|MadSci Network: Physics|
This is a very interesting question you asked. I had an idea about what the answer was, but after some digging, I found a lot more about it than I expected! I guess we'll both learn something from this one. In short, yes light from different lasers can interfere - in fact, it is done quite often (I will explain this more later). I do not have the "Feynman Lectures" that you quote from, so I could not read more on that topic. However, what you read seems to be alluding to the Heisnburg Uncertainty Principle. This essentially says that you can't possibly distinguish between the photons and still have the interference pattern exist. The minute you make a measurement of the photon, the wavefuction will collapse........to put it more colloquially, you can't have your cake and eat it too! ;-) The reason you will not see this happen with classical waves is because this phenomenom is a quantum effect. Quantum effects only come into play when you deal with atomic level actions.....the effect breaks down in the macroscopic world. Now, getting back to the lasers, two different lasers will never have the same output. They have different power source and will have numerous other minute differences that will lead to slightly different linewidths. (Linewidths are the range of output wavelengths that a laser produces. Its profile looks similar to a Bell Curve - the center frequency will be the characteristic wavelength of the laser. eg. a HeNe laser has a center frequency of 632.8nm) To quote a friend of mine (Kent Nickerson): When "two equal powered monochromatic beams (as is produced with a beamsplitter from a single monochromatic source) combine (as done in an interferometer) they yield a static interference pattern. This is the same as the "standing wave" pattern observed when a radio frequency signal reflects back upon itself. If one of the sources (radio, optic, or any wave source) differs from the other in frequency by dF, the pattern will modulate at frequency dF." (dF equals the difference between the center wavelenths of the two lasers) This modulation is a result of the superposition of the two waveforms. The low-frequency wave will serve as an envelope modulating the high-frequency wave. (Pedrotti, 1993) The two combined will produce a "beat" frequency that is equal to dF. A good way to imagine this is to think of two tuning forks of slightly different pitches. If you hear them individually, you will just hear the tone of that individual fork. If you hear them together, you will hear the two tones, along with a sinusoidally cyclical combination of the two waves combined (sounding sort of like a "whomp, whomp, whomp......"). Along with this superposition, the waves will also produce an interference pattern. However, like my friend Kent said above, due to the superimposed waves, the pattern will modulate at a frequency of dF. Finally, also courtesy of my friend Kent, is a novel application for this principle: One "application involves the testing of fast photodetectors for response to light modulated at, say, 50 GHz. This is very difficult - both in finding a laser that can be modulated that quickly and generating a 50 GHz electrical drive for the laser. However, the optical frequencies of a laser radiating at 800nm and a similar one tuned to 800.1nm differ by 50 GHz. The 50 GHz modulated interference pattern made by shining both these beams onto the detector can be used to test the detector." I hope this explaination has helped you. If you have any other questions please feel free to email me. Kurt Frost firstname.lastname@example.org REFERENCES: Pedrotti, Frank and Leno Pedrotti. 1993. Introduction to Optics. New Jersey: Prentice Hall. Dr. Dan Cassidy. Deparment of Engineering Physics, McMaster University, Hamilton, Ontario, Canada. Personal Conference. Kent Nickerson, M.Eng (EE). RF Researcher. Waterloo, Ontario, Canada. Personal Conference.
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