Re: the dark matter analogy. I think the analogy works well, but would just like to point out that even in theories where dark matter doesn’t interact even with the weak force, and there is some other force that it does interact with that’s analogous to electromagnetism, so it could bind together to form an earth-like planet, it still interacts with gravity, and if this earth-sized dark matter planet really did overlap with ours, we’d feel it’s gravity and the earth would seem to be twice as massive as it is. Or, to state it slightly differently, the actual earth would be half as massive as we measure it to be. But that would be inconsistent with what we know of its composition and density. We know the mass of rocks, and the measurement of the mass of a rock of a particular size wouldn’t be subject to this error, so we can rule out there being a dark matter Earth coincident with ours.
This isn’t in any way a criticism of what I found to be a brilliant piece. And I’m not even sure that it’s reason enough not to use that particular analogy, which otherwise works great.
Re: the dark matter analogy. I think the analogy works well, but would just like to point out that even in theories where dark matter doesn’t interact even with the weak force, and there is some other force that it does interact with that’s analogous to electromagnetism, so it could bind together to form an earth-like planet, it still interacts with gravity, and if this earth-sized dark matter planet really did overlap with ours, we’d feel it’s gravity and the earth would seem to be twice as massive as it is. Or, to state it slightly differently, the actual earth would be half as massive as we measure it to be. But that would be inconsistent with what we know of its composition and density. We know the mass of rocks, and the measurement of the mass of a rock of a particular size wouldn’t be subject to this error, so we can rule out there being a dark matter Earth coincident with ours.
This isn’t in any way a criticism of what I found to be a brilliant piece. And I’m not even sure that it’s reason enough not to use that particular analogy, which otherwise works great.