Kind: captions
Language: en
This is a video of light traveling
through a bottle at 250 billion frames
per second. And here's that same video,
but now with the camera moving.
You can see it sweeps across the scene
faster than the laser pulse itself.
Which means this camera must be
traveling faster than light. So, how is
this possible?
Well, in this video, I want to show you
three unusual ways of stopping time and
what you can see if you just keep
slowing down. From a century old
technique that still beats modern
slow-mo cameras all the way to a massive
quadrillion frames per second camera
that captures electrons whizzing around
molecules.
By the 1920s, electric motors were the
new standard for powering factories and
mills. But many of these motors also
came with a flaw. They were sensitive to
fluctuations in the electrical grid. A
power surge, like from a lightning
strike, made them behave unpredictably.
So, one MIT engineer named Harold Doc
Edertton set out to find a solution.
He had a setup that could induce these
power surges in a lab. But no matter
what he tried, Edertton just couldn't
see what was going on with the motors
because the machines would spin too fast
for the human eye to see. And cameras at
the time offered no help. Their exposure
times were too slow. So any photograph
of a running motor would come out
blurry.
But one day, Edertton noticed that every
time he triggered a power surge, his
equipment gave off a bright flash of
light. And when that flash hit the
motor, the moving parts appeared to
stand perfectly still, as if frozen in
time, which gave him an idea. He could
turn off all the lights in the room, set
up a camera, and leave the shutter open.
And since there was no light, no image
would form on the film. But then if he
could illuminate the motor with a very
brief and very bright flash like the
ones his equipment gave off, well then
he would get a sharp photograph.
All Editton needed was a way to reliably
create these flashes.
So he started by using a high voltage
power source to load electrons onto a
capacitor where they piled up onto one
of the plates. But because there was an
insulator slotted between the two sides,
the electrons couldn't just jump to the
positive side to balance out the
charges.
The only way for them to get there would
be to travel through the rest of the
circuit. And the circuit was
intentionally designed so that electrons
would have to cross a glass tube filled
with a non-conducting gas like argon or
xenon. On their own, they would not have
the energy to get through that gas. So,
Edgutin added a trigger that sent a high
voltage pulse through a wire wrapped
around the tube.
and the electric field from that pulse
would rip electrons off the gas atoms
inside the chamber, ionizing the gas and
turning it into a conductor. In that
instant, the charge stored in the
capacitor would surge through, heating
the gas to around 10,000 Kelvin, nearly
twice as hot as the surface of the sun.
This would produce a very bright and
very brief flash of light lasting just
10 micro seconds. Then the electrons
would recombine with the gas atoms,
stopping the current, and the circuit
would go dark again. This was Edertton's
strobe.
By the early 1930s, he was eager to test
it outside the lab. So, he packed up a
strobe and hit the road with his wife.
When he saw a random factory, he pulled
over, got inside the nearest phone
booth, and called up the facto's
president and asked him something like,
"Do you happen to have any motors in
there that don't work right? I'd like to
show you something." More often than
not, he ended up inside setting up the
strobe next to one of the motors. The
workers would watch as Edertton froze
the motor in time, allowing them to take
sharp pictures of the gears in motion.
Edertton isn't the first to make a
strobe. Rather, he took new bits of
technology that existed so he could make
a better strobe. A strobe that was
brighter, shorter flash duration. But he
was not unique in that. There were lots
of electrical engineers in the world at
that time who could have done that.
>> No. What Edertton uniquely brought to
the table was his eye for photography.
>> He took photos of synchronous motors and
I think in part because he just thought
synchronous motors were cool. One day he
showed his wife the 300th photo of a
synchronous motor.
>> Mhm.
>> And she said, "Harold, can't you take a
picture of something a little more
interesting?"
>> And so he did.
>> Tennis balls pancaked against the
racket, hummingbirds frozen in time.
He was one of the first to really start
using strobes to communicate what's
happening at these time scales we can't
see. He would do this through like Life
magazine, National Geographic magazines.
These magazines in the 30s40s were
essentially the social media influencers
of the day. He just had this eye for
composition.
