NEW YORK - A cafe customer fed up with cell phone chatter sits in a bubble of blissful silence as nearby patrons puzzle over dead handsets.
A man tries to take a secret snapshot with his camera phone but gets only a blank screen.
A priest imbues his church with a new energy - the electromagnetic kind - to keep his sermons serene and free from beeps, chirps and ring tones.
These are glimpses in a war of gadgets quietly playing out around the world.
As millions embrace the freedoms of mobile communications, some people and companies are pushing back against the tide. They are fighting technology with technology, using detectors, jammers and other gizmos to defend privacy, security and sometimes sanity.
Jamming cell phones is illegal in the United States, but with pocket-size jammers sold online by foreign companies and even on eBay, and the military and governments using such devices, the wireless fight has been engaged.
"It's like the battle between the radar detector and radar guns. It keeps on escalating," said Jeff Kagan, an independent telecommunications analyst based in Atlanta. He said the need for such devices is prompted by the "double-edged sword of technology."
"The inventor of the cell phone never thought about the fact that people would be using them constantly and impinging on other people's privacy," he said.
"The inventor of the camera phone never thought about the fact that they would be used in locker rooms and other inappropriate places."
How it works
Jamming a cell phone, essentially a two-way radio, is relatively straightforward.
Jammers typically disrupt the communication between handsets and cellular towers by flooding an area with interference or selectively blocking signals by broadcasting on the same frequencies that phones use.
Some jammers have to be as smart as cell phones, which try to increase power or hop to other radio channels to avoid interference.
Depending on their power, jammers can disrupt communications in an area spanning a few yards or across several miles.
Commercial jammers have been sold overseas for years, and some Internet postings offer instructions on building homemade models.
The Federal Communications Commission prohibits people in the United States from building, selling, operating or importing radio-jamming devices. People who violate this provision of a 70-year-old communications law face up to a year in prison and fines of $11,000 for each violation.
FCC officials say they have received very few complaints about jammed cell phones and have never taken action against anyone for that violation.
Those in the jammer industry say people use low-powered devices with little fear of reprisals, because it's difficult, if not impossible, for a caller to distinguish between a jammed signal and an ordinary cell phone dead zone.
Despite that, the U.S. law deters jammer use and limits its spread among consumers, said Kagan, the analyst.
The wireless industry says jamming devices endanger the public.
"One-hundred and fifty million Americans rely on wireless phones. If those phones are jammed, doctors might miss calls from hospitals or parents could miss emergency calls from baby sitters," said Travis Larson, spokesman for the Cellular Telecommunications and Internet Association.
Larson said that to cut down on the annoyances of cell phone use, customers should use their "mute button, volume control, vibrate mode, voice mail and an on-off button" when appropriate.
He said jamming a specific area where silence is expected, such as a movie theater, is still a risk, even with warning signs.
"Jammers may leak into other adjacent frequency bands, blocking public safety radio signals used by police officers and firefighters," he said.
But safety concerns, courtesy suggestions and the law haven't stopped people from buying jammers.
The British company Global Gadget UK Ltd. sells to people in other countries an array of jamming and detection products, including a portable jammer disguised as a cell phone that can disrupt cellular communication up to 45 feet away.
"You will be able to silence those anti-social types who insist on using their mobile phones in the most indiscreet way," an Internet ad for the product says. "The beauty is that they will not know it is you that has switched them off! All they will see is that their signal has dropped."
Michael Menage, the company's director, said he sells hundreds of pocket jammers to people in the United States, his biggest market. Each costs about $320.
"They're illegal to use over there. People are still quite keen to buy them," Menage said. He said he has sold to the U.S. military but that most customers are individuals or small businesses tired of cellular distractions.
He said some businessmen use jammers to keep meetings quiet or to disable possible eavesdropping devices, particularly cell phones rigged to be bugs.
Also illegal in England, jammers are more widely used in other parts of the world, including the Middle East, Africa, Asia and some parts of Europe. They are blocking phone calls in theaters, restaurants, libraries, prisons and religious buildings like mosques and churches.
Jamming also has long had a military and security function.
Law enforcement officers have used jammers to block mobile communications in dangerous situations, such as isolating hostage takers or protecting government officials on the move.
Jamming is used by U.S. troop convoys in Iraq to protect against remotely detonated roadside bombs. Some bombs use cell phones as wireless triggers, and a jamming signal can stop or delay an explosion.
Pakistani intelligence officials say jamming devices in the motorcade of Pakistan's President Pervez Musharraf delayed the detonation of a huge bomb that blew up moments after his car passed over a bridge Dec. 14.
Other alternatives are available to block, detect, deter or selectively disable cell phones in ways that avoid breaking the law in some developed nations.
No law prohibits blocking signals with thick walls or metal mesh cages surrounding or built into structures. Some hospitals use such cages to protect sensitive medical equipment from radio interference.
In the future, another option might be wood paneling infused with magnetic particles that can block radio waves. Japanese researchers, who announced their invention in 2002, said they hope it will be available in hardware stores.
BlueLinx Inc., based in Charlotte, N.C., is developing a product called Q-Zone, which tells phones equipped with Blue- tooth wireless technology to lower their volume or switch to vibrate when they pass into certain areas, said company Vice President Mary Beth Griffin.
Another way to deter cell phone use is a cellular detector that flashes lights, sounds an alarm or speaks a polite reminder when it senses an active phone within a certain range.
Starport International Ltd., another British company, distributes the Cellphone Detector Plus, a $450 gadget manufactured by Redmond, Wash.-based Zetron Inc.
The devices are used in hospitals, concert halls and theaters, serving as an electronic version of a "no cell phones" sign, said Jonathan Lemel, president of Starport International.
He said companies concerned about industrial espionage and the U.S. military also are customers.
But even devices meant to protect people from disturbances can quickly turn around and affect privacy.
Lemel said the detectors can be used in factories or warehouses, where employees are forbidden to make calls during bathroom breaks. The detectors are installed behind a false ceiling with a loud disembodied voice surprising and chastising those making calls.
"It's like a voice from on high," he said. "It has a very good effect, that does."