Quantum gravity: we explore spin foams and other potential solutions to this enduring challenge

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Hello, and welcome to the Physics World weekly

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podcast.

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I'm Hamish Johnston.

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Earlier this autumn, I had the pleasure of

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visiting the Perimeter Institute

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for Theoretical Physics in Waterloo, Canada,

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where I interviewed four physicists

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about their research.

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This is the second of those conversations

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to appear on the podcast,

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and it's with Bianca Dietrich,

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whose research focuses on quantum gravity.

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Our chat is coming up after this message

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from IOP publishing,

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research highlights into your favorite search engine,

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or follow the link in the notes for

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this podcast.

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Albert Einstein's

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general theory of relativity does a great job

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at explaining gravity,

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but it is thought to be incomplete

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because it is incompatible with quantum mechanics,

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which itself

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is widely considered

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to be one of science's most successful

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theories.

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Developing a theory of quantum gravity is a

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crucial goal in physics,

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but it's proving to be extremely difficult.

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We discuss the challenges

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and the possible ways forward

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in this podcast.

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I'm at the Perimeter Institute in Waterloo,

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Ontario,

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and I'm very pleased to be joined by

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Bianca

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Dietrich.

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Hi, Bianca. Welcome to the podcast.

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Hi, Amish. Very nice talking to you.

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And, Bianca, we're going to talk about quantum

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gravity,

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which I have to admit is probably the

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one

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thing in physics that I know very little

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about. So you have you probably have to

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be very patient with me

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about this.

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So quantum gravity is is

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a very important field of research,

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particularly because we don't have a theory of

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quantum gravity, do we? So

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why don't we have one? Is why is

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it so difficult?

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Yeah. So that's correct.

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We possibly don't have a theory of quantum

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gravity

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that, you know, enough people are happy with.

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I don't know.

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And so there's a well, there's a number

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of reasons you might have to stop me.

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Gravity turns out is, is very different from

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the three other interactions.

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We know of the strong,

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interaction,

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the weak interaction,

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and the electromagnetic

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interaction.

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I mean, one big reason is that it

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has a much larger and more complicated,

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symmetry

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group. There's

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a,

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independence of, choice of coordinates or diffeomorphism

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group,

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compared to, you know, just kind of Poincare

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asymmetries

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and,

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and,

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gauge series

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gauge symmetries of particle physics.

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And,

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indeed,

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gravity

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conceptually

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is

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according to Einstein's theory

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about the geometry of space and time.

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And it turns out because of this

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kind of invariance and the coordinates, you know,

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which is not

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asymmetry the other interactions

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do have. So usually, we quantize them on

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a fixed space and time background.

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But in the case of gravity, we don't

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have a fixed space and time background. It's

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a dynamical entity.

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And so there are many deep conceptual questions.

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And in fact,

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it's that if we do have a quantum

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theory of of gravity,

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we do also expect that we will have

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a quantization of space and time.

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And so that would kind of completely change

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our notion of order space and time.

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It also shows us that we possibly

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well, we need possibly new quantization techniques.

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And in fact,

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we could not apply many of the techniques

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which are which we rely very heavily on

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in the case of standard quantum field theory,

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so for the other interactions.

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To give you some examples, we know that

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if we, you know, perturbative quantum field theories

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that was used for a long time to

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treat particle physics,

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This field, in the case of gravity,

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for for some technical reasons, usually is expressed

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as not renormalizable.

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Another example is that, what what we also

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use a lot

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for particle physics or for condensed matter

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is a a very,

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a trick which is

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very useful,

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and that's that we,

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rotate

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the time

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in,

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in the complex plane, the so called rotation.

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And that allows us,

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you know, that turns

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series where we have complex amplitudes in the

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sense of,

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amplitudes with the phase where we have interference

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and so on to turn into statistical amplitudes.

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And,

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that allows us to do Monte Carlo simulations.

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And, you know, QCD,

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well, is is more nonperturbative

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than, say, electromagnetisms,

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but to understand QCD, what do we do?

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Latest simulations versus Monte Carlo simulations.

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If you do not have access to to

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these,

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Monte Carlo simulations, for instance, we kind of

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get into problems which are also hard to

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solve in the case of QCD,

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which is kind of,

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if you if you do want to do

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real time simulations. So that is also a

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big issue in condensed matter and and QCD

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or the sign problem in QCD.

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And it is part of the problem also

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that

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we don't have

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experimental

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data. You can't test your theory by doing

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an experiment like you can in I mean,

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on one side matter or particle physics.

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You know, on one side, we have lots

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of experimental data because we know that's a

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road exists.

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I will come to that.

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But we don't have so much new input,

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which we cannot explain by the standard series.

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So it's really astonishing that, like, standard cosmology,

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you know, which, to some degree is is

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based on,

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linearized quantum field theory on curves based.

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We can explain

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lots of our observations

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in cosmology and that,

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even so we know, okay, there's a big

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bang and

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and,

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but we would need to go to higher

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order and perturbation theory, for instance, to to

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see effects from quantum gravity.

