On Jul 7, 6:02 pm, stevendaryl3...@yahoo.com (Daryl McCullough) wrote: > harald says... > > >On Jul 7, 1:46=A0pm, stevendaryl3...@yahoo.com (Daryl McCullough) wrote: > >> harald says... > > >> >On Jul 6, 5:18=3DA0pm, stevendaryl3...@yahoo.com (Daryl McCullough) wrot= > >e: > >> >> harald says... > > >> >> >The twin scenario was presented by Langevin in 1911 to show that > >> >> >physical acceleration is "absolute", even more so with SRT than with > >> >> >Newton's mechanics. > > >> >> What does that mean? As I said, proper acceleration (as measured by > >> >> an accelerometer) is absolute, but coordinate acceleration is > >> >> certainly not. > > >> >It means that you agree on that point with Langevin. > > >> Well, it's hard for me to believe that Einstein was unaware of the > >> fact that an accelerometer can measure accelerations. > > >Einstein was as aware as most physicists that an accelerometer does > >not distinguish between an acceleration and a gravitational field; > >however, he pushed that idea to the extreme. > > Then I'm *not* disagreeing with Einstein. As I said, *proper* > acceleration (acceleration relative to freefall) is certainly > detectable, and Einstein agrees with that.
OK - I understood "proper" as in SRT.
> Of course, you *can* describe a path with coordinates. You > can describe a road by giving two functions lat(s) and long(s), > which specifies the latitude and longitude as a function of the > distance s along the road. > > >> The associated coordinate acceleration being zero is dependent > >> on a choice of a special coordinate system. > > >Yes. But what was your point? > > That the notion of "straight" versus "nonstraight" is *not* > dependent on a coordinate system.
It's definitely the case for "straight" trajectories, which are for example straight relative to an inertial system but not relative to a rotating system.
> Whether a path is straight > (for Euclidean geometry) or inertial (for relativity) is an > intrinsic property of the path, and a path doesn't change from > straight to nonstraight when you change coordinate systems.
> As I said, there is a special set of coordinate systems > (Cartesian coordinate systems, in the case of Euclidean > geometry, inertial coordinate systems, in the case of relativity) > such that straight paths or inertial paths are particular > simple: In such a coordinate system, an inertial path can > be written as: > > x(t) = x_0 + v_x t > y(t) = y_0 + v_y t > z(t) = z_0 + v_z t > > where x_0, y_0, z_0, v_x, v_y, and v_z are constants. > Straight paths can *only* be written that way if you > are using a Cartesian inertial coordinate system.
Ah - you used the right key words here; now we agree! :-)
> >> Whatever was meant by his generalized principle of relativity, > > >You mean that you really did not know, and that you still don't - even > >after reading all his explanations?! > > Well, it seems to me that you don't understand what Einstein > meant.
I understand why he agreed to call the clock exercise a "paradox" and an "objection" against his theory, which required to be solved. It appears that you still don't understand why, and I don't think that adding more words will help.
> In General Relativity, there *is* no "force of gravity". There are > only inertial forces which appear whenever an observer is accelerating > relative to freefall. That doesn't mean that gravitation is undetectable, > just that a gravitational *force* is undetectable. Gravitation in GR > is manifested through curvature, through the fact that the local standard > for freefall (inertial motion) changes from location to location. Unlike > Newtonian physics or Special Relativity, there is no longer a global notion > of an inertial frame. > > >> The modern way of looking at it is that "inertial forces" are > >> felt whenever the observer is accelerating *relative* to freefall. > >> Einstein originally thought of the equivalence principle differently: > >> He thought that an object accelerating in a gravitational field felt > >> two different kinds of forces: (1) inertial forces due to acceleration, > >> and (2) gravitational forces. These two forces canceled in the case > >> of freefall. > > >??? I strongly doubt that. Reference please! > > I cannot find an online reference, but it occurs in a discussion > by Einstein of his "elevator" thought experiment.
As far as I remember, he held that an object accelerating in a gravitational field feels no force at all; does it make a difference?
> >> >According to his theory, we are entitled to say that such an object > >> >is *not* (properly) accelerating but that instead a "real" > >> >gravitational field is induced through the universe which accelerates > >> >all the *other* objects. > > >> I think you are confusing the physical content of Einstein's theory > >> with the way he chose to describe it. > > >The purpose with which you and I try to describe things here is to > >make the physical content of what think clear to the other. Do you > >seriously believe that Einstein tried to do the opposite, to hide the > >meaning of his words? > > No, what I'm saying is that in your case, Einstein failed to > communicate (to you) what he meant.
Not Einstein, but we to each other. However, it just got better!
> >Good, we are making progress. :-) > >Einstein held that, as he put it, acceleration is "relative": > >according to his theory we may just as well claim that the traveler is > >*not* physically accelerated, contrary to Langevin's and your claim. > > No, you are confused. As I have said, there are two different notions > of "acceleration": (1) proper acceleration (acceleration relative to > the local standard for freefall) and (2) coordinate acceleration > (acceleration relative to whatever coordinate system you are using). > Einstein and I are in complete agreement that for the traveling > twin, proper acceleration is nonzero, while coordinate acceleration > is zero (using the appropriate noninertial coordinate system). So > where is the disagreement? There is none.
There is no disagreement on that point. What about the induced gravitational field?
> >He thought to solve the problem by saying that at the turnaround > >(according to the stay-at-home), the traveler may consider himself as > >remaining in place against an induced gravitational field that > >appears. > > And certainly he may, in the sense that he may choose a coordinate > system in which he is always at rest. The notion of being at rest > is relative to a coordinate system in relativity.
He only may do so if his induced gravitational field can be held to be, as his theory claims, "physical", and propagating according to the same laws of physics as all other gravitational fields.
> >> A lot of the confusion in physics discussions are because people are > >> caught up in interpreting *words*, as if we are analyzing some holy > >> text. I don't *care* what words Einstein, or anyone else, uses. > > >In that case we have nothing to discuss, > > Are you saying that you had no point other than complaining > about Einstein's way of describing his theory?
?! I have no complaints at all. My point, about which *you* "complained", was the simple fact that the "clock paradox" concerns the General PoR; that is irrelevant for SRT.
> General Relativity describes what happens when > you take clocks and move them about, move them up and down in a > gravitational field. It describes how mass affects gravitational > fields, and how (indirectly) it affects the behavior of clocks. > It describes how electromagnetic waves change frequency as they > pass near massive bodies. It describes how massive bodies orbit > one another. What other physical meaning could you possibly ask for????
I don't ask for anything; Einstein provided more!
> If you are asking, not about General Relativity, but the General > Principle of Relativity: that isn't a theory of physics, it is > a heuristic, or a philosophical position, or metaphysics. It has > no physical meaning, except to the extent that it guides us in > coming up with better theories of physics.
I rarely saw a more aggressive criticism against Einstein's theory. :-)