Usually the operator can see only the simulated display, and has no other visual referents. Because of this, the frame rate must be high enough to give smooth, nonflickering animation, and any perceptible transport delay can cause nausea and disorientation. However, the virtual world is not required (or expected) to look like the real world, so the simulator may be able to do less work to prepare the display.
Silicon Graphics systems, with their excellent graphic and audio capabilities, are well suited to building virtual reality applications.