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My questions on gravity

  1. Feb 1, 2010 #1
    Dear reader, here are my questions:

    What have we proven about gravity? I don't understand gravity, (I'm at a grade eight level in science, without any knowledge of physics what-so-ever). I asked a person, if the sun were to disappear scenario (after watching a youtube clip) some think I'm stupid for believing (more like inquiring about) in fabric of space affecting gravity (all new to me), and if would take eight minutes for disappearance of the sun to cause the earth to go off orbit. You could by breaking this down to me in the lowest of laymen terms for understand this.

    I don't even now what radiation, or gravitons, or formula, my head is spinning in questions: I feel so terrible, I have gone to high school yet. Could teach the facts of physics and gravity. I hate being ignorant.

    So here is their reasoning: [if you could simple, clear, undeniable statements to refute this I would appreciate this]

    HIS FATHER: if a huge curve in space caused be the sun creates a well that the earth orbits along, how does the moon continue to rotate on the far side of the "well" . Also it seems to me that if the sun, spherical would create "wells" for the planets that are not circular. The planets travel in ellipses, not circles, and must slow down and speed up depending.

    HIS FATHER: I also didn't like how a four dimensional model was demonstrated using only one plane of space. Why would the suns mass conveniently exert pressure in the fabric only in a downward direction conveniently from the perspective of the earth and the viewer? The mass "well' should be created in 360 degrees if there is a 3 dimensional fabric exerted upon equally from all directions.

    HIS FATHER: Also if gravitons exist and have no mass they would care less about the universal speed limit, "c".

    HIS FATHER: Seems to me a black hole's mass would create the hugest well in the fabric of all objects in the universe, allowing objects to rotate away and light to travel the farthest, yet the opposite happens, and light can't even escape.

    HIS FATHER: Also, if the earth creates a well large enough for the moon to ride its rim 240,000 miles away, why does the atmosphere choose to stay within 80,000 feet of the earth, with the fabric being displaced for it to freely travel 100's of thousands of miles farther?

    HIS FATHER: also, since you quoted a scripture [Is 40:22; Ps 102:25, 26], does this model harmonize itself with the scripture [Job 26:7] that says the world is being suspended on nothing?

    HIS FATHER: Also, if the rim in this fabric is what the earth is riding within, how can it be displace by the sun for earth, but also have rims for the two inner planets.

    HIS FATHER: The overall problem with this model is looking at celestial objects as bowling balls on a trampoline. But do you know what makes a bowling ball create that curve on a trampoline? GRAVITY. A model in space that works as though their is gravity underneath an invisible trampoline. GRAVITY to explain gravity. Physics gets WAY too theoretical for its own good sometimes. They used Newton to get to the moon. Funny, cause Einstein posited this way before that, but it has no practical use.

    I asked my physics teacher today. He thinks that Einstein is one of the greatest physicists and geniuses of all time, and he still disagrees with this particular aspect of general relativity. He maintains that the earth would instantly leave orbit on a tangent.
    17 minutes ago •

    HIM: why are mercury and venus along the same vertical plane as earth then? why wouldn't mercury and then venus and then earth, in that order, be on different planes from lowest to highest?

    HIM: OK check out this side view of the solar system, doesn't look like the planets are rotating in a curve made by a bowling ball on a trampoline.

    HIS FATHER: ent to my inbox: Got to remember that the 4 dimensional fabric of space model was shown in a two dimensional grid. General relativity also says that nothing exceeds the speed of light, but photons have mass, and are thereofore hindered by this barrier in space time. If gravitons exist and have no mass, such hinderance does not exist for gravity. Magnestism and light, as I said earlier are stopped by the limitations of electromagnetic radiation, and can be measured, but gravity shows no evidence of emitting anything measurable. In fact, special relativity is applied as an algorithm in computers on gpu's, because gravity bends electromagnetic radiation, so wins in that armwrestle? So gravity may work in tandem with electromagnetic radiation, but it can also work independently of it. Consider a black hole, again gravity works against the speed of light and wins.

    Please write back, once again thanks.


