nitsuj said:
Agreed, we are describing the same thing then (spacetime).
Ok, good.
nitsuj said:
Your initial retort implied light cone diagrams precede observations.
I didn't say anything about light cone diagrams; you did. I talked about "causal structure", which, again, is distinct from how we visualize or draw it.
nitsuj said:
First point as an executive summary regarding the diagrams
Please stop using the word "diagrams". Once again, I am *not* talking about diagrams or visualizations; I am talking about the geometry that the diagrams or visualizations refer to. To make an analogy, if I were to talk about the distance from New York to London, you are talking as if I mean a map that shows New York and London with a line between them.
nitsuj said:
Eventually is not a definable region.
Sure it is; it's just your future light cone at a given event on your worldline. When I say the Andromeda galaxy can emit signals that we will eventually receive, I mean that the worldlines of those signals intersects our future light cone somewhere.
nitsuj said:
Any object in the “elsewhere” (spacelike) region we cannot not observe
You are confusing objects and events, and also confusing objects themselves with the signals they emit. An "object" is not a single point in spacetime; it's a worldline or world tube. And to observe an object, we don't have to actually intersect its worldline or world tube; we just have to intersect the worldline of some signal emitted from the object's worldline or world tube.
nitsuj said:
…and it goes without saying as at the diagram time, i.e as the different regions have been defined by the diagram.
Once again, the diagram does not define the regions; the regions are there whether or not we draw a diagram of them, just as New York and London are there with a definite distance between them whether or not we draw a map showing them.
nitsuj said:
we have to split it up into “diagrams” to examine something like causal structure.
This is a little better because you are only saying we need diagrams to "examine" the causal structure; but it's still wrong, we can examine the causal structure of spacetime without drawing diagrams or visualizing anything. Diagrams are not the only tool in the physicist's toolbox. They are often very helpful tools, yes, but that doesn't mean they're necessary.
nitsuj said:
Yes objects receding at greater then c are the same as the elsewhere region
No, they're not. The Andromeda galaxy's world tube has a large portion that is in our "elsewhere" region, but the Andromeda galaxy is not receding from us faster than light; in fact it's moving towards us.
It is true that the objects receding greater than c are *in* our "elsewhere" region, but what you said implies that the two things are identical, which they're not. You state it better a bit further on:
nitsuj said:
The causal connection is broken forever, not just now. that said you may think this is me kitty footing around semantically; The horizon is another way of saying forever spacelike.
Yes, this is true for the cosmological horizon; we can't send light signals to a galaxy beyond our cosmological horizon, and that galaxy can't send any light signals to us either. (It's worth noting that it is *not* true for a black hole horizon; that type of horizon is one-way, objects inside can't send light signals out, but objects outside *can* send light signals in.)
nitsuj said:
The length MUST contract to maintain causal structure. It absolutely must contract, therefore it is absolutely length contracted.
So again, what causal properties of an object are different when it's length contracted, compared to when it's not? If you can't show any, then I'm still not buying this way of describing it. The word "absolutely" is really what I'm objecting to; as I said before, it gives entirely the wrong impression, that length contraction is frame-independent when it's not.
nitsuj said:
From a causal perspective means let the logic of causal structure “rule” or precede odd things like contraction/dilation.
But the way you put it, in what I quoted just above, makes it seem like the contraction/dilation precedes the causal structure; you say "the length MUST be contracted to maintain causal structure". If the causal structure precedes the contraction/dilation, then it would seem better to say something like "the causal structure of spacetime requires that objects appear length contracted/time dilated when they are moving relative to us".
nitsuj said:
If they “measure” spacetime between the objects within the Earth, my body or atoms then there is a spacetime interval there. As simple as that, I’m not sure how you interpret what I said to mean there is no spacetime between objects. That was kind of a strawman there.
I didn't interpret you to be saying there was no spacetime between objects; I interpreted you to be saying (or at least implying) that there was no spacetime *within* objects. You said "we measure the spacetime
only between things" (emphasis mine). Unless by "things" you mean "only elementary particles" (and even that is problematic, since we don't know for sure that all elementary particles are really point particles), that statement implies, to me, that you are ignoring the possibility of there being spacetime within objects. (Not that you were unaware of the possibility, but that you ignored it in making your statement. But for this discussion, I don't think it should have been ignored.)
nitsuj said:
Yes, and we assume gravity is spacetime distortions. We assume it’s the spacetime.
