coherence.
Light from a bulb is not coherent, so it interferes with itself and spreads out.
Light from a bulb is not coherent, so it interferes with itself and spreads out.
That is, in Physics, how would one take an EM wavelength/set of wavelengths/frequency/set of frequencies (or even more broadly speaking, "energy") -- and make it coherent?
https://en.wikipedia.org/wiki/Coherence_(physics)
See https://www.eso.org/sci/facilities/paranal/telescopes/vlti/t...
(I wish I could find a better writeup of the spatial interferometer, it's actually a pretty simple concept and a simple experiment but I've never seen it explained very well, even in print, when I was studying physics.)
1) Did you mean laser or antenna?
2) By single signal, did you mean single frequency? (If so, I get it. If not, please elaborate...)
>"The lightbulb isn't because individual electrons are being shaken randomly by thermal noise. The laser is coherent because it is taking advantage of stimulated emission which makes the output photons coherent with the environmental field."
3) If thermal noise is the reason that a lightbulb's light cannot be made coherent -- then could you suggest a method whereby the thermal noise in the lightbulb could be removed such that the light emitted could be made coherent?
4) What do you mean exactly by "environmental field"? (A Google search for that term in the context of Physics -- seems not to yield any results -- but then again I lay no claim to being the best Google searcher out there...)
(2) the signal of a radio transmitter is (usually) more or less a sine wave that is either modulated by varying the amplitude or the frequency. You could feed the same signal to multiple antennas. For instance in this photo
https://mastodon.social/@UP8/111780254496170231
there is a radio antenna used for emergency responder comms. Note that there are several arrays of antennas stacked on top of each other. If you feed the same signal into an array like that the radiation pattern becomes focused around the horizontal plane so that energy is not thrown into the ground and the sky.
(3) It is the shaking by random vibrations that makes the black body radiation of a light bulb. If you stopped that shaking there wouldn't be any light.
(4) By "environmental field" I mean the electromagnetic field inside the laser that an active molecule or atom inside the laser experiences.
Note if I hooked up 50 antennas to the same oscillator that would be coherent, but if I hooked up 50 antennas to 50 different oscillators that would be incoherent.
Take a look at
https://en.wikipedia.org/wiki/Phased_array
there are two ways to build a phased array. A passive phased array has one transmitter and an collection of phase shifters that delay the signal to create a controlled wavefront. In the first case the emissions of all the antennas are coherent because they come from the same oscillator, in the second case the antennas are coherent because the oscillators are synchronized to a common timebase and controlled by a computer.
Note the "magic" of that kind of phased array is similar to the "magic" of a hologram (they do similar things to wavefronts.) Light other than laser light has a certain amount of coherence though it is a complicated subject, see
https://www.nature.com/articles/s41598-017-06215-x
Note in Figure 10 they show that you can get enough coherence out of an LED to make a hologram. When they use a real laser the picture is really sharp but you see a speckle pattern that's caused by interference of the light with surface roughness. The LED image is blurry but doesn't have the speckle.