Phil Plait

In other news…

This was fun: a reader asked if aliens were in a galaxy 66 million light-years away, how big a telescope would they need to see dinosaurs on Earth?

Guess!

No; bigger. Try again. Nope, still bigger. Third try? Nope. Way, WAY bigger.

https://www.scientificamerican.com/article/could-aliens-in-another-galaxy-see-dinosaurs-on-earth/

Could aliens in another galaxy see dinosaurs on Earth?

How big would a telescope need to be to see Earth’s dinosaurs from 66 million light-years away? Think big—and then think bigger

Scientific American
Feb 20 at 4:15pmWeb
Show threadRobin BartonFeb 20
@badastro wild ass guess here but about the size of our solar system???
Show threadMichael BuschFeb 20

@Robo105

Considerably larger than that.

There is also an additional complication that @badastro did not have the space to get into:

The interstellar and intergalactic medium will eventually blur the wavefronts, making imaging impossible.

Show threadRobin BartonFeb 20
@michael_w_busch @badastro Well there goes my business idea of selling videos of other planets as reality TV
Show threadMichael BuschFeb 20

@Robo105 @badastro

There are other tricks one could play, such as using a star as a gravitational lens to magnify the image of the Sun and Earth.

But we cannot change the laws of physics.

Show threadCliff'sEsportCornerFeb 20
@badastro ehh it doesn't work that way
Show threadDavid NashFeb 20

@badastro Phil Plait doubtlessly knows all this, so this is very much a "yes, and..."

Those of us who are amateur astronomers can guess just how extreme the size needed must be, even without knowing the exact numbers. We look at, say, Mars, and even with a big telescope (by amateur standards, that is -- let's say 20 inches or 0.5 meters in diameter), we're doing well to see Earth-continent-sized regions of the planet as distinct entities, and then mostly because of color contrast, not specific surface details. Under some very specific conditions, you can see smaller things, but again only by virtue of contrast. The smallest thing you can see on Mars with large amateur gear is Olympus Mons (and it's not recognizable as a mountain, just as a differently colored dot), which is roughly (in linear dimensions) the size of France.

That's for a planet a few light-*minutes* away, just to see things at best on the scale of an extremely large mountain. For millions of light years, and things on the scale of a T. rex, you realize you have to scale things up by an almost unfathomable amount even before you reach for the calculator.

Show threadRay McCarthyFeb 20
@badastro
short answer:
Impossible.
I've done sums for RF, being originally a Communications Engineering Engineer.
We can maybe spot evidence of industrialisation via planet transiting star (very powerful TX) using Spectroscopic analysis.
Getting radio transmissions or seeing a city? No. Not even for neighbours in our Galaxy.
Show threadMichael BuschFeb 20

@raymaccarthy @badastro

I am personally responsible for radio transmissions that would be detectable to our current telescopes at distances of tens of thousands of lightyears.

But only if a hypothetical alien watcher should happen to be looking in the right direction at exactly the right time.

Taking pictures of dinosaurs is many orders of magnitude harder.

Show threadRay McCarthyFeb 20
@michael_w_busch
What power, frequency & aerial/dish gain your end? Even 10 ly would be amazing.
Show threadMichael BuschFeb 20

@raymaccarthy

The Arecibo planetary radar ran at 900 kilowatts of 2380 MHz on a 300 m dish.

The Goldstone Solar System Radar is 450 kilowatts of 8560 MHz on a 70 m dish.

The range limit is set by the bandwidth, which we can control down to <0.01 Hz.

Show threadRay McCarthyFeb 20

@michael_w_busch Thanks.

Sad about Arecibo.

I'll do sums later. I used to be paid to do terrestrial link and Geosat link budgets on 920 MHz to 22 GHz.

Show threadFlipper 🐬🏳️‍🌈Feb 20

@badastro well, the Rayleigh criterion says that it should be bigger than 1.22 lambda / theta, where lambda is the wavelength of the light and theta is the angular resolution of the observed object (in this case a dinosaur of order of 10m)

This gives:
D = 1.22 * 0.5x10^-6 / inv sin(3.4x10^-14)

I can't get any inverse sin calculator to return a result for such a small value that wasn't zero, so I'm going to estimate it as being approximately 10^-14.

So my guess for the telescope diameter is anywhere between a million kilometers and infinity.

Edit: I read the article and I think they got the 66ly in metres wrong (they said it was 6.6x10^23m) -- but they use the angular resolution as 10^-21 radians which increases my calculation by 10^7.