Scientists Capture a “Sonic Boom” of Light | Hacker News Reader

Scientists Capture a “Sonic Boom” of Light

smithsonianmag.com

97 pointsjonbaer39 comments

.mov files available here http://advances.sciencemag.org/content/suppl/2017/01/13/3.1....

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

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...

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?

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

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.

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

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.

>> 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.

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.

Beware the usage of the term "speed of light" here. There is two things: the phase velocity and the group velocity.

I think you mean when an electron (or other charged particle) travels through water.

     Is there a theory for what "effect" would be
     produced by a theoretical object going faster than 
     light?

There really isn't an effect. The speed of light (in a vacuum) is just the speed of causality. The fabric of space-time will bend around you to ensure you can never get there. Objects without mass move at the speed of light, and no other speed.

Now if you don't mean in a vacuum but in a medium like water, or air, or ice, etc.

That is https://en.wikipedia.org/wiki/Cherenkov_radiation

No, there's no (experimentally supported) theory that would work at all with an object going faster than light. It just isn't in the geometry - asking for a hyperbolic rotation that stretches your motion's vector past c is like trying to turn an apple around in your hand in a way that makes it twice as big.

What they're doing here is based on the fact that you can make light move at under c if you force it to travel through a dielectric material. The c in E=mc^2 is the speed of light in a vacuum: not necessarily through a carefully designed atomic obstacle course.

Light always travels at c. It may appear to an observer to travel slower than c if it is being absorbed and emitted through media along the way.

It's not due to absorption/re-emission. The vacuum permittivity and permeability of free space is different than a medium.

https://en.wikipedia.org/wiki/Vacuum_permittivity

https://en.wikipedia.org/wiki/Vacuum_permeability

Free space has an electrical resistance of about 376 ohms.

https://en.wikipedia.org/wiki/Impedance_of_free_space

>Free space has an electrical resistance of about 376 ohms.

Impedance, to be precise. If free space had electrical resistance of 376 ohms, our world would be much stranger place than it is now.

The speed that light propagates is not always C. Light travels at C in a vacuum, but at a slower speed in any medium, e.g. water. Objects, and light itself, can travel faster than "the speed of light in water" in water. https://en.wikipedia.org/wiki/Cherenkov_radiation

> A "sonic boom" of light seems nonsensical

The experiment specifically used mediums, where the speeds are not the speed through through the vacuum, the report from which smithsonianmag took the info and provides the link is somewhat more precise:

http://www.livescience.com/57572-ultrafast-camera-captures-s...

"study lead author Jinyang Liang, an optical engineer at Washington University in St. Louis, and his colleagues designed a narrow tunnel filled with dry ice fog. This tunnel was sandwiched between plates made of a mixture of silicone rubber and aluminum oxide powder."

"Then, the researchers fired pulses of green laser light — each lasting only 7 picoseconds (trillionths of a second) — down the tunnel. These pulses could scatter off the specks of dry ice within the tunnel, generating light waves that could enter the surrounding plates."

"The green light that the scientists used traveled faster inside the tunnel than it did in the plates. As such, as a laser pulse moved down the tunnel, it left a cone of slower-moving overlapping light waves behind it within the plates."

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

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.

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)

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?

Heard of Cherenkov radiation? http://en.wikipedia.org/wiki/Cherenkov_radiation

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), 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.

is the same thing? Cherenkov radiation with the "photonic mach cone"

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

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

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?

Luminal boom, surely?

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 :)

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!

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

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

As someone working in adtech, some of the megabytes of JS you get sent involves viewability tracking to catch stuff like this. Sometimes you'll even have two or more copies, one from the advertiser and some from the companies in between.

No major advertiser actually trusts what publishers claim - even the big ones like FB and YouTube though they are much less likely to be actively gaming you. That's why there's a lot of tracking JS in pages for viewability to catch stuff like this.

It's called "outstream" video ads. (instream would be a pre-roll where it plays over another video you are wanting to watch)

The publisher has not control over this and probably doesn't realize it's happening, it's purely the 3rd party ad provider they are using that is controlling that.

There are standards in which most ad companies follow (user must be 25% in view, ads must be muted...etc), but recently a large number of Israeli based ad firms have begun running outstream ads without these standards. The purpose is to drive up the view and completion rates. This gives them access to larger media buyers and higher fill rates (filling video ads for every page load is very hard to achieve)

Reference: years of being in the video ad industry

If they were smart, they'd be working at Google, I guess.

By the time I got there it was something about Germany surrendering or a wedding. Here, a video that's more or less the article; https://www.youtube.com/watch?v=LrVoW097kUQ

You can disable JS and make a keybinding for youtube-dl. I hit F4 and X launches https://github.com/jakeogh/youtube-dl-wrapper

Looks like lots of other people had the same idea[0]

[0] http://www.alexa.com/siteinfo/smithsonianmag.com

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