| Depends what you call a signal. Imagine the earth was just a bit more focused on science, united, and thought a bit longer term (on the order of 100k years). Living sustainable, but curious and motivated to explore. Imagine an earth where literacy, science, and peace were priorities. All countries joined a UN like organization and decided to spend 1% of the planets resources on exploring. Assuming earth level tech and the next 1000 years of progress (.000007% of the life of the universe) we manage a ship/converted asteroid/similar that can travel for 500 years at 2% of the speed of light with some combination of power solar power lasers orbiting the sun, solar sails, nuclear power (ion, explosion and/or pulse), or antimatter. That gets us to the nearest 10 stars. Hard to say what earth technology will be like in 1000 years, but no new physics are needed. Would that be a huge ship with kitchens, gyms, entertainment etc? Or maybe just some autonomous AI with some genetic samples, some robots, and a 3d printer? After another 1000 years things have improved further, and slowly spreading across the galaxy at a few % of the speed of light becomes ever more practical. Maybe just enough so that a 2nd solar system in the galaxy an start launching ships. Said 2nd solar system could make it dramatically easier for travel between those two systems since they could use a laser to help decelerate the incoming light sails. Those huge solar arrays + lasers for pushing a large ship up to a few % of the speed of light make for a hell of a beacon. Even unintentionally it should be visible from quite a distance, even by astronomical standards. A ring of solar panels near a sun could provide considerably more power than is available on earth today, even crazy inefficient things like producing antimatter might become viable. Things like geo-engineering become feasible, some distant civilization might detect things like an implausibly quick change in the atmosphere of mars. Or unusually energetic particles coming out of ships trying to accelerate or decelerate to a few % of the speed of light. As technology progresses we might even be able to focus the energy coming from the sun more directly to directly push ships to other stars. After all solar power -> laser is horribly inefficient. Distant observers on the same plane as our solar system might even start to notice the particular pattern caused by the solar/whatever energy collection. Earths atmosphere might start to exhibit unusual characteristics. Even nuclear propulsion becomes common and we start mining the outer reaches of the solar system that might exhibit some unusual characteristics. Of course any similar civilization in the galaxy could similarly expanding and eventually the two civilizations would get close enough to notice each other, even without deliberate signals. |