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We've never seen an atom. But we know what they look like. - Video học tiếng Anh
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We've never seen an atom. But we know what they look like.
We've never seen an atom. But we know what they look like.
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Subtitles (402)
0:00
- Thank you to Planet Wild for supporting PBS.
0:04
Take a look at this apple.
0:06
Now look closer and closer and closer.
0:11
When you see an apple,
0:13
what you're really seeing is the light
0:14
that's bouncing off the apple and into your eye.
0:17
Some of the visible wavelengths of light get absorbed
0:20
by the apple and the leftovers enter your eye
0:23
and your brain says, I'm looking at a red apple.
0:26
Now imagine for a second
0:27
that you could zoom in on your vision beyond
0:29
what a camera could do.
0:31
You'd see little bacteria and fungi crawling around.
0:34
Zoom in a bit more, and you'll see all these densely packed
0:37
red cells filled with water and sugar molecules,
0:41
but keep zooming a bit more and you'd see nothing.
0:45
Why can't we see any farther?
0:48
All these molecules in the apple,
0:50
and in fact, everything else that you see
0:52
around you are composed of atoms,
0:55
but atoms are so small that visible light just wraps
0:58
around them like an ocean wave engulfing a grain of sand.
1:02
You're probably like, wait, I've used a microscope.
1:04
Maybe you've seen images of what atoms
1:06
and molecules look like.
1:08
We've even split the atom.
1:10
You'd think we've actually seen one,
1:13
but that's actually not the case.
1:15
This is a paradox.
1:16
If everything around us is ultimately made of atoms,
1:20
but we can't see atoms, then how can we see anything?
1:24
We're gonna take a trip into a very small world
1:27
and find out just how weird that answer is.
1:37
Hey, smart people, Joe here. Alright, first things first.
1:40
We've already established that we can't see atoms
1:42
with our eyes, so it's worth asking
1:45
how do we even know that they exist?
1:47
Luckily, we have thousands of years of clues.
1:51
The story starts with Greek philosopher Democrats in the
1:54
fourth century BCE Democritus proposed
1:57
that all matters composed
1:59
of tiny eternal building blocks called atomos.
2:03
That means "uncuttable"
2:05
and those invisible atoms were worrying about in an infinite
2:08
void all across the universe stacking together
2:11
to form stuff.
2:12
Honestly, not bad for nearly 2,500 years ago,
2:16
but for most of human history,
2:17
the Adams's view was condemned
2:19
because most people sided with Aristotle.
2:22
He thought that matter could be infinitely divided
2:25
and was composed of just four elements, earth, fire,
2:28
air, and water.
2:30
It sounds like he was into Pokemon,
2:32
although in the east, some schools of Buddhism, Hinduism
2:35
and Islam had developed their own notion
2:37
of indivisible atoms.
2:38
Of course, none of these ancient thinkers had any way
2:41
to measure or really even consider the world at
2:45
that tiny scale.
2:46
So this was really all more of a thought experiment
2:48
to argue about, well, I don't know,
2:51
wearing togas and drinking wine.
2:53
But starting in the 16 hundreds, Adams jumped from the realm
2:56
of philosophy and religion into the world of science.
2:59
That's when this guy first estimated how big an atom was.
3:04
He measured how much incense had to be burned
3:07
before it could be smelled across a big chapel,
3:10
and he calculated how many incense particles he started
3:13
with, and surprisingly, he wasn't that far off.
3:16
Then in the early 1800s, John Dalton realized
3:19
that the chemicals in familiar liquids
3:21
and gases, well, they always contain whole number ratios
3:25
of different elements like
3:27
how water is always two parts hydrogen to one part oxygen
3:30
and not like one and a half parts oxygen.
3:34
This was proof that there must be some smallest unit
3:37
of matter that acts like Lego building blocks
3:41
to make all the stuff around us.
3:43
A few decades later, we caught a glimpse
3:45
of those Legos in action.
3:47
In 1827, the botanist Robert Brown was looking at grains
3:51
of pollen floating in a perfectly still puddle of water,
3:54
and he noticed that they'd always jiggle randomly,
3:57
and Albert Einstein later showed
3:59
that the pollen's spontaneous little wiggling was actually
4:02
caused by collisions with invisible water molecules.
