Hello, and welcome to the Physics World weekly
podcast.
I'm Hamish Johnston.
Earlier this autumn, I had the pleasure of
visiting the Perimeter Institute
for Theoretical Physics in Waterloo, Canada,
where I interviewed four physicists
about their research.
This is the second of those conversations
to appear on the podcast,
and it's with Bianca Dietrich,
whose research focuses on quantum gravity.
Our chat is coming up after this message
from IOP publishing,
which brings you Physics World.
Discover cutting edge science in minutes.
IOP publishing's new progress in series
Research Highlights website
offers quick, accessible summaries
of top papers from leading journals,
like reports on progress in physics
and progress in energy.
Whether you're short on time or just want
the essentials,
these highlights help you expand your knowledge on
leading topics.
Perfect for busy researchers,
curious minds, and anyone who wants to stay
informed
without the deep dive.
To start reading, just type
progress in series
research highlights into your favorite search engine,
or follow the link in the notes for
this podcast.
Albert Einstein's
general theory of relativity does a great job
at explaining gravity,
but it is thought to be incomplete
because it is incompatible with quantum mechanics,
which itself
is widely considered
to be one of science's most successful
theories.
Developing a theory of quantum gravity is a
crucial goal in physics,
but it's proving to be extremely difficult.
We discuss the challenges
and the possible ways forward
in this podcast.
I'm at the Perimeter Institute in Waterloo,
Ontario,
and I'm very pleased to be joined by
Bianca
Dietrich.
Hi, Bianca. Welcome to the podcast.
Hi, Amish. Very nice talking to you.
And, Bianca, we're going to talk about quantum
gravity,
which I have to admit is probably the
one
thing in physics that I know very little
about. So you have you probably have to
be very patient with me
about this.
So quantum gravity is is
a very important field of research,
particularly because we don't have a theory of
quantum gravity, do we? So
why don't we have one? Is why is
it so difficult?
Yeah. So that's correct.
We possibly don't have a theory of quantum
gravity
that, you know, enough people are happy with.
I don't know.
And so there's a well, there's a number
of reasons you might have to stop me.
Gravity turns out is, is very different from
the three other interactions.
We know of the strong,
interaction,
the weak interaction,
and the electromagnetic
interaction.
I mean, one big reason is that it
has a much larger and more complicated,
symmetry
group. There's
a,
independence of, choice of coordinates or diffeomorphism
group,
compared to, you know, just kind of Poincare
asymmetries
and,
gauge series
gauge symmetries of particle physics.
And,
indeed,
gravity
conceptually
is
according to Einstein's theory
about the geometry of space and time.
And it turns out because of this
kind of invariance and the coordinates, you know,
which is not
asymmetry the other interactions
do have. So usually, we quantize them on
a fixed space and time background.
But in the case of gravity, we don't
have a fixed space and time background. It's
a dynamical entity.
And so there are many deep conceptual questions.
And in fact,
it's that if we do have a quantum
theory of of gravity,
we do also expect that we will have
a quantization of space and time.
And so that would kind of completely change
our notion of order space and time.
It also shows us that we possibly
well, we need possibly new quantization techniques.
And in fact,
we could not apply many of the techniques
which are which we rely very heavily on
in the case of standard quantum field theory,
so for the other interactions.
To give you some examples, we know that
if we, you know, perturbative quantum field theories
that was used for a long time to
treat particle physics,
This field, in the case of gravity,
for for some technical reasons, usually is expressed
as not renormalizable.
Another example is that, what what we also
use a lot
for particle physics or for condensed matter
is a a very,
a trick which is
very useful,
and that's that we,
rotate
the time
in,
in the complex plane, the so called rotation.
And that allows us,
you know, that turns
series where we have complex amplitudes in the
sense of,
amplitudes with the phase where we have interference
and so on to turn into statistical amplitudes.
And,
that allows us to do Monte Carlo simulations.
And, you know, QCD,
well, is is more nonperturbative
than, say, electromagnetisms,
but to understand QCD, what do we do?
Latest simulations versus Monte Carlo simulations.
If you do not have access to to
these,
Monte Carlo simulations, for instance, we kind of
get into problems which are also hard to
solve in the case of QCD,
which is kind of,
if you if you do want to do
real time simulations. So that is also a
big issue in condensed matter and and QCD
or the sign problem in QCD.
And it is part of the problem also
that
we don't have
experimental
data. You can't test your theory by doing
an experiment like you can in I mean,
on one side matter or particle physics.
You know, on one side, we have lots
of experimental data because we know that's a
road exists.