    Space can be appropriately likened to a fabric. Physicists also liken space to fabric to explain the way gravity works. The nature of gravity results due to the way it curves. The fabric works as a surface for objects having significant mass in the physical universe. So if planets & their moons... were caught in the path of a gravity wave they would experience space itself contracting and expanding (much like a sheet vibrating). In contrast to two objects linked together with a string. Experts Stephen Hawking professor in the field of cosmology agree on this matter. This differs from the Vedic priest's turtle explanation. The untimely disappearance of our sun would cause a ripple in the fabric of space. The ripple from the grip from the force of gravity (resulting from the curve) which cannot exceed faster than the speed of light itself ("a universal speed limit") Regardless, gravity is just amazing.

    Please write me back, and remember I'm just a youth, so please keep it simple and dumbed-downed English will be best.
  2. jcsd
  3. Feb 1, 2010 #2
    Einsteins theory of gravitation is the current accepted model of gravity. It has a lot of support from experiments and observations. Any massive body affects the curvature of spacetime around it. If such a body moves, its effect on spacetime will move together with it, but the change in curvature of spacetime will be more delayed the farther away from the massive body you look, because the changes in spacetime only travel at the speed of light.

    Close to earth where the moon moves, the curvature caused by the earth is much stronger than that caused by the sun. The planets travel in ellipses in the sphericaly symmetric well. Think of a rolling marble on a 2d curved sheet. If you don't give it the exact speed it needs for a circular orbit, it will instead move like an ellipse. Faster when it is closer to the origin, and slower when it is farther out. This same thing happens in 3d space.

    It is a simple model that illustrates the same effect as the 3d curvature. As you say, in reality the curving effect happens in all directions from the sun. The 2d sheet illustrates the same point, and it is easy to draw it.

    The hypothetical graviton in physics is assumed to follow the same restrictions as the other massless particles, i.e. travel at the speed of light.

    I'm not sure I understand this. The "well analogy" is supposed to visualize that objects tend to move towards the well. Close to a black hole, the well becomes so steep that nother can escape it. It is related to the finite speed of light.

    The moon travels so far away because at the speed it has, it will follow an orbit around the earth, without falling in or disappearing away. If you give it more speed, it escapes from earth. If you stop it or reduce its speed, if falls in closer to earth. Similarly, objects on the surface of the earth are moving with a speed that is to slow for them to escape gravity. If the earth had started to rotate much faster, and objects on its surface lso, they could be thrown off the surface. The same would happen with the atmosphere. The moons position has to do with its origins. In principle the moon could have been much closer, but then it would move much faster around. As it is now, it is far away and uses a month of time per revolution around earth.

    I don't know what this refers to. But I don't think bible doesn't includes any nontrivial truths about gravity or cosmology.

    Sorry, I don't understand this.

    There is no trampoline. It is only a 2d analogy because humans cannot visualize the curved 4D spacetime, and it is difficult to visualize a curved 3d space :-)

    Changes in gravity travel only at the speed of light. So there would be a delay, if the sun suddenly disappears. Actually, there would be the same delay before you would see that the sun disappeared, because light also travels at the speed of light (of course) :-)

    I'm not completely sure I understand this question. The reason all planets move in aproximately the same plane is that the solar system started as an object with a lot of spin. The "centrifugal force" will make a spinning object that is not solid tend to morph into a spinning disc.

    There is no trampoline, it is only a lower-dimensional visualization, as mentioned above. The reason for the planets being approx. in the same plane is the spin of the solar system, which was also present early on when the solar system was formed (spin is conserved in time).

    Photons do not have mass, they are massless. The speed barrier is there for all objects, massive or not. Therefore gravitons also move at the speed of light. Massive objects can move at any speed lower than the speed of light. Light always moves at the same speed, and so do gravitons if they exist.

    There are lots of good popular science books about gravity. And when a physicist says "gravity is like a sheet of rubber", it doesn't mean that gravity works exactly like a sheet of rubber, but it is only a simple analogy so that everyone can understand the general idea of what happens in 3d space. It's not easy to draw a curved 3d space, although I have seen a simple example that uses a 3d grid to illustrate it. Here is one attempt:
    http://superstruny.aspweb.cz/images/fyzika/astronomy/gravity3d.gif [Broken]

    Mathematically it is not so difficult. But sometimes we want to make pictures :-)

    But if you write more questions next time, please consider splitting it up in a few different posts, and put it in the correct section. E.g. this one should have been in the section for gravity or cosmology, not general physics.

