4 ms·

I see confusion about reference frame. It is perfectly okay to say something is happening at 1 million light years away. And there is no ambiguity within our pr

I see confusion about reference frame. It is perfectly okay to say something is happening at 1 million light years away. And there is no ambiguity within our presumed Earth reference frame. Of course we won't know for another million years if such a claim is true. But if it is, it is perfectly sane to say it happened a million years ago when we do observe it. The point is given a reference frame, it is perfectly fine to specify an event with location and time, so long as you specify both location and time. These have nothing to do with light cones. [Add] To give another more relevant example if we observe a signal today coming from a location one million light years away, there is no ambiguity to say it happened a million years ago at such and such location. Anyone in a different frame can compute the time and location of the event unambiguously for their own frame, knowing the relative velocity between the frames.

gregdunn 8y ago
> It is perfectly okay to say something is happening at 1 million light years away. Well, maybe. Maybe not. Carlo Rovelli would argue that time doesn't exist like that (or at all) and that saying that something is happening now when it is beyond our lightcone would be a meaningless statement, and a lot of physicists would agree with him. (A lot would disagree, too.) Reference frames only clear this up if time does exist in a meaningful way and 'now' is more than a construction of our own observation.
- raattgift 8y ago
  > (or at all) Rovelli's position is that time is not fundamental, not that it does not exist. It emerges from something more fundamental. That is, he argues that there exists a theory (or family of theories) from which one can derive both General Relativity and Relativistic Quantum Field Theory in their effective limits, and that such a theory does not need to be paramaterized by time (or more broadly, does not need to explicitly reference the notion of time at all). More precisely, he and Connes propose[1] that statistical regularities in a generally covariant quantum system (our block universe) are just as much "laws" as dynamical relations like F = ma or F = dp/dt = \gamma(v)^3 m_0 a_parallel + \gamma(v)^1 m_0 a_perpendicular. There are macroscopic constraints and a low-entropy initial condition in the universe that takes our system away from being in just any configuration with equal probability, and towards a configuration with an entropy gradient. From that entropy gradient we can recover a notion of time, and even coordinatize it. Time emerges in a block universe (and indeed most subregions thereof) arranged in this way. > something is happening now when it is beyond our lightcone Galaxies probably don't cease to exist at the moment they cross out of our causal cone with the metric expansion, or soon afterwards. We exist even though there are observers who saw our ancestors cross out of their causal cones. Do you think the density and spectrum of the CMB evolve very differently for them and for us? Do you assert that such a question is meaningless? Do you think Rovelli does? (If so, why do you think that?) - -- [1] A. Connes, C. Rovelli, "Von Neumann Algebra Automorphisms and Time-Thermodynamics Relation in General Covariant Quantum Theories", Class. Quantum Grav. 11:2899–2918, 1994. https://arxiv.org/abs/gr-qc/9406019 https://arxiv.org/abs/gr-qc/9406019
  - gregdunn 8y ago
    >Rovelli's position is that time is not fundamental, not that it does not exist. It emerges from something more fundamental. Well, I'm a layperson, and I'm guessing you are a physicist (or at least much more inclined towards it than I am), so I'm hesitant to argue here, but... If that's what Rovelli means, he really should say that :) I've got The Order of Time in front of me and in it, as well as plenty of articles and interviews (targeted at laypeople), he says time doesn't exist, uses phrases such as 'A world without time', etc. Good portions of the book are prose where he muses on things like the meaning of life if time doesn't exist. He calls our perception of it an illusion. > our block universe In Order of Time he specifically says '[Our world] is not a "static" world, or a "block universe"'. I feel disconnected here, because I'm definitely not able to argue with what you're saying, but Rovelli's words from a book 23 years newer than your referenced article, seem to directly contradict what you're arguing - he explicitly states he does not believe we live in a block universe. >Galaxies probably don't cease to exist at the moment they cross out of our causal cone with the metric expansion, or soon afterwards. We exist even though there are observers who saw our ancestors cross out of their causal cones. Do you think the density and spectrum of the CMB evolve very differently for them and for us? Do you assert that such a question is meaningless? Do you think Rovelli does? (If so, why do you think that?) No, I am saying specifically that saying something is happening /now/ when it is outside of our light cone is a meaningless statement. It's not a matter of whether or not something exists when outside of our light cone (Though in some ways, as it can never effect us, it might as well not), but whether or not something distant can happen now. It doesn't even need to be outside of our light cone - just far off. 'A present that is common throughout the whole universe does not exist. There is a present that is near to us, but nothing that is "present" in a far-off galaxy. The present is a localized rather than a global phenomenon." Early on in the book, Rovelli argues that the concept of "now" or the present really only applies on a scale that's about the size of the Earth. As for what I believe, I'm not sure. I think Rovelli makes compelling arguments. But I could say the same about Smolin and others. I think I lean towards Rovelli's interpretation, or at least what I (hopefully!) understand of it. I'm perfectly willing to accept that you could be totally correct here and that I am wildly off base - but what I don't understand is why Rovelli's non-science paper writings seem to argue very differently than what you are saying. Am I misinterpreting them? Is he dumbing things down for the layperson? Why does he explicitly say we do not live in a block universe, and that time doesn't exist?