I will come to that.
But we don't have so much new input,
which we cannot explain by the standard series.
So it's really astonishing that, like, standard cosmology,
you know, which, to some degree is is
based on,
linearized quantum field theory on curves based.
We can explain
lots of our observations
in cosmology and that,
even so we know, okay, there's a big
bang and
and,
but we would need to go to higher
order and perturbation theory, for instance, to to
see effects from quantum gravity.
So this is certainly true, but one example
is that we have actually already observations,
in the kind of Planck regime where we
do where where some series, for instance, predicted
that something could happen,
in particular with regard to kind of Lorentz
breaking
and possible Lorentz deformation. And so there were
kind of
results from looking at gamma ray bursts,
which are like gamma rays coming from
sources which are very far away.
And so there you have these gamma photons,
which travel very long distances.
And so the idea is there because they
travel long distances, they have a long time
to accumulate effects which come from having some,
for instance, quantum space time.
But the simplest models were basically ruled out.
I mean, the kind of
but the regime was indeed basically that they
could already test
planck scale physics.
So that's certainly
well, I mean, if you would have more
of an experimental
input, yeah, we
possibly, that would reduce, the space of possibilities.
And,
I guess human fantasy is very large, so
it's kind of many possibilities are explored.
But I want to also express that, you
know, quantum gravity is not useless.
So I think first quantization attempts go back
to the nineteen twenties.
But an enormous
part of
what have became standard techniques
in in modern physics
has has started with the motivation
of gravity or quantum gravity.
You know, general relativity was the first example
of a gauge theory.
And so, like,
techniques for the quantization of gauge theories
were developed
with, with motivation to do quantum gravity.
The Newton Newton developed her
formalism, the Newton serum,
because she was asked by Herbert about gravity.
So there are lots of examples.
A more recent one is a topological phases
in condensed matter.
So first topological model was written down in
1968
by Pozano and Reggi
as a mode, you know, as a model
for, two plus one dimensional quantum gravity.
And this kind of in recent years, well,
variance of this model have become very popular
in, in condensed matter. So
there's lots of, ideas which were developed
in the push to understand quantum gravity, which
have become completely standard and, and and standard
tools.
And so I think I've worked for quantum
gravity, but it's been a boon for physics.
For, you know, for the for the larger
physics community.
And, indeed, you know, even me personally, I
developed, like, tensor network tools, and we see
the,
like, also the difficulties in doing quantum gravity.
So we try to understand it
somewhat similar to condensed matter
model.
However, different from condensed matter, we would have
to, bridge an enormous amount of scales
that's kind of from the Planck scale, which
is 10 to the minus 35 meters, which
we do indeed do not have so much
experimental access to
to kind of the cosmos.
So, basically,
the idea would be to have a theory
which is valid over all scales, which, you
know, usually we don't have and which is
a very hard problem.
And there are many condensed matter models even
in two dimensions, which we cannot solve.
Like, the quantum gravity models we have, you
know, they are far more complicated than con
condensed matter models,
like, four dimensions.
We have to do them in real time.
We cannot do the recoitation.
So computationally,
it's also an extremely
hard problem. Mhmm. Yeah. We're kind of
we don't have too many tools to do
real time simulations,
and we would need enormous computational
power.
And so maybe that will be in our
future. Maybe we have quantum simulations,
but these are some of the reasons. You
know? So but then there's a push to
develop computational tools,
which was part of my research, and, you
know, these are also useful for
as a a port, like, just for condensed
matter, for instance. And, Bianca, I wanted to
ask you about your research here at the
Perimeter Institute. You lead
the from
discretum to continuum
initiative.
What are the goals of this initiative?
Yeah. So,
the the discretum comes from the fact that,
part of my research is about,
nonperturbative
models of quantum gravity,
which often have discrete aspects. So they're either
you start
directly with kind of discrete building blocks.
Then if it comes to spin forms or
loop quantum gravity,
it's in fact that discreteness is a kind
of derived. It comes to the forms of
formalism
that,
certain geometric quantities
of discrete spectra.
And so, in fact, you have also discrete
building blocks.
And then the big question is, well, it's
it's very similar to to condensed matter, you
know, where you have
atoms or your your fundamental particles, and you
try to understand
the screw the interaction, you know, what is
the material
or who if you have water,
eventually, you want to derive
the flow equations for water.
How does it behave? And so to get
to the continuum limit to show that you
get,
well, space time as we know it, looking
somewhat smooth and continuous.
And so that is a is a
aim of the initiative.
