Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

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.

share|improve this question
    
Related: physics.stackexchange.com/q/11940/2451 –  Qmechanic Jan 21 '13 at 0:51
    
Yes. It is possible to do so. –  Monster Truck Jan 21 '13 at 1:03
3  
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
    
There is a huge difference between detecting total illumination and resolving images. Astronomers are detecting light echos of events, but there is a lot of integration (averaging) going on. You can get a (time convoluted) light curve, but resolving detail seems out of bounds. –  Ross Millikan Jul 26 at 4:00

3 Answers 3

Rather late, but as LDC3 resurrected this question from 2013....

This has already happened. Radio waves reflected from what is presumed to be an asteroid swarm about 25 LY away have returned to earth and been identified as early TV broadcasts. Quite appropriately, the first identified signals were episodes of Dr. Who.

However, radio and visible light are rather different things. If you are expecting to watch Julius Caesar walk up the steps of the Portico, it won't happen. We can barely do that from low orbit.

share|improve this answer
1  
Do you have a citation to back up the claim that we have detected our own radio signals? I'm reasonably sure we couldn't detect our own radio signals even 1 ly away with even the most powerful radio telescopes. –  Chris White Jul 26 at 2:09

There are several reflectors that were placed on the moon during the Apollo mission and the number of photons detected (after being reflected) is very small compared to the number of photons the laser emits. See the section "Details"

share|improve this 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.

share|improve this answer
2  
Just a quibble: the dark side of the Moon usually refers to the side that never faces Earth, even though this surface is regularly illuminated by the Sun. The reflecting surface you're talking about is the colloquial "light side of the Moon", just at a moment when it is not illuminated by the Sun. Still, +1. –  Kyle Jul 25 at 21:49

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.