# Relativistic Lagrangian

1. Nov 11, 2007

### fikus

How do we explain the fact that if we want to get Hamilton's equations out of Lagrangian:
$$L=-mc\sqrt{(\dot x^\mu \dot x_\mu)}$$,
where dot is noting derivative in proper time, we get that Hamilton's function equals zero ?
That same Lagrangian of course gives right equations of motion in Lagrange formulation.

Thanks

2. Nov 12, 2007

### pervect

Staff Emeritus
I'm not sure how to summarize the issue in a post, but it's talked about in "Goldstein, classical mechanics" pg 326, section 7-9, "Covariant Lagrangain formulations".

I suppose the shortest summary would be

3. Nov 12, 2007

### country boy

My copy of Goldstein may be old, but in it the section you refer to is 6-6 on page 207. The footnote on page 209 deals with the relativistic Lagrangian. The key is that the square root (equal to c and thus a constant) has a functional dependence on the velocities u that, through the variational calculus, gives the equations of motion.

4. Nov 12, 2007

### shoehorn

The answer is actually very, very simple. The Lagrangian is invariant under reparametrizations along the world line which preserve the parameter values at the endpoints.. A simple consequence of this is that the Hamiltonian is identically zero.

The same sort of thing pops up all over physics but is, frustratingly, rarely emphasised in courses in analytical mechanics.

5. Nov 12, 2007

### pmb_phy

I disagree. I calculated Hamilton's function here

http://www.geocities.com/physics_world/sr/relativistic_energy.htm

Set U = 0 and you'll get the same Hamiltonian function that you do. The Hamiltonian function in this case is merely the kinetic energy of the particle plus its rest energy. It is not zero as you claim it is.

If you believe that there is an error in my web page then please point it out. I always appreciate people finding errors in my website. It is of great use since in that way people are catching those errors that I became blind to in the derivation. They are extremely few though.

Pete

6. Nov 13, 2007

### pervect

Staff Emeritus
It's already been pointed out that your webpage disagrees with Goldstein. I'm not sure what more you need? Offhand, I'd expect this is a case of working out the answer to a different (though similar) problem than the OP was asking about, as one can certainly get a relativistic energy function (h) and hence a Hamiltonian (H) that is nonzero by parameterizing with coordinate time rather than proper time.

Last edited: Nov 13, 2007
7. Nov 13, 2007

### jostpuur

You mean
$$\dot{x}^{\mu} = \frac{1}{\sqrt{1-|v|^2/c^2}}(c,v)^{\mu}?$$

Then we have
$$L=-mc\sqrt{\dot{x}_{\mu}\dot{x}^{\mu}} = -mc^2$$

This looks quite useless Lagrange's function.

Wouldn't
$$L=-mc^2\sqrt{1-|v|^2/c^2}$$
be a better Lagrange's function?

8. Nov 13, 2007

### shoehorn

Apologies, I forgot to point out in my reply above that the OP's Lagrangian
is (obviously) incorrect. Taking units where $c=1$ one has

$$L=-m(1-\dot{x}_{a}\dot{x}^{a})^{1/2}.$$
If one introduces some parameter $s$ along the world-line, it's
obvious that one has a constraint of the form

$$\tilde{\mathcal{H}}=\pi_{t}+(\pi_{a}\pi^{a}+m^{2})^{1/2}=0,$$

where $\pi_{a}$ and $\pi_{t}$ are the canonical momenta. It's then
trivially easy to show that the constraint $\tilde{\mathcal{H}}=0$
is actually equivalent to the Hamiltonian

$$\mathcal{H}=\pi_{\alpha}\pi^{\alpha}+m^{2}=0.$$
The real reason why the Hamiltonian vanishes identically is because
the canonical action can be expressed in a form which is invariant
under reparametrizations of the path which preserve the values of
the parameter at the endpoints. This is a very, very well known result
in analytical mechanics and is deeply related to, for example, Jacobi's
principle and Dirac's analysis of constrained Hamiltonian systems.
As I said earlier, any decent course in analytical mechanics will
labour this point ad infinitum.

9. Nov 13, 2007

### pmb_phy

I have no recollection of such a comment and I see no inconsistency between those calculations and Goldstein/Lanczos. If you can't point to an error yourself and give a page number etc. then please stop making these unfounded erroneous claims. Even so, just because something disagrees with Goldstein doesn't mean that its wrong. Perhaps you're refering to the relationship between U and V. If so then that comes from Lanczos. Its too bad that you still make claims rather than simply ask first.

Note: The energy function h in my link above has the same value as the Hamiltonian. The only difference is that h is a function of generalized position and generalized velocity while the Hamiltonian is expressed as a generalized position and canonical momentum. There is a slight difference between that page and Goldstein in that I liked some of what Lanczos has done and have encorporated that into my page.

Pete

Last edited: Nov 13, 2007
10. Nov 13, 2007

### jostpuur

Have I understood something wrong when I get
$$\dot{x}_{\mu}\dot{x}^{\mu} = c^2?$$

I think this is a mistake. The dot is not supposed to be a derivative in proper time, but just in the local time of the chosen arbitrary frame. The we get
$$\dot{x}_{\mu}\dot{x}^{\mu} = c^2 - |v|^2$$

..... I immediately assumed, that the proper time is the proper time of the particle. Isn't that the common terminology?

