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Quantum gravity: the missing link
· Open question
Open questionNobody knows the answer.
The still-missing theory that would describe gravity using the rules of quantum mechanics.
See it on the Science map →How it works
General relativity treats spacetime as smooth and exact. Quantum theory says everything is fuzzy and probabilistic. Each works superbly in its own domain, but where both matter (black hole centers, the Big Bang) they give nonsense together. Treating gravity like the other forces produces infinities that cannot be removed. Leading candidates include string theory and loop quantum gravity; none has been confirmed.
Why it matters
Without it we cannot describe the birth of the universe, the inside of black holes, or say what space and time fundamentally are.
Think of it like
Like two leases for the same space written under different laws; each is valid alone, but they conflict at the shared wall.
How we know
No direct evidence yet. Its effects should appear around the Planck scale, about a quadrillion times beyond the energies of the Large Hadron Collider. Tabletop tests of gravity and entanglement are proposed.
Still a mystery
Is spacetime grainy, made of strings, or emergent from quantum information? Will any experiment ever decide?
Connections (8)
▸General relativity: gravity is curved spacetime clashes with quantum theory in Quantum gravity: the missing linkOpen question
The relationship
General relativity treats spacetime as a smooth, exact stage that bends. Quantum theory treats everything as fuzzy and uncertain. Gravity's source is energy, and in quantum theory energy fluctuates, so spacetime itself should fluctuate. When physicists apply the standard quantum recipe to gravity, the calculations produce infinities that cannot be removed. The two theories cannot both be right as written at very small scales.
How the link was found
The problem was recognized in the 1930s. 't Hooft and Veltman (1974) and Goroff and Sagnotti (1986) proved that the straightforward quantum version of gravity breaks down.
Is it proven?
The infinities are mathematical results, widely checked. In practice the clash only matters at extreme scales like black hole centers and the Big Bang, where no experiment can yet reach.
Open questions and mysteries
Which theory has to give way: quantum mechanics, relativity, or both? Is spacetime itself emergent from something deeper?
▸The black-hole information paradox is a test case for Quantum gravity: the missing linkOpen question
The relationship
The paradox arises exactly where gravity and quantum theory both matter, so solving it means knowing, at least in part, how they combine. Any proposed theory of quantum gravity must explain what happens to information in an evaporating black hole. It works as a stress test: a thought experiment precise enough to rule out candidate ideas that answer it wrongly.
How the link was found
Clear from Hawking's 1976 paper; Juan Maldacena's 1997 AdS/CFT correspondence made it a concrete test, offering a setting where gravity is equivalent to ordinary quantum theory.
Is it proven?
AdS/CFT strongly indicates information is preserved, at least in simplified model universes. That is mathematical evidence, not an experiment, and our universe differs from those models.
Open questions and mysteries
Does the AdS/CFT lesson carry over to our expanding universe? What does it say spacetime is made of?
▸Singularities: where physics breaks demands Quantum gravity: the missing linkOpen question
The relationship
Relativity predicts infinite density at singularities, which signals that the theory has been pushed past its limits. At such extreme compression, quantum effects must dominate, so a description that includes both is needed. Physicists expect quantum gravity to replace the infinity with something finite, much as quantum mechanics explained why electrons do not spiral into atomic nuclei.
How the link was found
The need became sharp after the Penrose (1965) and Hawking–Penrose (1970) theorems showed singularities are unavoidable in classical relativity.
Is it proven?
That relativity fails there is mathematically clear. That quantum gravity fixes the problem is a widely held expectation, not yet demonstrated.
Open questions and mysteries
Will singularities be smoothed into a bounce, a smallest grain, or reveal that space and time are not fundamental?
▸The Planck scale: the smallest meaningful size is where we need Quantum gravity: the missing linkOpen question
The relationship
At ordinary scales, gravity is negligible for particles and quantum effects are negligible for planets. At the Planck scale, about 10^-35 meters or 10^19 times a proton's mass-energy, both become strong at once. Neither theory alone can describe what happens there. Quantum gravity is, by definition, the theory needed to describe physics at this scale.