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So this is certainly true, but one example

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is that we have actually already observations,

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in the kind of Planck regime where we

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do where where some series, for instance, predicted

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that something could happen,

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in particular with regard to kind of Lorentz

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breaking

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and possible Lorentz deformation. And so there were

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kind of

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results from looking at gamma ray bursts,

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which are like gamma rays coming from

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sources which are very far away.

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And so there you have these gamma photons,

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which travel very long distances.

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And so the idea is there because they

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travel long distances, they have a long time

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to accumulate effects which come from having some,

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for instance, quantum space time.

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But the simplest models were basically ruled out.

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I mean, the kind of

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but the regime was indeed basically that they

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could already test

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planck scale physics.

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So that's certainly

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well, I mean, if you would have more

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of an experimental

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input, yeah, we

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possibly, that would reduce, the space of possibilities.

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And,

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I guess human fantasy is very large, so

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it's kind of many possibilities are explored.

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But I want to also express that, you

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know, quantum gravity is not useless.

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So I think first quantization attempts go back

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to the nineteen twenties.

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But an enormous

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part of

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what have became standard techniques

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in in modern physics

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has has started with the motivation

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of gravity or quantum gravity.

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You know, general relativity was the first example

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of a gauge theory.

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And so, like,

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techniques for the quantization of gauge theories

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were developed

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with, with motivation to do quantum gravity.

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The Newton Newton developed her

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formalism, the Newton serum,

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because she was asked by Herbert about gravity.

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So there are lots of examples.

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A more recent one is a topological phases

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in condensed matter.

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So first topological model was written down in

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1968

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by Pozano and Reggi

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as a mode, you know, as a model

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for, two plus one dimensional quantum gravity.

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And this kind of in recent years, well,

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variance of this model have become very popular

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in, in condensed matter. So

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there's lots of, ideas which were developed

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in the push to understand quantum gravity, which

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have become completely standard and, and and standard

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tools.

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And so I think I've worked for quantum

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gravity, but it's been a boon for physics.

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For, you know, for the for the larger

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physics community.

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And, indeed, you know, even me personally, I

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developed, like, tensor network tools, and we see

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the,

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like, also the difficulties in doing quantum gravity.

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So we try to understand it

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somewhat similar to condensed matter

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model.

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However, different from condensed matter, we would have

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to, bridge an enormous amount of scales

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that's kind of from the Planck scale, which

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is 10 to the minus 35 meters, which

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we do indeed do not have so much

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experimental access to

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to kind of the cosmos.

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So, basically,

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the idea would be to have a theory

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which is valid over all scales, which, you

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know, usually we don't have and which is

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a very hard problem.

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00:11:46,190 --> 00:11:48,269
And there are many condensed matter models even

286
00:11:48,269 --> 00:11:50,575
in two dimensions, which we cannot solve.

287
00:11:51,514 --> 00:11:53,675
Like, the quantum gravity models we have, you

288
00:11:53,675 --> 00:11:55,995
know, they are far more complicated than con

289
00:11:56,075 --> 00:11:57,375
condensed matter models,

290
00:11:57,835 --> 00:11:58,975
like, four dimensions.

291
00:11:59,434 --> 00:12:01,295
We have to do them in real time.

292
00:12:01,355 --> 00:12:02,654
We cannot do the recoitation.

293
00:12:03,290 --> 00:12:04,110
So computationally,

294
00:12:04,570 --> 00:12:05,710
it's also an extremely

295
00:12:06,170 --> 00:12:08,990
hard problem. Mhmm. Yeah. We're kind of

296
00:12:09,610 --> 00:12:11,610
we don't have too many tools to do

297
00:12:11,610 --> 00:12:12,910
real time simulations,

298
00:12:13,370 --> 00:12:15,790
and we would need enormous computational

299
00:12:16,410 --> 00:12:16,910
power.

300
00:12:17,894 --> 00:12:19,975
And so maybe that will be in our

301
00:12:19,975 --> 00:12:21,995
future. Maybe we have quantum simulations,

302
00:12:23,254 --> 00:12:25,174
but these are some of the reasons. You

303
00:12:25,174 --> 00:12:27,174
know? So but then there's a push to

304
00:12:27,174 --> 00:12:28,794
develop computational tools,

305
00:12:29,269 --> 00:12:31,669
which was part of my research, and, you

306
00:12:31,669 --> 00:12:33,610
know, these are also useful for

307
00:12:34,230 --> 00:12:36,309
as a a port, like, just for condensed

308
00:12:36,309 --> 00:12:38,629
matter, for instance. And, Bianca, I wanted to

309
00:12:38,629 --> 00:12:40,710
ask you about your research here at the

310
00:12:40,710 --> 00:12:42,570
Perimeter Institute. You lead

311
00:12:43,269 --> 00:12:43,929
the from

312
00:12:44,414 --> 00:12:46,034
discretum to continuum

313
00:12:46,414 --> 00:12:46,914
initiative.

314
00:12:47,534 --> 00:12:49,475
What are the goals of this initiative?

