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? Before we discuss those angry birds, I’d
like to thank Angry Birds 2 and Rovio Entertainment for inspiring and sponsoring this video. It’s amazing how captivated we are by flinging
birds and tumbling tower physics. Millions of people alter their bird’s trajectory
to compensate for the kinetic energy of air blown by fans to perfectly knock out the block
holding up the precarious structure. Download the upgraded edition of the classic
game by clicking the link below, play the map, compete with others online and power
up your birds. Thanks again to them for supporting Vsauce2,
have fun playing Angry Birds 2, but now we have to get back to our troubled truck driver. 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. Here we go. It didn't really collapse as much as I thought
it was going to. Here we go. It still didn't really collapse the way I
wanted it to. Hello. Welcome back. If you wanna watch more Vsauce2 videos just
click over there and if you're not subscribed to Vsauce2 then I suggest you give that a
shot. Just click the thing that says subscribe,
it's like over there. And um, it's like a free wonderful gift for
your computer, and your YouTube account, and your mind. And really the mind is the greatest gift of
all. Bye.
Edit: It plays loudly at 8:58!!!!
Canyon by Julian Emery.