Re: Is it possible to make things invisible?

When you see something, it is because light from that object travels along 
a path -- usually a straight line -- and enters your eye. The light that 
comes from the object can be either light that is generated by the object 
itself, or light that has come from somewhere else, and been reflected from 
the surface of the object.

For something to be invisible, it must not reflect any light off its own 
surfaces, and it must let light from objects behind it pass straight 
through into your eye. Glass or water or transparent plastic will let light 
pass through them, but you can usually see glass or water or transparent 
plastic because of surface reflections. If you design to cut down surface 
reflections as much as possible, you can make glass or water or plastic 
nearly invisible (but not quite).

Unfortunately, you cannot make an electronic computer completely out of 
transparent materials. It is a complicated story, but computers work with 
electricity, and light is connected with electricity, and so some of the 
components you need for a computer must be non-transparent to have the 
right electrical properties.

So how else could you make something invisible? Well, you could use four 
mirrors to block out the light coming from the object, and bend the light 
coming from behind the object around it:

-->---->----\    (object)    /---->---->----> observer
            |                |
            |                |
            \---->---->---->-/

Or you could use some similar but more complicated arrangement with lenses 
and prisms. The trouble with all of these sorts of arrangements is that 
they only make something invisible from a single direction, and although 
they make the object itself invisible, the two mirrors on the side are 
clearly visible.

The best solution I can think of is to put a super duper colour video 
camera behind the object, and an extra super duper LCD display screen with 
no visible edges in front of it. The video camera can take pictures of 
whatever is behind the object, and pass them along a wire to the LCD 
display, which can screen them in front of the object. And it is such an 
extra super duper LCD display that you can't tell the difference between 
the display and the real objects that are hidden behind it. Even that one 
only works from one direction though. You might be able to set up several 
cameras and screens to provide half-way round invisibility, but you cannot 
get all-round invisibility that way.

Would it be dangerous? Well I think that sort of arrangement could be 
cooked up without there being any electrical or radiation danger. But there 
would be an enormous moral danger that you would be tempted to cheat if you 
had this invisible computer! And a practical danger that any teacher that 
wandered around the room rather than just standing at the front would 
easily see your machine, and you might get into trouble.

John.