Thank You to Morning Brew for supporting PBS. On the Mediterranean shore
of Egypt some 2000 years ago, a scholar was mapping what he thought to
be pin pricks in the celestial sphere. He made careful note of the position of a bright
star that hung just above the southern horizon. But never imagined that he was gazing on a multi-star system the stellar that
hosted a world much like the Earth. A world that would be humanity’s first destination
when we would finally venture to the stars. Alpha Centauri is the first and brightest
of the constellation of the centaur, and the nearest star in our galaxy of 200 billion. From Ptolemy of Alexandria's
first record in his Almagest, more than a millennium and a half passed before we started to realize its incredible significance. By the end of the 17th century, the star had
long since slipped below the horizon of Alexandria in its galactic wanderings. But it shone bright from southern India, and
there a Jesuit priest peered at Alpha Cen through a newly invented device - the telescope. Father Jean Richaud saw two stars instead
of one, these days dubbed Rigel Kentaurus and Tolimar, a binary pair bound in an 80
year waltz. This telescope thing improved over the next
century and a half, allowing astronomers to watch alpha-cen sway relative to the background
stars as Earth orbited the Sun. That parallax revealed the system to be close
- closer than any other - but still more than four light years distant. The two stars were so near and so familiar
- both within 10% or so of our own Sun’s mass. Perhaps they also bore planetary systems - and
even planets like the Earth. Dreams of an interstellar humanity started
with the twins at the centaur’s foot. But our greatest hope of another Earth isn’t
with the twins. Forward another century, and a Scotsman named
Robert Innes photographed the southern stars from Johannesburg to track their galactic
wanderings. The fast-moving alpha-cen system was a prime
target, but Innes found something utterly unexpected - this wasn’t a binary system
after all - it was a trinary. A faint red star crawled on a vast, half-million
year orbit around its brighter siblings. For the entire history of humanity that orbit
has placed this red dwarf closer to the earth than its companions. So it was named Proxima Centauri, or just
Proxima - our solar system’s true nearest neighbour. At first Proxima’s proximity was mostly
a curiosity. Red dwarfs are the smallest, coldest, and
faintest of the stellar menagerie. This one was barely a star at all, really,
at only 12% of the Sun’s mass and only a little larger than Jupiter. Hardly exciting compared to its vibrant, Sun-like
siblings. But as the decades passed, we began to see
how special Proxima really was. The clue to its uniqueness was in its emission
lines - sharp spikes in its spectrum resulting from electron transitions in the atoms and
molecules of the star’s atmosphere. Proxima’s emissions lines seemed to shift
back and forth from the wavelengths dictated by the laws of physics. And it took until … well, now, to finally
understand the mystery of Proxima and the true magnitude of the importance
of the Alpha-Cen system. In 2016, Spanish astronomer Guillem Anglada-Escudé
and the Pale Red Dot team figured it out: Proxima had a planet. Or more technically an exoplanet - for extra-solar
planet. Planets don’t really orbit stars. A planet-star pair mutually orbits its shared
center of mass - its barycenter - which is usually deep inside the star. Planets make stars wobble, and that motion
induces something called Doppler shift in the star’s emission lines. Essentially, the wavelengths of all the star’s
light are stretched as the star moves away from us and compressed as it moves towards
us, causing emission lines to oscillate in wavelength very, very slightly - just like
with Proxima. This way of finding exoplanets is called the
radial velocity method. So named because the Doppler effect is only
produced by the component of the star’s velocity that’s either directly towards us or away
from us - in the “radial” direction. We couldn’t see a planetary system that’s
“face-on” - fully in the plane of the sky. But the more edge-on the orbits, the more
motion there is in the radial direction, and so the more chance of spotting the wiggling
spectrum. Now the most prolific method for finding
exoplanets has been the transit method, in which planets are identified by their dimming of their parent
star as they cross or transit the face of that star from our perspective. That restricts the transit method to planetary
systems that are almost perfectly edge-on. Nonetheless the Kepler mission found 2600+
