The Great Filter is the idea that, in the development of life from the earliest stages of abiogenesis to reaching the highest levels of development on the Kardashev scale, there is a barrier to development that makes detectable extraterrestrial life exceedingly rare. The Great Filter is one possible resolution of the Fermi paradox.
https://en.wikipedia.org/wiki/Great_Filter
The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high likelihood of its existence. As a 2015 article put it, “If life is so easy, someone from somewhere must have come calling by now.”
https://en.wikipedia.org/wiki/Fermi_paradox
Personally I think it’s photosynthesis. Life itself developed and spread but photosynthesis started an inevitable chain of ever-greater and more-efficient life. I think a random chain of mutations that turns carbon-based proto-life into something that can harvest light energy is wildly unlikely, even after the wildly unlikely event of life beginning in the first place.
I have no data to back that up, just a guess.
Sadly it may be the speed of light.
All these intelligent species are simply trapped in their own solar systems for all eternity by an unbreakable natural law.
AFAIK there is no known energy source that would keep a generation ship powered for the duration of an interstellar flight.
The person to whom you responded is half right. The speed of light is half of the barrier to interstellar travel. Entropy is the other half.
Would you need a power source? If you aim your ship correctly, then put everything alive into cryo, the ship could go completely dark, vent all heat and become a frozen rock. Then after [very long time] the ship enters the vicinity of a different star and can be reactivated and unfrozen using solar energy. You dont need energy to maintain cryo if the whole ship is at 1° kelvin.
(Of course that relies on cryo sleep being possible)
Also, you’d need to know for certain that the planet you’re sending your generation ship to is habitable for your species. While this may be technologically trivial for a society that can build a functional generation ship, the timescales for such projects (literally hundreds or even thousands of years from the launch of the probe to the yes/no signal) makes it extremely difficult to actually organise.
Plus isn’t the rate of expansion of the universe increasing? So at some point, even going at light speed, your destination will recede faster than you can travel.
Not really, no. Generational ships might make colonizing the nearest star systems possible, but even colonizing our own galaxy would require some kind of suspended animation. The milky way is between 100,000-200,000 light years in diameter so even at the speed of light, you’re looking at a travel time that is ~33-66% of the time that humanity has even existed(homo sapiens are currently estimated to have become a distinct species 200,000-300,000 years ago)… just to go to ONE star system out of the hundreds of BILLIONS that exist in our galaxy. You’re gonna need generational ships so self-sustaining and capable that the generation that actually arrives at the destination will have long forgotten the point of the trip and might not want to leave the comfort of the ship.
Still, colonizing our own galaxy is at least theoretically possible, given enough time. The real filter is just how unimaginably large the universe is. The vast, VAST majority of the observable universe is FOREVER out of our reach, as it is expanding away from us faster than the speed of light. Then there’s the unobservable universe, which could literally be infinitely bigger than the observable universe for all we actually know.
That’s why faster than light travel is the holy grail. Without it, we’re just kind of stuck.
Imagine if wormholes had zero constraints on the physical location of the other side of the wormhole though. We could open a portal to OUTSIDE the observable universe. What a mindfuck. We might even find a false vacuum decay racing towards us at the speed light, or regions of space that are contracting instead of expanding, or initiate a new big bang by opening a wormhole to an area of space where that hasn’t happened…we could travel to a point where we can watch the milky way get formed, since the light of its formation is just reaching that region of space. If it turns out the heat death of the universe is just a local phenomenon, we could continue expanding forever beyond it. World without end.
Generational ships wouldn’t have to reach the edge of the galaxy, just the next planetary system. There’s no reason civilization needs to remain centered on Earth, either. Think of it as a wave traveling outward, where it eventually reaches the edge, by many smaller hops. It will also eventually reach earth, where they might wonder at signs of a prehistoric civilization. Actually, think of it like the Middle East, where empires rise and fall, crusades and jihads burst through, religions rise out of nowhere, people speak many different languages. A galactic civilization could be dynamic and ever changing, distance can make us strangers to each other, the fate of any planet matter only to its inhabitants and neighbors
Until we figure out how that is possible outside of theory, it is just that. We have no plans that address actually keeping a spaceship working on such a timescale, and keeping the crew alive on top of it.
