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Properties Of Gravity

  1. Sep 6, 2007 #1
    Gravity seems to be a very unusual force. From what i understand:

    Gravity can bend space-time
    Gravity occurs with an object with mass
    Gravity can attract objects through barriers
    Gravity attracts each unit mass of an object with the same force.

    Can people add and correct any misunderstandings please.
  2. jcsd
  3. Sep 6, 2007 #2


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    And gravity is always attractive, there are no positive/negative masses.

    This is what has made gravity so hard to combine with other forces in a single theory.
  4. Sep 7, 2007 #3
    Gravity is probably my favorite subject - the more we learn about it the more fascinating it seems to become. I have some corrections and additions for your list, I hope and expect the real experts around here will have more.

    Let's begin by clarifying the nature of gravity in the light of General Relativity, which is by far the most successful model for gravity that we have - after a century, every test of GR has only validated it further*.

    According to GR, gravity isn't really a 'force' at all (the idea of gravity as a force refers to the old Newtonian model of gravity), instead, gravity -is- the curvature of spacetime. Basically, the presence of mass/energy appears to distort the physical dimensions of spacetime. So a mass in a gravitational field isn't experiencing a 'force' at all...it
    s simply following the geometry of the spacetime 'background' that it exists in. This explains why all masses 'fall' at the same rate. Another way of saying this is that inertial mass is equivalent to gravitational mass.

    Another slight clarification would be to say that mass and energy are gravitationally equivalent: not only does matter 'bend' spacetime, energy does too. For example, when you wind up a clock, it actually becomes slightly heavier. Another example would be a battery: a battery weighs more when it holds a 'charge' (ie when it possesses chemical potential energy) than when it's charge has run out. We can't measure the difference in mass in these cases, but we know it's true because the same principle applies to other cases that we -can- measure directly.

    Here's where it gets -really- interesting: there are actually -three- forms of gravitational field, and they're analogous to electromagnetic fields. The gravity that you and I know and love, is the 'static' gravitational field. It bears a resemblence to a static electrical charge, except with gravity, there's apparently only one form of 'charge' instead of two, and that one 'charge' attracts other like 'charges' (unlike electircal charges where likes charges repel).

    Now if you think that accelerating a 'gravitational charge' (ie mass/energy) might create an effect analogous to a magnetic field, you'd be right. It turns out that a mass in motion generates a gravitomagnetic field (also called the 'Lense-Thirring effect' or 'frame-dragging'). Don't get confused - the magnetic field created by a moving electrical charge, and the gravitomagentic field created by a moving mass are completely different phenomena. But they do share some facinating similarities.

    There's more. It appears that an -accelerating mass- will produce a 'gravitoelectric' field, which follows the rules analogous to electromagnetic induction.

    Wikipedia has a nice introductory page deduicated to these 'gravitoelectromagnetic' phenomena: http://en.wikipedia.org/wiki/Gravitoelectromagnetism

    And many physicists are anxiously awaiting on the analytical results of the Gravity Probe B satellite experiment, which was created at Stanford University to measure the gravitomagnetic field of the Earth (the results are due in the next few months): http://einstein.stanford.edu/

    But we already have compelling evidence of gravitomagnetism from astronomical observations of black holes and quasars: http://www.physorg.com/news99917013.html

    Now if we want to go way out to the leading edge of scientific investigation of gravitoelectromagetism, we'll find the work of European Space Agency physicists Martin Tajmar and Clovis de Matos, who shook the theoretical physics community last year with the tentative announcement that they may have created a gravitomagnetic field in a laboratory experiment by rapidly spinning a superconducting ring of metal. *Early indications are that for reasons still unknown, the intensity of this gravitomagnetic field may be about 30 orders of magnitude stronger than the field intenisty predicted by General Relativity under their experimental conditions - this would be, to the best of my knowledge anyway, the -first- time GR failed to predict the magnitude of a gravitational effect to within a neglible error. These researchers feel that their work may be the key to creating a quantum model of gravity, which is pretty much the Holy Grail of theoretical physics nowadays.

    Here's their fascinating and controversial paper from last spring: http://arxiv.org/ftp/gr-qc/papers/0603/0603033.pdf

    So we're only now beginning to examine the subtle intricacies of gravitational phenomena. Many feel that the current work will herald a revolutionary era of new applied physics akin to the days of Faraday, Maxwell, and Tesla, when applied electromagnetism changed the world.

    Perhaps you'll agree as you delve deeply into this riveting subject...
  5. Sep 7, 2007 #4


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    Awesome post, Demon. :cool:
    I've never heard of that ESA experiment. While I don't have time now, I look forward to reading that link. I probably won't understand most of it, but it certainly seems fascinating from the introduction.

    The only thing that I would like to mention is that the theoretical 'negative matter' would actually produce repulsive gravity. As far as I know, there hasn't been any physical evidence that it actually exists.
  6. Sep 7, 2007 #5


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    Gravity does NOT "bend space time"- gravity IS bent space time.
  7. Sep 7, 2007 #6
    Are there any strong and (more directly) weak force analogs to gravity?

