Most popular articles about quantum gravity, they first say,
oh, we all know that quantum theory is not really, you know,
doesn't go well together with general relativity.
And then they say, well, here are the two main contenders:
it's string theory, and it's loop quantum gravity.
These were not so simple theories.
But in reality, I mean, research has really moved away from these.
There other ways to look at the world that are maybe more fruitful,
and therefore, you know, maybe more correct.
And now we are going back to, you know, good old quantum field theory.
Go back to kind of minimal ingredients.
Is there anything new?
Well, the answer is
that it's not only extremely rich dynamics,
and totally unexpected things that happen,
but there is also kind of you can, you know, visible the mathematics that belongs with that.
My name is Renate Loll. I am a professor of theoretical physics
at Radboud University in the Netherlands.
And my main interest is to try and understand what quantum gravity is.
Quantum gravity is really the fundamental theory, quantum theory
underlying classical general relativity.
General relativity has given us like black holes,
a dynamical universe
with a big bang, gravitational waves,
amazingly successful. Now, what I would say we have to go one step further.
So the crucial thing that you want to have from the theory of quantum gravity is,
it must be the case that you recover, you know,
some of the properties of the classical theory.
Do you then see any evidence that interactions happen over long distances?
When you zoom out, you should get out something that's four dimensional:
three space plus one time.
The idea is really, to understand quantum gravity from
very elementary principles that we all know.
The idea has a name, it's called Causal Dynamical Triangulations,
which just describes basically the ingredients we are using.
Causal refers to the fact that we put by hand a causal structure into it,
a time flow.
Causal structure is present, at the most microscopic level of stuff, I'm just putting in.
Dynamical means I'm paying attention to the dynamical character of geometry
when I write down and solve the path integral,
because the path integral is a way, you know,
to take a quantum superposition of curved space-times.
And you need a handle on how do I describe these space-times.
And the triangulations just tell you, you know, what I've done to
make these spaces or space-times
amenable to computation. You know, I,
I put a triangular mesh on them.
Triangles have just,
they explore curved spaces in a quite effective way.
If you take kind of these basic ingredients, and you know,
tell your computer what to do.
Ask the right questions, we so far,
are getting very interesting results out.
The real breakthrough was: there is a universe that comes out.
It seems to have a non trivial shape, that was something that
one could relatively easily understand, the overall shape of
the universe we saw.
And it had a kind of blob shape.
So when we looked at it, you know, in time, so we could monitor things in time,
you could see it, it wanted to start kind of in a point.
And then the spatial universe kind of grew.
And then after it later times, it collapsed back to a point.
So we were able to show that, indeed, the behavior, you know, of this
quantum space, and that was sitting in our computer,
generated, according to the rules
what we now called CDT
on sufficiently large scales became a,
looked like a four dimensional object.
That had never happened before.
This is a priori in these models is absolutely, it's not a given at all.
So the question is, if you have a true quantum theory of gravity,
can we go one step further?
That's where my research is now going.
To understand can we possibly predict
what the very early universe should have looked like?
It's very interesting work,
it's like the work of an experimentalist almost,
you know, to tease out these properties.
It's just so rich and so many things can happen and we've just scratched the surface
of all the things Is that can happen. So it's just the beginning of an,
I would say, probably a very interesting and non-trivial story of,
you know, where quantum gravity maybe is going.