Originally posted by Uncle Al:
A force field has magnitude and direction associated with its every contained point. That is the whole of it.
Well, although what you say is correct, it may not accurately answer his question.
If instead of a force-field, goofkid means an invisible or transparent 'sheild' of some type, then such things DO exist.
A faraday cage is a way to screen out stray electromagnetic radiation, usually microwaves or radiowaves. Essentially its a wire mesh. Your micro-wave oven has one of these in the glass of the door (metal screen with holes in it).
There are also mu metal cages which can 'block' magnetic fields also. Actually they work by inducing a magnetic field with opposite sign to the intruding field which is being blocked, so there is no net field.
However, you can walk right through such mu metal cages as the gaps between the metal coils can be large. However, to a magnetic field the cage would act like a real barrier.
Even ordinary glass is a sheild of some kind. It blocks objects with mass, such as water, air molecules, hands, etc, and it also blocks some radiation (UV, IR), but it passes visible light right through.
So it depends what you're trying to block with a 'force field'. If you're trying to block visible light, you'd pick one kind of sheild. If you're trying to block magnetic fields, you'd pick another, if you're trying to block longer wavelength light (radio/microwave) you'd pick a different kind of sheild.
If you're trying to block physical objects, like a baseball, then you need to put a physical sheild in the way. Or you can get tricky and ionize the baseball (put a charge on it) and deflect it using voltages on metal plates. Electrons are accelerated in a cathode ray tube (old-style TV set) by a similar effect. By putting a metal plate in the way of a beam of electrons, and by holding it at a high enough voltage, electrons can be 'deflected' without ever touching anything. They can also be deflected by putting a magnetic field nearby.