There is no single agreed-upon answer to what reality fundamentally is, but modern physics and philosophy have narrowed the field to a handful of serious possibilities. What’s remarkable is how far these ideas have moved from everyday intuition. Over the last century, experiment after experiment has shown that reality at its deepest level behaves nothing like the solid, predictable world we experience. The objects you see, the space you move through, even the flow of time may be very different from what they appear to be.
Reality Is Not What It Looks Like
Start with the most unsettling finding from biology. Cognitive scientist Donald Hoffman and colleagues at UC Irvine used evolutionary game theory to ask a simple question: did natural selection shape our senses to perceive reality accurately, or just to survive? They proved what they call the Fitness-Beats-Truth theorem, which shows that organisms tuned to track survival payoffs will, over evolutionary time, drive organisms tuned to perceive objective truth to extinction. The more complex reality gets, the stronger this pressure becomes. As the range of possible perceptions grows, the probability that survival-based perception dominates truth-based perception approaches 100%.
What this means is striking. Your brain did not evolve to show you what’s really out there. It evolved to show you what keeps you alive. Colors, shapes, sounds, the feeling of solidity under your feet: these are more like icons on a desktop than photographs of an external world. You interact with the icons because they’re useful, not because they resemble the underlying circuitry. So any serious investigation into the nature of reality has to start by acknowledging that human perception is, at best, a simplified interface.
Quantum Mechanics Broke the Old Picture
Classical physics described a universe of solid objects moving through space according to fixed laws. Quantum mechanics demolished that picture. At the subatomic level, particles don’t have definite properties until they’re measured. They exist in overlapping states of possibility, described by mathematical wave functions, and only “choose” a definite outcome when something interacts with them. This isn’t a gap in our knowledge. It’s how nature actually works.
The most powerful evidence comes from tests of Bell’s inequality. In the 1960s, physicist John Bell devised a mathematical test that could distinguish between two views of reality: one where particles carry hidden instructions (local realism), and one where they don’t. If local realism were true, measurements on distant particles would always obey a specific statistical limit. Quantum mechanics predicted that entangled particles could violate that limit.
Over 50 years of increasingly rigorous experiments have confirmed the violation. A 2023 experiment using superconducting circuits, evaluated over more than one million trials, found a Bell inequality violation so extreme that the probability of it being a statistical fluke was less than 1 in 10^108. That number is, for all practical purposes, zero. The experiment closed all known loopholes, meaning the result can’t be explained by timing tricks or detector inefficiencies.
The implication is profound. Reality does not consist of independent objects carrying definite properties at all times. Instead, distant particles can share correlations that have no explanation in any local, predetermined framework. The universe is fundamentally nonlocal: what happens here can be instantaneously correlated with what happens far away, not through any signal, but through the structure of reality itself.
What Quantum States Actually Are
Even physicists disagree about what quantum mechanics is telling us. One increasingly influential interpretation, called QBism (short for Quantum Bayesianism), takes a radical position: quantum states are not objective features of the world. They are an agent’s personal degrees of belief about what will happen next. A wave function doesn’t describe a particle the way a blueprint describes a building. It describes your expectations.
Under QBism, a measurement doesn’t reveal a pre-existing value. The outcome is created in the act of measurement. This dissolves many of quantum mechanics’ famous paradoxes, but at a cost: it means there is no single, observer-independent quantum state of the universe. Reality, at least as described by physics, is participatory. You are not a passive observer reading off facts about an external world. You are an agent whose interactions with the world partly constitute it.
Information as the Foundation
Physicist John Archibald Wheeler proposed a concept he called “it from bit,” suggesting that the physical world (the “it”) ultimately arises from binary yes-or-no answers at the quantum level (the “bit”). In this view, information is not something that describes reality. Information is reality. Every particle, every force, every event reduces to answers to yes-or-no questions posed by measurement.
This idea has gained traction through the holographic principle, which emerged from theoretical work on black holes and string theory. The holographic principle states that all the information contained in a volume of space can be fully encoded on its boundary, like a two-dimensional surface. Physicist Juan Maldacena’s work showed a precise mathematical correspondence between a higher-dimensional space and a lower-dimensional boundary theory. If the holographic principle applies to our universe, then the three-dimensional world you experience may be, in a rigorous sense, a projection of information stored on a distant two-dimensional surface. The “depth” you perceive may not be fundamental.
Is Reality Made of Math?
MIT physicist Max Tegmark has pushed the information idea even further with what he calls the Mathematical Universe Hypothesis. His claim is that physical reality doesn’t just obey mathematical laws. Physical reality is a mathematical structure. There is no underlying “stuff” that the math describes. The math is all there is.
Tegmark proposes four levels of parallel universes that follow from this idea. Level I is the least controversial: standard cosmological models predict an infinite universe where every possible arrangement of matter occurs somewhere, including an identical copy of you roughly 10^(10^29) meters away. Level II allows for regions with different physical constants and even different numbers of spatial dimensions, generated by cosmic inflation. Level III corresponds to the many branches of quantum mechanics, where every possible measurement outcome is realized. Level IV is the most radical: every self-consistent mathematical structure exists as its own physical reality, with its own laws of physics.
Whether or not you find this compelling, the hypothesis highlights something genuinely strange. Physics has become so mathematical that the boundary between “a model of reality” and “reality itself” has blurred almost beyond recognition. The equations don’t just predict what happens. In many cases, the equations are the most precise description of what exists.
The Simulation Possibility
Philosopher Nick Bostrom’s simulation argument approaches the question from a different angle. It doesn’t prove we live in a computer simulation, but it presents a logical trilemma: at least one of three propositions is almost certainly true. Either nearly all civilizations go extinct before developing the computing power to simulate conscious beings, or nearly all advanced civilizations choose not to run such simulations, or we are almost certainly living in a simulation right now. There is no fourth option. If you believe advanced civilizations are likely to exist and likely to be curious, you’re pushed toward the third conclusion.
The argument is harder to dismiss than it sounds because it rests on probability, not physics. If even a small fraction of advanced civilizations run detailed ancestor simulations, the number of simulated beings would vastly outnumber “real” ones. A randomly chosen conscious being would almost certainly be simulated. The discomfort most people feel with this conclusion doesn’t constitute a counterargument.
What We Know for Certain
Amid all this theoretical variety, a few things are well established. The geometry of the universe is flat to extraordinary precision. Data from the Planck satellite mission measured the curvature parameter at 0.001 ± 0.002, consistent with perfect flatness. This means the universe is not curved back on itself like a sphere, and will expand forever.
Nonlocality is real. Entangled particles violate Bell’s inequality in loophole-free experiments, ruling out any theory where reality consists of independent objects with pre-existing properties communicating through local signals. Whatever reality is, it is not a collection of separate things sitting in space, each minding its own business.
Perception is not a window onto truth. Evolution shaped your senses for survival, not accuracy. The colors, edges, and surfaces you experience are a species-specific interface, not a faithful copy of what exists.
Beyond these empirical anchors, the nature of reality remains genuinely open. The competing frameworks (information-based, mathematical, holographic, participatory, simulated) are not just armchair speculation. They are active research programs with testable predictions, internal consistency, and deep connections to established physics. What they share is a common theme: the solid, observer-independent, intuitively obvious world of everyday experience is almost certainly not fundamental. Something stranger lies beneath it, and we are still working out what it is.