If humans developed faster than light travel and decided to set up space telescopes to see historical earth, how car back could we see the planets history? I once heard if you tried to develop a lens big enough to see the dinosaurs 70 million years ago, the lens would be so big it would collapse into a black hole. How far. Back could we see within the laws of physics?
If humans developed faster than light travel, how far back in time could we see with space telescope
You cannot break the laws of physics in your premise and then ask for a scientific prediction of the outcome. Once you suggest faster than light travel, you have left this universe behind, and you’re in the land of magic and make-believe. You can suggest literally any answer you wish, and it has just as much justification as any other.
So my answer is “giraffe pants”.
I agree with Smegs in this case. Normally I think OPs should be able to frame a hypo anyway they want but if we can engineer FTL travel we could probably overcome the big lens issue.
Well, replace the FTL travel with aliens. If someone is sitting on a planet 65 million light years away right now with a big enough telescope, could they be looking at dinosaurs? If not, how close are the closest aliens observing us who are able to make out any detail?
I don’t know but your username makes me think you may have the answer already. 
You’re kind of ignoring the basic question being asked here. One could simply ask how far back an alien civilization located millions of light years away could see Earth’s history using a telescope.
Then it becomes a question of what you mean by “Earth history” and about what kind of resolution you can achieve with your telescope. You’ll be able to observe the arrangements of continents from further away than you can resolve dinosaurs.
There are multiple theoretical ways that faster than light travel could be possible. Having said that, if it helps the question assume an alien civilization wants to look at earth via a telescope with enough resolution to make out what is happening on the planet. How far back can they see?
I hope you don’t hurt your wrist patting yourself on the back for that comment.
How big of a telescope do we need to resolve this much detail around Alpha Centauri? That will tell us how big to see 4.3 years into the past.
Or us, looking at a planet 65 million light years away.
You’re not going to resolve anything much on the planet’s surface - not because of the magnification factor (although that is a big problem), but because very few photons are getting here from there. You can theoretically overcome that by using long exposures, but then you’re not going to see anything that is moving (so if the planet is rotating, that’s also a problem.
Not really a problem; you can just use the black hole’s gravitational field for a lens. There have been serious, real world proposals to use the gravity of the Sun as a telescope lens.
Interestingly, FTL communications could also enable this apparent paradox – if there are aliens we can talk to ~2000 light years away, and we could send data back and forth instantaneously, if they had good telescopes they could send us video of Jesus.
What we really need to do is find an alien species that is close enough for good resolution, and that has been observing us for a long time already. Maybe they have archive footage of dinosaurs, and Jesus, on their alien YouTube.
Of course, if we do meet aliens, I think it shows a pretty appalling degree of self-centeredness and lack of curiosity on our part if “do you have pix of Jesus?” is the first ting we ask them.
Here’s a side by side comparison of what we could see on Pluto via the Hubble Space telescope compared to an image from New Horizons.
We were seeing 8 minutes into the past with those Hubble images. There could have been dinosaurs and alien cities covering Pluto and we would never have known, until New Horizons. And that’s only 8 minutes away.
Alpha Centauri is 4.3 light years away. So we’re seeing it as it was 4 years ago. Except we have no idea if there are any planets orbiting in the Alpha Centauri system. If such a planet existed we currently don’t have a telescope that would be capable of detecting it. If in the future we had a super-mega space telescope maybe we’d be able to detect one as a single pixel. That would be information from 4 years ago, and the information would be “there is a body there”.
Given the laws of physics only a few photons reflected off a distant planet could ever reach Earth. Far enough away, like in another galaxy, and it’s likely that no photon reflected off a planet would even enter our solar system. Which means that even with a magic telescope the size of the solar system we wouldn’t be able to image that planet, even as a single pixel.
I’m not an astronomer, but I know how to use Fermi approximation. No one else has attempted to give a real answer yet, so let’s go.
First, how big of a lens can we build? Carbon nanotubes are the strongest material we have, so let’s assume the aliens invent a clear material with the same strength. The breaking length under 1 g of pull for carbon nanotubes is around 4000 km. Our space telescope would be orbiting in space with no external gravitational pull, but 4000 km is bigger than the moon. I think it’s safe to assume it would start to deform under its own weight at some point (the moon having about 1/6th the gravity of earth). Let’s say our alien engineers can build a 10,000 km diameter lens, or 10^9 cm. That’s about the size of earth, but not as massive.
The Dawes’ limit for the lens is 10/10^9 = 10^-8 arcseconds. Some dinosaurs are pretty big, so lets say we are happy to be able to resolve features 1 meter across. That means we have to be about 10^13 meters or closer, or only 9 light hours.
So, it seems as though you would at best only be able to see hours into the past, not months or years. Light is fast.
Good analysis. But suppose we could build a solar-system sized telescope. Neptune is 4.5 x 10^12 m away, so let’s make the telescope a nice round 10^13 meters in diameter, or 10^15 centimeters, to match your units. It doesn’t collapse because of reasons.
That gives us 10^6 more resolution than your puny Earth-sized telescope.
Wesley, I hope there was a behind that. Smeghead’s post wasn’t snarky. The last part was a joke on himself, that the answer and question, like all good threads here on wild science hypotheticals, “pro” or “con,” is not a matter of stern intellectual inquiry in a symposium.
And I hope I haven’t gone soothing troubled waters where there aren’t any.
You didn’t see Smeg’s reply coming cause you weren’t moving fast enough.
He did, but he saw it as Smeg patting his chest.
And I think “looking for giraffe pants” should be the standard answer to “what good is all this theoretical mathematics and physics research” type questions.
I don’t think there is any theoretical limit on the size of a telescope. A reflecting telescope can be made up of multiple mirror segments, all controlled to form one single mirror. Each mirror segment can be an independent spacecraft, capable of precision formation flight. In theory, you can make such a telescope arbitrarily large - it could be light-years in diameter, if you had the time and resources to construct such a thing.
Although knowing where to aim it would be a huge problem - if the telescope were light-years across, it would take years for the control signals to reach all segments, and may more years for the whole thing to stabilize. Don’t even think about re-aiming the telescope to a different part of the sky, that would take millennia.