3 ms·

What I was trying to express by "inky void, not even a star" was that the void didn't even include anything resembling an aether. "You are the only body in that

What I was trying to express by "inky void, not even a star" was that the void didn't even include anything resembling an aether. "You are the only body in that void" is meant to include non-material bodies. All this passage is trying to establish is that in order to distinguish constant motion from absolute stillness, there has to be something else to measure against. Whether or not our universe contains something that can serve as an absolute frame of reference is not the point -- yet. So yes, it really is meant to be talking about a self-evident fact (once you think about it). But you're right; I'm not a physics teacher. I'm willing to concede that I underestimated the potential stickiness of this point, and/or that I failed to overcome my handicap in expressing this point.

jules 13y ago
It's only obvious because that's what you were taught. Aristotle thought that a moving body would come to a halt by itself, unless you keep pushing it. He thought that this was a law of physics. This didn't require any aether or other substance. It was almost 2000 years later that Galilei came with the idea bodies set in motion will remain in motion, and that the fact that they usually stop was solely the result of friction. But even then it is not clear that there isn't some other method by which you can find out your absolute velocity. It was only in 1887 that the Michelson-Morley experiment showed that you can't do that with light! So this is not a triviality that can be deduced by logical reasoning, it's an experimental result of physics. This makes you wonder which falsehoods we currently take for granted, that later generations can't even fathom anymore.
- breadbox 13y ago
  I find myself disagreeing still. I would say that the idea that you can't measure your own motion without something else to measure against, is obvious to the casual person. (Whether that "something" is a physical object, the background radiation, the luminiferous aether, or whatever.) The possibility that it might not be true is actually far less intuitive, since it would require there to be some intrinstic frame of reference that is impossible to leave behind, even in a thought experiment.
  - shardling 13y ago
    You can measure acceleration without needing a special frame. More significantly, you can measure rotational motion without something to measure against. See that Feynman quote I linked in my original post; he discusses exactly this point.
    - breadbox 13y ago
      Which is exactly why that passage confined itself to talking about constant motion.
      - shardling 13y ago
        But the argument you used would seem to apply to both of those thing. That's what makes it flawed -- you claim it is obvious, when in fact this line of argument is very subtle and often misleads laymen. (Notice that in this very thread there seem to be people who think you couldn't detect acceleration.)
  - jules 13y ago
    So you're saying that what laymen mean when they intuitively understand that you can't measure your own motion is something else than what is meant by a physicist in the context of relativity. In particular a physicist means that you can't do that even when you have access to light, or those other things. A layman understands that you can't measure whether you're moving without access to those things, but he does not understand that it's still impossible with access to those things. In that case you cannot appeal to layman intuition on the former to conclude the latter, which is exactly what's necessary for an understanding of relativity. Also, even with this interpretation it's questionable. In Aristotle's mind you can determine whether you're moving or not, even if you are completely alone in the universe. You take an object that you have with you (e.g. your t-shirt), throw it in a random direction with a specified velocity relative to you. According to Aristotle both you and the watch will decelerate. But since the deceleration is relative to the absolute reference frame in which you will eventually come to a halt, you can deduce whether you're moving from the relative deceleration between you and your watch (although interestingly if the deceleration has a special form, then you cannot determine whether you are moving, but that is again a non trivial fact that is not obvious to a layman). To Aristotle, and the people after him until Galilei, the idea of a special reference frame was perfectly intuitively obvious! (though, of course, incorrect)
- saalweachter 13y ago
  This still happens in cartoon/TV/movie physics: the engines of a space ship cut out, and the space ship coasts to a halt (even when the space drive is obviously Newtonian, and not magic).

jules 13y ago

It's only obvious because that's what you were taught. Aristotle thought that a moving body would come to a halt by itself, unless you keep pushing it. He thought that this was a law of physics. This didn't require any aether or other substance. It was almost 2000 years later that Galilei came with the idea bodies set in motion will remain in motion, and that the fact that they usually stop was solely the result of friction. But even then it is not clear that there isn't some other method by which you can find out your absolute velocity. It was only in 1887 that the Michelson-Morley experiment showed that you can't do that with light! So this is not a triviality that can be deduced by logical reasoning, it's an experimental result of physics. This makes you wonder which falsehoods we currently take for granted, that later generations can't even fathom anymore.

breadbox 13y ago
I find myself disagreeing still. I would say that the idea that you can't measure your own motion without something else to measure against, is obvious to the casual person. (Whether that "something" is a physical object, the background radiation, the luminiferous aether, or whatever.) The possibility that it might not be true is actually far less intuitive, since it would require there to be some intrinstic frame of reference that is impossible to leave behind, even in a thought experiment.
- shardling 13y ago
  You can measure acceleration without needing a special frame. More significantly, you can measure rotational motion without something to measure against. See that Feynman quote I linked in my original post; he discusses exactly this point.
  - breadbox 13y ago
    Which is exactly why that passage confined itself to talking about constant motion.
    - shardling 13y ago
      But the argument you used would seem to apply to both of those thing. That's what makes it flawed -- you claim it is obvious, when in fact this line of argument is very subtle and often misleads laymen. (Notice that in this very thread there seem to be people who think you couldn't detect acceleration.)
- jules 13y ago
  So you're saying that what laymen mean when they intuitively understand that you can't measure your own motion is something else than what is meant by a physicist in the context of relativity. In particular a physicist means that you can't do that even when you have access to light, or those other things. A layman understands that you can't measure whether you're moving without access to those things, but he does not understand that it's still impossible with access to those things. In that case you cannot appeal to layman intuition on the former to conclude the latter, which is exactly what's necessary for an understanding of relativity. Also, even with this interpretation it's questionable. In Aristotle's mind you can determine whether you're moving or not, even if you are completely alone in the universe. You take an object that you have with you (e.g. your t-shirt), throw it in a random direction with a specified velocity relative to you. According to Aristotle both you and the watch will decelerate. But since the deceleration is relative to the absolute reference frame in which you will eventually come to a halt, you can deduce whether you're moving from the relative deceleration between you and your watch (although interestingly if the deceleration has a special form, then you cannot determine whether you are moving, but that is again a non trivial fact that is not obvious to a layman). To Aristotle, and the people after him until Galilei, the idea of a special reference frame was perfectly intuitively obvious! (though, of course, incorrect)
saalweachter 13y ago
This still happens in cartoon/TV/movie physics: the engines of a space ship cut out, and the space ship coasts to a halt (even when the space drive is obviously Newtonian, and not magic).