Lets say mankind is gifted by some benevolent aliens a one use only time machine.
Here are the rules
It can go only backwards in time
It can go as far back in time as you want
It will send you a 2 hour film instantaneously of the exact time and place you have specified
It will destroy itself after relaying the film and not return back to the present time
So how will mankind use this?
Lets say we want to know how the pyramids were built. As the build date is still unknown we could be sending the time machine back to a time when it was already built or just an empty desert space. This is just one example.
Is there any other event from the past that mankind has enough accurate knowledge of its date that acquiring knowledge about it will give us vast benefits?
Although the rules don’t specifically state it, it sounds like the intention is that no interference with the timeline is possible, so presumably it winds up intangible or something, observing without interacting.
Which means I don’t see any real purpose in sending it back to anything on earth. There’s nothing in the history of this planet that we both have a precise time and location for, AND would be useful and interesting enough, I don’t believe. We couldn’t send it back to observe the moment that the primordial soup spontaneously evolved life, because we have neither a time nor a location. Even most known, confirmed historical events aren’t particularly well documented as far as time and location, and those that are…well, we don’t need much info on them, because they are well-documented.
The only thing I can think of, therefore, is if we are able to pin down the time and location of the big bang/beginning of the universe with enough accuracy to send it to observe. Of course, if it just relays back a ‘film’ with no detailed measurements, then even that would be pointless.
Anything far enough back in time to be interesting, is too far back to be sure of getting a particularly interesting two hour slice. So we should try to send it back to some highly-populated region in a little-known historical time, and get a ‘slice of life’ from their civilisation. The Roman republic could be a good choice, or very early China, but I’d favour sending the probe back to the center of the Olmec civilisation in South America, and seeing what we could pick up
Moment zero of the initial big bang expansion. Hmm, wait… is this recording device limited to visual frequencies that the human eye can see directly? Is it, itself, impervious to things like heat and extremes of gravity and pressure and whatnot?
There are numerous famous unsolved murders, disappearances, historical treaties, betrayals, mysteries that could benefit from an answer. Kennedy, Madeleine McCann, JonBenet etc etc.
But other than answer the mystery for a select group of people, and probably bring closure/justice, it is not likely to benefit mankind as a whole.
The bigger questions are about older great civilisations and how they built their monuments with very little in terms of scientific advancements. But the narrow window of sight that this gift offers requires very precise timelines for its use.
They had enough ‘scientific advancements’ to build them, or they wouldn’t have been built. You are aware that stone age people are genetically almost* identical to ‘modern’ humans, i.e. they were about as smart (or dumb) as we are.
*Do mutations like lactose tolerance wind up in the DNA?
It wasn’t specified in the OP that we had to go back to earth. Suppose we sent it back to the aliens’ home planet, in the two-hour window in which the time machine was created… thereby learning how to build another time machine.
Which doesn’t rule out someone breaking it trying anyway. “We just need to modify the self destruct mechanism to kill Hitler. It will work…trust me!” You just know there would be people arguing for exactly that.