4:06
This meant that atoms were real physical objects,
4:10
but what are those atoms actually like in the 1890s,
4:13
JJ Thompson put some electricity in a vacuum tube
4:16
and he saw a stream of light stretch between the two ends,
4:19
and he could bend that beam with magnets, which meant
4:23
that even neutrally charged atoms must be made
4:26
of some smaller bits, some
4:28
of which are negatively charged electrons,
4:32
and Thompson proposed that those electrons were scattered
4:35
around the atom like chocolate chips in a cookie.
4:38
Well, that picture also turned out to be wrong,
4:40
but it does sound delicious.
4:42
Later, Ernest Rutherford shot a tiny,
4:44
positively charged particles at a sheet of gold,
4:48
and he noticed that occasionally instead of passing
4:50
through the particles bounced off.
4:52
This showed that the atom has a dense,
4:54
positively charged nucleus at the center, while the rest
4:58
of it is mostly empty space.
5:00
If an atom or the size of a baseball stadium,
5:03
the nucleus would be about the size of a baseball.
5:07
Okay, what about the electrons then?
5:09
How do the chocolate chips get distributed if the cookie is
5:12
mostly empty space?
5:14
Niels Bohr proposed an idea in the early 1900s.
5:17
He heated up hydrogen
5:18
and he noticed that it gave off these distinct
5:21
and predictable colors of light.
5:23
So he drew a model
5:24
where electrons live in these distinct tracks
5:27
around the nucleus like planets orbiting a star,
5:31
and that's how we arrived at the emoji friendly atomic model
5:34
that we're all familiar with,
5:35
which is also wrong, but we'll get back to that.
5:39
So those experiments gave us a pretty good mental picture
5:42
for how an atom should look.
5:44
We just need to take a real picture of one
5:47
and then see if it lines up right not so fast.
5:50
You see, the problem is atoms are really tiny.
5:54
One atom is about as small compared to an apple
5:58
as an apple is to the entire earth.
6:00
The smallest thing
6:02
that a human eye can resolve is about one
6:04
tenth of a millimeter.
6:05
That's about the width of a human hair.
6:08
Visible light itself has a wavelength about a thousand times
6:11
smaller than that--a few hundred nanometers.
6:15
That's about the size of a, a typical bacterium like E. coli.
6:18
But the diameter of a single atom is thousands
6:21
of times smaller than visible light itself.
6:23
Any light that we can see physically cannot bounce off
6:28
of an atom, so it is forever invisible no matter
6:31
how far we zoom in.
6:33
But what about all the light that we can't see?
6:37
What about all the rest of the electromagnetic spectrum?
6:40
In the 1890s,
6:41
scientists accidentally discovered x-rays a much higher
6:45
energy form of light with a wavelength
6:47
that's 10,000 times smaller than what we can see.
6:51
That means it can pass straight through our skin
6:54
and leave shadows of dense solid objects like bones.
6:59
A few decades later, scientists started taking x-rays
7:02
of molecules too.
7:03
This famous x-ray image taken by Rosalind Franklin in 1951,
7:08
for example, that's what inspired the realization
7:10
that DNA has its twisted double helix shape,
7:14
but even x-rays are not small enough to see atoms,
7:18
so scientists had to try something even smaller.
7:22
Electrons themselves,
7:24
remember these cool toys with all the pins.
7:27
That is basically how an electron microscope works.
7:30
It shoots a beam of electrons at something
7:33
and it maps out the surface by looking at
7:36
how those electrons bounce off or pass through.
7:39
Electron microscopes can produce impressively creepy 3D
7:43
images of very tiny objects
7:45
and can even map out the rows
7:46
of atoms in something like a crystal,
7:48
but they still lack the resolution
7:51
to make out individual atoms.
7:53
To do that, we have to move from seeing to feeling
7:58
more advanced electron microscopes.
8:00
They hover an ultra thin charged needle over the material
8:04
they're trying to see, kind of like a record player.
8:07
By tracking how electrons and what you're trying to see tug
8:11
or vibrate the tip, they can resolve individual bumps
8:15
of atoms.
8:17
By adding a little extra voltage,
8:19
we can also drag individual atoms around.
