Thank you to CuriosityStream for supporting PBS. If the earth was one giant atom, its
nucleus could fit inside a baseball stadium! Everything outside the
stadium—the rest of the planet? That’s where the electrons live. In
a sort of wave-y, quantum-y cloud. The stuff that makes up stuff doesn’t contain
much actual stuff. Huh. But if an atom is just a miniscule nucleus surrounded by a wave-y,
quantum-y cloud of mostly nothing, kinda makes you wonder: Why doesn’t light woosh right through the
atoms in bricks, or steel, or chocolate ice cream? Why aren’t you and I transparent? [MUSIC] Hey smart people, Joe here. So, why aren’t we transparent? Well, we are. If you’re an x-ray! Our bodies just
aren’t transparent to visible light. Of course, visible light and x-rays are both just
different forms of electromagnetic radiation, with different wavelengths and
energies. So what’s the difference? Well, have a seat, because
glass is in session (BOO) Glass is transparent to visible light.
If we zoom down to the atomic level, we see glass is made up of a bunch of
silicon and oxygen atoms. Same as this stuff, sand! When that sand was melted down into
a liquid, those molecules left the nice, perfectly repeating crystal shape they were
living in, and went wild. Until we cooled them down really fast, and they froze
in place, in a sort of organized jumble. All those atoms are surrounded by
wave-y, quantum-y electron clouds. But the electrons around a nucleus can’t be
just anywhere. They live on specific energy levels - think of them as different
distances from that tiny nucleus. When a photon comes by, with
exactly the right amount of energy, it gets absorbed, bumping an
electron to a higher energy level. But if that photon doesn’t have just the right
amount of energy it passes right by. Woosh! Imagine I’m an electron. I’m hanging out here,
at a low energy level. I want to move up there. To higher energy levels. To make it happen,
I have to have just the right amount of oomph in my jump. Too little, and I don’t make
it. Too much, and well… oops. Just right... For the particular atoms that make up
glass, the energy levels are so far apart that visible light doesn't have enough energy
to boost those electrons up to the next level. That’s why visible light passes right through! But
photons of UV light do have the right amount of energy to power up those electrons, and they
get absorbed. Which is why glass is opaque to most UV! And why it’s hard
get a sunburn through a window. How transparent something is depends on this
relationship between light energy and an atom’s electrons. Different elements have different
energy requirements for their electrons to absorb light. Like how when visible light hits my
atoms, it’s absorbed. Some light might get through a few top layers of skin cells, but within a few
millimeters all the photons get gobbled up. That’s why I’m not transparent. But hit me with higher
energy waves, like X-rays, and I am transparent. Glad we cleared all that up (BOO) But, thinking about how atoms are wave-y,
quantum-y mostly empty clouds makes me wonder something else: Why am I even here? Why aren’t
the mostly empty atoms in my feet passing right through the mostly empty atoms in the ground,
sucking me into Earth’s superheated iron core? Why can I sit on a chair, kick a ball, or
smash those like and subscribe buttons? Why can I touch anything? Let’s say I would like to boop this
snoot. My finger—or “booper”—and the snoot are both made of
about a gajillion atoms, give or take a squadrillion. And all those atoms
are surrounded by negatively charged electrons. As the two objects get close enough together,
the negatively charged electron clouds at both surfaces repel each other, thanks to
what’s called electrostatic repulsion. The actual boop itself, the sensation of touch, is caused by an actual force from this repulsion,
acting on pressure-sensitive nerves in my skin. Kind of like how we’re never really aware of the
atmosphere until there’s wind pushing against us. Touching something doesn’t really mean decreasing
the distance between me and something else to zero, it’s just getting my atoms and that other
object’s atoms as close as the electrons—and physics—will allow. Of course, there is
one more way that electrons can interact. It is actually possible for two negatively
charged electrons to occupy the same quantum-y cloud energy level… as long as
they have opposite directions, or signs, for a property called “spin”.
And sometimes electrons in two different atoms can be squished close enough
that their wavy-ness even overlaps! That’s the reason covalent chemical bonds
exist, which is pretty convenient. Like the ones in your body. All those
molecules that keep you alive, full of atoms, absorbing visible light photons. And just
like the bonds between oxygen and silicon, in this camera lens, in the fiber optic cables
of the internet, even in the glass of the screen between us right now, that are letting
photons of visible light pass right through, which I hope is making this touching bit
of science just a little bit clearer. Stay curious.
