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21

None of the interesting equations in physics can be derived from simpler principles, because if they could they wouldn't give any new information. That is, those simpler principles would already fully describe the system. Any new equation, whether it's the Navier-Stokes equations, Einstein's equations, the Schrodinger equation, or whatever, must be ...


18

They are derivable from classical mechanics using either the continuum or molecular points of view. Starting with a continuum view, one applies conservation of mass, momentum, and energy to a control volume and the result is the Navier Stokes equations. The Navier Stokes equations, in the usual form, apply to Newtonian fluids, that is fluids whose stress ...


12

I once asked Putterman after a similar colloquium what he meant by this statement, and his answer was "long time tails". Long time tails are fractional powers that appear in the long time behavior of correlation functions, see, for example, here and here. These fractional powers are seen in molecular dynamics (they are more difficult to see experimentally), ...


5

A theory is a set of statements that is developed through a process of continued abstractions. A theory is aimed at a generalized statement aimed at explaining a phenomenon. A model, on the other hand, is a purposeful representation of reality. As you can see, both share common elements in their definitions. What differs one from the other (in my opinion) ...


5

In physics and engineering, we often abstract and idealize a physical problem to gain insight into the physics, e.g., infinite plane of charge, infinite line of charge, point charge, etc. Now, it goes without saying that if these idealizations didn't represent good approximations of relevant physical systems, they wouldn't be used. With regards to your ...


4

If a theory is not built on a solid foundation of (semi-rigorous) mathematics and a well-defined physical idea, the chances of it being accepted by the majority of physicists as a valid physical model are extremely small. If one wants to build up a theory of physics purely from philosphy, one will into some significant problems. After all is said and done, a ...


4

I was taught that the Standard Model was a misnomer; that it ought to be called the Standard Theory. I'm inclined to agree, though theories and models are both indispensable in science. Ultimately, the purpose of a model is provide local understanding of a particular phenomena. A model: Typically considers only fields, objects or quantities relevant to a ...


3

If a theory never connects with experimental reality then no matter how neat it is it will eventually be dropped. This is the problem that much of theoretical physics based around the various incarnations of String Theory finds itself. A theory is in one very real sense a data compression and predictive algorithm. It needs to join the experimental dots in ...


2

Astrophysicists have been looking at electron positron annihilations in the cosmos The Universe viewed trough INTEGRAL: the first complete map of the sky at the electron-positron annihilation energy (Credits J. Knödlseder - CESR - September 2005). If there existed regions in the sky where antimatter was aggregating, the interface between matter and ...


2

There are three reasons why mathematics is stated as an incomplete description of physics. I list them in order from pragmatically physical to more philosophical. Any calculation, any actual prediction of physics is based on a mathematical description that is known to be a mere approximation. You could conjecture that you have the complete list of ...


2

I could give an example of what people mean when they "say": ... metric tensor depend on the local coordinate system and therefore are not intrinsic to the surface Take for example the Schwarzschild metric. We have $$ds^2 = -\left(1-\frac{2m}{r}\right)dt^2 + \left(1-\frac{2m}{r}\right)^{-1}dr^2 +r^2(d\theta^2 +\sin^2\theta d\phi^2) $$ If you read this ...


1

Classical physics describes the movement of the center of gravity of extended bodies, which, when poorly taught, in the mind of the student becomes equivalent with "classical physics being a theory of point particles". That, of course, is utterly false, even on the level of the classical description. A center of gravity is a vector, not a point. ...


1

Whenever you come up with a theory (eg: Newtonian mechanics), it has some physical domain of validity, and then you come up with the next (better) theory (eg: relativity), and so on. This process might not have a "fixed point". At least if you had a fixed number of things to explain, then you might be able to consider iteratively simplifying it to an ...


1

There are strong constraints on antigravitating antimatter, because it could, in principle, be used, to create a perpetual motion machine. 1) Use energy $E$ to create a particle/antiparticle pair at height $h_{i}$ 2) Raise the particle/antiparticle pair to a height $h_{f}$. This takes zero work, because the antiparticle will be pushed up in the ...


1

Are theories that are based on the philosophy acceptable in Physics? Sure, but only if backed by maths and experiment. The prime example would be General Relativity, the development of which was guided by a whole bunch of principles (Mach's principle, equivalence principle, principle of covariance). In contrast, Einstein failed to develop a unified ...


1

How accurate are differential equations really, and to what accuracy can we predict future circumstances and events from them? Why does it matter that they are "differential equations"? Differential equations are just one type of model. The question is how accurate are theoretical models. The answer is, the ones you learn about in high school / college ...



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