I would guess, "probably not" for a variety of reasons.

First, the ionosphere bounces a lot of radio traffic right back down to the planet, amateur radio operators (like me) use that to talk to stations farther away on planet but it doesn't go into space.

Second the frequencies that do travel through the ionosphere are typically aimed at satellites (geosynchronous or lower) and are scaled to have a level of power that gets there reliably but without an overly large margin for error.

Third, the Sun is a very "noisy" source of RF energy and someone looking this way at us would also be looking at the Sun as well (if you're looking at transits). In terms of radio frequencies that they could perhaps analyze it would require a lot of filtering.

The analysis we've done on transiting planets so far is based on spectroscopy using the star as a huge light source and looking for how that light is filtered by the 'edges' of the planet. (so through the atmosphere of the planet).

If they are doing that, then the change in atmosphere composition over the last 1000 years seems like it would be detectable.

I would also imagine that in addition to those three, regular old-fashioned attenuation would degrade the signal at significant enough distances as well, no? Or is that not an issue in the vacuum of space?

My understanding is that the attenuation at distance is a function of geometry: https://en.wikipedia.org/wiki/Inverse-square_law

Yep. It's why pretty much every force follows the inverse square law, whether it's RF signal strength, sound volume, electromagnetic attraction or gravity.

Sort of, the strong and weak nuclear forces don't.

> the change in atmosphere composition over the last 1000 years seems like it would be detectable

Interesting to think what conclusions would be drawn in these scenarios. Obviously atmospheric composition has drastically changed over the eons - I wonder what sort of civilization could detect these kind of longterm changes, and if they wouldn't have better observational capabilities than what we assume here given they have persistent knowledge over "long" timespans

I _think_ even just seeing Oxygen (in O2 form) should be fairly suggestive, because it's not stable in the presence of a whole bunch of other things.

And the CFC's would be definitive proof.

https://arxiv.org/abs/1406.3025

Yeah, but you might not be looking for those specifically.

Oxygen however. Just the fact that our atmosphere is as far out of equilibrium as it is.

I wasn’t clear at all, but I certainly agree it would be clear life was on earth from such data. I am less clear how much about our society you could meaningfully conclude from it is all, but perhaps it’d be clear from our pollution exactly what we’re manufacturing/burning

Unless the observers' planet also had ancient, non-renewable carbon deposits (which required very specific conditions) then atmospheric pollution might not even be a concept to them.

They might notice a change in our atmosphere's composition, but honestly a 200 ppm to 400 ppm change of some trace gas over the course of two hundred years could have many much simpler explanations than "intelligent life dug up all the carbon and burned it".

It has, but something fundamental to our atmosphere (even before the great oxygen holocaust) is that it has lots of potential for 'exchange'; ie, its not fully reduced. Part of our earth system depends on the reduction side (us, heterotrophs, etc..) and part depends on things being available to oxidize (co2, previously in greater quantities n2). The fact that both are observable and present would indicate that our atmosphere is being maintained at a stable(ish) state far from equilibrium.

I've also read that modern digital communication signals would appear very similar to normal random RF noise.

it's important to remember that the Earth emits 100x the radio wave energy than the sun.

At some point between 50 and 100 light years the Omni directional radio waves blend with the background radiation.

I have seen projections of "plausibly easy" detection up to 50 light years. They difference between the number of stars that are 50 light years vs 40 light years is about 2x, so the calculation of how many stars are in range is actually very sensitive to emission range. There are 64 stars 50 ly out, and almost 500 up to 100ly out.

That's still not that many though, given the 500 billion stars in our galaxy. It seems only plausible that an ET intelligence would detect us if they have seeded the galaxy with probes - in which case their would probably be one listening in our own system anyway.

What kind of things do you talk about with radio stations so far away?

By far most long distance traffic is your/their callsign ("AB1CD"), your/their rough location ("location is Cape Canaveral, florida", or just "QTH is EL98"), and signal quality/strength ("five nine"). For contests a sequential number ("number 23"). Outside of contests, first names, power/transmitter/antenna used, and the weather.

Some people start longer discussions ("ragchews") from there. Though this is more common in regional communication. Topics can be all kinds of things, like exciting new ham radio topics ("there's a new ham radio satellite going up!"), where they've travelled (especially nice if you've been where the other person lives), their life history, health (not dead yet! me neither!) etc. Lots of random small talk. The main rule is to stay away from anything that could cause disagreement, like politics and religion.

This may not sound terribly interesting, but keep in mind that for many, this is really just the last step after a lot of time spent e.g. building and setting up new equipment or whatever. Also depending on propagation conditions (which vary wildly), just understanding the other person is a challenge in itself. The actual talking is often more of a side benefit.

Above is mostly assuming SSB (voice). Other modes are different. For example, with CW (morse) exchanges will generally be more concise. Then there's more "useful" modes, for example you could send/receive email over Winlink via a digital mode like VARA. This doesn't really benefit from extreme reach, but depending on which bands you have antennas for, the next station might be e.g. a thousand miles away, so this is more of a mid-range thing. In contrast, some digital modes like WSPR focus entirely on propagation research (/ antenna testing), and _automatically_ communicate the callsign, transmit power, location and _nothing else_, but getting (automated) reports from stations on the other side of the globe that have heard your very modestly powered transmissions feels pretty rewarding.

