Thanks to curiosity stream for supporting PBS Digital Studios. The Silence of the galaxy and the resulting Fermi paradox Has perplexed us for over half a century But our most recent surveys of the Milky Way Finally allow us to draw scientific conclusions about this depressingly persistent absence of aliens When Enrico Fermi uttered the words "Where is everybody?" He was succinctly summarizing what has become known as the Fermi paradox in short in a galaxy of hundreds of billions of stars Each of which having billions of years to spawn life and civilization, isn't it odd that none have made themselves apparent to us Enrico Fermi was supposedly using this as an argument against the plausibility of Interstellar travel. If aliens can travel between the stars why haven't they visited? But the paradox is much broader than this There are several ways that an advanced civilization could give away their presence radio transmissions, robotic probes, or star blotting solar arrays. A series of very recent surveys of our galaxy reveal none of the above while at the same time proving the abundance of potentially habitable worlds. The Fermi paradox has become only more paradoxical and these surveys are finally powerful enough to draw some serious conclusions about the rarity of advanced civilizations and about the chance of us ever becoming one. Before we get to that Let's see what the state of the art in astronomical surveys is telling us Nasa's Kepler telescope has discovered 2652 alien worlds to date Spotted as they marched across the face of their parent star in chance transit alignments This number allows us to figure out the fraction of stars that have planets Essentially all the stars do. It also allows us to figure out that there are something like 40 billion habitable planets in the Milky Way By habitable. I mean rocky planets the right distance from their star to have liquid water TESS, the Transiting Exoplanet Survey Satellite, is now in orbit and taking over from Kepler It'll find something like 20,000 new worlds telling us once and for all how common earth-like planets really are And how many of these planets have life? We have absolutely no idea We have no way to even start to guess that yet To answer this question We'll need to look at the composition of their atmospheres We talked about this before. Life is expected to massively alter the chemical composition of its home world's atmosphere We can't see this effect on earth-like planets yet But the James Webb Space Telescope - launched in a year or so will get close to doing so There is a good chance we'll have found hard evidence of extraterrestrial life in the next decade or so, but that's regular old non technological life Surely a high-tech civilization would be easier to spot We've been actively watching for radio transmissions from other worlds for half a century. We've seen nothing Even the famous Wow signal has likely been explained as a mission from a pair of comets That said our search hasn't even been sensitive enough to see human level transmission in the nearest star system we're probably only sensitive to deliberate beamed communication of which there are none, but Radio transmissions may not be our most likely first encounter with et Assuming any of those guys are like us at all they'll want to expand into and alter their galactic environment and Because they almost certainly have a head start on us of at least thousands of years that galactic gentrification Might be visible to us you might remember Tabby's star - the strange star in the kepler field that showed these bizarre dips in brightness some very large collection of Something is passing in front of the star Thrillingly, one proposed explanation was an alien mega-structure perhaps a vast fleet of solar collectors built by the local inhabitants Sadly, this is not the case follow-up observations revealed that the wavelength dependence of the dips is consistent with Dust so likely natural space junk not any type of artificial structure as we like to say on space time It's never aliens Until it is for example you these alien solar collectors may be our best shot at spotting other civilizations an Advanced civilization may launch so many solar satellites that they substantially block the light from their own star Let's call such a solar fleet a Dyson swarm after the Dyson Sphere which is the next level up a structure that completely encases a star a Dyson swarm would cause an Unusual dimming of its parent star and also cause an unusual increase in infrared light due to waste heat shed by the structure These are things we can look for. Kepler saw none of this but actually the best instrument for finding Dyson swarms is the European Space Agency's Gaia mission a team of astronomers Scoured the Gaia data looking for stars that were unusually faint for their stellar type They looked at 8,000 stars and exactly one of those stars had a brightness significantly lower than expected however They also figured out that the offending star has a binary companion Which may have caused a wobble in the Stars position and that in turn may have messed with the Stars parallax distance determination and the determination of its power output I.e. the star may just be further away than expected not wreathed in a swarm But Gaia's upcoming third