Last edited: Nov 13, 2007
11. Nov 13, 2007

### pmb_phy

You've got it correct so no worries.
You got it right the first time. The dot is with respect to proper time, and not coordinate time.
Yes. That is correct.

Pete

12. Nov 13, 2007

### jostpuur

But isn't it the

$$-mc^2\sqrt{1-|v|^2/c^2}$$

that is the free particle Lagrange's function, and not

$$-mc^2?$$

Last edited: Nov 13, 2007
13. Nov 13, 2007

### pmb_phy

Hmmmm! You have a valid point there. Let me think about it. In the mean time check out an application of the Lagrangian at

http://www.geocities.com/physics_world/gr/geodesic_equation.htm

for geodesic motion. You can use proper time for the affine parameter $\lambda$.

Pete

14. Nov 13, 2007

### jostpuur

If we start with the Lagrange's function

$$L=-mc^2\sqrt{1-|v|^2/c^2}$$

then the Hamiltonian is

$$H = \frac{\partial L}{\partial v^i} v^i \;-\; L = \frac{m|v|^2}{\sqrt{1-|v|^2/c^2}} \;+\; mc^2\sqrt{1-|v|^2/c^2} = \frac{mc^2}{\sqrt{1-|v|^2/c^2}}$$

I'm very confused about the claim that the Hamiltonian should be zero.

15. Nov 13, 2007

### pervect

Staff Emeritus
The problem is that you (and probably Pete) are working a different problem than the OP is.

I assume you don't have Goldstein. (I'm surprised Pete doesn't, though, or that if he does, he doesn't just look it up).

I can quote some of the relevant sections, but the entire explanation is just too long to type in.

Here's some of the motivation:

Note that the OP's expression for the Lagrangian is Goldstein's eq (7-162) in the second edition (for those people who have this textbook), and was derived by transforming the action intergal over t from one over an arbitrary parameter $\theta$.

Goldstein also goes on to point out that (using slightly different notation)

$$L = \frac{1}{2} m u_i u^i$$

works correctly as a Lagrangian, which differs by not having a minus sign, and being a quadratic expression, and that there are an infinite number of other possibilities.

Yep.

Last edited: Nov 13, 2007
16. Nov 13, 2007

### andrewj

a point not gonna work in a holographic universe

17. Nov 13, 2007

### jostpuur

I don't have it.

So I'm lost with the notation or something else. One step at the time: Was this right?

$$\dot{x}^{\mu} = \frac{1}{\sqrt{1-|v|^2/c^2}}(c,v)^{\mu}$$

18. Nov 13, 2007

### pmb_phy

No. You think Goldstein disagrees with my web page. But you are most certainly wrong. In fact you never really quoted Goldstein. You gave your version of what you thought a summary should look like. However there is nothing in that part of Goldstein which corresponds to your claim This is not a phenomenon of relativistic physics per se: .

Nothing in Chapter 7 disagrees with my web pages of this. In fact I learned a great deal of relativistic Lagrangian mechanics from Goldstein.
I don't need anything. You're making unfounded assertions which you are unable to back up and thus you have deluded yourself to thinking that you made a meaningful statement which wasn't very meaningful or useful at all.

I'll tell you what you need though. You to pay more attention to what you're reading. Since you're referring to the second edition of Goldstein (which you really should have stated which version you're using. I have all three, you seem to have the second edition while Country Boy has the first edition) then, if you had actually checked my web site against the relevant pages/equations of Goldstein then you'd see that the Lagrangian Goldstein gives in his 2nd Ed. in Eq. (7-136) is identical to the Lagrangian as I wrote it, i.e. L = K - U, in the first sentance of

http://www.geocities.com/physics_world/sr/relativistic_energy.htm

where after Eq. (1) I wrote where K = $-m_0 c^2\sqrt{1-\beta^2}$ and U is a function of both position and velocity (in all generality that is). The Hamiltonian for this system is not zero.

The answer to the OP's question is that there are two Lagrangians used in relativistic Lagrangian mechanics. One from the non-covariant form of the equations and one from the covariant equations. I created another page somewhere else to explain how this is done from both ways, i.e. by the non-covariant method and by the covariant method. However I couldn't find it this afternoon. I found it a little while ago an it appears that the page has been corrupted. Since I'm rebuilding the entire website I may have deleted some equation GIFs which are missing. The page is at

http://www.geocities.com/physics_world/em/relativistic_charged_particle.htm

I will attempt to restore it tonight. In the future please don't paraphrase and please post the exact equation numbers in your source and in my pages which you are criticizing.

Pete

Last edited: Nov 13, 2007
19. Nov 13, 2007

### pmb_phy

I'm just being nitpicky here, however that is not the Hamiltonian. You'd have to have that function in terms of position and momentum.

Pete

20. Nov 14, 2007

### shoehorn

This is most certainly not the answer to the OP's question. As I have pointed out several times in this thread, the answer to his question is the ability one has to cast the action in a particularly special reparametrization-invariant form. Once again: this is a point of central importance in analytical mechanics and in physics more generally since it is closely related to our freedom to construct explicit, rigourous examples of gauge theories.

And by the way, your claim that there are "two Lagrangians" used in relativistic Lagrangian mechanics is absurd. For each two such Lagrangians you give me, I, and everyone else, can give you an infinite number of equivalent Lagrangians.

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