Given the brief text, I'm not clear on how t' Hooft's idea is different than orthodox GR:
> Prof. Gerardus 't Hooft had brought up something interesting in his 2008 paper "A locally finite model for gravity" that "... absence of matter now no longer guarantees local flatness..." meaning that accelerations can be present in spacetime without the presence of mass. Wow! Isn't this a precursor to propulsion physics, or the ability to modify spacetime without the use of mass?
I don't know about the speculations, but the basic question of curvature in the absence of mass is totally orthodox general relativity.
It's a very widespread folk myth, even with physicists, that absence of matter means asymptotically flat spacetime, but it's not absolutely true.
I had thought John Baez discussed this on sci.physics ages ago, but after I couldn't find that, this is the first formal reference I dug up to support that:
"Relativity And Geometry"
by Roberto Torretti, 1983
Page 199: Chapter 6, Gravitational Geometry
> First several new non-flat matter-free solutions of the field equations have been discovered, which show that an empty universe built in accordance with General Relativity can be a very lively place, the interplay of pure gravitational energy being enough to produce some remarkable phenomena if a handful of test particles is supplied.[31]
[footnote [31] on page 334 discusses "Friedmann's equation", with a limit tending to 0 in the case of interest]
Maybe the point is that the orthodox stuff is usually concerning an empty universe solution? I had thought there were non-empty universe solutions that behaved this way as well, but I might be misremembering.
Massless GR is sometimes called "pure gravity", and is something people are still interested in (or were, last time I was paying attention) [1]. Kind of remarkable that even the base-case has so many open issues. I should re-read that Torretti book, I remember liking his style but that some of the math was over my head at the time.
> Prof. Gerardus 't Hooft had brought up something interesting in his 2008 paper "A locally finite model for gravity" that "... absence of matter now no longer guarantees local flatness..." meaning that accelerations can be present in spacetime without the presence of mass. Wow! Isn't this a precursor to propulsion physics, or the ability to modify spacetime without the use of mass?
I don't know about the speculations, but the basic question of curvature in the absence of mass is totally orthodox general relativity.
It's a very widespread folk myth, even with physicists, that absence of matter means asymptotically flat spacetime, but it's not absolutely true.
I had thought John Baez discussed this on sci.physics ages ago, but after I couldn't find that, this is the first formal reference I dug up to support that:
"Relativity And Geometry" by Roberto Torretti, 1983
Page 199: Chapter 6, Gravitational Geometry
> First several new non-flat matter-free solutions of the field equations have been discovered, which show that an empty universe built in accordance with General Relativity can be a very lively place, the interplay of pure gravitational energy being enough to produce some remarkable phenomena if a handful of test particles is supplied.[31]
[footnote [31] on page 334 discusses "Friedmann's equation", with a limit tending to 0 in the case of interest]
Maybe the point is that the orthodox stuff is usually concerning an empty universe solution? I had thought there were non-empty universe solutions that behaved this way as well, but I might be misremembering.