# Is it feasible to detect light emitted from planet earth 2,000 years ago by reflection from an object situated 1000 light years away?

The possibility though remote, is intriguing as we may be able in the future to actually "see" our own planet's history. Though sounding science fiction, if we are able to detect bodies in space that are able of reflecting light emitted from our planet earth, using amplification systems and filters,this may - if possible, give us a tool of utmost importance.

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–  Qmechanic Jan 21 '13 at 0:51
Yes. It is possible to do so. –  Monster Truck Jan 21 '13 at 1:03
We'd want to calculate the number of photons that are incident & then reflected from the earth (from the sun, the brightest source in our vicinity), then assume a $\frac{1}{r^2}$ law to see how many of those reflected photons hit the distant planet, then determine what fraction of those photons are reflected from the distant planet, give it another $\frac{1}{r^2}$ law on the way back, and see how many photons get back to us. I'm guessing it's <<1 per year, but haven't done the math. At that rate, it's going to be hard to separate that expected photon each year from noise. –  Will Cross Jan 21 '13 at 3:55
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## 1 Answer

Not in any practical sense. First of all, the intensity will be very low: inverse-r-squared going out to the reflector, and inverse-r-squared coming back. As @WillCross pointed out, it will be at very low levels. Secondly, the resolution will be very poor, as there's nothing to focus it and the subtended angle is very very small (in both directions).

Consider that we already do have such a reflector, about 1.5 light seconds away: the dark side of the Moon, illuminated by Earthshine. It somewhat lights up the lunar surface, and variations in the total illumination can be detected that correlate to cloud cover (increased reflection) on Earth. Even being so close, there's no way to see any sort of useful images as you envision. Maybe you could detect a massive nuclear war by reflections from the Moon or nearby planets, but at light year distances, it would be hard to sort out from the noise.

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