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?

  • 2
    $\begingroup$ Unless there is a mathematical expression behind these words, they don't seem to mean too much. $\endgroup$
    – safesphere
    Oct 18, 2018 at 21:41
  • $\begingroup$ $ $ Which page? $\endgroup$
    – Qmechanic
    Oct 18, 2018 at 21:46
  • $\begingroup$ 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? $\endgroup$ Oct 18, 2018 at 21:49

5 Answers 5


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.

  • $\begingroup$ 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." $\endgroup$
    – Nick Gall
    May 15, 2019 at 20:01

To me blurring as Rovelli uses the term is a reference to the fact that we do not see order in certain states ....such as a shuffled card deck where the red and black cards might have been mixed together. However that shuffled deck may have another specific order that would only matter if one were looking for it. For example a lottery ticket that loses is no different than any other losing ticket (even if it's in sequence) so all those losing tickets get "blurred" together whereas the winning ticket is the only one that matters. How we define disorder or entropy is a result of the "blurring" by which we view all states as equivalent if they are not the order we are looking for. There is a mathematical way to express this....the chance that certain states would exist vs others without the input of heat or energy but it's over my head to get into that. If I'm wrong about this than I understood that book even less than I thought I did!

  • $\begingroup$ It is somehow as to say that a broken glass can't reconstruct itself, but it can't broke exactly in one broken state, neither. (sort of thinking / question). $\endgroup$
    – Alchimista
    May 22, 2021 at 13:20

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.

Read the review linked above, see what you think.

  • $\begingroup$ @user203555 Thanks for pointing to that review. I think your summary is correct. It seems that Rovelli has discussed "blurring" over and over again in the book, but when we look at what we understand him to imply, it's not new at all to anyone who has basic knowledge of time in the relativity sense. $\endgroup$ Oct 19, 2018 at 15:45
  • $\begingroup$ In a book that's beautiful (at least in its English translation), Rovelli describes the thermodynamic nature of time, but seems to be saying that the characteristics we look at to determine the direction of its flow may not always be the appropriate ones for the context, which may broaden the meaning of "low entropy" in cosmology. Re popular relativity, however, he's not supportive of the "block universe" view of time, but sees time as emergent in a way that seems to shift it even further from the absolutism of Newtonian physics than it had been in GR. $\endgroup$
    – Edouard
    May 4, 2019 at 15:43

I think the "blurring" idea (as a macrostate) may have implications for psychology and the study of attention and perception. At least, that is my question here. I understand that this is a physics-related question and there are "levels" issues in applying a core concept from physics (entropy) to human psychology. So my question is meant to open the conversation aside from this point. Rovelli's use of Horace's "Odes" throughout the book encouraged my interdisciplinary thinking here. I am not convinced that his use of the Ode is merely a literary device.

Basically, human attention (and its ontogenic and phylogentic evolution) is about the systematic reduction of blurring and the growing ability to apply any number of mental processes with greater degrees of freedom. More attention capacity = greater ability to reduce any distractions, to focus, and reduce the blur. If this parallel works, than the human perception of "time" at the macro-level is based on attentional capacity. The historical development of clocks -- and more recently, digitization of time -- plays a critical role in attentional capacity and direction: how culture/society manages, masters, and influences individual and societal use of time.

A lot of work has been done on the consciousness of temporality that could further help unpack the definition of blurring -- see https://stanford.library.sydney.edu.au/archives/fall2017/entries/consciousness-temporal/


Entropy of a blob of milk mixed in a cup of coffee is higher than that of a blob of milk separated from a cup of coffee. However, the entropy of one specific time frame of milk mixed in coffee is no different from the entropy of one specific time frame of milk separated from coffee because each of them are single configurations of the two macro-states. In other words, if we do not distinguish all the configurations into two macro-states, but see each micro-states individually, there will be no differences in entropy, because the number of states are 1 for all micro-states. However, because we macroscopically distinguish all micro-states into two macro-states(blurred images), we get the notion of entropy and their differences.

Let's compare a deck of card in ordered and mixed forms. If we only distinguish each card by their colors, red and black, the cards in ordered fashion, reds for the first 26 cards and blacks for the last 26 cards, will have lower entropy than them in mixed order. However, if we truly distinguish each card independently, not only by the colors but also the shape, and the numbers, any order of cards are unique, so the entropy difference between each state will be zero. Hence, the difference in entropy arise only when we do not distinguish each cards independently but only by their colors, etc. In other words, the difference in entropy arise only when we distinguish cards, not by all measures but by blurring, only red and black, etc.


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