# Is there a general law for physics?

Here is why I pose this question, as I advance in physics more and more this idea haunts me and in fact helped me figure out and memorize many physical laws. It seems to me that there is a relation between all the laws of physics, something that is a general law that determines how should all other laws be generally stated. I can't express this law in a complete sentence and I'm not able to verify if it has an exception or not but I will try to explain this by examples to show what I really mean:

## Mechanics:

Oscillations: It seems that the whole process happens because the object is trying to get back into its initial state i.e the origin.

## Electricity:

Lenz's law: An induced electromotive force (emf) always gives rise to a current whose magnetic field opposes the original change in magnetic flux.

in other words: the current is trying to get its initial state by resisting the change of current.

## Atoms

Radiation and Nuclear reactions happen because the atom is trying to be stable.

Of course there are many other examples but I'm just stating few to make my point. The question is: is there something like that in physics? if not can you give me any counter example on this?

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The minimization of what is called the 'action' might be what you are looking for. This principle yields the equations of motion for whatever system the action describes, although I definitely wouldn't say it comes even close to describing all of physics. –  Danu Mar 1 '14 at 19:32
I don't get why is this question downvoted, it seems a good one after all. –  Mark Mar 1 '14 at 19:39

The principle of stationary action is what you're looking for.

You can construct a quantity called the Lagrangian, which is kinetic energy of the system, minus the potential energy of the system, namely:

$$\mathcal{L} = T-V$$

It is a function of position and velocity and for example, for a particle on a line, with a force acting on it, such that $F = -\frac{dV}{dx}$, you have $$\mathcal{L} (x,\dot{x})= \frac{1}{2}m\dot{x}^2 - V(x)$$ If this wasn't already abstract enough, the Lagrangian is important, because we're interested in its integral from time $t_1$ to $t_2$, namely:

$$\mathcal{A} = \int\limits_{t_1}^{t_2} \mathcal{L} (x,\dot{x}) dt$$ It is called the action, and it's the "thing" nature tries to minimize or, more precisely, to make stationary (meaning maximize or minimize). What does that mean? Well, it means that, for a particular system, nature chooses such a Lagrangian, which will give a stationary value when integrated between two fixed points.

So, as you might have guessed, the goal of the game is to find the Lagrangian which will minimize (stationarize?) the action.

The Lagrangian for a particle on a line is an extremely simple case, in general it doesn't have to be kinetic minus potential energy and, generally, it's also an explicit function of time, which means that some terms can depend on time not just through position and velocity depending on time.

How to get the equations of motion out of a Lagrangian? You use the Euler-Lagrange equations: $$\frac{d}{dt} \frac{\partial \mathcal{L}}{\partial \dot{q_i}} = \frac{\partial \mathcal{L}}{\partial q_i}$$ What's that $q$? Those are generalized coordinates, they can be Cartesian coordinates but they can be all sorts of different coordinates, whatever works the best.

Try out the equations on my example of a particle on a line.

You might be thinking, why the Lagrangian, what does that have to do with anything and how do we even get them? Well... mostly quantum mechanics and guesswork. After all, classical mechanics is only a limit of quantum mechanics and therefore it has to obey its underlying principles.

Although the Lagrangian is also used in quantum mechanics, there's an even more elegant concept, the Hamiltonian and Hamiltonian mechanics formalism, which basically sets the rules.

Bottom line, you can view it as this: $$\text{constructing a theory} \longleftrightarrow \text{finding the Lagrangian}$$

If you want a classical intuition for why is it kinetic energy minus the potential energy, you might want to read the article "Gravity, Time, and Lagrangians", Huggins, Elisha, Physics Teacher, v48 n8 p512-515 Nov 2010.

