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    $\begingroup$ By permanent length contraction, you presumably mean a change in the proper length of an object. And that's clearly possible both within relativity and outside it. For example, if I crush a soda can, it'll end up permanently shorter than before. $\endgroup$
    – knzhou
    Commented Oct 8, 2019 at 1:17
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    $\begingroup$ In both relativity and outside it, this kind of permanent length change can occur if you squeeze an object hard, and the object doesn't bounce back. It really has nothing to do with length contraction. Length contraction is a property of a frame you use to describe an object, not the object itself, so it undoes itself perfectly as long as you don't squeeze or crush the object in the common sense way. $\endgroup$
    – knzhou
    Commented Oct 8, 2019 at 1:18
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    $\begingroup$ If it's from a science popularizer like Brian Greene, I wouldn't really listen to it. These folks almost always put out oversimplified explanations. I rarely read a paragraph from them that doesn't have some error in it. $\endgroup$
    – knzhou
    Commented Oct 8, 2019 at 1:39
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    $\begingroup$ There's a variant of this scenario that doesn't involve compression. A train travels through a tunnel. At the two ends, there are two enormous guillotines. When the train is completely in, they go down simultaneously and just miss the train; then immediately retract. The train passes safely. Now, in the train frame, the tunnel is shorter then the train; if the two guillotines come down at the same time, the train is destroyed. Both can't be true. But, in train frame, they don't drop at the same time, one drops and retracts, train passes, then the other does the same behind it. $\endgroup$
    – Filip Milovanović
    Commented Oct 8, 2019 at 2:02
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    $\begingroup$ @knzhou I wouldn't describe Brian Greene as a science popularizer as much as a prominent physicist who also does science popularization. I can't remember ever reading a paragraph of his that's unambiguously wrong (as opposed to a defensible simplification that glosses over some subtleties). Greene knows his stuff. $\endgroup$
    – tparker
    Commented Oct 8, 2019 at 22:30