(ambient music) - Well, we used to look up in the sky and wonder at our place in the stars. Now we just look down and worry
about our place in the dirt. - In antiquity, our ancestors
looked up to the stars and saw a perpetual, eternal canvas, one inhabited by gods,
deities, and mythical legends. The heavens were immortal,
untouched by the ravages of time that would cause our buildings
to crumble, our art to fade, and our faculties to decline. On earth, everything dies, but those sparkling stars
seem to be the exception, a source of solace in a brutal cosmos. When we first learn that
stars die, it troubles us. It feels wrong. Children often panic at the thought, but truly, the light in our sky, the engine of our planet's biosphere, is gradually stepping
towards a dramatic end, albeit one billions of years from now. Before it dies, the sun
will boil off our oceans, vaporize our planetary surface, and eventually smother
the earth into itself in a final embrace. And it's not just the sun. Whilst the pathways might be different, a similar fate awaits all stars. And one day in the far, far future, there will be no lights in anyone's sky, a dark veil that swallows
the cosmos whole. But there are some who resist. After all, it's human nature. Even with our own lives, we're compelled to try
and extend our lifespans through medicine,
improved health, and diet. So could we somehow extend
the lifespan of our sun? Could we buy ourselves more time through our ingenuity and technology? Thinking of our planet's demise, usually folks assume
that it's the engulfment by the sun that defines the end, for in around seven and
a half billion years, the sun will appear unrecognizable as a swollen red giant star, one that has devoured
the inner solar system. But actually, earth's
engulfment by the sun is not really the problem. Well, don't get me wrong,
that is definitely a problem, but actually, the end for
complex life is far nearer, because as the sun ages,
its luminosity rises, and it's that increased power output that really spells the doom for life. You see, this leads to greater
insulation on the earth and hence greater weathering. I mean, essentially, more
energy means more evaporation, which leads to more precipitation. Rain dissolves carbon
dioxide out of the air, forming carbonic acid, and it's that acid which eventually leads to the formation of solid carbonates and limestones. The net effect of this is
that making the earth warmer causes carbon dioxide levels to drop. Yes, I said drop. And no, before you start, this won't save us from climate change, because this process takes
millions of years to complete, although there are some ideas
to artificially speed up, known as enhanced weathering. Now, whilst lower carbon
dioxide levels might sound great in our current era of a climate crisis, it's actually the death
now for complex life. In just under a billion years from now, which is still some six
and a half billion years before the sun engulfs the earth, CO2 levels will drop close to zero, terminating photosynthetic life and collapsing the global food chain. This is the end of complex life on earth. I've skipped through the
gory details of our demise, but if you have the stomach
for the more detailed account, then be sure to check
out our previous video, "Watching the End of the World." What all of this means is
that if we want to maintain a habitable earth for longer,
then we need to somehow keep the irradiation striking the
earth stable over that time, and the sun doesn't wanna
cooperate with that. So that begs the question, is there something that we could do, or perhaps something that our more technologically advanced
descendants might be able to do? There are basically three options. Option one is just to pack up and move the entire population
to another solar system, a journey of daunting
scale and complexity. In that choice, we sacrifice our home, watch it burn to the ground
as we sail away into the sky, a final goodbye to the
parent that gave us life. But options two and three refuse to let the earth die so easily
and seek to preserve it. Starting with option two, the idea here, somewhat audaciously, is to
physically move the earth further back from the sun. And it's this idea which has garnered the greatest scientific
