Conservation of kinetic energy between neutral fundamental particles in a vacuum?

Discussion in 'Physics & Math' started by one_raven, Aug 4, 2022.

  1. one_raven God is a Chinese Whisper Valued Senior Member

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    The observation that gravitational waves also travel at the speed of light lends credence to concept of a speed limit for kinetic energy.
     
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  3. exchemist Valued Senior Member

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    According to relativity, which has never been faulted, nothing, not even information, can travel faster than c. So it is not just EM radiation, it is anything at all.

    But now you are veering into fairly wild speculations about dark matter for some reason. Forgive me if I do not join you on that journey.
     
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  5. one_raven God is a Chinese Whisper Valued Senior Member

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    Please refer to the Cosmological Crisis. General Relativity plainly incomplete, as it can't account for the observed expansion of spacetime (among other things). Besides, we can apply neither GR nor SR to objects that don't seem to interact with EM radiation. Implying Relativity is infallible is nothing more than dogmatic regurgitation of an incomplete model.

    Feel free to exit the discussion. There's nothing holding you here. I do appreciate the back-and-forth. I appreciate when people challenge me to learn and consider more. I will say, however, that asking questions of the unknown is far less egregiously speculative or fanciful than arbitrarily ascribing properties that we have no way yet of directly measuring.
     
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  7. exchemist Valued Senior Member

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    That's a total misunderstanding on your part. You can - indeed must - apply GR and SR to everything. Whether EM radiation is involved is neither here nor there. Nor is it the job of relativity to account for the expansion of spacetime.The job of relativity is to provide the concept of spacetime in the first place.
     
  8. one_raven God is a Chinese Whisper Valued Senior Member

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    There are a lot of physicists who disagree with your assessment. Fact is, GR predictions fail at the galaxy cluster scale and at the quantum scale. Either it's incomplete, or our other models, means and modes of measurement are incorrect/incomplete. I'm trying to keep an open mind. Thanks for the chat.
     
  9. exchemist Valued Senior Member

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    No physicist thinks GR and SR apply only to objects that interact with EM radiation.
     
  10. one_raven God is a Chinese Whisper Valued Senior Member

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    Apologies. I didn't mean to say it doesn't apply to them at all, nor am I saying relativity is "wrong". I am saying that if they're not impacted by EM radiation, and we know that GR is incomplete because it fails to accurately predict at the quantum (and super-massive) scale.

    Where many physicists would disagree is you apparent believe in the completeness and universality of relativity, especially GR. It falls short of painting an accurate picture of gravity at the ends of the scale. It's that incompleteness that led us to posit the idea of Dark Matter, in the first place. Therefore, attempting to apply GR to objects that we know so little about their properties isn't valid, until and unless we can validate it, especially because it's the limitation in GR that led to the supposition of it in the first place.
     
  11. exchemist Valued Senior Member

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    That is wrong. Dark matter is hypothesised from the observed rotation rates of galaxies. This has nothing to do with relativity.
     
  12. one_raven God is a Chinese Whisper Valued Senior Member

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    Really? So they didn’t use relativity to make the predictions that we measured to be different than the predictions?

    I think you should revisit the so-called “Crisis in Cosmology”.

    If you’re interested, this is a decent article: https://www.space.com/why-is-there-a-cosmology-crisis

    Dr. Becky Smethurst is also a great resource with an approachable manner of helping to grasp these things:
     
  13. DaveC426913 Valued Senior Member

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    We should clear here. Relativity does not "fail" to predict the rotations of galaxies yet. For all we know, Dark Matter really is a thing and relativity is predicting it perfectly.

    Only if we eliminate DM as an explanation will relativity have to have re-examined. And even then, it won't have failed,it will simply need tweaking, as the best theories often do.
     
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  14. one_raven God is a Chinese Whisper Valued Senior Member

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    Agreed.
     
  15. exchemist Valued Senior Member

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    Correct. They did not use relativity to predict the rotation rates of galaxies, nor in applying the virial theorem. Both are just Newtonian mechanics.

    So dark matter is required by Newtonian mechanics. Of course, if Newton's law of gravitation ceases to apply, in ways that GR can't account for, then, sure, we may find GR's model of gravitation needs to be changed. But at present the physicists are putting their money on dark matter being real, rather than finding fault with GR. This is just Ockham's Razor.

