I apologize in advance if this comes off as a silly question, but I’m geniunely curious as to whether this is hypothetically possible, even if it’s not practical or realistic.
I remember from childhood science classes that it takes 8 minutes for light from the sun to reach our eyes here on Earth. That means that when we look up at the sun (setting aside the ensuing ocular damage for the moment), we’re actually seeing the sun as it was eight minutes prior – peeking at the past, in essence. So then I began to wonder: if we had the technology to stick a giant (I’m assuming it would have to be giant) mirror the same distance away from the Earth as the sun is, and if we had some enormously powerful telescope with which to view this mirror, would we then be able to examine life on Earth as it was happening precisely sixteen minutes (or would it be eight minutes) ago? If we could get a mirror sufficiently large enough, would current technology allow us to see any portion of Earth in any detail (such as cars moving on roadways) in this mirror?
Taking this to a much greater extreme if my prior assertions are true, my real curiousity would be about our ability to see thousands or millions of years into our past if an advanced civilization many light years away had happened to place such a mirror facing Earth a significant distance away from us eons ago. Just a mental exercise, I suppose.
Unless you could get the mirror to the desired location faster than the speed of light, all you could use it for was to allow future generations to see the earth as it is now.
My implication for the truly distant mirror was that it was placed there eons ago, although having faster-than-speed-of-light technology would solve that issue (and a number of other issues) as well.
Yes, a mirror as far away as the sun would allow us to see the Earth as it was 16 minutes ago. But really far away mirrors won’t be much help in figuring out who killed Kennedy.
A telescope looking into a mirror that far away would be like a telescope looking at Mars, or Jupiter. We could see continents, clouds and such, but not much beyond that. And increasing the power of the telescope only helps you so much.
Think about what it means when we “see” something. Light travels from the sun and bounces off objects on Earth. That light radiates out into space in all directions. This means that the amount of light reaching your eye from an object decreases with the cube of the distance from the object. So something twice as far away radiates 1/8th the amount of light into your eye. So you see how this goes. Light is quantized, the smallest unit of light is one photon. So unless a photon of light strikes the object and radiates straight back into your eye you won’t be able to detect the object. And a dim object that is far away might be so dim that it is statistically impossible for even a single photon from that object to reach your eye/telescope. So it wouldn’t matter how large the telescope is, you’ll NEVER be able to see that object.
Even beyond that, there are all sorts of other radiation sources in the universe, so even if one or two photons from the object hit your detector you won’t be able to tell them from the noise.
So say you point a telescope at Alpha Centauri, 4 light years away, and start looking for planets. Those planets should be brightly illuminated by Alpha Centauri A and B and should in theory be detectable. The trouble is that the Alpha Centauri A and B are putting out so much light that detecting the reflected light from a planet is like expecting to be able to detect the light from a candle held in front of a searchlight. The stars radiate millions of times more energy than the planet would. So detecting the planet is impossible with a simple optical search. And viewing the Earth by looking at a mirror 4 light years away will be similarly impossible, because all you’ll see is the reflection of the Sun.
The sun and earth are about 93 million miles apart. So if we looked in your mirror, earth and its reflection would be (effectively) 186,000,000 miles apart. I do not know the altitude of any spy satellites, but they are surely much closer than the moon, which is 240,000 miles away. Thus, the picture quality from a spy satellite would be at least 800 times as good as from your mirror. A better guess is that such pictures are taken from a very low orbit, like 100 miles or so, making them 1,860,000 times as good as your mirror.
If ya did hang a mirror out in space, wouldn’t ya sorta screw up the planet, by giving us a double dose of light/heat/radiation than we’d normally get?
Not really. The e.m. radiation has had to travel an extra 186,000,000 miles (to reach the distant mirror, then us) so it’s attenuated by a largish amount, and then you’re only getting a mirror’s worth of it, not all of the Sun’s radiating surface. That’s the $0.99 answer; we can do the math if you really like.
