
Video filmed at four trillion frames per second captures light in a flash - chriskanan
https://www.nature.com/articles/d41586-019-01625-5
======
tlb
This is more impressive than previous claims of trillion FPS cameras, because
it captures it all sequentially from a single event. Other such claims were
based on capturing one frame at a time of a repeating event with different
time delays.

~~~
NullPrefix
You could use an array of cameras and capture the same event at different
times

~~~
gridspy
You see that light pulse that they captured?

Your camera synchronization signal in the wire is traveling about 2/3 of that
speed. Can you imagine all the cameras in your array receiving and
interpreting that signal at the same time?

~~~
kevin_thibedeau
You pretrigger the cameras while accounting for the delay.

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davesque
I'm confused how you could film a light pulse this way. It seems that the
light pulse would itself have to be emitting light somehow. I'll assume it was
travelling through a gas that scattered it.

~~~
jerf
"The imaging system captures a light pulse (in a slowed video) as it passes
through a material, exits the material (at the dashed yellow line) and bounces
off a mirror (solid yellow line) back into the material."

~~~
identicalgamer
Is this trying to imply that it is capturing the scattering off of the Frenel
diffraction? I don't understand at first glance how this paper is capturing
the light without having a sensor in the path of propagation.

~~~
jerf
The light is being put through a translucent medium, like shining a laser
through milk or smoke or something, so you can see the scattered light.
Conventional/non-QM-based light physics can handle this situation.

If you mean, how are they seeing the non-scattered light, you are correct that
they are not.

~~~
HillaryBriss
> light is being put through a translucent medium

at certain points, the light pulse seems to "leap ahead" in fits and starts on
its journey. is that because of the way the medium scatters the light or
something?

~~~
mirimir
I think that it's because the speed of light inside the translucent medium
(left side) is lower than in air (right side).

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cr0sh
I'd be curious to see the dual-slit experiment done using this camera as the
"observer".

I'm not sure it'd yield anything we don't already "know" \- but it might be
worth trying just because.

~~~
lqet
I wondered exactly the same thing (see my other comment). However, I am not
sure if they can actually film single photons. My school physics are a bit
rusty, but iirc, quantum effects will not be relevant if you actually fire a
light "impulse" consisting of many, many photons. However, as I have asked in
my previous comment, it would be interesting if you could actually see the
light impulse being "split up" and interfering with itself.

~~~
foxyv
You can detect single photons. There are off the shelf detectors for available
for experimenting with photon entanglement and parametric down conversion. We
had one at my university.

[https://en.wikipedia.org/wiki/Photon_counting](https://en.wikipedia.org/wiki/Photon_counting)

~~~
lutorm
You can't make movies with them though... It would make an expensive camera.
;-)

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ebg13
> _The imaging system captures a light pulse (in a slowed video)_

I like that they remind us that the video is slowed down and that time doesn't
actually move at 10 picoseconds per second.

~~~
idlewords
If it did move at 10 picoseconds per second, how would we know?

Whoa. _Stares at hand_

~~~
snowwrestler
Makes me think of those videos of house plants, which we normally think of as
still objects. But with a time-lapse video, you can see them moving around a
bunch during 24 hours.

[https://www.youtube.com/watch?v=z0I3174S8Xg](https://www.youtube.com/watch?v=z0I3174S8Xg)

~~~
phyzome
Heck, with the right plants you can see them grow over the course of minutes:
[https://www.brainonfire.net/blog/2018/03/29/seeing-plants-
mo...](https://www.brainonfire.net/blog/2018/03/29/seeing-plants-move/)

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freedomben
This article caused me to remember something I heard on a Youtube video and
wanted to follow up on. Essentially, is there (and if so what is) the
framerate of the universe? It makes logical sense to have a minimum bundle of
time based on Planck's constant, so I did some googling and found the answer
(I have not checked this math)[1]:

> _The Planck time is 5.39 × 10-44 seconds. No measurable time can be shorter
> than that according to quantum physics._

Converting to FPS, that gives us:

> _One thousand eight hundred and fifty-five billion billion billion billion
> frames per second. 18.55 septillion FPS!_

So if like me you wondered if a trillion FPS is close to the maximum possible
frame rate in the universe, the answer is nope!

