Why haven’t we
unified physics?

Two theories work astonishingly well

Modern physics has two main frameworks for describing fundamental interactions. Each explains an enormous range of observations.

For most calculations, we can use whichever framework matters most. Gravity between individual particles is extremely weak, so particle experiments can usually ignore it. For a planet’s orbit, quantum effects on its gravitational field are negligible.

This separation is why physics can be so successful without a final unified theory. We do not need to solve quantum gravity to explain an atom or navigate a spacecraft. CERN’s overview of the Standard Model describes this division.

Nature does not keep them separate everywhere

A black hole can compress matter into conditions where both gravity and quantum effects should matter. Tracing cosmic expansion backward also takes us toward an early universe where our present approximations become unreliable.

In those situations, choosing one framework and ignoring the other stops being enough. We need a description that can handle both at once. That is the problem of quantum gravity.

The difficulty is built into what gravity is

Ordinary quantum field theory describes fields on spacetime. It uses that spacetime to say where events occur, how far apart they are and which events can influence others. General relativity makes spacetime itself a physical system that changes.

Imagine matter in a quantum superposition of two different locations. Each location would correspond, classically, to a different gravitational field. What describes the gravity of the superposition? If geometry also behaves quantum mechanically, the distances and times used to describe the experiment become part of the quantum problem.

We therefore face a deeper task than adding one more particle to a list: the framework must describe the quantum behavior of the arena in which the other particles live.

What “unified” means here

A complete account of fundamental physics needs gravity and quantum matter to fit together consistently. Quantum gravity is a necessary part of that goal. It would not automatically explain every particle mass, solve dark matter or prove that all forces are manifestations of one force.

The obvious calculation works—up to a point

Physicists can quantize small disturbances of gravity and calculate their effects at low energies. We already have a useful, limited quantum theory of gravity.

This is an effective theory: a description organized for a certain range of conditions. Its leading terms give Einstein’s gravity, with smaller corrections added as needed. At any chosen low-energy accuracy, only a limited number of ingredients matter. Donoghue’s work explains why this remains predictive.

The trouble appears when we try to extend the same calculation to arbitrarily high energies. Increasingly many corrections become important. Straightforward quantization requires an endless sequence of additional terms, so it no longer gives a complete high-energy theory from a finite set of measured inputs. Physicists call this non-renormalizability.

It tells us that this method has reached its limit. It does not tell us what comes next. New ingredients might appear, spacetime might have a deeper description, or gravity might behave consistently in a regime this approximation cannot capture.

Internal problems tell us where to look

General relativity can predict singularities: situations in which its description of spacetime becomes incomplete. Black-hole evaporation raises another puzzle. Quantum calculations predict radiation, but the original treatment appears to lose information about what formed the hole, conflicting with ordinary quantum evolution.

These are powerful theoretical clues. We have not observed a black-hole singularity or the complete evaporation of an astrophysical black hole. A calculation failing in an inaccessible regime does not supply the same guidance as a reproducible measurement of how nature behaves there. Harlow’s lectures explain the black-hole problem.

The missing ingredient is experimental feedback

New theories become much easier to develop when nature supplies a specific discrepancy to explain. A result tells physicists which assumptions to question and gives them something against which to test the replacement.

That is one common route. Another begins with a theoretical prediction and ends with an experiment capable of looking for it. Both routes eventually depend on observations that distinguish alternatives.

Electromagnetic waves, antimatter and gravitational waves also show the power of predictions that eventually meet experiments. The routes and waiting times differed; a preexisting anomaly was not required in every case.

Quantum gravity has a weaker connection between proposals and tests

We have strong reasons to expect our description to be incomplete, and many mathematical ideas about its replacement. What we lack is a widely accepted observation that clearly isolates quantum gravity and selects among those ideas.

It would be too strong to say that there are no testable proposals. Specific models can be constrained, and experiments are being developed. The difficulty is obtaining a prediction that is distinctive, measurable and convincingly tied to the deeper theory. The quantum-gravity phenomenology roadmap surveys these efforts.

If several approaches reproduce everything we can currently measure, agreement with those measurements cannot choose between them. Mathematical consistency eliminates some possibilities; it has not uniquely identified nature’s answer.

The clearest effects may lie far beyond our reach

Quantum effects on spacetime are expected to become strong near the Planck scale. This scale comes from combining the constants governing quantum mechanics, gravity and the speed of light.

A GeV is a unit of particle energy. Higher collision energies generally let us probe shorter distances. The Planck length is about 1.6 × 10⁻³⁵ meters. It is a theoretical scale, not an experimentally established smallest unit of space. NIST and CERN give these scales; CERN’s LHC overview gives the collider benchmark.

