DaveC426913 said:
Explain this one again?
The Earth revolving around the sun causes parallax of the nearby star against the backdrop of the galaxy, meaning we can measure its distance.
Statements like that are frame-dependent. You have chosen the frame of the Sun, in which the Earth moves. But in the frame of the Earth, the Earth does not move. This is a perfectly inertial frame in general relativity, because the Earth is a free particle with no forces on it following an inertial path, and the spacetime around it is curved by the same history that brought us the Sun (sticking to the equations of GR rather than sociological causal judgements about what "causes" that curvature). This spacetime history will also dictate that light from another inertial frame, that of the white dwarf, will appear to "wiggle". This wiggle is a property of the fact that the light is emitted by the white dwarf and absorbed at Earth, it doesn't have any meaning in any other context that isn't purely sociological. Since the Earth is a key player in that relationship, whatever may be said to be Earth's motion is also causative of the apparent wiggling of the white dwarf. If we strip away all the sociology and just stick to the equations of the physics, then the wiggle is a property of the frames involved and the spacetime that connects them, and so is a wiggle of neither the Earth nor the white dwarf, the only thing wiggling is the angle of the light relative to a fixed standard of reference on the Earth, as described by the equations of general relativity.
How exactly is that a gravitational effect? Even 'in a sense'?
The only label we have for the things that general relativity does to completely inertial objects is "gravitational". So if two inertial objects exchange light signals that appear to wiggle, then this is a gravitational effect, and GR will describe it fine no matter what frame of reference we adopt-- including one in which the Earth is perfectly stationary. Frames of reference in GR are local, not global, so it is perfectly appropriate to consider both the Earth and the white dwarf to be stationary in their own frames, and GR will tell us how to connect signals between them based on the spacetime that connects them. There is very little left that can be called a "cause", other than the history of the spacetime, and the equations of GR. The mass of the Sun is mostly what is important in what the spacetime is doing that is responsible for the wiggle, so it could also be said that the wiggle is "caused" by the mass of the Sun coupled with the choice of the two relevant reference frames-- the white dwarf and the Earth. You need all three to get the wiggle, so will the true "cause" please stand up! Yet had the researchers said that the mass of the Sun causes the light to wiggle, it would have created even worse confusion.
(Of course, you might be tempted to point out that nothing in this problem manifestly requires general relativity, so if we adopt a more approximate but more generally understood theory, like Newton's, it will a spawn a different language for talking about the "cause" of the wiggle. This is perfectly normal and is part of what I'm saying-- we choose the language when we choose the theory, and choices of theory are essentially a part of the "sociology" of attributing causes. None of this contradicts your point that the article can lead to pseudoscientific confusions, I just find it interesting how slippery is the whole issue of causation in physics.)
