Carlo Rovelli, loop quantum gravity and point particles

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.
  • #1
TheHeraclitus
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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?
 
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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.
 
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  • #3
TheHeraclitus said:
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

andSo 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.
 
  • #4
TheHeraclitus said:
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?

TheHeraclitus said:
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.
TheHeraclitus said:
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!
TheHeraclitus said:
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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FAQ: Carlo Rovelli, loop quantum gravity and point particles

Who is Carlo Rovelli?

Carlo Rovelli is an Italian theoretical physicist and writer who is best known for his work in the field of quantum gravity. He is one of the founders of loop quantum gravity, a theory that aims to reconcile general relativity with quantum mechanics. Rovelli is also a popular science communicator, having written several books on physics and the nature of time.

What is loop quantum gravity?

Loop quantum gravity (LQG) is a theoretical framework that attempts to describe the quantum properties of gravity. Unlike string theory, which posits that the fundamental constituents of the universe are one-dimensional "strings," LQG suggests that space-time itself has a discrete structure at the smallest scales, composed of tiny loops. This approach aims to unify general relativity with quantum mechanics without requiring extra dimensions or other exotic constructs.

How does loop quantum gravity differ from string theory?

Loop quantum gravity and string theory are both attempts to develop a theory of quantum gravity, but they take very different approaches. LQG focuses on quantizing space-time itself and suggests that space-time has a discrete, granular structure. In contrast, string theory posits that the fundamental particles are not point-like but rather one-dimensional "strings" that vibrate at different frequencies. Additionally, string theory often requires additional spatial dimensions beyond the familiar three, while LQG does not.

What are point particles in the context of quantum physics?

In quantum physics, point particles are idealized objects that have no spatial extent; they are zero-dimensional and occupy a single point in space. Examples include fundamental particles like electrons and quarks in the Standard Model of particle physics. These particles are considered to have no internal structure and are treated as mathematical points in theoretical models.

How does loop quantum gravity address the concept of point particles?

Loop quantum gravity primarily focuses on the quantization of space-time rather than the particles themselves. However, it provides a framework in which the interactions of point particles can be understood within a quantized space-time. In LQG, space-time is composed of finite loops, and particles interact with this discrete structure. This approach offers a different perspective from traditional quantum field theories that treat space-time as a continuous backdrop.

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