4 ms·

Here is a cosmological issue from Noether's Theorem. An expanding universe shows time asymmetry, therefore it might not have conservation of energy. This looks

by btilly 2y ago

Here is a cosmological issue from Noether's Theorem. An expanding universe shows time asymmetry, therefore it might not have conservation of energy. This looks like it actually happens. Photons going through empty space go through cosmological redshift, reducing their energy over time. The energy does not appear to go anywhere - it is just gone. I have no idea why this example is not more widely discussed.

nyrikki 2y ago
You are confusing the map for the territory. Under our best current theory (map) General relativity, total energy might not be conserved globally, the divergence of the stress-energy-momentum tensor is zero, meaning that energy is conserved locally within a small region of spacetime. Physics is about producing models that make accurate predictions, it is a map, not the territory itself. The 'crisis in cosmology' e.g. Hubble tension is most likely a sign that current models of the universe are incomplete. Energy is conserved in static spacetimes and asymptotically flat spacetimes. The Friedmann-Robertson-Walker spacetimes that cosmology often uses are not static nor asymptotically flat. It is widely discussed, but all models are wrong, some are useful. Noether's theorm is a power tool to find useful models.
- mjburgess 2y ago
  > Physics is about producing models that make accurate predictions Very few models in physics ever make accurate predictions -- only in very limited experimental circumstances, mostly ones inaccessible at the time these models were developed. The ability to craft these experimental conditions, which enable accurate prediction, is predicted on the models actually describing reality. How else would one control the innumerable number of causes, and construct relevant devices, if these causes did not exist and the devices werent constructed to measure reality? No no, the hard sciences are not concerned about prediction at all. They are concerned about explanation -- it is engineers who worry about predictions, and they quickly find that vast areas of science -- esp. physics -- is nearly impossible to use for predictive accuracy.
  - adrianN 2y ago
    Classical mechanics makes sufficiently accurate predictions to enable essentially all the engineering we do and was invented after we have been engineering things for a few millennia.
    - mjburgess 2y ago
      If you construct highly controlled experimental conditions predicated on classical mechanics being true, then in those scenarios, the model predicts. But in almost all cases it fails to predict, because the situation is vastly too complex to model. You are only able to construct devices (eg., steam engines, baloons, etc.) which are "simple" in the relevant ways, because classical mechanics successfully explains real properties of objects. If it didn't, you'd have no idea how to take an ordinary situation like, "dropping some objects off a cliff" into one where you could actually predict where they will land (ie., by waiting for a day with no wind, by shaping the objects to limit drag, and so on --- without controlling for these accidental features, you'd not be able to predict where anything would land other than "down there somewhere").
      - adrianN 2y ago
        We have been dropping artillery shells relatively accurately using classical mechanics for a couple of decades now.
        mjburgess 2y ago
        By constructing a gun and a shell -- this is my point. We engineer situations we can predict.
  - nyrikki 2y ago
    Western reductionism/Laplacian determism was falsified several times by counterexamples like quantum superposition, Cantor diagonalization etc. The System Identification Problem has also been shown to be equivalent to the halting problem. https://philarchive.org/rec/DIEEOT-2 https://philarchive.org/rec/DIEEOT-2 The common joke about spherical cows in physics also points to the predictive, descriptive nature of the field. The equivalence of various QM interpretations also points to the scientific realist views as being incorrect. Rice's theorm, Gödel, Wada property etc... also demonstrate the problems of confusing the map with the territory. There are further topics like indecomposable continua that arise frequently and naturally in nonpathological dynamical systems. Especially with time delayed ODEs, Hamilton systems etc? Are you arguing that Hamilton systems aren't 'physics'? The value of western reductionism is finding 'effective procedures' but teaching it as being reality is more about didactic convention and convenience. 'Hard science' is a term for study the universe through theories, hypotheses and experiments. It is still about making predictions that match observations. This it is descriptive vs prescriptive.
mjburgess 2y ago
I think it's widely known amongst physicists that energy conservation doesn't hold at cosmological scales. I've not heard of redshift being a case of this -- I'd imagine because the scales at which conservation breaks down are, to my recollection, none where you could observe red-shifted photons, or anything at all because these "scales" entail causal isolation. Eg., two areas of the universe which are totally causally isolated from each other, may across them, violate various laws of conservation. However I do not recall seeing any reason for the latter claim, and it's something I took to be implied about the kinds of conservation violation that GR entails (ie., GR is a locally-conservative theory).
- whatshisface 2y ago
  The necessity of time symmetry for energy conservation can be a little overstated. As long as the laws are holonomic, there will be a conserved quantity corresponding to the motion. You can call that quantity the energy. It won't be conserved if you jump forwards in time without allowing the state to change, but that can't ever happen, so is it really an issue?
  - mjburgess 2y ago
    The problem with 'motion' as the load-bearing property here is that there's an infinite number of derivatives at stake: x', x'', x''', etc. -- what is "the motion" ? What is supposed to be conserved about that motion? The intuition here seems to be that there's a continuity of some property involved in the "transmission of matter over time" which is unbroken, but its not clear what this is supposed to imply. It doesnt seem to imply, for example, that the universe operates like a closed system of motion simply because this property (whatever it is) is unbroken. There can be "global motion" without the need for discontinutiy/randomization/discretizatino/etc. in the trajectories of matter.
Iolaum 2y ago
In those scales (cosmology+Field Theory) energy is derived and defined as the "thing" that is conserved after you apply time symmetry. Also you kinda want time symmetry in a physical system otherwise you have no guarantee that today's laws of physics will be valid tomorrow.
- btilly 2y ago
  As much as we want time symmetry, we still live in an expanding universe. So we have at least some asymmetry.
  - Iolaum 2y ago
    Time symmetry applies to the physical laws governing the universe, not the distribution of mass in it. For a simpler example look at Newton's 3 laws. They have time symmetry but still allow for things to move around!
    - btilly 2y ago
      It also doesn't apply to the structure of space-time. Which is not a situation that Newton's laws can deal with.
ajkjk 2y ago
It's widely discussed in physics, I don't know why you would have the impression it isn't.
invalidator 2y ago
"as the Universe expands, photons lose energy. But that doesn't mean energy isn't conserved; it means that the energy goes into the Universe's expansion itself, in the form of work." -- https://www.forbes.com/sites/startswithabang/2015/12/19/ask-ethan-when-a-photon-gets-redshifted-where-does-the-energy-go/ https://www.forbes.com/sites/startswithabang/2015/12/19/ask-...
- btilly 2y ago
  That only works if radiation density and expansion are connected. General Relativity says that they aren't.
  - mnky9800n 2y ago
    How does general relativity say that?
    - btilly 2y ago
      The term for the stress-energy tensor has the wrong impact on expansion for a simple relationship of any kind. You can rescue it with pseudo-tensors, but then you force the existence of a preferred reference frame. Which undermines the principles of GR. Some still argue for that. Others don't.
    - deleted 2y ago
      [deleted]
layer8 2y ago
Wave-function collapse also violates conservation of energy. (Unless you believe in Many-Worlds, that is.)
- whatshisface 2y ago
  A state that can have the expected value of the energy change post measurement isn't considered to have an energy, rather an expected energy.
  - layer8 2y ago
    This is about the energy of the system as described by the Schrödinger equation.
    - whatshisface 2y ago
      If a measurement changes the expected value of the energy, one or both of the initial and final states won't be time-invariant, and you're talking about the time-invariant Schrodinger equation.