That was in particular to to develop
numerical methods,
and we made progress there.
But we made also recent progress in actually
showing that,
the spin forms which
are mentioned there
do indeed seem to have,
a suitable continuum limits.
Yeah. I wanted to ask you, Bianca, about
spin foams. They sound intriguing.
What what is a spin foam model,
and, how could it provide insights into quantum
gravity?
So, yeah, spin forms come from an approach
where we really take,
Einstein's theory of general relativity
as describing
the gravitational interaction or the gravitational force as
a property of space and time or the
geometry of space and time.
So we take this geometry of space and
time as our dynamical field,
and,
this geometry is then quantized.
And as a result of this procedure,
it turns out that,
indeed, as I mentioned,
for instance, areas and volumes
appear in in
discrete
this discrete spectra. So they kind of come
all in little chunks.
So that's the foam bit, the the little
chunks? That's a foam bit. So the the
the little chunks.
And, basically, what the spin form
model provides is an kind of a quantum
amplitude
for this chunk.
More specifically, what is the geometry of this
chunk? So probability amplitudes for
how big this chunk is.
And so that's one of these,
aspects that,
it's like how big and what is the
scale of this chunk.
But it's a dynamic variable, whereas usually in
physics and all other
quantum field series, it's something which we kind
of,
dial in from the outset.
And so that's one of these
interesting aspects which make it a bit challenging.
But we have these amplitudes of each chunk,
and the key difficulties then is to show
that if we kind of consider lots and
lots and lots and lots of these chunks,
we do get something, you know, like a
smooth space time with gravitational waves.
So there is a kind of computational
or as a,
the the well,
the key aim
to to show and then to show also
what would be effect effects behind having gravitational
waves.
I see. And
you you're originally from Germany, but you've been
associated with the Perimeter Institute for twenty two
of its twenty five years.
What initially attracted you to the institute, and
and what does it offer
physicists who,
who are interested in quantum gravity? You can
use this as a recruitment pitch
for hiring new colleagues if you'd like. I
I, you know, I came the first time
as a as a PhD student,
but with my supervisor, it's always good to
be kind of spatially near to your supervisor.
So,
but I also heard
at that point, Perimeter Institute was a very
new institute,
and, it was doing, you know, really research
in quantum gravity, which
well, certainly around 2003,
I guess, was done only at very few
places.
And,
well, even being
quite young, I was already, like, very interesting
people there in quantum in quantum gravity
research.
And so that also then motivated me, you
know, to come back as a postdoc
and to come back as a faculty. I
mean, first of all, kind of,
we have
a we we are excellent in quantum gravity
research.
We
have
a
large group, excellent people. And, historically,
indeed,
from from 2001
on, since since the institute was founded,
we produced,
like, lots of postdocs, which went on to
be faculty in other places. So kind of,
it's really center for quantum gravity research. But
beyond that, we
have lots of other theoretical physics
directions.
For instance,
you know, in trying to develop numerical methods,
I was interacting with condensed matter people here.
There's strong gravity,
which also there's lots of close interactions with
quantum gravity or cosmology.
So it's great to have all these different
fields,
because,
you know, very interesting things happens
happened at these intersections.
I see. I mean, your your responsibilities
here, do you I mean, primarily, you you
do research. But do you do you do
any teaching, you know, for example, at the
University of Waterloo, or is is it mostly
a
research position?
Yeah. I'm research faculty, so,
well, I don't kind of
have to do teaching.
I did
I did talk
in in the in the outside master
course, also at some point at AIMS in
Africa. So I, I did it a couple
of times.
Currently, I'm also faculty chair, so kind of
busy with There's lots of administration.
It was administration
things.
And so
yeah. So there's kind of
some teaching we do on,
on
on particular master on a master basis and
PhD level basis.
But, otherwise,
it's very focused on on research.
And and finally, we talked a bit about
the difficulty
of getting
data, experimental data on quantum gravity. You mentioned
the Planck scale and, you know, having to
think about photons travelling across the universe to
Yep. But to maybe see
something.
Does that discourage you sometimes when you're when
you're doing your theoretical work? Or
do you do you I suppose you have
to have faith in the mathematics. Do you?
Is that Yeah. I guess it depends. It's
it's possibly, you know, different generations have different
expectations.
So,
I'm possibly coming from a generation which is
possibly okay. It takes a long time to
kind of do quantum gravity, maybe, like, twenty
years before me. People
are expecting
the solve it in ten years.
