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Zachary
2004-Jan-04, 01:36 PM
I was just thinking about gravity and why a) it has no 'opposite', e.g: planets never repel each other, and b) what causes gravity? is there something similar to a magnetic feild? Anyone know the answers to these?

Cheers,
Zachary.

Glom
2004-Jan-04, 02:33 PM
I was just thinking about gravity and why a) it has no 'opposite', e.g: planets never repel each other,

Why should it?


and b) what causes gravity? is there something similar to a magnetic feild? Anyone know the answers to these?

A mass causes a gravitational field. According to quantum predictions, it is caused by the exchange of gravitons between the masses.

John Kierein
2004-Jan-04, 04:14 PM
I have a paper in the book "Pushing Gravity" that explains gravity as being caused by the push of very long wavelength cosmic background radiation. This makes the graviton be a hole in this background caused by the "shadow" cast by a mass. Thus the quantization of gravity is like the absence of a photon; similar to a "hole" in semiconductor theory. Others have this idea too. Modern theorists say that this mechanism in a shorter wavelength part of the spectrum causes galaxy formation. (Hogan and White nicknamed their theory "Mock Gravity" and it made made the cover of the journal Nature.) Others like Lyman Spitzer and Fred Whipple also say this caused planetary formation. My graviton is consistent with the characteristics required of quantum theory. It was first proposed by Charles Brush in 1910 although pushing gravity has a very long history as documented in Pushing Gravity. Newton's protege Favio was a proponent, but the most famous one was the Swiss physicist LeSage and many such pushing theories are often called "Lesagian".

Normandy6644
2004-Jan-04, 05:08 PM
Zachary,

Gravity is a rather odd force. I too share your question of why gravity has no opposite, though it appears it might have just that regarding the expansion of space. It's a topic of current research that is quite interesting. As to how it works, think of a bowling ball placed on a taut sheet. The bowling ball "curves" the space around it, creating a kind of "dent" in the fabric of space. When other lighter objects pass near the ball, they are subject to this gravitational field and start to orbit accordingly. I hope that helps!

tazmandevil3
2004-Jan-04, 06:06 PM
Of course there is an opposing force to gravity - Repulsion force (for anyone who followed the Planet X bullflop while it was going strong)! :roll:

Koan
2004-Jan-04, 06:39 PM
The recent (re)introduction of the concept of "dark energy" into cosmological theory was necessary to explain the apparent speeding up of the expansion of the universe. The effect of this dark energy is to provide an extra "push" outward against gravity. There is a thread currently running on it.

http://www.badastronomy.com/phpBB/viewtopic.php?t=10237&sid=97d3d0e8a8a9551be1557d78 5a95d4cf

At one time, it was suggested that matter and antimatter might exert antigravity forces on each other. But not being a particle theory physicist, I donít know if that has been proven or disproved or is still on the fence. Any one know, or do I have to go surfing ?

ToSeek
2004-Jan-04, 10:10 PM
At one time, it was suggested that matter and antimatter might exert antigravity forces on each other. But not being a particle theory physicist, I donít know if that has been proven or disproved or is still on the fence. Any one know, or do I have to go surfing ?

I don't think it's ever been part of mainstream theory that antimatter would have an antigravity effect.

Koan
2004-Jan-04, 10:35 PM
I think I heard that idea tossed around way back 30 years ago when they were trying to figure out where all of the antimatter went that would have balanced the mass of matter that was in our part of the universe.

The thinking was that since a particle and its antiparticle are created simultaneously from the collision of two high-energy photons, then many of the pairs should have almost instantly annihilated themselves, unless they could have gotten away from each other. Someone [no idea who] suggested that maybe the particle/antiparticle pair experienced an antigravity that pushed most of them far enough apart to prevent their contact, and eventually pushed the antimatter far beyond our part of the universe.

I'm not sure 30 years ago it was easy to distinguish from the outside which theories came out of mainstream thinking as compared to stream-of-consciousness speculation. I imagine one would have to have been an expert in the field to really be able to separate the two in those days.

By the way, do we know where the antimatter really did go ?

Cougar
2004-Jan-04, 11:15 PM
Someone [no idea who] suggested that maybe the particle/antiparticle pair experienced an antigravity that pushed most of them far enough apart to prevent their contact, and eventually pushed the antimatter far beyond our part of the universe.
I believe antimatter has opposite charge, but as previously mentioned, there is no opposite to gravity, so antimatter has the same gravitational effect as matter, and the above speculation is false.

