# What does Carlo Rovelli mean by "blurring"?

In Rovelli's book The Order of Time, he often refers to blurring. Can you help me to understand what he means?

He says we observe the universe from within it, interacting with a minuscule portion of the innumerable variables of the cosmos. What we see is a blurred image. This blurring suggests that the dynamic of the universe with which we interact is governed by entropy, which measures the amount of blurring.

What does this really mean?

• Unless there is a mathematical expression behind these words, they don't seem to mean too much. Oct 18, 2018 at 21:41
•  Which page? Oct 18, 2018 at 21:46
• Page 154 for that particular quote, but he uses blurring throughout the book. From what I've read, I almost get the feeling that he is saying we can't see the motion of individual atomic particles, we just see a blurred image of them. But that can't be right, can it? Oct 18, 2018 at 21:49

Blurring is Rovelli's "lay reader friendly" term for coarse graining. The following passage from his book makes this clear:

It follows that the notion of certain configurations being more particular than others (twenty-six red cards followed by twenty-six black, for example) makes sense only if I limit myself to noticing only certain aspects of the cards (in this case, the colors). If I distinguish between all the cards, the configurations are all equivalent: none of them is more or less particular than others.[18] The notion of “particularity” is born only at the moment we begin to see the universe in a blurred and approximate way.

Boltzmann has shown that entropy exists because we describe the world in a blurred fashion. He has demonstrated that entropy is precisely the quantity that counts how many are the different configurations that our blurred vision does not distinguish between.

In endnote 18, he says this:

The definition of entropy requires a coarse graining, that is to say, the distinction between microstates and macrostates. The entropy of a macrostate is determined by the number of corresponding microstates. In classic thermodynamics, the coarse graining is defined the moment it is decided to treat some variables of the system as “manipulable” or “measurable” from outside (the volume or pressure of a gas, for instance). A macrostate is determined by fixing these macroscopic variables.

Thus, saying "entropy exists because we describe the world in a blurred fashion" is Rovelli's "lay reader friendly" way of saying that "[t]he definition of entropy requires a coarse graining".

To read the gist of the argument that Rovelli makes in his book in the original scientific language, see Is Time’s Arrow Perspectival?, in which he uses the term coarse graining throughout.

• Note that although Rovelli primarily identifies "blurring" with macrostates, he does mention in passing that quantum indeterminacy is another form of blurring: "The intrinsic quantum indeterminacy of things produces a blurring, like Boltzmann’s blurring, which ensures—contrary to what classic physics seemed to indicate—that the unpredictability of the world is maintained even if it were possible to measure everything that is measurable. Both the sources of blurring—quantum indeterminacy, and the fact that physical systems are composed of zillions of molecules—are at the heart of time." May 15, 2019 at 20:01

He seems to me, reading this review: Rovelli: Physics and Philosophy to be referring to three effects:

Blurring due to the delay in the light from an object necessarily taking time to travel to our eyes, so there is "blurring" of particular point in spacetime for all observers. So the notion of an exact "now" is gone.

The collapse of the notion of “the present” Rovelli considers to be “the most astounding conclusion arrived at in the whole of contemporary physics”.

Blurring due to GR related time delays near massive objects.

And finally:

Indeed, Rovelli points out that our entire conception of reality is blurred, necessarily – we can only discern the big events, not the infinitesimally small – and that this blurring effect offers a kind of mediation between the counter-intuitive quantum world and the Newtonian world in which we live.

The book, which I have started to read, is a translation, (so I would allow for that) and Rovelli is definitely, (imo), writing for a wider audience, and in a more literary style than you will normally find in popular science books.

With regard to entropy, I can only offer my own guess: he is trying to include the disorder inherent in thermodynamic processes to all aspects of the reality we experience.

Yuk, that's a bit flowery, sorry, but I think that's what he's getting at.