# View the past by reflection? [duplicate]

I want to postulate that I have a sort of "time machine" that uses reflections from nearest planets to image places on Earth at a time in the past (2 x distance of reflector in light years). Is this possible?

Reflections from Mars would show what was happening 25 min ago (depending on exact locations in orbits). If possible, there is also a second mirror that reflect the first reflection, so 50 minutes in the past could also be viewed.

• Welcome to Worldbuilding. This looks more like the introduction of a question, and any question is missing. Please fix it, else it may be closed.
– L.Dutch
Aug 20, 2017 at 3:47
• L.Dutch did not understand the questions, but I got an answer that said current technology could not isolate the signal wanted from the other signals that would be received. That may be a temporary problem and static pictures may soon be possible? Aug 22, 2017 at 17:33
• To get it right, you want a constant stream of the past at specific fixed delay?
– xpy
Sep 11, 2018 at 9:20
• Related: worldbuilding.stackexchange.com/questions/122987/… This post does not involve a mirror, but in my answer I explain that you would only be able to see certain things depending on the atmosphere and position of objects. The accepted answer explains that hardly any light would actually reach that far. Sep 11, 2018 at 12:23
• @SZCZERZOKŁY This question is OLDER. This cannot be a duplicate. That question was previously closed as a duplicate of this one. Please stop voting this as a duplicate, and check timestamps. Sep 11, 2018 at 12:43

I have a sort of "time machine" that uses reflections off nearest planets to image places on earth at a time in the past (2 x distance of reflector in light years). Reflections off Mars would show what was happening 25 min ago (depending on exact locations in orbits),

The problem is that the intensity of the reflection will fall off as $\frac 1 {r^2}$, which means it's very unlikely you'll get even single photons returning from Mars.

So you could not form an image from such a "reflection" or realistically receive data from a reflection from another planet.

And remember that this reflection has to be distinguished from the light Mars itself is reflecting from its surface. And the light from Mars would swamp any reflected light from Earth.

So this is physically unrealistic.

but would there be a second weaker reflection of the first reflection, so 50 minutes in the past could also be viewed.

In principle you could have repeated reflections, but they could never be detected. It's going to suffer the same two way signal loss as the original signal. And it's going to be "buried" in the much, much more powerful signal from Mars itself.

• Might want to consider the possibility of a large diffraction-limited retroreflector, which would not have the same signal falloff limitation. Sep 5, 2018 at 6:11
• @Nathan Tuggy or perhaps spreading a few thousand micro-sattelites to collect photons over a wide area and create a good image. Or since the OP wants to "look back in time", he could just hang a lot of camera's viewing the earth and record it... Sep 6, 2018 at 19:48
• @Demigan: satellite spread doesn't fix the problem of photon collection: it just means you can use mass production to get the same total area, possibly reducing costs somewhat, maybe even by an order of magnitude. But a large-diameter diffraction-limited beam reduces the necessary collection area by many orders of magnitude. Sep 7, 2018 at 2:15
• @Nathan Tuggy a friend of mine helps design sattelites and explained the possibility of micro sattelites. Hang a couple of thousand in space away from any lightsource with thousands to millions of miles inbetween them, point them at the point you want to see and create an image. You would be able to make Google earth quality images from alpha centauri, the spread helps collect enough light. Sep 7, 2018 at 7:45
• @Demigan: Spreading satellites does nothing for light collection. It can be helpful, but only because you can compute interferometry and use that to resolve finer angles as though you had a sensor with the same diameter as the spread, but sparse: it still has the same total collection area as the sum of the individual imagers. For some astronomical purposes, light collecting area is less important than resolution, so having the equivalent area of a 5-meter telescope stretched across a million km can be a reasonable tradeoff. Sep 7, 2018 at 7:50