exoplanets this way, and extrapolating from that revealed that most stars host planetary
systems. Now that we know this, the radial velocity
method can step in and potentially catch many more systems - as long as they’re not perfectly
face-on. The radial velocity method is emerging as
a competitor to the transit approach due to rapid improvements in a class of
highly specialized spectrograph. OK, so, equipped with this cool new tool,
our intrepid team of exoplanet hunters realized that Proxima’s shifting emission lines made
sense if the star was wobbling in a tight circle, moving at about a mile per hour over
a period of 11 Days. That would then be the length of the year
of a hypothetical exoplanet responsible for that motion. Such a short orbital period, combined with
the star’s mass, gave them an orbital radius for the exoplanet of around 20 times smaller
than the Earth’s. That sounds perilous, until you realize that
Proxima’s energy output is nearly 600 times lower than the Sun’s. That places the new exoplanet exactly in the
habitable zone of the star - just the right distance for the intensity of the star’s
radiation to potentially allow water to exist in liquid form. There was one more stunning calculation to
come: with the distance between exoplanet and star combined with the speed of the wobble,
astronomers could calculate its mass. It’s practically the same as the Earth’s. Maybe 10-50% more massive, but almost certainly
a rocky world, of the type sometimes inhabited by bipedal apes. Suddenly, after centuries of pondering the
alpha-centauri system, it’s full significance became clear. The nearest habitable exoplanet orbited the
nearest star. Surely we’d identified the first port of
call for our interstellar future. The discovery of this exoplanet - Proxima
Centauri B - or Proxima-B - was just the first of this little star’s surprises. In 2019 a new team reported the discovery
of a second exoplanet, its signature also embedded in the slow dance of Proxima’s
emission lines. Proxima C is much bigger than it’s earth-like
neighbour, at 7 times earth’s mass. It’s also way further out, with 5-year orbit
at the same distance as Mars. And then in 2020 a third exoplanet was tentatively
identified. The prospective Proxima D is just a quarter
of Earth mass and orbits once every 5 days, well inside the orbit of Proxima B. That’s
interior to the habitable zone, making this prospective exoplanet literally boiling hot. Followup observations in January this year
somewhat solidified the measurements of Proxima D. And that brings us to today, approaching 2
millennia since Ptolemy’s observation. We have a bona fide planetary system in at
least one of the stars in the Alpha-Cen system. And there are admittedly much
more tentative detections of planets around its Sun-like siblings - a Neptunish body may have been imaged around
Rigel Kentaurus and an Earth-sized body may have transited Toliman on an orbit that would
fry it to a crisp. Those are quite speculative. But Proxima B is almost certainly real, and
so weirdly similar to the Earth. But is it similar enough to have an atmosphere? Oceans? Life? Let’s take a closer look. One very important point is that Proxima B
is probably tidally locked to its star. It’s close proximity to the star means strong
tidal forces, which will have forced the planet’s rotation period to be in resonance with its
orbital period. The simplest case is for the length of the
planet’s day to be the same as its year - both 11.2 earth days. That would keep the same side of the planet
facing the star at all times. One side baked and the other frozen,
and perpetual twilight at the boundary. This situation doesn’t sound conducive to
life for many reasons. For one thing, unless there’s a lot of atmospheric
circulation, that permanent night could cause the atmosphere to collapse, freezing to
the surface. But it may also be that the large temperature differential drives powerful
atmospheric convection - planet-wide gales of extraordinary strength
that could distribute heat from the day-side to the night. Or if the planet has significant oceans, these
could also distribute heat. It’s also possible for a planet to be tidally
locked without a perpetual day and night. Other orbital resonances can occur - for example,
2 days per year, 3 days per 2 years, etc. Proxima B may be in one of these. However it’s worth noting that any resonance
besides permanent day and night would result in massive ocean tides, assuming the exoplanet
has an ocean to be pulled and squeezed by the nearby star. On the other hand, we might actually hope
for a 1-to-1 resonance. In that case, the dark side might be the only
