Rough probability that aliens would visit the Earth?

[consuli]

A more pessimistic one is that technological civilizations are common but do not last long enough to develop advanced space travel

In case of us "advanced space travel" means bridging 4.2 light years to Proxima Centaur.
Anglada-Escudé, Guillem; et. al. (2016). "A terrestrial planet candidate in a temperate orbit around Proxima Centauri". Nature. 536 (7617): 437–440. arXiv:1609.03449. Bibcode:2016Natur.536..437A. doi:10.1038/nature19106. PMID 27558064.

And aliens in the milkyway have comparable distances to the next habitable planet.

How much time do we have still left, to create such an advanced space travel technology, before Earth will be overcrowded and get sunk in war for resources? 200 years?

Consuli

 

[stefan r]

... 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.

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?

... 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.

...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 10⁷ and 10⁸ times the current energy output of human civilization (i.e. 10^-5 solar flux, 4 x 10²¹ 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 10¹⁹ 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 10¹⁶ watts not enough energy to build and launch an interstellar fleet? If 10¹⁶ 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.

 

[consuli]

I think a lot of good points have been made in this lively discussion, I wasn't aware before. I want to thank every partipicipant for his/her good contributions.

What I take out of it, that the rough probility that aliens would visit Earth is pretty low, because an enormous number of conditions have to be all met, that this would happen.

For some kind of interim-summary I take the conditions I mentioned in my intial posting and I add the new conditions you mentioned to them.

Interim-Summary

So far, the following conditions have to be met, that aliens could/ would visit earth:

• There must to be a habitable birth planet for them (but only a very low proportion of planets is habitable at all)
• Biological life (in form of bacteria) must be swapped to their planet
  
• Their birth planet must stay habitable over millions of years
• But at the same time, the living conditions on the planet must be dynamic enough, so that the dominant (less intelligent) species is killed from time to time, so that the evolution race can start again for other newer (more intelligent) variants of the species (so that a sophisticated species is created, that is intelligent enough to understand cosmos and quantum physics, that would allow them to leave their planet, before it gets polluted and/or overcrowded, and/or war conflicts start) [Contributions from BWV]
  
• The habitable birth planet must not be larger than 1.3 Earth masses (otherwise it would not be possible to get to space with chemical fuel rockets and following a lot of important physics experiments in space to conclude from for cosmos and quantum physics would not be possible) [Contributions from Al_, cosmik_debris]
• The first "planet hop", that has to be accomplished to found an interstellar civilization, will be roughly in the range of 4 light years (deducting from our situation; distance Earth - Proxima Centaur)

• Among a lot electromagnetic backround noise they must pick up one of our electrmagnetic signals
• And they have to search a very very huge universe for us
• And if they find a signal from us, it would be maybe already thousands of years old, and we may not exist any more

• Pseudo-light-speed traveling (travelling with a significant proportion of light speed) must be physically possible
• There has to exist a space ship, that can resists particles, that hit the ship with a proportion of light speed; a grain of sand with 10% of light speed would cause a (nuclear) explosion of roughly 3 tons TNT [contributions from nikkkom]

• The aliens must gain a huge benefit to do such a (up to 10'000 years long) exertion journey to us (or must have another very strong motivation to do so) [contributions from usually_wrong, stefan_r]

Consuli

 

[nikkkom]

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.

I said "smaller hops", not "smaller stars".

 

[stefan r]

...

• The habitable birth planet must not be larger than 1.3 Earth masses (otherwise it would not be possible to get to space with chemical fuel rockets and following a lot of important physics experiments in space to conclude from for cosmos and quantum physics would not be possible) ...

Nuclear thermal rocket engines were tested by the USA in the 60s. NTRs were never launched but they should work. A small scale Project Orion rocket was launched using dynamite instead of nuclear blasts.

1.3 Earth masses is not a hard number for chemical rockets. The rotation rate and atmospheric thickness is important too. Larger planets are certainly harder to launch off of than smaller planets. With a specific rocket system and the planet's characteristics you can calculate if the payload mass is 0.

