Vsauce!
Kevin here..
Imagine a guy transporting thousands of birds in an enclosed truck, stopped before crossing
an old, dangerous bridge.
His avian cargo makes the truck too heavy to safely cross.
So how could he make it lighter?
The birds.
What if… he banged on the side of the truck, scared the birds inside, and caused them to
fly around?
If they’re in the air they’ll weigh nothing and he can safely cross the bridge!
Right?
The scared, angry birds are flying inside his truck as he slowly drives over the bridge…
Here we go.
Uh oh!
Don’t worry, don't worry.
Uh.
I'm sure the truck driver and the birds are fine, the bridge not doing quite as well.
Why didn’t his plan work?
Because he didn’t account for the unseen physics of bird flight.
Excuse me, bridge.
To fly, a bird has to generate enough lift and thrust to counteract drag and its body
weight.
We can calculate that by making weight W, which equals .3 -- a constant value related
to the “angle of attack,” which is the angle of its wings relative to the air, usually
6 degrees or so… multiplied by the airspeed V times the mass flow of air, which is airspeed
V times air density d, multiplied by S, which is the surface area of their wing… y’know
what… ?
Look, calculations aside, I like to think of this riddle as thousands of uncaged birds
just freaking out and flapping their wings inside in this truck.
Some birds are generating a downward force on the air to create the thrust and lift that
gets them up off the truck bed.
Some are flying horizontally, using thrust and lift to stay in the air, and... others
get in the air and free fall, which reduces the overall load in the truck by their full
bird weight.
The important part is that even though the truck driver can’t see it, air is a fluid,
and these birds are throwing physical forces around in every direction as they move within
that fluid.
Since he’s got thousands of birds in a closed truck, those forces are going to balance out
so that the truck’s weight is basically the same with the birds flying around as it
was with them just sitting there.
It’s easy to understand his logic, though.
Thousands of birds are heavy.
Get them up in the air and they won’t be.
But seeing the unseen, like the forces of flight, requires a high degree of abstract
thinking… and that’s a pretty new concept.
Not like last year new but like 50,000 years ago new, when Homo Sapiens separated itself
from other hominins by entering the era of “Behavioral Modernity.”
Abstract thinking is really hard to do.
We’re thousands of years past the Stone Age, and we still encounter everyday paradoxes
because we so often miss what we can’t see.
The truck driver’s eyes couldn’t show him all the complexities of bird flight physics.
I can’t see time.
That’s not a weird thing to admit, nobody can.
Sure, we can look at a clock, or notice the effects of time on things like the changing
of seasons, but we can’t actually see the dimension of time.
We see representations of time.
And even those can be deceiving.
Here’s an example.
You hate your evil identical twin.
You want him out of your life, at least for a while.
So, you put him on a ship to the nearest planetary system while you stay on Earth.
My evil identical twin is named Bevin.
That’s short for Bad Kevin.
Bevin.
Alpha Centauri is about 4.4 light years away.
If I put Bevin on a ship called Vspace2 and sent him off at, say, .8c -- that’s 80%
of the speed of light -- he’ll get to Alpha Centauri in 5.5 years.
If Bevin plants a flag and flies back, he would return in 11 years.
For me.
11 Kevin years.
Distance -- and also time -- seems to work differently for Bevin.
From his point of view, traveling at .8 times the speed of light makes Bevin’s distance
contract to just 2.64 light years… which means it takes him just 3.3 years each way.
To Bevin, his trip only took 6.6 years.
My 11 years of freedom from Bevin were only 6.6 years of banishment to him, which means
my identical twin is now 4.4 years younger than me.
That unseen element of time just… wasn’t the same for us.
Since we can't actually travel at luminal speeds -- yet -- we have to explore special
relativity in the lab and in our minds.
And sometimes that's a good thing, because seeing can deceive us -- it's why I burnt
my hamburger.
The higher the heat, the better, right?
I could see the outside of the burger cooking fast in the pan.
But that char is actually making heat conductivity to the center happen less smoothly.
Instead of engaging the Maillard reaction that combines sugars and amino acids to bring
out the flavors, higher heat just… scorches the outside without cooking the center evenly.
It tastes bad and the inside is raw.
We can get food poisoning because our eyes aren’t very good at thermodynamics.
But what if you’re the hamburger and you want to stay uncooked?
Metal.
Even though metal conducts heat to cook things like burgers, metal also protects against
unseen heat.
Thermal energy that comes in the form of infrared radiation exists just below red on the visible
spectrum of light.
And infusing protective gear with reflective metal repels infrared radiation… like this
aluminized emergency blanket, which reflects about 95% of radiated heat.
NASA invented metallized polyethylene terephthalate
for space, but I use it to look nice.
When it comes to birds in a truck, evil twins traveling the cosmos, or cooking burgers,
what you see is not always what you get.
Here.
I’ll draw two straight lines.
There.
Now just count the lines.
1, 2….3.
Even what appears to be the simplest visible truth can be filled with complexity that we
can only see when... we admit that we can't.
And as always -- thanks for watching.