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Why is Maxwell's theory so hard to understand? (2007) [pdf]

damtp.cam.ac.uk

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Re: Why is Maxwell's theory so hard to understand? (2007) [pdf]

#111
post #48
post #16

Earlier quoted context omitted.

>"E&M is far from "simple". It contains special relativity, for starters." A theory (which is just a set of assumed first principles and rules of logic) can be simple but allow you to deduce vast complexity from it. In fact, it is ideal for a theory to be as simple as possible. The "game of life" is not really a theory, but it demonstrates that simple rules can lead to surprising complexity: https://en.wikipedia.org/…

I reflexively downvote the game of life. I have never heard of a single useful application or analogy of “finite automata.” Yes, there are speculations about extremely small Planck lengths, by no new physics has panned out. Finally I am sad that one of the greatest computer scientists and polymaths, Ed Fredkin, got sucked into this. We all have weaknesses.

It’s remarkable how many silent downvotes you get when you tweak something that people think is cool. C’mon guys. Refute what I said.

Re: Why is Maxwell's theory so hard to understand? (2007) [pdf]

#112
post #48

Earlier quoted context omitted.

I reflexively downvote the game of life. I have never heard of a single useful application or analogy of “finite automata.” Yes, there are speculations about extremely small Planck lengths, by no new physics has panned out. Finally I am sad that one of the greatest computer scientists and polymaths, Ed Fredkin, got sucked into this. We all have weaknesses.

Surely you mean cellular automata, not finite automata.

Yes, you are right. My ignorance shows, but I am still hanging on to my opinion of cellular automata.

Re: Why is Maxwell's theory so hard to understand? (2007) [pdf]

#113
post #92
post #90

Earlier quoted context omitted.

So what are the equations for those two charges?

For any number of particles, the equation for each of them is this one here: https://en.wikipedia.org/wiki/Covariant_formulation_of_class... Edit: Coupled differential equations. If I have an equation for x in terms of y, and one for y in terms of x, then in total I have a set of equations for x and y. e.g.: dx/dt = y dy/dt = -x then the solution is x = C e^(i t) y = i C e^(i t) with C a constant determined by the in…

As a depressed MIT freshman I modeled my mood as a pair of coupled equations. I got growing oscillations!

Re: Why is Maxwell's theory so hard to understand? (2007) [pdf]

#115
post #14

E&M is far from "simple". It contains special relativity, for starters. Also it is incompatible with thermodynamics: solving this problem is why Planck invented quantum mechanics. Also point charges have infinite energy: this problem leads to renormalization theory. Also it introduces gauge invariance, an essential but complex part of all modern theories. And lastly, the mathematics of E&M is a big step up from Newto…

special relativity is also simple, once again its the differential equations that provide a difficult explanation.

Re: Why is Maxwell's theory so hard to understand? (2007) [pdf]

#116
post #16
post #14

E&M is far from "simple". It contains special relativity, for starters. Also it is incompatible with thermodynamics: solving this problem is why Planck invented quantum mechanics. Also point charges have infinite energy: this problem leads to renormalization theory. Also it introduces gauge invariance, an essential but complex part of all modern theories. And lastly, the mathematics of E&M is a big step up from Newto…

>"E&M is far from "simple". It contains special relativity, for starters." A theory (which is just a set of assumed first principles and rules of logic) can be simple but allow you to deduce vast complexity from it. In fact, it is ideal for a theory to be as simple as possible. The "game of life" is not really a theory, but it demonstrates that simple rules can lead to surprising complexity: https://en.wikipedia.org/…

you are being downvoted because of a problem with the nature of HN echo chamber being a symptom. if the voting/karma system was improved we wouldnt have problems with this kind of downvoting

Re: Why is Maxwell's theory so hard to understand? (2007) [pdf]

#117
post #91
post #61

Earlier quoted context omitted.

