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The sun attractive to the earth and holds us in orbit.

  1. Oct 5, 2003 #1


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    posted by MARCUS.

    if it was only energy density that would be mysterious enough but have you registered that it is, to a small extent, the pressure inside the core of the sun that makes the sun attractive to the earth and holds us in orbit.
    i missed this bit, can lambda be stressed? something akin to the
    action of ice crystals forming in water, with the resulting
    thermal currents.
    ive made ten hats today.
    Last edited by a moderator: Feb 6, 2013
  2. jcsd
  3. Oct 5, 2003 #2


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  4. Oct 5, 2003 #3


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    Re: stress

    wolram you restless and unorthodox critter, I am going to have to turn you over to Nereid because she will know how to handle you in your fits of temperament

    what in gods name do you mean about having made 10 hats?

    I have over and over again written down the second Friedmann equation and it has a pressure term just as important as the energy density term. Have you never looked at the equation. Have you not noticed it? It has been staring everybody in the face since 1920 or whenever. heres second Friedmann:

    3(a''/a) = - 4 pi (rho + 3 pressure)

    a is the scalefactor which by increasing shows the expansion of the universe and the only reason you never used to hear about the pressure term in VERBAL descriptions is because the pressure of nearly empty space was thought to be negligle, and then oops in 1998 they found out it wasnt and it made a difference

    well, that Friedmann equation is just a simplified boiled down version of the old Einstein eqution that says curvature of space is proportional to a combination of stress and energy called the "stress energy tensor" and that means both pressure (in a region) and energy density (in the region) cause the curvature around that region

    so it isnt just the energy concentrated in the sun it is also the pressure in the sun, that makes the sun attractive.

    and when people give VERBAL descriptions of this they dont bother to tell you about the pressure contributing because in the sun, as it happens, the energydensity is so much bigger that you can neglect the pressure term.

    A naive person may occasionally say "Ahem, let us DEFINE energy as that which causes gravity. It is analogous to electrical charge in electrical attraction and in a manner of speaking, ahem, is the 'charge' of gravity." But this is silliness because it mixes up energy and pressure and does not give a practical operational definition of energy. Fundamental physical quantities have to have operational definitions that dont mix up stuff and that you can actually use. I just keep quiet when I hear such talk.

    Remember, its not merely mass that causes gravity. Mass is inertia and there is stuff that has no inertia but still attracts
    The heat energy in the core of the sun contributes to its attractiveness. All the energy in the sun contributes----the em cee squared energy of the matter in it AND the heat and xrays and everything boiling inside it, all the forms of energy
    And it is also not merely the concentraton of energy that causes gravity. It also is the pressure in there.

    we really really have a mystery. if we knew what space was maybe we could figure WHY the concentration of energy and pressure in the sun bends the space where the sun is and makes light and everything else go in curves

    i just went out in the garden and looked at the goldfish in the pond and realized that I dont know much more than they do about it

    and you, you say, made 10 hats today

    Oh, BTW, what you should do is, if you dont believe what i said about pressure contributing to gravity you should ask someone like selfadj or nereid. Just write a simple PM, does pressure contribute to gravity. It wont take any time and they will write back and say, very simply, yes.

  5. Oct 6, 2003 #4


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    sorry i need a holiday, thanks for kick up BS marcus.
  6. Oct 6, 2003 #5


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    dammit wolram it is not BS

    you can have two stars with the same amount of material
    so that you think they should both exert the same amount of pull
    and if one star is set up so there is more pressure in its core
    it will have more gravitational pull JUST ON ACCOUNT OF that pressure

    and pressure is a significant contributor to the gravity of very dense stars----think I read somewhere it figures in neutron star gravity because they have so much pressure inside

    so its not just mass and its not just energy it is also pressure,
    that gives things their gravity
    and you mind your manners and dont use bad language
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