Considering we haven’t seen any generational alien ships visit, it seems like nobody else has figured it out yet, either.
There doesn’t need to be more to it than that. The observable universe is over 93 billion light years in diameter. That means even at the speed of light, it would take over 6.5x longer than the universe has even existed for anything to cross that distance… except the universe is expanding faster than the speed of light, so actually you need to go significantly faster than light to make it across. FTL is, sadly, still firmly in the realm of science fiction, so to the best of our current knowledge most of the universe is permanently inaccessible.
I think that the great filter/fermi paradox is a combination of two facts,
- Our entire radio output (the only example we have to go by) is pitiful compared to the sun, like a candle in front of a flood light, you’ll only be able to see it so far before it’s completely drowned out. After a few dozen light years our radio output is less than the margin of error of a stars detectable radio output.
- As a civilization advances it must reduce radio leakage. As data gets more important, it gets more important that you’re not wasting energy moving it around. Narrow beamed radio transmission becomes the norm and even less radio signals escape the system than when radio was messy and overpowered.
They’re not missing or gone, they’ve just moved beyond messy radio signals. Even we tightened up our radio emissions in a little over a century. Most of what we watch or listen to comes to us via fiber, cable, or short range transmissions like cell phone towers and Wi-Fi.
My favorite filter is the amount of phosphorous in the universe. Earth has an unusually high amount, and it’s vital for life. I like this one, because as more stars die, the amount of phosphorous goes up, implying we won’t be alone forever.
Anyway, look up “Issac Arthur” on YouTube for HOURS of content about the Fermi paradox and potential great filters.
I’m gonna add to this by saying phosphorus may be my favorite, but I think the most likely filter is just time, twice.
Do you know how unlikely it is that earth has been habitable for so long? Do you know how long life was single-celled? One of the theories for how advanced (eukaryotic) cells formed was the combination of at least three different branches of life into the same cell! Archaea (cell wall), bacteria (mitochondria/chloroplasts), and viruses (nucleus). Do you know how unlikely that sounds? Do you know how long it would take for that to happen randomly? Most planets probably aren’t even habitable for that long. Once we became eukaryotic, we started progressing much faster.
Then, keep in mind, the life has to continue to exist for billions of more years while it waits for the advanced life to happen again within the same section of the galaxy. So, time is two filters - both behind us and in front of us.
Throwing in another similar YouTube suggestion of John Michael Godier
An alternative is we are among the first. Third generation stars are the ones that have planets with enough heavy elements to allow for complex chemistry. Sol (our star) is thought to be among the first batch of third generation stars in our gallexy.
Light speed does seem to be the upper speed limit for the universe. Talking that into account we probably haven’t had a chance to see other early life as it would likely be spread pretty thin right now.
Honesty, I don’t think that there is a Great Filter. The Fermi Paradox strikes me as not very well-reasoned. A whole hell of a lot of things would have to go exactly right for civilizations to make contact, rather than it being the default assumption. There are lots of filters, not just one Great one.
But the closest to a Great Filter is that space is really, really. stupendously big. The chances of even detecting each other across such distances is vanishingly small, much less traversing them. Add in the difficulty of jumping the metabolic energy gap to become complex life, and that could reduce the density of civilizations down to a level that they’re just not close enough to each other in spacetime to admit even the possibility of contact. And we’re hanging our hat on some highly-speculative concepts like alien mega-structures harnessing whole solar systems to allow detection.
I think a lot of persnickety, smaller filters combine to make interstellar contact between civilizations against long odds. Perhaps the best we’ll get is spectral signatures from distant planets that are almost-conclusive proof of some sort of life.
I think at some point, almost certainly not in our lifetimes, we’ll detect the spectroscopic signatures of a planet that has an atmospheric makeup that HAS to be from life, but with no detectable signs of any civilization. Just nonsentient life. And we may never be able to get there.
I think you’re probably closest. There aren’t “filters” so much as we live in a universe that can only support life on a highly contingent basis, entirely by accident, at random intervals. It’s filters all the way down, really. None of us are getting out alive, might as well enjoy it while it lasts.
