Quantum Gravity and the Hardest Problem in Physics | Space Time

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Matt O'Dowd (1:01)

Between them, general relativity and quantum mechanics seem to describe all of observable reality. And yet, they can't be simultaneously true. They must be united in a deeper yet undiscovered theory.

After a century of work by the greatest minds in all of physics, why does this union still elude us?

The first few decades of the 20th century was a time of miracles for physics. First, Einstein's relativity utterly changed the way we think about space, time, motion and gravity. Then, the quantum revolution of the 20s and 30s overturned all of our intuitions about the subatomic world. Together, general relativity and quantum mechanics have allowed us to explain nearly every fundamental phenomenon observed, and they have predicted many unexpected phenomena that have since been verified.

And yet, these two theories contradict each other in fundamental ways. In the century since that golden era of physics, we've been trying to reconcile the two without success.

But today on Space Time, I'm going to begin our discussion of the great quest for this union, the quest for a theory of quantum gravity, and for a theory of everything. This is a big topic, so in this episode, I want to give you the motivation. What exactly are the conflicts between general relativity or GR and quantum mechanics? I'll save the solutions for future episodes. Let's start with summaries. General relativity, GR, is Einstein's great theory of gravity. In it, the presence of mass and energy warp the fabric of space and time, and the motion of objects is thereby altered. This results in the effect we perceive as gravity.

General relativity incorporates the earlier special relativity, which describes how our perceptions of space and time also depend on motion. Unlike the earlier ideas of Isaac Newton, in which space and time are treated as separate and universal, special and general relativity blend them together into a combined mutable space-time. Where general relativity describes the universe of the large and the massive, quantum mechanics talks about the subatomic world. It describes particles as waves of infinite possibility whose observed properties are intrinsically uncertain. Our experience of the universe appears to be plucked from this landscape of possibilities in strange but mathematically predictable ways. That math started with the Schrödinger equation, which tracks these probability waves through space and time. But the Schrödinger equation treats space and time as fundamentally separate in the old-fashioned Newtonian way.

So clearly there's a problem.

We already talked about how Paul Dirac fixed part of the problem with a relativistic wave equation for the electron. Nowadays, modern quantum field theories fully incorporate the melding of space and time predicted by special relativity. And yet, they still don't directly incorporate the warping of space and time predicted by general relativity. This causes issues, some mild and fixable, others catastrophic. Starting with the mild, we have the black hole information paradox. We've gone on about that at length. The black holes of pure general relativity swallow information in a way that can remove it completely from the universe. Especially when those black holes evaporate via Hawking radiation. That's a big conflict with quantum theory right there, which tells us that quantum information should never be destroyed. But that same Hawking radiation offers part of the solution to the information paradox. Following the work of Hawking, Jacob Bekenstein, Gerardo Twift and others, it has...

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