Dude, spare us the “I’m a physicist” shtick, if you were a physicist we’d see that by the quality and detailed knowledge shown in your writing and you wouldn’t need to keep telling us “I’m a physicist” in a need to give yourself extra gravitas. You’re taking Relativity a little too literally, yes in Relativity, for any object “calculations” are made relative to the motion of other objects, as opposed to a absolute standard of “rest”, according to Special Relativity there is no absolute standard of rest. But there are still real CAUSES of the motions we see. If I have a ball tied to a 3 foot long string and I start to swing the ball over my head like a rodeo rope, so that the ball is revolving around a center axis on my right hand, we would say that the motion of that ball is EXPLAINED by my hand swinging the string in a circular motion, which is causing the ball to trace an orbit around the center axis of my hand. Only a simpleton would get lost in the minutiae of arguing that the ball isn’t necessarily in an orbit around my hand because all motion is relative. Now let me ask you, when we observe the strange motion of the planet Mars in the night sky, where it wanders in one direction for a few months, and then one week turns around and wanders in the opposite direction, how do you EXPLAIN this motion?
Exactly right. Excellent post.
People who play the “Relativity” card often have no idea about either Special or General Relativity. I see this thing about how ‘it’s all relative, man’ so we can abandon trying to give up discerning the causes of observed motions, and that’s just naive and wrong.
First of all, the idea that the laws of motion are the same in inertial frames goes back to Galileo - and the transformations between one inertial frame and another is known as the Galilean transformation. The fact that it was generally known by Newton’s time that velocities are relative to reference frames and to one another doesn’t invalidate his explanations that form the foundations of celestial mechanics.
Second, people who don’t understand classical mechanics, never mind relativity, fail to make a distinction between kinematic and dynamic transformations. You don’t need Einstein to tell you that you can write down the equations of motion in any frame you like (that’s a kinematic transformation), but that where masses and accelerations are concerned, the interaction of bodies is not arbitrary with respect to inertial frames (that’s dynamics, and that’s what’s used to predict orbital mechanics). When the rabbit jumps six feet into the air, it is not dynamically correct to claim that the rabbit remains at rest and the earth leaps six feet away from the rabbit - and that’s true in classical mechanics and in special and general relativity. (OK - the precisely pedantic formulation is that the rabbit leaps six feet and the earth something like a trillionth of an angstrom)
Third, people who don’t understand the physics wrongly claim that the notion that there is no absolute reference frame in GR means that all frames are equivalent. They are not. It is true that uniform motion can only be measured with respect to other bodies, ie uniform motion is purely relative, but accelerations and rotations (which embody accelerations) can be measured, by measuring forces in a co-moving frame, without reference to other bodies. Accelerations are therefore not relative in the same sense that uniform motion is - we can unambiguously tell whether a body accelerating or rotating.
We observe in celestial mechanics that the sun, planets, stars, galaxies and so on, are undergoing accelerations owing to the gravitational forces from other bodies in the universe (or to put it in general relativistic terms, the bodies are following geodesics in a spacetime which is curved as a consequence of the presence of masses).
Nothing in SR or GR frees us from the scientific responsibility to explain the *causes *of motions of bodies in space. If one is asked to explain the cause of the five earth-sun Lagrange points, there’s no point shrugging and saying “it’s all relative, buddy, so these points are the same as any other” - they are not. Just as the Lagrange points are explained by invoking gravitational dynamics, so other observed dynamic behaviour requires explanation.
Alec
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