[1]: [http://www.librador.com/2009/01/16/The-frame-rate-of-the-
uni...](http://www.librador.com/2009/01/16/The-frame-rate-of-the-universe/)

~~~
klank
Don't confuse "our math breaks down at this point" with "it can't physically
happen".

Planck time is the time at which, if you want to do physics, you have to take
into account quantum mechanics and gravity (i.e. understand quantum gravity).
Something we don't know how to do. But it doesn't necessarily mean time is
discrete. Maybe it is. Maybe it isn't.

[https://en.wikipedia.org/wiki/Planck_time#Physical_significa...](https://en.wikipedia.org/wiki/Planck_time#Physical_significance)

~~~
pier25
Does the same reasoning apply to the Plank length or is that one discreet?

~~~
lodi
As I understand it, currently everything points to space and time (really,
spacetime) not being discrete. I'm copying and pasting a previous reply I made
the last time this came up.

\---

I'm no physicist, but the layman explanation I've heard is that the concepts
of Planck length and Planck time do not imply that spacetime is discretized
into little "voxels"; instead these quantities are just limits on the
uncertainty of any possible measurement.

As I understand it there are two contributing facts:

a) the Heisenberg uncertainty principle states that there's a tradeoff between
certainty in position vs momentum, so if you're more certain in a particle's
position you're less certain of its momentum. (For photons, momentum is
proportional to frequency, i.e. wavelength, and frequency is proportional to
energy.)

b) By mass-energy equivalence, anything with energy has mass, therefore higher
frequency photons are more "massive". A single photon of sufficiently high
frequency would form a black hole.

Putting those two together, to measure distances accurately you need higher
and higher frequency photons with shorter and shorter wavelengths. For
example, radar creates blurry images at ~5cm wavelengths, while ordinary
photographs can be razor sharp with wavelengths of only ~500nm. The Planck
length is just the wavelength at which the photon would have so much energy
that it would collapse into a black hole and break our current mathematical
models. That's why it's nonsensical--with current models--to talk about
lengths smaller than a Planck length, but it doesn't mean that space itself is
quantized. Similar argument for time.

(Also, the same logic applies to other particles like electrons, protons, and
even up to macroscopic items like baseballs; everything has a wavelength...)

~~~
posix_compliant
Semi-related question: if photons can have various frequencies, does that mean
protons can have various frequencies?

~~~
skygazer
AKA temperatures?

~~~
posix_compliant
I think temperature is a property based on the average kinetic energy of
particles in a system. The frequency of a proton, I imagine, would be an
intrinsic property.

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Twirrim
CalTech films light at the speed of 10 trillion frames a second! The "Slow Mo
Guys" went to caltech to take a look:
[https://www.youtube.com/watch?v=7Ys_yKGNFRQ](https://www.youtube.com/watch?v=7Ys_yKGNFRQ)

~~~
srcmap
I used to work on HP way back in 1991 on equipment that measure timing of
signal to 7.9 pico second resolution. This is very cool.

Love to see an ifixit tear down video on that CalTech camera. :-)

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MattSteelblade
I'm blown away by this. If I'm understanding this correctly, this picture[1]
is light traveling for a distance of about 3/8ths of an inch.

[1] [https://media.nature.com/w800/magazine-
assets/d41586-019-016...](https://media.nature.com/w800/magazine-
assets/d41586-019-01625-5/d41586-019-01625-5_16737810.gif)

~~~
SilasX
Wait, the light is all incoherent before it passes the dotted line, then
coherent until it bounces and passes back through? What's going on?

~~~
improv32
I believe the area between the dotted and solid lines is a mirror. The blob of
light is visible because it's passing through some medium that scatters it,
then when it disappears it has been absorbed by the mirror, then we see it
reemitted shortly after.

~~~
hcs
The solid line is the mirror, the dotted line is the edge of the scattering
medium. I assume there is air or vacuum in between.

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raziel2701
I wonder how the authors feel that their work is so good that the website for
Nature has a piece on it but not good enough to be published in a Nature
journal.

~~~
busyant
Maybe the authors didn't want to submit to Nature?

Also, not a physicist, but my understanding is that PRL is prestigious in its
own right.

~~~
makerofspoons
When I worked in a physics lab a PRL publication was seen as a big deal.

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lqet
So, looking at the video, I am curious: is there a video where they fire a
light impulse against a double-slit experiment setup? Would it be possible to
see the light being "split up" and interfering with itself?

~~~
simias
You can't see the light not going into the camera (obviously) so what you see
is some light from the beam being reflected into the sensor. You can't take a
picture of a photon, if that makes sense.

So in the double slit experiment you could see the path the light took only if
you allowed for a portion of that light to be reflected towards the camera.
But then if you did that you wouldn't end up with anything particularly new or
interesting as far as I can imagine.

