Aliens Observing from 100 Light Years Away

Your answer to Ted, who asked if aliens observing us through a telescope from a planet 100 light years away would see Earth as it was a century ago, was well-researched and (I’m sure) accurate, but I don’t think you addressed his question. He wasn’t asking about the refractory power of telescopes, he was asking a hypothetical–IF they could see the details of our daily life, would it appear to them that we were back in 1915, still embroiled in World War I. It seems to me that it would. 100 year-old-light waves would transmit 100-year-old data.

I agree.

You could, for example, posit that the aliens sent out probes long enough ago that one of them is close enough to resolve details on Earth. Assuming the probe’s communications are limited to light speed, it would take 100 years for its data to reach home, and the aliens would be looking at what was going on 100 years ago. The probe could also potentially relay transmissions it picks up from us, and once the aliens figure out our formats, they could listen to and (eventually) watch what we were sending out. Of course, if the aliens were advanced enough to send out such probes centuries ago, one presumes they would understand the lag and realize that the information is historical.

Now I’m contemplating a story in which the probe’s transmissions become the alien equivalent of reality TV, with a lively betting market surrounding it. A clever con-alien gets his tentacles on some prototype FTL tech and tries to use it to win wagers. Shenanigans ensue because our view of history isn’t always accurate.

Would you like to provide a link to the column in question for the rest of us?

Never mind, I found it - Would aliens looking at us from 100 light-years away see earth as it was a century ago?

And yeah, looks like Cecil pretty much failed to answer what was being asked.

Not really. It’s obvious that yes. the aliens would be seeing the Earth as it was 100 years ago. That’s what “100 light years away” means. But just saying “Yes” wouldn;t be a very interesting answer, so explaining why in practice you couldn’t see much is far more informative.

There is a way that aliens could view the Earth, or conversely, that we could view planets around nearby stars, at reasonable resolution. It is usually called a Gravitational Lens Telescope. The idea is to use a star as the objective lens, and a conventional telescope as the eyepiece. The star’s gravity bends light around it, so a telescope sitting at the right place can sample the light coming around the star and reconstruct an image of what is on the other side. With a telescope like Hubble, a coronagraph disk, and the right software, you could get Landsat-quality pictures of planets around nearby stars. You could also listen in on cell phone calls, signal the aliens with a low power laser, and have all sorts of fun.

There are a few problems with this idea, but they are solvable technological challenges. The minimum distance for the telescope to sit from our Sun is about 540 AU, and the seeing would be ideal around 1000 AU (93 billion miles). To get a spacecraft out that far would require fusion thrusters, which we don’t have right now, but in theory we could make. The spacecraft would need a fair sized nuclear reactor to power itself and send images back to Earth at high bandwidth.

These challenges are all solvable in the near term (e.g. in the lifetimes of most people reading this) if we want to spend the money. Each telescope would only be able to view one target star system, since the alignment is critical. I suspect no-one will want to spend the money until potentially life-bearing planets are found around nearby stars by other means.

The images you’d get that way would also be extremely distorted, to the point that you couldn’t, even in theory, extract images of anywhere near the quality your resolution would suggest. But you might be able to make out large features like continents. Smaller features would depend on the assumptions you’re using in the reconstruction of the data.