    Last edited by a moderator: May 4, 2017
  4. Feb 1, 2010 #3
    Thank-you for your help. However, it wouldn't hurt be open minded to vast seas of knowledge contain in ancient Hebrew scriptures. My questions now are how can effective explain, that a) gravitons would follow the speed barrier. b) and any ways to show proof of our theories.
  5. Feb 1, 2010 #4


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    You will not be satisfied with the reaction to ancient religious scripture here. PF is dedicated to established, accepted science from accepted sources.

    At best, you will be challenged to back the claims up with experimental evidence and will not be granted any premises unless compellingly backed up. Moreso, while not directly a banned topic, it will more than likely result in a locked thread.

    There are other fora out there that might be more open to your ideas.

    You have been given a heads up. Proceed at your own risk.
  6. Feb 1, 2010 #5


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    We are working on observing gravitational waves and determining their velocity. Gravity is an exceedingly weak force, and is so far at the very limits of our instuments.
  7. Feb 1, 2010 #6
    I am not well versed in GR so hopefully a more knowledgeable member of this forum can check me. But I find this to be a fundamental problem with the way GR is described to laymen for example on the history channel. Whoever "his father" is has not in fact discovered any kind of flaw in general relativity, and the conclusion here is completely wrong. Once you interpret gravity as being curved space time, there is no other manifestation of gravity present in the model. Rather, the depression in space time is a potential well, and things will thus fall into the depression not because gravity is "acting twice" but because of the tendency to fall into a state of minimum potential energy. Like I said I would love to be corrected especially on the last sentence. But I feel like this is a fundamental difficulty with the way GR is often explained and the source of many misconceptions.

    Edit: My main point is that you should probably place a little more faith in the cornerstones of physics than you have. Any problem you can come up with regarding general relativity almost certainly stems from a misunderstanding of the theory.
    Last edited: Feb 1, 2010
  8. Feb 1, 2010 #7


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    Yep, you are correct. Issuecryer has spotted the flaw in the rubber sheet analogy.

    Issuecryer: it is a flaw in the analogy of the model, not in the model.
  9. Feb 2, 2010 #8


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    Yes you are correct, the "balls-roll-into-dents-on-rubber-sheet-analogy" is circular and lacks the crucial time dimension. The model of gravity in GR is more like this:
  10. Feb 2, 2010 #9


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    There is a lot of confusion between the simple model of a dented rubber sheet which represents the force on a small ball because of the reduction in potential as it gets nearer the heavy one in the middle and the effect of a mass on the space nearby. None of these analogies should be taken too literally - if they could, the Maths of GR would be a piece of cake.

    If one wants to introduce biblical ideas into Science discussions then those ideas must be subjected to the same requirement of confirmation by experiment / observation as the modern theories. Otherwise you just have to leave them out.

    Remember, if one of us had to go back 4000 years and explain to people of the time, in a few words and using only contemporary terms and knowledge, something like Cosmology, he'd end up using exactly the same flawed analogies as you tend to find in religious texts - and it would not make any better sense than what you can read in the Bible. Being devil's advocate (or possibly the opposite) you could say the fact that Bible Science reads as nonsense, so often, doesn't mean it wasn't imparted by a higher intelligence - just that the people who received it were too ignorant to grasp it.
  11. Feb 2, 2010 #10
    Well said. However, what do you say to a person: who believes gravitons are not bound to the speed barrier, and the fabric of space effecting how gravity behaves is fiction?... And if the sun were to magically vanish, it wouldn't take 8 minutes for the earth to go off its orbit? And that Einstein's position has no practical use?... Argh!!!

    Okay, let's focus on his illustration of the "armwrestle," the big questions: 1. If gravitons exist, can we proof they are bound to the speed barrier? 2. If they don't exist, could we still prove gravity follows the speed barrier?...
    Last edited: Feb 2, 2010
  12. Feb 2, 2010 #11


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    There may yet be some hope - as long as they keep reading these forums?
  13. Feb 2, 2010 #12
    An interesting examination of the "speed" of gravity and whether gravitational equipotential surfaces are Lorentz covariant is in the following link. He does not arrive at a conclusion but does consider some interesting approaches:


    For all I know, there could be research out there that's figured this out. If so, anyone have a link?
  14. Feb 2, 2010 #13


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    That's a 'fun' and well informed link. I couldn't find any of what is says in the Bible, though so should I doubt it?
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