~~~
lqet
Of course, you are right! I intuitively thought of the light impulse emitting
further photons in the direction of the camera, which makes no sense.

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theWheez
Can anybody comment on what is meant by a "frame" here?

My intuition tells me that what we consider a "frame" in our daily experience
(24 up through maybe 144) would be pretty different from what this "frame"
would be, in terms of how it is captured and how it is subsequently rendered.

Any ideas?

~~~
jerf
We do not usually consider a frame duration to measure a length, because for
most of the durations we use the frame's size is absurdly large in practical
terms, ranging from thousands of miles to larger than the diameter of the
earth. So we can take a shortcut of identifying a frame as some area from the
camera's point of view identifying an infinite, instantaneous volume in front
of us, and don't consider the fact that light isn't instantaneous and in fact
has a transmission time.

Frames this short mean that there is a sort of volumetric aspect to what is
being catured. From the point of the camera, when light collection begins an
area sweeps out in front of the camera and back in time at the speed of light,
and when the light collection stops, the end of the area sweeps out from the
camera lens in the same direction. The result of this is a spherical shell
volume travelling backwards in time that represents the spacetime locations
that can emit light and appear in the frame [1]. The resulting picture is an
integration of the photons emitted in this 4D spacetime volume, in the
direction of the camera, projected onto a 2D image.

(That's assuming a point-like camera and instantaneously turning the camera on
and off; in reality it'll be fuzzier but the principle holds.)

The difference is that at these incredible fast speeds, the resulting 4D
spacetime volume is of human size in most directions in the places we care
about [2], measured in human-sized measuring units like "centimeters" rather
than "light-seconds". Normally we can neglect thinking about this volume
because we simply spray so "much" out that we don't have to think much about
capture exactly what we want, in much the same way that in normal day-to-day
life we tend to act as if lightspeed is simply infinite. In this case, we
actually have to think about it to get what we want.

A similar effect can be seen in network equipment, for what it's worth; at the
highest network speeds, we've now significantly passed the point where a given
bit being transmitted is now a human-sized fraction of a wire. If you could
"snapshot" a 100Gb network cable, you could see bits on the wire. If I'm
getting my math right, each bit is on the order of 2-3 centimeters, give or
take the medium not being fully light-speed.

[1]: It may be more intuitive for a moment to instead imagine that the camera
is starting to emit the shell at the time that it is turned on, and stops when
turned off, _forward_ in time, as that creates a more intuitive initial image
of what-seems-to-be-cause _preceding_ what-seems-to-be-effect. Imagine the
camera emitting light instead of collecting it. It may be easier to then
imagine this shell going out farther and farther, getting larger as it goes
(and the light inside dimming as it has to fill that volume with the same
amount of energy it started with), indefinitely out into the universe. But
since the camera is _receiving_ instead af _transmitting_ photons, the reality
is the same image, just with time reversed; as the time goes farther back, the
shell is farther away from the camera and larger.

[2]: Technically it extends all the way in the direction the camera is all the
way out in space back to the Big Bang or the CMB or something, but unless
you're deliberately photographing space, and why you'd do that with this
camera I have no idea, you don't have to worry about that.

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Causality1
This technology could give us the opportunity to verify a ton of our physical
and chemical models with direct observation. The demo may not be exciting but
if they can scale it down you can expect a lot of exciting discoveries. I
can't wait to see what we're wrong about.

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sdegutis
> _The imaging system captures a light pulse (in a slowed video)_

Good to know that's not the actual speed of light!

Anyway, it looks like the photons are "crawling" by their patterns of speed,
rather than traveling at a fixed speed.

~~~
jolmg
I would think that effect is due to impurities or lack of uniformity in the
material which causes some spots to light up more than others.

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hndamien
Is there any explanation for why it is so jerky?

~~~
MattSteelblade
Sampling rate isn't high enough is what I'd imagine, even at 4 trillion FPS.

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objektif
Looks like a matplotlib chart turned into an animation.

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korethr
I've had conversations with a guy who works at one of the companies that makes
high-speed cameras. One of the interesting thing to come out of those
conversations is that below a certain threshold of frame duration (which is
related to, but not necessarily the recriprocal of the frame rate) a camera
with that capability becomes subject to ITAR, because supposedly said camera
could be used to develop nuclear weapons.

My intuition is that the camera described in this article would be subject to
ITAR were it developed in a western nation. But, I doubt China gives any fucks
about that.