Most of these pictures were taken in the
1930s. And yeah, it seems easy enough to
swing a racket and it's easy enough to
press a button on the strobe. But how do
you time the strobe to go off exactly as
the racket hits the tennis ball?
>> That is the million-dollar question,
right? You have a strobe that turns on
and off in half a millionth of a second.
That's nice. How do you get it to go off
the right half millionth of a second?
Cuz there are a lot of them, right?
>> And the answer is we use sound. So we're
going to try and recreate one of
Edertton's photos. Popping a balloon and
freezing it in time. Is it okay if we
walk through the setup as
>> I think that would make perfect sense.
Why don't you blow up a balloon? Step
one is you set up the experiment or in
this case the performer and the next
thing you want to do is frame the image.
>> So you're framing now before the balloon
pops to get focus and
>> exactly. If you can't get a good photo
with nothing happening, adding the
motion will not help. And so the next
step will be to get the strobe uh in the
right spot. Now, the strobe is set up
with a trigger unit with a microphone
and uh when a sharp sound hits the
microphone, the trigger unit sends a
signal to set off the strobe. We're
going to turn the lights out and then
we're going to open the shutter of the
camera, but there won't be an image
because the room will be dark and I will
say 3 2 1 pop. And when I say pop, pop
the balloon with an upward motion. And
when the sound from the pop hits the mic
after a minor delay, the strobe will
fire. The camera will capture that image
for the 1/100,000th of a second it's
lit. All right, we ready? Lights out,
please. And 3 2 1 pop lights.
>> Oh, there you go. Oh, you look very Mhm.
Do not mess with this man. Mm-m. Nope.
>> Oh, it's awesome. You can see inside the
balloon. That's really cool.
>> Okay, here's another photo we took. Can
you guess what this is? This hovering
white orb.
Here it is just a moment later. It's
like a little sombrero.
>> It is. Yes.
>> That orb is a drop of milk falling onto
a plate.
>> That's a pancake. But you'll notice the
little drops are all spreading out.
>> Yeah. And it's so so crisp.
>> Come around. Have a look here. Right.
It's translucent here. You're seeing
through it.
>> Oh, wow.
>> Now, once Edertton showed the world how
powerful strobe photography was, he
attracted some unexpected attention. In
1939, a US major named George Goddard
walked into Edertton's lab unannounced.
He was working in the Army's
photographic lab, developing ways to
photograph enemy movements from a plane
during the nighttime. The old way of
doing a night reconnaissance photograph
was to fly over the site at high
altitude and drop a flare on a parachute
and then the reconnaissance plane had to
fly in under the flare where it would be
silhouetted where you could shoot at it.
>> Big problem,
>> right? So totally exposed. Goddard
wanted a safer way. So he asked Edertton
whether he could develop a strobe
powerful enough to illuminate the ground
from a plane that was a mile or so up in
the sky. A strobe that would be bright
enough to take a reconnaissance photo.
Edjertton pulled out some paper, did a
few calculations, and said, "We can do
that." The flash released about 60,000
jewels in a single millisecond, a peak
power of roughly 60 megawatt, which is
comparable to the output of a large
solar farm.
>> 1 2 3 push.
The flashlight was quickly utilized in
World War II, and it allowed the Allies
to take pictures of Normandy the night
before D-Day. This way, they could
confirm the German troops were
unprepared for the attack. It's hard to
ignore just how sharp these strobe
photos are, especially the ones Edertton
took in the 1930s. So, we got a research
grade slow-mo camera from 2020 that
shoots at 20,000 fps, and we're going to
compare its quality to Edgerton's
technique by shooting a bullet through a
playing card. So, let's do this slowmo
camera first.
>> 3 2 1
Let's see the video.
>> It's great to see how long the the top
part, which is now levitating, stays
now. Yeah.
Okay. And now let's do the same with
Edertton's method.
>> Lights going out.
Shutter.
>> Shutter open.
>> 3 2 1.
>> Okay. I think I saw it.
That's cool.
>> Oh, focus is amazing. The the edge of
the the card is is beautiful. You see
this ghost effect? There's a card here.
Ah, you do see the ghost effect and
that's because you open the shutter and
it was a second or two before I actually
fired the gun. There's enough stray
light in the room to give you a faint
exposure there.
