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I am probably going to be critizied for this, but

  1. Sep 16, 2005 #1
    Does anyone have the guts to think that gravity is not the universal force that holds planets and stars in place? I mean, gravity is an attractive force, but is there more than one force than just gravity that holds the universe toghether?
     
  2. jcsd
  3. Sep 16, 2005 #2
    i dont exactly now what you are trying to get at, as far as planet and stars, comets, and other celestial objects, well basicaly there orbits and postitions. then yes Gravity is the only thing holding them in their respective places/locations however that place is also not said to be infinite. gravity pulls, repels, and astonomers have been sing examples of that for a long long time.
     
  4. Sep 16, 2005 #3
    I dod not ask because I did not think gravity was the force that holds all stars. I asked because I read this article along time ago about astrophysics, where two astrophysicists from India had recently found a force that proves that gravity was not the force that holds planets and stars in orbit, at least the stars. I don't know what to believe so I came to ask here. Here's the address...

    http://www.zetatalk.com/usenet/use90089.htm

    Like I said, I do not know what to believe, since we have Relativity as the current theory, so please tell me what you think.
     
  5. Sep 16, 2005 #4

    mathman

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    After a cursory examination of the zetatalk reference, it seems to me that it is mostly hot air. I doubt if anyone would take it seriously.
     
  6. Sep 16, 2005 #5
    To illustrate a single instance of the nonsense in this document:
    "As Stellar Repulsion Force is a nuclear oriented force (SRF = MC2) its intensity is million times greater than Gravitational intensity (G = M) in the Sun and Stars. Therefore, ACTIVE FORCE exerted by the Sun and Stars is FORCE OF REPULSION - not Gravitation."
    If you can work your way past the clumsy phrasing what they appear to be saying is that since the magnitude of the SRF is proportional to the square of the speed of light and the mass, whereas gravitational attraction is proportional only to mass, then then the SRF must be very much greater than gravity. Even if there were such a beast as the SRF they conveniently overlook that all of the sun (or any star) is responsible for its gravity, yet only a vanishingly tiny part of it is engaging in a nuclear reaction at any one time.
    In my view it didn't take guts to propose this hypothesis, rather an absence of any intellect. I think you can safely ignore it.
     
  7. Sep 17, 2005 #6
    Thanks for the help, guys. I would not know what to believe without your help.
     
  8. Sep 17, 2005 #7

    SpaceTiger

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    And that is exactly why I despise online crackpots. When the scientific community rejects them, they hawk their theories to laymen who don't know any better. I like to think that one of the main purposes of science is to give the general public an understanding of the universe in which they live. It's much more difficult to do that over top of a background of self-obsessed incompetents with inordinate amounts of free time. Admittedly, real scientists could put more effort into spreading real science, but that's no excuse for the crazies. It is possible that some enthusiastic fringe researcher will come up with a ground-breaking idea, but they should always go through official channels first. If it's right, experiment will eventually demonstrate that fact and the theory will be accepted. Only then is it ok, IMO, to start hawking it as "truth".

    Sorry for the rant (it's not directed at moderman), but this sort of thing irks me a lot. Perhaps I just foolishly hope that my work (and that of my fellow astronomers/astrophysicists) will actually contribute something to the world.
     
  9. Sep 17, 2005 #8

    Chronos

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    Gee, ST, you're being rather delicate in your criticism of snake oil salesmen. Crackpottery is to science as pornography is to art.
     
  10. Sep 17, 2005 #9

    pervect

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    Reminds me of a song by Tom Lerher

     
  11. Sep 18, 2005 #10

    DrChinese

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    And THAT is why I like PhysicsForums!

    And I am especially glad that the same crackpots cannot peddle their wares here.
     
  12. Sep 18, 2005 #11

    pervect

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    Except, of course, in the occasional paid advertisment :-(.

    I used to be a big fan of unmoderated forums, but the fate of public forums like sci.physics.relativity has convinced even me that we _need_ some moderation to have useful discussions.

