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I was reading Wheeler & Taylor's Spacetime Physics. The authors mentioned about tidal effect, as well as all physics laws are same in free-float frame.

I am left wondering if tidal effect will make the laws different in a same reference frame - suppose two people are in same free-float frame under the influence of a black hole; however, one person floats (unfortunately) right next to a black hole, and another person floats too, but with super far distance from the black hole. Surely, the former misfortunate person has a very different experience than another - he/she probably will even be killed due to the tidal effect!

Consider another experiment: suppose a train vertically falls into the Sun. Let there be two floating people in the train. One at the top "drops" one ball, another at the bottom "drops" another ball. And they observe the balls. They all claim that the balls are perfectly in inertial frame - they do not find the balls accelerate. But overall the distance between two balls increase due to the tidal effect! Can we say (is it plausible) that "the train is a free float frame"? If ONE person (either at the bottom or top) drops TWO balls instead, he will never observe the distance between the two balls increase!

I am left wondering if the physics laws are still same in this case? Would it be more proper to say "all physics laws are same locally in all free-fall frames"?

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    $\begingroup$ There is a nice article by Robert Forward (of the mass detector) discussing the higher order effects present in various orbital scenario, the limits they put on attempt to take advantage of "zero" gee (he uses the term "micro-gravity" throughout as I recall), and schemes for canceling out the leading terms. Those schemes figure prominently in his novel Dragon's Egg. $\endgroup$
    – dmckee --- ex-moderator kitten
    Apr 12, 2019 at 1:25

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The Einstein equivalence principle states that locally the physical laws of special relativity hold. Locally means in a sufficiently limited region of spacetime, as for both spatial and time extension.

The tidal effect applies to a macroscopic object, if its dimensions cover a region where the curvature variation is not negligible.

Definitely the term "Local" has to be emphasized for a correct reading.

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  • $\begingroup$ Thanks!! I suppose the authors intensionally make their readers to ponder this with no explicitly mentioning this. Now I am wondering another (closely related) question: what do we mean by "physics laws" when it comes to macroscope ? (say, cosmos?) $\endgroup$
    – Shing
    Apr 11, 2019 at 16:16
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    $\begingroup$ On a cosmological scale an inertial reference frame is not defineable. That is why you need general relativity. $\endgroup$
    – Michele Grosso
    Apr 11, 2019 at 16:41

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