Can all of you who have your microphones on mute yourselves? It will help with the audio a lot.

can people mute their microphones?

Doesn’t this mean that all of the US population growth shown in the previous slide is due to immigration, not fertility?

Does this approach assume that all food grown is perfectly distributed? As far as I know, today's hunger problem is based largely on a supply chain issue and not because we don't grow enough food.

Aren’t there other constraints more stringent than land area? Those post-green revolution agricultural capacities use a lot of fixed nitrogen (and pollute a lot of coastal waters), deplete water tables, poison areas with pesticides, etc….

(today in India they are hitting these limits - depleting water tables, increasing pesticide use to try to compensate for reduced yields, poisoning water tables with pesticide, etc…

It may well be that *most* energy intensive civilizations have short lifetimes, but that the galaxy is still full of very long lived civilizations. If the transition to very long lifetimes is low but finite, then this is the most likely situation…

(the probability of the transition, that is…)

So in this case (the most likely case IMHO) then our own likely longevity is not correlated with the likely existence of long-lived civilizations…

This makes sense to me, I think we are already in the middle of this transition (as some of the earlier slides showed)

We are *trying*

Indeed, there is a whole genre of sci-fi where the Earth is an overpopulated environmental dystopia or even uninhabitable, while a small fraction of humanity has sets up settlements spreading across the galaxy. The two scenarios are not mutually incompatible, even within a species.

I mean the population curve is bending over, and I think the watts per person are as well (though not the total watts), perhaps that is a little more complicated.

Given the proximity of the tidally-locked planets in compact systems like TRAPPIST-1, I’ve always enjoyed the idea that a civilization might observe technosignatures like city lights on a nearby planet before they had explored the entire surface of their own.

^ Good point Jessie! May be there will be different timescales for civilizations to develop within a system.

Would NO2 be a long lived signature? I tend to think of anything that is “pollution” as inherently short-lived, not the hallmark of a sustainable civilization, and therefore not really observable. But perhaps this is not true about NO2?

@Jessie, I’ve been look at the solar gravity lens focus (ours is out beyond 500 au), and have similarly wondered if there were inhabited planets around stars that naturally resided in an orbit that would be in their star’s gravity lens focus.

NO2 is short-lived, unless a civilization colonizes a nearby planet or lives sub-surface. We never know what is the frequency of mediocre technological civilizations !

*not never....we do not know I should say

@Seth - are there any star/planet mass ratio combinations that put the planet in the HZ?

David - probably would be short lived - unless it was intentional, like in a terraforming project.

(I guess it’s just the mass of the star, not the planet)

@Seth I don’t think so: 500 au is very far away, so there is very little free energy out there for an inhabited planet to use. I think the distance to the lens is also rather insensitive to the mass of the star (I think the distance to the lens scales as R/M, which is roughly constant for stars on the MS).

I’d have to dig out my plots (I can email them to you), but if I remember correctly, it only occurred naturally for fairly high mass star.

@Mark - so not a good technosignature?

What if you took a useless planet in your own system (ethically suspect concept I realize - but one where you couldn’t live and didn’t want to do anything and there was no indigenous life) and put some really bizarre and highly observable molecules in its atmosphere, as a beacon of anomaly?

@David - Might be a semantic thing, but I would think a terraformed planet would be a perfect techno signature. Mars with 100 ppm SF6 would be far more “observable” than a few ppt of CFC or NO2 on Earth

@David: we discuss these things in our paper.

@David: Yes, I think of this as the “Praxis scenario”: an “industrial” planet that has no biosphere but a thick technosphere, like if we covered Mercury with solar panels and supercomputers.

Good! Can’t wait to read it.

@Jason - it does have a mass dependence. I think the HZ and the SGL cross at 80 stellar masses! It is more interesting if we consider planets orbiting white dwarfs.

@Jason - I was wondering if this might be done on a planet for no other reason than to stand out in techno signature searches. Kind of absurd. Stanislaw Lem type scenario...

The idea of using “dead” planets as industrial centers seems like a really useful idea.

@Seth I’m not sure if we expect *any* rocky planets to be orbiting 80 solar mass stars?

the length of time the earth has had a detectable biosphere is roughly 2 billion years, the amount of time with a detectable technosphere (AFAWK) is ~10K years. That is a 5x10^-6 ratio from the only case we have....

@Gavin, unless there are statistically more technospheres....but they have to be really large in number.

@Gavin: Doesn’t that assume only one technological species every arises per planet? I would think that if one arose, another is more likely to follow.

That is r selection, but point taken

@jason. well yes. no need to assume that, but no actual evidence to the contrary so far... ;-)

Gavin has a upcoming seminar about this topic.

technospheres without a biospheres sound fascinating, they could even be the industrial sites that are utilized in the beginning of building a Dyson swarm

Thank you for the talk!

I have to run, thanks so much Jacob!

Nice talk, thank you!

Have to leave, thank you for the talk!

@Gavin it’s long been in the literature (at least since the 1960s) the idea that technosignatures will be completely undetectable unless they are much more long lived then ours is currently - because of that ratio...

Thanks for your interesting talk, Jacob! This raises many interesting questions to think about. One os them is the factor of cosmic catastrophes that need further study.

Technofossils

Are there any non-astronomical ways to empirically test the great filter?

Thanks for a great talk Jacob!

And thanks to the organizers for starting this series!

Thanks for interesting talk!

Thank you!

Thank you Ravi & Jacob!

thanks for organizing this!!!!