What is OSETI?
OSETI is an acronym for Optical Search for Extraterrestrial Intelligence,
which is a relatively new adaptation of the Radio SETI projects currently in progress.
The primary difference between Optical SETI and Radio SETI
is that Optical SETI searches for pulses of laser light in or near
the visible portion of the light sprectrum, while Radio SETI searches
for signals within the radio and microwave portions of the light spectrum.
For a more detailed look at OSETI, and for answers to such questions as
"Is discovering extraterrestrial life likely to happen?",
please read on or jump to any one of these sections:
Origins of SETI and OSETI
The original idea to search the skies for signals from intelligent life elsewhere
in the universe came from Giuseppe Cocconi and Philip Morrison. Cocconi and Morrison
published a paper in the journal Nature
in 1959. The paper outlined the possibility of interstellar communication with
MASERs operating in the 21 cm range
based in the frequency of hydrogen oscillations, the only known astronomical
microwave wavelength at the time.
Even though the invention of the LASER
came only a year later, the first LASERs were low-powered devices compared
to the much more highly evolved radio wave devices. This is the most likely
reason that earlier searches for extraterrestrial intelligence were conducted
in the radio portion of the light spectrum instead of the visible portion
of the light spectrum.
Since then, LASER technology has improved dramatically, and there is now
substantial evidence that interstellar communication may perhaps better
be performed in the optical portion of the light spectrum. Consequently,
several research institutions are initiating optical versions of the original
Optical SETI versus Radio SETI
Several arguments exist for the choice to search for signals in the optical region
of the electromagnetic spectrum
over the radio portion. These reasons stem primarily from the benefits for another
civilization to send a beacon or signal in the visible rather than in the microwave.
Briefly, some of the reasons include:
- Visible light-emitting devices are smaller and lighter than microwave
or radio-emitting devices.
- Visible light-emitting devices produce higher
and can consequently send information much faster.
- Interference from natural sources of microwaves is more common
than from visible sources.
(See the technical paper
on OSETI for additional details.)
- Naturally occurring nanosecond
pulses of light are mostly likely nonexistent.
Now in more detail: Most of the benefits of operating in the visible spectrum
are the result of light having a smaller wavelength
and higher frequency than
microwave radiation. For instance, compared to the large antennae needed for radio
wave emission, lasers are extremely small and light. In addition, due to the higher
frequency of light waves (1.4 gigahertz
versus 430 terahertz for hydrogen
microwaves and red light), carrier signals with lasers can be much higher frequencies
allowing extremely fast data transfer.
The second reason to prefer an optical signal to a radio one may be even more
important. Unlike microwaves, where sudden signal spikes can come from things
such as spark plugs, brief spikes or pulses of visible light are rare in the universe
as we know it. Therefore if a laser pulse is detected, the probability is higher
that it is actually from intelligent life and not from a natural astronomical event.
In fact, nanosecond spikes of light are thought to be nonexistent in the universe.
Frequently Asked Questions
What power would a transmitting laser's beam need to be in order for us to detect it?
Light coming from a star alone filtered to one part in 10,000 amounts to 4
Joules of energy every nanosecond.
Therefore, a laser signal coming from near the star must exceed 4 Joules within
a nanosecond in order for it to outshine the star. Modern lasers designed
for nuclear fusion can exceed this power requirement by some 300,000 times
(see the technical paper).
Even if the exact frequency of transmission is unknown, such a laser would still
outshine the sender's star by 30 times during the nanosecond pulse without
What is the maximum distance from which we could detect a signal?
Establishing contact as far away as 100 light-years
should present no problem. Using research grade equipment, detecting signals
from distances up to 1000 light-years are feasible.
For what time period is each star examined?
Some debate lies within the details of the question of how long each star
should be exmined. If we assume that the sending civilization will try
to communicate with the nearest 1,000 likely candidates for intelligent life,
using modern apparatus capable of redirecting a laser 10 times per second,
the civilization could illuminate all 1000 solar systems in 100 seconds.
If this is the case, then we could expect to receive a pulse once every 100 seconds.
However, if the number changes from the 1,000 to the 1,000,000 closest candidates,
then it would take about 28 hours to illuminate the entire field. This would mean
that we would have to observe each of a million stars for over a day each;
not a feasible research goal. However, it is highly likely that the transmitting
civilization might be using multiple transmitters equipped with technology that might
allow it to illuminate more than 10 stars per second, thereby reducing the time each
star needs to be scanned to a more realistic figure.
As for how long we actually examine each star: about ten minutes.
The duration for which we examine each star is determined by a non-OSETI
project at Harvard, which is searching nearby stars for extrasolar planets.
The Harvard OSETI project piggybacks on this other project, so their OSETI
project examines each star that the other project examines, for however
long they examine each star. They typically examine each star for ten minutes
or more, depending upon the brightness of each star. Since we'll be observing
each star simultaneously with Harvard, we'll also be examining each star
for about ten minutes, longer for dimmer stars.
What stars are being scanned?
Stars similar to our own make likely candidates for having intelligent,
communicative life. The closest stars with or capable or having planets
and supporting life should be given priority over stars with short life spans
and other stellar objects such as close binary stars, which are unlikely
to have planets capable of sustaining life.
The list of stars believed to be likely candidates for having intelligent,
communicative life, is a subset of those that are being searched for extrasolar
planets. We are examining the same stars that the extrasolar planet search
based at Harvard is searching.
