Lol sorry about that oneiromancer… I would be a bad teacher I know
I’ll do my best
First you need to know a few key elements which help to describe the EM wave. When talking about waves, I only mean the sine waves:
Amplitude: the heigth of the wave at a specific place.
Wavelength: the distance between two adjacent wave tops.
Phase: To understand what this is, you have to depict a sine wave as a circle (see pict). The phase of the sine wave at a certain point then describes the position of this point on the corresponding circle (measured in degrees). When the sine wave has gone through 360° (this is one cycle), it comes back to its starting point.
Frequency: The number of cycles in one second, thus it depicts how often the wave goes round the 360° circle in one second.
Interference happens when two or more waves with the same frequency overlap at some point. Then you get constructive or destructive interference patterns (as explained in a previous post).
Now, what exactly is coherence? To answer this, you have to know the origin of light waves. In short, light waves (which consist of photons) are emitted by electrons inside atoms, when these electrons fall back from a higher energy level to the ground level. The electrons in the ground level have a lower energy, thus in order to jump from a higher level to this ground level, they have to release the surplus of energy. And it’s exactly this surplus which can be found back in the photon. The energy of the photon thus corresponds to the difference in energy between the higher energy level of the electron and the atom’s energy ground level. This also means that photons have a well-determined energy, so they must have a well-determined length. That’s why light doesn’t exist of one gigantic wave beam, but in fact consists of trillions of small wave trains. Each wave train then corresponds to one single transition of an electron to the ground level of an atom. The why and how of these energy levels doesn’t matter here, but this is the basic explanation of why light exists.
So hopefully you now see that a flashlight sends out trillions of wave trains, each with a different length, because a flashlight consists of trillions of atoms with lots of different energy levels. Thus, the resulting wave trains also have a different length.
Back to coherence now… There are two options:
1. Suppose you have two waves, both with the same frequency, but with a different phase towards each other. At a certain point you decide to measure the phase relation between the two waves. You get a certain result. A few seconds later, you measure it again. If you get the same result, it would mean that the difference between the phases of both waves remained constant during that time period. Both waves now show a clear temporal coherence. Their phases are related towards each other, in this sense that they remain constant during a certain period of time. The longer this time period is, the more coherence. If the phase relation between the waves has changed during this time period, then there’s no clear relationship between both phases. This is incoherence. If however there’s only a small difference between both phases, then they show a partial coherence.
As previously explained, in reality lightbeams consist of many short wave trains. Normal lightsources don’t show a clear relation between the points in time when those different atoms emit their photons ( = wave trains). This means there’s also no clear phase relation between the wave trains. Temporal coherence only occurs when there’s a clear phase relation between waves during a time period, thus when there exists a certain relationship between the moments when the electrons of the different atoms fall back to the energy ground level.
2. Suppose you have a light source which consists of more than one atom. This means that the wave trains, emitted by the different atoms, reach a certain point in space, not necessarily with a similar difference in phase. If an atom lies a bit further away from that specific point in space, than wave trains have to travel a bit longer before they get there. And so they have a different phase than wave trains coming from atoms which lie a bit closer to that point. Only wave trains which are coming from atoms which lie at a same distance from the spatial point show a clear phase relation (they’re always in phase towards each other). This is simply because these wave train have to travel the same length to get to the spatial point. In this case, there’s spatial coherence: you look at wave trains from a specific spatial point at one single moment. So you see that the bigger your light source, the less coherence. 100% pure coherence is only possible when your light source consists of one atom: all the wave trains coming from this atom are ofcourse in phase towards each other, and they have to travel a same distance to the spatial point.
So if we relate all this to the shared dream discussion, you see that coherence between two or more brain wavefunctions creates the best conditions for interference to occur. To get constructive interference between them, they have to be in phase, thus radiating a certain amount of coherence. Otherwise you get an irregular pattern of phase shifting between the waves, thus making if very difficult to establish a stable interference connection between the brains, which lasts for a certain time period.
Lol am I still speaking Chinese?