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So I've heard that when a photon flies by a atom excited enough to release a photon there's a good chance it will. Because Photons want to be together and have the same direction etc?

Is this true? And then, why is it like this?

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Because they are so radiant. –  Mew Jan 10 '13 at 23:38

3 Answers 3

Did you here that in the context of lasers or BEC?

Photons are bosonic particles, this is generally what people refer to when they say 'they want to be together'.

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I think what you are talking about is the stimulated emission of radiation. This is part of the process that occurs in a LASER and in fact, gave the LASER it's name - Light Amplification through the Simulated Emission of Radiation. When an atom is in an excited state it can spontaneously decay to a lower energy state with the emission of a photon of a particular frequency. If a photon of that same frequency passes an excited atom it can stimulate the atom to emit a second photon in the same direction and with the same phase (and frequency) as the original photon. According to Wikipedia:

In optics, stimulated emission is the process by which an atomic electron (or an excited molecular state) interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field. A photon created in this manner has the same phase, frequency, polarization, and direction of travel as the photons of the incident wave. This is in contrast to spontaneous emission which occurs without regard to the ambient electromagnetic field. However, the process is identical in form to atomic absorption in which the energy of an absorbed photon causes an identical but opposite atomic transition: from the lower level to a higher energy level. In normal media at thermal equilibrium, absorption exceeds stimulated emission because there are more electrons in the lower energy states than in the higher energy states. However, when a population inversion is present the rate of stimulated emission exceeds that of absorption, and a net optical amplification can be achieved. Such a gain medium, along with an optical resonator, is at the heart of a laser or maser. Lacking a feedback mechanism, laser amplifiers and superluminescent sources also function on the basis of stimulated emission.

Stimulated emission was a theoretical discovery by Einstein [1] within the framework of quantum mechanics, wherein the emission is described in terms of photons that are the quanta of the EM field. Stimulated emission can also be described classically, however, without reference to either photons, or the quantum-mechanics of matter.[2]

The reason why two photons can have exactly the same frequency, phase and location in space is because they obey Bose-Einstein statistics. Photons have spin 1 which is why they are Bosons with Bose-Einstein statistics. If the photon were a fermion (like the spin 1/2 electron) simulated emission would not be possible since it is impossible for two fermions to occupy exactly the same quantum state.

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Any particle in a system (like a photon or an electron) can be in many different energy state. You may be familiar with the energy states of atoms, these are the states occupied by electrons orbiting the nucleus.

But electron are fermions and they don't want to be together (in fact they can't). In order to minimize the energy of the atom any additional electron will occupy the lowest energy state available (but only if this energy state is empty).

On the opposite photons are bosons and they can be together. They can occupy the same energy state as the other photons. If you consider a system with many photons, any additional photon will occupy the lowest energy state wether it is already occupied by another photon or not. This is done in order to minimze the energy.

However the answer to your first question :

Is this true?

is "yes".

The answer to your second question :

And then, why is it like this?

is more complicated. The answer to the question "Why photons are bosons?" is very difficult. But in a general way bosons can be seen as the way for fermions to communicate. It is true in the case of photons. Photons are the way for electrons to exchange energy and thus interact.

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