How the link was found
Planck defined the units in 1899; Bronstein (1936) and later John Wheeler in the 1950s argued they mark where gravity becomes quantum.
Is it proven?
Simple dimensional reasoning makes it strongly expected. Direct test is far beyond reach: the Planck energy is about a quadrillion times what the Large Hadron Collider produces.
Open questions and mysteries
Could quantum gravity effects show up at lower energies, perhaps through subtle effects on light from distant explosions?
▸Quantum gravity: the missing link has a candidate in String theorySpeculative
The relationship
String theory replaces point particles with tiny strings. One vibration of a closed string behaves exactly like the graviton, so gravity appears automatically. Because strings have size, they smear out the point-like collisions that create infinities when gravity is quantized naively. The theory appears to give finite answers where the straightforward approach fails, making it the most studied quantum-gravity candidate.
How the link was found
Scherk and Schwarz, and Yoneya, in 1974 noticed strings contain a graviton; Green and Schwarz's 1984 work made the theory consistent.
Is it proven?
No experimental confirmation. Evidence is mathematical: finite calculations, the 1996 black-hole entropy match, and AdS/CFT. Its main successes are in idealized settings.
Open questions and mysteries
It is best defined for universes unlike ours, with negative vacuum energy. How to apply it to our expanding universe is open.
▸Quantum gravity: the missing link has a candidate in Loop quantum gravitySpeculative
The relationship
Loop quantum gravity takes a different route: instead of adding new ingredients, it applies quantum rules directly to the geometry of space in general relativity. The result is a space built from discrete chunks of area and volume. It aims to remain faithful to relativity's principle that space is not a fixed stage but dynamic.
How the link was found
Abhay Ashtekar's 1986 reformulation of relativity; Carlo Rovelli and Lee Smolin developed loop methods from 1988 onward.
Is it proven?
No experimental confirmation. It gives discrete area and volume and a finite entropy for black holes, but has not shown that ordinary smooth space emerges at large scales.
Open questions and mysteries
Can it reproduce general relativity at everyday scales? Can it include the other forces of nature?
▸Quantum gravity: the missing link is needed to answer What came before the Big Bang?Open question
The relationship
Relativity predicts the universe began from a singularity, where the theory itself fails. To ask what came before, or whether "before" means anything, requires a theory that works in that extreme. Proposals built on quantum gravity give different answers: a bounce from an earlier universe, time emerging from space, or a quantum birth from nothing.
How the link was found
Discussed since the 1960s; concrete proposals include Hartle and Hawking's no-boundary idea (1983), loop quantum cosmology's bounce (2000s), and string-based cyclic models.
Is it proven?
None of the proposals is confirmed. Some predict faint patterns in the cosmic microwave background or primordial gravitational waves that future telescopes might look for.
Open questions and mysteries
Is the question even meaningful? Would quantum gravity allow any information about "before" to reach us?
▸Quantum gravity: the missing link is needed to describe Planck epoch: the first 10⁻⁴³ secondOpen question
The relationship
In the Planck epoch, energy is packed so densely that gravity is as strong as quantum jitter. Spacetime cannot be treated as a fixed stage; the stage itself fluctuates. Only a theory that makes gravity quantum could describe it, including whether time had a beginning. Candidates tell different stories: loop quantum gravity suggests a bounce, string theory offers colliding branes, and Hartle and Hawking a rounded-off start.
How the link was found
Bryce DeWitt, working with John Wheeler, wrote the first equation for the quantum state of a whole universe in 1967 (the Wheeler–DeWitt equation). James Hartle and Stephen Hawking applied it to the beginning in 1983.
Is it proven?
Not proven. There is no accepted quantum gravity theory and no observation from this era. The need follows from the Planck-scale argument.
Open questions and mysteries
Can any relic of this era survive inflation to be measured? If not, the earliest moment may stay permanently beyond observation.