# Insights Relativistic Work-kinetic Energy Theorem - Comments

1. Aug 23, 2015

### bcrowell

Staff Emeritus
2. Aug 23, 2015

### Jano L.

I have trouble seeing what is the problem you're trying to solve. The relation $W=Fd$ is definition of work, not the work-energy theorem. The work-energy theorem says work equals change in kinetic energy of the particle. This follows mathematically from the equation of motion $md(\gamma v)/dt = F$ and Einstein's definition of energy $E=\gamma mc^2$.

3. Aug 23, 2015

I see the $way of writing formulae does not work well, so here's my comment in plaintext: I have trouble seeing what is the problem you're trying to solve. The relation W=Fd is definition of work, not the work-energy theorem. The work-energy theorem says work equals change in kinetic energy of the particle. This follows mathematically from the equation of motion md(γv)/dt=F and Einstein's definition of energy E=γmc^2. 4. Aug 23, 2015 ### pervect Staff Emeritus I'm not too sure my thoughts will be helpful for the intended purpose (which I assume ultimately involves a simple presentation of special relativity), but perhaps they'll provide some insight. And they can be expressed pretty briefly. As long as special relativity has a non-trivial Hamiltonian, we can write Hamilton's equations: $$\frac{\partial H}{\partial q} = \dot {p}$$ Now, if we can also identify H with energy, q with position, and$\dot{p}$with force, then we have the work-energy theorem, the rate of change of the energy with position must be equal to the force. 5. Aug 23, 2015 ### bcrowell Staff Emeritus Right, I should have notated this as$\Delta E=Fd$. There are two disadvantages to your method. (1) In the 1905 paper, Einstein uses$\Delta E=Fd$to prove$E=m\gamma c^2$, so he can't use the latter to prove the former. (2) Your method doesn't work for massless particles. 6. Aug 23, 2015 ### Jano L. I do not know what Einstein was getting at there. I am not sure proving the relation$E=\gamma mc^2$makes sense. I think this is just a definition of energy, same as$\frac{1}{2}mv^2$is definition of kinetic energy in classical mechanics. 7. Aug 23, 2015 ### bcrowell Staff Emeritus This is an interesting approach, but if we examine it carefully I think it's not quite as much of a slam-dunk as it might seem. And it does have the disadvantage that it won't work as a presentation at the undergraduate level, since a physics major might not be exposed to Hamiltonian mechanics until after taking their upper-division SR course. Spelling out the steps in more detail, I think we would have the following: (1) The action has to be Lorentz-invariant and additive, and the only possibility that seems to present itself is$S=(\ldots)m\int_{t_1}^{t_2} d\tau$, where ... represents a constant. (2) Working backward from this, infer that the Lagrangian for a free relativistic particle in one dimension is$L=(\ldots)m/\gamma$. (3) Find the conjugate momentum, which is$p=(\ldots)m\gamma v$. (4) Interpret$dp/dt$as a force. (5) Calculate the Hamiltonian. (6) Identify the Hamiltonian with the energy. (7) Use Hamilton's equations to associate$\partial H/\partial x$with minus the force. The first thing to note is that this is much, much longer than the simple derivation I gave (the second of the three sections in my original post). The next problem is that we have foundational issues in steps 1 and 4. Maybe there's an explicit uniqueness theorem one could give at step 1? Otherwise it's just a plausibility argument, with no a priori guarantee, for example, that the final result will come out consistent with Maxwell's equations in the case of a charge moving in a field. In step 4, we have to decide what is the best definition of force. One could argue that this definition is good, because by the time we get to step 7 we will have shown that it preserves the form of the work-energy theorem without correction. But this is a weak justification if we aren't at step 7 yet, and in fact we have a different definition of force, the four-force, which a priori would be preferable because it's tensorial. At step 6, we have to check some technical criteria. A general philosophical objection to the whole thing is that Hamiltonian mechanics lacks manifest Lorentz invariance at every step of the way. In particular, time is treated as a parameter rather than a coordinate. 8. Aug 23, 2015 ### bcrowell Staff Emeritus Definitions are like babies. They have to come from somewhere. Einstein is proving that this equation is the only one for a massive particle that is consistent with Maxwell's equations. In any case, this is not a definition of energy, since it doesn't apply to massless particles. 9. Aug 25, 2015 ### exponent137 In Classical mechanics we do not need$E=Fd$, that we can calculate$E\propto v^2$. The same is valid in SR. Thus I agree with BCrowell and disagree with Jano L.$E\propto v^2## can be obtained with help of quadratic forms.

