Author: Joseph Lazio <jlazio@patriot.net>

There are two possibilities for sending information to other
technological civilizations over interstellar distances: send matter
or send radiation.  The focus in SETI has been on detecting
electromagnetic radiation, particularly radio, because compared to all
other known possibilities, it is cheap, easy to produce, and can
travel across the Milky Way Galaxy.

Compared to radiation, most matter has a distinct disadvantage: it is
slow.  Radiation can travel at the speed of light whereas (most)
matter is constrained to travel slower.  Distances between stars are
so large, it makes no sense to use a slow mode of communication when a
faster one is available.  The speed at which spacecraft travel is the
primary justification why there is little effort spent within the SETI
community searching for interstellar spacecraft (that and the fact
that there is no evidence that there are any such interstellar
spacecraft from other civilizations in our vicinity).  A secondary
justification is that spacecraft are relatively expensive.  The launch
of a single Earth-orbiting spacecraft can cost US $100 million.  It
is difficult to imagine building and launching a fleet of interstellar
spacecraft for US $500 million, yet this is the estimated cost of a
next-generation radio telescope capable of detecting TV signals over
interstellar distances.  It is possible that future technology will
make spacecraft cheaper.  It is difficult to imagine a technology that
would make spacecraft cheaper without also lowering the cost of a new
telescope.

Although chunks of matter, i.e., spacecraft, seem a rather inefficient
way to communicate across interstellar space, what about a beam of
matter.   Most often suggested in this context is a beam of neutrinos.
Neutrinos are nearly massless so they travel at almost the speed of
light.  They also interact only weakly with matter, so a beam of
neutrinos could cross the Milky Way Galaxy without any significant
absorption by interstellar gas and dust clouds.  This advantage is
also a disadvantage:  The weakness of their interaction makes it
difficult to detect a beam of neutrinos, far more difficult than
detecting a beam of electromagnetic radiation.

(A beam of electrons or protons could be accelerated to nearly the
speed of light and would be far easier to detect.  However, electrons
and protons are charged particles.  When travelling through
interstellar space, the direction of their travel is influenced by the
magnetic field of the Milky Way Galaxy.  The Milky Way's magnetic
field has "small-scale" irregularities in it that would divert and
scatter such a beam.  The result is that one could not "aim" such a
beam in any particular direction [except possibly to the very closest
stars] because its actual path would be influenced by the [unknown]
direction[s] of the magnetic field it would encounter.)

The known forms of radiation are electromagnetic and gravitational.
Electromagnetic radiation results from the acceleration of charged
particles and is used commonly:  Radio and TV broadcasts are radio
radiation, microwave ovens produce microwave radiation, X-ray machines
produce X-ray radiation, overhead lights produce visible radiation,
etc.  Gravitational radiation results from the acceleration of massive
objects.  Gravitational radiation has never been detected directly,
and its indirect detection resulted in the 1993 Nobel Prize.  Gravity is
a much weaker force than electromagnetism.  Thus, detectable amounts
of gravitational radiation result only from events like the explosion
of a massive star or the gravitational interaction between two closely
orbiting neutron stars or black holes.  Again, it is possible that a
future technology might result in gravitational radiation becoming
easier to detect.  It is still difficult to imagine that it would not
also result in electromagnetic radiation.

Of the various forms of electromagnetic radiation---radio, microwave,
infrared, visible, ultraviolet, X-ray, and gamma-ray---only radio and
gamma-ray can cross the Milky Way Galaxy.  The other forms suffer
varying amounts of absorption by interstellar dust and gas clouds
(though they could still be used to communicate over shorter
distances).  Gamma rays are extremely energetic and are produced by
events like the explosion of nuclear bombs.  Radio radiation is far
less energetic.  Thus, to send the same amount of information requires
far less energy (i.e., it's cheaper) to send it via radio than gamma
ray.

The above are merely plausibility arguments to suggest why radio is
likely to be a preferred method of communication among technological
civilizations.  Of course, they may reason that they are only
interested in communicating with other civilizations technologically
advanced enough to transmit and detect neutrino beams or gravitational
radiation (or maybe even some undiscovered method).  If so, the
existing radio SETI programs are doomed to failure.  Nonetheless, it
does seem sensible to search first using the most simple technology.