data release will expand this study from 8,000 to a million stars If they're out there, we'll find them Eventually, it's still early days now hunt for ET But maybe we can at least agree that if there are any technological civilizations in our neighborhood of the Milky Way They are keeping to themselves That still seems surprising given those 40 billion possible starting points for life in the Milky Way After all would only take one of these worlds to spawn an expansionist species Or even a single individual among that species to launch a single self-replicating probe give them a million year head start on us just one ten thousandth of the age of the Milky Way and Someone should have colonized the galaxy by now. Yet we see nothing but detecting nothing is still data Give astronomers data and they will write papers a number of attempts have been made to address the Fermi paradox based on this new data This is often done in terms of the famous Drake Equation Which tries to estimate the current number of communicative civilizations? by combining a series of factors, there are Astrophysical factors like the rate of star formation and the fraction of these stars that form habitable worlds biological factors like the frequency of the appearance of simple life and the proportion of those that lead to intelligent life and sociological factors like the fraction of intelligent life that transmits radio signals and how long that communicative civilization lasts The biological and sociological factors are so poorly understood That estimates of the number of civilization vary between none to tens of thousands but now that we have a much better understanding of the Astrophysical factors and are also pretty sure that there aren't tens of thousands of advanced civilizations We can actually start to constrain those biological and sociological factors Astronomers Adam Frank and WT Sullivan did this in 2015 They found that as long as the odds of any given how tall planet spawning a technological species is greater than one in sixty billion Then humanity is not the first to appear in the Milky Way Let's take us even further Assuming there aren't tens of thousands of civilizations existing before us we must have beaten some steep odds it must be harder than one in a million to from damp rock to rocketship Luis Anchordoqui and collaborators found a less stringent limit on this likelihood by assuming that a lot of life gets wiped out by gamma-ray bursts a type of cataclysmic exploding star We covered those before also These researchers estimate that in order for us not to see any aliens the chance of going from space rock to high-tech Society must be less than half a percent Now that sounds high But remember it's only for the planets not wiped clean by gamma-ray bursts whether it's gamma-ray bursts or something else our failure to find aliens tells us there must be a developmental bottleneck or colloquially a great filter that seriously limits the milky way's production of highly visible Galactic Civilizations Optimists like to think that humanity has already passed the great filter That is perhaps it's one of the biological factors of the Drake Equation that keeps the number of aliens low So watery planets may be abundant But maybe there's a step in that slow Evolutionary crawl from primordial chemical soup to techno monkey that makes advanced civilizations hellishly unlikely This is something we can come back to but you can look into the rare earth hypothesis or the gaian bottleneck hypothesis for some plausible proposals If the great filter is behind us, that's good. It means we made it It also means we have a long lonely future as one of the galaxy's only advanced species But hey one is better than none, right? But our modern understanding of the Fermi paradox Permits a second interpretation To wrap up let's consider the possibility that lots of planets produce civilizations at around our level and They wipe themselves out with such thorough Reliability that literally none ever leave a mark on the galaxy Some scientists have been inspired by the persistent absence of evidence for aliens to investigate this possibility Statistically, how likely is it that we wipe ourselves out? Some scientists argue that deliberate annihilation of all humans isn't just possible. It's likely Joshua cooper and John Sotos point out that the technology needed to create an extinction-level event will inevitably fall into the hands of many individuals over time one whack job or Impulse-control challenged individual will push the metaphorical big red button Now these researchers focus on genetically engineered viruses But the principle applies to any doomsday tech nuclear weapons, genocidal nanobots, willful or negligent environmental destruction, you name it the researchers argue that all emerging technological civilizations will go through this phase of massive access to potentially cataclysmic technology Depending on the destruction probability. Let's call it the wacko factor You might expect civilizations to last from decades to thousands of years But even thousands of years is brief enough to help explain the Fermi paradox Well, that's a nice sobering thought to end on but I should also point out that this same massive access to technology that could kill us may also get humanity