Comment on Which is it Lemmy?

• Bytemeister@lemmy.world ⁨3⁩ ⁨days⁩ ago

  Alright. Cube is moving at 0 m/s (or at rest) and the portal moves over the cube. How fast is the cube moving?

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  • ericwdhs@discuss.online ⁨3⁩ ⁨days⁩ ago

    0 m/s relative to the chosen inertial reference frame, 1 m/s relative to the portal entry, and 1 m/s relative to the portal exit.

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    • Bytemeister@lemmy.world ⁨3⁩ ⁨days⁩ ago

      Woah woah woah woah. I thought relative movements were not real. We’re talking in terms your worldlines here.

      This was you BTW…

      A reference frame in that sense is a useful convention but doesn’t actually signify something in reality

      So please, I ask again, how fast is the box moving when the portal is passed over it at 1 m/s?

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      • ericwdhs@discuss.online ⁨3⁩ ⁨days⁩ ago

        Sorry. Lol. I used some shorthand I didn’t think I’d need to defend based on earlier statements, but for clarity:

        1. Movement relative to other objects is in itself not a real thing. 2) Movement relative to local spacetime geometry is always real. 3) Spacetime geometry is usually continuous. 4) The usual continuity of spacetime allows for the convention of defining movement relative to other objects with the underlying connecting spacetime element left unstated. 5) Breaking any underlying assumptions about spacetime continuity also breaks the convention, but the convention is still valid across spaces where the underlying assumptions are not broken.

        Expanding out my last statement:

        1. The box is initially moving 0 m/s in the inertial reference frame we defined to be at rest.

        2. The portion of the box that has not yet crossed the portal boundary is moving 1 m/s in the inertial reference frame the entry portal is currently at rest in (valid due to the continuity in spacetime across the measurement between them).

        3. The portion of the box that has already crossed the portal boundary is moving 1 m/s in the inertial reference frame the exit portal is currently at rest in (valid due to the continuity in spacetime across the measurement between them).

        There are additional possible declarations to make if the portal boundary is a non-zero volume instead of a plane of instantaneous transition, but the games’ interpretation of portals constrains that to either be zero or close enough to zero to not matter for the end result. Let me know if you want me to expand on that too.

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