    What are some simple introductions to GR using Einstein notation? Misner, Thorne and Wheeler's Gravitation is a bit above my head.
  8. Sep 7, 2007 #7


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    So what it the cause of acceleration of masses towards (assuming both objects in free fall) each other if it isn't a force?
  9. Sep 8, 2007 #8
    In the old Newtonian model, space and time, the ‘background’ of action in the universe, were immutable and Euclidean. Put simply, the shortest distance between two points was always assumed to be a straight line, and any change in motion was attributed to forces acting on a mass.

    General Relativity threw that idea right out the window. Einstein proved mathematically that space and time were, well, ‘flexible.’ And his theory described exactly how much spacetime (now unified under his model) ‘curves’ when matter is present. Another way of saying this is that spacetime ‘couples’ with matter/energy (mass and energy also became united within his theory – dude was a genius, you see). So gravity isn’t the action of a ‘force,’ it’s simply 4D geometry disregarding our Euclidian expectations of nature.

    This all seemed pretty outrageous at the time Einstein presented this weird geometric solution for gravity. But not only did this crazy idea work, it worked better than the Newtonian gravity model…it explained the at-the-time inexplicable little anomaly in the precession of the perihelion of Mercury, among other things. Physicists were used to thinking of gravity as a force acting against a ‘flat’ background. That is, Newtonian gravity was a ‘background independent’ theory of gravity. Einstein explained that matter wasn’t experiencing a ‘force of gravity’ at all, but rather, it was simply following -the shortest distance between two points- in a curved 4D spacetime.

    It may be useful to consider the situation of a falling body within a gravitational field. This is where we really see that there isn’t a force involved.

    Imagine that you’re a cold rock in space drifting toward the Earth at a constant velocity (we’ll neglect the minor fluctuations in velocity induced by other nearby planets and such).

    As you approach the Earth, your velocity toward it increases. It increases a little bit at a time when you’re far away, but the rate of the increase continues to grow as you get closer…but you don’t feel any force ‘pushing’ or ‘pulling’ on you at all. Your trajectory may also change – it seems like the Earth is coming at you faster and faster, and it’s even putting itself more and more directly in your path! Hark! From your point of view, you're not acccelerating toward the Earth - it's accelerating toward -you-. Even as you begin to enter the Earth’s atmosphere, you still don’t feel any ‘force of gravity’ at all – not the slightest nudge toward the Earth. In fact, the only force you’ll feel is a slight ‘push’ from directly ahead of you, from atmospheric friction. And finally, you’ll feel a force directly ahead of you as you rudely impact the Earth.

    So the only force we feel here on the surface of the Earth, is the force of the dirt beneath our feet stopping us from conforming to the dictates of the curved spacetime all around us. If there were a hole all the way through the Earth, we’d move through it to the other side of the planet and back and forth…without ever feeling any kind of accelerating force: we’d feel ‘weightless’ and unperturbed at all times (except for incidental air friction effects).

    Why? Because from our point of view, we’re actually stationary on this crazy, twisted background of 4D spacetime. In freefall, the only reason we think we’re moving is because the other objects around us don’t share our unique little frame of reference. But if we were wearing blinders, there’d be no way to tell whether we were drifting in space, orbiting a planet, or plunging toward a black hole, because as we’ve seen, gravity isn’t a force.
    Last edited: Sep 8, 2007
  10. Sep 8, 2007 #9
    Oops i meant mass and energy bends space time.

    Thanks alot guys for the clarification on such an unusual subject!
    Last edited: Sep 8, 2007
  11. Sep 9, 2007 #10
    Anytime, Morga. I forgot to mention gravity waves - we haven't seen those directly yet, but we can infer their existence from observing regions of unusually intense gravitational interactions, such as binary star systems containing neutron stars, and stars being pulled into black holes. Gravity waves radiate from extremely violent events, and they're basically a ripple in the spacetime metric of reality.

    It might also be worth noting a special case called 'gravitational lensing.' It wasn't long ago that we first observed that we could see light from distant objects being distorted by intense gravitational fields lying directly between the Earth and a distant light source, just like light gets bent through a magnifying lens. But today astronomers are using the gravitational lens phenomenon fairly routinely to detect the presence of 'dark matter.'

    I've often wondered if someday we'll find a way to artificially engineer gravitational fields to produce this effect to make the perfect space-based telescope.
  12. Sep 9, 2007 #11


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    Replace that rock with a very long rod pointed at the earth, and the rod will experience a tension due to the variation of strength of the gravitational field
  13. Sep 9, 2007 #12
    Some questions for the great physics people.


    In that picture, it shows spacetime being warped. The gravity. Well, two questions about that.

    1) Why does the object, or earth, or my coffee cup, or me, or anything bend spacetime? What causes it?

