10 ms·

Carlo Rovelli on challenging our common-sense notion of time

110 points by Deinos 8y ago

gtrubetskoy 8y ago
Would it be fair to say that the second law of thermodynamics is only "a law" given our human perspective of the direction of time? If time has no "arrow", but a memory is only possible in non-decreasing entropy which is our perspective, there can be another perspective in which the big bang is the future and our future is the past, only we cannot comprehend it because a memory is not possible in decreasing entropy?
- danharaj 8y ago
  The second law of thermodynamics is about entropy, which is an observer dependent quantity. It is the discrepancy between what one knows about a system (e.g. its temperature) and its state (e.g. the position and momentum of every particle in a classical gas). A being that knows the exact state of a system does not observe entropy increasing or decreasing. At least classically. I don't understand the rest of your question.
  - SomeHacker44 8y ago
    Digging deep into my memory of classical mechanics, I think this is along the right lines. However, I believe (seem to recall) entropy is an actual measure of the number of ways a system can be ordered in its details and still produce the same generalized outcome. A crystalline solid has relatively low entropy because you can be reasonably sure where each nucleus is, as they are highly ordered. A gas or plasma can have the nuclei distributed nearly randomly. Both systems can be measured generally (stochastically, or of their overall or average properties) but the number of ways you can organize all the nuclei to get those results very different between the two systems. Feel free for someone more knowledgeable to correct or expand.
    - dodobirdlord 8y ago
      That's basically the gist of it. The entropy of a system is proportional (via Boltzmann's constant) to the natural log of the number of available microstates, where microstates are configurations the system could be in, and available denotes that they have the same total value for all conserved quantities as the aggregate system. Consequentially the system is free to spontaneously transition between any of its available microstates.
- rixed 8y ago
  Even if we were not able to tell which is which between the past and the future (as in Feynman's dropped egg though experiment), it would still be paradoxical that we could still easily _distinguish_ them in most macroscopic changes, given that's supposed to be caused by a superposition of time symmetrical microscopic changes. Even with no observer, no past and no future, why are most macroscopic events asymmetric?
jnurmine 8y ago
I am not really skilled in physics and have never understood why time would be a tangible dimension like, say, width or height, instead of a mathematical construction to argue about change. I get it that it is convenient to talk of change in something, like movement from state a to state b, under the guise of wrapping it in "time", but is there some physical argument in support of time in general? Honest question. Living beings getting older is their cells becoming more and more inefficient in division and cell repair. One could likely achieve the same effects with chemicals, but doing so would not mean time has run faster. The concept of time for humans seems to be all about observable change, which needs an observer with a memory to compare the current state with previous state to be able to say: time has passed. A rock changes via erosion and such, but it has no memory, and cannot observe anything. Does a rock feel time? Of course not. Does it exist "in time"? Does it, without an observer that somehow measures the flow of time (via changes in Cesium atoms or something)? Or is the rock just existing and under the whims of all forces of nature that might impact it and change it into smaller pieces and eventually to sand, and so on. I guess my question is: what exactly is time, physically, and why should it have to exist as some sort of a physical process in the first place.
- ksdale 8y ago
  I know next to nothing about physics, but I think time is more concrete than that. There have been experiments in which they've measured discrepancies in different clocks according to what physics predicts. That is, time passes differently according to your position/speed in space. In that sense, time isn't just arbitrarily declared by the guy holding the clock. I'm not sure if that addresses your concerns at all...
  - mrhappyunhappy 8y ago
    So, we know nothing.
    - goatlover 8y ago
      We know more about time than people prior to Einstein.
mathgenius 8y ago
> Unlike general relativity, quantum mechanics, and particle physics, thermodynamics embeds a direction of time. This is the bit that irks me. Quantum mechanics, the real QM that physicists actually use, involves collapse of the wavefunction. This is absolutely time-asymmetric. But in all these discussions of "why is time one-way?" this never seems to be mentioned. Apparently QM is not a real theory, and just a placeholder until we can work something out properly. Irk!