by btilly 2y ago

nyrikki 2y ago

You are confusing the map for the territory. Under our best current theory (map) General relativity, total energy might not be conserved globally, the divergence of the stress-energy-momentum tensor is zero, meaning that energy is conserved locally within a small region of spacetime. Physics is about producing models that make accurate predictions, it is a map, not the territory itself. The 'crisis in cosmology' e.g. Hubble tension is most likely a sign that current models of the universe are incomplete. Energy is conserved in static spacetimes and asymptotically flat spacetimes. The Friedmann-Robertson-Walker spacetimes that cosmology often uses are not static nor asymptotically flat. It is widely discussed, but all models are wrong, some are useful. Noether's theorm is a power tool to find useful models.

mjburgess 2y ago
> Physics is about producing models that make accurate predictions Very few models in physics ever make accurate predictions -- only in very limited experimental circumstances, mostly ones inaccessible at the time these models were developed. The ability to craft these experimental conditions, which enable accurate prediction, is predicted on the models actually describing reality. How else would one control the innumerable number of causes, and construct relevant devices, if these causes did not exist and the devices werent constructed to measure reality? No no, the hard sciences are not concerned about prediction at all. They are concerned about explanation -- it is engineers who worry about predictions, and they quickly find that vast areas of science -- esp. physics -- is nearly impossible to use for predictive accuracy.
- adrianN 2y ago
  Classical mechanics makes sufficiently accurate predictions to enable essentially all the engineering we do and was invented after we have been engineering things for a few millennia.
  - mjburgess 2y ago
    If you construct highly controlled experimental conditions predicated on classical mechanics being true, then in those scenarios, the model predicts. But in almost all cases it fails to predict, because the situation is vastly too complex to model. You are only able to construct devices (eg., steam engines, baloons, etc.) which are "simple" in the relevant ways, because classical mechanics successfully explains real properties of objects. If it didn't, you'd have no idea how to take an ordinary situation like, "dropping some objects off a cliff" into one where you could actually predict where they will land (ie., by waiting for a day with no wind, by shaping the objects to limit drag, and so on --- without controlling for these accidental features, you'd not be able to predict where anything would land other than "down there somewhere").
    - adrianN 2y ago
      We have been dropping artillery shells relatively accurately using classical mechanics for a couple of decades now.
      - mjburgess 2y ago
        By constructing a gun and a shell -- this is my point. We engineer situations we can predict.
- nyrikki 2y ago
  Western reductionism/Laplacian determism was falsified several times by counterexamples like quantum superposition, Cantor diagonalization etc. The System Identification Problem has also been shown to be equivalent to the halting problem. https://philarchive.org/rec/DIEEOT-2 https://philarchive.org/rec/DIEEOT-2 The common joke about spherical cows in physics also points to the predictive, descriptive nature of the field. The equivalence of various QM interpretations also points to the scientific realist views as being incorrect. Rice's theorm, Gödel, Wada property etc... also demonstrate the problems of confusing the map with the territory. There are further topics like indecomposable continua that arise frequently and naturally in nonpathological dynamical systems. Especially with time delayed ODEs, Hamilton systems etc? Are you arguing that Hamilton systems aren't 'physics'? The value of western reductionism is finding 'effective procedures' but teaching it as being reality is more about didactic convention and convenience. 'Hard science' is a term for study the universe through theories, hypotheses and experiments. It is still about making predictions that match observations. This it is descriptive vs prescriptive.