    Sutter's article is to do with the discrepancy in estimates of the Hubble constant when made by different methods. He does not suggest that relativity is particularly called into question by the discrepancy. It's just that something in current physics and cosmology seems to be wrong, if the interpretation of these measurements is valid. That could be relativity, or it could be one of the other assumptions. He concludes by saying he personally thinks it is more likely not any of these things but the interpretation of the measurements that is wrong in some way.
     
    Last edited: Aug 6, 2022
  16. Beaconator Valued Senior Member

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    And maybe fusion first starts with dark matter. Spreading 100% efficiency to a world of 97%
     
  17. one_raven God is a Chinese Whisper Valued Senior Member

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    I’m not sure I follow. Care to expand?
     
  18. Beaconator Valued Senior Member

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    If dark matter was theoretically first it might be 100% efficient because theoretically it be neither hot nor cold therefore incapable of friction until enough masses.
     
  19. DaveC426913 Valued Senior Member

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    There are some words missing from this. Mostly the ones that make it make sense.
     
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  20. Beaconator Valued Senior Member

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    Is dark matter hot or cold?
     
  21. exchemist Valued Senior Member

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    That's a tangentially interesting question. I would say cold, on the basis that it does not seem to emit as a black body.

    But I suppose if it is some new form of matter, it might be possible for it to have a temperature (i.e. for particles of it to have kinetic energy and undergo collisions with neighbouring particles), without creating any radiation.
     
  22. Beaconator Valued Senior Member

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    I found this nugget of enlightenment

    “In the cold dark matter theory, structure grows hierarchically, with small objects collapsing under their self-gravity first and merging in a continuous hierarchy to form larger and more massive objects. Predictions of the cold dark matter paradigm are in general agreement with observations of cosmological large-scale structure.

    In the hot dark matter paradigm, popular in the early 1980s and less so now, structure does not form hierarchically (bottom-up), but forms by fragmentation (top-down), with the largest superclusters forming first in flat pancake-like sheets and subsequently fragmenting into smaller pieces like our galaxy the Milky Way.”-Wikipedia

    As I suspected….

    Could structures not form from both simultaneously? One or the other would put a stamp on the age of the universe.

    Yet dark matter not having a temperature would not only account for its lack of interaction with ordinary matter, but enter the universe into an ageless entity
     
  23. James R Just this guy, you know? Staff Member

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    one_raven:
    I don't really understand your idea about a "speed" for kinetic energy. We can certainly speak about the rate of energy transfer: that's the physical quantity called "power".

    In a collision between two particles, the rate of energy transfer will depend on the details of the collision and how long the collision lasts over all.
    GR is the theory which originated the whole idea of an "expansion of spacetime" in the first place. I don't know what you mean by saying GR "can't account for" the expansion, exactly. GR describes the expansion quite appropriately. That's what GR is: a description of spacetime.
    Where did you get that idea? GR is primarily a theory of gravity. It describes how masses interact. Electromagnetism is quite compatible with GR. In fact, Einstein's original motivation for SR was to make spacetime work correctly with Maxwell's equations of electromagnetism.
    How so? What specific failures are you referring to?
    GR isn't a quantum theory, if that's what you're getting at. It's limits in describing very high gravity on very small length scales are well understood. I'm not sure what you see as the problems at the large end of the length scale. Can you explain?
    No. As exchemist said (I think), dark matter was initially proposed to explain unexpected observations in galactic rotation curves. GR didn't come into it. But if there's one thing we know about dark matter is that it gravitates. GR has no problem coping with the gravity of dark matter. The problem is that we don't yet know what dark matter is, exactly. When we identify it, I'm confident GR will accomodate it. This is assuming, of course, that dark matter really is a thing and we're not making some other kind of error in physics that nobody has yet managed to explain.
    Theoretical physics is what theoretical physicists do. It's certainly worth attempting to apply GR to various hypothetical dark matter models. By doing that, we can help to narrow down the possible candidates for dark matter and lock down some other parameters. In other words, the theoretical work helps to guide the experimental investigations (and vice versa). This is how science is done, business as usual.
     

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