If we shine a laser through a double slit set up (I use a leveling laser and two slits cut into black tape), we get a nice, static pattern of light and dark. We can measure the distance between peaks and distance from the slits to the wall, and so calculate the wave length, as a nice high school experiment.
But when we get into the math details, we usually assume that the waves are standing waves, i.e., non-moving peaks. Is the laser light a standing wave? If not, why is the interference pattern so stable instead of having the peaks slide around. If the laser light is a standing wave, why? I usually associate standing waves with things that are fixed at either end (like a guitar string).
Thank you in advance for any insight you can provide. I did review other questions, such as Why aren't interference patterns wiped out by random phase shifts?, where the thread ends with the unresolved comment that "BTW the interference pattern of water waves along an observer line moves to the right and left. The intensity distribution of light is static. That's strange." I guess I am asking why that strange result occurs.
Addendum: thank you to the respondents. It makes sense that any any location, the path length differences are constant, so the light interaction is unchanging. The other pieces that differentiates the pattern from that of water is that we can see the water waves oscillating from peak to trough, so that they always appear to be changing.
