How does gravity repel?


by shaan_aragorn
Tags: gravity, repel
waht
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#19
Feb13-08, 04:12 PM
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Gravitational slingshot also gives illusion of a mysterious repulsive force. When a space probe approaches a giant planet like Jupiter on a certain tangent, it swings around and gains incredible speed. Upon leaving the planet, one would assume the gravity would slow down the space probe by pulling it inwards, but that is not true. The probe leaves the system at a higher speed as if it was repulsed.
tony873004
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#20
Feb13-08, 04:37 PM
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Quote Quote by waht View Post
Gravitational slingshot also gives illusion of a mysterious repulsive force. When a space probe approaches a giant planet like Jupiter on a certain tangent, it swings around and gains incredible speed. Upon leaving the planet, one would assume the gravity would slow down the space probe by pulling it inwards, but that is not true. The probe leaves the system at a higher speed as if it was repulsed.
That's a good point. Jupiter seemingly "pushed" the New Horizons spacecraft onto a faster trajectory to Pluto.
DaveC426913
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#21
Feb13-08, 07:37 PM
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Quote Quote by waht View Post
Gravitational slingshot also gives illusion of a mysterious repulsive force.
It's only mysterious if you're not paying atttention.

It's not gravity creating the acceleration, it's the planet's own motion through the solar system.


Quote Quote by waht View Post
When a space probe approaches a giant planet like Jupiter on a certain tangent, it swings around and gains incredible speed. Upon leaving the planet, one would assume the gravity would slow down the space probe by pulling it inwards, but that is not true.
It most certainly is true. It pulls every bit as much when the probe is receding as when it's approaching.


Quote Quote by waht View Post
The probe leaves the system at a higher speed as if it was repulsed.
No, it leaves the system at a higher speed as if it traded some momentum with the planet, which is now moving at a slightly slower pace through the solar system.
waht
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#22
Feb13-08, 08:25 PM
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Quote Quote by DaveC426913 View Post
No, it leaves the system at a higher speed as if it traded some momentum with the planet, which is now moving at a slightly slower pace through the solar system.
I was just alluding to the fact that gravitational slingshot can appear to be an effect of a gravitational repulsion. It is well understand, and it's a consequence of conversation of momentum. Total momentum of the system is constant before and after the pass. But the probe steals a little bit of momentum from Jupiter, which is manifested in a greater relative velocity for the probe, and less so for Jupiter. Since the mass of Jupiter is extremely large compared to the probe, we won't detect any changes in the planet's motion.
DaveC426913
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#23
Feb13-08, 09:25 PM
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Quote Quote by waht View Post
I was just alluding to the fact that gravitational slingshot can appear to be an effect of a gravitational repulsion. It is well understand, and it's a consequence of conversation of momentum.
Then isn't this just misleading?
waht
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#24
Feb14-08, 10:12 AM
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Quote Quote by DaveC426913 View Post
Then isn't this just misleading?
I think it's a plausible hypothesis that gravity can be repulsive if you didn't understand classical mechanics. Once you introduce CM, the hypothesis is proven wrong.
DaveC426913
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#25
Feb14-08, 05:40 PM
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Quote Quote by waht View Post
I think it's a plausible hypothesis that gravity can be repulsive if you didn't understand classical mechanics. Once you introduce CM, the hypothesis is proven wrong.
But why would we offer hypotheses born of ignorance? They come here for answers.
waht
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Feb14-08, 08:04 PM
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Quote Quote by DaveC426913 View Post
But why would we offer hypotheses born of ignorance?
Unintentionally if it came down to it.
Alex48674
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#27
Feb16-08, 09:49 AM
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So can gravity repel ='[?

I thought at 80%plank high density it could? Or something like that =]
marcus
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Feb16-08, 01:56 PM
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Quote Quote by Alex48674 View Post
So can gravity repel ='[?

I thought at 80%plank high density it could? Or something like that =]
that is the result that Ashtekar's group has gotten in many of the cases they have studied, in the context of Loop Quantum Cosmology. That doesn't mean its right, but it is suggestive.

We know that classic (unquantized, vintage 1915) GR is wrong at very high density because it develops singularities.

So ever since Dirac (for over 50 years) people have been trying to quantize GR in the expectation that it would get rid of singularities----like where GR breaks down at bang and hole.

Ashtekar's group has a pretty good model that they can run in the computer and vary the conditions, so they can try case after case. It is a comparatively straightforward quantization of GR that gives you a wave function for the size of the universe and observables for things like density. Not a lot of extra junk---no extra dimensions, branes and made-up stuff.

Their quantized cosmology agrees with standard almost exactly a few moments after the start of expansion----it has the right classical limit. It only disagrees with classic picture within a few planck time intervals of the actual start of expansion.

It consistently says there was a bounce which occurred at a density of around 80 percent Planck. It goes back in time to before the start of exapansion and says there was a collapsing phase.

These are the most highly cited papers in quantum cosmology at present. Ashtekar's group and also work by Bojowald along similar lines. It is up to you how seriously you take it---it might be wrong, and a dead end, it might be on the right track.

All we know is that their research is hot right now----since 2005 or earlier----and when they quantize Gen Rel in the most straightforward manner applied to early universe it turns out that quantum effects make gravity repellent at near-Planck densities and tend to cause a kind of collapse rebound that kicks off the expansion phase.
marcus
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#29
Feb16-08, 02:06 PM
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if anyone is curious here are some papers by Ashtekar, Bojowald, and others
this is a Spires keyword search for recent (post-2005) Quantum Cosmology papers
ordered by how much the papers have been cited in other research
(reflecting how useful or significant the results are considered to be by the QC community)

http://www.slac.stanford.edu/spires/...tecount%28d%29

Under each paper listed there is a place where you can click on "ABSTRACT"

if you click there, you get a brief summary of the paper and a link to download PDF if you wish. It saves time to read the summary first to see if you really want to download. In any case it is free. But on the other hand technical papers usually only have a small amount at the beginning and end which is readily accessible. So it is not everybody's favorite food.

anyway with some qualification what Alex said is right, gravity does seem to repel at high densities if you go by the quantized General Relativity (quantized gravity law) used in the currently leading line of quantum cosmology research. It happens due to quantum corrections--terms in the Hamiltonian that become important at high density. And the new stuff has not been tested observationally, so you have to decide for yourself how seriously to take it.
Abbas Sherif
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#30
Feb18-08, 02:23 PM
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Quote Quote by DaveC426913 View Post
Wild, unfounded speculation will not help answer the OP's question.
Dave, we are all learners and knowledge seekers. you guys are opportuned to be in a country that provides all the necessary resources needed for these researches.I'm form Africa. You use the internet on a daily basis. I use it once in a while. If you feel mine is a speculation and its unfounded,look for the book, Physics for Scientist and Engineers by Paul A. Tipler and read on the chapter, evolution of the universe and about the lepton era. Be good guy
DaveC426913
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#31
Feb18-08, 03:18 PM
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Quote Quote by Abbas Sherif View Post
If you feel mine is a speculation and its unfounded,look for the book, Physics for Scientist and Engineers by Paul A. Tipler and read on the chapter, evolution of the universe and about the lepton era. Be good guy
Why? Does it introduce observational evidence that the rest of the world is unaware of, that supports the proposed theory?

Or is it just postulating a speculation that is not founded upon evidence?
Kry Havoc
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#32
Feb23-08, 04:59 AM
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lets not forget the theory of gravitons.
for those who don't know some scientist's believe that there are particles no charge, no mass, and carry energy/gravitational energy, this is almost identical to photons, particles that have no mass or charge but they carry electromagnetic energy,


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