Also, remember that gravity waves move at the speed of light; you can't detect a photon's gravity from a distance without waiting for the same light speed delay that you would get from the photon itself.
I've wondered about that: The moving particle, photon, electron, neutron, is throwing off a gravity wave as it is moving if we don't have a gravity wave detector detecting that wave. So, just having the detector, maybe a second away, collapses the wave function?
My guess is that as a wave function is split into multiple parts going off in different directions and getting far apart, the particle is not yet "localized" in any of the parts. The idea that there is a particle, "localized" but we just don't yet know where it is I have a tough time accepting. Instead, until there is an interaction that transfers the energy, although maybe not the gravitational energy, there is no localization. Or, all the parts of the wave function both feel and generate gravity until the collapse from an interaction with, say, a detector.
Or, if only one part of the wave function has the particle, then when two parts of the wave function are combined, as in just Young's double slit, we should not get the interference we do get. Or, it seems we get the interference of the wave function parts, then get the detection; there never was anything localized until the detection at which time all the parts of the wave function have to collapse everywhere, instantaneously, even across 1 billion light years -- no I don't like to believe that, but I'm just looking at the interference of two parts of the wave function in Young's double slit: If the particle was really in just one of the two parts of the wave function, then tough to believe in the interference we do see.
This gets to to crux of the issue the author (and others in contemporary physics, Hawking included for that matter) was raising. We can conjecture, via theoretical physics, a whole slew of possible models of the universe that combine general relativity and quantum mechanics. But, there is a lack of experimental evidence (or interpretation of existing results) to resolve the ongoing "debates". And there hasn't been any real progress, in that respect, in the past 80 years.
We need to test the edge cases of both theories as best we know how. For example, tunnel to the center of the earth (or another planet?) and verify our existing models in the gravity well, maybe repeat the double-slit experiment remotely while we're there. I'd say a task much more difficult, engineering-wise, then flying a human being to another planet and back.
Not necessarily. Many kinds of motion do not cause gravitational wave emission. And even the kinds that do don't cause enough to detect with our best current technologies, by many, many orders of magnitude. You basically need huge masses in a situation where they are in very tight orbits, like merging black holes or neutron stars.
But in the Michelson interferometer, the Fabry-Perot, etc., the parts of the wave functions pass close to lots of mass, get reflected by mirrors, etc. without the wave function collapsing. So, a part of a wave function should be able to pass near a gravitational wave detector and be detected without causing the wave function to collapse?