On 7/12/2020 11:50 PM, Alan Grayson wrote:
There can be because it's consistent with the equations. A black hole doesn't include any matter. General relativity is non-linear, that's why there can be non-flat cosmologies that contain no matter. Of course there may be some different, better theory in which spacetime can't be curved without matter...but it seems unlikely since we have good evidence that gravitational waves exist. BrentYes, good evidence that gravitational waves exist, but as far I know they're always associated with material interactions such as collisions of black holes. In the case of EM waves, I'd be more receptive of your claim that they can exist independent of charges and/or currents, but as far as I know there's no evidence of that. AG
But a collision of black holes does NOT involve matter. Black holes (as far as the theory goes) are purely geometric things, i.e. made of empty space.
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