mjburgess 2y ago

I think it's widely known amongst physicists that energy conservation doesn't hold at cosmological scales. I've not heard of redshift being a case of this -- I'd imagine because the scales at which conservation breaks down are, to my recollection, none where you could observe red-shifted photons, or anything at all because these "scales" entail causal isolation. Eg., two areas of the universe which are totally causally isolated from each other, may across them, violate various laws of conservation. However I do not recall seeing any reason for the latter claim, and it's something I took to be implied about the kinds of conservation violation that GR entails (ie., GR is a locally-conservative theory).

whatshisface 2y ago
The necessity of time symmetry for energy conservation can be a little overstated. As long as the laws are holonomic, there will be a conserved quantity corresponding to the motion. You can call that quantity the energy. It won't be conserved if you jump forwards in time without allowing the state to change, but that can't ever happen, so is it really an issue?
- mjburgess 2y ago
  The problem with 'motion' as the load-bearing property here is that there's an infinite number of derivatives at stake: x', x'', x''', etc. -- what is "the motion" ? What is supposed to be conserved about that motion? The intuition here seems to be that there's a continuity of some property involved in the "transmission of matter over time" which is unbroken, but its not clear what this is supposed to imply. It doesnt seem to imply, for example, that the universe operates like a closed system of motion simply because this property (whatever it is) is unbroken. There can be "global motion" without the need for discontinutiy/randomization/discretizatino/etc. in the trajectories of matter.

Iolaum 2y ago

In those scales (cosmology+Field Theory) energy is derived and defined as the "thing" that is conserved after you apply time symmetry. Also you kinda want time symmetry in a physical system otherwise you have no guarantee that today's laws of physics will be valid tomorrow.

btilly 2y ago
As much as we want time symmetry, we still live in an expanding universe. So we have at least some asymmetry.
- Iolaum 2y ago
  Time symmetry applies to the physical laws governing the universe, not the distribution of mass in it. For a simpler example look at Newton's 3 laws. They have time symmetry but still allow for things to move around!
  - btilly 2y ago
    It also doesn't apply to the structure of space-time. Which is not a situation that Newton's laws can deal with.

ajkjk 2y ago

It's widely discussed in physics, I don't know why you would have the impression it isn't.

invalidator 2y ago

"as the Universe expands, photons lose energy. But that doesn't mean energy isn't conserved; it means that the energy goes into the Universe's expansion itself, in the form of work." -- https://www.forbes.com/sites/startswithabang/2015/12/19/ask-ethan-when-a-photon-gets-redshifted-where-does-the-energy-go/ https://www.forbes.com/sites/startswithabang/2015/12/19/ask-...

btilly 2y ago
That only works if radiation density and expansion are connected. General Relativity says that they aren't.
- mnky9800n 2y ago
  How does general relativity say that?
  - btilly 2y ago
    The term for the stress-energy tensor has the wrong impact on expansion for a simple relationship of any kind. You can rescue it with pseudo-tensors, but then you force the existence of a preferred reference frame. Which undermines the principles of GR. Some still argue for that. Others don't.
  - deleted 2y ago
    [deleted]

layer8 2y ago

Wave-function collapse also violates conservation of energy. (Unless you believe in Many-Worlds, that is.)

whatshisface 2y ago
A state that can have the expected value of the energy change post measurement isn't considered to have an energy, rather an expected energy.
- layer8 2y ago
  This is about the energy of the system as described by the Schrödinger equation.
  - whatshisface 2y ago
    If a measurement changes the expected value of the energy, one or both of the initial and final states won't be time-invariant, and you're talking about the time-invariant Schrodinger equation.