out on a limb without a wave to hop on

I probably slid through 'local realism' a bit too quickly.

It's a general approach to physical descriptions. A sort of philosophy about how the universe works. It means that for some chunk of the universe, what happens next to that chunk is purely dependent upon that chunk's properties and immediate surroundings. The various fields that we talk of : electromagnetic, gravitational, nuclear etc are part of that immediate surroundings.

However these fields are extended entities that span across - in theory at least - the entire universe. They are hence a component to be reckoned with for all surroundings of all chunks. The field is the connection apparatus that links what we like to think of as a cause 'over here' having an effect 'over there'.

So for the apple falling toward the Earth ( when not otherwise held in position ) we reason thus :

- the Earth has produced a contribution to the gravitational field.

- the apple is sitting in the gravitational field.

- the subsequent mechanics of the apple ( moment to moment ) depend upon it's response to the gravitational field at it's momentary position.

.... this is logically symmetric for the apple influencing the Earth's motion, so I could correctly say :

- the apple has produced a contribution to the gravitational field.

- the Earth is sitting in the gravitational field.

- the subsequent mechanics of the Earth ( moment to moment ) depend upon it's response to the gravitational field at it's momentary position.

But the magnitudes are far different and this comes back to the mass of the objects. The apple is tiny, the Earth is huge ( relatively ). So the Earth is making a large field contribution for which the smaller apple will display a large response. Obversely the apple is making a tiny field contribution for which the large Earth will display a small response. Thus we say, as a language construct :

'the apple falls to the Earth'

and not :

'the Earth falls to the apple'

... even though theory says that both occur. This becomes more evident in generality ie. systems containing bodies of comparable mass. The solar system is our best studied example of this. Interestingly it is often said that 'the Sun is the centre of the solar system'. It is better to say that the Sun is the dominant mass in the solar system, and that it may be simplest to model/consider that system with Sun-centric co-ordinate systems. Basically that is what Copernicus said/did. Note that Copernicus had even more epicycles than Ptolemy - I mention that as it is often erroneously claimed that Copernicus eliminated them. He didn't. He shifted the 'origin' of the co-ordinates.

[addendum]
Sharp punters will spot a difficulty here. What is the contribution of the apple to itself ? Meaning that if it produces a field component then how does it react to that ? This is generally known as 'self-energy' and is also a conundrum with other physical fields as well. Ways to manage it :

- ignore it, thank you very much. Just phrase your thinking and the maths especially to not include the apple-influencing-itself component.

- sweep it under the rug. This is the QED contribution of Feynman/Schwinger/Tomonaga called 'renormalisation'. The trick* here is to just assign some total field effect at the surface of some ( vanishingly ) small volume around the point of interest, make that value c/w measurement, and don't look inside that surface.

- worry about it more than that. Come up with a better theory. Strings ? :-) :-)
[/addendum]

Cheers, Mike.

* To be a good boy I must admit : I am giving an equivalent explanation here. The full horror is to offset one high-order effect with another, which in turn implies I am limiting the energies to be nett-summed, and that in turn means not getting in too close ....

Yes. Two actually. The weak and the strong.

The weak has been combined with electromagnetism as the 'electroweak' force. It is involved in nuclear decays for instance. The basic event ( at everyday energies ) is the decay of a down quark into an up quark with the release of an electron and a neutrino type. This is why a neutron ( 2 downs + 1 up ) all on it's own will decay to a proton ( 1 down + 2 ups ) in about a quarter of an hour, on average. Inside nuclei there are various similar reactions. It is called 'weak' due to it's intrinsic strength being above gravitation but below electromagnetism. It is very short range of the order of nuclear dimensions.

The strong is the strongest of all. It's range is of the order of a proton/neutron width. It has this weird behaviour of getting weaker if real close in and getting stronger further out. To separate a group of quarks to individual ones would require infinite energy. So that doesn't happen. What does happen if separation is attempted is the creation of more quarks. So they've only been seen in relatively stable groups of two or three, sometimes as very short lived 'jets' of very many. A legitimate question is 'how do we know what quarks are if we never see them alone ?' The answer is moderately complex. If one fires, say electrons, at protons then there will be close encounters b/w the electrons and the quarks and the results can be studied. Over very many such interactions one can deduce the quark parameters that would consistently give the observed outcomes.

[ One especially interesting result here is that a down quark has mass/energy of ~ 5 Mev and an up has ~ 2 Mev. So a neutron's mass/energy from such 'valence' quarks ought be ~ 12 Mev. This is way lower than the ~ 940 Mev as measured 'on the outside' as it were. The remaining mass/energy is that of the strong interaction, or put another way, the energy of the gluons .... ]

Electromagnetism is also long range.

Excellent! :-)

I've delayed answering this because I've had to think 'well, what is the problem here?'. Exactly why* does action-at-a-distance ( call it AAAD ) fail ? I'm going to have a longer think about that before replying. I want to identify and isolate the key components better ...

It's fine. You ask good questions ... :-)

Cheers, Mike.

( edit ) So the forces are ranked ( strongest to weakest ) :

strong nuclear > electromagnetism > weak nuclear > gravity

... with the method of ranking being a wee bit obtuse in everyday terms, but is defined in terms of constants that determine the degree of 'coupling' between the matter particles and the relevant force carrying particles. NB also that matter particles are under the heading of 'fermions' with the force mediators being 'bosons'. Essentially fermions cannot be stacked together in the precisely the same place : if there are two or more, one of more of them has to move on. Which is good as the matter of the entire universe would wind up all in the one spot otherwise. Whereas bosons can be grouped together, or not, as you please.

* Such exercise is good for the psyche and soul. I've been reading a recent book by Jim Baggott called Farewell to Reality: How Modern Physics Has Betrayed the Search for Scientific Truth ( it seem you can only publish a book - perhaps in the USA at least - only if it has several titles ). While naturally bagging string theory as prediction failure par excellence, it also has a crack at a number of other theory topics too. It's not so much that theory awaits proof/disproof on some experimentalist's workbench, but that some theoreticians actually believe physical demonstration may be foregone and the moniker of 'science' still applied. It seems the string theory rot is spreading. Recurrent readers of my posts will note that I do keep going on about this. This is probably my medical training & background. I literally cannot imagine practising medicine in a likewise reality-be-damned manner ....

Now that I come to think of it : I started the medical course in January 1980 .... oooh, I see your point. :-0

Cheers, Mike.

( edit ) ... and this from a guy who used to jump out of planes for a living ? Like that's easy, right? There is no way that you'd miss the planet. Your work's already done for you from the get go ... a lazy use of gravity if you ask me ! :-):-)