Re: defining laws in isolation or 'no other force acting'
<hibbsa-/[email protected]> 08 Jan 2014 08:45:49 -0800
| Newsgroups | gmane.science.physics.fabric-of-reality |
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| Message-ID | <[email protected]> |
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<p> <br>
<br>
---In [email protected], <david.deutsch@...> wrote:<br>
<br>
> On 6 Jan 2014, at 12:23, <hibbsa@... mailto:hibbsa@...> <hibbsa@... mailto:hibbsa@...> wrote:<br>
><br>
>> ---In [email protected] mailto:[email protected], <david.deutsch@...> wrote:<br>
>> <br>
>>> In that sense, the laws of constructor theory *are* about isolated systems: the substrate plus constructor.<br>
>> <br>
>> The isolated system doubles as a better way to make clear there aren't other influences acting. Certainly there are applications where the boundary has independent value....say in how the major problems of mechanical engineering were solved (which were fundamental in nature)<br>
>> <br>
>> But, take Newton's laws of motion. Say, an object will remain at rest or continue the way its going at constant velocity, WHEN no other forces are acting. <br>
>> <br>
>> Does that say that when forces are acting, inertia and the other laws aren't also acting? No one would say that. Newton uses forces acting and that law in combination to get orbital behaviour, show conservation of momentum hence explain elliptical orbit and so on. <br>
><br>
> Newton's first law specifies the behaviour of isolated systems.<br>
><br>
> Newton's second and third laws specify the behaviour of all systems, isolated or not. Indeed, the first law follows from the second, if one adopts Newton's conception of what it means for a force to 'act'.<br>
<br>
This is fine, but the first law is a good example of why it is useful for definitional reasons to invoke an isolated situation. <br>
<br>
True physical situations of absolutely no forces acting are probably quite rare in the universe, across the end to end path of any object. The value of the law as stated is primarily because it isolates a physical law that always acts the same, regardless of other factors in play.<br>
<br>
Another way to come at this would be to point out that it's theoretically plausible to instantaneously define boundaries arbitrarily. <br>
<br>
Which is not to say that a non-trivial generalization of the nature of, say, energy, is not one of the major promises of CT. But only that, the way you characterize it in the quote I took, does not seem to distinguish the trivial from non-trivial generalization. <br>
<br>
For example, on the same grounds you suppose the generalizations that you do, what is to rule out a more general statement of Newton's first law along the lines of "not only when no forces are acting...but..." ? </p>
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