Infinite gravity

[Strange]

Just now, zapatos said:

Airbrush asked "Would all matter in the universe already be in "gravitational contact" with all other matter in the universe, ever since the beginning?"

You responded "yes".

Ah, sorry. My fault. I read (or interpreted) that as "all the matter in the observable universe". And responded on that basis.

Matter outside the observable universe was not in "gravitational" (causal) contact with matter near us. (It starts getting confusing, because matter some distance away from us has its own observable universe sphere around it, which overlaps with our observable universe but includes matter that is outside our observable universe...)

[zapatos]

1 minute ago, Strange said:

Ah, sorry. My fault. I read (or interpreted) that as "all the matter in the observable universe". And responded on that basis.

Matter outside the observable universe was not in "gravitational" (causal) contact with matter near us. (It starts getting confusing, because matter some distance away from us has its own observable universe sphere around it, which overlaps with our observable universe but includes matter that is outside our observable universe...)

Okay, thanks. Is it safe to say that objects that were once in the observable universe but are now outside the observable universe due to expansion, would start out having gravitational impact on us but lose that gravitational contact sometime after they leave the observable sphere? And if so, would we lose gravitational contact at the same time we lose visible contact? 

[MigL]

There is a difference between the 'fossil' field and 'changes' to the gravitational field.
Changes propagate at the speed of information, c , while the fossil field is pre-existing, and no new information needs to be transferred.
IOW changes have to obey causality, and have a limited range of observability that is time variant, because of the limited speed of light, while the pre-existing fossil field is simply the existing space-time curvature, and if our measurements could be made accurate enough, we could know the 'overall' curvature of the universe at large ( not just the observable part ).
Our current best measurements indicate the Universe is essentially flat ( to a very high degree ), indicating that it is extremely large compared to the observable part we see, such that curvature is trivial ( analogous to the Earth appearing flat at short ranges ), or, it was extremely 'fine-tuned' at the beginning, such that even after 13+ billion years of expansion, it is still essentially flat.

[zapatos]

That helps a lot. Thank you.

[MigL]

As to the question of range of gravity...

Lets consider again QFT, the 'marriage' of Quantum concepts with Special relativity.
All forces are thought to be mediated by virtual bosons, such as photons, W and Z particles, gluons, and gravitons.

Photons and gravitons are massless, and must travel at c. They can also be extremely low energy/long wavelength such that when we apply the Heisenberg Uncertainty Principle to them, as their energy approaches zero ( wavelength approaches infinity ), the HUP says they can exist for a time approaching infinite.
IOW they can approach infinite distance when their strength approaches zero because there is no time constraint; and this is evident in the inverse square law which the obey.

Now consider the weak force, mediated by massive virtual W and Z particles. The fact that they have to have a minimum amount of mass-energy to exist means that their time is limited by the HUP. Since their mass-energy cannot approach zero, they can't have an amount of time approaching infinite to travel to infinite ranges. The distance they can reach is limited by the HUP and SR ( as they are subluminal ).

The same analysis can be done for the strong force when it is modelled as a Yukawa potential with massive Pion exchange. But I'm incapable ( without doing some research ) of doing this analysis with massless gluons mediating the color interaction and residual strong force between protons and neutrons.

[beecee]

On 5/2/2019 at 9:36 PM, Strange said:

We have a scientific theory in which gravity has unlimited range.

If you want to come up with an alternative mathematical model where that is not the case, but which is still consistent with all other evidence, then go ahead. 

You will also need to describe how your limited range of gravity can be tested by experiment or observation. 

Also I would presume that the property of nonlinearity that applies to gravity, would also suggest it has a potentially infinite range...Is that a valid assumption?

[michel123456]

12 hours ago, MigL said:

Photons and gravitons are massless, and must travel at c. They can also be extremely low energy/long wavelength such that when we apply the Heisenberg Uncertainty Principle to them, as their energy approaches zero ( wavelength approaches infinity ), the HUP says they can exist for a time approaching infinite.

Interesting, never thought of that. Doesn't that mean that if you want to represent a photon in a spacetime diagram it will be represented as a line (and not as a point) parallel to the time axis?

Edited May 8, 2019 by michel123456

[Strange]

10 hours ago, beecee said:

Also I would presume that the property of nonlinearity that applies to gravity, would also suggest it has a potentially infinite range...Is that a valid assumption?

I don't think so. Newtonian gravity as infinite range but does not have those sort of non-linear effects.

43 minutes ago, michel123456 said:

Interesting, never thought of that. Doesn't that mean that if you want to represent a photon in a spacetime diagram it will be represented as a line (and not as a point) parallel to the time axis?

Parallel to the time axis would imply no motion through space. They will be a line at 45° (a stationary object would be a line parallel to the time axis and any moving object would be between those two extremes).

[michel123456]

23 hours ago, Strange said:

Parallel to the time axis would imply no motion through space. They will be a line at 45° (a stationary object would be a line parallel to the time axis and any moving object would be between those two extremes). 

Sorry, yes at 45°. But I mean: the photon IS literally the line. It is not a point-like object traveling along the line. It IS the line, a two-dimensional object with 1D in time & 1D in space.

Edited May 9, 2019 by michel123456

[Strange]

10 minutes ago, michel123456 said:

Sorry, yes at 45°. But I mean: the photon IS literally the line. It is not a point-like object traveling along the line. It IS the line, a two-dimensional object with 1D in time & 1D in space.

Yes. Well, a line is a 1D thing (in 4D space) not 2D.

Every object object is represented as a line (and in the case of point-like objects it will be a 1D line - no width.).