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Old 11-09-2019, 09:06 AM
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How far away in the universe could we detect a similar civilization to ours?


If there was an earth-like populated by a species exactly as advanced as humans are today, how close would they have to be for us to detect them, and vice versa?
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Old 11-09-2019, 09:19 AM
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“Earth-like” should have been followed by “planet” in the OP.
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Old 11-09-2019, 09:51 AM
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That depends on whether they (we) are deliberately trying to make contact, or just going by incidental radiation.

The incidental answer is "we can't, even if they were at the nearest star".
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Old 11-09-2019, 11:50 AM
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You might do some reading on SETI (Search for Extraterrestrial Intelligence):
https://en.wikipedia.org/wiki/Search...l_intelligence
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Old 11-09-2019, 11:59 AM
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Originally Posted by Chronos View Post
That depends on whether they (we) are deliberately trying to make contact, or just going by incidental radiation.
Obviously “it depends”. The OP is asking how possible it is given the best circumstances.

Quote:
The incidental answer is "we can't, even if they were at the nearest star".
Oh, then it doesn’t matter at what the circumstances are.

Last edited by hajario; 11-09-2019 at 12:04 PM.
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Old 11-09-2019, 12:34 PM
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That's the answer for incidental radiation (like picking up on ordinary TV broadcasts). We could do it if we were actively trying, but then you have to ask how much resources we're trying with. And also depends on us aiming our antennas at each other to fairly high precision.
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Old 11-09-2019, 01:06 PM
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Given that we are both actively trying with direct antennas, how far away could we detect them?

Given that they are like us with satellites and broadcast media, how far away could we detect them if we were looking in the right place?
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Old 11-09-2019, 02:59 PM
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Earlier thread. (There have been pretty recent ones, but this is what I googled up.)
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Old 11-09-2019, 03:31 PM
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Given that we are both actively trying with direct antennas, how far away could we detect them?
But that involves another assumption. That the other intelligences are even aware of the universe outside of their own planet.

We live on a planet with partial cloud cover that allows us to think about the stars, what they are and who might be out there.

Intelligences of an aquatic planet with life or one with permanent cloud cover might never even be curious about the outside.
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Old 11-09-2019, 03:34 PM
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I'm pretty sure that if they're at the same stage as us, they'd be curious about what you find when you go up. After all, we've been in the Marianas Trench [/straightline]
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Old 11-09-2019, 05:49 PM
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Since they're "exactly as advanced as humans are today," they've already broken through any water/air or cloud barriers, and have already explored other planetary objects in their system, if any.
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Old 11-09-2019, 08:34 PM
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Since humanity seems intent on suicide, I'd say no comparable civilizations will last long enough to contact another.
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Old 11-10-2019, 03:03 AM
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The hard upper limit is however long they've been sending signals off-planet, so if they've been sending out radio signals for 100 years then 100 light years is the cut off. In reality, any signal would become indistinguishable from background noise well before that.
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Old 11-10-2019, 07:33 AM
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Originally Posted by Jackknifed Juggernaut View Post
If there was an earth-like populated by a species exactly as advanced as humans are today, how close would they have to be for us to detect them, and vice versa?
There are various scenarios, with widely-varying detection ranges: incidental RF leakage, intentional directional RF beams, intentional laser communications aimed at specific star systems, etc.

At a given level of technology it varies based on funding. E.g, the proposed Cyclops radio telescope system could supposedly detect 1000 megawatt beacons out to 1000 light-years or eavesdrop on the electromagnetic "garbage" of technical societies out to about 100 light-years: http://www.xenology.info/Xeno/24.2.5.htm

The 100-meter-diameter Green Bank radio telescope and 64-meter Parkes dish in Australia are currently engaged in a 10-year search for extra-terrestrial signals. It's detection capability is described as "If a civilization based around one of the 1,000 nearest stars transmits to us with the power of common aircraft radar, the GBT and the Parkes Telescope could detect it."

https://public.nrao.edu/news/gbt-breakthrough-listen/
100-meter Green Bank dish: https://public.nrao.edu/wp-content/u...BT_4_large.jpg

64-meter Parkes Australia dish: https://upload.wikimedia.org/wikiped...background.jpg

The current Veritas optical search could detect extra-terrestrial laser communications using technology similar to ours out to about 1,400 light years: https://earthsky.org/space/breakthro...chnosignatures

Re unintentional RF leakage signals from earth, there have been widely varying estimates of detection ranges by extra-terrestrials if using our own technology levels:

"Dr. Seti" (Dr. H. Paul Shuch) estimates 1,000 light years: http://www.setileague.org/askdr/howfar.htm

However Dr. Seth Shostak estimates only 1 light year: http://www.universetoday.com/10229/d...ns-about-seti/

I don't know why the wide range of estimates by people working in the field. One may be using unintentional TV signal leakage (so-called "I Love Lucy" case), and the other unintentional cold war radar. In the TV case, the AM TV signals were transmitted continuously using several thousand watts, and were mildly directional in a horizontal plane. IOW the radiation pattern was designed to *not* go into space.

