If a camera was placed 1 light year away from Earth with a high
enough definition, could it be used to spy on events that took place on
Earth one year ago? And, if so, could this technique be used to record
our past by sending an array of such cameras to the appropriate
distance in order to capture momentous events in Earth's history?
All cameras (and our eyes) record the past simply because it takes a finite time for the light to get from the object to the lens. However, even given magic lenses that could provide the resolution needed for a distance of one light year, the system still fails. Since you then have to get the information back, you are adding at least another year's wait. Why not simply place the camera in earth orbit and have a delay of a few milliseconds?
If your thought is to capture something after the fact (i.e. get pictures of a massive eruption) then it still fails since you cannot get the camera to catch up with the light carrying the image
If “Theoretically” means “Not really”, then distant observers theoretically
could watch ancient Earthly events; after all, light in transit is a data storage
medium. We only ever see things in the past anyway: other planets we see as
they were minutes or hours ago, stars years, galaxies millions or billions of
years, the moon just over a second ago, and people nanoseconds or microseconds
ago. But from space you see only
“momentous events” outdoors, from above, and with no sound track. From just a
light-year away we could hardly detect Earth, let alone fine details such as
whether the planet has iced over or blown up.
But chasing after history by overtaking ancient outward-bound light,
then stopping to watch as the light catches up? No. At present FTL travel seems
impossible. Even if you converted our whole galaxy into propulsive energy, it
would be too little to accelerate a single camcorder to light speed, never mind
beyond. So, to laugh at the errors of the past, resign yourself to sending out
your equipment in anticipation, then to watching lousy camera angles and
vanishingly faint views with lousy resolution, presenting our current future as
our future past.
Yes it could. This is what astronomers do all the time to watch momentous events that took place in stars millions of year ago. It is amazingly convenient that our telescopes are already positioned far enough away from those stars to see this old light arriving (not to mention keeping the temperature of the astronomer somewhat below that of the star).
To take a photograph in five minutes time of Earth bound events from last year all you would have to do is get your camera one light away from Earth in the next five minutes - which could be tricky as it would have to go considerably faster than the light it is trying to photograph.
However, you could put the camera in position now, wait a year for the light from today's momentus events to reach it, photograph the light that arrived, then radio the image back and wait a year while the radio waves got here and voila! images of the past.
Alternatively you could just take a snap and stick it in album for a couple of years.
However, you would first of all needed to have developed faster than light (FTL) space craft which is a long way off if not actually impossible. But, you would also need luck, for instance if you wanted to photograph something you might not be able to, there could be a cloud in the way.
Oh, also, if you did have an FTL space craft, there could be an issue with transparent materials and their fragility and thickness, though, at the time of development that probably won't be a problem.
Also, the further away you get, the less light you would get from Earth and therefore, you would also need a highly sophisticated camera, well, such as Hubble.
Overall, theoretically possible, but only practically possible if you happen to have a camera with extreme zoom and sensors and a rocket which goes FTL and has a window. Oh, and a lucky rabbit's foot, or a camera with light sensors able to view through water vapour, ash and whatever else might be up in the atmosphere, oh, and perhaps planets and stars etcetera, otherwise the alignment for the exact time might not be quite correct considering a light year is 5,878,499,810,000 miles. Or alternatively a time machine, which might, incidentally, be easier to create than the technology needed otherwise.
Since such a camera does not presently exist ,and it will take at least 1 year to get it there and install it ,such a camera will only capture events later than now. It will do no more and at much greater cost than a camera installed here and now with a long recording loop delay