>> Also, we still used a microphone to time
the bullet even though it's faster than
sound.
>> So, here's the gun. It fires the sound
of the gun comes out, but the bullet is
coming out ahead of the sound. But the
bullet is supersonic and a supersonic
object moving through the air creates a
sound, a sonic boom.
>> Now you can pick up that sonic boom with
a microphone. And by moving the
microphone physically along the
trajectory, you get to time where the
bullet is going to be when the strobe
goes off.
>> I think it's a brilliant way to solve
the problem. And and I get to say that
because I did not invent it.
>> Edertton was very inventive and had
projects all over the place. he was
teaching at MIT, but then he had
companies for things like underwater
cameras and he was making movies and oh,
he even won an Oscar. So, if there's
anything he wanted to do, he sort of
just did it. And uh I feel like I'm
quite the opposite. You know, when I
joined Veritassium back in 2023, I
started off as a researcher. I was
fact-ecking videos and setting up
shoots, but then Derek and the other
writers suggested I should make a video
of my own. And I remember thinking,
"Yeah, I don't know, maybe one day." Uh
but they kept pushing for me to do it.
And I'm so glad that they did because
when Dererick and I made my first video,
I fell in love with it. So, if there's
anything you're putting off, you should
just go do it. And if that something is
a project you want to set up as an
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Hostinger for sponsoring this part of
the video. And now, let's go look at
pictures of that card we shot. Okay, so
here are the two techniques side by
side. And here's Edertton's original as
well.
So, why does a research grade camera
from 2020 struggle to get the same
resolution as a camera from decades ago?
Well, that's because there are really
two resolutions we're working with here.
A spatial resolution, or how many pixels
your image has, and a temporal
resolution, which dictates whether you
capture only one frame, like a strobe
photo, or a progression of frames, like
our high-speed video. The problem is
that more often than not, the hardware
is limited so that you have to trade one
resolution for the other. High pixel
count or high frame rate. The
fundamental limit you hit is how fast
you can get pixels off the sensor. And
that's why there's a maximum speed to
read every pixel. And then to go any
faster, you have to not read out all the
pixels. So this camera will give you a
million frames a second, but you're like
16 by 128 pixels. And that's not much of
an image.
>> Okay. So, there's always this trade-off.
Either you go for very high pixel counts
and bring the frame rate down at the
edger center. We push this as far as it
goes with one frame and that's it. But
you can also push it the other way. One
pixel and very high FPS. One trillion
FPS in fact. But wait, it's just one
pixel. What can you do with one pixel?
Great question. The cameras that I can
show you today are cameras that You're
right. They can really only see one
pixel at a time, but they can see close
to a trillion frames per second. And
what that lets you do is ultra slow
motion videos showing light actually
traveling. Here's that video of light
traveling through a soda bottle. You can
see the actual wavefronts propagating
underneath the bottle and even how the
light bounces off the cap.
And even though this looks like a normal
video, you can take it with a camera
that only sees one pixel. Here's how you
do it. A single pixel camera is one that
captures just one thing, how many
photons land on the sensor. And the
sensor here is typically sensitive
enough to register when even a single
photon hits it. And it can count those
incoming photons around a trillion times
a second. So each bin and technically
each frame is roughly one picoscond
long. In that time, even light itself
only travels around 0.3 mm.
Sounds impressive, but we've actually
had this tech in many phones for years
now. It's just LAR. You shoot out a
pulse of light, it bounces off, and you
time how long it takes for that pulse to
come back. And from that, you get the
distance to the object that it bounced
off of. But this is all you need to take
a speed of light video. We have a a
setup here that is basically a scaled
down room. And we just have some
different shapes in it. So we have a
conosphere. We have a mirror in the
back. And finally, we have the
veritassium and the camera culture
logos. We want to see light propagating
through a scene.
