Imagine we’ve made contact with some friendly aliens outside the solar system. Communication is obviously slow, considering how far they are away, but we realise which planet they’re from and they, us. If they have incredibly powerful telescopes, and they train them on us, they’re going to see Earth in the past, rather than us now. Depending on how far they are away, it could be any part of human history or prehistory.
Here’s my question; is this, in theory, possible? Not the aliens part. But the concept of using a good enough telescope, far enough away, to be able to see historical Earth with any kind of accuracy or usefulness?
I’m an engineer on a NASA weather satellite program. We basically have a telescope in near Earth orbit looking down on Earth at all times. It’s one of the best wide-field telescopes ever made. It can pick out weather and ground features at a resolution of one pixel every 750 meters, in a swath nearly 3000 kilometers wide.
With the exception of the freakishly high-resolution Earth obs satellites like GeoEye or optical spy satellites (with ridiculously small fields of view), it’s probably one of the best telescopes ever pointed at Earth.
And the features of human civilization barely register. Agriculture shows up, because it covers large areas and has vegetation patterns that can be distinguished from wild vegetation. If you know the particular infrared and visual spectral characteristics to look for. Meaning, you already understand how human agriculture works (what we plant, how we plant it, where we plant it).
From enough light-years away that it’s a historically notable light-speed time offset? No way.
We can cut out the aliens, with their unpredictable cooperative reliability, and simply go far out into space and mount mirrors, and then look at ourselves in the past in those mirrors. Or, use digital technology in retransmitting the signals back, so they would even be delayed by buffering. I watch sports events “from the past” every night on my computer.
But we’d only be able to see back as far as the point when the mirrors were installed. Or rather, the point X years after the mirrors were installed, assuming the mirrors are X light years from Earth.
As far as the OP goes, if the aliens have contacted us already, then by definition they are close enough that they are seeing the Earth as it was after they first contacted us. Barring FTL communication, of course.
Since you are saying “communication is slow”, I would assume no FTL communication. Therefore, if they are far away enough to be able to see our paleolithic past, they won’t be able to see (or hear from) our technological present for many thousands of years. They could send a message to the caveguys they see, which we might receive now, but they won’t get our response for many mor thousands of years. Doesn’t sound like much of a dialog to me.
There are limits to resolving power of telescopes based on aperture and distance. There is a nice link to minutephysics that discusses this with regard to Legolas of LOTR.
I suppose a giant system of linked optical telescopes, maybe bigger than a solar system could in principle begin to resolve details of extra solar planets, but without mathing it, I guess that for any reasonable historical distance (say more than 1-20 light years) you’ll probably see the earth worse than did the astronauts on the moon.
Even if there is no upper limit on telescope size, I wonder if relatavistic considerations would limit the ability to combine the image, and therefore limit resolution.
Then, as good and altruistic cosmic citizens interesting in furthering interstallar archaeology, we ought to sending real-time video of other planets into space, planets that do not yet have the capacity to do so on their own, so that millions of years from now, civilizations that will have evolved on those planets can go out into space and retrieve them still lumbering along at the speed of light.
It is too late to capture images of our primordial earth, but we can at least make the gesture on behalf of others less fortunate than ourselves. Just think how lucky we would be, if someone else had thought to do that for us.
There are a large number of Earth surveillance satellites in orbit with optical capabilities more precise than 750 m, including commercial smallsats (Skybox) and nanosat (Planet Labs “flocks”', Spire). Unlike weather surveillance, which needs a wide field of view but only needs low resolution to identify weather patterns, these commercial imaging systems used to provide data to GIS users such for agriculture, traffic management, forest fire and disaster relief, et certera require higher resolution (down to ~6 m for optics mounted within the 2U to 3U CubeSat platform, even better for deployable optic systems).
To return to the o.p., at interstellar distances beyond a few parsecs, light coming from the Earth will be less than one photon per second for any array less than planetary size, not even accounting for absorption by the interstellar medium. Similarly, radio signals will be so faint at interstellar distances that picking up distinct transmissions is virtually impossible unless they are extremely high power, direction signals… And of course, if you are communicating with radio or optical electromagnetic signals, the other images or signals the aliens will receice will be contemporary with them. To see “back into the past” they would have to go outward faster than light to catch up with thr historical “light surface”. It’s probably just easier to send the the Gilligan’s Island Complete Series Collection.
Even with an infinitely powerful telescope, you can’t see the past.
If you traveled to another star system at the fastest speed theoretically possible (i.e. very close to the speed of light), and then pointed a telescope at Earth, you’d get a view of the Earth just after you left. Because you’re seeing the light that left Earth just after you did, and is catching up with you.
Similarly, if you sent a message to an alien on a different star system and said “please take a picture of our planet and send it to us”, you’ll get a picture of the present day Earth. Because, again, their telescope will be seeing the light that left Earth just after your message.
The only way to observe the past is to travel faster than light and overtake the light that already left earth (which of course is theoretically impossible), or to find an alien civilization who has already been observing Earth and get their old images.
At some sufficient distance, would there be enough photons of light to resolve an image? Isn’t this similar to how at even four light years out, there’s no antenna that would be able to pick up a television broadcast of The Honeymooners. or even Orson Welles “War of the Worlds?” on radio?
(assuming of course, that such an antenna was there in the late 50’s or early 40’s, respectively)
That was the premise of a short story I once wrote in high school!
An alien time capsule of our world. The only problem was that it used some incredibly advanced technology to capture very accurate and detailed images of what was happening on the ground from orbit both in the Earth’s long, long past, and much more modern times. Caesar crossing the Rubicon, T-Rex roaming the Americas, Richard I taking an arrow to the arm, etc.
In the end chaos ensues as governments and religious organizations realize that things were about to be revealed, that they would rather people not know. In the end the Russians end up shooting down the probe.
Ahhh, to have the time to write such things again…
