I saw the WTC searchlights tonight from the corner of 2nd ave and 17th St at ground level (20-30 blocks NE of ground zero). I was surprized that the lights didn’t appear to be going straight up. Instead, the beams appeared angled towards my vantage point, although not so much so that they reached directly overhead. The sky was cloudless tonight and the beams are very powerful, so I figured that maybe after the beams reach a certain height, my perspective became distorted.
Are the beams set straight up? If so, what is the nature of the distortion I observed?
My guess is that the beams are not at a constant width, but diverged (became wider) slightly as it went higher up. Your brain thinks it is a constant-width beam, but since it looks wider at the top than it expects, your brain concludes that it must be tilted towards you.
Even if the beams were straight, it’s quite possible that your perspective was distorted. It’s a common phenomenon for a straight line which seems to go on forever will appear curved. Searchlights are an excellent example of this.
Achernar is correct. The beams are straight and, because of your vantage point, they appear to be angled to a spot directly above you. This is mostly because your brain is not able to discern the distance between you and a point on the beam high above you, as compared to a different point higher on the beam (got it?). This lack of perspective (no background) causes your brain to think that the points on the beam are pretty much the same distance from you as all the others - and the illusion of the bent light.
As an example, as a college student in the dorms, my pal and I often saw a green shaft of light “pointing” to a spot above our dorm building. One night we went to investigate. We found the source, a small building on campus about half a mile away. We quizzed the grad student who came out of building on what the deal was and he told us (IIRC) that they were pumping a laser 40 miles into the atmosphere to ionize particles and guage the signal return - or something like that. (My pal and I figured that that had to be one mutha of a laser and spent quite some time later that evening trying to wing a frisbee through the beam to see what would happen.)