That gap makes direct access extraordinarily difficult. And the comparison is generous: a proton’s collision energy is shared among its constituents, so the energy available to a particular elementary collision is lower.

The Planck scale is not a rule saying that every observable effect must wait until that energy. Some models predict effects at lower energies, tiny effects might accumulate across cosmic distances, and precise laboratory experiments could test gravity’s quantum character. But nature has not guaranteed an accessible signal that distinguishes the underlying theories.

This creates a frustrating situation: the existing theories work best where we can test them, while their deepest problems arise where we have the least access.

We already know physics is incomplete in other ways

The absence of a decisive quantum-gravity signal does not mean that all observations fit a finished picture. Several gaps are already clear.

See CERN on neutrino masses, ATLAS on dark matter and CERN on the Standard Model’s limits.

These are different kinds of incompleteness. Neutrino masses already provide a measured departure from the minimal Standard Model. Dark matter provides gravitational evidence for a missing component. Neither observation, by itself, tells us how to quantize spacetime.

A theory could explain neutrino masses while leaving quantum gravity untouched. Another could solve part of quantum gravity while saying little about dark matter. We do not yet know how many of these problems share a solution.

What would let us make progress?

We need a tighter connection between the questions a theory answers and the measurements an experiment can make.

A reproducible anomaly that narrows the possibilities

A useful discrepancy must survive checks for instrumental errors and ordinary physical explanations. It must also constrain the replacement: saying “something is missing” leaves far more possibilities than measuring exactly how a prediction fails.

A distinctive prediction we can actually test

A candidate theory becomes much more informative when it predicts something that competing accounts do not, within reach of an experiment. Fitting known results is essential, but many different microscopic theories can share the same large-scale behavior.

A smaller experiment that answers a smaller question

One proposal asks whether the gravitational interaction between two tiny masses can make them quantum-entangled—give them correlations that cannot be described by independent quantum states. With other interactions excluded and the interpretation’s assumptions justified, that could constrain classical accounts of gravity. It would not by itself reveal the final theory. Bose and colleagues’ proposal illustrates this approach.

Astrophysical observations and cosmology offer other tests of particular models. These are active research routes, with difficult interpretation problems. The 2025 roadmap describes their possibilities and limits.

Knowing that a theory must be incomplete is much easier than discovering the specific way nature goes beyond it.

That is why quantum gravity remains open. We can identify deep consistency problems. We have candidate ideas. We still lack the combination of a sufficiently complete theory and decisive experimental guidance that would let us identify the answer.

At the edge of the question: could spacetime emerge?

The proposed answers differ in what they keep and what they replace. String theory introduces extended quantum objects whose excitations include gravity. Loop quantum gravity explores quantum states of geometry. Other approaches ask whether gravity itself becomes well behaved at high energies. These remain research programs, with different achievements and unresolved problems. A comparative review surveys their cosmological connections.

Why people speak of string theory’s “failure”

String theory raised hopes that a single framework could explain gravity, particles and their interactions. After decades of work, it has not produced a distinctive, experimentally confirmed prediction establishing it as the theory of our universe. One obstacle is its vast “landscape” of possible solutions: different ways of arranging its extra dimensions and fields can produce different low-energy physics. Without a compelling way to select the solution describing our universe, deriving a unique set of observable predictions becomes difficult. CERN’s discussion of the landscape explains this problem.

The failure is to fulfill that ambition so far. Experiments have not disproved the whole framework, and its mathematical achievements do not establish that nature uses it. String theory has supplied important tools for studying black holes and connections between gravity and quantum physics, including holography. Its unresolved status illustrates the central challenge of this page: a framework can yield deep insights while still lacking the experimental evidence needed to identify it as the correct description of reality. A report from Strings 2024 describes both the progress and the predictive gap.

Could geometry itself be a consequence?

One more radical possibility is that spacetime is a collective phenomenon. Temperature emerges from the behavior of many molecules; perhaps distances and geometry emerge from relationships among deeper quantum variables. We do not yet know what those variables would be in our universe.

Holography gives a concrete reason to investigate this. In special theoretical models, a quantum theory without gravity corresponds to a gravitational spacetime with an extra dimension. Some entanglement properties of the quantum system correspond to geometric areas in that spacetime. Maldacena’s proposal and Ryu and Takayanagi’s relation establish the setting for this work.

Those models do not yet give a demonstrated description of our universe. They show that the question “What is spacetime made of?” can have mathematical substance. A future theory might retain pieces of these ideas, but expecting a synthesis is speculation.

The obstacle remains the same: an intriguing explanation must eventually become a sufficiently precise theory that nature can confirm or rule out.

Terms used on this page