# Why do Lasers work? **URL:** \ **Category:** Factual Questions\ **Created:** [May 20, 2022, 2:04am UTC](https://boards.straightdope.com/t/why-do-lasers-work/964731 "2022-05-20T02:04:25Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [May 20, 2022, 2:04am UTC](https://boards.straightdope.com/t/why-do-lasers-work/964731/1 "2022-05-20T02:04:25Z")
Not to be needlessly [Kirkian](https://youtu.be/jg1mYsIrFPs?t=57), but I know _how_ a laser works - an material is pumped with energy, putting into a metastable state, and then as the molecules return to a lower energy state, they emit radiation. This radiation then stimulates other molecules to also fall to the lower-energy state, resulting in a cascade of radiation, all in phase. But - _why_? Why does a photon of the correct wavelength stimulate a molecule to emit a like photon? ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [May 20, 2022, 2:14am UTC](https://boards.straightdope.com/t/why-do-lasers-work/964731/2 "2022-05-20T02:14:06Z")
Ultimately, I think, because photons are bosons - particles that have the oroperty that the existence of one or more in a particular state increases the likelihood of another taking on that state. Beyond that it’s all math. ---
**Author:** ![Bootb](https://avatars.discourse-cdn.com/v4/letter/b/e19b73/32.png) [@Bootb](https://boards.straightdope.com/u/Bootb)\ **Post date:** [May 20, 2022, 5:11am UTC](https://boards.straightdope.com/t/why-do-lasers-work/964731/3 "2022-05-20T05:11:29Z")
The way that I remember it being explained in my high school chemistry class is that atoms “want” to be in their lowest-energy state that’s stable. So if you pump energy into them, they’re going to get rid of it at their first opportunity. ---
**Author:** ![Velocity](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/velocity/32/18006_2.png) [@Velocity](https://boards.straightdope.com/u/Velocity)\ **Post date:** [May 20, 2022, 6:55am UTC](https://boards.straightdope.com/t/why-do-lasers-work/964731/4 "2022-05-20T06:55:39Z")
> [@Andy\_L](#): > > Ultimately, I think, because photons are **bosons** - particles that have the property that the existence of one or more in a particular state increases the likelihood of another taking on that state. Beyond that it’s all math. I misread this as “because photons are bosses” and now I think it makes intuitive sense that way too. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [May 20, 2022, 11:32am UTC](https://boards.straightdope.com/t/why-do-lasers-work/964731/5 "2022-05-20T11:32:04Z")
There is a discussion beginning on Page 163 in [Itzykson and Zuber](https://archive.org/details/quantumfieldtheo0000itzy) in which they come up with an expression for the scattering matrix for a quantized electromagnetic field interacting with a classical source. When you turn on your source, the average number of radiated photons does not change(!), but the probabilities for emission increases. In their notation (J(k) is the Fourier transform of some classical current j(x) and S=\exp\left[-i\int d^4x A\_{\text{out}}(x)\cdot j(x)\right] and \left| \bar n\_{\text{inc}}\;\text{in} \right\> = \frac{1}{\sqrt{\bar n\_{\text{inc}}!}} {\left[\int d\tilde k\sum\_{\lambda=1,\,2}f\_\lambda(k)a^{(\lambda)\dagger}(k) \right]}^{\bar n\_{\text{inc}}} \left| 0\;\text{in} \right\> ), the probability of getting 1 photon is approximately p\_{0\to 1} = \int d\tilde k\,\left(\bigl|J\_1(k)\bigr|^2+\bigl|J\_2(k)\bigr|^2\right) while p\_{\bar n\_{\text{inc}}\to\bar n\_{\text{inc}}+1} = \int d\tilde k\sum\_{\lambda=1,\,2}\bigl|J\_\lambda(k)\bigr|^2 + \bar n\_{\text{inc}} {\left| \int d\tilde k\sum\_{\lambda=1,\,2}f\_\lambda^\*(k)J\_\lambda(k)\right|}^2 so the presence of photons in the initial state increases the probability of emission, even though the average number of radiated photons does not change. ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [May 20, 2022, 1:37pm UTC](https://boards.straightdope.com/t/why-do-lasers-work/964731/6 "2022-05-20T13:37:59Z")
That makes sense. ---
**Author:** ![JWT\_Kottekoe](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@JWT\_Kottekoe](https://boards.straightdope.com/u/JWT_Kottekoe)\ **Post date:** [May 20, 2022, 3:49pm UTC](https://boards.straightdope.com/t/why-do-lasers-work/964731/7 "2022-05-20T15:49:51Z")
I don’t have time to explain it now, but I will just note that stimulated emission is a classical effect. Spontaneous emission is a quantum mechanical effect. ---
**Author:** ![Vlad\_Igor](https://avatars.discourse-cdn.com/v4/letter/v/7feea3/32.png) [@Vlad\_Igor](https://boards.straightdope.com/u/Vlad_Igor)\ **Post date:** [May 21, 2022, 12:08am UTC](https://boards.straightdope.com/t/why-do-lasers-work/964731/8 "2022-05-21T00:08:39Z")
From what I remember, the emitted photons don’t always stimulate other molecules to emit the same photon. The stimulation has to come from the external source that is driving the material to lase. When the material absorbs the incoming energy, it has to do something with it. That action is a transfer of electrons to a specific higher, more energetic state. However, that orbit is unstable, so the electron falls back to its ground, or “normal” orbit. The energy released when the electron falls is a quantum of energy with a specific amount of energy and wavelength. The physical contruction of the laser (here I’m thinking of gas HeNe or solid ruby lasers) forces the photons to align by wavelength, and when they have enough collective energy, they escape the material as a single wavelength, coherent light beam. ---
**Author:** ![DocCathode](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/doccathode/32/18773_2.png) [@DocCathode](https://boards.straightdope.com/u/DocCathode)\ **Post date:** [May 21, 2022, 1:06am UTC](https://boards.straightdope.com/t/why-do-lasers-work/964731/9 "2022-05-21T01:06:22Z")
Laser’s DON’T work. If you believe they do, you’ve been duped by the Conspiracy. This is why that whole ‘the wildfires were started by space lasers’ thing was so laughable. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 21, 2022, 11:57am UTC](https://boards.straightdope.com/t/why-do-lasers-work/964731/10 "2022-05-21T11:57:23Z")
> [@beowulff](#): > > Why does a photon of the correct wavelength stimulate a molecule to emit a like photon? I’m not a laser guy, but I think that this might be the root of the misunderstanding: What kind (i.e., wavelength, because there no other “kind” for a photon) is emitted is governed by the material, not by the stimulating photon. If you take a ruby or whatever, and stimulate it with a blue photon, you won’t get stimulated emission of more blue photons; you’ll just get no emission at all.