrinsic nature. Physical explanation replaces nouns with verbs: what things are is a product of what their components do. String theory may not be right, but it illustrates the trend. According to it, the vast zoo of particle species are different vibration patterns of a single type of primitive and featureless thing called a string. Taken to its logical end, this reasoning suggests that no nouns will be left at all.

Some philosophers conclude that the entire category of “thing” is misguided. According to a view known as structural realism, relations are the primary ingredient of nature, and what we perceive as things are hubs of relations. This view has its oddities, however. What differentiates physical from mathematical objects or a simulation from the original system? Both involve the same sets of relations, so there seems to be nothing to tell them apart. And if there are no nouns, then what is acting out the verbs? Is physics built on quicksand?

It is not just the physics problems that make physicists wonder whether they are on the right track. Many have gotten interested in consciousness, drawn by the so-called hard problem of consciousness. The methods of science seem inherently incapable of describing subjective experience. Our inner mental life is hidden from external observation and does not seem reducible to mathematical description. It strikes many researchers as an unnecessary add-on with no place in the physical scheme of things. By this argument, some researchers say understanding the mind could demand some new principle of science or new way of thinking. Physicists are intrigued that their basic picture of the world could be missing something so important.

That is not the only reason that physicists have been giving thought to the mind. The multiverse is one example of how we may perceive a filtered version of reality, and once you start down this path of wondering how truth might be skewed, you might entertain possibilities that make the multiverse sound tame. Immanuel Kant argued that the structure of our minds conditions what we perceive. In that tradition, 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, among others, have argued that we perceive the world as divided into objects situated within space and time, not necessarily because it has this structure but because that is the only way we could perceive it.

Just because our brains navigate the world successfully does not mean they capture its structure faithfully. In machine learning, researchers have found that computer systems are often better at making predictions or controlling equipment when they eschew direct representations of the world. Similarly, reality might be completely unlike what our minds or our theories present to us. Scholars such as philosopher Colin McGinn and Harvard University psychologist Steven Pinker have suggested that our particular style of reasoning is why we find consciousness so hard. Perhaps one day we will construct artificial minds that see right through the problems that stump us, although they might get hung up on those we think are easy.

If anything restores confidence that truth is within our grasp, it is that we can divide and conquer. Although “real” is sometimes equated with “fundamental,” each of the multiple levels of description in science has an equal claim to be considered real. Therefore, even if things vanish at the roots of nature, we are perfectly entitled to think of things in daily life. Even if quantum mechanics is mystifying, we can build a solid understanding of the world on it. And even if we worry that we aren't experiencing the fundamental reality, we are still experiencing our reality, and there's plenty to study there.

If we find that our theories are clutching at vapors, that's not a bad thing. It's reminding us to be humble. Physicists can be full of themselves, but the most experienced and accomplished among them are usually circumspect. They tend to be the first people to point out the problems with their own ideas, if only to avoid the embarrassment of someone else doing it for them. No one ever said that finding the truth would be easy.