yuiop said:
I have read that MWI predicts that gravity is quantised. Is this prediction unique to MWI? If for example, someone discovered a quantum theory of gravity that was correct, that MWI would be declared the only valid interpretation of QM and that interpretations such the Copenhagen interpretation would be ruled out?
Dunno yuiop about QG, but the following bit under 'Common objections and misconceptions' at
http://en.wikipedia.org/wiki/Many-worlds_interpretation may be helpful:
" * We cannot be sure that the universe is a quantum multiverse until we have a theory of everything and, in particular, a successful theory of quantum gravity.[60] If the final theory of everything is non-linear with respect to wavefunctions then many-worlds would be invalid.[1][3][4][5][6]
MWI response: All accepted quantum theories of fundamental physics are linear with respect to the wavefunction. Whilst quantum gravity or string theory may be non-linear in this respect there is no evidence to indicate this at the moment.[13][14]"
So if anything a final QG theory looks likely to kill MWI. Jumping in where angels fear to tread, my main objection to MWI is the reliance on absolute linearity of QM. Hard to believe anything in nature is truly absolute, but without that, the exponential explosion in number of 'worlds' MWI predicts can last forever? How would things not finish up sooner rather than later in some kind of 'gridlock' - is it really possible to pile an infinite number of quantum states on top of each other? Or is there some other way out here?
EDIT: This may be the source you were thinking of:
http://www.hedweb.com/manworld.htm#exact
"Q38 Why quantum gravity?
Many-worlds makes a very definite prediction - gravity must be quantised, rather than exist as the purely classical background field of general relativity. Indeed, no one has conclusively directly detected (classical) gravity waves (as of 1994), although their existence has been indirectly observed in the slowing of the rotation of pulsars and binary systems. Some claims have been made for the detection of gravity waves from supernova explosions in our galaxy, but these are not generally accepted. Neither has anyone has directly observed gravitons, which are predicted by quantum gravity, presumably because of the weakness of the gravitational interaction. Their existence has been, and is, the subject of much speculation. Should, in the absence of any empirical evidence, gravity be quantised at all? Why not treat gravity as a classical force, so that quantum physics in the vicinity of a mass becomes quantum physics on a curved Riemannian background? According to many-worlds there is empirical evidence for quantum gravity.
To see why many-worlds predicts that gravity must be quantised, let's suppose that gravity is not quantised, but remains a classical force. If all the other worlds that many-worlds predicts exist then their gravitational presence should be detectable -- we would all share the same background gravitational metric with our co-existing quantum worlds. Some of these effects might be undetectable. For instance if all the parallel Earths shared the same gravitational field small perturbations in one Earth's orbit from the averaged background orbit across all the Everett-worlds would damp down, eventually, and remain undetectable.
However theories of galactic evolution would need considerable revisiting if many-worlds was true and gravity was not quantised, since, according to the latest cosmological models, the original density fluctuations derive from quantum fluctuations in the early universe, during the inflationary era. These quantum fluctuations lead to the formation of clusters and super-clusters of galaxies, along with variations in the cosmic microwave background (detected by Smoots et al) which vary in location from Everett-cosmos to cosmos. Such fluctuations could not grow to match the observed pattern if all the density perturbations across all the parallel Everett-cosmoses were gravitationally interacting. Stars would bind not only to the observed galaxies, but also to the host of unobserved galaxies.
A theory of classical gravity also breaks down at the scale of objects that are not bound together gravitationally. Henry Cavendish, in 1798, measured the torque produced by the gravitational force on two separated lead spheres suspended from a torsion fibre in his laboratory to determine the value of Newton's gravitational constant. Cavendish varied the positions of other, more massive lead spheres and noted how the torsion in the suspending fibre varied. Had the suspended lead spheres been gravitationally influenced by their neighbours, placed in different positions by parallel Henry Cavendishs in the parallel Everett-worlds, then the torsion would have been the averaged sum of all these contributions, which was not observed. In retrospect Cavendish established that the Everett-worlds are not detectable gravitationally. More recent experiments where the location of attracting masses were varied by a quantum random (radioactive) source have confirmed these findings. [W]
A shared gravitational field would also screw up geo-gravimetric surveys, which have successfully detected the presence of mountains, ores and other density fluctuations at the Earth's surface. Such surveys are not sensitive to the presence of the parallel Everett-Earths with different geological structures. Ergo the other worlds are not detectable gravitationally. That gravity must be quantised emerges as a unique prediction of many-worlds."