7.10 Radio Waves and Biological Systems
221
passing wave, whose cross-sectional area can be much larger than the antenna wire
area itself. This means your cell phone antenna can be relatively small in area. 19
7.10 Radio Waves and Biological Systems
Until Maxwell’s predictions, life forms on Earth were oblivious to radio waves. With
wavelengths much larger than typical organisms, radio waves from outer space did
not have much effect, nor did organisms on Earth evolve to use radio waves to
communicate.
These days, humans blast radio waves to space, making this planet an obvious
location of intelligent life, if any other such intelligent life forms were to monitor
our transmissions. Our planet radiates billions of watts of man-made radio waves
into space, containing our signals. 20 An additional one billion watts in radio waves
radiate from currents in aurora near the Earth’s poles. 21 We expect advanced
civilizations to use radio to transmit and receive information, although, once they
start using global wires and fibers, their intelligent radio radiation with high power
would diminish. Moreover, unless their transmitters focused directly toward Earth,
radio noise from stars would obscure the signal, even over the distance to nearby
extrasolar planets.
It is also possible to safely transfer large amounts of power via radio and
microwaves. Solar power collected in space or on the Moon can be sent to Earth over
microwave beams, and collected by elevated wire grid antennae covering several
acres of land. These grids would let sunlight through, so cows could eat grass under
them.
Small remote sensing devices, such as chemical, electrical, thermal, and video
probes, encapsulated in pills or implantable devices, are routinely used. They
transmit information and receive instruction via radio waves.
Radio waves have a relatively low absorptivity in biological tissue. However,
those individuals with implanted pacemakers have to be cautious around radio
transmitters. The stray signals can induce currents in any implanted metal, including
the wires of the pacemaker. These currents, even though small, may be enough to
interfere with the operation of the pacemaker, or stimulate the heart into arrhythmia
through the wires. Pacemakers themselves have an average current drain of only
about 20 microamps.
19 For cell phones, the length of the antenna can also be reduced by making the antenna from a
substance with a large electric permittivity, decreasing the resonance length.
20 TV signals in the 100 MHz range at one time dominated our radio radiation, spreading in a bubble
with a radius in light-years equal to today’s date minus 1950.
21 Donald Gurnett, The Earth as a Radio Source: Terrestrial Kilometric Radiation, J Geophys Res
79, 28 (1974).
221
passing wave, whose cross-sectional area can be much larger than the antenna wire
area itself. This means your cell phone antenna can be relatively small in area. 19
7.10 Radio Waves and Biological Systems
Until Maxwell’s predictions, life forms on Earth were oblivious to radio waves. With
wavelengths much larger than typical organisms, radio waves from outer space did
not have much effect, nor did organisms on Earth evolve to use radio waves to
communicate.
These days, humans blast radio waves to space, making this planet an obvious
location of intelligent life, if any other such intelligent life forms were to monitor
our transmissions. Our planet radiates billions of watts of man-made radio waves
into space, containing our signals. 20 An additional one billion watts in radio waves
radiate from currents in aurora near the Earth’s poles. 21 We expect advanced
civilizations to use radio to transmit and receive information, although, once they
start using global wires and fibers, their intelligent radio radiation with high power
would diminish. Moreover, unless their transmitters focused directly toward Earth,
radio noise from stars would obscure the signal, even over the distance to nearby
extrasolar planets.
It is also possible to safely transfer large amounts of power via radio and
microwaves. Solar power collected in space or on the Moon can be sent to Earth over
microwave beams, and collected by elevated wire grid antennae covering several
acres of land. These grids would let sunlight through, so cows could eat grass under
them.
Small remote sensing devices, such as chemical, electrical, thermal, and video
probes, encapsulated in pills or implantable devices, are routinely used. They
transmit information and receive instruction via radio waves.
Radio waves have a relatively low absorptivity in biological tissue. However,
those individuals with implanted pacemakers have to be cautious around radio
transmitters. The stray signals can induce currents in any implanted metal, including
the wires of the pacemaker. These currents, even though small, may be enough to
interfere with the operation of the pacemaker, or stimulate the heart into arrhythmia
through the wires. Pacemakers themselves have an average current drain of only
about 20 microamps.
19 For cell phones, the length of the antenna can also be reduced by making the antenna from a
substance with a large electric permittivity, decreasing the resonance length.
20 TV signals in the 100 MHz range at one time dominated our radio radiation, spreading in a bubble
with a radius in light-years equal to today’s date minus 1950.
21 Donald Gurnett, The Earth as a Radio Source: Terrestrial Kilometric Radiation, J Geophys Res
79, 28 (1974).