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Here we will review the results of the coupled mass problem,
Example 1.8.6 from Shankar. This is an example from classical physics
which nevertheless demonstrates some of the essential features of coupled
degrees of freedom in quantum mechanical problems and a general approach
for removing such coupling. The problem involves two objects of equal mass,
connected to two different walls and also to each other by springs. Using
F=ma and Hooke's Law (F=-kx) for the springs, and denoting the
displacements of the two masses as x1 and x2, it is straightforward
to deduce equations for the acceleration (second derivative in time,
and
):
The goal of the problem is to solve these second-order differential
equations to obtain the functions x1(t) and x2(t) describing the
motion of the two masses at any given time. Since they are second-order
differential equations, we need two initial conditions for each variable,
i.e.,
,
and
.
Our two differential equations are clearly coupled, since
depends not only on x1, but also on x2 (and likewise for
). This makes the equations difficult to solve! The solution was
to write the differential equations in matrix form, and then diagonalize
the matrix to obtain the eigenvectors and eigenvalues.
In matrix form, we have
|
= |
|
(3) |
where
.
Since this 2x2 matrix is real and symmetric,
it must also be Hermitian, so we know that it has real eigenvalues,
and that the eigenvectors will be linearly independent and can be made
to form an orthonormal basis.
Equation 3 is a particular form of the more
general equation (in Dirac notation)
|
(4) |
where we have picked a basis set which we will call
,
where
|
(5) |
represents a unit displacement for coordinate x1, and likewise
|
(6) |
represents a unit displacement for coordinate x2. Clearly any state
of the system (x1, x2) can be written as a column vector
|
(7) |
(as in eq. 3),
which can always be decomposed into our
basis as
|
= |
|
(8) |
or
|
(9) |
Hence, eq. 3 can be considered a representation
of the more general eq. 4 in the
basis.
If we assume
the initial velocities are zero, then we should be able to predict
x1(t) and x2(t) directly from x1(0) and x2(0). Thus, we
seek a solution of the form
|
= |
|
(10) |
where
G(t) is a matrix, called the propagator, that lets us
get motion at future times from the initial conditions. We will have
to figure out what
G(t) is.
Again, the strategy is to diagonalize
.
The point of diagonalizing
is that, as you can see from
eq. 3, the coupling between x1 and x2 goes away if
becomes a diagonal matrix. You can easily
verify that the eigenvectors and their corresponding eigenvalues,
which we will label with Roman numerals I and II, are
This new basis, the eigenvector basis, is just as legitimate as
our original
basis, and is in fact
better in the sense that it diagonalizes
.
So, instead of
using the
basis to obtain
eq. 3 from eq. 4, we
can use the
basis to
obtain
|
= |
|
(13) |
so that now
depends only on ,
and
depends only on
.
The equations
are uncoupled!
Note that we are now expanding the solution
in the
basis, so the components
in this basis are now
and
instead of
x1 and x2:
|
(14) |
Of course it is possible to switch between the
basis and the
basis.
If we define our basis set transformation matrix as that obtained by
making each column one of the eigenvectors of
,
we obtain
|
(15) |
which is a unitary matrix (it has to be since
is Hermitian).
Vectors in the two basis sets are related by
|
|
|
(16) |
In this case,
U is special because
;
this doesn't
generally happen. You can verify that the
matrix,
when transformed
into the
basis via
,
becomes the diagonal
matrix in equation 13.
The matrix equation 13 is of course equivalent to the
two simple equations
and you can see that valid solutions (assuming that the initial velocities
are zero) are
where we have defined
So, the
basis is very special,
since any motion of the system can be decomposed into two decoupled
motions described by eigenvectors
and
.
In
other words, if the system has its initial conditions as some multiple
of
,
it will never exhibit any motion of the type
at later times, and vice-versa. In this context, the special
vibrations described by
and
are called
the normal modes of the system.
So, are we done? If we are content to work everything in the
basis, yes. However, our
original goal was to find the propagator
G(t) (from
eq. 10) in the original
basis. But notice that we already have
G(t) in the
basis! We can simply
rewrite equations 19 and 20 in matrix
form as
|
= |
|
(23) |
So, the propagator in the
basis is just
|
= |
|
(24) |
To obtain
G(t) in the original basis, we just have to apply the
transformation
|
(25) |
noting that this is the reverse transform from that needed to bring
from the original to the eigenvector basis (so that
U and
swap). Working out
was a problem
in Problem Set II.
Finally, let us step back to the more general Dirac notation to point
out that the general form of the solution is
|
(26) |
and actual calculation just requires choosing a particular basis set
and figuring out the components of
and
and
the matrix elements of operator
in that basis. Another
representation of operator
is clearly
|
(27) |
as you can check by evaluating the matrix elements in the
basis to get
eq. 24. Thus
Next: Summary
Up: Time Evolution
Previous: Introduction
C. David Sherrill
2000-05-02