defining laws in isolation or 'no other force acting'

Post by h***@public.gmane.org
Hi DD, hope you are well,
Quoting from your CT paper
"2.2 Constructor-independent laws of physics
In physics, too, most laws are about substrates only: they hold for all possible
constructors. But the prevailing conception disguises that. For example, one way of
formulating a conservation law in the prevailing conception is that for every isolated
physical system S, a certain quantity Q(S) never changes. But since an isolated
system never deviates from a particular trajectory, almost all its attributes (namely,
all deviations from that trajectory) remain unchanged, not just conserved quantities.
For instance, the total energy of the atoms of an isolated crystal remains unchanged,
but so does their arrangement, and only the former invariance is due to a
conservation law. In constructor-theoretic terms, the arrangement could be changed
by a constructor while the energy could not.
More generally, what makes the attributes that we call conserved quantities
significant is not that they cannot change when a system is isolated, but that they
cannot be changed, whether or not the system is isolated, without depleting some
external resource."
Reading this I'm curious, DD, why in your judgement, assumptions about isolation, or 'no other forces acting' and so on, tend to be used when defining laws?

They are used when the laws are about isolated systems. We can't choose what the laws we discover are going to say.

Post by h***@public.gmane.org
I think it's because that is the simplest, by far clearest, way to be very clear what the law actually is.
It allows a statement like this "for any isolated physical system S, a certain quantity Q(s) remains constant" instead of much longer statements that have to enail that energy or whatever, cannot be created or destroyed...but it can seem to be but what's going on there is energy is being transported from somewhere else....
As in Newton's laws of motion with assumptions about no other force acting...'. He wasn't saying that was the only time the law was true.
I appreciate this is a very marginal aspect of your CT work, but I do think you are operating a misunderstanding here and the other instances like perpetual motion etc.
Or perhaps I am....I shall look out for what you have to say

In that sense, the laws of constructor theory *are* about isolated systems: the substrate plus constructor.

-- David Deutsch

h***@public.gmane.org

2014-01-06 12:23:40 UTC

Hi DD, hope you are well, Quoting from your CT paper
"2.2 Constructor-independent laws of physics
In physics, too, most laws are about substrates only: they hold for all possible constructors. But the prevailing conception disguises that. For example, one way of formulating a conservation law in the prevailing conception is that for every isolated physical system S, a certain quantity Q(S) never changes. But since an isolated system never deviates from a particular trajectory, almost all its attributes (namely, all deviations from that trajectory) remain unchanged, not just conserved quantities. For instance, the total energy of the atoms of an isolated crystal remains unchanged, but so does their arrangement, and only the former invariance is due to a conservation law. In constructor-theoretic terms, the arrangement could be changed by a constructor while the energy could not. More generally, what makes the attributes that we call "conserved quantities" significant is not that they cannot change when a system is isolated, but that they cannot be changed, whether or not the system is isolated, without depleting some external resource."
Reading this I'm curious, DD, why in your judgement, assumptions about isolation, or 'no other forces acting' and so on, tend to be used when defining laws?

They are used when the laws are about isolated systems. We can't choose what the laws we discover are going to say.
I think it's because that is the simplest, by far clearest, way to be very clear what the law actually is. It allows a statement like this "for any isolated physical system S, a certain quantity Q(s) remains constant" instead of much longer statements that have to enail that energy or whatever, cannot be created or destroyed...but it can seem to be but what's going on there is energy is being transported from somewhere else....
As in Newton's laws of motion with assumptions about no other force acting...'. He wasn't saying that was the only time the law was true. I appreciate this is a very marginal aspect of your CT work, but I do think you are operating a misunderstanding here and the other instances like perpetual motion etc.
Or perhaps I am....I shall look out for what you have to say
In that sense, the laws of constructor theory *are* about isolated systems: the substrate plus constructor.

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)

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.

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.

David Deutsch

2014-01-06 13:41:42 UTC

Post by David Deutsch
In that sense, the laws of constructor theory *are* about isolated systems: the substrate plus constructor.

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)
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.
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.

Newton's first law specifies the behaviour of isolated systems.

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'.

-- David Deutsch

h***@public.gmane.org

2014-01-08 17:50:17 UTC

Post by David Deutsch
In that sense, the laws of constructor theory *are* about isolated systems: the substrate plus constructor.

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)
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.
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.

Newton's first law specifies the behaviour of isolated systems.
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'.

In a post that will arrive with this one, I suggested it was theoretically plausible to arbitrarily draw boundaries, say instantaneously. I left the implication as I saw it, implicit. Perhaps I should try to explain it instread.

The relevance is that the choice of where to draw the boundary when defining a principle may involve considerations as to the clearest way to communicate something. In this tense, the drawing of a boundary is part of the same toolset that might also see an overall picture represented by more than one principle (in newton's case three).

In another tense, the overall reach of a theory could be seen as represented by a boundary also. But this boundary is not typically part of the explicit statement of a theory. This is the one that we cannot choose in terms of what a theory says.