We assume it when we construct the theory, I suppose. But we don't assume it when we make measurements, such as the measurement of tidal gravity that I described. Once again, that measurement shows distortion of something--freely falling worldlines that start out at rest relative to each other don't stay that way. But the objects being measured show no distortion, so the only thing left that can be distorted is spacetime. That's not an assumption, it's a conclusion from the evidence.
nitsuj said:
We cannot measure nothingness (spacetime).
I agree you can't measure nothingness (at least, classically you can't; quantum mechanically is a whole other can of worms...). But spacetime is not nothingness; that's why we can measure it.
nitsuj said:
We can measure just the physical stuff that spacetime is in between.
We can *directly* measure only the objects, not spacetime itself, yes. But we can certainly measure spacetime indirectly, just like we measure lots of things indirectly. Nobody has ever directly measured a quark, but we can measure them indirectly and so we know they exist.
nitsuj said:
And we can use two measures to determine the physically fundamental “distance” (interval) between those objects. You say that’s measuring spacetime, I can say that’s measuring the relationship between the two objects…not the spacetime itself.
If spacetime geometry were fixed, as in special relativity, I would buy this point of view. But spacetime geometry is not fixed; it's affected by the presence of matter and energy. Once again, consider the measurement of tidal gravity that I described. The objects themselves are not affected (yes, in principle they could be if the tidal gravity were strong enough or the objects were large enough, but we're talking about a case where the objects are small enough that we can't measure any change in them during our experiment). But their rate of separation is not always the same; it depends on what masses are present. So something must be changing, and we call that something "spacetime".
nitsuj said:
for all I know GR says explicitly (mathematically) that spacetime is something physical.
AFAIK yes, that is the mainstream view in GR. (Bear in mind, also, that GR is not a quantum theory, so our current belief is that it is not fundamental; it is a low-energy classical limit of a quantum theory of gravity that we don't have yet. Quantum mechanically, the view that spacetime is something physical is even harder to avoid, because you can have quantum fluctuations in spacetime even when there is no matter or energy present, and those fluctuations can have physical consequences, just as with other quantum fields.)
nitsuj said:
That’s measuring two rocks, you say that’s measuring the nothingness between them.
It's directly measuring the two rocks, and indirectly measuring the spacetime geometry. See above.
nitsuj said:
What value does the additional component of Spacetime is something physical add to this scenario?
Good question! This is really the crux of the issue. There are probably a number of possible answers, but here's mine:
- It allows us to eliminate direct action at a distance; we don't have to view the Earth as directly affecting the rocks without anything physical between them. The Earth affects spacetime, and spacetime affects the rocks; spacetime is basically a field, just like other fields, and GR is a field theory of gravity, with the same advantages as other field theories compared to "action at a distance" theories like Newtonian gravity.
This probably serves as a comment on much of the rest of your post.
nitsuj said:
to your point about the Dynamic EFE SET; Is there some property of spacetime itself that must be “input” into the dynamic equation that can calculate gravity?
Yes: you have to know how much spacetime curvature is produced by a given quantity of stress-energy. In other words, when we write the EFE in conventional units:
$$
G_{\mu \nu} = \frac{8 \pi G}{c^4} T_{\mu \nu}
$$
that ##8 \pi G /c^4## on the RHS is a property of spacetime.
nitsuj said:
Hopefully PeterDonis my replies don’t make you nauseous or exhausted, or that I miss your points because of semantics.
Thanks for playing
You're welcome! I think this is a good discussion, and the questions you're asking are worth asking.
To clarify I mean that spacetime really doesn't seem to have any effect on matter. Perhaps more diplomatically, spacetime seems physically elusive
) the wrong impression. Length contraction is frame-dependent, not "absolute".
. Downside to living within the "speed is allowed" zone is it gives goofy measures of spacetime if there is comparative motion, length & time give different results, the interval is the same. Why? Because spacetime is physically even more simple than both length & time, 