Also note that there are phenomena that give you long range, like grayline DX (good propagation along the day/night line), tropospheric ducting (bounces earth->sky->sky->sky->...->earth) and moonbounce/EME (earth->moon->earth). Trying these is fun in itself even without meaningful information exchanged. There's also ham radio satellites; including one in geostationary orbit (Es'hail 2). If you hit one, you can hit more or less your entire continent. There the fun is using the satellite; the reach is generally a welcome side effect.

Sometimes very silly things like "How's the weather our there?" But often sort of radio nerdy things like "I've got 25 watts going into an end fed quarter wave antenna. What signal level are you receiving?" :-) I have been trying to bounce a 40M (7MHz) beacon signal up to Portland OR for a friend of a friend who is testing their own SDR receiver design.

The opening pages of Kristen Radtke's Seek You talk about her father’s (unknown to her until her uncle told her about it) ham radio hobby when he was younger. A lot of it is pretty much making contact (CQ) and possibly then the exchange of postcards acknowledging the contact. It's definitely its own sort of thing.

From the 1960's to the present, the Ballistic Missile Early Warning System radars have been broadcasting enormous amounts of coherent power on specific, non-natural frequencies into outer space: if we were to detect a signal like that we'd almost certainly determine it was artificial.

You can see the list of stars which would have recently started seeing the first glimpses of the BMEWS Line radars here: https://en.wikipedia.org/wiki/List_of_star_systems_within_55...

Obviously many other radars (DEW, Hen House, etc.) are doing the same thing, but BMEWS has continuous operation and power on its side.

In theory yes but in reality Earth rotates and orbits the Sun. Any particular observer outside our solar system has a very low probability of catching any one of those beams at the moment it's photons are reaching them.

There's also little chance of those beams will ever hit the same observer. The inverse square law also means that a potential observer would need to be in a relatively small radius from our system.

I hope that little forest of stars is not just dark, but actually empty.

And those signals have travelled 60 light years. Our Galaxy is 100 000 ly accross…

How far away will the signal be detectable above the background noise?

If you'll excuse the self-link, I have written about this at length: https://www.damninteresting.com/space-radio-more-static-less...

In short, even our largest, most sensitive radio telescopes are far too small to detect omnidirectional (e.g., FM radio) leakage from Sol's nearest celestial neighbor. Unless an exoplanet directs a coherent radio beam at Earth, we'd need an antenna larger than the planet to detect a signal amid the noise.

As I understand it the amount of time we were tossing radio into space was quite brief, and more modern radio traffic is directed at the ground (since after all if you're leaking signals into space you're wasting power).

One of the papers cited in the OP argues that a good indicator that there's something interesting about a planet is the combination of molecular oxygen and methane in its atmosphere - the former is a strong oxidiser, while the latter is a reducing gas, so there's some dynamic system producing free methane in the atmosphere.

The inverse square law really dilutes out most signals - https://www.quora.com/How-far-do-radio-signals-travel-into-s...

No, our radio emissions would not be trivially detectable. In fact they would be phenomenally difficult to detect even from our nearest stellar neighbor, Proxima Centauri. If you take the most powerful BBC antenna [1], then a James Webb-sized telescope placed near Proxima Centauri, and pointing straight at the Earth would receive less than 5 photons per second from that antenna.

Here's the calculation, in case you're curious: one light year is about 10^16 meters; a sphere of radius 4.25 ly has an area of 2 10^34 m2. The BBC antenna emits in the 200 kHz band, so a single photon has an energy of (plank const) x (frequency) = (6.6 10^-34) x (200 10^3) = 1.3 10^-28. The antenna has a power of 500 kW, so that translates into 3.8 10^33 photons per second. That means at Proxima's distance each square meter will be crossed by a photon about once every 5.4 seconds. The James Webb telescope has a receiving area of 25 m2, so you end up with a bit less than 5 photons per second.

[1] https://en.wikipedia.org/wiki/Droitwich_Transmitting_Station

To put it generally: How far could we be from each other and still detect ourselves?

They are trying to communicate with us with Quantum Entanglement. Until our physics progresses, so as to understand that Quantum Entanglement does not limit you to transfer of information at the speed of light, we are considered a level IV Civilization.

The current agreement is that level IV Civilizations should stay "Uncontacted" :

https://en.wikipedia.org/wiki/Uncontacted_peoples

What if they had an array similar to the one used in "Contact" to receive the response?

Our radio waves have traveled for about 200 light years. Proxima centauri, our closest star, is located at 4246 Light years from us.

Very, very unlikely that someone can hear us, even if they tried. Space is huge.

Proxima centauri is actually ~4 light years away. Space is really big, but there are a handful of stars in relative vicinity of us (under 250 ly)

https://en.wikipedia.org/wiki/Proxima_Centauri

http://www.icc.dur.ac.uk/~tt/Lectures/Galaxies/LocalGroup/Ba...

I think your Alpha Centauri distance is off by 3 orders of magnitude :)

Edit: I think there are thousands of stars within 200ly of Earth.

Here's an interactive 33ly radius model: https://gruze.org/fly_10pc/ - left click + drag, mousewheel, right click.

Could be more efficient, but this interactive does the trick; shows how loaded our neighborhood is.

Going to hamburger menu -> settings -> show labels brings it to a crawl, though filtering objects helps that a bunch.

Showing only stars with known planets is rather interesting.

This is really cool! Definitely going to be a time sink for me!