Loop Quantum Gravity Explained

SPEAKER_1 (0:00)

This episode is brought to you by Accenture. When you're advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales. Using automation, analytics, and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

SPEAKER_2 (0:31)

Strength is one of the most important investments in your long-term health. That's why I love Orange Theory Fitness.

Every one-hour Orange Theory workout helps you build muscle, burn fat, and take control of your health. Guided by a coach every step of the way. No wondering what to do next. No intimidating gym floor. Just strength built for life. Visit orange theory.com/spotify to learn more and get your first class free. Terms and conditions apply. See website and participating studio for details.

Matt O'Dowd (1:00)

It's time we talked about loop quantum gravity. What exactly is it? What are the loops? And can it really defeat string theory in our quest for a theory of everything?

The holy grail of physics is to connect our understanding of the tiny scales of atoms and subatomic particles with that of the vast scales of planets, galaxies, the entire universe.

To connect quantum physics with Einstein's general theory of relativity. Our search for a theory of quantum gravity is a century old, and we've talked about it quite a bit already, including what's probably the lead contender, string theory. But string theory isn't the only game in town, or so some physicists believe. There may be another way to reconcile the physics of the tiny and the gigantic, another way to a theory of quantum gravity that avoids a lot of the conceptual baggage, like tiny wiggling strings made of coiled up extra dimensions. That other way would be loop quantum gravity, and today we're going to find out exactly what it is. Back in the day, we talked about why combining quantum mechanics and general relativity was so hard. For example, there's the fact that general relativity, or perhaps quantum mechanics, breaks down when we think about the extreme densities of the black hole or the big bang singularities. But there are more fundamental conflicts, namely, background independence and the problem of time. Now, I'll mention the problem of time briefly in a little bit, but the real focus is going to be on this background independence thing, because this is what really inspired the invention of loop quantum gravity. So what is it? Quantum mechanics, and indeed most theories in physics, involve a set of equations describing how stuff moves around, exerts force, etc.

on some background coordinate system, like actors on a stage, where the actors are particles and wavefunctions and fields, and the stage is the coordinates of space and time. In quantum mechanics, that stage is flat and static and isn't influenced by the actors. It requires some giant hacks to even attempt regular quantum calculations in a non-flat geometry.

In short, quantum mechanics is not background independent. General relativity, on the other hand, has to be background independent, because that's what its equations do. They change the background. They describe how the presence of mass and energy warp the fabric of space-time. Our background coordinate system itself becomes a dynamic entity. More precisely, the metric, the object encapsulating the geometry and the possible structure of space-time, evolves in the equations of GR. So those equations need to work regardless of that background. In string theory, a type of background independence emerges in an abstract space of moving.

SPEAKER_1 (4:00)

This episode is brought to you by Accenture. When your advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales, using automation, analytics and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

SPEAKER_2 (4:31)

Strength is one of the most important investments in your long-term health. That's why I love Orange Theory Fitness.

Every one-hour Orange Theory workout helps you build muscle, burn fat, and take control of your health, guided by a coach every step of the way. No wondering what to do next. No intimidating gym floor. Just strength built for life. Visit orangetheory.com/spotify to learn more and get your first class free. Terms and conditions apply. See website and participating studio for details.

Matt O'Dowd (5:00)

Strings, and with that comes a gravitational field. But for that to work, first you need those strings to exist, and we don't know if they do.