Quote:
Originally Posted by Moontanman
How difficult would it be to detect laser light from another star? Detection of even a quick flash of laser light would be proof positive of an alien civilization. Would it be possible to set up a back yard type detector to look for laser pulses over interstellar distances?
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I doubt this would work.
The
McDonald Laser Ranging Station’s 15 gW attowatt (
) laser currently used for the
Lunar Laser Ranging Experiment detects on the order of 1 photon per second return reflection from the retroreflectors placed on the Moon by the Apollo missions. So, if some folk at
Alpha Centauri, about 4.4 LY distance (about 1 billion times the distance of the Moon) were doing something like the LLRE, and unintentionally illuminated the Earth, they would have to keep the beam on for about 37 billion years for telescope like MLRS’s 30 inch one to detect a single photon.
There’s nothing recognizable about a single photon of laser light (unless, as in the case of the LLRE, you know exactly what wavelength to expect) against the black body spectrum of background starlight. What makes laser light recognizable is getting a lot of photons of the same wavelength. So the beam of a distant laser like our most powerful astronomical lasers is “washed out” into noise at distances more than a few times the distance to the moon.
The MLRS’s laser beam is actually set to disperse (widen) about 1000 times more than its best beam. This is done to assure that it hits its target lunar retroreflector given the unavoidable jitter of its mount and aiming mechanics, analogous to widening the choke on a shotgun to make it easier to hit a moving target, so in principle, the beam from an extra-solar laser like it could be about a million times stronger. We’d then need only 37 thousand years to expect to catch one of it’s photons. If we had a giant space telescope with a primary mirror/lens about the diameter of the Earth, we could expect about 40 photons/sec, possible enough to detect the laser. However, it’s fairly inconceivable that such a laser would accidentally stay focused on the Earth even long enough for one photon to be received, and giant space telescopes are certainly beyond the grasp of backyard astronomy
Quote:
Originally Posted by Moontanman
Laser pulses would be unlikely to be false positives, could it be done at least as easily as a radio telescope set up?
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The optics are simple enough – any optical telescope with a spectrometer will do – but as noted above, false positives – distinguishing a single photon of laser light from billions of identical photons of ordinary starlight – would be nearly impossible, unless you knew in advance its exact wavelength.
I can only imagine a couple of scenario where we might detect an extrasolar laser beam.
One would be if it came from gigantic laser intentionally aimed at the Sun in order to propel a spacecraft similar to
laser lightsail propelled Prometheus imagined in the hard science fiction of Robert Forward. In this scenario, such detection would give us a few decades advance notice to expect visiting explorers.
Another would be if the ETs had detected us, and
wanted to make their presence known. A laser like the MLRS’s could illuminate the earth with a density-rate of about 10 photons/s/m^2, roughly the same as the LLRE return reflection. If this were the case, however, and the ETs guessed that we were monitoring the sky with projects like
SETI, they’d likely chose instead signal via radio frequency, rather than laser.
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A killer signal/noise problem, via scaling up LLRE specifications
