Hacker News Reader

Comment by dr_dshiv | Hacker News Reader

And yet he totally shuts down the question of why positive and negative charges attract… which I’ve always been dying to understand.

https://www.physicsforums.com/threads/why-do-positive-and-ne...

View on HN

Change your reference frame. Assume both charges are moving at constant speed != 0 in the same direction. From this view they are not charges but opposite currents. We know that opposite currents repel [1]. Therefore, one could conclude that opposite charges repel. It's this kind of question that the interviewer should ask.

[1] https://www.quora.com/Why-do-two-wires-with-current-flowing-...

The difficulty is that eventually you reach a level where there are no satisfying answers to this sort of question. There are a lot of interesting levels to explore between “why did aunt Minnie go to the hospital” and fundamental physical principles, but for things like “why do magnets repel?” (or “why do unlike charges attract?”), these questions are getting pretty close to the point where the only answer is “we don’t know, that’s just what we see happens when we watch the universe work.”

You can go farther and get stuff about the action in quantum electrodynamics, but these don’t really provide any more satisfying answers, they just lead to more questions like “why do systems follow trajectories of stationary action?” and “why is the lagrangian for electromagnetism what it is?”

But how do you know where the limit is? Isn’t it a bit like asking why the planets move in epicycles and the response being “it’s just the way it is”?

Yeah, and I don’t mean to suggest that there are no further levels to discover and understand or more fundamental properties to explain the ones we know, just that there is a limit to what we know, and that at some level things which are experimentally observed are assumed. IMO these are always excellent questions, and there are people looking to find explanations and more fundamental physics, but this is increasingly difficult as current models explain current observations very well and it is hard to go further with experiment.

Even if there are further levels to be understood, those probably won’t lead to any more satisfying answers, just a swap of one axiom to ponder with a more fundamental one.

I think it is like asking about epicycles prior to Kepler, or asking about Kepler’s laws prior to Newton (and on). There are new and better explanations, but always more questions than answers, and some things that just are (as far as we know).

You mention stationary action.

Interestingly, F=ma and Hamilton's stationary action are mutually derivable.

The usual presentation is to show that Hamilton's stationary action implies the newtonian formulation. Interestingly, it is also possible to start with F=ma, and move in all forward steps to Hamilton's stationary action. No additional assumptions are required.

I created a series of interactive diagrams to make Hamilton's stationary action entirely transparent.

http://www.cleonis.nl/physics/phys256/energy_position_equati...

(Of course, quantum mechanics is the overall deeper theory; we assume that classical mechanics holds good because classical mechanics is a limiting case of quantum mechanics; at scales large enough that quantum effects average out the outcome in terms of quantum mechanics converges onto the outcome in terms of classical mechanics.)

This boils down to what magnetism is. My favorite explanation is the Kaluza-Klein hypothesis that explains magnetism as an emergent effect of space geometry. If this explanation is accepted, we still need to answer what causes things to move forward and "execute" these maxwell/KK equations. And that's a great mystery.

Reply on news.ycombinator.com