315
00:12:50,495 --> 00:12:51,315
Yeah. So,

316
00:12:53,054 --> 00:12:55,695
the the discretum comes from the fact that,

317
00:12:57,559 --> 00:12:59,480
part of my research is about,

318
00:13:00,279 --> 00:13:00,779
nonperturbative

319
00:13:01,240 --> 00:13:02,779
models of quantum gravity,

320
00:13:03,559 --> 00:13:07,179
which often have discrete aspects. So they're either

321
00:13:07,720 --> 00:13:08,460
you start

322
00:13:09,000 --> 00:13:11,414
directly with kind of discrete building blocks.

323
00:13:13,334 --> 00:13:15,254
Then if it comes to spin forms or

324
00:13:15,254 --> 00:13:16,154
loop quantum gravity,

325
00:13:16,934 --> 00:13:19,334
it's in fact that discreteness is a kind

326
00:13:19,334 --> 00:13:21,014
of derived. It comes to the forms of

327
00:13:21,014 --> 00:13:21,514
formalism

328
00:13:21,975 --> 00:13:22,214
that,

329
00:13:23,254 --> 00:13:24,554
certain geometric quantities

330
00:13:25,095 --> 00:13:26,394
of discrete spectra.

331
00:13:27,580 --> 00:13:29,259
And so, in fact, you have also discrete

332
00:13:29,259 --> 00:13:30,080
building blocks.

333
00:13:31,179 --> 00:13:34,620
And then the big question is, well, it's

334
00:13:34,620 --> 00:13:37,340
it's very similar to to condensed matter, you

335
00:13:37,340 --> 00:13:38,320
know, where you have

336
00:13:39,035 --> 00:13:42,154
atoms or your your fundamental particles, and you

337
00:13:42,154 --> 00:13:43,055
try to understand

338
00:13:43,434 --> 00:13:45,355
the screw the interaction, you know, what is

339
00:13:45,355 --> 00:13:46,014
the material

340
00:13:47,355 --> 00:13:49,215
or who if you have water,

341
00:13:49,835 --> 00:13:51,509
eventually, you want to derive

342
00:13:52,549 --> 00:13:54,250
the flow equations for water.

343
00:13:54,870 --> 00:13:57,750
How does it behave? And so to get

344
00:13:57,750 --> 00:13:59,750
to the continuum limit to show that you

345
00:13:59,750 --> 00:14:00,250
get,

346
00:14:03,029 --> 00:14:05,590
well, space time as we know it, looking

347
00:14:05,590 --> 00:14:07,049
somewhat smooth and continuous.

348
00:14:08,235 --> 00:14:10,175
And so that is a is a

349
00:14:11,115 --> 00:14:12,254
aim of the initiative.

350
00:14:13,115 --> 00:14:15,215
That was in particular to to develop

351
00:14:16,154 --> 00:14:17,295
numerical methods,

352
00:14:18,634 --> 00:14:20,250
and we made progress there.

353
00:14:21,690 --> 00:14:24,009
But we made also recent progress in actually

354
00:14:24,009 --> 00:14:24,830
showing that,

355
00:14:26,009 --> 00:14:27,309
the spin forms which

356
00:14:28,009 --> 00:14:29,070
are mentioned there

357
00:14:29,450 --> 00:14:30,909
do indeed seem to have,

358
00:14:31,529 --> 00:14:33,284
a suitable continuum limits.

359
00:14:34,084 --> 00:14:36,904
Yeah. I wanted to ask you, Bianca, about

360
00:14:37,044 --> 00:14:39,065
spin foams. They sound intriguing.

361
00:14:40,164 --> 00:14:42,264
What what is a spin foam model,

362
00:14:42,644 --> 00:14:45,284
and, how could it provide insights into quantum

363
00:14:45,284 --> 00:14:45,784
gravity?

364
00:14:47,389 --> 00:14:49,970
So, yeah, spin forms come from an approach

365
00:14:50,190 --> 00:14:51,629
where we really take,

366
00:14:53,309 --> 00:14:55,490
Einstein's theory of general relativity

367
00:14:56,269 --> 00:14:57,009
as describing

368
00:14:58,509 --> 00:15:01,375
the gravitational interaction or the gravitational force as

369
00:15:01,375 --> 00:15:03,695
a property of space and time or the

370
00:15:03,695 --> 00:15:05,235
geometry of space and time.

371
00:15:06,254 --> 00:15:08,335
So we take this geometry of space and

372
00:15:08,335 --> 00:15:10,514
time as our dynamical field,

373
00:15:11,615 --> 00:15:12,115
and,

374
00:15:12,815 --> 00:15:14,434
this geometry is then quantized.

375
00:15:16,480 --> 00:15:19,059
And as a result of this procedure,

376
00:15:20,799 --> 00:15:22,179
it turns out that,

377
00:15:22,639 --> 00:15:24,100
indeed, as I mentioned,

378
00:15:24,639 --> 00:15:26,500
for instance, areas and volumes

379
00:15:27,065 --> 00:15:28,365
appear in in

380
00:15:29,304 --> 00:15:29,804
discrete

381
00:15:30,264 --> 00:15:32,424
this discrete spectra. So they kind of come

382
00:15:32,424 --> 00:15:33,804
all in little chunks.

