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Now, maybe this is a result of my poor understanding of spacetime, but a theoretical warp bubble contracts space in front, and expands it in the back. Positive energy always contracts, so I get the need for negative energy, but why or, more accurately, how is it supposed to work in the ring shape. Basically, I have 2 questions.

  1. How is a ring of negative energy density supposed to generate a positive gravity well? It's not like a positive ring generates a negative one.

  2. How does a ring even know where the 'front' is?

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    $\begingroup$ Like all impossible physical machines the Alcubierre drive starts with some form of unobtainium. $\endgroup$
    – FlatterMann
    Commented Nov 2, 2022 at 12:49

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It's not really a ring but a toroid (page 10), and it's the initial speed that determines the direction of the movement, see an article by the author of the idea.

The Alcubierre metric is a kind of Gullstrand-Painlevé metric.

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