In 2016, U.S. Embassy staffers and their families in Cuba started experiencing strange symptoms: headaches, dizziness and insomnia, almost all of which were triggered by unusual, high-pitched sounds, like buzzing, humming or grinding metal.
The cause of these symptoms has puzzled experts for more than a year, but now, secretive "microwave weaponry" is emerging as a top suspect, according to The New York Times. The problem is, while everyone agrees that microwaves, bizarrely, can make people hear sounds, it's far from settled whether they could cause the kind of damage experienced by the Americans in Cuba.
"It isn't even close to a plausible theory," said Kenneth Foster, a professor of bioengineering at the University of Pennsylvania. [27 Oddest Medical Cases]
The Cuban medical mystery began in November 2016, when staffers from the U.S. Embassy started reporting intense mechanical sounds in their homes and hotel rooms. A March 2018 study published in the journal JAMA on 21 of the 24 individuals thought to be exposed confirmed a range of long-term symptoms, including trouble with balance, headaches, hearing loss, sleep problems and trouble concentrating.
"These individuals appeared to have sustained injury to widespread brain networks without an associated history of head trauma," the authors of that study concluded.
So what could have caused brain injury without head trauma? Investigators have considered the possibility of sonic weapons or mass hysteria. According to ProPublica, FBI agents even questioned an insect biologist on the theory that the noises the diplomats heard could have been made by cicadas or other insects.
Several scientists, though, think that microwaves are the most likely culprit. The case, as argued by bioelectromagnetic researcher James Lin, a professor emeritus at the University of Illinois at Chicago, is based on the 1961 discovery that people can hear microwaves.
Hearing microwaves doesn't make much sense on the surface: Microwaves are electromagnetic energy that doesn't require a medium, like air or water, to transmit. Acoustic signals, on the other hand, are vibrations that do require a medium. Our ears, of course, are designed to pick up acoustic signals, not electromagnetic ones.
However, Lin told Live Science, people can hear microwaves that are focused in a beam and pulsed rapidly — a phenomenon discovered in 1961 by a biologist named Allan Frey. It works like this: Each tiny microwave pulse, lasting 10 microseconds or so, hits the tissues of the head and heats them up an infinitesimal amount, about a millionth of a degree Celsius. Scientists have mathematically calculated this number, rather than measuring it directly.
"There is no instrument that I know of that can measure [the change]," Lin said.
These micropulses are quite different from the nearly continuous waves that heat food in a microwave oven, he said. The itsy-bitsy — but very rapid — temperature change they generate expands the water in the tissues of the head enough to generate vibrations — an acoustic wave. This acoustic wave then travels through the soft tissue into the denser bone, which transmits the wave to the inner ear. From there, the sound wave is transformed into a nerve impulse, in the same way typical acoustic sounds are.
Typically, this sound is subtle, often compared to the click of two rocks knocking together underwater. Lin said an intense enough microwave beam, focused by a radar dish, could generate an acoustic wave inside the head big enough to damage brain tissue or the inner ear.
"To me, based on all the reported information, I don't think there is any other way that could happen," Lin said. "People are in the same room, [and] some people don't hear it; some people hear it. How would you target it? It has to be a beam of microwaves."
The case against microwaves
Then again, maybe not. It's impossible for a beam of microwave pulses to do auditory damage without essentially vaporizing the target, Foster told Live Science.
"You'd have to increase the intensity from anything that's been experimented by maybe seven or eight orders of magnitude, and then the subject would just go up in smoke," said Foster, who researched the possibility of microwave weapons for the U.S. Navy starting in the 1970s. The heat generated by the microwave pulses would simply be too intense, he said.
The military has tried to develop microwave devices to generate unpleasant or irritating sounds over the years, Foster said, but there is no way to get intense enough sounds to physically harm someone with excessive sound pressures without physically harming them with heat. The military does have an electromagnetic weapon called the Active Denial System, which uses millimeter-scale waves in longer pulses to beam heat at targets at a distance. These waves heat the water in the tissues, creating a burning sensation, but the heat penetrates only a fraction of a millimeter into the skin and dissipates quickly when the target escapes the beam. The system was used briefly by the U.S. military in Afghanistan in 2010 but proved unwieldly for combat. The vehicle-mounted equipment is bulky, expensive and difficult to repair, Foster said. [Flying Saucers to Mind Control: 22 Declassified Military & CIA Secrets]
Another strike against the microwave theory is that the Associated Press released a recording of the sounds linked to the diplomats' symptoms in 2017. A microwave-generated sound can't be recorded or heard anywhere outside the target's own head.
"That is one thing I'm trying to tie down," Lin said. "Some media reports say they recorded the sound waves and so forth, so that doesn't seem true to me."
Regular old acoustic sounds are a more likely culprit for the diplomats' woes than a microwave attack, Foster said. In March, researchers from Zhejiang University in China and the University of Michigan found that they could re-create sounds like the ones released by the Associated Press with the intersection of ultrasound waves, which are normally too high-frequency for the human ear to hear.
Ultrasound is used for a variety of purposes, from room-occupancy sensors to rodent-repelling devices, the authors wrote. Or, Foster said, there could be spycraft that the intelligence community would rather not discuss openly; ultrasound could be used in listening devices covertly installed in the diplomats' homes or hotel rooms.
"If that's what's going on," he said, "there's no way the government would tell you that story, so instead they're letting reporters go on about microwave hearing and all that sort of stuff."
Editor's note: This article was updated at 3:35 p.m. to clarify that it might be possible to create bothersome noises with harmless microwave pulses, but not to harm a person with microwave-generated sound pressures without first harming them with heat.
Originally published on Live Science.