>> So the way we do that is we shoot out a
really short laser pulse that hits just
one point in the scene. And that laser
pulse has a ton of photons in it. Those
photons will hit an object, scatter
everywhere, and we want to see what that
scattering looks like at all these
portions of the scene. To start, our
single pixel camera will point at the
top left corner. So, when a pack of
photons from the laser pulse hit this
random spot on the wall, reflect into
the corner, and finally bounce into the
camera, the sensor is going to pick up
their signal at a trillion frames per
second, but that signal will be pretty
faint.
So, the problem here is that we're
exposing for such a short time that you
actually just don't get that much photon
return. What we do is we actually take a
bunch of measurements and then group
them all together and that gives us
actual usable information about, you
know, how far away the light traveled in
a scene. Then you move the camera
slightly and repeat the experiment. So
you shoot out a laser pulse, let it
scatter off the same spot and record the
signal at this new position. You do that
over a million times and then move the
camera again until you record a whole
grid of points.
>> You're literally going just one pixel at
a time.
>> One pixel at a time. That's the caveat.
Yeah, exactly. We actually have uh two
mirrors here that let us steer the beam
uh you know left and right and up and
down. So by turning where the mirror is,
we can turn where the sensor looks. The
most important thing for this technique
to work is that the scene has to play
out pretty much exactly the same every
time you move the sensor. Because if it
didn't, then every pixel would tell a
different story. It's like if I try to
record this section four separate times
and use a quarter of each to fill in the
screen, I would get a garment.
Thankfully, the laser pulse in our scene
scatters pretty predictably.
That's what lets us get unlimited
resolution. So, we can basically say,
"Let's scan as many points in the scene
as we need." That gives us good spatial
resolution. The more points you scan
along this grid, the higher your final
resolution. If you want 4K, you simply
scan a 4K grid of pixels. It's just
going to take more time.
>> The nice thing is light is fast. So, we
can do this as fast as the mirrors can
move. Within just a couple of minutes,
the sensor captures millions of laser
pulses across this whole grid.
>> So, this is now everything compiled
together for a time of flight.
>> Exactly. This is everything put
together. So, what we're looking at now
is going to be um again for a fixed
laser spot. So, I'm going to click play.
>> Oh, camera cultural logo.
>> Mhm. Yeah.
>> All of this was less than 8 n of time.
And here's another scene we shot.
Now you can also take this a step
further by rotating the scene and
recording multiple points of view. We
have this algorithm that kind of takes
this coke bottle video capture from
different views and is able to create
these flythroughs uh being able to see
the light propagate from any direction
and like flying through the scene as
it's happening.
>> Oh, that's so cool.
So some things that are interesting to
note is because we're moving towards the
right and the light is propagating
towards the right but we're moving kind
of faster in this visualization this
wavefront appears stationary as you can
see.
>> Oh yeah that is kind of breaking
physics. I mean you are moving the
camera faster than light.
>> Yeah exactly. Yeah it's kind of
mindboggling almost.
So this is a fish tank that we put a
mirror into and this diffused reflector.
So you'll see a pulse of light will
enter the fish tank. It will reflect of
the mirror and then hit the diffused
reflector.
>> That's crazy.
>> This is a defraction grating. So it's
kind of it defracts the light into
different modes.
>> Those are the different modes. That's
insane. My first impression was, "Oh,
these are just simulations from I I
don't know, Unreal Engine 5, but this is
like real data.
>> It's like the bullet time video in
Matrix. You've probably seen it."
>> No way. Is your motivation the Matrix?
By the way, these videos were created at
the University of Toronto and MIT. But
Brian from Alpha Phoenix actually built
one of these speed of light cameras in
his garage, which is mad. You should go
check that out. So, those are the two
extremes. strobe photography on one side
and a trillion fps on the other. But if
you combine both, you actually get to
see what electrons are doing.
Even though what that means exactly is
debated.
You say electrons act like waves. No,
they don't exactly. They act like
particles. No, they don't exactly. We
can write mathematical expressions and
calculate what the thing is going to do
without actually being able to picture
it. Do electrons exist?
>> How truthful do you want me to be?
>> Now, we still don't have a video or
photo of electrons, but this might be
the next best thing. And to pull it off,
we had to build big. Really big.
Okay. We're driving down what up until
2017 was the world's straightest object.
>> So, we've been driving for like what,
fiveish minutes?