    The key to sucessful moderation is in the quality of the moderators. We are very fortunate here at Physics Forums to have good moderators, ones who don't tolerate nonsense and disruptive posters but still have a very "light hand".
     
  13. Oct 1, 2005 #12
    okay...let's look at what you're saying.

    you say universe stays in one piece becasue gravity makes it stay that way.
    however gravity is a force that pulls things into one gigantic sphere if it is the only thing to act (example: in space water will form spheres rather than any other shapes becasue of quite a few forces, molecular and external, including gravity.)


    but if you recall Newton's law says that one body will keep on moving unless there's a force to stop it from moving. F=ma where Force = Mass * Acceleration is the formula to describe this.

    but although gravity should pull the universe back into one spot, precisely in the same place where Big Bang occured (assuming matter distribuited equally) the universe is in fact expanding. it's slowing down? it's accelerating? i don't know. however, gravity is not the only thing keeping it together.

    if it were, the universe would have an expansion with a constant acceleration in the negative direction, making it eventually stop, then reverse, then fall back into the particle that caused the big bang.

    i doubt that gravity is only thing affecting universe. at that level of distance vs. mass, i doubt gravity even plays a role.

    gravity between two any objects in any spot can be calculated by

    m*g= G *m1*m2/d^2

    m*g is the force of gravity. it is always the mass of the object times the gravitational constant, and it's also called weight.
    it is equal to G which is a constant 6.67*10^-11 * mass of one object * mass of second object divided by the distance between the two Squared.

    it can be derivated endlessly for speed, time acceleration, whatever you need but basically you can calculate the force with which a dust particle on pluto attracts the black hole across the universe. according to newton's third law the force that one attracts the other is reciprocal. (opposite and equal).
     
  14. Oct 1, 2005 #13

    HallsofIvy

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    Yes, that's (trivially) true. And what was your point in saying it?
     
  15. Oct 30, 2005 #14

    Nereid

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    One corollary to this is that you (a person, any person) can refer folk with interesting questions to PF, knowing that:
    A. Their question will be treated with respect
    B. They will get a good answer (or answers), to whatever depth they wish to pursue it
    C. Nonsense and crackpottery will get short shrift (hopefully, without too much scorn; innocent enquiries are most definitely NOT the peddling of snake oil).

    Another corollary is that fringe scientists will quickly discover PF, and will try very hard to get their ideas into on-going discussions (any discussions; many have the 'Madonna' view of PR - there is no such thing as 'bad PR'). Thank goodness for PF's IR policies! :approve:
     
  16. Nov 1, 2005 #15
    Did they forget about the existence of gravitationally bound binary stars?
    Further, their statements about how energy is released from fusion is right up there with the Time-cube guy. Simulations of stellar interiors depend on gravity, and no repulsion force, and are accurate to within 1% of helioseismology measurements. A repulsion force like what they are talking about would literally blow stars apart.
    For example:
    [tex]
    \frac{dP}{dr} = -G \frac{M_r \rho}{r^2} [/tex]
    [tex]
    \frac{dM_r}{dr} = 4 \pi r^2 \rho [/tex]
    [tex]
    \frac{dL_r}{dr} = 4 \pi r^2 \rho \epsilon
    [/tex]
    Are the basic equations of stellar structure. The first equation is derived from:
    [tex]
    dm\frac{d^2r}{dt^2} = F_g + F_{p,t} + F_{p,b}
    [/tex]
    Which means that the net force on a mass portion [tex]dm[/tex] is equal the the sum of gravity, downward pressure on the top of the portion, and upward pressure on the bottom of the portion. If we add a repulsive force [tex]F_r[/tex] we get:
    [tex]
    dm \frac{d^2r}{dt^2} = F_g + F_r + F{p.t} + F{p,b}
    [/tex]
    which becomes
    [tex]
    dm \frac{d^2r}{dt^2} = -G \frac{M_r dm}{r^2} - A dP + F_r & dm = \rho A \dr [/tex]
    [tex]
    \rho A dr \frac{d^2r}{dt^2} = -G \frac{M_rdm}{r^2} - A dP + F_r [/tex]
    [tex]
    \rho \frac{d^2r}{dt^2} = -G \frac{M_r}{r^2} \rho - \frac{dP}{dr} + \frac{F_r}{A dr}
    [/tex]
    Now, the condition for hydrostatic equilibrium, that is stable stars that don't blow apart or implode is:
    [tex]
    \frac{d^2r}{dt^2} = 0
    [/tex]
    So, we get:
    [tex]
    -G \frac{M_r}{r^2} \rho - \frac{dP}{dr} + \frac{F_r}{A dr} = 0
    [/tex]
    Now, we know what the pressure gradients inside the sun are due to helioseismology measurements. That means, that for the sun to be stable, it would have to be far, far more dense in order to counteract that mysterious repulsion force (which they never give an expression for, despite 23 years of work). However, if the star were that dense, the planets, governed by gravity, could not be in their current orbits.
    Either the star blows apart, or the planets need to start flying through the sky a lot faster. Neither is the case.
     