attention in recent years. In 2001, Don Korycansky
authored a provocative paper that suggested that
our distant descendants could indeed pull this
off using asteroids. The idea starts by first
finding a very large asteroid, something like the size of 324 Bamberga, but one in a highly
elliptical and distant orbit, such that it periodically swings into the inner solar system
every few millennial or so. The asteroid's orbit is then manipulated, perhaps using reflective paint, or even just smashing into it, like the DART Mission recently did. Either way, this manipulation
causes the asteroid to make a near pass of the earth. In doing so, the asteroid
will whip around the earth, performing a gravitational sling shot. In such exchanges, orbital
energy is transferred between the asteroid and the earth, with the direction depending
upon the angle of attack. Korycansky cleverly arranges the slingshot such that the asteroid's
orbital energy is lowered, but the earth gains. Now, because the earth is so much more massive than the asteroid, this has a huge effect
on the asteroid's orbit, but just marginally increases the earth's orbital distance around the sun. Congratulations, you have just moved the earth back a tiny bit, and thus slightly reduced its temperature. Now, to really make a difference, we're gonna have to do this many times, so rinse, wash, repeat over and over. And the real elegance of this scheme is that that asteroid can
be reused to this end. The first step is that
the asteroid's orbit is nudged with thrusters towards Jupiter, where it performs another slingshot to regain its lost energy. Additional adjustments are then made to the asteroid's orbit
to ensure that it returns back to the earth in its next roundtrip. And in fact, the authors
propose that the outer planets, such as Saturn, could help here. For an asteroid with a
period of, say, 6,000 years, it would perform 1 million of these close encounters with the earth over the next 6 billion years. Each one pushes the
Earth's orbit back a little until we reach an orbital distance comparable to Mars's current location. That distance is important, because in 6 billion years' time, the sun will be about
2.2 times more luminous. So pushing the earth square
root 2.2 times further out, which is where Mars lives now, means our irradiation stays
the same as the modern value. The scheme is brilliant, and I remember being quite
inspired by this paper when I first read it many years ago, just the idea that we
can use the components of our solar system to
modify our environment. Astroengineering. In fact, many have even suggested that we could use this
to solve climate change in the short term. However, it has to be said that this idea comes with
some considerable challenges. Perhaps most alarmingly,
we're deliberately aiming one of the largest asteroids
in the solar system to close proximity of
our fragile home planet. Indeed, that asteroid
would need to be so large that if the unthinkable
happened and we miscalculated, leading to a strike on the earth, it won't just trigger a mass extinction. It would completely
exterminate life on earth. Game over. The authors are certainly aware of this, and acknowledge it in a rather haunting final sentence of their paper, saying, "This danger cannot be overemphasized." In this scheme, it's also deeply unclear what happens to Mars,
because we're pushing earth further back into a Martian-like orbit, and two planets cannot stably occupy the same orbit at the same time. The authors also briefly comment on this, somewhat enigmatically, by stating, "The fate of Mars in this
scenario remains unresolved." This orbital overlap is important, because we know that
the inner solar system can easily become chaotic. For example, Batygin and
Laughlin showed in 2008 that instabilities are
plausible on a time scale of just 1 billion years from now. And now, in this asteroid
deflection scheme, we're really stirring the pot by pushing planets around
into unnatural positions. Okay, so you might think
that we are finally done with our list of side effects here, but there is just one other small thing. I mean, it's just a minor hiccup, really. We lose the moon. Now, maybe you're okay with
losing the moon, but I'm not. - I got nukes out the yin-yang. Just let me launch one, for Christ's sake!
