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That's no moon, it's a space station: the
Death Star. But could we build one in the
near future? Even if we could amass the
vast quantities of raw materials
required and, perhaps more importantly, be
able to pay for it. We still run into
two problems:
how do you power the Death Star and where
does the gravity come from? Rewatching
The Force Awakens though gave me an idea to
solving both these problems in one
fell swoop and then it became obvious,
the clue is literally in the name.
What if at the heart of the Death
Star is a star?
Sorry i'm getting a call. Wait, Martin are
you saying that the Death Star could be
like a miniaturised Dyson Sphere with an
artificial star in its core thereby
acting as both the power and gravity
source?
Yes that's exactly right David Kipping
from the Cool Worlds channel. That's
actually not as wild as it seems. Physicists
like Freeman Dyson have proposed that
advanced civilizations might build giant
structures around their star as a way of
siphoning off all of the stars energy.
Instead of being the size of a small
moon like the Death Star
physicists usually imagine these Dyson
Spheres to be the size of the Earth's
orbit around the Sun. But such a
colossus has some physical problems.
First off the Dyson Sphere is
gravitationally unstable meaning even a
tiny push on the edge
of the dyson sphere will cause it to
plunge and crash into the star. Second by
being a rigid shell the material of the
dyson sphere itself would be under immense
compressive strain from the star. In
fact it would be equivalent to building
a tower on the earth which was a hundred
thousand kilometers high. Martin, I have a
bad feeling about this. Is this going to scupper your
plans for the DeathStar? Actually
they're not a problem for a few clever
reasons, which I'll get onto later,
first though some numbers. According to the
Death Star plans (what have you done with
those plans) the space station measures
up at a hundred and twenty kilometers in
diameter and the reactor core is about a
tenth of that. Now applying Newton's
inverse square law of gravity for the
strength and then integrating over the
entire space station, if we want an
average gravitational field of 1g that
reactor core has to be two times ten to
the twenty kilograms in mass that's just
370 times smaller than the mass of our
moon. But of course that's just the
average gravity, stand just outside of
the reactor core and there would be 31g
gravitational fields and go to the
outermost floor and you're talking about
a third of a g, the same
gravitational strength as on Mars. Given
that most humans can only withstand up
to 5g and only fighter pilots can withstand
continuous 9 g's of acceleration you and
I best stay clear of the first 2000
levels of the death star while TIE pilots
dare only go down to floor 1150. But
could the death star itself even
withstand
the stresses involved? Well the
gravitational stresses on the materials
would be given by this formula and while
that does depend on the density of the
specific material used, it's usually in the
region of about a few giga pascals here.
Materials like steel and titanium have
compressive strengths which are only a few
times less than this, meaning that they
would implode in on themselves due to
the gravitational forces... so that's not
great!
There is a way around this though, future
wonder materials such as graphene could
totally withstand the stresses involved
so problem averted.
Well that's the gravity sorted, but what
about the power. As our death star's core
is an artificial star we can use power
from fusion of hydrogen plasma into helium.
We haven't cracked fusion power just yet.
At the moment we tend to put in more energy
heating the plasma and trying to confine
it then we actually get out from the
thermonuclear reactions. Naysayers
always claimed that fusion is 20 years
away, but plasma physicists think that
the key to cracking fusion power is by
going bigger and our next big experiment
is ITER which will be a third of the
volume of Olympic swimming pool. Not to
brag but my hypothetical death star is a
quarter of the volume of lake Michigan.
So how much fusion power would we expect
to get out? (If you only knew the power)
Well our death star's reactor core is
incredibly dense, it's 1100 times denser
than the core of the Sun and about
a tenth of the density of a white dwarf star.
For fusion to be occurring we need
temperatures of at least 10 mega Kelvin.
Putting those together the reaction rate
gives us 25 exawatts of power, that's two
million times the power consumption of
human civilization. But at those densities
and pressures not only do we have to
worry about the plasma's thermal pressure
of 300 trillion atmospheres trying to
expand the plasma core, the matter is so
tightly packed
that the electrons' wave functions
would ordinarily overlap one another.
Pauli's exclusion principle doesn't like
that and it provides an extra outward
force, the electron degeneracy pressure,
and that measures up at a whopping five
quadrillion atmospheres. In order to
prevent our death star for being
swallowed whole from the inside we need
to counteract those outwards pressures
but perhaps we can use a trick that
actual stars do that we could never use
on fusion experiments here on earth:
use the star's own gravity to keep the
plasma in check.
Plugging everything in it turns out whilst
we've gone pretty damn big with the
Death Star,
we've not gone big enough. Gravity can
only account for point naught naught naught naught
three percent. But there may be another
solution, what if instead we use magnetic
fields in the way that we did for
lightsabers? That could be the sort of
active confinement system to get around
the usual Dyson Sphere collision
problems when just using gravity. So
let's place a magnetic bottle at the
heart of the reactor core keeping all that
plasma at bay using the magnetic pressure.
Plugging in the numbers it
turns out we just need a magnetic field
of 36 million Tesla, that's a lot of
Tesla. To put that into context the
strongest magnetic field ever created by
us on earth was 45 Tesla.
I mean we're talking in the range
of neutron stars and magnetars here. So
to make a death star turns out we just
need to be able to create some of the
strongest magnetic fields in the
universe and there you have it.
Thank you so much for watching all of
this video don't forget to check out the
companion video to this on the cool
world's channel,
hey David how did I end up in your
screen? It's a trap,
well no not really but if you want to
hear more about how astronomers could
use current telescopes to detect an
object like the Death Star then make
sure you look at the link the end of
this video for more. So do check that one
out and don't forget you can like this
video, you can share it around and you can
of course subscribe for more stuff from
me. Until next time.