Quantum gravity

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 wanna 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.

Any energy must cause spacetime curvature.

If not, you could build perpetual motion machines, for example, using the Casimir effect.

In quantum gravity, gravity itself becomes an excitation in our quantized spacetime.

The energy of those excitations should themselves precipitate more spacetime curvature represented as further excitations.

Uh, you know, d- d- Einstein's theory of gravity doesn't apply very easily to the quantum world, and quantum mechanics doesn't lend itself to the very large.

You know, there are, uh, exceptions, so Stephen Hawking famously came up with an equation that did include both quantum mechanics and general relativity, but it wasn't a unified, what we call a theory of quantum gravity just yet.

So the most famous example is string theory.

Yes.

[laughs] Uh, people who say they understand it don't.

They understand how to solve the mathematics.

At the moment, it's a very powerful mathematical theory, and it's one of the candidate ideas for a theory of quantum gravity, a so-called theory of everything, um, because the idea is that it wants to...

So quantum mechanics describes three of the four fundamental forces of nature.

The mission for this deep dive is, well, it's incredibly ambitious, but also deeply necessary.

We're looking at how a single elegant mathematical theory could simultaneously solve dark matter, dark energy and quantum gravity.

All at once.

All at once.

Well, the catch is a phantom.

Literally, it is known in physics as the ghost problem.

And it stems from the holy grail of modern physics, which is quantum gravity.

Right.

NASA awards something, this interesting name, Inflection.

They gave them $20 million to work on a quantum gravity sensor called the Quantum Gravity Radiometer Pathfinder, which is a mission to develop a space-based quantum gravity sensor.

Didn't know we needed that But apparently we do right Okay, I put that in my car.

What can I do with it, right? I'll fight a crime

Just basically not counting how many estates, so-called microstates, are the same big thing.

And this entropy, if it's real, it actually tells us something very deep about quantum gravity, not just quantum field theory in curved space then.

Because it's actually counting the total number of states that would be called the black hole by some outside observer.

And these states are states of this deeper theory that we don't have.

So we don't know what is the fundamental theory that unites quantum mechanics and gravity, but somehow magically we know how many states it has.

and and it's it's it's one of these magical things that if you want either depending on who you talk to for example if you talk to bill andrew he would say it's a fantasy or bob also some kind of traditional relativity said no it's not really entropy you don't really know my theory of quantum gravities but another most physicists say string theories for example they would they would tell you now this is this is our most direct clue into the nature of quantum gravity and In fact, a string theorist, I think to this day, believe that the biggest success of a string theory is they could explain where this number comes from, because the number that basically Hawking first, in fact, it was Beckenester and Hawking, they did complementary work that got this entropy from basically temperature and mathematics of general relativity.

A string theorist in the 90s managed to reproduce the same number from the mathematics of a string theory by counting basically the number of modes of strings, basically, in some versions of black holes.

Now, why did I mention this entropy thing? If you believe this entropy thing, in fact, the entropy formula says that the area of the horizon counts the number of states, the number of microstates.

Conjecture Institute Fellow Spotlight, Antonia Weber.

Both Ends of the Quantum Gravity Spectrum.

Author, Logan Shipkin.

For more, visit conjectureinstitute.org.

Both Ends of the Quantum Gravity Spectrum.

Discovering Physics' Most Infamous Clash.

Conjecture Institute fellow Antonia Weber's love of physics began in her high school class with problems whose solutions are well understood, such as, why don't electrons fall into the nucleus? Now she spends her days investigating problems at the cutting edge of fundamental physics.

With her work on the BMV experiment, Antonia contributed to our understanding of how quantum theory and gravity interface in the low-energy domain.