- jblow 8y ago
  Many physicists do not believe that collapse of the wavefunction is a thing.
  - deleted 8y ago
    [deleted]
  - mathgenius 8y ago
    Ok, but why is there no discussion at all? I stand by my irk.
    - whatshisface 8y ago
      You can get every conclusion you want out of QM without any wavefunction collapses, so there's no need to discuss it (depending on your purpose) and if you're an MW-er you'd argue that, further, there's no reason to believe that it even happens. Since there are several self-consistent "piles of words" that all talk about the same math but contradict each other, (Does the wavefunction collapse? Does 1600s literature simulate the simulacrum?) not talking about it seems completely justified.
      - mathgenius 8y ago
        > You can get every conclusion you want out of QM without any wavefunction collapses I don't think so. > "piles of words" I definitely agree about the piles. Maybe ordinary language is just to feeble for this stuff.
deleted 8y ago
[deleted]
interfixus 8y ago
> In fact, clocks tick slower when they are in a stronger gravitational field They shouldn't, if they are properly made. They should tick and measure at their usual rate. Their local timeframe is out of whack with somewhere else, but that's got nothing to do with the clocks.
- bagacrap 8y ago
  I don't know what you're basing this assertion on, but "The Most Unknown" highlights NIST and CU researchers who are building hyper accurate clocks with the express purpose of using them as mass detectors.
  - interfixus 8y ago
    Exactly. Hyper accurate clocks. Don't gain and don't lose. Which is why you may use them to detect differences in the flow of time in various locations. It's just one of my peeves, "clocks slowing down in hight gravity". Nonsense, it's time itself which "slows down".
    - johnalamosisi 8y ago
      wat
- gregdunn 8y ago
  It's unfortunate you're getting downvoted, because this is correct. For anyone that is dubious: Black holes are one of the common ways this gets discussed, because it's such an extreme example, and you can read more about it at http://www1.phys.vt.edu/~jhs/faq/blackholes.html#q11 http://www1.phys.vt.edu/~jhs/faq/blackholes.html#q11 - this goes into a bit about photons and the event horizon - you can ignore that and just consider it in regards to the time dilation effects of gravity.
  - interfixus 8y ago
    Thank you. It is faintly disturbing how many downvotes short, factually incontrovertible comments may often garner on HN.
rbrbr 8y ago
This guy uses rather confusing descriptions of his theories and ideas. I respect his approach but other are much better in explaining these topics.
- keithpeter 8y ago
  Rovelli mentions his paper from 2009 with the title 'Forget Time' in the video. The paper can be downloaded from [1]. I enjoyed the presentation. Talking for an hour on this kind of material with just a piece of string and a couple of watches is fairly impressive. The thought experiment with the wooden box and green/red balls (in two size categories unrelated to their colour) was thought provoking. I'd like to see someone define/construct two sets of macrostates that give a different time scales in a system with one set of microstates. For all I know someone may have actually done that. The paper referenced does have a section on the 'thermal time hypothesis'. [1] https://arxiv.org/abs/0903.3832 https://arxiv.org/abs/0903.3832
kwhitefoot 8y ago
If you found that interesting you might also enjoy Rovelli's defence of Aristotle's physics: https://arxiv.org/abs/1312.4057 https://arxiv.org/abs/1312.4057
eveningcoffee 8y ago
We know that the universe went through various very fast state changes in its very first fraction of seconds. Given that the gravity must have been immerse then how should we understand this time?
- gregdunn 8y ago
  >Given that the gravity must have been immerse then how should we understand this time? We don't really know. There are speculative theories, but they don't all agree. It's very difficult because the temperature would have been so hot at the very start that instead of four fundamental forces (of which gravity is one), we would have had only one. There's also the Hartle-Hawking State, which is a theory that proposes at the very "start" there simply was not time, only space, and that time coming into existence was part of the process that occurred during the planck epoch. There's also the Grand unification epoch, inflationary epoch, electroweak epoch, quark epoch, and hadron epoch that occurred in the universe before a second had passed, all with very different implications for how physics could have worked, so the answer would change multiple times during that period.

