How large a space telescope do we need to see exo-civilizations?

[magnemoe]

1 hour ago, Exoscientist said:

 
 Thanks. I didn’t think of the uplinks. Even though it might not be possible to decrypt the signals, it might possible the spectrum is so unusual to be unlikely to be natural.

   Bob Clark

But they are weak compared to TV broadcasts and directional and moving. 
I say look for an biosphere firsts. I say this is easier than radio unless aliens try to contact us actively and will also much easier for planets to get an oxygen atmosphere than an advanced civilization. 

Then you have 

That one is seriously weird. My guess it was close to an neutron star merger some years ago so it scoped up lots of heavy elements including island of stability elements who decayed into stuff we know. 
Most likely an error so verification is always important. 

[kerbiloid]

Variants:
1. It's a destroyed alien storehouse.
Either by an accidental accident, or by a accident caused by a coming revision from the Dept. of Logistics.

2. It's a candy house trap.
Young interstellar civilizations come here, then - BANG!

[magnemoe]

7 hours ago, kerbiloid said:

Variants:
1. It's a destroyed alien storehouse.
Either by an accidental accident, or by a accident caused by a coming revision from the Dept. of Logistics.

2. It's a candy house trap.
Young interstellar civilizations come here, then - BANG!

I vote 1b, fun story, back during WW 2 US upgraded their shore batteries because of an risk of Japanese raids. You had some panics like the LA air attack who was people probably shooting at clouds or US planes. Once the AA guns started firing the rest joined in. 
Obviously the shell from the small ones had to come down somewhere, the heavier ones had timed fuses but not 20-30 mm. 
Now in 44 the threat of Japan doing raid or even shore bombardment on US west coast was laughable, but the batteries still got ammo for practice, but cleaning the guns was an pain so the simply stored most in some old magazines, for previous generations of guns. 
Then the war ended they just hide the entrance to these magazines. 
50 years later some wanting to develop the area got an surprise  in lots of cordite and old shells. 

[Exoscientist]

On this seminar series:

Technosignature seminar series.
https://seec.gsfc.nasa.gov/News_and_Events/technosignatureSeminars.html

I found a talk:

Simulation of the Earth’s radio-leakage from mobile towers as seen from selected nearby stellar systems Ramiro Saide, SETI institute January 17, 2024

An article describing their work is here:

Can ET Detect Us? May 2, 2023
https://www.seti.org/press-release/can-et-detect-us

It has a link to the research article.

They first look at the case of total mobile(cell phone) tower emissions. They’ll extend it to total mobile(cell) phones, powerful civilian and military radars, and television and radio transmitters in follow-up work.

They estimate total mobile towers are in the range of 4 GW total and at Barnard’s star distance of < 10 lightyears away would need 100 times more sensitivity than the Square Kilometer Array(SKA), equivalent to a 1 kilometer wide radio telescope. This would be 10 kilometers across. But when you add in together the other radio sources it might be smaller than this.

 There was a study on the upcoming Square Kilometer Array(SKA) that it could detect an airport radar 200 lightyears away:

This Radio Telescope Could Detect Alien Airports.
https://youtu.be/ayqyb8XCtE0

There are 260,000 stars within 200 lightyears of Earth.

  Robert Clark

Edited Tuesday at 12:29 AM by Exoscientist

[magnemoe]

38 minutes ago, Exoscientist said:

On this seminar series:

Technosignature seminar series.
https://seec.gsfc.nasa.gov/News_and_Events/technosignatureSeminars.html

I found a talk:

Simulation of the Earth’s radio-leakage from mobile towers as seen from selected nearby stellar systems Ramiro Saide, SETI institute January 17, 2024

An article describing their work is here:

Can ET Detect Us? May 2, 2023
https://www.seti.org/press-release/can-et-detect-us

It has a link to the research article.

They first look at the case of total mobile(cell phone) tower emissions. They’ll extend it to total mobile(cell) phones, powerful civilian and military radars, and television and radio transmitters in follow-up work.

They estimate total mobile towers are in the range of 4 GW total and at Barnard’s star distance of < 10 lightyears away would need 100 times more sensitivity than the Square Kilometer Array(SKA), equivalent to a 1 kilometer wide radio telescope. This would be 10 kilometers across. But when you add in together the other radio sources it might be smaller than this.

 There was a study on the upcoming Square Kilometer Array(SKA) that it could detect an airport radar 200 lightyears away:

This Radio Telescope Could Detect Alien Airports.
https://youtu.be/ayqyb8XCtE0

There are 260,000 stars within 200 lightyears of Earth.

  Robert Clark

Cell signals is interesting they are very diffuse but also directional but direction is pretty horizontal so seeing the US from above you will not get much, you will get more signals if east or west coast is on the horizon then viewing earth from far away. The WOW signal was someone painting an asteroid or some unidentified space stuff with an powerful targeting radar. 

[Superpluto126]

The Question depends on mostly how sure you want to be. We could, theoretically, find traces of life with out current day technology through Spectrographs and studying the composition of Exoatmospheres.

There is a proposal called a 'Solar Lens' Telescope that uses the Suns warping of gravity as a Telescope Lens, potentially allowing us to greatly enhance our visual study of nearby stars, However to accomplish this you need to travel very far out (Far past Pluto types of far) which in itself requires a breakthrough in engine technology, and I'm unsure of the tech you'd need for the Telescope itself.

[magnemoe]

On 1/27/2025 at 10:38 PM, Superpluto126 said:

The Question depends on mostly how sure you want to be. We could, theoretically, find traces of life with out current day technology through Spectrographs and studying the composition of Exoatmospheres.

There is a proposal called a 'Solar Lens' Telescope that uses the Suns warping of gravity as a Telescope Lens, potentially allowing us to greatly enhance our visual study of nearby stars, However to accomplish this you need to travel very far out (Far past Pluto types of far) which in itself requires a breakthrough in engine technology, and I'm unsure of the tech you'd need for the Telescope itself.

Detecting life is much easier than civilizations until it has significant structures in space,  a bit below Kardashev 1 below would be minimum.  The reflected light from an planet is many order of magnitude stronger than any radio signal we can create, not even sure how strong an focused beam would be after many light years, benefit of radio is that its easy to filter out the no signal parts. 
Pollution is kind of an iffy techno signature, they tend to be short lived as technology moves on, its also something alien life often could create naturally and you know that the planet has life if you have so good atmospheric readings, 10-30% oxygen and an nice temperature and you are pretty sure. 

Think you need to get out to 550 AU to use the sun gravitational lensing not sure how good an telescope you need but don't think you need an very large one as its the secondary mirror. 
Its something to do after we find the first planet with clear life signatures.  As I understand you would get an 100x100 pixel view of an exoplanet. 

You can get life without oxygen but it would probably be pretty primitive as oxidation is an excellent way to get energy. You can get oxygen without life but its mostly based on water breaking into oxygen and hydrogen, the hydrogen escapes. probably that happened on Venus, It also happen on Earth but at an low rate. 

 

Edited Tuesday at 10:59 PM by magnemoe