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Please Tell Me What You Mean About "spacetime"

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Posted by M.W.Pearson on December 24, 2003 20:50:46 UTC

Hi Wanda,
You wrote:
Spacetime is real.
You are confusing coordinates with spacetime.

That was the last time in your note that you mentioned the term "spacetime."

Wanda wrote: Einstein made it clear that space is real when his Theory of General Relativity predicted that the force of gravity can be replaced by geometric variations of flat space. That leaves open the question of whether time ia a real dimension or just a representation. Time certainly exists but it may not be a dimension."
I don't deny that space is real. I even admit that abstractions are real. The x,y axis
is one way to express our abstraction about spacetime. Now, what do you think is precisely the difference between a "real dimension" and "just a representation?" What substance does a "real dimension" have which is not also then double-defined as also being a specific form of matter or energy?

Wanda wrote:
LQG has further solidified the real existence of space by proving that it is quantized and independent of coordinate system. That is, the spin network that underlies space and time do not have apriori location in space. Location is derived from the network, as well as time."

Please trot out the clear definition of "spin network?" (Please?) To demonstrate one thing, such as space is independent of coordinate system and hence not measurable, one must have a measurement system which has failed and an explanation why it fails.

Space is only independent of coordinate system as long as we do not know how to plot it. We do indeed super-impose coordinate systems on space to map them. A football field is a crude example. If you are saying that universe-level space cannot be mapped, then at least I know what you're saying, though I don't believe it. I agree there is no coordinate system in physical space, as far as I have heard, except wjhen they have conditionally and temporarily (and by definition) been super-imposed by minds -- that is, "coordinate systems" are all abstractions. They are real, but the territory in space which they purport to map does not necessarily contain the numbers and hashmarks (what a surprise on that, eh? :) )

Wanda wrote:
Scientific American has a very readable account of LQG by Lee Smolin. A more advanced discussion can be found in the link I provided to on the Big Bang Forum.

I will await your clear exposition on the three questions I asked or an admission that spacetime is the x,y axis and that I do not seem confused after all.

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