...

• The first "planet hop", that has to be accomplished to found an interstellar civilization, will be roughly in the range of 4 light years (deducting from our situation; distance Earth - Proxima Centaur)

Stars are frequently closer than 4 light years. Alpha centuari will be within 3 light years in 25,000 years. Gliese 710 is supposed to come within a quarter of a light year 1,280,000 years from now. Scholz' star came within a lightyear 70,000 years ago. Over a few million years most stars should have encounters much less than a light year. The direction is random.

If you have a specific destination in mind it is not likely that there is an advantage to stopping in route. Stopping a spaceship consumes as much delta-v ( basically fuel) as launching a spaceship.

...

• Among a lot electromagnetic backround noise they must pick up one of our electrmagnetic signals
• And they have to search a very very huge universe for us
• And if they find a signal from us, it would be maybe already thousands of years old, and we may not exist any more

They can detect the ozone layer as Earth transits the Sun. Humans may have this capability when (if?) the James Webb Space Telescope finally launches. Earth could have been an object of interest 2.5 billion years ago when oxygenating bacteria switched the atmosphere. The atmosphere became more extreme 800 million years ago. Plants colonizing land caused a significant change to the appearance of the surface. The beginning of agriculture may also be a recognizable technosigniture.

Launching a bigger telescope is many orders of magnitude easier than launching an interstellar probe. A probe is many orders of magnitude easier than sending an occupied ship. A civilization capable of interstellar travel should have telescopes a million times bigger than James Webb or Hubble. Also if you have a civilization on several stars you may be able to do interferometry with a light years wide base line.

The electromagnetic signals are new to the 20th century. NAZI propaganda from the 1930s is going to be the first thing detectable. It has only traveled ~85 light years. Stars further away have not heard from us yet. Cold war early warning radar is the loudest signal we sent in random directions over a period of time.

...

• Pseudo-light-speed traveling (travelling with a significant proportion of light speed) must be physically possible
• There has to exist a space ship, that can resists particles, that hit the ship with a proportion of light speed; a grain of sand with 10% of light speed would cause a (nuclear) explosion of roughly 3 tons TNT [contributions from nikkkom]

• The aliens must gain a huge benefit to do such a (up to 10'000 years long) exertion journey to us (or must have another very strong motivation to do so) [contributions from usually_wrong, stefan_r]

I believe you are mix and matching ideas and responses. In one scenario you have aliens who recognize something significant about Earth or the Sun and send a mission to investigate or colonize. In another different scenario the aliens are spreading colonies everywhere across the habitable zone of the Milky Way. Our solar system just happens to be located inside the Galaxy that they intend to explore/farm.

10% light speed, 0.1c, is not fast enough for any star to respond after hearing a radio signal except for Proxima, Alpha Centuari, Barnard's, and Luman 16. Wolf 359 and Wise 855-074 are maybes. Sirius would need 0.12c and would need to have had the ship ready to go. The possibility of aliens responding to radio signals with a colony ship is hard to believe. Extremely unlikely, odds close to 0.

Ice age glaciers and other types of weathering can erase evidence of a fairly large research outpost. Equipment dumped on the ocean floor 10 million years ago would be buried under sediment. There are good reasons to leave an ecosystem alone. Earth has very little value to a star-faring species. We currently have no way of measuring how many dwarf planets should be in our solar system. If a few dwarf planets were consumed for reaction mass the solar system would look a lot like what we see now.
The likelihood that migrating aliens appeared right now (last 100 years) is nearly a million times lower than the probability that migrating aliens passed through during the last 100 million years. That does not mean that either are likely. The 10,000+ years per light year is fast enough for a species that started spreading out across the Milky Way. The distance to center is 26,000 light years from the Sun. Some stars are moving faster than 0.001c,1000 years per light year. Barnard's star is 6 light years away and has radial velocity 0.00036c. If they spread out in both the inward and outward directions the galactic rotation would smear them across the entire disc in 200 million years. The Milky Way's thin disc formed over 8 billion years ago. Sun-like stars with Earth-like planets should have started appearing with the thin disc. Earth has been around for 4 billion years before our civilization emerged. That leaves a 4 billion year window for aliens to evolve and start spreading across space. It is also possible that Earth could have evolved intelligence faster (or slower). A 200 million year time span is not very long on the cosmic scale.

 

[newjerseyrunner]

The most obvious question about aliens visiting Earth is... why? Why spend the resources building a huge ship capable of maintaining a crew for a rediculously long and difficult journey when a probe the size of a golf ball could do the job? If you station a small probe on the surface of the moon, it could watch everything humanity does with almost no chance of being discovered.

If order to reach space faring, a species must understand the scientific method. They’d understand that you can’t observe a species naturally if you interact with it, and the most likely reason they would visit would be curiosity.

 

[BWV]

people who do they "relativity crusade" by hasty mindless quashing of anybody who dares to say anything about possibility of faster than light communication will look like retarded morons in less than 30 years from now.

Perhaps no more than you look like a classless twit using language like that. Not sure what the moderation policy is here, but I would not tolerate these pejoratives any more than I would racial slurs.

BTW ‘retarded moron’ is redundant

(Wiki)
Moron is a term once used in psychology and psychiatry to denote mild intellectual disability. The term was closely tied with the American eugenics movement. Once the term became popularized, it fell out of use by the psychological community, as it was used more commonly as an insult

(And the same has happened to the word ‘retarded’

 

[Vanadium 50]

Ah yes..."they laughed at the Wight Brothers". Problem with that argument is they also laughed at the Three Stooges.

 

[berkeman]

Not sure what the moderation policy is here, but I would not tolerate these pejoratives any more than I would racial slurs.

@Prometeus has left the building. That was the sound of the door closing that you just heard.

 

[consuli]

The most obvious question about aliens visiting Earth is... why? Why spend the resources building a huge ship capable of maintaining a crew for a rediculously long and difficult journey when a probe the size of a golf ball could do the job? If you station a small probe on the surface of the moon, it could watch everything humanity does with almost no chance of being discovered.

If order to reach space faring, a species must understand the scientific method. They’d understand that you can’t observe a species naturally if you interact with it, and the most likely reason they would visit would be curiosity.

Because of your very good points

I am changing

• The aliens must gain a huge benefit to do such an (up to 10'000 years long) exertion journey to us (or must have another very strong motivation to do so) [contributions from usually_wrong, stefan_r]

to

• The aliens must gain a huge benefit to do such an (up to 10'000 years long) exertion journey to us (which we, as less developed species cannot provide) [contributions from usually_wrong, stefan_r, newjerseyrunner]
  
• Or they must have another very strong motivation to do an exertion journey, especially as scientists would prefer to send a unmanned remote probe (cheaper and and more exact natural observation without interference [contributions from usually_wrong, stefan_r, newjerseyrunner]
• Thus, only the motivation of curiosity and sensation does remain to do an exertion journey to us [contributions from usually_wrong, stefan_r, newjerseyrunner]

just to aggregate into handy bullet points.

Consuli

 

[bahamagreen]

The Drake and similar estimates put focus on resulting quantity, but with enough time, a very few instances of intelligence emergence, or even one, might achieve unimaginable progress.

We have found 11 billion year old rocky planets.

On our own 4.6 billion year old rocky planet:
- life (replicating molecules subject to selection pressure) emerged after only a half billion years
- multicellular microscopic worms emerged after another three billion years of development
- man emerged after only one more billion years

Note the projection:
replicating molecules to the worm - 3 billion years
the worm to man - 1 billion years
This happening on one of the 11 billion year old rocky planets, they would have had and would continue to maintain a 6.4 billion year head start and lead over us. An intelligence with billions of years of development ahead of us may not be constrained by any of our present of foreseeable limitations. We can't even imagine the form or capabilities of intelligence so advanced*.

* Douglas Adams may have been only half joking when he wrote,"...a Hooloovoo is a super-intelligent shade of the color blue."