In the case of cold war missile-detection radar systems, they were essentially beamed straight into space at extremely high power levels. The Cobra Dane radar transmitted at 15.4 megawatts peak, and similar radars had an approx. 3db beam width of 2 degrees. That is fairly broad vs a 100-meter steerable dish, but it's still 42 db gain, which would equate to roughly 193 gigawatts peak effective radiated power.

https://upload.wikimedia.org/wikiped.../Cobradane.jpg

I don't know what the max RF input power is for Arecibo, but that 300-meter dish would produce about 71db gain at 1.4Ghz (21 cm hydrogen). If 15.4 megawatts input was used, the effective radiated power would be about 193 terawatts.

Another major difference in detection capability over the years is not the antenna or modulation but increasingly sophisticated techniques for extracting weak signals out of background noise, and over a wide range of frequencies. In the real world it's very different than listening on headphones, as depicted in the movie "Contact". It's more like the Large Hadron Collider where oceans of data are produced, filtered and analyzed over long periods by supercomputers.

LHC computer room: https://cds.cern.ch/images/CERN-CO-1...ile?size=large

Calulators:

https://www.satellite-calculations.c...te/gain_bw.htm
http://www.sengpielaudio.com/calcula...lification.htm
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Old 11-10-2019, 08:10 AM
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To be clear, in Contact, Dr. Arroway was not listening to the headphones for any scientific purpose. She was doing it because she felt like it, because she found the white noise relaxing. Until the day it turned out to not be white noise.
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Old 11-10-2019, 03:13 PM
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Earth has a buttload of satellites and assorted space-junk orbiting it. Could we look at a similar planet and think Hey, that's not natural?
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Old 11-10-2019, 04:02 PM
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We cannot even see entire planets in other solar systems. (They have all been detected indirectly, not by imaging them.) We certainly could not see satellites around them.
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Old 11-10-2019, 04:39 PM
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We cannot even see entire planets in other solar systems. (They have all been detected indirectly, not by imaging them.) We certainly could not see satellites around them.
I was thinking more along the lines of what the satellites blocked or reflected, like light from the star or natural radiation from the planet.
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Old 11-10-2019, 05:21 PM
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I was thinking more along the lines of what the satellites blocked or reflected, like light from the star or natural radiation from the planet.
They'd have to be humongously bigger than any satellite we've ever launched. In other words, a megastructure. That was the wild speculation about Tabby's star, but it's since been shown to most likely be caused by dust.
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Old 11-10-2019, 07:30 PM
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Could we use spectroscopy to detect industrial pollution? Are things that are unlikely to be natural, CFCs for example, detectable at a distance?
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Old 11-11-2019, 06:59 AM
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We're just barely getting to the point where we could detect Earthlike oxygen levels on another planet (I'm not sure if we've succeeded yet, but that's the goal). Trace substances like CFCs would be far, far more difficult.
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Old 11-11-2019, 09:20 PM
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We're just barely getting to the point where we could detect Earthlike oxygen levels on another planet (I'm not sure if we've succeeded yet, but that's the goal). Trace substances like CFCs would be far, far more difficult.
Would an abundance of free O2 be indicative of life of some kind (not necessarily industrial or even intelligent)? Is it only possible due to some ongoing process like photosynthesis?
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Old 11-11-2019, 10:44 PM
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Would an abundance of free O2 be indicative of life of some kind (not necessarily industrial or even intelligent)? Is it only possible due to some ongoing process like photosynthesis?
There is one situation where there could be free oxygen and no life. A planet covered with ice, water or some combination of them and a star that produces lots of UV light. The free oxy would come from photodisociation of water molecules where the hydrogen escapes the atmosphere. It has to be pretty much all covered with something that doesn't react with O2 i.e. water and/or ice. Extensive amounts of rocks would be a oxygen sink, so lots of land is out.

I would expect such a situation to be largely self-limiting, since the free oxygen will form an ozone layer that will block most of the UV. I don't know what oxygen level it would more or less stabilize at, but I suspect it's below the 20% we have here on Earth.
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Old Yesterday, 09:38 AM
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But yes, it being a strong (though not perfect) indicator of life is why we're trying to be able to detect free oxygen. Though of course, one could also have life without oxygen, and we have no idea how common that might be.
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