    Last edited: Nov 1, 2005
  17. Nov 2, 2005 #16

    Kurdt

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    I did not read the article but from what I can tell of the replies here it seems the guys that wrote it are probably getting confused with the radiation pressure and the pressure of the hot gas that mainly stops ordinary stars from collapsing on themselves. This pressure is maintained by the nuclear reactions in a stars core but seeing as only 10% of a star like our own reacts say in an approximate 10 billion year life then this pressure merely exists to impose a steady state equilibrium on the star. The force would blow a star apart if 10% of the matter reacted on a timescale much shorter than that (obviously depending on stelaar mass because i have quoted those values for our own star but all are in equilibrium in a similar fashion.).

    Anyway, if it took them 23 years to come up with something that everyone already knew about anyway and then claim its a new fundamental force then I really need not say much more.
     
  18. Nov 3, 2005 #17
    I don't understand the question....what do you mean in place?

    I think also that gravity is not the only thing acting in the universe ,keeping it in "in one piece". Just remember the Einstein "biggest error" (in his own words) , that is the cosmological constant. It appers when solving Einsten-De Sitter-Friedman-more people "non-static universe" equations.


    But maybe i'm going to far from your question, if you mean just why stars don't collapse, well, stellar interior structure equations give the answer (franznietzshe posted them, i think).
     
    Last edited: Nov 3, 2005
  19. Nov 6, 2005 #18
    Gravity is very mind boggling, but it's best to start with understanding accepted theory before one lets their feelings direct thier sense of reasoning about reality. If what the site says is true, then planets don't need to orbit the sun, they can just sit there. Also, why doesn't the sun repel the matter that it's made of? Why doesn't it simply explode. If the planets attract the sun, then they will pull the sun apart too. This is simply junk. It's a romance explored by an ostrich with his head in the sand. I myself think that there is a big piece missing from accepted theory, but this particular theory doesn't agree with reality.
     
  20. Nov 17, 2005 #19
    just a question i had in mind while reading everyone's comments.
    is orbit stronger than gravity?
    every planet has their own gravitational pull, some that are greater than others and yet none are crashing into each other. i know that another force in the opposite direction pulls it back into place so the planet stays in between but for example, the moon isn't flying off towards to the sun when the sun has a greater gravitational force than the earth. is it because the moon is in orbit around earth and that is why gravity is not strong enough to break that orbit?
     
  21. Nov 18, 2005 #20

    Orbits are maintained by gravity. You're seperating two things that are the same. There is no such thing as an "orbit force". The earth's gravity is stronger on the moon that the Sun's because the earth is much closer. But if you notice, while orbiting the earth, the moon is also orbiting the sun.
     
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