# Carlo Rovelli, loop quantum gravity and point particles

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• TheHeraclitus
In summary, Carlo Rovelli's book Reality Is Not What It Seems discusses loop quantum gravity and the concept of space as a quantum field. Rovelli suggests that space is made up of "nodes" that have volume and shape, unlike other quantum fields which are described as point particles. This raises questions about how loop quantum gravity deals with these particles, but Rovelli's popular-level explanation should be taken with a grain of salt. He also addresses the issue of Lorentz violations in this model and points out that point particles are not a part of quantum field theory.f

#### TheHeraclitus

TL;DR Summary
How are particles described in loop quantum gravity?
I am reading a popular-science book Reality Is Not What It Seems by Carlo Rovelli, one of the founders of loop quantum gravity.
He writes:
The nodes of the graph represent the discrete packets of volume and, as in the case of photons, can only have certain sizes, which can be computed using Dirac's general quantum equation. Every node n in the graph has its own volume vn: one of the numbers in the spectrum of the volume.
and
The nodes are the elementary quanta of which physical space is made. Every node of the graph is a "quantum particle of space".
and
the gravitational field is space. Space is no longer different from matter. It is one of the "material" components of the world, akin to the electromagnetic field. It is a real entity which undulates, fluctuates, bends and contorts.

So basically, space (spacetime) is just another quantum field like all the others, and the quanta of this field is the nod. Nods have volume, which means they must be extended in dimensions, they must have shape. But all quanta of the other fields, for example photon, electron, quark are described as point particles, they have no volume. It seems absurd to me that every quantum field has point particles except gravitational field which has extended particles.

Rovelli also claims there is minimum length in nature, but point particles would be smaller, they are dimensionless.

So, how does Rovelli and Loop quantum gravity deal with these particles?

Particles are "pointlike" in standard quantum field theory in the sense that their wavelength can be arbitrarily small. But this is a problem of standard theory, because it leads to UV divergences. According to loop quantum gravity, particles are actually not "pointlike" in that sense, which is a sort of improvement over the standard theory. But note that it is a popular-level explanation, similar to the book you have been reading, so should be taken with a grain of salt.

PeroK, ohwilleke and malawi_glenn
Summary: How are particles described in loop quantum gravity?

I am reading a popular-science book Reality Is Not What It Seems by Carlo Rovelli, one of the founders of loop quantum gravity.
He writes:

and

and

So basically, space (spacetime) is just another quantum field like all the others, and the quanta of this field is the nod. Nods have volume, which means they must be extended in dimensions, they must have shape. But all quanta of the other fields, for example photon, electron, quark are described as point particles, they have no volume. It seems absurd to me that every quantum field has point particles except gravitational field which has extended particles.

Rovelli also claims there is minimum length in nature, but point particles would be smaller, they are dimensionless.

So, how does Rovelli and Loop quantum gravity deal with these particles?
One big difficulty with a node-like model of space-time is that this can give rise of Lorentz violations (i.e. deviations from special relativity) if taken too literally. But, this problem is not, in general, a problem with all models in which there is a minimum length that do not rigidly try to fit space-time into a fixed discrete network.

So basically, space (spacetime) is just another quantum field like all the others
The other fields are defined at each point on spacetime. How can spacetime be a field defined on itself?

But all quanta of the other fields, for example photon, electron, quark are described as point particles.
They aren't. Especially the photon! They are states of the relevant quantum field.
It seems absurd to me that every quantum field has point particles except gravitational field which has extended particles.
Given it's not part of the theory it may well be absurd!
So, how does Rovelli and Loop quantum gravity deal with these particles?
He doesn't have to, as point particles are not part of quantum field theory.

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