I'll bite: everything having to do with gravity is marvellous!

I had a more detailed response, an ode to gravity, if you will, but my computer ate it. In short, gravity amazes me because it is one of the few things in physics where you very quickly run into "I don't know" as the answer to most of your questions. It's one of those rare fields (heh heh) in which a non-scientist with a little bit of knowledge can easily ask intelligent questions that a scientist can't answer.

"Marvellous" is a good description indeed.

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"It's turtles all the way down."

Here's the thing though.

Gravity is NOT a force (Mass just 'falls' down the Gravity well, and light is barely curved by that gravity well because it is going sooo fast) and yet everytime we see explanations, like the ones above, they are 'assigning' 'energy' to it, making it a 'force'.

And, how does "Gravity making Gravity" (in all the ways described above), which is a 'local consideration', not violate the Laws Of Thermodynamics?

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

The "amazing things" being noted here about gravity are what led a fellow named Einstein to finally arrive at General Relativity, which is just so darned beautiful and elegant you just can't stand it. The more I learn about it, the less and less amendable I am to modifying it (and the measurements back that up more and more -- for example, the moon follows the path predicted by General Relativity to within *2cm*. That last amazing precision comes from lunar laser ranging).

General Relativity's explanation for all this is gravity is geometry. Mass-energy (and momentum) shape the geometry of space-time and everything simply follows inertial paths through that "curved" geometry. The accelerations you're all thinking about here from Newton are mere "coordinate accelerations", something relative to a particular choice of coordinates. In General Relavity, F = ma, by 'a' here is the proper acceleration defined by some rather complex math, known as "covariant derivatives" yada yada.

While all that fancy stuff is important, qualitatively it's really very simple, proper acceleration is deviation from the geodesic, which is intertial path through space-time. Sitting here on the ground, your proper acceleration is "up", as your geodesic wants to fall toward the center of the earth at about 1g. The real force, the force "you feel", F cause a deviation from your geodesic (relative to your own local ruler and clock) of 'a', such that F = ma still holds, but 'a' is not what you originally thought it was.

There are roughly three forms of the equivalence principle, three "strengths". The weak form is all about the above, inertial mass is equivalent to gravitational mass, which is a Newtonian way of stating that everything follows its geodesic.

Now, the next strength, the "medium" Equivalence Principle, sometimes called the Einstein Equivalence Principle, is that all non-gravitational physics (EM, chemistry, nuclear, everything) *locally* is the same in free fall as it is moving inertially in flat space-time. IOW, if you're in a black box, "feeling no force", no local non-gravitational experiment can tell if you're falling in a gravitational field or just floating free in no gravitational field. And you can turn that around and find the equivalence of being stationary in a gravitational field and an accelerating in flat space-time as well.

The Strong Equivalence Principle, the highest strength, applies that to local gravitational physics as well. IOW, a local Cavendish experiment in free fall is the same as one in flat space-time. And so you can't tell the difference in a black box even with local gravitational experiments.

The latter is quite amazing indeed. General Relativity sastifies that, which is quite amazing considering its non-linear character. And the reason for that gets pretty deep. The consequences of that, insofar as tidal forces can be ignored, means that local gravity is independent of the "background gravity". That is our solar system, in free-fall orbit in the Milky Way's background gravitational field, behaves the same way as it would by itself in otherwise empty space.

EM fields will not do that at all. A local system of charges will generally behave differently in a background field than without. And so one can detect the background by doing local experiments (in principle).

The big difference is plus and minus charges. Apply a background E field, and the + wants to go opposite from the local - charge. And second is the charge to mass ratio of the particles. Now, you might think that if you constrain the system to only one polarity of charge and require the charge to mass ratio be constant, you might get Strong Equivalence Principle like behavior.

You won't, and radiation is the killer. Accelerating charge radiates and so the background field would break the equivalence by changing the radiation.

Note in General Relativity, radiation comes in at high order derivatives, the quadrapole term, not dipole. And so constant acceleration in GR does not cause radiation, but "jerk"; this is also the reason gravity is so good at hiding the propagation delay and makes it look like it is acting instantaneously. And that's the also the main trick gravity uses to maintain the Strong Equivalence Principle. And of course the two are different aspects of the same thing.

And yet another way to put this is that the gravitational field has to include information about the *acceleration* of the source as well as its position and velocity. EM fields do not do that.

Yet another way to put it is in terms of the (v/c) ratio of the sources. With EM, radiation effects go as (v/c)^3 to first order. With gravity, it's two more powers higher, (v/c)^5.

It turns out the Strong Equivalence Principle is not easy to sastify with just any arbitrary theory of gravity. Modifications to GR usually will destroy it (but be very close in the weak field limits, as they must to give anything close to what is observed on local systems).

Because it's so difficult to hold, many think a quantum theory of gravity will have to violate it in general. But, of course, it will have to hold in the weak field limit (and weak here will get pretty strong, the violations would be in very extreme quantum regimes).

Anyway, if that turns out to be the case, one could in principle determine the background field. And that will actually give some of our Geocentrist types fits actually.

