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Arken
2007-Sep-06, 07:36 PM
Hello. I was thinking about the use of gravitational lensing to find planets and wondered if it could (at least somewhat) get around the inverse square law problem with searching for intelligent radio signals? After all, if light from a lensing event can be magnified, why not other parts of the spectrum?
So should SETI be looking for these lensing events too or am I totally off base here?

antoniseb
2007-Sep-06, 07:42 PM
So should SETI be looking for these lensing events too or am I totally off base here?
- The lensing does occur in all parts of the electromagnetic spectrum, though it doesn't amplify the signal all that much, and lensing events happen very rarely (notice how many planets we've discovered this way, in spite of having observed millions of stars almost daily this way.
- Most planets observed by lensing events are very remote.
- The amplified signal is not from the planet, but from the light of a star behind it.

So in short, it is an interesting thought, but it won't help.

Arken
2007-Sep-06, 08:03 PM
Thanks for that bit of education. :)

Tim Thompson
2007-Sep-06, 10:42 PM
Hello. I was thinking about the use of gravitational lensing to find planets and wondered if it could (at least somewhat) get around the inverse square law problem with searching for intelligent radio signals?
Unfortunately, there is no way around the inverse square law. Gravitational lensing magnifies the visible light of the star, but that light intensity falls off as an inverse square. However, you are correct to assume that planets can be detected by gravitational lensing. At least one planet has been discovered that way (OGLE 2005-BLG-390Lb (http://en.wikipedia.org/wiki/OGLE-2005-BLG-390Lb), about 5.5 Earth masses; Uranus is 5.9 Earth masses), and astronomers are working on techniques to discover Earth mass planets by gravitational lensing, even as we speak (An anomaly detector with immediate feedback to hunt for planets of Earth mass and below by microlensing (http://cul.arxiv.org/abs/0706.2566); M. Dominik, et al., Monthly Notices of the Royal Astronomical Society 380(2): 792-804, September 2007).

I was actually unaware of this until I happened to notice the paper in the current issue of MNRAS. I had already seen your question, but was unsure how to answer. When I saw this paper, I remembered your question at once, and that led me to actually pay attention. So today we both learn something about detecting planets.

As for SETI, detecting planets is only one part of the job. The radio signals that SETI projects look for are all clearly distinguishable as of non-natural, intelligent origin. Find a signal, and you have found ET. Finding a planet, on the other hand, only tells you where to look with the radio telescope to find ET. Then you have the same old inverse square once again.

Nereid
2007-Sep-08, 02:58 AM
There are, no doubt, a number of 'second guessing' propositions that tie SETI to gravitational lensing.

For example, 'if there is an ETI at least as advanced in their understanding of GR as we are, then perhaps they deliberately beam encoded messages, in powerful radio/microwave/IR/optical/{insert your fave here} blasts, at times when relevant alignments are such that (very distant) G type stars will receive them, at strengths way beyond inverse-square'.

But then we could extend this, and say ....