>> Yeah. And we we are all like I want to
say about halfway there. We
>> I thought we were at the G. Oh, nope.
There is a lot more there.
>> All right, let's do it.
>> Let's go see it.
>> This is Slack, a US national lab that
houses a 3.2 km long, perfectly straight
electronic accelerator.
>> Wa, that is so long down there.
>> Then that like it just continues like
this all the way down.
>> The noise you hear is exactly 120 hertz.
That's the frequency at which electron
pulses are generated underneath this
building. 120 pulses a second. And this
is the sound of the equipment that
accelerates them to over 99.999992%
the speed of light.
And this lets you see electron clouds
move around molecules essentially,
>> right? So why would you care? Because
essentially electrons create the fields
in which everything else happens.
molecular bonds break and form because
the electrons essentially give them a
push to do so. Right? So the electrons
are responsible for everything that you
see in nature and being able to look
into their motion is the most
fundamental way of studying materials
and matter.
>> Now to achieve this you need a
nanoscopic equivalent of a strobe. So
you first feed these relativistic
electron pulses through a set of devices
called undulators. They're stacks of
magnets spaced only a few millimeters
apart with alternating poles. So the
first pair has the north pole facing the
electron pulse from above and the south
from below. Then the second pair flips
and so on. Now because the electrons are
traveling through a magnetic field, a
force called the Laurent force will push
them in a direction perpendicular to
both their velocity and the magnetic
field lines in accordance with the left
hand rule. So at one magnet pair the
electrons will curve clockwise and at
the next pair counterclockwise and so
on. This causes the electrons to wiggle.
Now since electrons carry a charge, this
wiggling motion causes them to emit
electromagnetic radiation. And even
though they oscillate at these
millimeter wavelengths because of the
magnet spacing, the wavelength of the
resulting em radiation is much smaller.
>> This is the fun thing about the theory
of relativity. If you travel at near the
speed of light, the length scales
contract. While that periodic structure
is microscopic for us, right? We see
each magnet. These are uh centime scales
for the electron because it's traveling
so fast. All of that space contracts,
>> right? And so it's actually oscillating
really really fast and these oscillation
periods are compressed and it means that
if you compare that to wavelength that
is in the X-ray domain.
>> Now that actually only gets your
wavelength part of the way to the true
X-ray regime. But if as an observer you
stand at the far end of the accelerator
all those electrons will be coming at
you at more than 99% the speed of light.
So in your reference frame any light
those electrons emit will additionally
be blueshifted producing X-rays as small
as 50 pometers in wavelength. Initially
these X-rays are created randomly along
the undulator producing an incoherent
light pattern. But soon after the
electric fields from the X-rays start to
interact with the electrons speeding
some of them up and slowing others down.
This causes faster electrons to catch up
with slower ones. So they get bunched up
into periodic structures. These parallel
sheets that are spaced at distances
exactly equal to the wavelength of the
X-rays. This is called microbunching.
Now these sheets of electrons emit light
as unified fronts. So the resulting
X-rays come out coherently as a laser
pulse. This dramatically increases their
intensity. And the pulses come out
incredibly tightly packed being only a
few phentocs long. And they can even get
as short as a couple hundred at
10 to the^ of -18. An absurdly quick
pulse. To put it another way, the atroc
is to the second what the second is to
the age of the universe.
>> On an autosecond scale, you can see
electrons zip around essentially atoms
and molecules.
>> That's insane. Yeah.
>> After the undulator, the X-ray pulses
are sent to experimental stations at the
end of the tunnel.
>> So where's the main X-ray uh beam? The
main X-ray beam is coming through this
tube over here.
>> Okay,
>> if you wanted to, it's a little harder
to see, but coming in through this tube
over here. So, this is the main place
where the X-rays come into the hutch.
And so, the X-rays focus into what we
call an interaction point.
>> Now, you fill this interaction point
with the molecules whose electrons you
want to study. Now we shine that X-ray
pulse on a molecule and when it hits the
molecule it will ionize the molecule and
it ionizes predominantly from these
inner shells from the very core parts.