My overall point, is that these two senses of a boundary might or might not be the same thing. Might or might not be represented by the boundary used to define the principle..even when that principle represents the theory as a whole.

If they are the same...then a more general statement that does away with the boundary is non-trivial. But if they aren't, a more general statement that does away with the boundary, may or may not be non-trivial.

For that reason, I would argue it is important to make the case for non-triviality as part of any generalization. It isn't clear to me that you do that in the paper which I brought down quotes from. Which doesn't mean what you did is non-trivial necessarily.

But even if it could be interpreted that way...it might be worth clearing up, since - good or bad - science seems to have an unstated convention of criticizing triviality by voting with its feet. Which can be damaging if the goal is about introducing a new field (which among other thing is about attracting and retaining research interest)

h***@public.gmane.org

2014-01-08 16:45:49 UTC

Post by David Deutsch
In that sense, the laws of constructor theory *are* about isolated systems: the substrate plus constructor.

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)
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.
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.

Newton's first law specifies the behaviour of isolated systems.
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'.

This is fine, but the first law is a good example of why it is useful for definitional reasons to invoke an isolated situation.

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.

Another way to come at this would be to point out that it's theoretically plausible to instantaneously define boundaries arbitrarily.

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.

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..." ?

h***@public.gmane.org

2014-01-19 07:26:32 UTC

Post by David Deutsch
In that sense, the laws of constructor theory *are* about isolated systems: the substrate plus constructor.

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)
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.
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.

Newton's first law specifies the behaviour of isolated systems.
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'.

Hi DD, Please excuse the delay responding to the point you made here. I had never thought of it before, so have had to spend some time thinking about this matter of the first law following from the second.

I might interpret this wrong, but the easiest way I can see you are right, starts with setting F=m.a = 0 (m !=0) then looking for conditions consistent with this state. From this, there is a chain of inferences - as rigorous as you like - that will, as you say, lead to the first law.

Whether this is trivial or non-trivial, is much more problematic, because the reasoning seems to reverse quite easily, such that the second law derives from the first. In both cases, it is necessary to have fore knowledge of both 'velocity' and 'acceleration' and the relationship between them.

One direction may be more or less intuitive than the other, but other than that it's hard not to get a draw between them. Reversibility doesn't make something trivial, but it does make the question of triviality or not, dependent on what significance you seek to attach in the first place. It's hard to attach significance of the form 'the first law follows the second' non-trivially, when there is reversibility.

Or maybe I've completely misconceived the derivation you use. I wait in hope of your response as ever.

Gary Oberbrunner

2014-01-20 22:11:09 UTC

Post by h***@public.gmane.org
the reasoning seems to reverse quite easily, such that the second law
derives from the first.

--
Gary

h***@public.gmane.org

2014-01-22 05:08:01 UTC

Post by h***@public.gmane.org
the reasoning seems to reverse quite easily, such that the second law
derives from the first.

The first law doesn't specify the relationship between force and
acceleration (except that there is one). For instance, F=m*a^2 would also
lead to bodies at rest staying at rest and bodies in motion staying in that
state of motion. (There's no acceleration in either case.) So you can't
derive the second law from the first, only the other way.

--
Gary

You might be tempted to kick yourself when you your brain realizes it's true before I can finish writing that "you got the problem right but the wrong context" as "F -> a" is the right relationship, but the way it could be in terms of "a" is the trivial context, and "m" is the intractable problem.

But there's a reason why you couldn't choose "m" even though "F -> a" , is literally substituting "m" for "->" (i.e. relationship) in a context that you've probably known since you were about 8.

But the intractability of "m" is that in the history of the end to end reasoning, "m" never existed, and there was never a way to bring it into existence.

Gary Oberbrunner

2014-01-31 11:01:16 UTC

Post by Gary Oberbrunner

Post by h***@public.gmane.org
the reasoning seems to reverse quite easily, such that the second law
derives from the first.

You might be tempted to kick yourself when you your brain realizes it's
true before I can finish writing that "you got the problem right but the
wrong context" as "F -> a" is the right relationship, but the way it could
be in terms of "a" is the trivial context, and "m" is the intractable
problem.
But there's a reason why you couldn't choose "m" even though "F -> a" , is
literally substituting "m" for "->" (i.e. relationship) in a context that
you've probably known since you were about 8.
But the intractability of "m" is that in the history of the end to end
reasoning, "m" never existed, and there was never a way to bring it into
existence.

Sorry, I have absolutely no idea what you're talking about here.

--
Gary

h***@public.gmane.org

2014-02-05 05:20:32 UTC

Post by Gary Oberbrunner

Post by Gary Oberbrunner

Post by h***@public.gmane.org
the reasoning seems to reverse quite easily, such that the second law
derives from the first.

Sorry, I have absolutely no idea what you're talking about here.

My bad - I'm not sure what I said either :O)

Gary Oberbrunner

2014-01-06 14:00:18 UTC

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