383
00:15:34,504 --> 00:15:36,504
So that's the foam bit, the the little

384
00:15:36,504 --> 00:15:39,544
chunks? That's a foam bit. So the the

385
00:15:39,544 --> 00:15:40,470
the little chunks.

386
00:15:42,389 --> 00:15:44,809
And, basically, what the spin form

387
00:15:46,389 --> 00:15:48,889
model provides is an kind of a quantum

388
00:15:48,949 --> 00:15:49,449
amplitude

389
00:15:50,389 --> 00:15:51,529
for this chunk.

390
00:15:51,990 --> 00:15:54,889
More specifically, what is the geometry of this

391
00:15:55,189 --> 00:15:58,174
chunk? So probability amplitudes for

392
00:15:58,875 --> 00:16:00,495
how big this chunk is.

393
00:16:00,875 --> 00:16:02,315
And so that's one of these,

394
00:16:05,274 --> 00:16:06,235
aspects that,

395
00:16:06,875 --> 00:16:09,195
it's like how big and what is the

396
00:16:09,195 --> 00:16:10,575
scale of this chunk.

397
00:16:10,970 --> 00:16:13,450
But it's a dynamic variable, whereas usually in

398
00:16:13,450 --> 00:16:14,990
physics and all other

399
00:16:16,250 --> 00:16:19,450
quantum field series, it's something which we kind

400
00:16:19,450 --> 00:16:19,950
of,

401
00:16:21,610 --> 00:16:23,070
dial in from the outset.

402
00:16:23,930 --> 00:16:25,470
And so that's one of these

403
00:16:26,834 --> 00:16:29,414
interesting aspects which make it a bit challenging.

404
00:16:30,434 --> 00:16:33,014
But we have these amplitudes of each chunk,

405
00:16:33,875 --> 00:16:36,194
and the key difficulties then is to show

406
00:16:36,194 --> 00:16:39,314
that if we kind of consider lots and

407
00:16:39,314 --> 00:16:41,909
lots and lots and lots of these chunks,

408
00:16:44,069 --> 00:16:46,549
we do get something, you know, like a

409
00:16:46,549 --> 00:16:48,889
smooth space time with gravitational waves.

410
00:16:49,909 --> 00:16:51,610
So there is a kind of computational

411
00:16:52,069 --> 00:16:52,889
or as a,

412
00:16:55,289 --> 00:16:55,850
the the well,

413
00:16:56,389 --> 00:16:57,529
the key aim

414
00:16:58,125 --> 00:17:00,945
to to show and then to show also

415
00:17:01,565 --> 00:17:05,025
what would be effect effects behind having gravitational

416
00:17:05,164 --> 00:17:05,664
waves.

417
00:17:06,204 --> 00:17:07,585
I see. And

418
00:17:08,125 --> 00:17:10,445
you you're originally from Germany, but you've been

419
00:17:10,445 --> 00:17:14,069
associated with the Perimeter Institute for twenty two

420
00:17:14,069 --> 00:17:15,849
of its twenty five years.

421
00:17:17,269 --> 00:17:20,069
What initially attracted you to the institute, and

422
00:17:20,069 --> 00:17:21,369
and what does it offer

423
00:17:21,829 --> 00:17:22,789
physicists who,

424
00:17:23,670 --> 00:17:26,394
who are interested in quantum gravity? You can

425
00:17:26,394 --> 00:17:28,174
use this as a recruitment pitch

426
00:17:28,954 --> 00:17:31,355
for hiring new colleagues if you'd like. I

427
00:17:31,355 --> 00:17:33,994
I, you know, I came the first time

428
00:17:33,994 --> 00:17:35,694
as a as a PhD student,

429
00:17:37,194 --> 00:17:39,994
but with my supervisor, it's always good to

430
00:17:39,994 --> 00:17:42,679
be kind of spatially near to your supervisor.

431
00:17:43,059 --> 00:17:43,559
So,

432
00:17:44,819 --> 00:17:46,119
but I also heard

433
00:17:46,500 --> 00:17:49,619
at that point, Perimeter Institute was a very

434
00:17:49,619 --> 00:17:50,359
new institute,

435
00:17:50,740 --> 00:17:53,880
and, it was doing, you know, really research

436
00:17:53,940 --> 00:17:55,275
in quantum gravity, which

437
00:17:57,195 --> 00:17:59,674
well, certainly around 2003,

438
00:17:59,674 --> 00:18:01,755
I guess, was done only at very few

439
00:18:01,755 --> 00:18:02,255
places.

440
00:18:03,515 --> 00:18:04,015
And,

441
00:18:05,674 --> 00:18:06,894
well, even being

442
00:18:07,369 --> 00:18:09,950
quite young, I was already, like, very interesting

443
00:18:10,009 --> 00:18:12,509
people there in quantum in quantum gravity

444
00:18:12,970 --> 00:18:13,470
research.