110 points by Deinos 8y ago

Would it be fair to say that the second law of thermodynamics is only "a law" given our human perspective of the direction of time? If time has no "arrow", but a memory is only possible in non-decreasing entropy which is our perspective, there can be another perspective in which the big bang is the future and our future is the past, only we cannot comprehend it because a memory is not possible in decreasing entropy?

danharaj 8y ago
The second law of thermodynamics is about entropy, which is an observer dependent quantity. It is the discrepancy between what one knows about a system (e.g. its temperature) and its state (e.g. the position and momentum of every particle in a classical gas). A being that knows the exact state of a system does not observe entropy increasing or decreasing. At least classically. I don't understand the rest of your question.
- SomeHacker44 8y ago
  Digging deep into my memory of classical mechanics, I think this is along the right lines. However, I believe (seem to recall) entropy is an actual measure of the number of ways a system can be ordered in its details and still produce the same generalized outcome. A crystalline solid has relatively low entropy because you can be reasonably sure where each nucleus is, as they are highly ordered. A gas or plasma can have the nuclei distributed nearly randomly. Both systems can be measured generally (stochastically, or of their overall or average properties) but the number of ways you can organize all the nuclei to get those results very different between the two systems. Feel free for someone more knowledgeable to correct or expand.
  - dodobirdlord 8y ago
    That's basically the gist of it. The entropy of a system is proportional (via Boltzmann's constant) to the natural log of the number of available microstates, where microstates are configurations the system could be in, and available denotes that they have the same total value for all conserved quantities as the aggregate system. Consequentially the system is free to spontaneously transition between any of its available microstates.
rixed 8y ago
Even if we were not able to tell which is which between the past and the future (as in Feynman's dropped egg though experiment), it would still be paradoxical that we could still easily _distinguish_ them in most macroscopic changes, given that's supposed to be caused by a superposition of time symmetrical microscopic changes. Even with no observer, no past and no future, why are most macroscopic events asymmetric?

jnurmine 8y ago

I am not really skilled in physics and have never understood why time would be a tangible dimension like, say, width or height, instead of a mathematical construction to argue about change. I get it that it is convenient to talk of change in something, like movement from state a to state b, under the guise of wrapping it in "time", but is there some physical argument in support of time in general? Honest question. Living beings getting older is their cells becoming more and more inefficient in division and cell repair. One could likely achieve the same effects with chemicals, but doing so would not mean time has run faster. The concept of time for humans seems to be all about observable change, which needs an observer with a memory to compare the current state with previous state to be able to say: time has passed. A rock changes via erosion and such, but it has no memory, and cannot observe anything. Does a rock feel time? Of course not. Does it exist "in time"? Does it, without an observer that somehow measures the flow of time (via changes in Cesium atoms or something)? Or is the rock just existing and under the whims of all forces of nature that might impact it and change it into smaller pieces and eventually to sand, and so on. I guess my question is: what exactly is time, physically, and why should it have to exist as some sort of a physical process in the first place.

ksdale 8y ago
I know next to nothing about physics, but I think time is more concrete than that. There have been experiments in which they've measured discrepancies in different clocks according to what physics predicts. That is, time passes differently according to your position/speed in space. In that sense, time isn't just arbitrarily declared by the guy holding the clock. I'm not sure if that addresses your concerns at all...
- mrhappyunhappy 8y ago
  So, we know nothing.
  - goatlover 8y ago
    We know more about time than people prior to Einstein.

mathgenius 8y ago

> Unlike general relativity, quantum mechanics, and particle physics, thermodynamics embeds a direction of time. This is the bit that irks me. Quantum mechanics, the real QM that physicists actually use, involves collapse of the wavefunction. This is absolutely time-asymmetric. But in all these discussions of "why is time one-way?" this never seems to be mentioned. Apparently QM is not a real theory, and just a placeholder until we can work something out properly. Irk!