Dark energy happens to work in opposition to gravity, but it is really not related. It is a separate, unrelated force. Gravity is known as the weakest of the four known forces - the weakest BY FAR. If dark energy is determined to be a fifth "force", then it would appear to be many, many orders of magnitude weaker than gravity.


By the way, do we know where the antimatter really did go?
It was annihilated when it came into contact with matter very early in the life of the universe. The entire amount of matter currently in the universe is the result of a slight imbalance of matter over antimatter due to a very minor quantum asymmetry that manifests only at very high temperatures and energies.

John Kierein
2004-Jan-04, 11:50 PM
I am not at all convinced that there isn't an equal amount of anti-matter as matter. We see the signal of electron positron annihilation at the top edge of the galaxy where it meets the intergalactic medium. The intergalactic medium may well be composed of equal amounts of electrons and positrons and we would never see 'em since they have no spectral signature. An antimatter galaxy would look just like a matter one. Just because our galaxy seems to have an excess of matter doesn't mean other galaxies don't have an excess of anti-matter. It's OK to date an anti-matter person, just don't kiss.

Cougar
2004-Jan-04, 11:55 PM
I am not at all convinced that there isn't an equal amount of anti-matter as matter.
A minimum of investigation into observations already made would show that this idea is untenable.

Manchurian Taikonaut
2004-Mar-05, 02:54 PM
Yes gravity is a strange thing


Recent cosmological observations extending the Hubble diagram of high redshift Type Ia supernovae performed independetly by two groups (the Supernova Cosmology Project [2] and the High-Z Supernova Team [3, 4] presented evidence that the expansion of the Universe is accelerating rather than slowing down. This fact combined with the Boomerang and Maxima-1 measurements of the first acoustic peak location in the angular power spectrum of the cosmic microwave background (CMB) [5, 6] point towards a standard cosmological model with critical density ( = m + = 1) and a dominant -like, "dark energy" component at the present epoch ( 0.7). This component could be produced by non-zero and positive cosmological constant with
http://nedwww.ipac.caltech.edu/level5/March01/Dark/Dark.html


The crux of all this is that globular star clusters in our Milky Way are 12 to 16 billion years old. So can the universe be younger than the stars it contains? No, but the results may mean that ours is a low-density universe whose expansion is only gradually slowed by gravity. Or Einstein's infamous "cosmological constant," which imbues empty space with antigravity-like properties, also might be involved
00


Riess and his team, which included Louis Strolger of the science institute and Alexei Filippenko of the University of California-Berkeley, used the Hubble to search for exploding stars, or supernovas, which are swept up in the dark energy's cosmic push. They discovered 42 supernovas in their survey area, including six of the seven most distant known.

But rather than seeing the changes in the push that many theorists had predicted, Einstein's steady, unchanging cosmological constant fits the data better than any of the alternatives.

"What we've found is that it looks like a semipermanent kind of dark anti gravity energy," Riess said. "It appears like it's been with us for a long time; if it is changing it's doing so slowly." {{{{
where M(r) = 4/3 (1 + 3w) r3. Notice that for w < -1/3 we have negative gravitating effective mass (antigravity) which can lead to accelerated cosmological expansion and to non-trivial dynamical effects on astrophysical scales. The accelerated cosmological expansion is obtained for w < -1/3 from the Friedman equations which for k = 0 imply


(4)
(5)

where RQ is the scale factor of the universe. In what follows we focus on the effects of dark energy with w = -1 (cosmological cosntant). The more general case of -1 < w -1/3 ([18]) will be discussed elsewhere.

Swift
2004-Mar-05, 03:00 PM
Gravity is a rather odd force. I too share your question of why gravity has no opposite, ...
But which of the forces has an "opposite". What is the opposite of the strong force or the weak force? Electromagnetism really doesn't have an "opposite" just because magnetized materials or charged particles can be attracted or repel it each. I've thought that the "weirdness" of gravity is that it is does not appear to be quantumized (is that the word) and that is has not been unified with the other forces (weak, strong, electromagnetism).

eburacum45
2004-Mar-05, 05:35 PM
The repulsive force which is causing the expansion of the universe can be regarded as having a kind of negative gravity;

Antimatter has positive gravity, and has normal mass, but there is a theoretical kind of matter called negative matter, which has negative mass, negative energy, negative gravity and negative inertia;

Negative energy is involved in the Cosmological Constant, Hawking radiation and the Casimir effect, but taking the idea just that little further to imagine real negative matter is pure science fiction;

which is why I enjoy the concept so much.

one link (http://van.hep.uiuc.edu/van/qa/section/New_and_Exciting_Physics/Antimatter/20020405173833.htm)