
Scientists Capture a “Sonic Boom” of Light - jonbaer
http://www.smithsonianmag.com/smart-news/scientists-capture-sonic-boom-light-180961887/?no-ist
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turbohedgehog
.mov files available here
[http://advances.sciencemag.org/content/suppl/2017/01/13/3.1....](http://advances.sciencemag.org/content/suppl/2017/01/13/3.1.e1601814.DC1)

edit: Movies S4-6 say they're corrupt in Firefox, but VLC plays them fine
(mp4v codec).

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saycheese
Here's an animated GIF of the "photonic mach cone" made from the still images
of the experiment:

[https://wordlesstech.com/wp-content/uploads/2017/01/First-
re...](https://wordlesstech.com/wp-content/uploads/2017/01/First-recording-of-
a-Photonic-Mach-cone-3.gif)

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tabeth
Is there a theory for what "effect" would be produced by a theoretical object
going faster than light? The relevance is that, I thought a "sonic boom" was
defined as what happens to an object and its surroundings after going faster
than sound. A "sonic boom" of light seems nonsensical, even after reading the
article. Isn't this just a cone shaped wave?

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cjslep
Cherenkov radiation[0] is when a photon is travelling faster than the speed of
light _through water_. To overly simplify, the speed of light in a vacuum is
different than the speed of light through a medium.

[0]
[https://en.m.wikipedia.org/wiki/Cherenkov_radiation](https://en.m.wikipedia.org/wiki/Cherenkov_radiation)

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FreeFull
It doesn't happen with photons. Photons themselves always travel exactly at
the (vacuum) speed of light, even in water, and they aren't charged particles.
Cherenkov radiation happens with charged particles like electrons.

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delecti
A photon's net speed through a medium is not always the vacuum speed of light.

> For example, the refractive index of water is 1.333, meaning that light
> travels 1.333 times faster in a vacuum than it does in water.

Light travels at about 0.75c in water.

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

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posterboy
I guess that's relative, depending on which photon is referenced. The
corollary is that from the photon POV, not its speed changes, but spacetime is
warped, so it sees time in its own frame of reference going faster and/or
distances shorter than we.

You are both correct, but on different levels.

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lippel82
>> not its speed changes, but spacetime is warped

No, in water, the spacetime is not significantly warped, certainly not warped
enough to slow down light. The different speed of light in a medium compared
to vacuum comes from the interaction between the photons and the medium. The
photon then, effectively, becomes a hybrid "photon-medium excitation" which is
an effective particle with an effective speed lower than the vacuum speed of
light.

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posterboy
Maybe not from our point of view, but from the POV of the photon the water is
moving with c_0 through the photon, so certainly relativistic effects apply.

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msds
The paper describing the camera is very interesting -
[https://www.ncbi.nlm.nih.gov/pubmed/25471883](https://www.ncbi.nlm.nih.gov/pubmed/25471883)

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CamperBob2
Isn't Cerenkov radiation effectively analogous to a sonic boom? You don't get
to see the nifty Mach cone, but I've heard it's the same basic phenomenon.

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Manishearth
Yes, this is the case. And Cherenkov radiation is visible, too. My first
reaction on seeing the title was "this has existed for years". I think here
they're actually talking about the setup used to image a "mach cone" in
cherenkov radiation (the use of dry ice creates a non-vacuum medium in which
the radiation may occur)

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dahart
Well, the article doesn't explain anything, but is the sonic boom a reasonable
analogy -- is that really what's happening here?

Maybe the trailing cone is simply light diffusion?

In any case the analogy feels weird because we don't call a sound wave a
"sonic boom", we call it "sound". Light itself wouldn't make a photonic boom,
something accelerating passing through warp 1 would, right?

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posterboy
Heard of Cherenkov radiation?
[http://en.wikipedia.org/wiki/Cherenkov_radiation](http://en.wikipedia.org/wiki/Cherenkov_radiation)

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dahart
I hadn't before this thread, but Cherenkov radiation does seem to slightly
better fit the comparison to a sonic boom than regular light scattering,
doesn't it?

After reading the paper this article is talking about
([http://advances.sciencemag.org/content/3/1/e1601814.full](http://advances.sciencemag.org/content/3/1/e1601814.full)),
what they are describing _is_ normal light diffusion:

"We have demonstrated ultrafast video recording of light-scattering dynamics
using LLE-CUP and visualized the propagation of a scattering-induced photonic
Mach cone as an instantaneous light-scattering pattern with a single camera
exposure."

Maybe it's reasonable, and maybe I'm being pedantic or even wrong, I'm not
entirely sure, but for some reason describing this as a "photonic mach cone"
or a "sonic boom of light" seems just a tad more sensational than
informational & accurate.

Unlike both Cherenkov radiation and sonic booms, the cone here would be caused
primarily by the scattering, which of course is slower than the primary wave.
The cone isn't a direct symptom of unimpeded light propagation, meaning the
angle of the cone represents the speed of scattering and not the speed of
light.

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Eerie
Sure missed an opportunity to call it "Sonic Rainboom".

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iamleppert
Prior using streak camera to capture up to 1 trillion fps:

[http://www.bbc.com/news/technology-16163931](http://www.bbc.com/news/technology-16163931)

Although this method required repeating the exposure multiple times from
different angles. I still don't understand how they are doing it, or why the
binary pattern is necessary. Wouldn't the DMD device be a limiting factor
here?

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sambe
Luminal boom, surely?

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abrown28
By the time I scrolled down the video had finished playing then went to the
next video. I then refreshed the page and scrolled down quickly only to find
they had added a big banner over the video telling about the next video in the
queue. I'm not sure I'm ever going back to smithsonianmag.com

EDIT: Capturing a light sonic boom is still pretty cool though :)

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dawnerd
Whats with this trend with sites having auto playing videos below the fold and
hidden away? Almost every news site does it now. Worst is when they have a
flash ad on top so all you see is the click to play overlay trying to figure
out where the video is. AHHH!

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infectoid
Maybe they are hacking the stats. "We had 95% of readers view the video. Give
us more ad dollrz!"

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dawnerd
Having worked on eHow I can say pretty confidently that's what they're doing.
Probably helps their ad sales people too.