survivable part of the planet, as long as there’s enough atmospheric cycling to keep
it warm. Why? In order for life to have a chance in this
system, it needs to be protected from the star itself. Don’t let their size fool you; there’s
nothing cute about red dwarfs. They can be angry, violent little monsters. Proxima is no exception. It’s what’s known as a flare star. Massive convection through the star’s body
generates crazy magnetic storms, which can cause the star to have powerful outbursts
- flares - that blast high energy particles and radiation through the planetary system. During flares, Proxima B is blasted with X-rays
and ultraviolet light and high energy particles. A sufficiently thick atmosphere and strong
planetary magnetic field could in principle protect any surface dwellers, who would then
get to enjoy pretty spectacular auroras. Auroras which may even be visible from Earth
by a near-future telescope. Despite this violence, Proxima really is faint. At Proxima B’s location there’s enough
light to keep water liquid but most of that is infrared. There’s far less visible light, so to our
eyes the star would appear very dim. Photosynthesis in these conditions would be
difficult. Native foliage would probably be black for
maximum absorption, and may have to rely on alternative photosynthetic pathways that can
make use of the infrared light. By the way, unlike the Sun which grows brighter
over time, red dwarfs fade. In order for Proxima B’s hypothetical oceans
and atmosphere to have survived this earlier, hotter period it would have to have formed
further out and then migrated inwards. That’s not crazy, because planetary formation
models indicate that there wouldn’t have been enough material so close to the star
for it to have formed in that position. Nonetheless, we’re focusing in on a relatively
stringent set of requirements for this exoplanet to be able to support life. But even if it’s a stretch, we need to imagine
all the ways that life could have formed there, because that’ll help us build the instruments
needed to detect signatures of life. We’re now building and planning a new generation
of giant telescope - 30 to 100 meters in diameter, which should be sensitive enough to detect
emission lines from molecules in Proxima B’s atmosphere, if it has one, or even the light
reflected from the planet’s surface. Both of these may bare the characteristic
signatures of life, as we’ve discussed previously. Ultimately we would want to visit our neighbour
to pay our respects. In fact, the mission is already being planned. It’s the breakthrough Starshot program - something
we’ve discussed previously. The plan is to use a giant laser array to
accelerate a train of mylar light sails to 20% the speed of light in the direction of
Proxima. They’ll each carry a tiny computer chip
sporting a miniature camera, to take fly-by pics as they zip through the system 20 years
after launch. There’s no update on when this might launch,
but not tomorrow. Hopefully one day we’ll be sending people
instead of computer ships, to study the system and maybe even settle there. After all, Proxima, with its many-trillion-year
lifespan will far outlive our own Sun. So let’s fast forward our story another
few millenia. A descendent of humanity stands on the black
grassy plains of Proxima B, which sway in the planet-wide gale. Four lights are visible through the thick
atmosphere - the gleaming white twins Tolimar and Rigel Kentaurus, the glowering red orb
of Proxima herself. But there’s another white star on the horizon,
but it’s slowly slipping away on its own orbit around the Milky Way. Perhaps this post-human Proximan will wonder
if the stories are true - that people began there, on a planet around Sol, Alpha-Centauri’s
nearest neighbor across spacetime. From Wall Street to Silicon Valley, and everywhere
in between, Morning Brew is a daily email newsletter that curates from major outlets
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stability we need to keep making this show - a stability much like the perpetual daytime
on Proxima B, and the many-trillion year lifespan of a red dwarf. And today I want to make a special shoutout
to Daniel Alexiuc, who’s supporting us at the big bang level. Daniel, we managed to find a loophole in the
international space treaty and have claimed ownership of Proxima B. As a sign of our enormous
appreciation, we’re giving you first pick of any continents discovered - assuming it
has continents. Offer is null and void if any sentient natives
are found. Travel expenses are not included. Otherwise, it’s yours to do with it as you
