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What is gravity?

  1. Jun 27, 2003 #1
    Apart from being the weakest force in the known universe... What exactly do we know about what gravity is?

    How does gravity make things combined together? How does gravity form planets/stars, etc...

    I understand the basic concept of gravity.. but I dont understand why things have it... does everything have gravity? do we have gravity? but we are so tiny that it is unmeasurable? or is it just things like stars, planets, and other giant physical objects?
  2. jcsd
  3. Jun 27, 2003 #2


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    the most accurate model we have of gravity is General Relativity which says gravity is embodied by the shape of space time----"gravity is geometry".

    The main equation is the 1916 so-called Einstein equation which says

    curvature = (8pi G/c4) energy density

    the expressions for the energy density and curvature are complicated but in answer to your question yes, all forms of energy are at least nominally included-----very tiny contributions can be neglected as in all physics. But the mass-energy in matter and the light-energy in light and the so-called dark energy which is inferred but not explained and even heat energy----all these things contribute to curving space-time through the total energy density on the RHS of the equation

    and all stuff follows the curvature---even light does not go in straight lines but is bent by the curvature of space. Planets go in their orbits because of the curvature. What feels like the force of your weight is the effect of geometry.

    It just happens that this model of gravity (not a force so much as an effect of geometry) works better----gets more accurate answers----than the old Newton force model with its rigid absolute space.

    Gravity needs to be analysed as the dynamic changing geometry in which other stuff exists and happens-----the shape of space itself is dynamic and evolving----the stage is a major participant in the play and not just a place where the play is performed.

    Better not to think of it as just "one of the forces".

    This however does not make it easy to understand! The 1916 "gravity is geometry" model (General Relativity) is the most correct one we have but that does not mean it is as easy as Newton's 1680s model. Unfortunately:frown:
  4. Jun 27, 2003 #3
    interesting... i probably only really understand about 1/8 of what you just said but that is more my fault than yours

    i guess gravity just confuses me... as it should.. since the great minds in physics and astronomy can't even completely crack it
  5. Jun 27, 2003 #4
    If the questioner is asking what the force carrier is - that is not known. "Gravitons" have been postulated as the field carriers, but this is all very much speculative. Without a link to quantum mechanics, postulation of force carrier characteristics is troublesome at best.
  6. Jun 27, 2003 #5
    so what your saying is we really don't know too much about gravity other than that it exists and what effects we have observed it has on other things
  7. Jun 27, 2003 #6
    It's a question of choice. If you like more Quantum Dynamics gravity is caused by the exchange of a particle (the graviton). If you like more General Relativity, is caused by the curvature of the space
    There are other minor theories, like Le Sage theory
    Last edited: Jun 27, 2003
  8. Jun 27, 2003 #7
    On the contrary. We know a great deal 'about' gravity. We know 'how much' gravity something might posess and how the universe will react to the existence of this gravity. We don't know, specifically, how gravity is 'created'. But this doesn't stop us from using it successfully. For modest uses, Newtonian theory (F=Gm1m2/r^2) will suffice. If you really want to get in the thick of it, you're going to need to learn GR. This, I might add, is not for the weak of heart, and probably a bit dramatic for an intro. Make sure you understand Newton's theory first, before you become obsessed with Einsteins. :wink:
  9. Jun 28, 2003 #8
    curvature = (8pi G/c4) energy density

    The primary problem with the view of gravity by Relativity is that the curvature geometry is static. That is it provides no motive force.

    An interstate highway has curves and cars traveling it follow the curves (most of the time) but the curves are not the reason cars run off the road laterally. The curve is merely static geometry. It takes energy to produce acceleration and motion.

    Relativity provides no motive source of energy.
  10. Jun 28, 2003 #9
    Re: curvature = (8pi G/c4) energy density

    then why if you drop something it falls, other than the force of gravity pulling it? what does relativity say about that? Isnt gravity creating the acceleration and motion?
  11. Jun 28, 2003 #10
    When you speculate that gravity is a weak force in the universe, you are indicating that you are not thinking logically about the subject and are merely conforming to commercial views which are intended to make you into mindless consumers and sycophants. The fact that all celestial bodies are possessors of gravity should indicate to you that gravity is THE underlying force in the universe and is therefore the strongest force in the universe, and not the weakest.
  12. Jun 28, 2003 #11
    Actually, that's a terrible simplification. We "know" a great deal about the "what" of gravity when dealing with our own immediate scale of reference. But the Theory of Gravity fails to apply at the more extremes of scales - such as the quantum and (galactic) macro scales - which are precisely where the major issues in physics actually are. There is also the great problem of not even being able to address the "why" of gravity.

    Sure, anyone here could spiel off about how much we "know" - but that would be ignorant of various highly significant outstanding issues. Without a generally accepted mechanism addressing the force carrier issue, and with the whole debacle over "dark matter" and the accelerated expansion of the universe due to claimed "dark energy", you can be absolutely sure that the Theory of Gravity is due for a significant overhaul, perhaps akin to the world-changing way in which Einstein did so with Newton.
    Last edited by a moderator: Jun 28, 2003
  13. Jun 28, 2003 #12
    That's a rather glib interpretation don't you think? Gravity IS the weakest of the fundamental forces when compaired the range verses strengh argument. All atoms demonstrate the strong nuclear force so I can very well say that the strong nuclear force is THE underlying force of the universe because without it we wouldn't have stable nuclei for gravity to influence. If anyone is showing ignorance it's you.

