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Imagine a spacecraft with a huge, shiny sail
attached, like a kite, accelerating through space.
But there's no wind in space.
This sail ship is pushed by sunlight.
Think about it.
Photons, or particles of light, have no mass,
and yet they can push you.
The push of a single photon of visible light
is tiny, less than 100 quintillionth
of the force of a snowflake falling on your cheek.
But let's put it this way.
If every single photon landing on your face
had the same momentum as a snowflake,
it would feel like all of the snow from Mount Everest
were falling on your face every second.
Standing in the sun wouldn't be so easy.
And there's no friction and space
to slow down the sail ship, so it would just
keep building up speed over time.
So how do we take this light push
and turn it into a light sail?
Well, when the photon reflects off of the light sail,
it's a simple transfer of momentum.
Think of billiard balls.
When the billiard ball gets hit by a cue ball,
it gets a kick, just like the light sail gets a kick
from a reflecting photon.
In addition, if the balls bounce off of each other,
the billiard ball moves much faster than
if, say, you put Velcro on both balls and they stuck together.
Similarly, a light sail works much better if it's reflective
and the photons bounce off instead of being absorbed.
And forward you go, with just the push of light.
Japan's IKAROS light sail uses this phenomenon
to even steer their aircraft.
What they do is they can change the reflectivity
on the sides of the solar sail in real time
using liquid crystals, similar to the crystals in your LCD
screen.
By changing the reflectivity around one edge
of the solar sail, they can make it
so that the shinier side is pushed
more than the other side, and thus pivot and steer the craft.
You don't have to move anything, and you can
steer using liquid crystals.
That's ingenious.
And fun, Icarus is a character from Greek mythology
that flew too close to the sun, and the wax on his wings
melted off.
Oops.
Now, I think the coolest thing about solar sails
is that light can push things at all.
It seems pretty straightforward.
The photons come in and pelt the light sail, pushing it forward,
just like air molecules in wind pelt a boat's cloth sail
and push it forward.
Right?
Well, no, because molecules have mass,
unlike photons which have no mass.
They are precisely zero kilograms.
And you learn in physics class that momentum equals mass times
velocity.
So no mass means no momentum.
But then, quantum mechanics class
comes along and tells you, well, actually there's
another way that you can get momentum,
and it's if something has energy.
And light does have energy, so it can have momentum.
Just another thing that quantum mechanics
came in and turned upside down.
And let me just say, solar sails aren't science fiction.
They are the now.
In 2008 NASA launched a small solar sail satellite
to orbit Earth, called the NanoSail-D. D for Dianna.
Just kidding.
Also in that year, Japan launched
its interplanetary explorer, IKAROS,
which made it all the way to Venus
in less than seven months.
This upcoming year, the Planetary Society
plans to launch LightSail, a crowdfunded satellite
the size of a loaf of bread, which seems pretty small,
but a smartphone now has more processing
power than the computer on the lunar missions had.
A rocket will carry LightSail into space, where
it will be ejected into orbit.
Then LightSail's estimated acceleration
under constant direct sunlight is
0.06 millimeters per second squared,
or an increase of about half a mile per hour every hour.
And the bigger you make your light sail,
the larger that acceleration.
In addition to light sails, we've
harnessed this property of light,
where it can exert a pressure and thus a force here on earth,
too.
Scientists have developed optical tweezers
using lasers to trap and study single biological cells,
to stretch single DNA molecules, and to trap single atoms.
So they're more of a trap than tweezers, but I digress.
So why light sails?
What do they add to space exploration?
Well, they're low cost, not too heavy,
and they last a really long time.
Spaceflight is really expensive and complicated,
so light sails are just another one
of those technologies inspired by physics
that aims to decrease costs and make spaceflight easier.
Plus, they're super cool.
It's a satellite being pushed by light.
What more could you ask for?
Thank you so much for watching this episode of Physics
Girl, and happy physics-ing.
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Squarespace, build it beautiful.
One more thing.
Solar sails sound like solar cells,
which are the photovoltaic components in solar panels.
But the two are distinct technologies, and actually
work because of different physics.
Solar sails are literally pushed by the photons,
while solar cells use the photoelectric effect
and absorb the photons and convert
that energy into electricity.
Solar sails are actually more efficient.
They can convert about 90% of that energy
into motion of the solar sail, while current photovoltaics can
only convert about 40% of the absorbed energy
into electricity.
But the photovoltaics actually absorb the energy photons,
so they can still produce and store more energy.
Now you know.