russ_watters said:
... It is not reasonable to assume that we will develop a method for traveling at 10% of the speed of light, cheaply, in the next decade or two (or fifty).
The assumption "interstellar travel is realistically possible" is not supportable by current knowledge of science. It may not be used as a basis for a line of logic about reality here. Further discussion of it may be deleted. [/mod hat]
The speed capabilities does not work as an argument against "interstellar travel is realistically possible". We have demonstrated rockets capable of 10
-4c. We do not need to prove that faster propulsion is possible. We only need to prove that intelligent life can survive for 10,000 years in space. There are a large number of scientists who consider life in space a possibility.
On NASA forums you can dismiss ideas for projects that would take 10,000 years or even 100 years. Physics is far more open. The floor of the Pacific ocean is sinking down the Marianas trench and the Atlantic sea floor is expanding. This is not useful information to an engineers or economists planning logistics of shipping lanes. Mentioning it during a discussion of TPP agreement would be rash. The continental drift rate is useful information for a paleontologist trying to explain why bones are located (or not located) at a particular dig site. Sea floor spreading is fast enough to be relevant to evolutionary theory over continental distances. Known, demonstrated rockets are easily fast enough to be relevant over interstellar distances. You could even limit the aliens to mechanical mechanism (slingshots catapult etc) and gravity assists.
The Milky Ways disc has been around for more than an order of magnitude longer than the travel time needed to colonize all of it using just the motion of the stars.
nikkkom said:
And unless they have *ridiculously awesome* interstellar drives, large interstellar "hops" would be still hard for them. It's likely they will prefer smaller ones. This means that they will not just travel to the selected "best" systems hundreds of light-years distant from one another - they will hop to nearest ones. This means almost every stellar system will be visited within the expanding "ball" of colonized space. This means they will visit Earth as soon as this expanding "ball" reaches Earth. No search is necessary.
I am not convinced by your argument. Alpha Centauri and Sirius have a huge advantage for interstellar travel. They can use the stars for gravity assist. If you have limited Delta-v capabilities but high impulse you would have a very strong incentive to go to Sirius and skip other nearby stars. Trying to slow down using Jupiter gives much less Oberth effect and is more difficult than what you could get using Sirius b. If using light sails Alpha Centuari might be competitive with Sirius. They are much better targets than the Sun.
Wikipedia says there are more than 25,000,000 asteroids in the belt larger than 100m diameter. Of that some 10% are metallic. Alien artifacts could be carbonaceous, silicate, and could be shaped like an average asteroid. For a thought experiment suppose that we place 10,000 20th century school buses in the asteroid belt. I am not suggesting that aliens have school buses or that they parked anything in the belt. I just want to know which telescopes would find one if there was a fleet of school busses. The thought experiment changes if you park them in the belt 1 million years ago. The paint is also a factor. New yellow buses should be much easier to detect than unpainted or dented/chipped buses. The buses should collect some of the same dust that collects on metal asteroids. Still, the question is "who in 2018 has the capability of finding one" and proving that it is similar to a bus?
nikkkom said:
... It's likely they will prefer smaller ones...
I do not believe that is true. You certainly did not provide supporting evidence. It is commonly assumed in science fiction.
If we assume that it is true then you gave a reason why the aliens are not observed in our solar system. There are many small stars to select from.
BWV said:
...Plenty of scientists have thought that K2 or K3 level civilizations are possible, but we will never know for certain until we either find one or become one
it does not seem that great of leap to think that barring some catastrophe, humans could reach that level
My car has a 15 gallon gas tank. If I was given a raise I would not upgrade to a 30 gallon tank. If I was given a $million or $billion I would still drive a sedan with a 15 gallon tank. Driving an tanker truck is not an improvement in my quality of life. Commuting further (or burning gas in place) is also not an improvement in my quality of life. A civilization is likely to increase energy consumption only if there is something that they gain by increasing their consumption. There needs to be some reason why they would struggle to use a larger fraction of the available luminosity.
The infrared flux around Alpha Centauri is between 10
7 and 10
8 times the current energy output of human civilization (
i.e. 10-5 solar flux, 4 x 10
21 watt). Of course dust is the most likely explanation for that. Even if some of the radiation was from an alien industry dust is still the most likely explanation for most of the zodiacal light. Many of our civilized activities generate dust.
If 10
19 watts is good enough to power a civilization capable of interstellar travel then most sun-like stars have more than enough infrared signature to mask civilized, star faring energy emissions. The small red dwarfs are a few orders of magnitude lower luminosity. That could mean no one is using the small red dwarfs, they use the red dwarfs for some things but not to continue expansion, or that they can expand using less energy. They could go through a temporary spike in energy output while launching ships and then cool off to the currently observed levels. What makes 10
16 watts not enough energy to build and launch an interstellar fleet? If 10
16 is enough energy and a colony ship the size of Texas is big enough then there is no evidence available suggesting that these are not common throughout the Milky Way.
There are good peer review papers discussing the possibility of planets around Alpha Centauri. I recall reading one that constrained the planets to less mass than 3x Jupiter. I am comfortable saying that it is highly unlikely that Jupiter radius space ships are orbiting Alpha Centauri. However, a Jupiter radius spaceship cruising near a Lagrange point orbit would not show up in the image taken by any of our telescopes. If we are uncertain about the presence of an Earth mass planet then we are also very uncertain about the presence of debris, pollution, and/or ruins on the surface of a planet.
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