Name: Quantum Gravity
ISBN: 9781500976125

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discoinvernoJan 2024

It is true that there is no experimental evidence, but I think there are some convincing arguments that something must happen at the Planck scale (for very short distances) in a full quantum-gravity theory.

Here are some quotes from "Covariant Loop Quantum Gravity", Rovelli and Vidotto (slightly redacted). I suggest the whole chapter 1, in particular 1.2 to get an idea of why fundamentally spacetime may be discrete.

"In general relativity, any form of energy E acts as a gravitational mass and distorts spacetime around itself. The distortion increases when energy is concentrated, to the point that a black hole forms when a mass M is concentrated in a sphere of radius R ∼ GM/c^2, where G is the Newton constant. If we take L arbitrary small, to get a sharper localization, the concentrated energy will grow to the point where R becomes larger than L. But in this case the region of size L that we wanted to mark will be hidden beyond a black hole horizon, and we lose localization. Therefore we can decrease L only up to a minimum value, which clearly is reached when the horizon radius reaches L, that is when R = L. Combining the relations above, [..] we find that it is not possible to localize anything with a precision better than the Planck length (~10^-35 m). Well above this length scale, we can treat spacetime as a smooth space. Below, it makes no sense to talk about distance. What happens at this scale is that the quantum fluctuations of the gravitational field, namely the metric, become wide, and spacetime can no longer be viewed as a smooth manifold: anything smaller than the Planck length is “hidden inside its own mini-black hole”."

"The existence of a minimal length scale gives quantum gravity universal character, analogous to special relativity and quantum mechanics: Special relativity can be seen as the discovery of the existence of a maximal local physical velocity, the speed of light c. Quantum mechanics can be interpreted as the discovery [..] that a compact region of phase space contains only a finite number of distinguishable quantum states, and therefore there is a minimal amount of information in the state of a system. Quantum gravity yields the discovery that there is a minimal length lo at the Planck scale. This leads to a fundamental finiteness and discreteness of the world."

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Anon84Dec 2022

If I had to pick one book from 2022, I would definitely go with Carlo Rovelli’s “The Order of Time” https://amzn.to/3W4PdVG

In the book, Rovelli explores the concept of time and how it has been understood by scientists and philosophers throughout history. An intellectually stimulating and not overly technical read about the concept of a universe without time and some of the latest developments in Loop Quantum Gravity (Rovelli’s field of work).

I also have a blog post on my Top 10 books from last year: https://medium.data4sci.com/top-10-books-we-read-in-2022-c3d...

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raattgiftNov 2018

I haven't read Rovelli's pop-sci book. Could you glance through the table of contents and/or the index and look for, among other things, "thermal time" and "time emerges in a world without time"? I would be astonished if he did not discuss these ideas at length in his book.

> Rovelli's words from a book 23 years newer than your referenced article, seem to directly contradict what you're arguing

Ok, let's deal with that. It's not like he stopped writing on the topic or substantially changed his position (rather than elucidating it).

You can click on arXiv:1505.01125, arXiv:1407.3384, and arXiV:0903.3832 in the search below; each paper directly supports what I wrote. The first two digits of any arXiv ref is the year of submission.

https://arxiv.org/search/advanced?advanced=&terms-0-operator...

Or alternatively you can take this approach, following his self-citations of Connes & Rovelli 1994:

https://scholar.google.com/scholar?hl=en&as_sdt=2005&sciodt=...

however not all of those are directly relevant to this particular argument.

> he explicitly states he does not believe we live in a block universe

Can you extract from the book the couple paragraphs around where he says that explicitly? I bet it'll be easy to clarify. I'll even take a preemptive guess.

We'll have to digress briefly into (hopefully scientific) philosophy.

The (eternalist) block universe point of view is essentially that the universe is deterministic: if you have the full details of the state of the universe at a particular time, you have the full details of the state of the universe at every other time too. This is fundamental to the https://en.wikipedia.org/wiki/Initial_value_formulation_(gen... about which Rovelli has written extensively (e.g. at https://arxiv.org/abs/gr-qc/0202079 and chapter 4 of his book Quantum Gravity, draft on his own page http://www.cpt.univ-mrs.fr/~rovelli/book.pdf ).

Unfortunately philosophers have several different versions of "block universe" with varying degrees of determinism. The growing block universe, for instance, holds that the past exists, that the present is an objective quantity, and that the future does not yet exist (as in it is not fully determined) at the the present of the growing block. Is it possible that Rovelli was rightly criticizing that, and you or he or his editor missed a "growing" in "growing block universe"?

Returning to his Quantum Gravity book, I'm happy to recommend §2.4 generally, although it is far from pop-sci. In 2.4.4 (p 58, pdf page 76) he distinguishes ten different notions of time. Clearly not all of these can be equally fundamental; his argument is that none of them is fundamental in that they are demanded by all possible theories compatible with General Relativity in its classical limit. However, as some of the ten are directly measurable, one cannot (and he does not) argue they they do not exist -- rather they emerge from correlations between observables.