445
00:18:14,730 --> 00:18:17,769
And so that also then motivated me, you

446
00:18:17,769 --> 00:18:19,309
know, to come back as a postdoc

447
00:18:20,714 --> 00:18:22,934
and to come back as a faculty. I

448
00:18:22,954 --> 00:18:24,394
mean, first of all, kind of,

449
00:18:25,674 --> 00:18:26,414
we have

450
00:18:26,954 --> 00:18:29,695
a we we are excellent in quantum gravity

451
00:18:29,755 --> 00:18:30,255
research.

452
00:18:32,474 --> 00:18:32,974
We

453
00:18:33,329 --> 00:18:33,829
have

454
00:18:34,450 --> 00:18:34,950
have

455
00:18:35,369 --> 00:18:35,869
a

456
00:18:36,289 --> 00:18:38,869
large group, excellent people. And, historically,

457
00:18:39,970 --> 00:18:40,470
indeed,

458
00:18:42,130 --> 00:18:44,609
from from 2001

459
00:18:44,609 --> 00:18:47,105
on, since since the institute was founded,

460
00:18:47,904 --> 00:18:48,565
we produced,

461
00:18:48,944 --> 00:18:51,585
like, lots of postdocs, which went on to

462
00:18:51,585 --> 00:18:54,244
be faculty in other places. So kind of,

463
00:18:55,105 --> 00:18:57,904
it's really center for quantum gravity research. But

464
00:18:57,904 --> 00:18:59,204
beyond that, we

465
00:19:00,029 --> 00:19:02,289
have lots of other theoretical physics

466
00:19:02,990 --> 00:19:03,490
directions.

467
00:19:04,430 --> 00:19:05,250
For instance,

468
00:19:06,430 --> 00:19:08,990
you know, in trying to develop numerical methods,

469
00:19:08,990 --> 00:19:11,625
I was interacting with condensed matter people here.

470
00:19:12,184 --> 00:19:13,325
There's strong gravity,

471
00:19:13,785 --> 00:19:16,744
which also there's lots of close interactions with

472
00:19:16,744 --> 00:19:18,285
quantum gravity or cosmology.

473
00:19:19,704 --> 00:19:21,625
So it's great to have all these different

474
00:19:21,625 --> 00:19:22,125
fields,

475
00:19:22,825 --> 00:19:23,325
because,

476
00:19:23,625 --> 00:19:25,500
you know, very interesting things happens

477
00:19:25,900 --> 00:19:27,680
happened at these intersections.

478
00:19:28,380 --> 00:19:30,480
I see. I mean, your your responsibilities

479
00:19:30,859 --> 00:19:33,500
here, do you I mean, primarily, you you

480
00:19:33,500 --> 00:19:35,500
do research. But do you do you do

481
00:19:35,500 --> 00:19:37,660
any teaching, you know, for example, at the

482
00:19:37,660 --> 00:19:40,220
University of Waterloo, or is is it mostly

483
00:19:40,220 --> 00:19:40,664
a

484
00:19:41,144 --> 00:19:41,644
a

485
00:19:42,025 --> 00:19:42,684
research position?

486
00:19:43,465 --> 00:19:45,725
Yeah. I'm research faculty, so,

487
00:19:47,384 --> 00:19:49,244
well, I don't kind of

488
00:19:49,545 --> 00:19:50,845
have to do teaching.

489
00:19:51,465 --> 00:19:52,125
I did

490
00:19:52,505 --> 00:19:53,309
I did talk

491
00:19:53,950 --> 00:19:56,130
in in the in the outside master

492
00:19:56,990 --> 00:19:58,750
course, also at some point at AIMS in

493
00:19:58,750 --> 00:20:00,990
Africa. So I, I did it a couple

494
00:20:00,990 --> 00:20:01,570
of times.

495
00:20:03,309 --> 00:20:05,865
Currently, I'm also faculty chair, so kind of

496
00:20:05,865 --> 00:20:07,884
busy with There's lots of administration.

497
00:20:08,585 --> 00:20:09,644
It was administration

498
00:20:10,265 --> 00:20:10,765
things.

499
00:20:11,865 --> 00:20:12,684
And so

500
00:20:13,305 --> 00:20:14,684
yeah. So there's kind of

501
00:20:16,025 --> 00:20:17,805
some teaching we do on,

502
00:20:18,730 --> 00:20:19,230
on

503
00:20:20,009 --> 00:20:22,329
on particular master on a master basis and

504
00:20:22,329 --> 00:20:23,630
PhD level basis.

505
00:20:24,890 --> 00:20:25,630
But, otherwise,

506
00:20:27,369 --> 00:20:29,149
it's very focused on on research.

507
00:20:30,250 --> 00:20:32,654
And and finally, we talked a bit about

508
00:20:32,894 --> 00:20:33,634
the difficulty

509
00:20:34,894 --> 00:20:35,795
of getting

510
00:20:36,174 --> 00:20:39,615
data, experimental data on quantum gravity. You mentioned

511
00:20:39,615 --> 00:20:41,934
the Planck scale and, you know, having to

512
00:20:41,934 --> 00:20:44,575
think about photons travelling across the universe to

513
00:20:44,734 --> 00:20:46,595
Yep. But to maybe see

514
00:20:46,974 --> 00:20:47,474
something.

