# Meaning of Lagrangian

1. May 15, 2006

### gulsen

I've been thinking on it for a while, and can't find a satisfying argument. What would L=T-V mean physically? And what would it's time integral mean? What (physically what) are we minimizing?

Following Feynman, it should be explained at freshmen level if we have understood it.

2. May 15, 2006

### lalbatros

Gulsen,

First, what's the meaning of "meaning"?
Do you want to understand how to use it?
Do you want to know why is physics like that?

By analogy, how would you answer the question "what is energy"?
Is energy a conserved quantity: that's not enough to define it.
Is energy the quantity conserved for a time-shift symmetric system: that's enough to define it, but are you happy with this meaning?

Note here that the Lagrangian is not a conserved quantity like energy.
Its meaning doesn't appear in a conservation law.
Rather, it appears in a "variational law" called the "least action principle".
Physics can be described (partly) by conservation principles: energy, momentum, ...
Alternatively, it can be described by (more general) variational principles: just as if nature was based on a kind of 'economical' principle.

I think that the meaning of a concept in physics lies only in its relation to other concepts. There is no "out of the box" understanding that would provide an all-encompassing meaning. There are still irreductible concepts.

Nevertheless a "strong meaning" to the lagragian is given by its relation to the action and the relation of action with quantum mechanics. The action is the quantity that is extremised by the actual path of particule in between two points. This was discovered a long a long time ago and derived from the more 'pictural' basis of classical mechanics (usual forces and Newton laws). It is striking that this was discovered much before quantum mechanics was discovered, a sort of scientific miracle.

You will find easily on the web the link between the least action principle and quantum mechanics. As a matter of fact, the principle of least action results from quantum mechanics approaching classical mechanics when the wavelengths of the wave-functions become 'small' (the object becoming "large"!). We may call that a meaning, since this shows that the Lagragian is related to the phases of the wavefunctions when wavelengths become very small.

But the best reading for that is Feynmann ...

But note an even more striking fact: variational principles don't only explain the structure of classical mechanics by its quatum legacy, it appears that variational principles are also within the structure of quantum mechanics itself.

And now a more speculative point of view:

Row Frieden speculated that the appearance of variational principles everywhere in physics has a fundamental reason: the information that can be obtained on a system need an interaction. Therefore, information cannot be considered as independent from physics itself, and this is reflected in the law of physics by the variational principles. It should be no surprise then that Roy Frieden gives a new meaning to the Lagrangian and also to the two terms of the Lagragian. Unfortunaly I got the impression that this new theory doesn't contribute yet to physics, it is more like an interpretation.

But is "meaning" not a synonim for "interpretation" ?

Last edited: May 16, 2006