-Richard

The trinity of equivalence suggests something else. Two identical targets, except for the orientations of their nuclearly polarized spins (with a substantial percentage of the nuclei polarized) will not fall at the same rate in a high strength magnetic field. To do so would show a lack of parity effects in neutral current scattering. Parity effects are universally seen in every weak interaction. The free fall, or Eotvos experiment must see a differential here, otherwise, type 2 supernovae would never eject their pulsars because of perfect symmetry between the two polar scattering modes.....but they do eject their pulsars, so there must be a polar preference, so the two targets must accelerate at different rates....hence the trinity of equivalence (Peterson,G., Gravity Research Foundation writing competition...many moons ago)...been there, done that. pete

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A third rate theory forbids
A second rate theory explains after the fact
A first rate theory predicts...A. Lomonosov

So does that mean that if there's a black hole or a neutron star or something else with a large magnetic field embedded in a molecular cloud, the ortho and para H2 will separate out, like a gigantic gas centrifuge? And that if the field is strong enough and the cloud cold enough, the orthos might align themselves to the field? (Probably have to be an impossibly cold cloud and an impossibly strong field... hydrogen is a fickle creature.) Oooh... or maybe they'd all start dancing around like little gyroscopes.

And forgive me if this is obvious, but how does this relate to the equivalence principles? Doesn't it follow directly from the fact that a particle with spin has a magnetic dipole moment (and that the spins are coupled)? Or are the alignments of the various dipole moments in an object a consequence of something deeper?

(P.S. Spin blows my mind. In grade three I learned about how you have to rotate an electron through 720 degrees before it looks the same. Now, that was when I thought an electron was a little planet orbiting a gigantic proton sun, but fifteen years later I learned a bit about spinors, and it still amazed me just as much... if not more.)

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"It's turtles all the way down."

snarkophilus. The intrinsic definition of North and South is traceable to the pole of the Co-60 nucleus that emits beta particles when a collection of them are polarized. The nucleus "knows" North and South as preferential directions..(Lee, T.D.K, Yang, C. N.) All weak interactions show a polar preference. Neutrinos and anti-neutrinos, and their W⁺, W^-, and Z⁰ modes of interaction also "know". So when there is a distinct collection of N-S entities which are not randomly distributed as a spin glass... parity effects show. Just as with Lee, and Yang...nobody thought to check till they suggested it, whereupon it was found in every run....nobody has checked my suggestion. (Lee's lecture at MIT's Physics Colloquiem was the best I ever heard...got me thinking). pete.
P.S. I'll bet a hot fudge sundae.

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A third rate theory forbids
A second rate theory explains after the fact
A first rate theory predicts...A. Lomonosov

trinitree88, which of the follow might offer tests of this idea (even if only in principle)?

binary pulsars (esp double pulsars): they're already excellent laboratories for testing GR, much better than we have in our solar system ... and many have intense magnetic fields (far more intense than anything we've ever likely to create here) ... and the weak force plays an important part in the stability of the objects themselves.

inspirals: these may well be the events which generate the first GW detected, by the likes of LIGO ... surely the waveform will have footprints of this idea all over it?

GRBs: a 'visible' counterpart to inspirals ... perhaps the inherent asymmetries will show up in the details of the light curve? int the polarisation by time plots?

AGN jets: if there are asymmetries, then they'll show up as asymmetries in the jets, somewhere, won't they?

more generally, accretion disks around a wide range of SMBH, both in terms of mass and spin, (and probably magnetic field strength too) ... should bear the footprints of this idea, n'est pas?

Nereid. Good questions.Tricky business indeed. The simplicity of the polarized target test is that targets are routinely polarized dynamically under conditions of high field strength and low temperature, and then may be stored or manipulated under conditions closer to room temperature and under lower field strengths before relaxing. This should enable a freefall test, particularly in a pair of identical chambers in a deep water pool....perhaps a quarry, where external rigging and floats could serve well as lab platforms.
Your universal choices pose the problem of unknown relative abundances of polarized species, and what species those might be, and whether or not the abundances are time varying, or the fields are constant....way beyond what I'm proposing as an experimental test.
1.Some suspicions that the process is at work in the sun partially driving convection zones, and contributing to the 22 yr sunspot cycle, have occurred to me before, and I have suggested it in other posts that a higher Reynolds number in pulsars may see vastly different time scales there for them to switch polarity. If that were to happen, the kinematics of lone nearby pulsars should show periodic transverse velocity variations....beyond their rotational hiccups. So, I'd venture that coalescing binary pulsars with the same magnetic polarity ought to show a distinct signature from those of opposing pairs, but the electromagnetic signature may far outweigh the weak signature, and as a chemist I don't venture into domains PhD's tread lightly in.
2.,3. In your inspirals and AGN jets, clear unobstructed views of both jets simultaneously can only be said to offer observational targets of opportunity.A lunar base telescope may be required for the detail necessary, and temporal corrections for the 'simultaneous' symmetries seen or not.
4. I see little correlation with mass, spin, magnetic field of SMBH....presuming they are unequivocally identified as such.

I'll stick to a doable physical setup for an experiment. If it fails, I'm wrong. If not, we'll see if somebody dares to risk a hot fudge sundae...(coffee ice cream, two scoops, whipped cream, fudge sauce, chopped walnuts, and two cherries..my daughter always fights me for them)...ciao. pete.

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A third rate theory forbids
A second rate theory explains after the fact
A first rate theory predicts...A. Lomonosov

Russ T.,

Just like when you go further down in the water, the pressure(or weight of the water) above you is greater and greater causing even more pressure(or weight). Distance from an object is proportional to pressure (or gravity).