>> The thing is core level electrons from
different elements have different
ionization energies. So if you want an
X-ray to eject a core level electron
from a nitrogen it needs around 400
electron volts whereas for an oxygen it
needs around 550.
So by tuning the X-ray energy to match
these ionization energies, you get to
choose which of these atoms within the
molecule the X-ray is going to ionize.
And any excess energy left after an
X-ray has ejected an electron will be
taken by that electron as kinetic
energy. Now once you ionize this
molecule, the kinetic energy will tell
you something about what's going on
around that that electron. Well,
electrons are not independent particles.
They talk to each other, right? they
have a negative charge.
>> So if you have a high electron density
around a particular atom, the core level
electrons will be bound less tightly to
the nucleus because of the presence of
all these other electrons around the
atom. So its ionization energy will
actually be slightly lower. Whereas if
you have a lower electron density than
average around an atom, those core level
electrons will be bound more tightly
with a higher ionization energy.
Therefore, when you measure the kinetic
energy of the electrons you eject, you
can use the difference between the input
X-ray energy and the output kinetic
energy to infer what that electron
density was. Now, once we can take a
picture of an electron density, we can
change what the molecule is doing, make
it do some process in time, and then we
can look at how electron densities
change. We have above us, we actually
have an entire laser hall. So, we
generate laser light. We bring it down
through tubes like here behind you or
over here on the ceiling. Traditional
laser light, these are infrared lasers.
We bring them onto this table and you
see all these boxes on this table. These
boxes are to condition the laser to give
it the properties that we want. We can
change the color of the laser. We can
change the polarization state. We can
change the duration of the pulse. So, we
sculpt these pulses. Then, we um have
them go co-propagating with the X-rays
into our target. And so now our our
first laser pulse will create some
non-equilibrium state in the molecule.
We'll drive some dynamics and then our
X-ray pulse will probe it. This X-ray
pulse ejects an electron from the
molecule after a time delay t. And by
measuring the kinetic energy of that
electron, you can study how the electron
density of the molecule reacted to the
trigger laser. So you get an atosecond
snapshot like a strobe of how the
molecule changed. Then you can get
another sample of the same molecule.
Shoot it with the laser again, but this
time increase the probe delay slightly
to t plus delta t. This will tell you
how the electron density changes a
little later. And you can keep
increasing this delay each time to get a
sequence of snapshots of how that
electron density evolves over time.
>> Isn't there a big assumption here that
every time you do it, you're expecting a
repeatable result from the molecules?
>> Absolutely. you need for your uh your
initiator to drive the same dynamics
over and over again. If you have a new
process happening every time, this
technique will fail.
>> But if the scene is repeatable, then
like the trillion fps camera, you can
use this technique to create a molecular
movie. And here the smallest amount by
which you can tweak the time delay for
the X-ray strobe is around 300 atconds.
So you can get frames that are only a
few hundred at apart. And if you stitch
those together, you get a movie that
technically runs at over a quadrillion
frames per second.
So this is a a movie of the dynamics
that we might like to image. So this is
a a small molecular system. This is
called para aminophenol. This
calculation was done by some of our
collaborators um in Madrid. um they had
calculated what is the response of this
molecule to the removal of an electron.
So they simulate an X-ray pulse coming
in and removing an electron. So the red
color here represents an increase in the
density of the electron and the blue
represents a decrease. And so we see
that when we've shined this X-ray pulse
onto this molecule and we've removed an
electron, we initiate some charge
distribution that starts to move across
the molecule. And so we want to image
this charge motion.
>> The video is a simulation, but it's been
validated by the experiments done at
Slack.
>> Our method for probing these seems to
work. We can compare to a handful of
points and say, "Oh, these look broadly
similar." Much after this few focs
to diverge from our our measurement. And
actually, this is the most exciting time
in science, right? When you have a
prediction and then you have a
measurement and they don't agree, that's
when you get really excited because you
just found something you didn't know
ahead of time. You couldn't have
predicted that.
>> I think the most powerful thing for me
here is we animate a lot of electrons,
right? And pretty much every Veritassium
video has electrons moving in some way.
So the fact that we're actually seeing
these electron densities move around,
>> I don't know. I think it's spectacular.
>> Absolutely.