Quantum behavior follows directly from uncertainty. This uncertainty does not refer to indecision about your next meal but to a fundamental property of the real universe around us as formalized by the Heisenberg Uncertainty Principle. That principle tells us, among other things, that reality is not sure whether things are here or there until someone or something observes them. Since observations determine outcomes, they play a central role in quantum physics and, consequently, in the lives we experience.

Uncertainty might seem rather ordinary and obvious at first. After all, when strolling in an unfamiliar park, one does not know where a tree, bird or another person is located until one looks. Scientists, however, always delve more deeply and want to know very precisely where objects are, as well as whether that tree, bird or other person even existed at all before observed.

One way scientists study uncertainty is via lab experiments that test whether intersecting light beams create particular bright and dark stripes called an interference pattern. The pattern is caused by interaction between light following different paths. To get a sense of this interference via analogy, imagine playing just about any ball game. If your opponent hits/kicks/throws the ball one way, you need to respond accordingly. Your opponent might do something else, of course, so you need a plan for that possibility, too. However, if your opponent has many possible plays and you ponder how to counter all of them at the same time, those plans will interfere with each other in your mind and confuse you. It is only after you observe what your opponent actually does that your single best response becomes clear.

Observation has also proved of utmost importance in quantum physics. Experiments involving light, such as one called the double-slit, yield different results depending on whether you observe the experiment while it is operating. Yes, your act of observation changes the outcome of the experiment! Result A occurs if you do not observe the process, result B if you do. One variant of this experiment known as the "delayed-choice quantum eraser" lets you delay your observe-or-not choice until after the experiment has run. Incredibly, its result is the same as if you had not delayed your choice! That is, the corresponding result (A or B) forms before you make your outcome-changing observation.

This implies the experiment "knew" from the start which viewing choice you would make at some arbitrary later time and somehow created the result to match it beforehand! This effect is not theoretical but rather has been demonstrated via experiments, though the description here has been simplified for introductory purposes. To try it at home, see Hillmer and Kwiat's "A Do-It-Yourself Quantum Eraser" in the May 2007 Scientific American.

How can an experiment "know" what you will do later? There are few scientifically-credible explanations. It would be helpful if science knew exactly what happens when we make an observation, an action that scientists call a measurement. Physicists have not yet agreed upon the process that occurs, leaving unresolved what is called the quantum measurement problem. Regarding experiments that yield surprising findings, many physicists choose to ignore the strangeness and instead proceed with related calculations because mathematics is more reliable than conjecture. Others propose models then invent experiments to explore them. Both approaches have value in the right hands.

One model that can account for the odd quantum eraser result is called superdeterminism. Under superdeterminism, all events proceed directly from prior ones without any possible variance. If superdeterminism is correct regarding the quantum eraser experiment, you did not have a choice about whether to look at the experiment's operation. Instead, your selection of whether or not to look was effectively predetermined long ago, as was the result of the experiment, so your choice and the experiment's result always match, regardless of which came first. That means under superdeterminism none of us has free will; instead, our lives play out more like a movie made long ago, with us as unwitting actors following predetermined direction toward a predetermined end result.

One argument against superdeterminism is quantum uncertainty itself, something physicists believe is universal for physical systems. If superdeterminism is the way reality operates, uncertainty, which is at the heart of quantum physics, would serve no novel purpose -- nor would the universe as it heads to a preset end result. Instead, physicists find uncertainty to be key rather than extraneous, which suggests to many that superdeterminism does not accurately depict reality.

Surveys indicate a different model has grown to be accepted by more professional physicists than any other: the Many-Worlds Interpretation (MWI) and related variants. In Something Deeply Hidden, physicist Sean Carroll views MWI as inevitable given just two essential quantum physics rules. MWI seems able to account logically not only for the delayed-choice quantum eraser experiment but also for other current puzzles in physics. Under MWI for the quantum eraser, there exists a "world" in which you choose to look, as well as another "world" in which you choose not to. Since both happen, there always exists a world in which the result of the experiment matches your choice.

Each world can be thought of as a sort of container holding a copy of the known universe, including you, with different outcomes possible in each copy. An observation definitively places you into the world that contains the object you have observed in the condition you have observed it. Such sharing of worlds by observer and observed can also account for what is called entanglement, the subject of the 2022 Nobel Prize in Physics. While MWI posits a world for every possibility, an open question is whether those worlds are mathematical chimeras, or instead do physically exist before you make a choice, or after, or both, or neither! Good science does not happen without some anal retentiveness.

