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>Tie two rocks together with a rope, he suggested, and go into deep space far from the gravitation of the Earth or the sun...Rotate the rope about its centre an
by trevyn 9y ago
>Tie two rocks together with a rope, he suggested, and go into deep space far from the gravitation of the Earth or the sun...Rotate the rope about its centre and it will become taut as the rocks pull outwards. The rocks will create an outward force pulling the rope tight. If one does this in an empty universe then what can it mean for the system to be rotating. There is nothing to measure rotation with respect to. Newton deduced from this thought experiment that there had to be something to measure rotation with respect to, and that something had to be space itself.
Eh? You measure the rotation against the thing that performed the “rotate the rope” step in the experiment.
Thought experiments that contain the step “now magically remove X from the observable universe” should be treated with caution! :-)
If you change it to just “There exists a universe empty except for a rotating set of rocks connected by a rope”, rotation needs no meaning as it’s one of the axioms. Also, there is no observer in that universe, so you’re making a pretty dangerous assumption about the ability to observe a universe without being a part of it.
- dullgiulio 9y agoWell, it's a thought experiment. You can think of the same with two rocket powered capsules, tied with a rope, so that nothing has to disappear. If they are rotating, they are rotating against something (space itself), if they are not rotating, rocket power is strangely not having any effect, which violates Newton's Third Law.
- trevyn 9y agoThat’s easy then; they’re rotating relative to the exhaust stream that they’re creating. Now if you’re talking quantum vacuum thrusters, exploring exactly how that works should also give you a good idea of what space is! ;-)
- mustacheemperor 9y agoBut, how could you measure or perceive that rotation, relative only to the exhaust stream created by each rocket? If you are picturing the rockets doing any more than pulling the rope taut with the force of their motion, ie the rockets are turning around like a wheel in your mental image, you are actually picturing them rotating relative to you, the stationary observer. The conundrum is: what is the rotation without the relative perspective of the stationary observer? What pulls the rope taut?
- trevyn 9y ago>What pulls the rope taut? Ok, this seems to be a clash of intuitions explained by reference frames — When you imagine an object in deep space or its own empty universe, you intuitively use an “inertial reference frame”, where “the physics of a system in an inertial frame have no causes external to the system”. I.e. you do not see any “fictitious” forces like the centrifugal force, so the rope would not be taut. https://en.m.wikipedia.org/wiki/Inertial_frame_of_reference https://en.m.wikipedia.org/wiki/Inertial_frame_of_reference If you are imagining the rope to be taut, you are imagining a force pulling it taut, but this force that you’re imagining is only explained by physics if you’re using a rotational (non-inertial) reference frame. So if your system has unexplained “centrifugal” forces, those forces can be modelled as the system rotating within an inertial reference frame, without having any evidence of any “non-rotating” matter. It just so happens that deep space is pretty close to an inertial reference frame, so your mind sort of makes that assumption. Or, in other words, you could measure and perceive the rate of the rope’s rotation by analyzing the forces in system, without having any physical external point of reference.
- deleted 9y ago[deleted]
- heavenlyblue 9y agoIf the rotation needs no meaning, what stretches the rope then?