jblow 8y ago
Many physicists do not believe that collapse of the wavefunction is a thing.
- deleted 8y ago
  [deleted]
- mathgenius 8y ago
  Ok, but why is there no discussion at all? I stand by my irk.
  - whatshisface 8y ago
    You can get every conclusion you want out of QM without any wavefunction collapses, so there's no need to discuss it (depending on your purpose) and if you're an MW-er you'd argue that, further, there's no reason to believe that it even happens. Since there are several self-consistent "piles of words" that all talk about the same math but contradict each other, (Does the wavefunction collapse? Does 1600s literature simulate the simulacrum?) not talking about it seems completely justified.
    - mathgenius 8y ago
      > You can get every conclusion you want out of QM without any wavefunction collapses I don't think so. > "piles of words" I definitely agree about the piles. Maybe ordinary language is just to feeble for this stuff.

deleted 8y ago

[deleted]

interfixus 8y ago

> In fact, clocks tick slower when they are in a stronger gravitational field They shouldn't, if they are properly made. They should tick and measure at their usual rate. Their local timeframe is out of whack with somewhere else, but that's got nothing to do with the clocks.

bagacrap 8y ago
I don't know what you're basing this assertion on, but "The Most Unknown" highlights NIST and CU researchers who are building hyper accurate clocks with the express purpose of using them as mass detectors.
- interfixus 8y ago
  Exactly. Hyper accurate clocks. Don't gain and don't lose. Which is why you may use them to detect differences in the flow of time in various locations. It's just one of my peeves, "clocks slowing down in hight gravity". Nonsense, it's time itself which "slows down".
  - johnalamosisi 8y ago
    wat
gregdunn 8y ago
It's unfortunate you're getting downvoted, because this is correct. For anyone that is dubious: Black holes are one of the common ways this gets discussed, because it's such an extreme example, and you can read more about it at http://www1.phys.vt.edu/~jhs/faq/blackholes.html#q11 http://www1.phys.vt.edu/~jhs/faq/blackholes.html#q11 - this goes into a bit about photons and the event horizon - you can ignore that and just consider it in regards to the time dilation effects of gravity.
- interfixus 8y ago
  Thank you. It is faintly disturbing how many downvotes short, factually incontrovertible comments may often garner on HN.

rbrbr 8y ago

This guy uses rather confusing descriptions of his theories and ideas. I respect his approach but other are much better in explaining these topics.

keithpeter 8y ago
Rovelli mentions his paper from 2009 with the title 'Forget Time' in the video. The paper can be downloaded from [1]. I enjoyed the presentation. Talking for an hour on this kind of material with just a piece of string and a couple of watches is fairly impressive. The thought experiment with the wooden box and green/red balls (in two size categories unrelated to their colour) was thought provoking. I'd like to see someone define/construct two sets of macrostates that give a different time scales in a system with one set of microstates. For all I know someone may have actually done that. The paper referenced does have a section on the 'thermal time hypothesis'. [1] https://arxiv.org/abs/0903.3832 https://arxiv.org/abs/0903.3832

kwhitefoot 8y ago

If you found that interesting you might also enjoy Rovelli's defence of Aristotle's physics: https://arxiv.org/abs/1312.4057 https://arxiv.org/abs/1312.4057

eveningcoffee 8y ago

We know that the universe went through various very fast state changes in its very first fraction of seconds. Given that the gravity must have been immerse then how should we understand this time?

gregdunn 8y ago
>Given that the gravity must have been immerse then how should we understand this time? We don't really know. There are speculative theories, but they don't all agree. It's very difficult because the temperature would have been so hot at the very start that instead of four fundamental forces (of which gravity is one), we would have had only one. There's also the Hartle-Hawking State, which is a theory that proposes at the very "start" there simply was not time, only space, and that time coming into existence was part of the process that occurred during the planck epoch. There's also the Grand unification epoch, inflationary epoch, electroweak epoch, quark epoch, and hadron epoch that occurred in the universe before a second had passed, all with very different implications for how physics could have worked, so the answer would change multiple times during that period.