- Sir! Are you suggesting that
we blow up the moon? - Would you miss it? Would you miss it? - Look, the moon isn't just a
pretty light in our night sky. Its existence may be
central to our survival, because, as shown decades
ago by Jacques Laskar, the moon is thought to stabilize
Earth's spin obliquity. That's the tilt which
gives us our seasons. Take the moon away, and climate
stability could run wild, transforming our planet into a nightmarish
environment for civilization. With options one and two covered, is there any other alternative? Is there some other way
that we could save our home? Well, that's where my
research group stepped in, the Cool Worlds Lab at
Columbia University. The asteroid deflection business made a lot of headlines
when it was first published, but buried in the literature
is an alternative, an idea that we felt was underdeveloped, and that is starlifting. Could we reduce the mass of our sun? The first published
discussion of starlifting was in a fairly obscure
conference proceeding in 1985 by David Criswell with, then, Greg Matloff reintroducing
the idea in 2017. Stars naturally lose small
amounts of mass all the time, in particular through their stellar winds. So Matloff proposed
enhancing that mass loss rate through the use of
lasers aimed at the star. One could also imagine
siphoning mass off the surface with a compact massive
object in close orbit, causing so-called mass overflow. Friend of the channel Isaac Arthur has a great video introducing
this work on starlifting that I'll link to down
below in the description. But despite this work,
it's really not clear how much mass we have to take off the sun, nor exactly how that
affects the solar output. Stellar evolution is an
notoriously challenging field, and I knew this wasn't gonna
be an easy problem to solve. Each year at Columbia, we
welcome new graduate students to our Astronomy Program,
and professors such as myself line up in a shark tank setting to pitch new research ideas to them. I remember last year how
I spoke to the students about all the different exoplanet projects we were working on, but I mentioned that, "Hey, on the side, we also like working on "these wacky little side projects." Afterwards, one of the students, the brilliant Matt Scoggins, came up to me and said, "Tell me more." So for the last year, Matt has been chipping away
at this problem with me, and we're thrilled to be able to share our new work with you today, research that finally
reveals what it takes to save not just ourselves
but the sun itself. Perhaps one day, then, just as the sun has provided life unto us, we can use our ingenuity
to return the favor and give back to the sun the gift of life. But how exactly do we propose to do this? - The key to making a star live forever, or at least a lot longer, is balance. You have to find just
the right mass loss rate such that the temperature of your planet remains the same at all times. In simple terms, if you
remove mass from the star, it reduces the gravitational pressure at the center, sort of like this. (balloon hissing) Reducing the pressure at the center reduces the rate of fusion, and reducing the rate of fusion reduces the luminosity output. This should intuitively make sense. We know that small red
dwarfs like Proxima Centauri produce far less output than our sun, but the other effect in play here is time. As stars age, their cores accumulate more and more helium ash in their centers. Since helium is denser than hydrogen, this raises the internal pressure, and hence increases the rate
of fusion, luminosity output. So our challenge is this balancing act where we need to find
the rate of mass loss such that these two effects
cancel each other out, and we produce a stable luminosity. You can immediately
see that we're creating a kind of strange star over time, because after doing this
for a sustained period, you have a much lower mass star, but one whose luminosity
equals that of our modern sun. And the only reason that
that statement is true is because this engineered star is what we call fairly evolved. It's aged to the point
where its output is enhanced and it's teetering on the
edge of becoming a red giant, but we keep spooning mass
off at just the right rate to prevent that from happening. You can see this behavior on this plot. Let's put a dot down for the sun and evolve it forward in time. Without any starlifting,
it would naturally do this, barely change in mass, but
increase in luminosity, represented by the
changing color shown here. Eventually, the track stops, because the star transitions
into a giant phase. We can also put on tracks for
stars of different masses, which are the other
horizontal lines shown here. However, in starlifting, we
play this game where we pull just enough mass to keep
the luminosity the same. All of these dots here
have the same luminosity as our modern sun, so we
just need to cause the sun to follow this new black
track, an engineered one. What's interesting is that
even with starlifting, the game eventually ends, because we're gradually
getting closer and closer to that red giant termination point. So the game eventually ends, but you can see that
we've actually extended the sun's main sequence
lifetime to 20 billion years, whereas it would naturally
only go to 10 billion. That's a 15 billion-year future
of stable luminosity for us. There's no need to move earth,
no need to do anything else, and the best part of this
is the mass loss rate required for this is pretty timid. It's only 2.5% the mass
of Ceres each year, which is the largest
asteroid in the solar system. This is way below the