off the earth and on to other planets So which will come first spreading human civilization across the galaxy or humanity's doom. I guess it depends on the relative probabilities Are we more inclined to acts of self-destruction and planetary sabotage or to acts of preservation and exploration Well, perhaps more importantly can we actually choose between these? Well, we sure can try the alternative may be eternal quiet across the lifeless reaches of space-time Thank you to curiosity stream for supporting PBS Digital Studios curiosity stream is a subscription streaming service that offers documentaries and nonfiction titles from a variety of filmmakers including curiosity stream originals, for example You can check out particle fever which follows the first round of experiments at the Large Hadron Collider and the eventual identification the Higgs boson particle You can learn more at curiositystream.com/spacetime and use the code spacetime during the signup process Last week we dipped our toes into string theory. You guys had a lot to say Sam Pollard asks, how does adding more particles require fewer dimensions? well, the answer is supersymmetry Now this is a type of gauge symmetry that in principle unifies fermions and bosons Add it to string theory and you get super string theory. Adding the symmetry gives you fermions But it also shaves off dimensions Now the details are way beyond a comment response But look into super string theory and supersymmetry In response to me saying that string theory has only one tunable parameter Michael Murphy sarcastically points out that 10 to the power of 500 is a little larger than 1 Now he's referring to the number of Calabi-Yau manifolds that string theorists need to search through to find the geometry of our universe's extra dimensions To be fair, it's not 10 to the power 500 different tunable dials There's only one dial with 10 to the power of 500 different settings on it. So that's just one free parameter It's not even the 19-ish free parameters of the standard model Now some string theorists would say that there are zero free parameters all of those 10 to the power of 500 different possibilities exist somewhere in the multiverse We just happen to be in the one that makes people Korbis Barkly and some others ask Whether if string theory makes no testable predictions shouldn't it be just called string hypothesis? Well, I'm going to quote vacuum diagrams on this one Scientists don't actually use the nomenclature that some popularizers of science often insist on Now that popular definition is expressed reasonably well on Wikipedia. A scientific theory is an explanation of an aspect of the natural world that can be repeatedly tested and verified in Accordance with scientific method using accepted protocols of observation measurement and valuation of results But actually we've always used the word theory to mean again quoting doctor diagrams Some framework that loosely described allows you to understand some aspect of the natural world while a hypothesis most often refers to a single claim rather than a coherent framework that typically comprises many claims Or to put another way. In colloquial science speak, a scientific theory is a well-developed suite of ideas laws models and predictive tools that attempts to explain an aspect of the natural world It should also have the possibility of making predictions even if they aren't verified yet There's no separate word for a theory that's not well accepted but there's still a huge difference between such a scientific theory and say the theory of intelligent design a Scientific theory is self-consistent and well developed Eduardo Martinez makes an interesting point does not the idea of shut up and calculate and we don't care what strings are made of sound a lot like the flaws that science finds in religion I guess by that you mean that people in science sometimes accuse religion of promoting belief without Analysis and evidence. Does science do the same? Surely sometimes But in the case of shut up and calculate I think science is going in the opposite direction promoting analysis without belief or rather valuing the capacity of science to make concrete observable predictions based on evidence based models and that any Interpretation beyond these observables is beyond the realm of science. I personally don't agree with that. I think being able to predict doesn't mean that you understand. I think understanding comes from interpretation of your models ,not the models themselves Archie Wahwah wonders why M theory is represented by five interlocking pringles Well rumor has it that Ed Witten came up with M theory while he was eating Pringles late one night He was struck by the potential of multi-dimensional Pringles to unify string theory M is for munchies
Follow-up response to comments and criticisms on this video, with new info about "soft" and "hard" filters is here, and absolutely must-see (it's only a short portion at the end and the link takes you right to it):
https://www.youtube.com/watch?v=ztFovwCaOik&t=823
I really love all the PBS Space Time series of videos. They are surprisingly high quality, and you can jump in anywhere to view any topic that interests you.
Great vid. But why does the host talk like someone is squeezing his balls every other second?