515
00:20:49,410 --> 00:20:51,970
Does that discourage you sometimes when you're when

516
00:20:51,970 --> 00:20:54,309
you're doing your theoretical work? Or

517
00:20:54,609 --> 00:20:56,289
do you do you I suppose you have

518
00:20:56,289 --> 00:20:58,369
to have faith in the mathematics. Do you?

519
00:20:58,369 --> 00:21:00,855
Is that Yeah. I guess it depends. It's

520
00:21:00,934 --> 00:21:03,414
it's possibly, you know, different generations have different

521
00:21:03,414 --> 00:21:03,914
expectations.

522
00:21:05,095 --> 00:21:05,595
So,

523
00:21:07,734 --> 00:21:10,214
I'm possibly coming from a generation which is

524
00:21:10,214 --> 00:21:12,855
possibly okay. It takes a long time to

525
00:21:12,855 --> 00:21:15,255
kind of do quantum gravity, maybe, like, twenty

526
00:21:15,255 --> 00:21:16,380
years before me. People

527
00:21:17,339 --> 00:21:17,920
are expecting

528
00:21:18,619 --> 00:21:20,079
the solve it in ten years.

529
00:21:20,779 --> 00:21:23,259
Well, there's certainly always a certain level of

530
00:21:23,259 --> 00:21:23,759
excitement,

531
00:21:24,859 --> 00:21:28,240
about upcoming experiments. So as I mentioned, indeed,

532
00:21:29,305 --> 00:21:31,164
where there's kind of a number of proposals

533
00:21:31,384 --> 00:21:31,884
to,

534
00:21:33,465 --> 00:21:34,924
to measure things.

535
00:21:35,625 --> 00:21:38,105
Are are there any upcoming experiments that you're

536
00:21:38,105 --> 00:21:40,105
really excited about, or or have you been

537
00:21:40,105 --> 00:21:42,079
involved in any proposals?

538
00:21:43,259 --> 00:21:45,259
I well, I have not been so I

539
00:21:45,259 --> 00:21:47,440
mean, I've coming from a more theoretical,

540
00:21:48,619 --> 00:21:49,980
directional sense of,

541
00:21:50,700 --> 00:21:53,019
for spin for models, a key aspect for

542
00:21:53,019 --> 00:21:55,065
us was to show that, you know, we

543
00:21:55,065 --> 00:21:55,784
get to,

544
00:21:56,744 --> 00:21:58,524
a smooth space time, which,

545
00:22:00,664 --> 00:22:02,845
you know, actually well, if I can

546
00:22:03,464 --> 00:22:06,284
say why why would that be a key

547
00:22:07,000 --> 00:22:08,779
He said it is, that

548
00:22:09,799 --> 00:22:12,619
this was tried very often with various models,

549
00:22:14,759 --> 00:22:17,480
but most models use this idea of a

550
00:22:17,480 --> 00:22:19,900
weak quotation, so so called Euclidean models.

551
00:22:21,184 --> 00:22:23,984
And also, you start with four dimensional building

552
00:22:23,984 --> 00:22:25,664
blocks. It turned out if you try to

553
00:22:25,664 --> 00:22:27,445
glue them together into simulations,

554
00:22:28,545 --> 00:22:29,684
statistical simulations,

555
00:22:30,945 --> 00:22:33,345
most of the resulting structures you saw at

556
00:22:33,345 --> 00:22:34,325
larger scales

557
00:22:35,289 --> 00:22:37,930
didn't look at all like smooth manifolds. So

558
00:22:37,930 --> 00:22:40,330
they looked like fractals, for instance. So we

559
00:22:40,410 --> 00:22:42,430
actually, technically, they're called like polymers.

560
00:22:44,730 --> 00:22:46,670
And so that's happened for for

561
00:22:48,424 --> 00:22:50,825
twenty years. So it was only recently we

562
00:22:50,825 --> 00:22:51,565
we see,

563
00:22:52,345 --> 00:22:54,265
we have one model where this does not

564
00:22:54,265 --> 00:22:55,545
happen. So it's kind of,

565
00:22:56,025 --> 00:22:56,765
what be

566
00:22:57,224 --> 00:22:59,164
a key aspect to show non perturbatively

567
00:22:59,465 --> 00:23:01,244
that we get a smooth space time.

568
00:23:01,940 --> 00:23:03,079
However, I made

569
00:23:03,779 --> 00:23:06,339
a particular progress with spin forms in a

570
00:23:06,339 --> 00:23:07,400
more perturbative

571
00:23:08,259 --> 00:23:09,640
approach, so kind of,

572
00:23:10,980 --> 00:23:12,839
and we could show that we get,

573
00:23:13,884 --> 00:23:16,045
a gravitational waves. Assuming that we get a

574
00:23:16,045 --> 00:23:18,045
smooth space time, we do get we can

575
00:23:18,045 --> 00:23:20,304
show that we have gravitational waves,

576
00:23:20,765 --> 00:23:23,345
and we get also corrections to the dynamics

577
00:23:23,565 --> 00:23:24,065
compared

578
00:23:25,164 --> 00:23:26,464
to general relativity.