There are many variations of the standard "double-slit" experiment. One type employs single photons, the smallest packet of light possible. As before, an interference pattern forms only if we do not observe the path each photon follows, but wait, this version of the experiment sends a single photon at a time. One photon has nothing with which to interact, so why does an interference pattern still form? Some construe that pattern as evidence of interaction of single photons from each of several worlds, an emerging interpretation known as Many Interacting Worlds (MIW). Under MIW, particles from different worlds interact and, among other consequences, produce interference patterns.

Why does the interference pattern go away if we observe the photon's path? According to the MIWOI ("mee-woy") version of MIW as developed by this author, observation stops interaction from worlds in which photons take other paths. As in the ball game analogy, once observation eliminates other possibilities, there remain no differing ones left to cause interference. The act of measuring reduces our set of worlds to only those that match our observation, a hypothesis that may prove an important part of the solution to the quantum measurement problem.

MIWOI combines the Many-Worlds Interpretation (MWI) with a re-emphasis on the Heisenberg Uncertainty Principle that is an often overlooked part of MWI. MIWOI contends uncertainty means the world a subatomic particle occupies is uncertain prior to its observation. Uncertainty therefore permits those particles to move between cohered (linked) worlds where they then randomly interact with other particles. That interaction causes differences to develop in various worlds even while those worlds remain cohered. Those differences are revealed by, among other things, interference patterns in double-slit experiments prior to decoherence-triggering observation.

Observation also plays a key role in what we perceive as the passage of time. According to MIWOI, when we make an observation, we establish the worlds in which we exist, eliminating from our reach those worlds different from what we observed. For example, if we flip a quantum coin (an imaginary coin free of outside influences) and see it results in heads up, from that point forward there remain no tails-up worlds accessible to us for that flip. We have a simple term for the set of prior results: we call it the past. Once other results become inaccessible to us, change is not possible, which is why the past forever remains as we observed it. Observation is the fundamental action by which the future becomes the past.

If quantum effects are universal, when we stroll in a park, why don't we see tree trunks making striped interference patterns like those in the double-slit lab experiment described above? There are multiple reasons. First, quantum effects originate at the realm of the very small, and usually smaller than what is visible through an ordinary desktop microscope. Molecules, atoms and electrons routinely exhibit quantum uncertainty, whereas something the mass of a flea egg is about the largest that can show uncertainty, and then only rarely.

Second, interference patterns can occur only where multiple different quantum outcomes are possible. Recall result A and result B in the double-slit description above. Scientists describe those two outcomes as being cohered (linked) together prior to observation. Your act of making an observation causes those cohered possibilities to decohere (unlink). The slightest interaction between observer and observed triggers such decoherence. Consequently, by the time you specifically look at something as large as a tree, the quantum uncertainty that would have yielded interference has already ceased because the tree's inanimate surroundings, or the "environment" as physicists call it, have observed the tree, with you as part of that environment.

For example, sunlight that bounces off the tree then reflects off a pond and touches your skin just as you enter the park counts as interaction between you and the tree, as do countless other similar sequences before you reach the park. This observation-triggered decoherence reduces all possibilities for the tree to a single one, leaving nothing different by which to make an interference pattern. By the way, evidence says decoherence can also occur without human consciousness as the trigger.

Though a whole tree trunk is too massive to readily exhibit uncertainty, the submicroscopic internal workings of its leaves are not. Quantum effects were found during the 21st century to enhance photosynthesis, the chemical process by which sunlight powers plants. Application of discoveries of this kind may increase useful energy collected by solar panels.

If all the possibilities, all the many worlds, are physically real rather than mathematical mirages, they might explain anomalous gravitational effects found by astrophysicists: galaxies act as if there exists far more matter than we can find, something therefore dubbed dark matter. What if dark matter is matter in other worlds? Ironically, the as-yet-unsuccessful search for it may be confounded by the search itself because, according to MIWOI, observations eliminate interaction from the other worlds where the dark matter resides. By looking, we cause decoherence that renders those other worlds inaccessible to us. The very act of looking directly for dark matter makes it disappear in the same way observing the double-slit experiment changes the results.

