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by tsimionescu
1696 days ago
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A field is, by definition, a physical quantity in space and time. The key idea of GR is that gravity is the curvature of space time. The electromagnetic field is not bent, light for example always travels in perfect "straight lines" in the curbed space time created by mass/energy (more specifically, light always follows the shortest possible length of space-time between two points, which, in un-curved space-time is a straight line, but is a curved line if space-time is curved). Do note that current quantum field theories do not work in curved space-time, so this may turn out to be wrong in certain crucial ways. |
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In general curved spacetimes. But that includes a lot of obvious unphysicality.
Modelling our universe, QFT in CS (the subject of textbooks, after all, like Birrell and Davies) works just fine away from strong curvature, all of which as far as we can tell is shrouded behind an event horizon or not-practically-observable in the very early universe.
You don't have to take my word for it. See https://en.wikipedia.org/wiki/Robert_Wald 's first three slides (after the title slide) at http://gravity.psu.edu/events/abhayfest/talks/Wald.pdf )
tl;dr: it is a fine effective theory, but not a good candidate for a fundamental theory.
(Also in your first paragraph you are implicitly carving up spacetime in to space + time, and not taking that into account in what you write about "straight lines". However, you've got one part right namely (paraphrasing the start, up to the second comma, of your parenthetical) the spacetime interval of a null geodesic).