579
00:23:27,589 --> 00:23:29,829
So I'm kind of in the process indeed

580
00:23:29,829 --> 00:23:30,329
to

581
00:23:30,869 --> 00:23:32,170
think about experiments.

582
00:23:32,549 --> 00:23:34,329
No. That's it. So so are those

583
00:23:34,630 --> 00:23:37,029
those deviations, are they something that could be

584
00:23:37,029 --> 00:23:37,849
seen by

585
00:23:38,309 --> 00:23:41,305
LIGO or maybe a future experiment? To do

586
00:23:41,305 --> 00:23:41,884
the calculations.

587
00:23:42,265 --> 00:23:43,945
Right. But that would be a you know,

588
00:23:43,945 --> 00:23:45,484
that would be, interest

589
00:23:45,785 --> 00:23:48,285
some some things to to explore.

590
00:23:49,865 --> 00:23:51,465
It's actually also turns out that,

591
00:23:53,065 --> 00:23:55,305
you know, spin forms are technically kind of

592
00:23:55,384 --> 00:23:55,884
it's

593
00:23:56,950 --> 00:23:59,130
in principle, is a penalty violating

594
00:23:59,750 --> 00:24:01,990
theory if a certain parameter is not has

595
00:24:01,990 --> 00:24:04,250
not a very particular value. And so

596
00:24:05,029 --> 00:24:06,490
this is something which

597
00:24:07,190 --> 00:24:09,990
could provide more of an experimental signature because

598
00:24:09,990 --> 00:24:11,674
everything else is not penalty

599
00:24:12,134 --> 00:24:12,634
violating.

600
00:24:13,095 --> 00:24:13,595
So,

601
00:24:14,375 --> 00:24:16,375
on the other hand, it's very constraining, so

602
00:24:16,375 --> 00:24:17,515
I can see that.

603
00:24:18,295 --> 00:24:20,075
But there's, of course, many other

604
00:24:20,855 --> 00:24:23,755
ideas and and and proposals

605
00:24:24,420 --> 00:24:26,440
of of experimental signatures

606
00:24:27,860 --> 00:24:28,259
that,

607
00:24:28,740 --> 00:24:29,320
are always

608
00:24:30,340 --> 00:24:33,619
seem to be very promising. And, so, and

609
00:24:33,619 --> 00:24:37,000
that indeed includes gravitational waves or probing,

610
00:24:39,015 --> 00:24:40,794
probing's a big bang via cosmology.

611
00:24:41,974 --> 00:24:43,894
So has that I mean, with the, you

612
00:24:43,894 --> 00:24:45,434
know, discovery of gravitational

613
00:24:45,734 --> 00:24:48,534
waves and also, yeah, I suppose the information

614
00:24:48,534 --> 00:24:50,294
that we have, you know, from the cosmic

615
00:24:50,294 --> 00:24:51,835
microwave background, etcetera,

616
00:24:53,140 --> 00:24:53,460
You know, the

617
00:24:54,259 --> 00:24:55,880
I suppose these really amazing

618
00:24:56,660 --> 00:24:57,160
measurements

619
00:24:57,619 --> 00:24:59,619
Mhmm. Are they I mean, is that something

620
00:24:59,619 --> 00:25:02,099
that peep people who work on quantum gravity

621
00:25:02,099 --> 00:25:04,740
are very excited about? Is this possibly opening

622
00:25:04,740 --> 00:25:05,559
up a new

623
00:25:05,974 --> 00:25:07,815
view on the universe that could be very

624
00:25:07,815 --> 00:25:10,394
useful to you? I mean, it's certainly certainly

625
00:25:10,455 --> 00:25:12,215
that's what we hope. You know, on the

626
00:25:12,215 --> 00:25:14,394
technical side, and the, you know, the gravitational

627
00:25:14,535 --> 00:25:15,515
wave observations

628
00:25:16,134 --> 00:25:18,089
are also highly technical in itself.

629
00:25:19,130 --> 00:25:21,609
So there needs to be, well, some people

630
00:25:21,609 --> 00:25:22,429
which bridge,

631
00:25:22,970 --> 00:25:26,029
like, the theoretical chronology side to the observational

632
00:25:26,250 --> 00:25:26,750
side

633
00:25:27,289 --> 00:25:29,710
because there are many questions about about noise.

