| > Perhaps space travel isn't easy or fast at all Except we have excellent knowledge about lower bounds on how easy and fast space travel can be. And its plenty fast enough to explore the entire galaxy on geological timescales. Voyager 1 is traveling at 17 km/s which would take it 1 light year every 17,700 years. Given that the galaxy is 100,000 light years across, that means that Voyager 1 could travel across the galaxy in 1.7 billion years. A spaceship built for that purpose would take vastly less time. Voyager weighed less than 800 kg. Using the [rocket equation](https://www.omnicalculator.com/physics/ideal-rocket-equation) and a realistic exhaust velocity for methane/oxygen of 3280 m/s, you can see that you could accelerate a voyager 1 sized craft to 3000 m/s with about 1200 kg of fuel, which is less than $1 of fuel, no joke (https://www.nextbigfuture.com/2022/02/spacex-reusable-rocket...). Of course it takes a lot of money to just get that fuel into orbit: $1500/kg with a falcon heavy. But that means you could send a voyager-1-sized craft to space with the fuel it needs to achieve 3000 km/s for a mere $3 million (not including the cost of the craft itself). And harvesting methane and oxygen on smaller planets would bring down that cost by a lot. At that speed, the craft could cross the entire galaxy in 9.6 million years. A blink of an eye on geological timescales. And if we really cared enough to spend more than a couple million dollars on this thing, we could get it there orders of magnitude faster using propulsion systems way less shitty than a methane-oxygen rocket. With just 1% of the energy the sun outputs in a single second, you could accelerate 100 voyager 1s to 1% the speed of light. In short, if you think space travel is a barrier, you're plainly very wrong. Even with today's technology we could send machines throughout the galaxy in a pretty short period of time. In the near future, that time will drastically reduce. > we can send and receive signals much easier than we can send and receive space crafts And yet there are vastly larger motivations for sending a spacecraft than sending a signal. You might not be aware that nearly all radio signals that escape the earth are not powerful enough to be detectable above noise further than 1/2 a light year out. https://astronomy.stackexchange.com/questions/33939/when-do-... There's no reason to believe we should be able to detect alien radio signals unless they are intentionally aimed at us and meant for us. Also, you can't see a radio signal that passed by 1 million years ago. Whereas you certainly would be able to see evidence of a von neumann swarm having done that. > Say only 2 or 3 planets in a given galaxy end up developing intelligent life, is that rare? Whether anyone would consider that "rare" or not is irrelevant. The puzzle would be: if even ONE intelligent speicies existed out there more than several million years ago, why haven't we seen evidence of their space craft? > maybe intelligent life is common enough but it tends to rarely progress past a certain point of development due to wiping itself out Yes, this is of course the premise of usual solutions to the fermi paradox. However, those solutions are often either "maybe nukes will destroy us" or "maybe something we don't know about yet will". Neither are super satisfactory as answers. |
We can send a small object anywhere, but we can't send enough of them everywhere. If an intelligent species directed such an object to our galaxy, how close would is need to be for us to detect it? They physical object alone would be near impossible without it being extremely close, and even if it was producing a signal to be detected there is a limit based on size and how much energy it has on board. Even that would be hard to detect at any sort of galactic distances.
>And yet there are vastly larger motivations for sending a spacecraft than sending a signal.
Unless the signal is directed, it covers an exponentially increasing area, which is also why it has limited range. There is a fundamental trade off between range and area, meaning that there is an upper bounds on the volume we can contact. How likely is life found in that volume?
>Whether anyone would consider that "rare" or not is irrelevant.
When it is part of the question, how is it irrelevant? If it was on the other side of the galaxy, how would they know to send someone to our solar system and how far could we view their spacecraft if they weren't in our solar system? And that still assume a neighbor in the galaxy. If they weren't, the space between galaxies greatly changes the equation.