SoftCollection LED Line program lets you make a virtual LED table in the air.
You can make simple hardware, as shown in the picture below, that consists of an 8 LEDs line, 9 resistors of 470 Ohm each, a button and a pendulum that presses this button.
If you will move LED line left and right 5-10 times per second and desired LEDs will shine at desired moments, you will see words in the air. There is still a problem of line synchronization so the mechanism of button and pendulum is needed. When LED line moves from right to left, the button must be released, but when the direction has been changed, the pendulum must press the button. Remember that wrong hardware or wrong connection may cause harm to your computer.