What kind of signal are we looking for?
We are looking for pulses of visible light, several nanoseconds
in duration. The amplitude of the pulse must significantly exceed
the strength of illumination from the actual star. See Ed Groth's
more detailed description of the signals
that we and Harvard are looking for.
What frequency of light are we looking for? Is there a preferred wavelength?
There is no known preferred frequency or wavelength of visible light.
Several have been discussed, such as several wavelengths that are
naturally absorbed by stellar or interstellar atoms and ions, and should
therefore not naturally be visible. However, unlike in microwave and radio
SETI, searching an exact frequency is not vital because the expected
transmitting beam should outshine the unfiltered light from the star
by at least 30 times.
Are there any natural phenomena that could trigger false readings?
None that we know of that we cannot compensate for.
- Cerenkov flashes can produce no more
than one photon during a nanosecond, not enough to render a significant reading
in the detector.
- Lightning from distant storms is also insignificant, since it is unlikely
to deliver pulses under a microsecond
- Atmospheric glow produces a steady stream of photons, but only about 100
per pixel per second in a 1 square meter telescope, which in a 10,000 pixel
detector would only result in .001 photons per nanosecond; not nearly enough
to render false readings. OSETI can theoretically be conducted during the day,
if the sky is clear.
- The high voltages used within our detectors cause electrical arcing
within the detectors, which in turn trigger the detectors. We therefore
use two detectors, and ignore all "detections" that do not occur in both
detectors simultaneously. Only coincident detections in both detectors
are considered as possible OSETI events.
- Cosmic rays striking the atmosphere cause cascades of high energy
particles and photons, which can in turn trigger our detectors if the
photons enter our telescope. Photons generated by one such cosmic ray
could not enter both our telescope and Harvard's telescope, so we and
Harvard compare the timing of our possible OSETI events to eliminate
such atmospheric sources. We detect about one possible OSETI event
per hour of observation.
Are there any other research institutions currently engaged in OSETI projects?
Yes. Please click here for links to the Web pages of other OSETI projects.
What kind of detector is being used?
A fast optical detection system designed by Paul Horowitz is being used with our
36" reflecting telescope. The detection system uses a beam splitter and two fast
photodetectors to check for coincident events (signals in both detectors simultaneously).
This eliminates several false detections resulting from events originating
in the detection system itself.
Is it likely that intelligent life exists on other planets in the galaxy?
One method of determining the number of potentially communicating planets in the galaxy
is to use the Drake equation
Using this simple calculation, the results of which varies greatly depending on what
numbers you use (such as the number of stars with planets, and the lifetime of a
communicating civilization), the number of actively communicating civilizations
in the galaxy can vary from one to tens of thousands. In a galaxy approximately
300 thousand light-years in diameter in which we are nearly centered, the chances
are high that if life exists throughout the galaxy, we would be within a reasonable
range to "hear" a signal from another civilization. In the end, the probability
depends on whom you choose to ask, but even if the chances of actually finding
intelligent life are small, the fact that it would be perhaps the greatest discovery
that mankind has ever made makes SETI, OSETI, and other such projects a worthy effort.
If another civilization was contacted, what would they most likely be like?
If another civilization was actually discovered, chances are they would be far more
advanced than us. This stems from the fact that the creation of technology is a rapid
and brief process when compared to the time scale of life itself. Life has existed
on Earth for thousands of millions of years, yet it is only in the past one or two
hundred years that communication technology has really started. In addition, that
progress of technology, unless interrupted by some sort of disaster, should continue
rapidly until civilization comes in equilibrium with its energy source. At this point
the evolution of technology should plateau for the duration of the planet's civilization.
In other words, the period of time during which a civilization evolves its communications
technology, which might last a few thousand years, is small compared to the millions
of years that a civilization might exist. Prior to the advancement of their technology,
it would be impossible for them to communicate with us. That leaves only the brief
period of technological evolution and the remaining plateau. Since the prior of these
two is relatively short in duration, we would most probably encounter a civilization
in the plateau, making them far more advanced than us. Another possibility however,
is that the civilization undergoes some sort of disaster during the technological
revolution resulting in a "spike" on a plot of technology versus time (see below).
This would mean that the time period during which the civilization could communicate
would be greatly reduced, making it unlikely that we would detect them at all.
Technical Papers on OSETI
- The Technical Case for Optical and Infrared SETI
A technical paper including some calculations, from 1998; PDF format, 219 KB
- Optical SETI at Harvard-Smithsonian
A paper given at Bioastronomy '99, including more project details and some results; PDF format, 132 KB
- An All-Sky Optical SETI Survey
A paper given at IAF Rio '00, including information about the planned all-sky survey; PDF format, 302 KB
- Is there "RFI" in Pulsed Optical SETI?
Preprint of a paper about background interferences for pulsed OSETI; PDF format, 137 KB
- Targeted and All-Sky Search for Nanosecond Optical Pulses at Harvard-Smithsonian
Preprint of a paper describing the OSETI projects and summarizing the results through mid-December 2000; PDF format, 536 KB
- Optical SETI at Princeton
Thesis by Natalie Deffenbaugh describing the Princeton OSETI project through April 2002; PDF format, 1.54 MB
- Search for Nanosecond Optical Pulses from Nearby Solar-Type Stars
Preprint of a paper describing the OSETI projects and summarizing the results through November 2003; PDF format, 418 KB
Links to Other OSETI Web Pages
Please direct comments about these Web pages to