    And how does physics make you a mindless consumer? Think before you type.
    Last edited by a moderator: Jun 28, 2003
  14. Jun 28, 2003 #13
    I would argue differently. GR is a theory that will probably tell the tests of time. Probably, I'm biased in my appreciation for GR, but I would be willing to bet that QM is what gets the overhaul. More to the point, while GR does fail at the quantum level (though here again, I would argue it works perfectly well in the macro universe) QM also fails at the macro scale.
    But I digress. Clearly the individual who was posing the question initally is not as versed as some others on this board. My reply was merely an attempt to elucidate our understanding of the phenomenon of Gravity. I was not attempting to make bold, sweeping assesments of our scientifically pure understanding :smile:
  15. Jun 29, 2003 #14
    An atom exhibiting a strong nuclear force is a localized phenomena, while gravity extends throughout large segments of space and affects large physical bodies by making them conform to movement patterns dictated by the gravity. The relevance there should be obvious to anyone who likes to think.

    No one suggested that physics turns people into mindless consumers. It was stated that the commercial view would have people achieve that exalted state. When you consider the countless books published about cartoon physics theories, you then get the notion of how much money is on the fringe of science. But the real money, the billions of dollar, comes from the taxpayers funding projects that are proclaimed to find the age of the universe and the beginning of time and other such nonsense.

    It's easy to see how the gullible people in society follow dolefully behind anyone who proffers a big lie. When they are told something that is so outrageous that they have no idea if it is true or not, they waive judgement on it and accept it without thinking deeply on the subject. That success of propaganda has recently been displayed before the world in Iraq, where the U.S. intentionally scared the populace so they could go in and steal the Iraqi oil. That same tactic of using the "Big Lie" has been applied to physics theory in the cases of the big bang, black holes, infinite density, string theory and other such nonsense that no person on this planet understands or has any chance of proving. While at the same time the commercial science community rejects all legitimate theory that casts doubt on their cartoon theories, such as Halton Arp's description of non-recession velocity as an explanation for redshift.
  16. Jun 29, 2003 #15
    WHY things have gravity is a question I cannot answer, but then there are few 'why' questions we CAN answer (Why do we exist? Why did the Big Bang happen? Why did life originate on the Earth?)

    Your second question, 'does everything have gravity?', is much easier to answer. Yes, everything has gravity, or, to be more precise, all matter and energy has gravity exactly proportional to its mass. That means that elementary particles (of which everything is made up of) have gravity. As humans consist of elementary particles, they naturally have gravity. Light is energy, so it also has gravity, it's just pretty much immeasurable.
  17. Jun 29, 2003 #16
    Wow, you stupped pretty low just to throw that Iraq comment in there didn't you. Ok, now on to Arp's 'physics'. True is, I don't buy it. Why? Well I can't find any basis for the apparent inherent redshift he so dearly hangs on to. He fails to realize that inflation is equal in all directions everywhere that's why astrophysicists get similar reading in different parts from the sky. Thirdly his reasoning for black holes makes no sense. Not everything goes IN. If you heat material to extremely high temperatures as is in the accretion disk around a black hole it gives off X-rays. The magnetic fields created are going to push that gas up into jets. That's materal NOT hitting the event horizon yet and still has a chance. All I see him doing is making assumptions without any math to back it up. And learning a bit of the physics of these things myself, I'll stick to the math behind it.
    Last edited by a moderator: Jun 29, 2003
  18. Jun 29, 2003 #17
    I'm reminded of Contact. You know the scene, where Dr. Drumland shuts down SETI because it has no practical value to the taxpayers.

    You've made HUGE, SWEEPING denials of some of physics most contemporary, promising, and above all respected theories since the turn of the last century. And perhaps I'm the only one, but I don't see a reason why. There is no evidence to suggest that Black Hole theory is wrong, and a plethora working in its favor. We've shown Universal evolution from up to millionths of a second after the Big Bang. We are on the verge of cracking the mysteries of the universe, and you're comparing it to a Nationwide conspiracy to get Iraqi oil. There aren't conspiracies of propaganda in physics. There are people who are a touch misinformed, like perhaps the poor soul who started this off and got more than he bargained for :wink: . But this is no reason to make ad hominem attacks just for the hell of it.
  19. Jun 30, 2003 #18
    lol... i just was curious about gravity... lol for some reason all the threads i start in this forum break out into harsh word battles...

    anyways.. i was just thinking about gravity one day so i figured i would as why things fall towards the center of our earth.. but things don't fall to the center of me.. know what im sayin...

    but i did get some good explinations of things
  20. Jun 30, 2003 #19


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    Let's start all over again.

    Kleinma, you wonder why things fall toward the center of the Earth, do you understand why, say, satellites like the international space station don't (seem to) fall? Let's build up a descriptive view of your idea of gravity before we get back to discussing current physical theories about gravity.
  21. Jun 30, 2003 #20
    umm well hmm...

    well i guess i would say that satellites are outside of earth gravitational pull where they can stay in space... but aren't they slowly falling? or is that just some satellites.. i think i read about that before... but i know they are still in earths gravity somewhat or they would not orbit right??
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