Now we can leave the philosophical.

The scientific content of his Quantum Gravity book's central conjectures are that not only is it feasible to write down physical behaviours without recourse to time as a fundamental quantity, those behaviours' descriptions take an especially simple form when one does so in the way he proposes through the course of the book. The first of these points is not really scientifically controversial.

> No, I am saying specifically that saying something is happening /now/ when it is outside of our light cone is a meaningless statement.

There is no single correct definition of now in a general curved spacetime; there is instead a democracy of "now"s. Each microscopic portion of your body has its own separate causal cone, and at any particular instant the ones on microscopic things near your scalp are spacelike separated from the ones near the soles of your feet. Does that make stubbing your toe meaningless?

> "The present is a localized rather than a global phenomenon"

It's an idiosyncracy rather than a phenomenon at all. By the time you have perceptual awareness of your toe-stub it is ancient history on the scale of electron interactions between your skin and the leg of the table or whatever, and indeed the daughter products of the scatterings (there will be at least some photons and gravitational waves shed) will have raced past your brain to as far as outside the orbit of the moon. You have free choice of "nows", and no fundamental reason to choose which of them to apply.

> localized

From a purely technical perspective, we can always apply Fermi coordinates to anything freely falling in a Lorentzian spacetime (as ours is, to the best of our ability to test directly or via astrophysical observation). This guarantees a minimum definition of "local". However, the coordinates are peculiar to the object; although we can certainly extend Fermi coordinates into a https://en.wikipedia.org/wiki/Local_reference_frame?oldforma... , frames of reference do not determine the physical behaviours. As you and I drift past each other in deep space, there are only personal reasons to choose a set of Cartesian coordinates with you at the origin over a set of Spherical coordinates with me at the origin.

If we drift past each other with high enough relative velocity, then even when we are extremely close to each other -- much closer than the scale of the earth -- your wristwatch now and my wristwatch now will not generally agree. (Assuming they are identically constructed, if they agree at some instant, they will disagree at other instants). This is true even if at all relevant times we are well within each other's causal cones.

So some ideas of "now" fail at scales much smaller than that of Earth's radius.

Others might survive at scales much larger: we've been doing Einstein-synchronization experiments since the Viking missions to Mars, and we can straightforwardly predict https://en.wikipedia.org/wiki/Barycentric_Dynamical_Time anywhere in the solar system. So we can say at time X TDB Mars-based spaceprobe should activate instrument Y. Although it might take us a couple minutes to verify that spaceprobe activated Y at X, we can be pretty confident before those two minutes expire as our TDB clock registered X, its TDB clock also registered X.

Likewise, we can synchronize in principle with a distant observer of the Cosmic Microwave Background such that we each agree to do something when the CMB's dipole-free spectrum is closest to a blackbody of X kelvins. Assuming honesty, we can be pretty confident that millions of years later we will see that their "CMB clock" registered X kelvins when they began the agreed action.

These are among the infinite supply of idiosyncratic "now"s that can be useful even if they are in no way fundamental. Utility surely implies meaningfulness, even if neither utility nor meaningfulness implies universality.

> As for what I believe, I'm not sure.

Good. Neither am I. Neither is Rovelli, if he is honest. There is a lot of room to research the various problems of time.

> I don't understand [...] why Rovelli's non-science paper writings seem to argue very differently than what you are saying.

As I said, I don't know what the non-science papers say exactly; if you feel like pasting extracts in reply, we can try to sort this out. Any of your guesses in your final paragraph could be right, or it could be something else.

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auxbussMay 2018

In Carlo Rovelli's, Reality is Not What it Seems [0], he speaks of this final point in its short, final chapter.

He starts by using an example from Plato's, Phaedo, where Plato acknowledges the limits of the knowledge of his time – in relation to Socrates being unsure about his "belief" in the Earth being a sphere.

Rovelli says:

"This acute awareness of our ignorance is the heart of scientific thinking. It is thanks to this awareness of the limits of our knowledge that we have learned so much. We are not certain of all which we suspect, just as Socrates was not sure of the spherical nature of the Earth. We are exploring at the borders of our knowledge."

As was Newton, Maxwell, Einstein, amongst others.

Rovelli goes on:

"Science is not reliable because it provides certainty. It is reliable because it provides us with the best answers we have at present. ... They are the best we have because we don’t consider them to be definitive, but see them as open to improvement. It’s the awareness of our ignorance that gives science its reliability."

Science is never settled. Rovelli's book is about Loop Quantum Gravity; as unsettled a field of research in science _today_.

[0] https://www.goodreads.com/book/show/29767627-reality-is-not-...

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