Well, there's certainly always a certain level of
excitement,
about upcoming experiments. So as I mentioned, indeed,
where there's kind of a number of proposals
to,
to measure things.
Are are there any upcoming experiments that you're
really excited about, or or have you been
involved in any proposals?
I well, I have not been so I
mean, I've coming from a more theoretical,
directional sense of,
for spin for models, a key aspect for
us was to show that, you know, we
get to,
a smooth space time, which,
you know, actually well, if I can
say why why would that be a key
He said it is, that
this was tried very often with various models,
but most models use this idea of a
weak quotation, so so called Euclidean models.
And also, you start with four dimensional building
blocks. It turned out if you try to
glue them together into simulations,
statistical simulations,
most of the resulting structures you saw at
larger scales
didn't look at all like smooth manifolds. So
they looked like fractals, for instance. So we
actually, technically, they're called like polymers.
And so that's happened for for
twenty years. So it was only recently we
we see,
we have one model where this does not
happen. So it's kind of,
what be
a key aspect to show non perturbatively
that we get a smooth space time.
However, I made
a particular progress with spin forms in a
more perturbative
approach, so kind of,
and we could show that we get,
a gravitational waves. Assuming that we get a
smooth space time, we do get we can
show that we have gravitational waves,
and we get also corrections to the dynamics
compared
to general relativity.
So I'm kind of in the process indeed
to
think about experiments.
No. That's it. So so are those
those deviations, are they something that could be
seen by
LIGO or maybe a future experiment? To do
the calculations.
Right. But that would be a you know,
that would be, interest
some some things to to explore.
It's actually also turns out that,
you know, spin forms are technically kind of
it's
in principle, is a penalty violating
theory if a certain parameter is not has
not a very particular value. And so
this is something which
could provide more of an experimental signature because
everything else is not penalty
violating.
So,
on the other hand, it's very constraining, so
I can see that.
But there's, of course, many other
ideas and and and proposals
of of experimental signatures
that,
are always
seem to be very promising. And, so, and
that indeed includes gravitational waves or probing,
probing's a big bang via cosmology.
So has that I mean, with the, you
know, discovery of gravitational
waves and also, yeah, I suppose the information
that we have, you know, from the cosmic
microwave background, etcetera,
You know, the
I suppose these really amazing
measurements
Mhmm. Are they I mean, is that something
that peep people who work on quantum gravity
are very excited about? Is this possibly opening
up a new
view on the universe that could be very
useful to you? I mean, it's certainly certainly
that's what we hope. You know, on the
technical side, and the, you know, the gravitational
wave observations
are also highly technical in itself.
So there needs to be, well, some people
which bridge,
like, the theoretical chronology side to the observational
side
because there are many questions about about noise.
No. But,
physically,
you know, there's this idea where,
yeah, for instance, with a cosmological
microwave background, so the elect
electromagnetic
radiation with electromagnetic
electromagnetic radiation, we can probe
early universe cosmology only so far
because we need well, it's only after, you
know, electromagnetic radiation decoupled from everything else.
Whereas this, kind of if we could, like,
gravitational wave background,
that goes much further to the towards the
Oh, I see. Yeah. The big bang. So
you would expect to
see much deeper to towards
the beginning of the universe,
the beginning of time. And so would that
take you into a regime where quantum gravity
was significant in terms of the evolution More
of the universe. Certainly more significant. Right. So
kind of
yeah. So
and,
yeah, certainly
more significant
and, with,
this kind of
now also microwave
measurements, which hopefully at some point,
we could
see nonlinear effects.
And also for the nonlinear effects,
we would see the quantum gravity would play
more of a role than form of the
linear linear
things. And, there are also predictions of
kind of predictions of, yeah, deviations for the
linear theory,
like the tilt of the spectrum and,
which currently my understanding, you know, we cannot
distinguish these effects, but,
hopefully, in the future, we will. Oh, wow.
So that well, that's really exciting then. I,
I mean, I suppose my view of quantum
gravity has always been, well, they'll never get
any experimental data. I mean, it sounds like
there could be lots coming through the pipeline.
Yeah. So so we you always hope that
this
people will always predict some things. Yeah. Well,
that Yeah. That's great. Thanks so much for
explaining that to us, Bianca. And, best of
luck with your research in the future.
Thank you. It was nice to talk to
you.
That was Bianca Dietrich
who spoke to me at the Perimeter Institute
for Theoretical Physics.
Thanks to Bianca for a fascinating
conversation
about quantum gravity.
And a special thanks to our producer Fred
Ailes.
We'll be back again next week. See you
then.