1 of 2

# Problem 2.1 in Birger Bergersen's and Michael Plischke's Equilibrium Statistical Mechanics

I have the solution to problem 2.1 from Bergersen's and Plischke's textbook which I don't quite understand.

I'll post the question itself and its solution.

a) Consider a harmonic oscillator with Hamiltonian $$H = 1/2(p^2+q^2)$$ show that any phase space trajectory $$x(t)$$ with energy $$E$$, on the average, spend equal time in all regions of the constant energy surface $$\Gamma(E)$$.

b) Consider two linearly coupled harmonic oscillator: $$H=1/2(p_1^2+p_2^2+q_1^2+q_2^2+q_1q_2)$$Express the phase space trajectory $$x=(p_1,p_2,q_1,q_2)$$ in terms of the initial values of the amplitude and phase of the normal coordinates. Show that there are regions of the constant energy surface which aren't visitedfor any particular trajectory $$x(t)$$.

Now, for the solution:

a) Let $$p=r\cos \phi ; q=r\sin \phi$$ The constant energy surface $$\Gamma(E)$$ in the $$q-p$$ plane is the circle $$r=const$$. The equations of motion can be written $$\dot{r}=0 ; \dot{\phi}=const$$, i.e., the phase space point moves with constant velocity and covers the whole "surface". It thus spends equal time in all regions of $$\Gamma(E)$$.

b) The solutions to the equations of motion can be written $$q_1=r_1\sin \phi_1 + r_2 \sin \phi_2$$ $$q_2 = r_1\sin\phi_1 - r_2\sin\phi_2$$ where $$r_1,r_2$$ are constants of the motion and $$\phi_1 = \sqrt{3/2}t + \phi_1(0)$$ $$\phi_2 = \sqrt{1/2}t + \phi_2(0)$$ The projection of the constant energy surface on the $$r_1,r_2$$ surface is the ellipse $$3/4 r_1^2+1/4 r_2^2=const$$ However, with given initial conditions only a single point on this ellipse is visited.

I don't understand the solution to part b), I assume that $$p_i = r_i \cos \phi_i$$, and then to derive $$3/4 r_1^2+1/4 r_2^2=const$$ I need to use the Hamiltonian given in b) and then equate what I get to some constant, but I get if I am not mistaken the following:

$$1/2(r_1^2+2r_1^2\sin^2 \phi_1^2 + r_2^2) = constant$$

Which is off by far, what do I need in order to derive this ellipse equation, I don't see how did they derive it out of thin air.