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It's a good article, but I have one big complaint: the terminology of bottum-up and top-down is reversed! In the context of politics, magister-dixit or top-dow

It's a good article, but I have one big complaint: the terminology of bottum-up and top-down is reversed! In the context of politics, magister-dixit or top-down has a negative connotation, and democracy or grass-roots or bottom-up has a positive connotation. In physics we should disregard the political connotation. Yes historically most new physics were top-down observations, i.e. noticing small (unexplained) deviations, and first modeling the deviation, and then eventually realising the underlying cause. A good example is the discovery that some planet motions: while most did seem to move in perfect circles, one of them had a measurable deviation, and after exhaustively trying to fit "state of the art" mathematics (degree 4 polynomialss or ovals), because Kepler incorrectly thought others would have already tried the simpler conic sections (ellipses for closed orbits). Only after the ovals kept failing did he try the too-obvious-otherwise-it-would-already-have-been-discovered conic sections, and found a very precise match. Examples of bottom-up physics are the early theories of statistical mechanics like Ludwig Boltzmann's, or the atomistic theory of chemistry: with little experimental evidence, they could derive physically realistic behaviours for large ensembles of particles, and only much later were molecules and atoms discovered. Bottom-up is postulating smaller particles and investigating how they would influennce the behaviour of bigger collections, and then trying to prescribe experiments to prove their predictions. So when the author of this article talks about bottom-up he is actually describing the return to top-down, explain as you measure deviations, and when he talks about top-down he seems to actually refer to the bottom-up postulates of new physics with hopefully falsifiable tests... Again, a good article, but its nomenclature of top-down and bottom-up seems reversed to me.

ssivark 8y ago
Not quite. When talking about bottom-up and top-down, one can talk about small to large scales in size (microscopic to macroscopic) or or can talk about small energies to large energies. The two descriptions are inversely related because small length scales correspond to large energies, and vice versa (roughly due to the Heisenberg uncertainty principle). Conventionally, when particle physicists talk about top-down and bottom-up, they are talking about energy scales: "top" is high energy, more fundamental, smaller distances and "bottom" is low energy, more emergent, larger distances.
- DoctorOetker 8y ago
  first, thank you very much for responding, but still I have the impression that bottom refers to the more fundamental perspective, and top refers to the more general perspective... i.e. the fundamental equations for electromagnetism are the maxwell equations with epsilon, mu and c for vacuum, and then one can use these as microscopic equation for other media and generate effective equations for other media so that the new maxwell equations are similar but modified, i.e. different scalar epsilon, mu, c or perhaps for birefringent crystals the epsilon and mu are genralized to matrices/tensors, or for nonlinear optics theres second and third order tensors... and those new effective equations are top and the fundamental is bottom... at least thats how I think most people would denote top and bottom... I kind of see what you mean, but I don't think top annd bottom on energy scales is very significant, I think the derivabilitiy of new effective constitutive equations/propeerties from fundamental equations/properties is more top vs bottom... but I agree, at a certain point its not a discussion of physics but just dictionary wars...

ssivark 8y ago

Not quite. When talking about bottom-up and top-down, one can talk about small to large scales in size (microscopic to macroscopic) or or can talk about small energies to large energies. The two descriptions are inversely related because small length scales correspond to large energies, and vice versa (roughly due to the Heisenberg uncertainty principle). Conventionally, when particle physicists talk about top-down and bottom-up, they are talking about energy scales: "top" is high energy, more fundamental, smaller distances and "bottom" is low energy, more emergent, larger distances.

DoctorOetker 8y ago
first, thank you very much for responding, but still I have the impression that bottom refers to the more fundamental perspective, and top refers to the more general perspective... i.e. the fundamental equations for electromagnetism are the maxwell equations with epsilon, mu and c for vacuum, and then one can use these as microscopic equation for other media and generate effective equations for other media so that the new maxwell equations are similar but modified, i.e. different scalar epsilon, mu, c or perhaps for birefringent crystals the epsilon and mu are genralized to matrices/tensors, or for nonlinear optics theres second and third order tensors... and those new effective equations are top and the fundamental is bottom... at least thats how I think most people would denote top and bottom... I kind of see what you mean, but I don't think top annd bottom on energy scales is very significant, I think the derivabilitiy of new effective constitutive equations/propeerties from fundamental equations/properties is more top vs bottom... but I agree, at a certain point its not a discussion of physics but just dictionary wars...