    2) If that's what cause gravity, what makes something follow that path?

    For example, in this image http://blazes816.com/spacetime.gif [Broken], why would the chunck of ice (the blue thing) follow the green path (roughly), and not the red one? Wouldn't gravity have to pull it down in that pattern?
    Last edited by a moderator: May 3, 2017
  14. Sep 9, 2007 #13
    Energy and Matter (Which is energy locked up) bend space time. Although i do not know why.....

    It doesn't follow a path it's traveling in a straight line. The space time is bent and it is traveling in a straight line through it.

    You have taken that picture out of context but i think what it means is that the red line is what we perceive, and the green is what has actually happened. So we can see the chunk of ice between the massive object because the light has traveled through bent space time around the object.

    That's my interpretation anyway.
    Last edited by a moderator: May 3, 2017
  15. Sep 9, 2007 #14
    All objects tested have followed the path predicted by general relativity. Mass or energy density warps spacetime. According to GR, objects moving between the same two points in spacetime, despite their having different velocities over different spatial distances, follow an identical, maximal path. Furthermore, general relativity is the most elegant, simple (and only) theory to make these predictions - as far as we can tell, without error.
  16. Sep 9, 2007 #15
    "we haven't seen those directly yet, but we can infer their existence from observing regions of unusually intense gravitational interactions"... why aren't these "inferences" good enough? Is there some quantity that watching a light beam bend on earth will allow us to measure, but that our astronomical observations won't? or is there some other possible explanation for the unusual interactions other than gravitational waves? What more do we need?
  17. Sep 9, 2007 #16


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    In a bit of a wrinkle, the folks at CERN intend to measure the gravitational infall rate of neutral anti-hydrogen and compare it to the infall rate of hydrogen. This is a critical test of the Weak Equivalence Principle.

  18. Sep 9, 2007 #17
    Consider a Minkowski (special relativistic, 4-dimensional Euclidean space) cone local to a point in spacetime. In an inertial frame, matter spreads out most symmetrically - through spacetime as concentric hypercircles on the cone, like ripples from a point on a pond. Under acceleration, though, the conic section tilts (since there is a spatially preferential increase in velocity), and we can section the Minkowski cone at a given time into hyperbolas, parabolas or ellipses. With the aqueous analogy, if you were to view the ripples lit from above onto a slanting pond bottom, they might appear as ellipses.

    Curvature in local spacetime, the ratio between changing velocity and associated spacetime surface area (in terms of c), is a constant relative to observer and object. Tracked in local spacetime, the trajectories of a bullet and a ball follow the same fundamental curvature due to gravity.
  19. Sep 10, 2007 #18


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    If gravity isn't a force, then why does it take a force to oppose it? For example, imagine a rocket "hovering" (as opposed to orbiting) somewhat near a planet using force to maintain it's distance from the planet. If gravity isn't the opposing force, then where is the equal and opposite force opposing thrust from the rocket's engine?
  20. Sep 10, 2007 #19
    Your intuition about netforces and motion breaks down in a gravitational field... think about it this way... in flat space-time (not in a gravitational field)... an object will move at constant speed until acted upon by an outside force, in space-time with a curvature of so and so (in gravitational field), an object will accelerate toward the center at so and so many m/s^2 until acted upon by an outside force.... the "natural state of motion" at a given dist. from the grav. field center is a certain acceleration, not constant speed.... it doesn't take a force to accelerate something in a gravitational field, just like it doesn't take a force to move something at constant speed in flat space-time, you have to rethink things

    You must change your definition of a force, here force isn't something that causes acceleration, force is something that opposes acceleration... (a common def. may be, force causes objects to move in unnatural states of motion and needed to keep them moving in these states of motion)

    The force on you while you're on the surface of earth (your weight) is caused by the earth pushing up on you preventing you from accelerating, not by gravity pushing down on you, you feel a net force up, opposing your acceleration... (this force is analogous to the car seat pushing up against you're back when your car is accelerating, preventing you from moving at constant speed).. note the different directions of the forces and how they correspond to the respective definitions above

    These notions are difficult to get used to because we simply don't think about them in this way, that's why it's simpler to just get rid of forces all together in General Relativity and instead talk about accelerations or curved paths
    Last edited: Sep 10, 2007
  21. Sep 10, 2007 #20
    The "hovering" is that force providing acceleration that opposes the rocket's tendency to follow a worldline, that is, to remain in an inertial frame. This natural state of the rocket is free-fall.

    The trick is to discern reaction from action, the impulse of the rocket from the "forces" of gravity. In the reference frame of the rocket, one cannot differentiate gravity's acceleration on it from that of hovering, just that they measure a certain value overall. Gravity can therefore be seen as a force (or equivalently, as energy density), but more intuitively both as spacetime curvature. Recall that the deaccelerating gasses of the rocket in vacuo follow their own worldlines with their own inertial frames. Curvature is universal, where forces can sometimes be fictitious or insufficient.
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