gregdunn 8y ago

> It is perfectly okay to say something is happening at 1 million light years away. Well, maybe. Maybe not. Carlo Rovelli would argue that time doesn't exist like that (or at all) and that saying that something is happening now when it is beyond our lightcone would be a meaningless statement, and a lot of physicists would agree with him. (A lot would disagree, too.) Reference frames only clear this up if time does exist in a meaningful way and 'now' is more than a construction of our own observation.

raattgift 8y ago
> (or at all) Rovelli's position is that time is not fundamental, not that it does not exist. It emerges from something more fundamental. That is, he argues that there exists a theory (or family of theories) from which one can derive both General Relativity and Relativistic Quantum Field Theory in their effective limits, and that such a theory does not need to be paramaterized by time (or more broadly, does not need to explicitly reference the notion of time at all). More precisely, he and Connes propose[1] that statistical regularities in a generally covariant quantum system (our block universe) are just as much "laws" as dynamical relations like F = ma or F = dp/dt = \gamma(v)^3 m_0 a_parallel + \gamma(v)^1 m_0 a_perpendicular. There are macroscopic constraints and a low-entropy initial condition in the universe that takes our system away from being in just any configuration with equal probability, and towards a configuration with an entropy gradient. From that entropy gradient we can recover a notion of time, and even coordinatize it. Time emerges in a block universe (and indeed most subregions thereof) arranged in this way. > something is happening now when it is beyond our lightcone Galaxies probably don't cease to exist at the moment they cross out of our causal cone with the metric expansion, or soon afterwards. We exist even though there are observers who saw our ancestors cross out of their causal cones. Do you think the density and spectrum of the CMB evolve very differently for them and for us? Do you assert that such a question is meaningless? Do you think Rovelli does? (If so, why do you think that?) - -- [1] A. Connes, C. Rovelli, "Von Neumann Algebra Automorphisms and Time-Thermodynamics Relation in General Covariant Quantum Theories", Class. Quantum Grav. 11:2899–2918, 1994. https://arxiv.org/abs/gr-qc/9406019 https://arxiv.org/abs/gr-qc/9406019
- gregdunn 8y ago
  >Rovelli's position is that time is not fundamental, not that it does not exist. It emerges from something more fundamental. Well, I'm a layperson, and I'm guessing you are a physicist (or at least much more inclined towards it than I am), so I'm hesitant to argue here, but... If that's what Rovelli means, he really should say that :) I've got The Order of Time in front of me and in it, as well as plenty of articles and interviews (targeted at laypeople), he says time doesn't exist, uses phrases such as 'A world without time', etc. Good portions of the book are prose where he muses on things like the meaning of life if time doesn't exist. He calls our perception of it an illusion. > our block universe In Order of Time he specifically says '[Our world] is not a "static" world, or a "block universe"'. I feel disconnected here, because I'm definitely not able to argue with what you're saying, but Rovelli's words from a book 23 years newer than your referenced article, seem to directly contradict what you're arguing - he explicitly states he does not believe we live in a block universe. >Galaxies probably don't cease to exist at the moment they cross out of our causal cone with the metric expansion, or soon afterwards. We exist even though there are observers who saw our ancestors cross out of their causal cones. Do you think the density and spectrum of the CMB evolve very differently for them and for us? Do you assert that such a question is meaningless? Do you think Rovelli does? (If so, why do you think that?) No, I am saying specifically that saying something is happening /now/ when it is outside of our light cone is a meaningless statement. It's not a matter of whether or not something exists when outside of our light cone (Though in some ways, as it can never effect us, it might as well not), but whether or not something distant can happen now. It doesn't even need to be outside of our light cone - just far off. 'A present that is common throughout the whole universe does not exist. There is a present that is near to us, but nothing that is "present" in a far-off galaxy. The present is a localized rather than a global phenomenon." Early on in the book, Rovelli argues that the concept of "now" or the present really only applies on a scale that's about the size of the Earth. As for what I believe, I'm not sure. I think Rovelli makes compelling arguments. But I could say the same about Smolin and others. I think I lean towards Rovelli's interpretation, or at least what I (hopefully!) understand of it. I'm perfectly willing to accept that you could be totally correct here and that I am wildly off base - but what I don't understand is why Rovelli's non-science paper writings seem to argue very differently than what you are saying. Am I misinterpreting them? Is he dumbing things down for the layperson? Why does he explicitly say we do not live in a block universe, and that time doesn't exist?