rate that Matloff predicts should be possible with lasers, yet more, the energy cost to do this is easily supplied by the star itself. Just harvest the sun's output
and feed a fraction of it back to create the necessary mass loss. So our paper shows, using the latest stellar evolutionary
models at our disposal, that starlifting can
sustain a stable luminosity for tens of billions of years. But much like the asteroid
deflection scheme, this is something with
considerable challenges. First, it isn't clear how we would actually implement starlifting. If we wanna keep the
material near the star, we need some sort of stellar excavator, or if we want to eject the
material, as Matloff suggested, would earth need to be protected from this increased stellar wind? Of course, there is also the
small issue of currently being nowhere near technologically
advanced enough to starlift, and it's unlikely to happen
in the next few millennia, but it gives us something to hope for. (ambient music) - Starlifting could be the key to sustaining the sun for far longer than its natural life allows. It's a staggering thought
that human ingenuity could one day greatly extend
the lifespan of an object that massively dwarfs
our tiny pale blue dot. Although our paper is largely satisfied with the result that
we could theoretically extend the sun's lifespan
by 10 giga-years, one can't help but ask,
"Could we go further?" Could we somehow tune this method such that we could reach
hundreds of billions, perhaps even trillions, of years? One option is to pick on
a different host star. The sun is rather old and massive, but what if we picked a
smaller red dwarf star instead? Certainly, one could apply
our method to smaller stars, but there's a limit to how far one can go. The candle that burns half as
bright burns twice as long, so the stars that will
last the longest naturally are the smallest red dwarfs, especially those barely massive enough for hydrogen fusion in the first place. Sadly, these skinny
little or late end dwarfs don't really work with starlifting, because these stars don't
have a lot of fat to spare. Remember, these stars
are barely massive enough for hydrogen fusion in the first place. When it comes to maximum life extension, we found that the best
stars were orange dwarfs, stars in between the mass of
our sun and the smallest stars. With initial masses of
about half that of our sun, there's plenty of excess fat
to skin off here over time. In fact, it's possible to get these stars to live for trillions
of years, and crucially, that's trillions of years
of stable luminosity output. So civilizations living
around these orange dwarves may have struck the cosmic jackpot, because their home star
could be engineered to live more than a trillion years. Who knows? Perhaps civilizations even migrate to these stars for that reason, but do we really have to abandon the sun to last a trillion years? Maybe not. One final trick we conceived combines the asteroid
deflection scheme from earlier with starlifting, a cosmic one-two knockout punch for longevity. So how does this work? One way to make the sun last far longer is to take off more mass
than we previously conceived. Remember, originally, we take off mass at just the right rate such that the sun's
luminosity remains stable. Instead, let's now take off more mass, which will let it live for longer. But the consequence of that is that the sun's
luminosity will now fall. This would ordinarily be very
bad news for life on earth, but using the asteroid
deflection scheme from earlier, we could move the earth,
but now not outwards, but rather inwards,
preventing freeze-over. By combining the two,
there's no clear barrier to having your cake and eating it, a sun that could last a
trillion years or more. Do other civilizations do this? Will we ever do this? Will our technology ever develop to the point where this is even possible? Those are all great questions, and ones that we do not
have the answers to. Research progresses in
step-by-step increments. Previous work showed
that we can imagine ways of starlifting mass off the sun, and our work shows how it could greatly extend our star's lifespan, but we've only really
scratched the surface of what's possible in astroengineering. And it makes you wonder,
could we even tell whether a distant star
was engineered or not? Sci-fi author Karl Schroeder once paraphrased Arthur
C. Clarke by saying that, "Any sufficiently advanced civilization "is indistinguishable from nature." The original version said "magic." In this research, we touched on some ideas for how to detect engineered stars, but frankly, it remains a major challenge. Ultimately, that means for now that more research is
needed down the road. This isn't the kind of research which is easy to fund, but to some of us, questions about life and the
universe are foundational. They're endlessly fascinating,
and if you believe that, you can support our work
by bumping up this video, subscribing, and, if
you have the resources, you can even support our research program by using the link in the description. If you have any thoughts or
questions about starlifting, please do put them down
below in the comment section. I'll do my best to get back to you. So until next time, stay
thoughtful and stay curious. (light electronic music) Thank you so much for watching
this video, everybody. If you enjoyed it, be sure
to like, share, subscribe, all the YouTubey stuff. And as I said, this research
was supported by our donors. Let me thank our latest two, that is, Richard Williams and Joseph Gillmer. Thank you so much for your support, guys. So have a cosmic awesome day out there, and I'll see you 'round the galaxy. (music continues)
