The equivalence principle?

[jajrussel]

I have seen a few references between the equivalence principle and free fall, but I seem to be having trouble making the connection? An accelerating rocket does not seem the same as an object in free fall. I've seen the drawings and yes the drawings make sense the rocket man is compared to a man standing on Earth. So why do they keep referencing free fall with the equivalence principle?

[Country Boy]

 

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 In order to answer this, I would have to know what you mean by "acceleration" and "free fall".  The way I understand them, an object "in free fall" is accelerating!

 

[StringJunky]

4 hours ago, jajrussel said:

I have seen a few references between the equivalence principle and free fall, but I seem to be having trouble making the connection? An accelerating rocket does not seem the same as an object in free fall. I've seen the drawings and yes the drawings make sense the rocket man is compared to a man standing on Earth. So why do they keep referencing free fall with the equivalence principle?

 

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All objects fall the same way under the influence of gravity; therefore, locally, one cannot tell the difference between an accelerated frame and an unaccelerated frame. Consider the famous example of a person in a falling elevator. The person floats in the middle of an elevator that is falling down a shaft. Locally, that is during any sufficiently small amount of time or over a sufficiently small space, the person falling in the elevator can make no distinction between being in the falling elevator or being in completely empty space, where there is no gravity.

We could imagine two apples floating on either side of the person; as the elevator approached the earth, the apples would approach each other. This happens because their paths, both toward the center of the earth, eventually converge. But this is not an effect that can be detected in a local experiment.

This statement of the equivalence principle makes an important suggestion. In special relativity--and all classical mechanics--we are used to the idea that objects travel at constant velocity unless a force acts on them. Now, if we can't locally tell the difference between falling in a gravitational field and travelling at constant velocity, then, locally, they must be the same thing. The paths of free bodies define what we mean by "straight" and if we observe an object deviate from constant velocity, it must be because spacetime itself is curved.

https://www.npl.washington.edu/eotwash/equivalence-principle