If dark matter "disappears" when we look, we should be able to detect the effect. I hypothesized via MIWOI during the 2010s that dark matter's influence decreases as observations reduce the number of worlds that interact. If correct, that would mean dark matter's effects are proportional to the number of worlds that remain cohered. I outline an experiment to test that in the book Reality's Prism. It exploits a loophole in reality to explore dark matter with and without making a decoherence-triggering observation.

That experiment has not yet been performed, but others have. One study found that galaxies that collided, and thereby observed each other and decohered, did show a localized reduction in dark matter, as MIWOI had predicted. This finding may tell us about the number of worlds that exist. If that number were truly infinite, any reduction in the count would leave an infinite number of worlds still cohered, which would leave dark matter effects unchanged. Instead, the study found dark matter was reduced, which suggests the number of worlds is not infinite.

Another MIWOI prediction was confirmed during 2024 by the James Webb Space Telescope (JWST). JWST found that large galaxies existed very early after our universe began, which some think is too soon for gravity to have brought them together. To explain this surprise, some scientists are looking to modify how gravity works. MIWOI's explanation is simpler, and is essentially the same as for the colliding galaxies mentioned above. Particles of the early universe did not yet have much time to interact, so they were still cohered with a greater number of worlds than today. More cohered worlds meant more accessible dark matter, so the total gravitational field it created was stronger, and that caused matter to gather into galaxies at a faster rate in the early universe than it now does.

Dark matter's influence on gravity tells us dark matter is real and has a physical presence, perhaps in other worlds. That suggests at least some of those other worlds contain actual, physical "copies" of you and me, with each of our doppelgängers as real as any other. Many of them would be doing virtually the same thing at the same time as each of us. Incredibly, due to interaction between worlds, you may be able to see the presence of your counterparts in the other worlds, as described next.

A colorful optical effect known as a glory, first recorded in China about 2000 years ago, appears as a halo around your shadow cast onto a cloud deck below when you stand on a mountain. Certain aspects of a glory halo cannot by explained without invoking quantum phenomena. That could be sunlight's interaction with the copies of you in other worlds who are standing in nearly the same place in their worlds, i.e. you are seeing the combined, interacting shadows of your copies, and they yours. This conclusion is speculative but sufficiently tantalizing to warrant exploration.

Note that, under this conclusion, your observation of a glory does not make the glory disappear for the same reason the observation of an interference pattern in the double-slit experiment does not make that pattern disappear. Just as for an interference pattern, a glory is a consequence of you not observing the path of light on its way to forming the glory. That lack of observation allows you to remain cohered with your counterparts in other worlds, which permits light from those worlds to interact and produce the glory. In the case of a glory, it's as if your body becomes part of a double-slit experiment.

The existence of countless copies of each of us, if correct, would continue science's long history of enlarging our surroundings while dethroning us from a vaunted place within. Nicolaus Copernicus described some 500 years ago how Earth is not the center of our solar system. Centuries later, we found our Milky Way galaxy is not the only galaxy. Now, it seems, each of us might not be our only representative. Instead, who we are would then be better described by the full range of our counterparts, a large number of whom are experiencing different paths through life via many worlds, each one as valid as the one reading this.

With or without many worlds, do we passively experience different life paths or actively choose those paths? Though some mystics claim quantum uncertainty means we can make anything and everything transpire at will, such control has never been demonstrated in physics. No one has found an ability to repeatably regulate uncertainty's randomness, much less induce results that are beyond scientifically possible outcomes.

Without uncertainty, every action follows from a cause, which itself follows from a prior cause, all the way back to the start of the universe. That is known as superdeterminism. Our brains, too, would be subject to that rigid sequence, leaving us no chance of free will because all our thoughts would be predetermined by prior causes. Human free will is possible only if something exists to break the rigid cause-and-effect chain. MIWOI says this is where quantum uncertainty comes in. Uncertainty may act like background noise within our brains, at maximum permitting us to freely choose among options or, at minimum, randomly nudging our thoughts toward or away from particular options without us realizing.

How? Let's use an analogy. Imagine attending a noisy party that has lots of concurrent conversations and music. The random bits of conversations and song lyrics make for a jumble of sounds, a significant background noise, yet from within that din, humans have the ability to choose one conversation, focus on it, and follow along while largely ignoring other sounds.