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Could you explain how is this related to nuclear decay? –  jinawee Mar 1 '14 at 21:12
I don't know enough to explain the details and I suspect it would take several thousand pages to explain them, but it all boils down to a similar story. Nuclear decays are the result of nuclear forces, which are described by the Standard Model of particle physics. But you can view the Standard Model itself as a Lagrangian. (en.wikipedia.org/wiki/…) –  Schlomo Mar 1 '14 at 21:22
Here is another good introduction to the Lagrangian and action. damtp.cam.ac.uk/user/tong/dynamics.html –  mmesser314 Mar 2 '14 at 4:10
@jinawee processes that create or destroy particles can be described by Lagrangian mechanics but you have to replace the particles by fields and the Lagrangian by a Lagrangian density. The result is quantum field theory. –  Brian Bi Mar 2 '14 at 4:55

Well, there is at least a principle which states that system "tries" to minimize it's energy.

As was also mentioned in comment above, in general I believe there is just Principle of least action, from which one can also derive conservation laws which are neccessary consequence of Lagrangian symmetry - Noether's theorem. (Action is integral of a Langrangian over time).

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Although there's no consensus, I think that the vast majority of my physics colleagues would answer your question with a "yes", but acknowledging that we don't yet know for sure what that general law (jocularly referred to as the "Theory of Everything" or ToE) is. The above-mentioned principle of least action is a good start, but it's not the final word, since it's for classical physics only. It can be elegantly incorporated into quantum mechanics using a mathematical tool known as the path integral, but we physicists as a community have still not succeeded in unifying quantum mechanics with Einstein's general relativity into a theory of quantum gravity that we can fully understand and test against experiment. String theory and loop quantum gravity are two popular competing contenders, but the jury is still out.

Schlomo Steinbergerstein mentioned a stone-age paper of mine discussing these issues, but I discuss all this in much more detail in my new book http://mathematicaluniverse.org

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It's still a very interesting paper, but I do have to read that book! :) –  Schlomo Mar 2 '14 at 14:51

No. Otherwise physics would have been nothing more than application of mathematics.

The closest to general laws is the summery. The conservation laws come from symmetry of different kind. Google for symmetry and momentum conservation for example.

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No. In your examples you imply a will of objects for stability. There is no such will and the examples are select by the fact they show stability.

Have a look at the second law of thermodynamics (I name it Death)...

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The second law of thermodynamics is not a fundamental law in the strict sense. It's there because of the conditions right after the Big Bang, it's basically a statement that time flows in a certain direction. We still haven't figured out why is that so, until then it's more of an empirical law, a fact about our observable universe. –  Schlomo Mar 1 '14 at 22:48
I'm not sure what to make of this comment. Observability seems to be a good thing to support a theory. And it is not that it wasn't derived some time ago in university using statistical mechanics. But that's not the point. The question is more of a philosophical one. You see a pattern for once, and you look for proofs. State the number 5 is holy and you'll see it everywhere. –  Lord_Gestalter Mar 1 '14 at 23:11
My point is this: en.wikipedia.org/wiki/T-symmetry –  Schlomo Mar 1 '14 at 23:26
My point is: Don't interpret physics philosopically. Please prove the second law of thermodynamics to be wrong by building a perpetuum mobile (2nd kind will do). Feel free reversing the time to do so ;-) –  Lord_Gestalter Mar 1 '14 at 23:48
It's not philosophy, it's theoretical physics. 2nd law of thermodynamics isn't wrong and nobody can prove it wrong. But it isn't strictly speaking, a "law", it doesn't arise as a mathematical consequence, it's postulated, either explicitly or via assumptions about time. It's a form of an empirical statement. I can postulate anything and put it in mathematical form and have my experiments support it, but it's not a strict law if it's not somehow a consequence of the most fundamental laws, roughly speaking. No philosophy, just physics. –  Schlomo Mar 2 '14 at 1:07

I think the most real general law in our current physics is: physics strongly favors behavior that can accurately and reliably be described by mathematics. There is no particular reason why the symmetry-breaking model at the top of this site should be a useful description of something that has actually happened, might have left comprehensible evidence of its occurrence, and could be coherently discussed as a sequence of causally linked events, except that reality seems to actually work that way.

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This is the sort of vacuous argument Max Tegmark has been pushing lately. –  Brandon Enright Mar 1 '14 at 23:36
Actually, Tegmark was much more precise, check out his paper arxiv.org/abs/gr-qc/9704009 Not as vacuous as you might expect! –  Schlomo Mar 2 '14 at 1:14