E&M is simple in the sense that the equations that govern the relationship between the charge distribution and the electromagnetic field are quite simple. And the equations governing how charges move in an electromagnetic field are quite simple. I think this is what Dyson has in mind. Understanding the implications and limitations of those equations is not simple at all. An even more blatant difficulty with E&M, rela…

> Given the trajectories of charges it will tell you what the electromagnetic field will be, and given the electromagnetic field it will tell you how charges will move. Unfortunately, these two parts of the theory seem to be incompatible, and the theory will not tell you how fields + charges will evolve in time. The Problem is not coupling charged matter fields to the EM field. You get a well defined set of coupled a…

what you expounding upon is [1] of the indicators that we dont completely understand physics at the "point charge" scale, and very possible there is no such thing as a point charge, rather there is a centroid of field intensty/probability I.E. a wave function. point charges are likely an overly simplified view, and artefactual convienience of extrapolation.

Re: Why is Maxwell's theory so hard to understand? (2007) [pdf]

#118
post #91

Earlier quoted context omitted.

> Given the trajectories of charges it will tell you what the electromagnetic field will be, and given the electromagnetic field it will tell you how charges will move. Unfortunately, these two parts of the theory seem to be incompatible, and the theory will not tell you how fields + charges will evolve in time. The Problem is not coupling charged matter fields to the EM field. You get a well defined set of coupled a…

what you expounding upon is [1] of the indicators that we dont completely understand physics at the "point charge" scale, and very possible there is no such thing as a point charge, rather there is a centroid of field intensty/probability I.E. a wave function. point charges are likely an overly simplified view, and artefactual convienience of extrapolation.

The problems in the classical theory are easily understood. The charge and current densities of point particles are not smooth functions but distributions (think of the Dirac δ-“function”). If they act as the sources of the EM field the EM field itself becomes singular. Now if you try to solve the full Maxwell equations including the backreaction of matter & radiation fields you would have to multiply distributions which is ill defined.

There are similar problems in the quantum theory but the divergences are less severe and can be dealt with in a systematic way. Most physicist believe they will totally disappear in some more fundamental underlying theory. From a mathematicians point of view there is the hope that at least some QFTs are finite and the divergences are just an artifact of the construction & pertubation theory.

Re: Why is Maxwell's theory so hard to understand? (2007) [pdf]

#119
post #24

Earlier quoted context omitted.

If LIGO confirmed "aether", then the measurements in the perpendicular arms would vary depending upon time (orientation of Earth in rotation, orientation of Earth around Sun, etc.). As far as I know, they very much do NOT vary--to an absolutely amazing precision. As far as I can tell, if the "aether" existed and we could detect it, we basically would have no hope of detecting gravitational waves. > Naively, spacetime…

But the arms are predicted to vary in those cases by the same theory that LIGO is confirming: our rotation causes frame dragging, and there's some weird Sun-Earth orbital relativistic effects as well, right? I know the Sun-Jupiter orbit produces a large portion of the estimated 5,000 watts of gravitational emissions given off by our solar system. My mental model of this is that it perturbs the aether ("spacetime") th…

I just noticed this:

> our rotation causes frame dragging

Not exactly; it's the choice of accelerated coordinates and pretending that the coordinates are freely-falling that manifests seemingly odd coordinate-dependent physical effects. One can resolve these by switching to freely-falling coordinates, or by not pretending that the accelerated coordinates are freely-falling. In practice this means doing Special Relativity calculations only in the Special background of the theory, and using the full covariant physics otherwise.

One can treat the Lense-Thirring effect as a distortion of a Special Relativistic background (or a different static background, like Schwarzschild), or one can determine the actual background from the distribution of stress-energy.

The first choice leads one to conclusions like the precession of test objects in polar orbits around axisymmetric bodies with nonzero angular momentum, and applying that to Earth and a satellite.

The second choice is more work, because angular momentum and axisymmetry are only two of the departures from Schwarzschild in the system. Crucially the contributions from surface bumps, masscons, the distribution of matter in the satellite, and so on are all small enough that it's fair to ignore them in the second case.

The difference is that the second case simply correctly calculates out the geodesic the satellite occupies in the real spacetime, while the latter calculates out an orbit that is simply wrong for the real configuration of the system and then applies corrections by bringing in pseudo-forces.

