Is There a Theory of Everything, and What Would a Graviton Be?

Short answer: not yet. There is no accepted theory of everything, and the graviton is a hypothetical particle that nobody has detected. This post is about an open question, and we label it that way throughout. It follows our explainer Do Black Holes Evaporate?, because black holes are where gravity and quantum physics are forced to meet. Our Graviton Whisper tee, a quantum-gravity design, is drawn from exactly this question.

What would a theory of everything have to do?

At minimum, describe all four fundamental forces in one consistent framework. Today they live in two. CERN puts it plainly: the Standard Model covers the electromagnetic, strong and weak forces, but “gravity, is not part of the Standard Model, as fitting gravity comfortably into this framework has proved to be a difficult challenge” (CERN^[1]). Gravity is described by Einstein’s general relativity instead. We cover the four forces in The Four Fundamental Forces.

The Stanford Encyclopedia of Philosophy calls quantum gravity “a physical theory (still ‘under construction’ after over 100 years) incorporating both the principles of general relativity and quantum theory” (Weinstein & Rickles, SEP^[2]). A theory of everything would go further and include the other forces too.

What is a graviton?

In the Standard Model, each force has a carrier particle: the gluon, the photon, and the W and Z bosons. CERN adds: “Although not yet found, the ‘graviton’ should be the corresponding force-carrying particle of gravity” (CERN^[1]). The SEP describes it as “the hypothetical massless, spin-2 particle responsible for mediating gravitational interactions” (SEP^[2]).

Haven’t gravitational waves already shown gravitons exist?

No. LIGO and Virgo detect gravitational waves as ripples in spacetime, the way a radio picks up a classical radio wave. That does not register single gravitons. The waves do test graviton properties, though:

• Mass. Combining binary black-hole events from GWTC-3, LIGO, Virgo and KAGRA bounded the graviton mass to m_g ≤ 2.42 × 10⁻²³ eV/c² at 90% credibility, assuming gravitons disperse like massive particles (LVK 2025^[13]). An earlier, looser bound from GW170104 alone was 7.7 × 10⁻²³ eV/c² (LIGO/Virgo 2017^[3]).
• Speed. From the neutron-star merger GW170817, gamma rays arrived (+1.74 ± 0.05) s after the gravitational waves. That limits the difference between the speed of gravity and the speed of light to between −3 × 10⁻¹⁵ and +7 × 10⁻¹⁶ times the speed of light (Abbott et al. 2017^[4]).

Both results agree with a massless graviton moving at the speed of light, but neither shows that gravitons exist.

Could we ever detect one graviton?

Possibly never. Freeman Dyson questioned whether any conceivable experiment could detect a single graviton. Tony Rothman and Stephen Boughn found that an idealized thought experiment could, but “when anything remotely resembling realistic physics is taken into account, detection becomes impossible, indicating that Dyson’s conjecture is very likely true” (Rothman & Boughn 2006^[5]).

Why is gravity so hard to unify?

It is absurdly weak

CERN notes that “a small fridge magnet is enough to create an electromagnetic force greater than the gravitational pull exerted by planet Earth” (CERN^[6]). Compare the electric repulsion and gravitational attraction between two protons. Both fall off as 1/r², so the distance cancels:

F_e/F_g = e²/(4πε₀ G mₚ²) ≈ 1.24 × 10³⁶

Here e = 1.602 176 634 × 10⁻¹⁹ C is the proton’s charge (NIST^[7]), ε₀ is the vacuum permittivity (8.854 × 10⁻¹² F/m), G is Newton’s gravitational constant and mₚ = 1.672 621 925 95 × 10⁻²⁷ kg is the proton mass (NIST^[8]). The electric force wins by about a trillion trillion trillion.

The usual method gives infinities

The obvious approach, treating gravity like the other forces and quantizing it, “turned out that the theory is not perturbatively renormalizable, meaning that there are ineliminable infinities” (SEP^[2]). The tricks that tame infinities in quantum electrodynamics do not work for gravity.

The test energies are out of reach

Quantum gravity effects are expected near the Planck scale, set by the Planck length:

ℓ_P = √(ħG/c³) ≈ 1.616 × 10⁻³⁵ m

where ħ is the reduced Planck constant and c the speed of light. NIST gives ℓ_P = 1.616 255 × 10⁻³⁵ m (NIST^[9]) and the Planck energy as 1.220 890 × 10¹⁹ GeV (NIST^[10]). The Large Hadron Collider collides protons at 13.6 TeV, “the highest energy ever reached in a Lab” (CERN^[11]). The Planck energy is about 9 × 10¹⁴ times higher. The SEP says this scale is so remote that testing quantum gravity “along standard lines is rendered near-impossible,” though it notes recent work suggesting the outlook may be more optimistic (SEP^[2]).

What are the main candidate theories?

