On 1/16/2014 1:40 AM, LizR wrote:
On 16 January 2014 19:20, meekerdb <[email protected] <mailto:[email protected]>> wrote:On 1/15/2014 7:44 PM, Stephen Paul King wrote:Dear LizR, But stop and think of the implications of what even Bruno is saying. *Space is completely a construction of our minds.* _There is no 3,1 dimensional Riemannian manifold out there_. We measure events and our minds put those together into tableaux that we communicate about and agree on, because our languages, like formal logical system, force the results to obey a set of implied rules. We formulate explanations, formulate models and look for rules that the models might obey. Hopefully we can make predictions and measure something...Sure we create models like spacetime - BUT we can agree on them and they are successful models both in prediction and in explanations leading to other models. You write *like this a new discovery.* The set of rules isn't implied, it's quite explicit: The model must be the same for everybody in every circumstance. Physicsis intended to apply everywhere. That's why momentum and energy are conserved. These models are our best guess about what's out there. So it makes no more sense tosay _"There is no 3,1 dimensional Riemannian manifold out there" _than to say "There is no computer monitor in front of me." or "I'm a brain in a vat." or "There is no refrigerator in my kitchen." If I remember correctly, momentum is conserved because space has no preferred direction
That's angular momemntum. Linear momentum is conserved because "space has no preferred position". I put that last in scarce quotes because in the context of "why are the laws of physics the way they are" it is an fundamental choice of our model that we don't want any preferred position, so in a sense we pick out that characteristic as physics and lump everything particular into geography.
Noether proved that if we wrote our laws to have a continuous symmetry (like translation invariance) there would necessarily be a corresponding conserved quantity.
Brent P.S. Do you know what conserved quantity corresponds to invariance under a Lorentz boost?
and energy is conserved because there is no preferred time. An insight we owe to one of my heroes, the wonderful Emmy Noether, if I'm not mistaken.
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