Why Quantum Gravity Doesn't Work

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Quantum mechanics and general relativity both describe nature with astonishing precision, but when we try to combine them using the same perturbative tools that work so well in quantum electrodynamics, the method breaks down.

In QED, the small value of the fine-structure constant ensures that higher-order corrections remain tiny, allowing us to make incredibly accurate predictions. For gravity, however, the effective coupling grows with the square of energy. As we approach the Planck scale, loop corrections become unsuppressed, renormalisation fails, and smooth spacetime stops being a useful description.

This does not mean physics itself collapses; it means our current mathematical framework reaches its limits. At low energies, quantum gravity works perfectly as an effective theory, giving small, well-defined corrections to Newton’s law. But at extremely high energies, perturbation theory loses predictive power, hinting that spacetime may be emergent from something deeper.

📚 Resources and References

Lectures on Particle Physics — David Tong
https://www.damtp.cam.ac.uk/user/tong/particle.html

Quantum Gravity at the Planck Length — Joseph Polchinski (SLAC Summer Institute Lectures)
https://arxiv.org/abs/hep-th/9812104

Particles, Feynman Diagrams and All That — Michael Daniel (IOP Physics Education)
https://doi.org/10.1088/0031-9120/41/2/001
https://faculty.washington.edu/seattle/physics541/%202010-reading/diagrams-3.pdf

Gravity as a Quantum Field Theory — Roberto Percacci (Symmetry)
https://www.mdpi.com/2073-8994/15/2/449

Quantum General Relativity and Effective Field Theory — John F. Donoghue
https://arxiv.org/abs/2211.09902

Introduction to Elementary Particles — David Griffiths (Wiley-VCH)
https://www.wiley.com/en-us/Introduction+to+Elementary+Particles%2C+2nd+Revised+Edition-p-9783527406012

String Theory (Cambridge Part III Lecture Notes) — David Tong
https://www.damtp.cam.ac.uk/user/tong/string.html

How Do Feynman Diagrams Work? — James Robert Brown (Perspectives on Science)
https://direct.mit.edu/posc/article/26/4/423/15455/How-Do-Feynman-Diagrams-Work

Why Quantum Gravity is Hard — David Wakeham
https://hapax.github.io/physics/hacks/qgrav/

Quantum Gravity — Carlo Rovelli (Cambridge University Press)
https://www.cambridge.org/core/books/quantum-gravity/9EEB701AAB938F06DCF151AACE1A445D

Introduction to the Effective Field Theory Description of Gravity — John F. Donoghue
https://arxiv.org/abs/gr-qc/9512024

QED: The Strange Theory of Light and Matter — Richard Feynman (Princeton University Press sample)
https://assets.press.princeton.edu/chapters/i2352.pdf

The Basics of Quantum Electrodynamics — Donie O’Brien (Trinity College Dublin)
https://www.maths.tcd.ie/~donie/includes/QEDintro.pdf

Quantum Gravity in Everyday Life — C.P. Burgess (Living Reviews in Relativity)
https://arxiv.org/abs/gr-qc/0311082

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