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Phy6explorer
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I was thinking about gravity when I thought that gravity only attracts.Why doesn't gravity repel?Do you think there is some unidentified example where gravity repels?
Phy6explorer said:I was thinking about gravity when I thought that gravity only attracts.Why doesn't gravity repel?Do you think there is some unidentified example where gravity repels?
neopolitan said:Is the OP asking "why, unlike other forces, is the force of gravity universally attractive?"
If so, isn't this wrong in itself, since three of the four fundamental forces are universally attractive, and only one either repulsive or attractive depending likeness or lack of it respectively (since electromagnetism is one phenomenon)?
And then, is this a relativity question at all?
Finally, the last poster doesn't seem to understand what naturally causes pressure. I assume he is talking about the phenomenon where a hot air balloon rises, which is due to a pressure effect. There is no negative pressure in the balloon (in fact the pressure is equal) nor is the phenomenon observed "gravitational repulsion" (it is merely the tendency for heavier things to sink, displacing the lighter balloon).
cheers,
neopolitan
Bose said:Gravity can repel if the pressure is negative
cristo said:Anti particles do not have negative mass!
Nonsense!Bose said:Gravity can repel if the pressure is negative
Bose said:Gravity can repel if the pressure is negative.
Astronuc said:Nonsense!
Astronuc said:Nonsense!
malawi_glenn said:http://supernova.lbl.gov/~evlinder/turner.pdf
Equation 1, maybe it was this that Bose was referring to?
Anyway, I have no clue what he meant, just a paper I found. I never heard of repulsive Gravity in my Cosmolgy courses..
Actually it is not nonsense, this is precisely what Dark Energy is all about, i.e. DE is a form of energy with negative pressure that is able to make the universe accelerate in its expansion due to the action of a repulsive gravitational force.Astronuc said:Nonsense!Bose said:Gravity can repel if the pressure is negative
Google on "Cern" "athena" "antihydrogen" for some pretty exciting stuff. They intend to make cold neutral antihydrogen and experiment with it. One of the tests is to see if the gravitational infall rate of antihydrogen is identical to that of hydrogen (Testing the weak equivalence principle). Also, CPT invariance requires that the masses of hydrogen and antihydrogen be identical and that they should be spectrally identical - more interesting tests to be made.kev said:In General Relativity the curvature of spacetime around a massive object determines the trajectory of a particle so that it follows a geodesic path largely determined by the particles velocity and independly of the mass of the particle (unless the particle's mass is so great compared to that of the massive body that it significantly distorts the spacetime curvature itself. For example a normal particle, a photon (with zero rest mass) and an anti particle (with negative mass??) all fall towards the massive body. I would be interested to know if General Relativity predicts that two anti-particles attract or repel each other gravitationally. If anti particles repel each other this would imply that anti particles can never clump together and form a massive anti-body or anti black hole, so maybe that is not correct because I have read on a number of occasions that black holes and even stars could be made entirely of antimatter. Is that true?
Astronuc said:Since the poster did not mention DE nor the 2nd Friedman equation, I took the comment in context of matter as we observe it here on earth, and was reflecting on another thread that was implying anti-gravity was somehow related to anti-matter.
Turner's paper makes the point - ". . . dark energy has the following defining properties: (1) it emits no light; (2) it has large, negative pressure, pX ~ [itex]\rho[/itex]X; and (3) it is approximately homogeneous (more precisely, does not cluster significantly with matter on scales at least as large as clusters of galaxies)."
I agree that the truth is obscurbed by simply saying "No" or "Nonsense" especially since there is at least several examples of "antigravity" postulated to exist (or did exist) in nature. One example is found in the Inflationary Universe Model where the universe expanded at an extremely accelerated rate. This is an example of "antigravity." I recall that this could have resulted by either a postive cosmological expansion or tension (negative pressure) by some bizarre field (scalar field?). Then there is the example of a vacuum domain wall which is an wall made of matter which has tension in it and is created by some sort of bizarre field as well. Then there is a cosmic string which has a positive engergy density and tension. The result is no gravitational attractipon at all since these effects (due to energy vs due to tension) cancel out.kev said:So you agree that the statement by Bose that "Gravity can repel if the pressure is negative" is not nonsense after all (in the cosmological scale)?
It should be noted that the OP asked "Do you think there is some unidentified example where gravity repels?" Giving an unqualified "No" answer, when you are actually privately restricting the conditions to those found on Earth on a small scale is obscuring the truth somewhat.
Giving an unqualifed example is incorrect. I can give plenty examples of repulsion/attraction with respect to electrical fields, magnetic fields, mechanical force which involve tension (positive stress) or compression (negative stress) which do not invoke gravity or anti-gravity. So in general to say strictly "gravity repels if pressure is negative", one would be incorrect.kev said:So you agree that the statement by Bose that "Gravity can repel if the pressure is negative" is not nonsense after all (in the cosmological scale)?
It should be noted that the OP asked "Do you think there is some unidentified example where gravity repels?" Giving an unqualified "No" answer, when you are actually privately restricting the conditions to those found on Earth on a small scale is obscuring the truth somewhat.
malawi_glenn said:Guys, drop it :-)
Gravity is a fundamental force of nature that describes the attraction between objects with mass. It is not repulsive because it is a result of the curvature of space-time caused by the presence of mass.
Gravity works by pulling objects towards each other due to the warping of space-time by the presence of mass. The more mass an object has, the stronger its gravitational pull.
Yes, there is overwhelming evidence for gravity being attractive. For example, the orbits of planets around the sun, the motion of objects falling towards the Earth, and the bending of starlight by massive objects all support the attractive nature of gravity.
In classical physics, gravity is always attractive. However, in theories such as Einstein's general theory of relativity and quantum mechanics, there are instances where gravity can be repulsive, such as in the case of dark energy.
Gravity affects the motion of objects by causing them to accelerate towards each other. The force of gravity between two objects is directly proportional to their masses and inversely proportional to the square of the distance between them.