Expedition to Europa

We would have no way to tell for sure.

I recall a Star Trek (?) episode what that was almost exactly what was happening - I recall that this powerful being was about to turn half the matter in the Sun into antimatter, which explosion would be a sight to behold - from a very great distance. This was averted when the powerful being's parents appeared and scolded their Child for playing with predators or the like.

Joe Gwinn

The experiment has been running for at least half a billion years, and probably some 3.5 billion years.

High school projects do seem to have short time scales.

Being small insignificant and with feeble radio transmitters. The only reason we are still in contact with the most remote ones is that ground based low noise amplifiers have improved enormously since their launch.

When it was operating and doing radar range imaging the EPR of the Arecibo dish was phenomenal along the direction it was pointing. Anyone in the beam would see a very distinctive flash potentially with obviously digital patterns in it if they were lucky.

There have been the odd WOW signal detected on Earth but nothing that stood up to detailed analysis or was reproducible. The pulsar trace was originally marked LGM in the margins since it's very precise repetitive signal looked artificial at first glance.

That was because they realised pretty quickly that what they were seeing was an inorganic reaction of peroxide and perchlorate ions in the soil with the nutrients provided. The next generation search for life on Mars will use stable isotopically labelled materials and stands a pretty good chance of detecting anything that is alive or nearly so.

There are hints that just maybe something is alive on Venus (unlikely as this sounds) phosphine has been detected in its atmosphere.

But we only really spot them when they get near enough to the sun and warm up enough to outgas. Considerable effort now goes into trying to find any and all Earth crossing asteroids that are big enough to matter and/or might one day be a threat to our planet.

There have been more than one WOW event seen but nothing in the same place twice and most of them have plausible astrophysical signatures.

Seeing a narrow band signal at a non-natural frequency or a modulated one would be indicative of a civilisation. Pulsars almost fitted the bill at the time they were discovered - tagged LGM in the margin.

They are still limited by the laws of physics as they apply in this universe unless they have found some much deeper understanding. You can put pretty good bounds on how far away people can watch terrestrial TV from but our planet was only non-thermal radio bright for less than a century. That is a blink of the eye in cosmological timescales.

Give or take an order of magnitude I expect most civilisations will go through similar phases of technology to us (unless they are aquatic).

Technological civilisations will only be non-thermal radio bright in an easily detectable way from after they invent AM broadcasting and before they invent spread spectrum UHF services or coax cable or fibre optics.

Interestingly the Austrian actress Hedy Lammar co-invented spread spectrum RF but is mostly remembered for her film roles.

Drake has a nice equation where some of the unknowns when he originally wrote it down have now become rather better hard numbers. It turns out that planets around stars are much more common than first thought.

Ones in the Goldilocks zone with all three phases of water are rarer.

I certainly wouldn't rule out the possibility of life on Mars. I've been down the UK's deepest salt mine and NASA have an experiment down there looking for microbes that can literally live in rock (*very* slowly).

Again they are trying to figure out how you would find and cultivate them on Mars if they were present. Nothing beats experiment.

Tardigrades are also pretty good at surviving space flight as are several lichens. Hard UV levels might be a bit much for terrestrial ones but if life did arise on Mars then something already there might have adapted when it slowly lost it's oceans, atmosphere and most of its water. Losing your magnetic shield is not good for a young planet...

It still puts quite hard limits on how small a device can observe the Earth even with diffraction limited optics. Monitoring radio traffic is much easier if the frequency is such that it escapes.

Indeed and remote sensing is getting very good now. I expect that someone will find a planet with evidence of an industrial atmosphere within the next century or so (maybe sooner).

That can be fraught with difficulties. I expect life as reproducing photosynthetic slime is extremely common anywhere that is stable enough to allow a billion years of reactions. Aqueous environment might not be strictly necessary but most other things have solid phases that sink.

They may have done but there are still hard limits on how big an antenna must be to obtain signal that is above the background noise floor.

Encounters between species with advanced technology don't usually end well for the ones still in the iron age when they come up against automatic weapons. If we ever meet alien space travellers we had better hope that they are friendly. Even so their advanced technology could wreak unintentional havoc.

Space is mind bogglingly *BIG* the gaps between stars are huge.

The only place where stars are close together are in globular clusters and in environment so odd that it is probably one of the places where you can pretty much rule out life evolving. Close three body encounters happen often enough to lob whole stars out at very high speed and leave the remaining ones ever more tightly bound. They are extremely pretty.

Arecibo's first ever deliberate message was sent to M13 it will take about 25k years to get there and another 25k for a reply if the intended recipients happen to notice it. Chances are there is no-one there.

Worth seeing first hand in a 15" scope or larger.