In the weak field limit, one can get completely correct coordinate-dependent results taking the former approach. Additionally, analogies arise in this approach that can lead to interesting intuitions. However, one should always check to make sure that the intuitions can be explained in terms of coordinate-independent formulations of physics.

> weird Sun-Earth orbital relativistic effects

If you calculate out the geodesics, no; all the parts of Earth down to its individual molecules and below "want" to travel on geodesics sourced by the system and do so unless the stronger three forces interfere with that "want" (and in bulk Beiglböck and Dixon show that the Earth has a coordinate-independent centre-of-mass that does travel on a timelike geodesic, and you arrive at it by considering the vector position of each particle).

This is a lot of work.

So instead, you can use a background like Kerr (or Schwarzschild or Minkowski) and correct the failure of the parts of the Earth (or the planet in the large) to travel on the geodesics of these backgrounds. The correction is typically done by introducing pseudoforces, pseudofields, and dynamics and potentials in these. They're pseudo because they vanish entirely in at least one frame of reference. (see https://en.wikipedia.org/wiki/Fictitious_force which is pretty decent).

> gravitational aether

One could do perturbative General Relativity by choosing Minkowski (flat) spacetime as the background \eta [0], and then recovering all the real motions of objects in a perturbation field h. That field, h, is a pseudofield with its own potentials and dynamics. It's just a set of first-order corrections to the background metric. The real metric g will be an expansion: g = \eta + h + O(h^2) + O(h^3) + ... where O(h^x) are higher-order correcting terms, and those are negligible for systems where stresses are weak and speeds are slow compared to c.

It is reasonable to think of h as having the properties of an aether in some cases: it can walk and quack like a real field, like the electromagnetic one in Maxwell's theory. However it can also be made to vanish entirely without removing matter and the gravitational potentials they source, and it is a pseudofield in the more fundamental theory of General Relativity. The Maxwell electromagnetic field is always present (you can only get rid of it by removing all charges and potentials), and obviously related fields are still there in the more fundamental theories of QED and the Standard Model (a QFT).

The specific theory of the luminiferous aether theory that Michelson-Morley surprised themselves by falsifying was that the aether was stationary and the Earth moved through it. There were comparable theories of a gravitational aether (https://en.wikipedia.org/wiki/Mechanical_explanations_of_gra...) that have never recovered many fairly easily observed behaviours of gravitating masses, so there was never really a surprisingly failed direct test of them.

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[0] More generally, as someone raised this with me before, one can write g^{(0)}_{\mu\nu} for an arbitrary choice of background instead of abusing the notation \eta_{\mu\nu} which (out of context) generally means only the Minkowski metric. I leave out the greek-lettered indices above.

Re: Why is Maxwell's theory so hard to understand? (2007) [pdf]

#120

Earlier quoted context omitted.

But the arms are predicted to vary in those cases by the same theory that LIGO is confirming: our rotation causes frame dragging, and there's some weird Sun-Earth orbital relativistic effects as well, right? I know the Sun-Jupiter orbit produces a large portion of the estimated 5,000 watts of gravitational emissions given off by our solar system. My mental model of this is that it perturbs the aether ("spacetime") th…

By aether, do you mean an underlying medium that allows forces to be propagated? I guess that using that definition, there is an 'aether'. The difference with the rejected concept of aether is that this "aether" is deformable by gravity, whereas the rejected one is not.

> underlying medium that allows forces to be propagated

General Relativity is a local theory concerned with the mechanisms that generate the metric, the geodesics implied by the metric, and the coupling of objects to those geodesics.

The relevant forces are those which accelerate objects into non-geodesic motion (or boost them from one geodesic to another). Those are local[1] as well: electromagnetism and the nuclear interactions. There's nothing like a luminiferous aether or underlying medium in the Standard Model, even if you look funnily at the gauge bosons -- they obey Lorentz covariance.

I made a couple of sibling comments to yours, one of which deals directly with your comment's parent's idea of a gravitational aether.

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[1] RM Wald, _Quantum Field Theory on Curved Spacetime and Black Hole Thermodynamics_ (University of Chicago Press, 1994). Cf. the top of page 6 of Hollands & Wald 2014, https://arxiv.org/abs/1401.2026

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