Why in the world would Einstein suggest....

There are no "conclusions" made by physicists about reality of faster/slower. Only statements about the equivalence of appearances or "observational constraints" in inertial reference frames.

The twin paradox (so called) was a curiosity raised by Einstein himself..And it is that first show of genius of the upsetting of "common sense" in physics which many find it difficult to cope with.

This means that Jack sees Jill’s light clock to be going slow—a longer time between clicks—compared to his own identical clock. Obviously, the effect is not dramatic at real railroad speeds. The correction factor is , which differs from 1 by about one part in a trillion even for a bullet train! Nevertheless, the effect is real and can be measured, as we shall discuss later.

It is important to realize that the only reason we chose a light clock, as opposed to some other kind of clock, is that its motion is very easy to analyze from a different frame. Jill could have a collection of clocks on the wagon, and would synchronize them all. For example, she could hang her wristwatch right next to the face of the light clock, and observe them together to be sure they always showed the same time. Remember, in her frame her light clock clicks every 2w/c seconds, as it is designed to do. Observing this scene from his position beside the track, Jack will see the synchronized light clock and wristwatch next to each other, and, of course, note that the wristwatch is also running slow by the factor In fact, all her clocks, including her pulse, are slowed down by this factor according to Jack. Jill is aging more slowly because she’s moving!

But this isn’t the whole story—we must now turn everything around and look at it from Jill’s point of view. Her inertial frame of reference is just as good as Jack’s. She sees his light clock to be moving at speed v (backwards) so from her point of view his light blip takes the longer zigzag path, which means his clock runs slower than hers. That is to say, each of them will see the other to have slower clocks, and be aging more slowly.

This phenomenon is called time dilation. It has been verified in recent years by flying very accurate clocks around the world on jetliners and finding they register less time, by the predicted amount, than identical clocks left on the ground. Time dilation is also very easy to observe in elementary particle physics, as we shall discuss in the next section.

If you took the clock that had been left on the ground and measured it on a flying jetliner, you're just switching to the other of the two frames of reference, not finding a 3rd-person, objective or somehow more real frame.

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If you took the clock that had been left on the ground and measured it on a flying jetliner, you're just switching to the other of the two frames of reference, not finding a 3rd-person, objective or somehow more real frame.

Not sure what you are saying here, FBM, but the Hafele-Keating (airplane) experiment has been discussed a fair amount in this thread already.

To sum it up, the faster-moving clocks did, in fact, show less time elapsed that the less fast (keep in mind that even the earth clock is moving).

This was, indeed, an objective fact, not a matter of appearance.

Had the guy on the faster moving airplane assumed that the earth clock was running slower, he simply would have been WRONG. He would not be "equally correct" as the guy on earth saying the plane clock had slowed down (he was RIGHT).

In what way objective? Was it measured in a third inertial frame?

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In what way objective? Was it measured in a third inertial frame?

Well, the explanation is complex, and raises additional questions about SR (again these have been discussed earlier, although I'm not sure I can locate the posts easily).

To make the point, it's as though two clocks were synchronized before take-off. When the plane landed, the two clocks were compared. The flying clock recorded less elapsed time than did the stationary earth clock.

Yes, and since they're measured in one of the two inertial frames, the results are not objective. They're subjective to that frame

...SR says that there is no grand, universally objective frame in/by which motion in every smaller frame can be evaluated in some ultimate manner, truer or realer somehow...

I don't follow you here. There is nothing "subjective" about what the hands on a clock read. Same if it's two clocks.

Well, the thing is, you don't need to know "absolute truth" to make a relative judgment. If I'm going down the road at 100 mph, then that 100 is relative to the earth's surface. It doesn't matter if my "absolute speed" was known to be 1000 mph, 10,000 mph, or whatever. I can still measure my speed relative to the earth. Whatever my "absolute" speed, it's 100 mph more than the earth's surface.

The very fact that the two clocks read different times attests to the frame-subjectivity of measurements made in those frames. If there were no frame subjectivity, the clocks would read the same.

Time slowing as you approach c

All three claim they have not moved a single inch during the last 10 minutes.

Can they all be correct?

Impossible to say, because all three claims are underspecified. "A single inch" --- relative to what?

But the clocks read what they read depending on which frame they were in

This phenomenon is called time dilation. It has been verified in recent years by flying very accurate clocks around the world on jetliners and finding they register less time, by the predicted amount, than identical clocks left on the ground

Had the guy on the faster moving airplane assumed that the earth clock was running slower, he simply would have been WRONG. He would not be "equally correct" as the guy on earth saying the plane clock had slowed down (he was RIGHT).

Had the guy on the faster moving airplane assumed that the earth clock was running slower, he simply would have been WRONG. He would not be "equally correct" as the guy on earth saying the plane clock had slowed down (he was RIGHT).

Their [sic] IS a preferred frame, implicitly posited by them, in every statement which starts out with a phrase "from A's frame of reference..."

I'm not sure why you think I'm troubled or perplexed

SR says nothing about someone in one reference frame making assumptions about what's happening in another one. Observations, yes, but not assumptions.

Also, we're slipping towards another ambiguity. A "preferred frame of reference" does not designate a frame preferred by the physicists

In theoretical physics, a preferred or privileged frame is usually a special hypothetical frame of reference in which the laws of physics might appear to be identifiably different (simpler) from those in other frames.

Although there is no preferred inertial frame under Newtonian mechanics or special relativity, the set of all inertial frames as a group may still be said to be "preferred" over noninertial frames in these theories, since the laws of physics derived for inertial motion only work exactly in this special category of frames.