Gravity only illusion due to expansion of universe?

[Dale]

Expansion of the universe may be isotropic (specifically in all directions), but it is not smooth. ... If concentrations of mass-energy did resist expansion, then gravity would then "point" along a line of increasing concentration, ie towards the (other) mass.

Sounds like a traditional "push gravity".

 

[Dale]

It's the appearance of the A(t) term that looks questionable, as if space has a preferred coordinate system in which it expands isotropically. I suppose it depends upon what motivates its inclusion. Is it ad hoc, to explain the expansion of the Universe; does it require a cosmological constant added to the Einstein tensor?

This kind of arbitrary function is fairly common and allows you to easily define a family of coordinate systems simply by choosing different functions. http://arxiv.org/abs/gr-qc/0311038" of the same thing for the Schwarzschild spacetime. In this paper the free parameter is a function of the radius instead of time, but it amounts to the same thing.

 

[neopolitan]

Sounds like a traditional "push gravity".

What I described was not at all like Le Sage's push gravity, which seems to introduce more forces. I looked only very briefly at a site dedicated to Wright's push gravity - it was enough to glimpse a large number of the words spelt in capitals to realize that I didn't want to read any of it. I did notice that it was an aether theory, which was enough in itself to dissuade me from reading further.

If there are reputable push gravity theories, I may be interested, but it seems they replace gravity with a repulsive force which is not what the OP here seemed to be considering and certainly not what I was hypothesizing about - I was thinking of whether the force could be an illusion in entirety, not replaced by another, rather more counter intuitive force.

Try thinking of this: one consequence of relativity, the one that really messed with Einstein's head is that it implies expansion, he worked tirelessly to try to get around that and later described that effort as his greatest blunder (and that is ok, science benefits enormously from blunders). Is it so unreasonable to ponder what would have happened if Hubble's work had come earlier than Michelson and Morley, and we knew that the universe was expanding but not that there was something odd about some of our late 19th century assumptions which included aether? Could we get from this universal expansion back to relativity and the understanding that aether is unnecessary, as opposed to from relativity to expansion? I think you should be able to.

For me the first step is to realize that the universe is expanding, but not all of it ... why is that?

It may be difficult to try the exercise properly, since there are huge temptations to take the shortcuts we know are there, but it might be worth the effort if we get the whole picture - well, maybe more of the picture or a different perspective on the same piece of the picture we already have

cheers,

neopolitan

 

[Dale]

What I described was not at all like Le Sage's push gravity, which seems to introduce more forces. ... I was thinking of whether the force could be an illusion in entirety, not replaced by another, rather more counter intuitive force.

Now it sounds like GR.

For me the first step is to realize that the universe is expanding, but not all of it ... why is that?

GR already describes that well.

I just fail to see (1) how this idea relates to established theories (2) what the motivation for this idea is.

 

[neopolitan]

For me the first step is to realize that the universe is expanding, but not all of it ... why is that?

I just fail to see (1) how this idea relates to established theories (2) what the motivation for this idea is.

In answer to (2)- the OP asked.

In answer to (1)- way ahead of you. I was pondering how I could answer this question last night. What you re-posted here was not originally posted freestanding. It followed a paragraph in which I asked:

Is it so unreasonable to ponder what would have happened if Hubble's work had come earlier than Michelson and Morley, and we knew that the universe was expanding but not that there was something odd about some of our late 19th century assumptions which included aether?

Then in a following paragraph I said:

it might be worth the effort if we get the whole picture - well, maybe more of the picture or a different perspective on the same piece of the picture we already have

Imagine you are doing some complex maths (like we did in the old days, by hand). Once you're finished you have a result. But is it right? How do you check? One way is to take the end result and work backwards. As a very simple example you have:

25+6=31 ... checking 31-6= 25 ... I seem to have it right

While my background is engineering, I have been forced to do some accounting from time to time and you quickly learn to balance your books, especially if you are using double entry bookkeeping. I see double entry bookkeeping as similar to what you and JesseM and Fredrik tend to do as a whole. You provide the geometric method for arriving at a result, and Jesse and Fredrik provide a simultaneity based approach (or whatever). The more different, valid ways you have at arriving at the same result, the more confident you are going to be that the end result is right.

As an aside, what you probably won't accept in double entry bookkeeping is the introduction of imaginary money, even if you remove it in a later ledger entry.

So, in answer to your question: how does this relate to established theories? It relates by giving us confidence that we have the whole picture, if you can start from different positions and arrive at the same result (effectively relativity), then you have more confidence in the end result, and possibly better understanding of how it can be interpreted.

The different starting positions that I know of are:

the two postulates,
the Minkoswki metric (here I mean the four-space geometric approach),
gallilean boost plus speed limited information (the gallilean boost assumes instantaneous transmission of information), and
universal expansion

Each of these allows you to arrive at the equations of relativity (at the very least SR), the last one also does allow you to consider gravity to be an illusion (which you indicate may be GR-ish).

There may well be other starting positions.

cheers,

neopolitan