|Two forms of the Schrödinger equation|
|Introduction · Mathematical formulation
In physics, specifically quantum mechanics, the Schrödinger equation is an equation that describes how the quantum state of a physical system changes in time. It is as central to quantum mechanics as Newton's laws are to classical mechanics.
In the standard interpretation of quantum mechanics, the quantum state, also called a wavefunction or state vector, is the most complete description that can be given to a physical system. Solutions to Schrödinger's equation describe not only molecular, atomic and subatomic systems, but also macroscopic systems, possibly even the whole universe. The equation is named after Erwin Schrödinger, who constructed it in 1926.
The most general form is the time-dependant Schrödinger equation, which gives a description of a system evolving with time. For systems in a stationary state, the time-independent Schrödinger equation is sufficient. Approximate solutions to the time-independent Schrödinger equation are commonly used to calculate the energy levels and other properties of atoms and molecules
Schrödinger's equation can be mathematically transformed into Werner Heisenberg's matrix mechanics, and into Richard Feynman's path integral formulation. The Schrödinger equation describes time in a way that is inconvenient for relativistic theories, a problem which is not as severe in matrix mechanics and completely absent in the path integral formulation.
The Schrödinger equation takes several different forms, depending on the physical situation. This section presents the equation for the general case and for the simple case encountered in many textbooks.
For a general quantum system:
The time independent equation, again for a single particle with potential energy V takes the form:
This equation describes the standing wave solutions of the time-dependent equation, which are the states with definite energy.
Following Max Planck's quantization of light (see black body radiation), Albert Einstein interpreted Planck's quantum to be photons, particles of light, and proposed that the energy of a photon is proportional to its frequency, one of the first signs of wave–particle duality. Since energy and momentum are related in the same way as frequency and wavenumber in special relativity, it followed that the momentum p of a photon is proportional to its wavenumber k.
Louis de Broglie hypothesized that this is true for all particles, even particles such as electrons. Assuming that the waves travel roughly along classical paths, he showed that they form standing waves for certain discrete frequencies. These correspond to discrete energy levels, which reproduced the old quantum condition.
Following up on these ideas, Schrödinger decided to find a proper wave equation for the electron. He was guided by William R. Hamilton's analogy between mechanics and optics, encoded in the observation that the zero-wavelength limit of optics resembles a mechanical system—the trajectories of light rays become sharp tracks which obey Fermat's principle, an analog of the principle of least action. A modern version of his reasoning is reproduced in the next section. The equation he found is:
Using this equation, Schrödinger computed the Hydrogen spectral series by treating a hydrogen atom's electron as a wave Ψ(x, t), moving in a potential well V, created by the proton. This computation accurately reproduced the energy levels of the Bohr model.
However, by that time, Arnold Sommerfeld had refined the Bohr model with relativistic corrections. Schrödinger used the relativistic energy momentum relation to find what is now known as the Klein–Gordon equation in a Coulomb potential (in natural units):
He found the standing waves of this relativistic equation, but the relativistic corrections disagreed with Sommerfeld's formula. Discouraged, he put away his calculations and secluded himself in an isolated mountain cabin with a lover.
While at the cabin, Schrödinger decided that his earlier non-relativistic calculations were novel enough to publish, and decided to leave off the problem of relativistic corrections for the future. He put together his wave equation and the spectral analysis of hydrogen in a paper in 1926. The paper was enthusiastically endorsed by Einstein, who saw the matter-waves as an intuitive depiction of nature, as opposed to Heisenberg's matrix mechanics, which he considered overly formal.
The Schrödinger equation details the behaviour of ψ but says nothing of its nature. Schrödinger tried to interpret it as a charge density in his fourth paper, but he was unsuccessful. In 1926, Just a few days after Schrödinger's fourth and final paper was published, Max Born successfully interpreted ψ as a probability amplitude. Schrödinger, though, always opposed a statistical or probabilistic approach, with its associated discontinuities—much like Einstein, who believed that quantum mechanics was a statistical approximation to an underlying deterministic theory— and never reconciled with the Copenhagen interpretation.
Schrödinger's equation can be derived in the following short heuristic way.
and to realize that since
and similarly since
so that, again for a plane wave, he obtained:
And, by inserting these expressions for the energy and momentum into the classical formula we started with, we get Schrödinger's famed equation, for a single particle in the 3-dimensional case in the presence of a potential V:
There are several equations which go by Schrödinger's name:
This is the equation of motion for the quantum state. In the most general form, it is written:
and the operator can be read off:
For a particle in three dimensions, the only difference is more derivatives:
and for N particles, the difference is that the wavefunction is in 3N-dimensional configuration space, the space of all possible particle positions.
This last equation is in a very high dimension, so that the solutions are not easy to visualize.
For a particle in one dimension,
But there is a further restriction—the solution must not grow at infinity, so that it has a either a finite L2-norm (if it is a bound state) or a slowly diverging norm (if it is part of a continuum):
For example, when there is no potential, the equation reads:
which has oscillatory solutions for E > 0 (the Cn are arbitrary constants):
and exponential solutions for E < 0
The exponentially growing solutions have an infinite norm, and are not physical. They are not allowed in a finite volume with periodic or fixed boundary conditions.
For a constant potential V the solution is oscillatory for E > V and exponential for E < V, corresponding to energies which are allowed or disallowed in classical mechanics. Oscillatory solutions have a classically allowed energy and correspond to actual classical motions, while the exponential solutions have a disallowed energy and describe a small amount of quantum bleeding into the classically disallowed region, to quantum tunneling. If the potential V grows at infinity, the motion is classically confined to a finite region, which means that in quantum mechanics every solution becomes an exponential far enough away. The condition that the exponential is decreasing restricts the energy levels to a discrete set, called the allowed energies.
for the complex field ψ.
This equation arises from the Hamiltonian
with the Poisson brackets
It must be noted that this is a classical field equation. Unlike its linear counterpart, it never describes the time evolution of a quantum state.
The Schrödinger equation has certain properties.
The probability density of a particle is . The probability flux is defined as [in units of (probability)/(area × time)]:
The probability flux satisfies the continuity equation:
For a plane wave:
So that not only is the probability of finding the particle the same everywhere, but the probability flux is as expected from an object moving at the classical velocity p/m. The reason that the Schrödinger equation admits a probability flux is because all the hopping is local and forward in time.
The Schrödinger equation does not take into account relativistic effects; as a wave equation, it is invariant under a Galilean transformation, but not under a Lorentz transformation. But in order to include relativity, the physical picture must be altered.
The Klein–Gordon equation uses the relativistic mass-energy relation:
to produce the differential equation:
which is relativistically invariant.
Some general techniques are:
In some special cases, special methods can be used: