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Statistics suggest that there is at least 1 planet per star. Multi-planetary systems are the norm, not a rarity. There is a huge amount of observational bias i

by joshvm 8y ago

Statistics suggest that there is at least 1 planet per star. Multi-planetary systems are the norm, not a rarity. There is a huge amount of observational bias in exoplanet research and given that we've "only" found a few thousand planets, it's naive to think our Solar System is unique. It's practically certain that with better instruments we'll find new planets in currently known systems. For example HD_10180 (7 planets currently) also has two potential gas giants. See also HD 219134. Also: stats of 100 billion to one, those are reasonable odds. That would make two species in the Milky Way, let alone the other billions-trillions of galaxies in the Universe.

btilly 8y ago
I suspect that you almost exactly reversed the point. The point of the Fermi paradox is that an advanced technological system should be able to colonize the whole galaxy in a geologically short time scale using Von Neumann machines. So if it had happened anywhere else in the galaxy, we'd expect to know about it because they would be already here. But they are not here. So we're probably the first in this galaxy. Given that there are around 250 billion stars in the galaxy, if the odds were above 1/100 billion per star, the observed outcome would be unlikely. So it is probable that the real odds are below 1/100 billion. I was offering potential requirements under which real odds on the order of 1/trillion could be justified. These odds are low enough that the observation "we're the first technological civilization in the Milky Way" is not a surprise and the Fermi paradox stops being a paradox. Neither the Fermi paradox nor my argument suggests that there aren't lots of other galaxies which contain advanced technological civilizations. The time for an advanced galactic civilization to cross intergalactic space is NOT a geological eye-blink. And almost all galaxies that we see are in other clusters, which are much, much farther off.
- Retric 8y ago
  I think the Von Neumann machine bit is a vastly larger issue than people assume. The Milky Way is 105,700 light years across. At voyager probe speeds that’s ~1.8 billion years. Stopping requires 4x as much energy, and cutting that time in half requires 4x as much energy. Worse slowing down to build a probe adds more time as does however long it takes to select targets, gather resources, and build several more probes. On top of a huge number of issues could slow things down such as lost probes, or needing to avoid the center of the galaxy resulting in an even longer trip. Not to mention collisions, at a minimum you need to survive grain of sand sized collisions, but interstellar space may contain significant numbers of much larger objects. Finally these probes would both come here, and stick around for us to see. A true space based civilization might have little interest in planets like ours, or have sent multiple probes to our solar system that failed. It seems possible for hundreds of civilizations to all be expanding across the galaxy even if we have seen none of them.
  - btilly 8y ago
    Using https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion we could, with currently conceivable technology, build spaceships that travel at several percent of the speed of light, and decelerate when they reach their goal. At 1% of the speed of light, we can cross go 100,000 light years in 10 million years. This is sufficiently short relative to the time that it took intelligent life to evolve that it would be a pretty good coincidence if there were two that just happened to reach that stage that close together. Note that with the magic of exponential growth, a few probes sent long distances and then spreading out locally should be able to reach every star in a galaxy with surprisingly little extra time over simply making the trip.
    - Retric 8y ago
      Nuclear pulse propulsion might be possible and it might approach the initial assumptions, but unless your talking ounces of cargo reaching and slowing down from 1% light speed is not feasible. We can’t rely on hand waving science fiction, they would need to send enough infrastructure to booststap manufacturing of a replacement probe. Let’s lowball that at 1 Billion tons of cargo and run through the rocket equation for 10,000 ISP. Even hand waving that we don’t have force fields to survive single gram impacts at 1% light speed. At those velocities anything is going to act like a massive shaped charge. Space is fairly empty, but when you start talking about the volume swept up across light years that’s really not a safe assumption. 70 years ago people generally assumed fusion was right around the corner. It’s only when you start building these things you get some idea of all the real limitations.
      - btilly 8y ago
        Sanity check. https://en.wikipedia.org/wiki/Project_Longshot https://en.wikipedia.org/wiki/Project_Longshot was a NASA design that could use nuclear pulse propulsion to deliver 30 tons of payload from here to Alpha Centauri at an average speed of 4.5% light speed. That is with acceleration and deceleration. The same technology could let us go much farther - just with coasting in the middle. Now it is absolutely true that we could not survive any significant impacts at that speed. However we also have data indicating that the interstellar medium is essentially empty. Here is a sanity check. Per http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.html http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.ht... the density of dark matter (that is, all the stuff that we can't see) is around 6x10^(-28) kg/cm^3. Since you arbitrarily said a gram, let's switch that to g/m^3, and we get 6x10^(-19) g/m^3. The distance to Alpha Centauri is on the order of 4x10^16 m. So a 1000 square meter surface should run into about a gram of "stuff". According to our best theories, most of it is currently unknown stuff, 99% of the rest is single atoms, and the vast majority of the rest is dust particles with sizes on the scale of waves of blue light. Dealing with that dust and hydrogen is a serious issue. But the odds of running into pebble sized objects at random are pretty low. Heck, even microbe sized objects will be vanishingly rare. True, it will be a disaster if we do. But that translates in practice into, "There will be a failure rate for probes." The size of a probe to bootstrap manufacturing seems likely to be a lot smaller than a billion tons of cargo. Don't forget, this is going to be encoded using technology that is as far beyond us as we are beyond building the pyramids. I think we can assume rather efficient storage/bootstrapping mechanisms to be available for us.
- joshvm 8y ago
  Well your statement was: > As a sanity check, the fact that no other intelligent life has been observed is evidence that it is unlikely. Plus some claims about the uniqueness of our solar system which are not necessarily true, given the small sampling of planet's we've found. There is so much bias in what we observe that I don't think you can make those statements, whether or not you're implying that a species would be galaxy-conquering. Given these biases I think the odds may well be much better than 100 billion - though I'm not current enough to judge whether that's an appropriate estimate based on curent astrophysics. There's so much uncertainty that it could be orders of magnitude different in either direction.

by joshvm 8y ago

btilly 8y ago

I suspect that you almost exactly reversed the point. The point of the Fermi paradox is that an advanced technological system should be able to colonize the whole galaxy in a geologically short time scale using Von Neumann machines. So if it had happened anywhere else in the galaxy, we'd expect to know about it because they would be already here. But they are not here. So we're probably the first in this galaxy. Given that there are around 250 billion stars in the galaxy, if the odds were above 1/100 billion per star, the observed outcome would be unlikely. So it is probable that the real odds are below 1/100 billion. I was offering potential requirements under which real odds on the order of 1/trillion could be justified. These odds are low enough that the observation "we're the first technological civilization in the Milky Way" is not a surprise and the Fermi paradox stops being a paradox. Neither the Fermi paradox nor my argument suggests that there aren't lots of other galaxies which contain advanced technological civilizations. The time for an advanced galactic civilization to cross intergalactic space is NOT a geological eye-blink. And almost all galaxies that we see are in other clusters, which are much, much farther off.

Retric 8y ago
I think the Von Neumann machine bit is a vastly larger issue than people assume. The Milky Way is 105,700 light years across. At voyager probe speeds that’s ~1.8 billion years. Stopping requires 4x as much energy, and cutting that time in half requires 4x as much energy. Worse slowing down to build a probe adds more time as does however long it takes to select targets, gather resources, and build several more probes. On top of a huge number of issues could slow things down such as lost probes, or needing to avoid the center of the galaxy resulting in an even longer trip. Not to mention collisions, at a minimum you need to survive grain of sand sized collisions, but interstellar space may contain significant numbers of much larger objects. Finally these probes would both come here, and stick around for us to see. A true space based civilization might have little interest in planets like ours, or have sent multiple probes to our solar system that failed. It seems possible for hundreds of civilizations to all be expanding across the galaxy even if we have seen none of them.
- btilly 8y ago
  Using https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion we could, with currently conceivable technology, build spaceships that travel at several percent of the speed of light, and decelerate when they reach their goal. At 1% of the speed of light, we can cross go 100,000 light years in 10 million years. This is sufficiently short relative to the time that it took intelligent life to evolve that it would be a pretty good coincidence if there were two that just happened to reach that stage that close together. Note that with the magic of exponential growth, a few probes sent long distances and then spreading out locally should be able to reach every star in a galaxy with surprisingly little extra time over simply making the trip.
  - Retric 8y ago
    Nuclear pulse propulsion might be possible and it might approach the initial assumptions, but unless your talking ounces of cargo reaching and slowing down from 1% light speed is not feasible. We can’t rely on hand waving science fiction, they would need to send enough infrastructure to booststap manufacturing of a replacement probe. Let’s lowball that at 1 Billion tons of cargo and run through the rocket equation for 10,000 ISP. Even hand waving that we don’t have force fields to survive single gram impacts at 1% light speed. At those velocities anything is going to act like a massive shaped charge. Space is fairly empty, but when you start talking about the volume swept up across light years that’s really not a safe assumption. 70 years ago people generally assumed fusion was right around the corner. It’s only when you start building these things you get some idea of all the real limitations.
    - btilly 8y ago
      Sanity check. https://en.wikipedia.org/wiki/Project_Longshot https://en.wikipedia.org/wiki/Project_Longshot was a NASA design that could use nuclear pulse propulsion to deliver 30 tons of payload from here to Alpha Centauri at an average speed of 4.5% light speed. That is with acceleration and deceleration. The same technology could let us go much farther - just with coasting in the middle. Now it is absolutely true that we could not survive any significant impacts at that speed. However we also have data indicating that the interstellar medium is essentially empty. Here is a sanity check. Per http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.html http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.ht... the density of dark matter (that is, all the stuff that we can't see) is around 6x10^(-28) kg/cm^3. Since you arbitrarily said a gram, let's switch that to g/m^3, and we get 6x10^(-19) g/m^3. The distance to Alpha Centauri is on the order of 4x10^16 m. So a 1000 square meter surface should run into about a gram of "stuff". According to our best theories, most of it is currently unknown stuff, 99% of the rest is single atoms, and the vast majority of the rest is dust particles with sizes on the scale of waves of blue light. Dealing with that dust and hydrogen is a serious issue. But the odds of running into pebble sized objects at random are pretty low. Heck, even microbe sized objects will be vanishingly rare. True, it will be a disaster if we do. But that translates in practice into, "There will be a failure rate for probes." The size of a probe to bootstrap manufacturing seems likely to be a lot smaller than a billion tons of cargo. Don't forget, this is going to be encoded using technology that is as far beyond us as we are beyond building the pyramids. I think we can assume rather efficient storage/bootstrapping mechanisms to be available for us.
joshvm 8y ago
Well your statement was: > As a sanity check, the fact that no other intelligent life has been observed is evidence that it is unlikely. Plus some claims about the uniqueness of our solar system which are not necessarily true, given the small sampling of planet's we've found. There is so much bias in what we observe that I don't think you can make those statements, whether or not you're implying that a species would be galaxy-conquering. Given these biases I think the odds may well be much better than 100 billion - though I'm not current enough to judge whether that's an appropriate estimate based on curent astrophysics. There's so much uncertainty that it could be orders of magnitude different in either direction.