Quantum uncertainty is like the noise at that party, and as best as physics has determined, it is truly random, i.e. non-deterministic. If the world a subatomic particle occupies is uncertain, MIWOI says such particles from other worlds must continually interact with our brain cells in a truly random fashion. That randomness may give us the chance to make a non-predetermined, free-will choice of what to focus upon in our life in the same way we can choose the conversation to focus upon at a party. While that cannot be proven, such choices remain a scientific possibility within a field that otherwise finds scant support for human free will.

Our brains are not empty space, at least we all hope, but empty space is easier to study. Physicists have found so-called empty space is not truly empty but more a cauldron of virtual particles that pop in and out of existence. Such appearance and disappearance supports the idea of particle movement from one world to another. You can see that interaction for yourself without any lab equipment: look around a very dark room after your eyes adjust and you will find the view is not entirely of uniform dimness. Instead, objects will appear slightly fuzzy as if you are gazing through falling snow. According to MIWOI, part of that fuzziness originates with particles from other worlds that stimulate your optic nerves. The same thing causes some of the visual noise in short-exposure photographs, both film and digital.

Let's return to decoherence, the unlinking that occurs upon observation. One of the most incredible things about quantum decoherence is that it happens instantly, or nearly so. Unlike light that takes time to propagate from one place to another, when decoherence happens, from the observer's perspective it does so everywhere at the same moment. This hypothesis led to consternation when introduced during the early 20th century, culminating in a high-profile debate between physicists Albert Einstein and Niels Bohr. The instantaneous nature of decoherence is now being exploited within quantum computers that can calculate far faster than conventional computers. Quantum effects are fueling the 21st century's revolution in high-speed computing destined to power future Artificial Intelligence.

Conventional digital computers are limited to trying one thing at a time at the speed of light, whereas quantum computers can try a huge number of possibilities at the same time, prune wrong ones, reduce them all to a single answer, and do it via the virtually-instantaneous quantum decoherence process. How is this doable? Recall under quantum physics that both result A and result B are possible simultaneously before observation. So, if a quantum computer operates unobserved, it can try both results at the same time. Given more qubits, a kind of memory, a quantum computer can try vastly more than two possible results at a time, and narrow them to one specific answer very quickly. Recall how the slightest observation causes quantum states to decohere. The premature observation of a quantum computer's memory causes decoherence that ceases a computation prior to completion. So, one engineering challenge is to isolate a quantum computer's workings so the surrounding environment, with us in it, does not observe them too soon.

For physicist David Deutsch in The Fabric of Reality, the operation of the universe is indistinguishable from that of a computer. That does not mean we exist as a computer simulation but rather hints we might be part of the computer itself. Those concepts have blossomed into the burgeoning field of quantum information, an interdisciplinary study involving physics, computing, mathematics, cryptology, philosophy, plus other areas. Important to the field is how observation cements one quantum outcome out of many possible. For example, to secure the transmission of a secret message, quantum techniques can make the act of observation change part of the message (give result B instead of result A, for example), and thus expose that an interloper had snooped.

Some believe information to be more fundamental to the structure of the universe than matter and energy. In a general sense, observation gathers information and, per quantum science, information cannot be destroyed. Consequently, the universe is gaining information as time passes, reducing uncertainty as a side effect -- or perhaps as the main effect; we are not sure.

Why is there uncertainty at all? Why might there be many worlds? Why is there something rather than nothing? "Why?" is perhaps the most important question in science but often one of the most difficult to answer. Philosophical speculation can sometimes suggest research topics. The worlds of MIW appear to serve as a near infinite number of containers for different outcomes prompted by random interaction between worlds. Those worlds provide room for every physically possible outcome to occur in at least one of them. In computing, trying every possibility is known as a brute force approach to answering a question. This suggests reality exists to answer a question by trying every possibility, gathering information along the way. We probably will never know what that question is, but the existence of such a question implies the existence of a questioner. Might that series of implications be a literal deus ex machina? There is no shortage of topics to ponder at the frontiers of science.

We establish our past via observation, whereas future possibilities remain vast. Since we have yet to find evidence of intelligent life elsewhere, humans might be the first to decrypt at least a bit of reality's mysterious clockworks. Looking ahead, objectivity is essential in research, even if findings disagree with a subjective preference for individuality, free will or purpose. Regardless of the nature of reality, I find it compelling that each of us has been afforded a view from the inside.