8:22
That's what allowed for this the tiniest movie ever made.
8:28
Then scientists figure out how
8:29
to put individual atoms in a sort of trap.
8:33
In 2018, this photo was taken using a normal digital camera.
8:37
A strontium atom was trapped in an electric field zapped
8:41
with a laser, and they took a long exposure photo
8:44
of the light that it radiated back.
8:46
If you look in the very center,
8:48
you'll see a pale blue dot floating in dark, empty space.
8:53
Sorry. Anytime I say pale blue, that voice just comes out.
8:56
Well, that right there is an atom or is it?
9:01
I mean, it's sort of like seeing the lights
9:03
of a faraway car at night.
9:05
I mean, you can't say you know what the car looks like.
9:08
So have you really seen the car?
9:10
That is something to fight about in the comments,
9:13
and that brings us to this image.
9:15
It's the most high res shot we've got of atoms.
9:18
It came from researchers at Cornell University in 2021,
9:22
and what they did was fire electrons at a material from
9:25
different angles and it recorded the way
9:26
that they bounced off.
9:28
Then they used fancy computer stuff to reconstruct
9:31
where the atoms must have been to create
9:34
that pattern resulting in this image.
9:37
So does this count as seeing an atom?
9:41
I mean, it sort of seems like being in a dark room
9:44
and shooting a bunch of bullets at a sculpture.
9:46
You can look at where the bullets landed
9:47
and use that to paint a picture of the shape.
9:51
But have you actually seen the sculpture?
9:55
Well, I guess it depends on what you mean by see.
9:58
A purist might say that seeing is capturing light
10:01
with one's bare eyes,
10:03
and in that case, we certainly have not seen an atom.
10:06
If you count the use of fancy machines to magnify
10:09
that light, then still no.
10:12
But can we see something by feeling it?
10:14
I mean, people with visual impairment can read braille
10:17
and they can visualize the 3D structure
10:19
of art pieces in tactile museum exhibits.
10:21
Is that any less seeing than seeing with our eyes?
10:25
I genuinely don't have the answer,
10:27
but it does feel sort of different.
10:30
But there's one more wrinkle when it comes
10:33
to seeing an atom,
10:35
and it's something that throws this whole
10:36
quest kind of on its head.
10:38
Remember, boar's model of the atom?
10:41
Well, it turns out that's also not how an atom works.
10:44
See, in the last century, we've learned
10:46
that atoms are actually much, much weirder than that.
10:49
This is the realm of quantum mechanics
10:51
where everything you thought you knew about the world just
10:54
goes right out the window.
10:55
There's this famous experiment
10:56
where scientists shoot single electrons at a screen
10:59
with two little slits cut out
11:02
and shooting one electron at a time.
11:03
At the screen, you'd expect
11:05
to see two little lines appearing on the
11:07
wall behind it, right?
11:09
But instead, what you actually see is a stripe pattern
11:13
that can only happen from each individual electron acting
11:16
not like a particle, but like a rippling wave.
11:20
It travels through both slits simultaneously,
11:23
and then each wave interferes with itself.
11:27
This is super weird, but what's even weirder?
11:29
When you install a little detector
11:31
to track which slit the electron passes through,
11:34
you see a different pattern on the back wall,
11:38
two distinct lines
11:39
as if the electrons were behaving like particles.
11:42
How do we make sense of this?
11:44
Well, physicists still aren't quite sure, to be honest.
11:49
They've been debating this one for almost a hundred years,
11:52
so don't feel bad if your head hurts too.
11:54
But they mostly agree that
11:55
before an electron is measured,
11:58
it doesn't actually have a position.
12:00
It exists somewhere in a cloud.
12:02
There's some probability that describes
12:04
where it could be if you were to measure it,
12:07
but that doesn't mean that it's actually there at
12:10
any given moment.
12:11
You can sort of visualize it like this,
12:13
where each dot represents
12:14
where the electrons might be measured.
12:17
The more tightly the dots are packed,
12:19
the more likely they are to be found in that place.
12:22
Well, you notice how those probabilities end up forming
12:25
orbit like shapes.
12:26
They're similar to the ones that we're used to seeing,
12:29
but there's some really important differences from
12:31
that old planet model, and they are very weird.