wish - name it, claim lordship over it, start your galactic empire there, or just use it
for weekend getaways. Its just a small token of our gratitude for
your support. Thanks again, Daniel. Today we’re doing comments on the last two
episodes: the one on objective collapse theories, where wavefunction collapse is explained as
a real, physical process. And the one on cosmic strings - topological
defects that may span the length of the universe. In the objective collapse episode we talked
about some field “hitting” the wavefunction to cause it to collapse, and Kadag asks what
exactly is doing the hitting. The answer is we have no idea. It has to be a field that has a non-linear
influence on the wavefunction. I’m afraid it’s hard to give a satisfyingly
intuitive description of what that means. But in general it means different branches
of the wavefunction are able to influence each other, which is not the case in standard
quantum mechanics. Check out our episode on the many-worlds or
Everett-Wheeler telephone for juicy details on non-linear quantum mechanics. And yes, one field that might be able to do
this is gravity, in which case the wavefunction is being “hit” by the non-linearities
across the wavefunction introduce by spacetime curvature. And on the subject of gravity, Jan Wester
asks why the curvature of spacetime can’t be in a superposition. Well we don’t know for sure that it can’t
- just that if you try to have a superposition of curvature you get unresolvable infinities
at large curvatures that produce hopeless contradictions between general relativity
and quantum mechanics. Essentially, quantum mechanics and quantum
field theory assume a well-defined underlying framework, upon which all the quantum weirdness
can play - those are the dimensions of space and time. Even including relativity and the shifting
nature of space into time and vice versa, you still have a consistent if mutable underlying
grid. As soon as that stage itself becomes undefined,
everything becomes much more difficult. Resolving that has been the major work of
all of our searches for quantum gravity and theories of everything. OK, on to cosmic strings. A few of you asked how it is that a topological
defect can decay. When those cosmic strings radiate gravitational
waves, how is the Higgs field supposed to smooth itself out? Two ways: if the loop shrinks itself down
to zero size then the Higgs field phase angles can match up. Also, the version of the topological defect
that I described in that episode - the simple vortex of phase angles making a loop around
the string - is the most simplistic version of this defect. In reality, the defects are probably more
complicated. Rather than being a simple knot in the field,
imagine something more like a shoelace knot - it can potentially untangle itself. This is a bit of a crappy analogy, so maybe
I’ll come back to it in another episode. Brandon Munshaw asks if the Higgs phase angle
something that could theoretically be measured? Or are cosmic strings the only indication
of a change in phase angle? The answer is yes and absolutely no. The phase angle is fundamentally unmeasurable
- just the the phase of the wavefunction - it’s a symmetry of the Higgs field and doesn’t
affect the behavior of the field. But the relative phase angle can in principle be measured - that is, we
can see the difference in the phase angle between two points in space. I don’t know how you’d do this besides
observing a cosmic string, but I suspect there are ways. Doctor Scoot asks whether cosmic strings could
explain the filaments and voids in the large scale distribution of galaxies in the universe,
and whether they could explain the cosmic microwave background and dark matter. So the answer is maybe to the first two - but
only if cosmic strings influence the CMB. These strings may have had sufficient density
at extremely early times to leave an imprint on the CMB- on the density fluctuations from
the early universe. Although that hasn’t been detected, at least
not yet. The later development of large scale structure
is defined by those density fluctuations, so in that sense cosmic strings could have
influenced it - but not directly in the sense of cosmic strings causing string-like structure
in galaxy distributions. As for dark matter - extremely unlikely - in
our most accepted understanding of cosmic strings there aren’t anywhere near enough
of them to account for dark matter. But seriously guys one of the weird things we
talk about will turn out to be dark matter. Or Aliens. And that's when our job will be done. But let's hope it's never either. But
wait what if aliens are dark matter!
