What hold galaxies together?

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Aug 3, 2022.

  1. Sailor Al
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    Sailor Al Senior Member

    Please post that as a comment on the Veritasium video and check the reactions!
     
  2. Alan Cattelliot
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    Alan Cattelliot Senior Member

    We ve made a great progress here in explaining how J.Biden cant fall up the stairs :)
     
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  3. Barry
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    Barry Senior Member

    If there is no gravitation attraction/force, I do wonder what causes the tides which are calculated months if not years ahead of time accommodating the gravitational and position effects of the sun and moon.
    Perhaps more space time unsubstantiated theories can explain this?
    Al?
     
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  4. Sailor Al
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    Sailor Al Senior Member

    There are currently three models that are used to predict the nature of time, distance and mass. None of them describe how it really works, they are just models that are effective in different situations, and these situations differ in their scale. And they do not overlap.
    • Newtonian, or "classical" mechanics is effective to predict human experience - on the scale of seconds, metres and kilograms. In Newtonian mechanics gravity is an acceleration.
    • General Relativity is effective for predicting nature on very large distance scales of parsecs, masses of stars and speeds close to the speed of light. In General Relativity, gravity doesn't exist. It is explained by the mass curving spacetime.
    • Particle theory works for really, really tiny, very, very light stuff. In Particle Physics, mass appears to be explained by the Higgs Boson and I think gravity once again becomes part of the model.
    That is not my “unsubstantiated theory”, it's mainstream physics.
    Tides are a phenomenon in human experience and can be accurately predicted using Newtonian mechanics, in which gravity is an acceleration. General Relativity wouldn't be an effective model to predict tides.
    The bending of light from a distant star around another star or galaxy, "gravitational lensing", was predicted by General Relativity. It can't be explained by Newtonian mechanics.
    I don't know enough about particle physics, but I'm pretty sure that the Higgs Boson can't be used to predict either tides or gravitational lensing.
    So, "horses for courses". You don't use a power saw to cut your meat, you don't use General Relativity to predict tides.
     
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  5. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Tides are not predicted by Newtonian mechanics. It seems you consider Veritaserum as a true source of knowledge. So go have a check.

    PS. You are still making a confusion between gravitationnal acceleration and gravity. Peter's pants are the pants of Peter. If Peter losses his pants, Peter still is Peter.
     
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  6. Sailor Al
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    Sailor Al Senior Member

    Of course they are. How do you think they derived these?
    upload_2022-11-21_9-42-24.png
     
  7. Alan Cattelliot
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    Alan Cattelliot Senior Member

    If you check the video, you will know by yourself. Don't be mistaken, I don't say that Gravity is not responsible for tides. I say that no one has ever used any newtonian mechanics to establish the tides table. Let me return your question : How do YOU think they are derived ? Be honest. Have you ever studied the tides on earth using the first, second and third law of Newton ? On which basis are you writing the sentence i've quoted from you ?

    Empirically ... Using statistics... In the essence, tides are computed the same way Ptolemee computed in his time the movement of the celestial bodies in his time ( or tried to ...) The tide predicting machine that is presented by Veritasum do exactly that. More parameters and many refinements are made in today's algorithm. That(s all.

    Views and opinions are to be shared on public forums, but making inaccurate assertions do not help at all.
     
  8. Sailor Al
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    Sailor Al Senior Member

    Don't get hung up on the "Newtonian" in "Newtonian or 'classical' mechanics". It's a lot more than the three famous Laws. It covers the gamut of classical mechanics, including simple harmonic motion, which underpins Kelvin's use of Fourier Transforms.
    My point was that in Newtonian or "classical" mechanics, gravity is an acceleration.
     
  9. Alan Cattelliot
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    Alan Cattelliot Senior Member

    In my opinion, language is everything. Like BARRY, I'd rather stay inline with the newtonian or "classical" mechanics formulation :

    G.m.M/r² [Kg.m.s-2] force of gravity between a point mass, M, and another point mass, m
    G.M/r² [m.s-2] acceleration due to gravity of a point mass m
     
    Last edited: Nov 21, 2022
  10. gonzo
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    gonzo Senior Member

    I think that using the terms "acceleration" and "force" when referring to gravity tends to create confusion for some. Acceleration, by definition, is a change is velocity. However, the attraction between bodies exists, and is measurable, even when there is no velocity or change of velocity between them.
     
  11. Will Gilmore
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    Will Gilmore Senior Member

    g=9.8m/s² isn't an illustration that gravity is an acceleration. The 'g' is a formulaic symbol for the acceleration caused by gravity. The big 'G' in Alan's quoted formula is called the Gravitational Constant and is really just a scalar to match up the units so we can translate them into a useful standard. If we were to use other units of force or mass or acceleration, the Gravitational Constant would need to have a different value. You could even have a unit of force where 'G' was unnecessary. These are formulas that model reality. Pete will be Peter no matter the pants. That doesn't mean we know him, even if he does wear familiar clothes.
     
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  12. gonzo
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    gonzo Senior Member

    Sailor Al is confusing a mathematical representation with reality; if reality actually exists. Science, in the strict definition of the word, is a myth. Myths are human constructs that explain reality in a way that is useful to conduct our lives. As long as they work, there is no need to change them for any practical reason. For example, the myth that the sun rises from the East and travels overhead to the West, works as well as the newest astrophysics theory for someone trying to figure out how many hours of daylight there will be.
     
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  13. Sailor Al
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    Sailor Al Senior Member

    I'm not sure why you are criticising me here.
    My statements at #109 clearly express the view that totally different models ("mathematical representations") can be used to interpret reality, and that the choice of which model to use depends upon which aspect of "reality" you are trying to interpret. I don't think I am confusing mathematical representations with reality there, I am pointing out three very different, but equally valid models (mathematical representations) of mechanics (reality).
    On this subject I think we agree.
     
  14. gonzo
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    gonzo Senior Member

    This is in part form post #113. Newton referred to gravity as an attraction. Acceleration is used to calculate that attraction, because it works well at low velocities.
     

  15. Will Gilmore
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    Will Gilmore Senior Member

    If gravity isn't a force, how does it accelerate objects? (Advanced) - Curious About Astronomy? Ask an Astronomer http://curious.astro.cornell.edu/physics/140-physics/the-theory-of-relativity/general-relativity/1059-if-gravity-isn-t-a-force-how-does-it-accelerate-objects-advanced
    More on this when I have time.

    -Will
     
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