Again, the answer is that they do. There are rather well-authenticated cases spanning a wide variety of “physical effects.” Car-stopping cases are one important class. UFOs have repeatedly been associated with ignition failures and light-failures of cars and trucks which came near UFOs or near which the UFOs moved. I would estimate that one could assemble a list of four or five dozen such instances from various parts of the world. Interference with radios and TV receptions have been reported many times in connection with UFO sightings. There are instances where UFOs have been reported as landing, and after departure, holes in the ground, or depressions in sod or disturbed vegetation patterns have been described. In many such instances, the evident reliability of the witnesses is high, the likelihood of hoax or artifice small. A limited number of instances of residues left behind are on record, but these are not backed up by meaningful laboratory analyses, unfortunately.
A physical effect that does not typically occur under conditions where the description of events might seem to call for it, relates to sonic booms. Although there are on record a few cases where fast-moving UFOs were accompanied by explosive sounds that might be associated with sonic booms, there are far more instances in which the reported velocity corresponded to supersonic speeds, yet no booms were reported. A small fraction of these can be rationalized by noting that the reporting witnesses were located back within the “Mach cone” of the departing UFO; but this will not suffice to explain away the difficulty. One feels that if UFOs are solid objects, capable of leaving depressions in soil or railroad ties when they land, and if they can dash out of sight in a few seconds (as has been repeatedly asserted by credible witnesses), they should produce sonic booms. This remains inexplicable; one can only lamely speculate that perhaps there are ways of eliminating sonic booms that we have not yet discovered; perhaps the answer involves some entirely different consideration.
If we include among “physical effects” those that border on the physiological, then there appear to be many odd types. Repeatedly, tingling and numbness have been described by witnesses who were close to UFOs; in many instances outright paralysis of a UFO witness has occurred. These effects might, of course, be purely psychological, engendered by fear; but in some instances the witnesses seem to have noted these effects as the first indication that anything unusual was occurring. A number of instances of skin-reddening, skin-warming, and a few instances of burns of very unusual nature are on record. These physiological effects are sufficiently diverse that caution is required in attempting generalization. Curiously, a peculiar tingling and paralysis seem to be reported more widely than any other physiological effects. A person who is almost unaware of the ramifications of the UFO evidence may think it absurd to assert that people have been paralyzed in proximity to UFOs; the skeptic might find it inconceivable that such cases would go unnoticed in press and medical literature. Far from it, I regret to have to say, on the basis of my own investigations. I have encountered cases where severe bodily damage was done, or where evident hazard of damage was involved, yet the witness and his family found ridicule mounting so much faster than sympathy that it was regarded wiser to quietly forget the whole thing. At an early stage of my investigations I would have regarded that as quite unbelievable; UFO investigators with longer experience than mine will smile at that statement, but probably they will smile with a degree of understanding.