Best regards

10. Aug 26, 2015

### vanhees71

Inspired by this thread and bcrowells remark in #8 on "massless particles" (which don't make too much sense in classical (i.e., non-quantum) relativistic physics anyway), I've extended the section on "naive dynamics" (i.e., dynamics not employing Hamilton's principle of least action) in my SRT writeup by giving a derivation of the work-energy theorem and also a formulation of the dynamics of massless particles:

http://fias.uni-frankfurt.de/~hees/pf-faq/srt.pdf

11. Aug 26, 2015

### bcrowell

Staff Emeritus
Why?

12. Aug 27, 2015

### vanhees71

Very simple: There are no classical massless particles, at least I've never heard that one has observed them yet. This makes much sense from the point of view of quantum field theory. The only candidate that comes to mind within the standard model would be a photon, but this is the quantum (one-particle Fock state) of the electromagnetic field, which is a massless spin-1 field and thus necessarily a gauge field, and you cannot define a position observable in the strict sense. So it is hard to imagine, how to get a classical particle limit. The classical limit is that of classical electromagnetic fields, i.e., coherent states with a large average photon number rather than classical point particles.

The other massless fields in the Standard Model are the gluon fields, but the corresponding quanta, the gluons, are confined, so one cannot expect to find them as particles in the sense of a classical limit.

The very last possibility would be that one of the neutrinos is in fact massless. It's hard to say at the present empirical status. If it would exist, it may be a candidate for something that could make sense as a classical approximation in the sense of a massless particle.

13. Aug 27, 2015

### bcrowell

Staff Emeritus
I don't think #12 is the same as saying that massless particles don't "make sense" classically. In any case, we deal with massless particles constantly in classical relativity, and we do it in practical contexts that relate directly to observation.

Last edited: Aug 27, 2015
14. Aug 27, 2015

### vanhees71

Can you give an example where one does this? Anyway, there's of course nothing wrong with it from a theoretical point of view. You can have perfectly valid equations of motion for massless particles.

15. Aug 27, 2015

### bcrowell

Staff Emeritus
Gravitational lensing would be a good example.

16. Aug 27, 2015

### vanhees71

In which sense has gravitational lensing to do with the motion of massless particles? Well, you might interpret ray optics (i.e., the eikonal approximation for classical electromagnetic waves) as massless-particle motion. In this sense the naive photon picture is not completely wrong...

17. Aug 27, 2015

### bcrowell

Staff Emeritus
Yes.

18. Aug 28, 2015

### vanhees71

Ok, so that's well understood by classical electromagnetism. No naive-photon picture is needed. What's usually done is just the eikonal approximation to classical electromagnetism (i.e., ray optics derived from wave optics). Photons in a curved background spacetime are a quite tricky business!

19. Aug 28, 2015

### bcrowell

Staff Emeritus
I would put it the other way around. Ray optics is all you need for the application I mentioned, so there is no need for classical electromagnetism or quantum mechanics. Ray optics is not a "naive-photon picture." In any case, I think this is of little or no relevance to the topic of this thread. If you want to start a separate thread on the status of massless particles in classical relativity, that might be more appropriate IMO.

Last edited: Aug 28, 2015
20. Aug 29, 2015

### vanhees71

Sure, ray optics is all you need for this purpose. For things like the redshift-distance relation in cosmology you need its proper definition via the eikonal approximation of the Maxwell equations in a given background spacetime (here the FLRW pseudometric), usually for spherical waves.

It's also historically interesting that in the 1930ies there was a debate about the correct interpretation of the Hubble redshift, where distinguished physicists like von Laue were involved. There they also had the debate, whether the already then favored derivation in the naive photon picture is correct. Finally Robertson showed, how to derive the relation within classical electromagnetics. It's downloadable from NASA (I think not behind a paywall):

Robertson, H. The apparent luminosity of a receding nebula Zeitschrift für Astrophysik 15, 1938, 69