With active shutter 3d yes, but with polarized 3d no. I think most movie theaters are polarized 3d.
Technically, I think those projectors can go up to 60fps in 3D, but I can't remember/easily find where I read it so I'm not that confident about that figure.
Using polarised 3D both eyes should still get 48fps they just receive different(ly polarised) light. TVs using active shutter glasses do reduce the frame rate by half but will usually be reducing it from 96Hz or more
By the way - in the cinema at 24fps each frame is usually flashed twice which encourages the psycho-visual system to fill in the blanks and smooth out the motion so it appears sharper and less blurred than it would on a LCD/Plasma TV screen that usually holds the image for the duration of the frame. Some high end models can do black frame insertion if they are using the right type of LED backlighting and are set to the right mode.
Which shot images have you seen played faster than 72 fps? I've seen one test played back at 120 fps, but it was in a camera r&d lab in germany. 72 fps is really more than enough. Consider that 3D projection actually projects 3 frames per each eye, so 3D@72fps would actually mean projector is churning out 432 frames each second. Or you're talking about games, which is another thing entirely since input is a part of experience too. In that case, I agree.
Finding a 'framerate' for the eye is difficult. Under certain conditions, 60hz (think florescent lighting) can be enough that your eyes won't notice a flicker. (your eyes hold a residual for some period of time, and you never see the darkenss). But if you are flashing a series of super-crisp photos, your eye is much more likely to pick up on stuttering because each photo has just as much light.
And then, if you have some action sequence in a movie where parts of the scene are really bright and fast-motion, and others are dim, the minimum amount of frames per second you need to create a 'perfect' experience for the eye is anybodies guess. It would probably also make a difference if you were a person used to looking for flaws in a picture, vs a person who is just trying to enjoy a movie and doesn't know anything technical about framerates, etc.
Regardless, all forms of currently available cinema have framerates with room for improvement.
No, it's not. The actual threshold is unknown, but it's generally assumed to be around 100fps. Like the threshold for color, it's a complex question.
You don't see stuttering in a movie (usually) because each frame has an exposure time that is almost the full duration of how long the frame will appear on screen. The result is that anything moving will blur on the film (instead of getting a crisp shot), and so motion sequences are much more natural.
I'm not entirely sure why 48fps was chosen, but I know that too much more and many of the projectors that are currently showing the HFR film would be unable to. It's also worth noting that in 3D, each eye only gets a new frame at half of the rate (because each eye only sees half of the frames), so each eye is getting refreshed at the rate of a regular movie.
I know that there were lots of other technical obstacles when filming in 48fps, such as color muting by the camera. This could have also played a role in the choice of keeping it at just 48fps instead of something higher.
I'm not entirely sure why 48fps was chosen,