Yes, I read that before I posted my question. I understand that it's due to the expansion of space. Still, those galaxies are moving (so to speak) faster than lightspeed with respect to us. Shouldn't their mass be rather ... large?
I understand that we're moving at that speed relative to them, and I assume we don't notice such a crushing mass because, locally, we're in free fall, so we wouldn't notice it. Nor would they, locally. But we should notice it as to them, and vice versa. Or so it seems to my limited understanding.
No; the galaxies are just sitting there minding their business; it's the expansion of the intervening space (between the distant galaxies and us) that creates the sense of movement.
Think of it this way; if no force acts on the mass in the galaxies, it's velocity hasn't changed, and thus there are no associated Special Relativistic effects, e.g. the effective mass of the galaxy doesn't get larger.
The relativistic effects that we DO see (redshift) are, as the article points out, due to General, not Special, Relativity, and are a consequence of the spatial expansion.
Yes, I read that before I posted my question. I understand that it's due to the expansion of space. Still, those galaxies are moving (so to speak) faster than lightspeed with respect to us. Shouldn't their mass be rather ... large?
I understand that we're moving at that speed relative to them, and I assume we don't notice such a crushing mass because, locally, we're in free fall, so we wouldn't notice it. Nor would they, locally. But we should notice it as to them, and vice versa. Or so it seems to my limited understanding.
The way I like to view it is this: Special Relativity has to do with observations from one frame of reference to another. That is the key: one observer in one (inertial) frame of reference observes something in another frame of reference, that is moving. So, when you consider the Lorentz expansion of the mass, it means that one observer, looking at an object in another frame of reference that is moving fast (relative to the first observer), notices an increase in mass. HOwever, the mass is unchanged for the second observer who is "moving" in the second frame of reference. Indeed, to him, it is the first observer that is moving fast (in the opposite direction) and it is the first observer who has the large mass.
The mass doesn't change, it is just the observer who sees it differently.
This last point is essential. Otherwise, I could "increase" the mass of the earth just by getting into a fast rocket ship and zipping by it.f
The way that I understand Special Relativity vs. General Relativity is somewhat different than the article. The article talks about "space" as if it were some kind of, well, ether. And the ether theory was discredited with the Michelson-Morley experiment.
The way that I understand Special vs. General Relativity is that Special Relativity is essentially a local phenomenon. That is, Special Relativity was derived, and the experimental basis for it was done in a local space. Indeed, the classic examples of Special Relativity all involve two frames of reference and two observers who observe one another as they zip by each other. Therefore, at the time of observation, they are close. Special Relativity simply wasn't derived for long distance scales.
This observation is crucial. If the General Relativity postulate is correct, then space is curved. However, for short distances, space appears Euclidean. The effect is identical to the observation that the Earth seems flat to us, because locally, the curvature is very small. Special Relativity was derived in a flat space. Locally, space is flat enough that the correction terms to the Special Relativity formulae is negligible. However, for large distances, the effect is that the Special Relativity formulae are no longer correct.