Portable radios click. Not to mention they broadcast on the waves outdoors. Putting a note is dangerous. Cell phones do not cut. And a scream would ruin everything. So how do you whisper a message to a colleague without being noticed?
How do you pass a message while you're in a crowded room without blowing up your cover?
How do you listen to music while studying under the watchful eye of your parents?
The Massachusetts Institute of Technology (MIT) has found a way.
They are transmitting lasers to people's ears.
There are no specialized headphones receiving the signal.
Only the original and naked Mark I ear canal.
And it's all because of something called photoacoustic effect.
The invisible laser interacts with the environment.
In particular, moisture.
Where the laser reaches the humidity of the air, its light is absorbed and transformed into sound waves. And sweeping the laser to different lengths at the speed of sound can be used to encode different audio frequencies.
"Our system can be used at some distance to transmit information directly to someone's ear," says project leader Charles Wynn.
The lasers were used before, to decode the vibrations of the voices as they hit surfaces like glass.
But it transmits silently.
It is something that whispers in the silence of a quiet night. It is also something that can convey a segmented message in a crowded room without anyone listening to the conversation.
And the effect only works at pre-programmed distances.
Someone walking through the invisible beam of light will not hear anything.
But will the laser fry your hearing?
No, the MIT team insists. The laser system used can not harm the skin or the eyes. And the sound that tickles the inner ear follicles is the same as any other.
It's just that it comes from the vibration of moisture that only your ears can detect.
"This can work even in relatively dry conditions because there is almost always a little water in the air, especially around people," says Wynn.
So far, lab experiments have been able to transmit clearly audible messages (60 decibels) for about 2.5 meters. They say the technology can be easily sized to cover greater distances.
"We expect this to become a commercial technology," says lead author of the study, published in the journal Optics Letters, Ryan Sullenberger. "There are many interesting possibilities and we want to develop communication technology in useful ways."