According to the SEP, “The two most popular approaches remain string theory and loop quantum gravity” (SEP^[2]). In string theory, particles are vibration modes of tiny strings, and one mode corresponds to the graviton. Loop quantum gravity instead aims at a quantum spacetime directly; the SEP notes it is often claimed that “the discreteness of area and volume operators are concrete predictions of the theory” (SEP^[2]). Neither approach has been confirmed by experiment.

Grand unification is a smaller, testable step

Grand Unified Theories (GUTs) try to join only the strong, weak and electromagnetic forces, leaving gravity out. Super-Kamiokande’s collaboration writes that GUTs “predict the convergence of the electromagnetic, weak, and strong interaction couplings at energies around ∼10¹⁶ GeV,” and that they “generically predict transitions” between quarks and leptons that make protons decay (Super-Kamiokande 2020^[12]). No proton decay has been seen. Their 2020 limit for the decay p → e⁺π⁰ is a partial lifetime longer than 2.4 × 10³⁴ years at 90% confidence (Super-Kamiokande 2020^[12]).

Extra dimensions at the LHC

One idea for gravity’s weakness is that “part of it spreads to extra dimensions.” If a collision made a graviton that escaped into one, detectors would see missing energy; CERN says physicists would then need to study it to tell “whether it is a graviton escaping to another dimension or something else” (CERN^[6]). No such signal is established.

Open point: where things stand

Unknown, and we will not pretend otherwise. No theory of everything is accepted, the graviton is hypothetical, proton decay is unseen, and the Planck scale is far beyond any collider. What we do have are hard limits: a graviton, if it exists, has a mass below about 2.42 × 10⁻²³ eV/c², and gravity travels at the speed of light to about one part in 10¹⁵. Black holes, where Hawking radiation mixes the two theories, remain the sharpest puzzle; see the black hole information paradox.

FAQ

Has the graviton been discovered?

No. CERN calls it “not yet found,” and the SEP calls it hypothetical. Gravitational waves are detected as classical waves, not as single gravitons.

Is string theory a theory of everything?

It is a candidate. It includes a graviton among its vibration modes, but no experiment has confirmed it.

What is the difference between a GUT and a theory of everything?

A GUT unifies the strong, weak and electromagnetic forces only. A theory of everything would also include gravity.

Why is gravity the odd one out?

It is far weaker than the others (about 10³⁶ times weaker than electric repulsion between two protons), the standard quantization gives infinities, and its quantum effects sit at energies far beyond experiments.

References

1. CERN, “The Standard Model”. https://home.cern/science/physics/standard-model
2. S. Weinstein & D. Rickles, “Quantum Gravity,” Stanford Encyclopedia of Philosophy (first published 26 Dec 2005; revised 26 Feb 2024). https://plato.stanford.edu/entries/quantum-gravity/
3. LIGO Scientific and Virgo Collaborations, B. P. Abbott et al. (2017), “GW170104: Observation of a 50-Solar-Mass Binary Black Hole Coalescence at Redshift 0.2,” Phys. Rev. Lett. 118, 221101 (arXiv:1706.01812). https://arxiv.org/abs/1706.01812
4. LIGO Scientific Collaboration, Virgo Collaboration, Fermi GBM & INTEGRAL, B. P. Abbott et al. (2017), “Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A,” ApJL 848, L13 (arXiv:1710.05834). https://arxiv.org/abs/1710.05834
5. T. Rothman & S. Boughn (2006), “Can Gravitons Be Detected?,” Found. Phys. 36, 1801–1825 (arXiv:gr-qc/0601043). https://arxiv.org/abs/gr-qc/0601043
6. CERN, “Extra dimensions, gravitons, and tiny black holes”. https://home.cern/science/physics/extra-dimensions-gravitons-and-tiny-black-holes
7. NIST CODATA 2022 recommended values: elementary charge. https://physics.nist.gov/cgi-bin/cuu/Value?e
8. NIST CODATA 2022 recommended values: proton mass. https://physics.nist.gov/cgi-bin/cuu/Value?mp
9. NIST CODATA 2022 recommended values: Planck length. https://physics.nist.gov/cgi-bin/cuu/Value?plkl
10. NIST CODATA 2022 recommended values: Planck mass energy equivalent in GeV. https://physics.nist.gov/cgi-bin/cuu/Value?plkmc2gev
11. CERN, “The Large Hadron Collider” (key facts). https://home.cern/science/accelerators/large-hadron-collider
12. Super-Kamiokande Collaboration, A. Takenaka et al. (2020), “Search for proton decay via p→e⁺π⁰ and p→μ⁺π⁰ with an enlarged fiducial volume in Super-Kamiokande I–IV,” Phys. Rev. D 102, 112011 (arXiv:2010.16098). https://arxiv.org/pdf/2010.16098
13. LIGO Scientific, Virgo and KAGRA Collaborations, R. Abbott et al., “Tests of General Relativity with GWTC-3,” Phys. Rev. D 112, 084080 (2025) (arXiv:2112.06861). https://arxiv.org/abs/2112.06861

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.