12:35
In a way, an atom actually is
12:38
and isn't mostly empty space.
12:41
It's filled with this cloud of electron possibility.
12:46
So what does this mean for our quest to see an atom?
12:51
Well, to see something, you have to interact with it,
12:54
whether that's bouncing light off of it
12:56
or feeling its electrons,
12:58
and quantum theory tells us that any time that we interact
13:02
with an atom that changes the way that the atom behaves.
13:05
So what does the atom look like when we are not watching?
13:09
That might be an impossible question to ask of the universe.
13:13
We can't know.
13:15
Okay, but that brings us back to that first question.
13:17
If atoms are so strange and elusive,
13:20
and if everything is made of atoms, then
13:23
how can we see anything?
13:24
I mean, if the bricks of the universe are invisible,
13:28
how can we see the house?
13:29
The answer is that seeing many atoms is very different than
13:33
seeing a single atom.
13:35
See, when atoms are clumped together into matter,
13:39
their electrons interact with each other
13:41
and they create this sort
13:43
of interconnected electromagnetic ocean.
13:46
When light hits the material,
13:48
it bounces not off individual electrons on individual atoms,
13:53
but off of that collective sea of electrons.
13:57
When we look at an apple
13:59
and we're talking about some 10 million, billion,
14:03
billion atoms all connected in a web of chemical bonds,
14:08
and collectively they can reflect light.
14:12
It's sort of like looking at an
14:13
image on your computer screen.
14:15
All the rich colors and contours
14:16
that you see are actually composed of tiny individual red,
14:19
green, and blue pixels.
14:21
You can't make out the individual pixels,
14:23
but you can see the bigger features that emerge from all
14:26
of their collective light,
14:28
just like the images on your screen.
14:30
The objects that we see in real life are just blurry
14:34
averages of what's really underneath.
14:37
We just have to accept that the world that we know
14:40
and see is one that emerges from a world
14:43
that we will never be able to observe directly.
14:47
The deeper we look into an apple, the less we seem to see,
14:52
but I mean isn't seeing the unseeable
14:54
what science is all about?
14:56
It's sort of what being a human's all about.
14:59
We may not have ever seen an atom,
15:00
but we still know that they're there.
15:02
We find ways to catch their ripples
15:05
and shadows and textures.
15:07
Eventually, we build up so much information
15:10
that we can predict with incredible accuracy.
15:13
What we're gonna see, I mean, compare this to say,
15:15
looking for Bigfoot.
15:17
I haven't seen him either, so I can't say whether
15:20
or not he exists.
15:21
Maybe he's invisible. Maybe he lives in Antarctica.
15:24
But if you brought me mounds
15:26
of repeated evidence from different locations
15:28
with no alternative explanation, so much
15:31
that you could predict
15:32
where Bigfoot's footprints will show up next,
15:35
then well even I might start to buy it.
15:39
That's exactly what we've done for atoms.
15:41
We can't see them,
15:42
but we still know them shockingly well,
15:46
probably better than we know ourselves.
15:48
By monitoring how often electrons
15:50
and an atom change orbitals when we blast them with a laser,
15:52
we construct clocks that can keep time for billions
15:55
of years without losing a second.
15:57
We can use the color of light given off
15:59
as electrons change orbitals to figure out
16:01
what makes up the atmosphere
16:03
of planets orbiting others' stars.
16:05
You are made of atoms.
16:07
Every cell in your eye
16:08
that processes light is made of atoms.
16:11
Your brain, which constructs the experience
16:14
of seeing is atoms.
16:16
We are the universe's atoms trying
16:19
to see ourselves and failing.
16:22
That's a very strange thought and weirdly humbling,
16:26
but if you look deeply enough at something even as simple
16:29
as an apple, it may just open up an entire universe
16:32
of secrets, or at least some very good questions.
16:35
Stay curious.
16:37
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18:02
I mean, I'd like to see an ai do this. Eat an apple, huh?
18:07
Try it computer. You don't even know what apple tastes like.
18:13
Mm. Play the movie.
18:18
Well, that's going in the end card. Great. Alright, cut.
18:21
I'm not supposed to eat the apple. No, right.