634
00:25:30,184 --> 00:25:30,924
No. But,

635
00:25:31,384 --> 00:25:31,884
physically,

636
00:25:32,265 --> 00:25:34,285
you know, there's this idea where,

637
00:25:37,304 --> 00:25:39,005
yeah, for instance, with a cosmological

638
00:25:39,625 --> 00:25:41,644
microwave background, so the elect

639
00:25:42,194 --> 00:25:42,694
electromagnetic

640
00:25:43,460 --> 00:25:45,080
radiation with electromagnetic

641
00:25:46,580 --> 00:25:48,119
electromagnetic radiation, we can probe

642
00:25:50,820 --> 00:25:53,559
early universe cosmology only so far

643
00:25:54,019 --> 00:25:56,724
because we need well, it's only after, you

644
00:25:56,724 --> 00:25:59,944
know, electromagnetic radiation decoupled from everything else.

645
00:26:01,444 --> 00:26:04,244
Whereas this, kind of if we could, like,

646
00:26:04,244 --> 00:26:05,625
gravitational wave background,

647
00:26:06,484 --> 00:26:08,964
that goes much further to the towards the

648
00:26:09,284 --> 00:26:11,284
Oh, I see. Yeah. The big bang. So

649
00:26:11,284 --> 00:26:12,450
you would expect to

650
00:26:13,649 --> 00:26:15,669
see much deeper to towards

651
00:26:15,970 --> 00:26:17,269
the beginning of the universe,

652
00:26:17,649 --> 00:26:19,889
the beginning of time. And so would that

653
00:26:19,889 --> 00:26:23,109
take you into a regime where quantum gravity

654
00:26:23,730 --> 00:26:26,289
was significant in terms of the evolution More

655
00:26:26,369 --> 00:26:29,923
of the universe. Certainly more significant. Right. So

656
00:26:30,017 --> 00:26:31,112
kind of

657
00:26:31,800 --> 00:26:32,634
yeah. So

658
00:26:34,315 --> 00:26:34,815
and,

659
00:26:35,914 --> 00:26:36,734
yeah, certainly

660
00:26:37,115 --> 00:26:37,855
more significant

661
00:26:38,154 --> 00:26:39,215
and, with,

662
00:26:39,914 --> 00:26:40,894
this kind of

663
00:26:41,195 --> 00:26:42,174
now also microwave

664
00:26:43,035 --> 00:26:45,539
measurements, which hopefully at some point,

665
00:26:46,480 --> 00:26:47,140
we could

666
00:26:47,840 --> 00:26:49,380
see nonlinear effects.

667
00:26:50,160 --> 00:26:52,100
And also for the nonlinear effects,

668
00:26:52,559 --> 00:26:55,059
we would see the quantum gravity would play

669
00:26:55,440 --> 00:26:57,039
more of a role than form of the

670
00:26:57,039 --> 00:26:58,180
linear linear

671
00:26:58,985 --> 00:27:01,965
things. And, there are also predictions of

672
00:27:03,225 --> 00:27:06,825
kind of predictions of, yeah, deviations for the

673
00:27:06,825 --> 00:27:07,725
linear theory,

674
00:27:08,904 --> 00:27:11,085
like the tilt of the spectrum and,

675
00:27:12,609 --> 00:27:15,089
which currently my understanding, you know, we cannot

676
00:27:15,089 --> 00:27:16,789
distinguish these effects, but,

677
00:27:17,170 --> 00:27:19,569
hopefully, in the future, we will. Oh, wow.

678
00:27:19,569 --> 00:27:21,730
So that well, that's really exciting then. I,

679
00:27:22,210 --> 00:27:23,890
I mean, I suppose my view of quantum

680
00:27:23,890 --> 00:27:26,049
gravity has always been, well, they'll never get

681
00:27:26,049 --> 00:27:28,815
any experimental data. I mean, it sounds like

682
00:27:28,954 --> 00:27:30,974
there could be lots coming through the pipeline.

683
00:27:31,755 --> 00:27:33,914
Yeah. So so we you always hope that

684
00:27:33,914 --> 00:27:34,414
this

685
00:27:34,714 --> 00:27:37,355
people will always predict some things. Yeah. Well,

686
00:27:37,355 --> 00:27:39,660
that Yeah. That's great. Thanks so much for

687
00:27:39,660 --> 00:27:41,980
explaining that to us, Bianca. And, best of

688
00:27:41,980 --> 00:27:43,759
luck with your research in the future.

689
00:27:44,220 --> 00:27:45,900
Thank you. It was nice to talk to

690
00:27:45,900 --> 00:27:46,400
you.

691
00:27:54,345 --> 00:27:56,125
That was Bianca Dietrich

692
00:27:56,585 --> 00:27:59,164
who spoke to me at the Perimeter Institute

693
00:27:59,625 --> 00:28:01,244
for Theoretical Physics.

694
00:28:01,865 --> 00:28:04,044
Thanks to Bianca for a fascinating

695
00:28:04,505 --> 00:28:05,005
conversation

696
00:28:05,544 --> 00:28:07,085
about quantum gravity.

697
00:28:07,730 --> 00:28:10,369
And a special thanks to our producer Fred

698
00:28:10,369 --> 00:28:10,869
Ailes.

699
00:28:11,570 --> 00:28:14,049
We'll be back again next week. See you

700
00:28:14,049 --> 00:28:14,549
then.