Does Gravity bend Gravity?

dipole

There are some close analogies between gravity and electromagnetic fields though when dealing with rotating masses which is probably where the OP is getting confused.

http://en.wikipedia.org/wiki/Gravitoelectromagnetism

Very fascinating stuff.

twofish-quant

Also, it's an interesting fact that if you write Einstein's equations in five dimensions, you get Maxwell's equations

http://en.wikipedia.org/wiki/Kaluza_Klein

This is a deep and profound fact although people aren't sure exactly what it means.

dipole

That's extremely interesting, it's as if the rotating mass somehow "brings out" whatever characteristics this fifth dimension has to produce the same results.

I'd love to do research in GR if I didn't care about ever finding a job. :)

B.M.Gray

A "gravitational collective" bending another, singular gravitational force. A very good question actually when one deeply thinks about this. I immediately look towards black holes for a possibility of two gravitational forces bending each other, and/or individually yielding to another gravitational source. A gravitational collective (as I call it just to simplify the meaning), such as the sun, moon, and planets within the inner and outer solar system would effect each others individual gravitational forces, yet, the effects are shared so this would happen as one whole collective. For example, the earths gravity effects the moon, and thus the moons gravity effects the earth (Jupiters gravity effects its moons, as well as every other planet in the system to separate but certain degrees). The stability of our system is due to the forces from each body acting on one another and therefore keeping each other 'in-line', such as the orbits, and planetary rotations.
My further question towards this topic would be how did our system balance itself out, down to the tiniest fractions of earths position relative to the sun, to the position of the moon to balance earth, to the rate of it's rotation and orbital path which are the effects which caused our apparently perfect 24 hour days, and 12 month calendars?
I could get into details on how seemingly perfectly placed each planet is but then this would be too long a post, so I'm simply asking was our goldilocks position simply a result of murphys law over time or something else?

bahamagreen

It's interesting you mention that. A lot of things kind of get "brought out" when looking at higher dimensions.

If you have four dimensions where three are space and one is time, a curious thing happens when you define the surface of a 4D "sphere" of constant radius where the definition is that all surface points have the same distance to the origin... d=sqrt(x^2 + y^2 + z^2 - c^2t^2).
Its three dimensional space projection, a 3d sphere, grows hyperbolically for t>0.

twofish-quant

You can get a Ph.D. and then work for an investment bank. When I interviewed for my current job, I was impressed when one of the interviewers grilled me with questions on numerical relativity. I was even more impressed when he pointed out that I flipped the names of two variables in my answer.

twofish-quant

If you try to do physics this way, you pretty quickly end up with equations that are completely unmanageable.

The way that people have worked the problem since the 19th century is to calculate things in terms of "fields." An object creates a gravitational, electromagnetic or whatever field, and the field then influences the behavior of other objects.

So if you have two objects, their gravitational fields will add up. And if you have two situations in which you have the same field, it doesn't matter what the original objects were.

Our days are 24 hours, because 24 is a nice round number. The number of lunar cycles for one solar cycle is roughly 12, but there is enough of a difference to give people lots of head aches.

Now there *are* situations called resonances in which objects do end up in perfect synchronization. For example, one revolution of the moon is one rotation. What happens is that you end up in situations where a synchronized system happens to be the state with the lowest energy, and that happens a lot in the solar system.

One possibility is the anthopic principle (i.e. we are finding that hot jupiters are pretty common in the universe, but if there were one in our solar system, we wouldn't be here to talk about it).

Chronos

Hot jupiters are a classic example of selection bias. They are relatively common because they are easiest to find.

Planetary spacing was an 18th century exercise in numerology resulting in the Titius-Bode law.
I know of no particular reason earth orbit could not be substantially different than it is and still be stable.

twofish-quant

And there's another likely selection bias in that any solar system with a hot Jupiter isn't going to have inner planets.

The assumption before exoplanet observation was that our solar system was typical, and even with our limited data, it's pretty clear that this is not the situation.

The planetary people that I know of strongly disagree with that. If you have Jupiter mass objects in the inner solar system then the inner solar system becomes wildly dynamically unstable. It turns out that one reason that objects in our solar system are relatively "well behaved" is that Jupiter and Saturn are in a rough resonance that circularizes both their orbits. If you didn't have that resonance then over the course of a billion years, there's really nothing to keep Jupiter from crashing into the inner solar system.

It turns out that it's very hard to keep N-bodies dynamically stable.

I know people who have at least speculated that Titus-Bode is an application of the anthropic principle. Most solar systems don't have well spaced planets, but solar systems without well spaced planets end up without astronomers.

Chronos

That is not in dispute.
While our solar system is not proven to be typical, neither is it proven atypical. We simply have not detected enough multi-planet systems to draw any such conclusion.Again, that is not in dispute. There just isnt space enough for other inner planets to achieve a stable orbit with an 800 pound gorilla in the room. References would be appreciated. I've occasionally seen similar claims on creationist sites, but, never seen it affirmed in mainstream literature

Vagn

It is part of the Nice Model, published in Nature in 2005.
http://www.nature.com/nature/journal...ture03539.html
http://www.nature.com/nature/journal...ture03540.html
http://www.nature.com/nature/journal...ture03676.html

Chronos

See http://en.wikipedia.org/wiki/Stabili...e_Solar_System for a counter example.

twofish-quant

That's not true. With Kepler, the number of exoplanets are in the thousands, and that's enough data to show that the solar system is not typical. Here is one paper that goes through the selection effects, and it was written with 2006 data.

Observational biases in determining extrasolar planet eccentricities in single-planet systems
http://arxiv.org/abs/1008.4152v1

Based on 2006 data, you could argue that as many as one third of large planets have circular orbits, but that's enough to make the solar system uncommon.

With the new Kepler data, the solar system also looks pretty uncommon since most of the planets have non-circular orbits....

The Exoplanet Eccentricity Distribution from Kepler Planet Candidates
http://arxiv.org/abs/1203.1631

The argument is whether the solar system is atypical-uncommon or atypical-rare....

The fact that most planets have eccentric orbits was a big shock. The belief in 1990 was that since gas and dust go into circular orbits in a disk, that they would end up with nice circular planetary orbits. This wasn't what people found....

as far as See http://en.wikipedia.org/wiki/Stabili...e_Solar_System for a counter example.

Our solar system is pretty stable. However what people are finding is that if you just put some random planets into a solar system, it's hard to keep them from hitting each other, and doing some very complex things.

http://arxiv.org/abs/0903.4700
Planet-planet scattering leads to tightly packed planetary systems

http://arxiv.org/abs/0801.3226
Extrasolar Planet Interactions

This is all starting to form a nice picture. Solar systems go through a phase in which planets are in eccentric orbits and they hit each other. Most never leave that state, but we were lucky, and our solar system ended up in a stable system.

http://arxiv.org/abs/0706.1235

From mean-motion resonances to scattered planets: Producing the Solar System, eccentric exoplanets and Late Heavy Bombardments