The problem of whether invisible men exist is a thorny one. We simply don't know, because they are invisible.
Until recently, the only solution to the problem of invisible man detection seemed to be the inelegant and unreliable method of observing the invisible man's effects on his surroundings, footsteps being the classic example. However, recent technological advances in quantum cryptography have at last provided a reliable, elegant, and surefire method to detect invisible men.
Of course invisible men can see--otherwise they'd step into things. Since sight requires light, invisible men must remove at least some light from their surroundings. Their pupils should therefore appear black. The invisible man has a way around this problem, however. If invisible men simply dublicate all light which hits their eyes, observing one photon and sending one on, they can remain concealed to the classical observer.
This is where quantum mechanics steps in. The classical observer is fooled by the duplication of the light--but it is not possible to duplicate quantum states. This fact allows quantum cryptography to insure that its information has not been intercepted. Since quantum cryptography encodes its information in photons, exactly the same technique could be used to insure that light crossing the room had not been intercepted by an invisible man.
Polarization of light can be measured using one of three bases: rectilinear, circular, and diagonal. Observation of any of these three states will change the state of the others. In quantum cryptography, data is recorded in polarization states so that an eavesdropper will change some of the data merely by observation, thus making their presence known. An invisible man detector would work by a similar method.
The detector would comprise two elements: a sender and a receiver. Each would generate the same random sequence from a shared seed. The sender would then send data taken from the random sequence, in a basis taken from the random sequence. The receiver would measure only that basis, and check for agreement. Disagreement between the data and the generated sequence would indicate that someone had observed the light. Someone...invisible!
This hypothetical invisible man detector could operate similarly to walk-through metal detectors. As it stands, an invisible man could simply walk into an airport, board a plane, and conduct terrorist activities. This crucial gap in our preparedness will be solved when a quantum detector is added to every airport, military base, and other sensitive locations.
Invisible man detection has gone a long way, from the clumsy mob actions of a hundred years ago to the sophisticated mob actions of today. The time has come to step into the 21st century with a quantum solution to a threat you'll never see coming.