I have a question regarding the generation of coherent light. Despite an EE degree, I never studied Lasers in college, and everything I know about them I learned in high school. So, I imagine that I have an overly simplified view of them.

As I understand it, the basic idea of a Laser is that you first excite the electrons in some material. On their own, the electrons would eventually fall back to their rest state, each producing a photon. But, you can coerce an electron into falling back to rest state by hitting it with a photon of the same frequency as the one it will generate. Then, you will have two photons of the same frequency. The real magic, however, is that the photons will also be in phase.

That's the part that bugs me. Doesn't this violate some causality rule? If the photons are in perfect phase, doesn't that imply that it took zero time for the electron to recognize the first photon, drop to its rest state, and produce the second photon?

I'm sure that there is an explanation of this that involves lots of math, but if someone can distill it into layman's terms, I'd appreciate it.

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:roll:

:rollseyes:

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Thank you for your informative and illuminating response.

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I agree Extravoice. Sarongsong, that was rude. VERY rude. Unfortunately I can't help you, Extravoice, but I want you to know you have my support.

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This might help you.
http://en.wikipedia.org/wiki/Laser#Laser_physics

Was that what you were looking for? Otherwise, can you specify?
I've taken photonics in uni, but it's been a few years.

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Sarongsong probably just had a bad day, and was a little frustrated

GS is for general science questions, and people like to help. However, we've had a lot of GS questions that were thinly disguised attempts to introduce Against the Mainstream (ATM) topics. Hence, the reaction when Extravoice suggested that lasers violate causality.

If I read the OP closer, I can see that Extravoice is just indicating ignorance of the subject. But that was already stated explicitly, so the jab at causality was really unnecessary. To be fair, I had the same reaction, I just didn't post it. Sorry about that. Unfortunately, the explanation in the OP is probably as close as I could come to "layman's terms", and anything more probably does involve math. Maybe that's what stymied Sarongsong. I hope jokergirl's link helps.

PS: looking at jokergirl's link reminded me of something that may help: resonance does not need instantaneous actions that violate causality, it merely has to have "good timing". Pluck a guitar string, and another might vibrate in resonance, without violating causality.

Thanks for your responses, jokergirl and hhEb09'1, and for your support ravens_cry.

Unless he/she states otherwise, I'll write-off sarongsong's response as being due to an overly sensitive ATM detector that gave a false-positive reading.

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I'll be watching this thread as powerful, and extremely narrow coherent beams can theoretically be produced by millions of laser diodes, if we can keep most of them phase locked. This is important as laser diodes are reliable, small and more efficient than other types of lasers. It appears the sychronizing light path needs to be of very precise length. My application of interest is SSP = www.spacesolarpower.wordpress.com

Today's google image is very appropriate.

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Extravoice

^That's the part that bugs me. Doesn't this violate some causality rule? If the photons are in perfect phase, doesn't that imply that it took zero time for the electron to recognize the first photon, drop to its rest state, and produce the second photon?

I'm sure that there is an explanation of this that involves lots of math, but if someone can distill it into layman's terms, I'd appreciate it.^

Don't worry about the rolling eyes.Happens to me often in the real (not internet)world.Usually turns out the eye roller not only does not know the answer but does not see the significance of the question or even understand it.

Layman's terms for me too please.

If the second photon comes out at exactly the same time as the original then there
may be a causality problem.
What if it is after the stimulating photon,in a row.?
If at the same time is it alongside or in the same place as the stimulating photon?.
How long is a photon?
How wide is a photon?
How high is a photon?
How many waves in a photon?.
Off to Google I think.

Google holiday logos:

Invention of the First Laser - May 16, 2008

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0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 ...

I think I have the answer, but someone please correct me if I am wrong:

The first photon is actually absorbed/destroyed by the atom it strikes. It doesn't bounce off. It is absorbed, and then (I presume) two new photons are released, simultaneously, a non-zero amount of time after the intial photon struck. So no causality contravention occurs.

I don't think this is correct. There's a reason why the word "stimulated" is used in the laser acronym. An electron is already in the excited state, so there wouldn't be an absorption. The passing photon stimulates that electron to descend, producing a second photon that is in phase with the first.

the absorb one, emit two actually come closest to my understanding of the process as well, based on what I understand of some of the Feinmanm lectures.
Apparently, movement of light through a transparent medium is best modeled as a whole series of such absorptions and re-emittances.
Adding that two photons are sometimes emitted wouldn't adjust that much and seems like an elegant (and therefore probably horribly misleading) explanation.

Another way of answering the initial question is that there's no violation of causality by zero-time reactions, if the space distance between the trigger and reaction is also zero.

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Trying to make sense of computers, The Error Log.

maybe the answer can be found here:
http://www.rp-photonics.com/encyclopedia.html

John the dataminer, or in this case, trace finder

That site doesn't seem to have an explanation for why both photons will be in the same mode, it only states that they they will.

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And the "driving on the freeway on a scooter" analogy still holds true because the pilots are sitting in 7 to 30 ton aircraft o' doom and you are running around them in your very own Meatbody, Mark I. Beep, beep.
Trying to make sense of computers, The Error Log.

Feynman

But which lectures? From the appendix, it looks like he mentions lasers in three places, only incidentally. The last, III - 9, is about masers, and he mentions the theory applies to lasers.

Feynman
That's what I get from getting used to using a browser with spell checking, and then I switch to one without . . ..

Anyway, it wasn't from a lecture on lasers per se, as far as I remember it was one on how light moves through a thing plate of glass where I got the impression the lower speed of light in glass was explained/modeled as multiple absorptions and reemissions of the photons, which made the absorb one, emit two look reasonable.

I fully acknowledge that this thinking has no basis in anything other than my vague recollection of a subject where I don't know the math and is therefore likely to be utter rubbish.

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And the "driving on the freeway on a scooter" analogy still holds true because the pilots are sitting in 7 to 30 ton aircraft o' doom and you are running around them in your very own Meatbody, Mark I. Beep, beep.
Trying to make sense of computers, The Error Log.

I didn't do well in that class so I had to enrole in Hooked on Photonics.
It was illuminating.

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Gimme a minute to read through Jay's latest observations...