# Tendency to drop down to a lower energy state.

1. Apr 20, 2012

### CarlosLara

Good evening. I have been wondering what causes quantum systems (or systems in general) to drop down to the lowest energy state. A hydrogen atom absorbs a photon, its electron "jumps" to a higher energy level, and that excited state lasts for only an instant before the atom emits the photon to return to the ground state. Why don't atoms stay excited? Why is there this tendency of systems to always try to occupy the lowest energy state? What causes this mechanism?

Thank you very much in advance.

2. Apr 20, 2012

### Curl

Because there is a *very* high probability that the electron emits the energy.
This is explained by statistical mechanics.

3. Apr 21, 2012

### sambristol

Lets leave the quantum arguments for a moment and consider any system.

The second law of thermodynamics states that the entropy of a system will be at a maximum when it reaches equilibrium. For almost every reasonable system this implies that the potential energy of the system will be at a minimum at equilibrium. Some physicists (R P Feynman for example) like to state this 'principle of minimum total potential energy ' is the fundamental law and work backwards to the maximum entropy principal. It does not matter which direction you go in the analysis the result is still the same that a system will minimise its potential energy in any way it can without violating other conservation laws.

You ask what causes this – there are all manner of arguments but I regard most at best dubious as they tend to introduce additional assumptions (a BAD thing to do – look up Occams Razor). I prefer to say that the second law of thermodynamics is a fundamental law of the way the universe works.

Coming now to the quantum case, all of the above applies, but you say that the electron 'instantly' drops back to its lowest energy state. This is not strictly true, the rate of transition depends on the energy difference between the states and the equation which governs this is called Fermi's Golden Rule (look it up).

Also sometimes an atom (or more often the same effect inside the nucleus) can't drop back without violating some other conservation law (often conservation of angular momentum). This is called a 'hindered decay' – look it up – which can have the system spend a very long time in the excited state.

Mmm I seem to have written a lot of words but I hope this helps.

Regards

Sam