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Comment by nuccy | Hacker News Reader

The title (likely intentionally) is misleading, it should say "travelling faster than light in a medium". Nothing here travels faster than light in vacuum.

BTW there are special types of telescopes used to observe gamma rays - they cannot see gamma ray directly but observe a flash of Cherenkov light of a cascade of charged particles created when gamma ray hits atoms in the atmosphere. Those telescopes are Imaging Atmospheric Cherenkov Telescopes [1].

1. https://en.wikipedia.org/wiki/MAGIC_(telescope) or https://en.wikipedia.org/wiki/VERITAS or https://en.wikipedia.org/wiki/High_Energy_Stereoscopic_Syste... or https://en.wikipedia.org/wiki/Cherenkov_Telescope_Array_Obse...

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>> The title (likely intentionally) is misleading,

That is because that is not the title...the title is: "What is Cherenkov Radiation?"

He obviously meant the title of the HN post? Why not straight up say that the article title is different and avoid the passive aggressiveness.

I did not meant it in that way...and I would bet, most did not interpret it as such...the passive aggressiveness is coming from you.

I meant it that the poster, changed the original title, what is a kind of editorializing. I thought of calling on the mods to edit it back, but judged that would be a kind of... semi-passive aggressiveness.

I don't think there's anything 'passive' about specifically asking for what you think should be done!

It came of kinda passive aggressive to me, to be honest!

> the passive aggressiveness is coming from you.

could you explain this? the comment you're replying to, how is it passive-aggressive? seems more regular-aggressive to me.

I see the passive aggressiveness only in your response, not the parent.

Your comment is a good reminder of why I love the HN community so much. Having acccess to knowledge in specific domains, like in this case, is truly a gem.

And thanks!

Maybe the coolest (heh) application of Cherenkov radiation is neutrino telescopes like IceCube, detecting tiny flashes of light in cubic kilometers of ultra-clear Antarctic ice, caused by vanishingly rare interactions between neutrinos and ordinary matter – and to exclude non-neutrino interactions, they look down, using the entire Earth as a shield guaranteed to stop anything that’s not a neutrino!

While I knew that IceCube existed it didn't occur to me until know that I have no idea how they filter out collisions from particles that comes from above or sideways. Is the resulting flash of light not omnidirectional? Is it polarized differently?

I believe they reconstruct the paths simply based on the timing of detection events. Muons created by neutrino interactions travel more or less linearly and create a "bow shock" of Cherenkov radiation; it's not a single flash of light, just like a sonic boom is not a single bang. The directions of electron and tau neutrinos are more difficult to reconstruct because the associated particles tend to scatter and bounce around.

One of the amazing things about Cherenkov radiation is that it is not omnidirectional. There are processes (fluorescence, or scintillation, for example) which result in isotropic emission of light from materials. But Cherenkov light is emitted along a conical surface about the axis of the charged particle's direction of travel. As a sister comment said, muons tend to move linearly, so this light stacks up in a particular direction. Electrons, particularly at lower energy, scatter more, and that stacking gets washed out a bit.

That directionality is very useful. The directionality of a muon or electron produced from a neutrino interaction is correlated with the direction of the incident neutrino. That's interesting for the astrophysical kind of questions that IceCube is addressing, and it's also useful in other contexts (background rejection for fundamental physics searches, monitoring of nuclear reactors via neutrinos, ...).

Cherenkov radiation is indeed polarized, but to my knowledge that hasn't ever been made use of.

Tangentially: there's also some really cool work being done on detectors which utilize both Cherenkov and scintillation light at the same time, exploiting the directional (and other) differences to extract the most information possible out of each event.

Right, so without knowing much about physics I'm guessing that momentum is preserved and IceCube works a little bit like a giant cloud chamber but looks for trails of Cherenkov radiation rather than an ionized particle trail.

The cone of Cherenkov radiation produces arcs of "bright pixels" in the detector.

There are very good illustrations of this if you search for "Super-Kamiokande Cherenkov ring." That's a Japanese neutrino telescope with a bunch of huge photomultipliers lining the walls of a giant underground cistern filled with water. Ice Cube has similar light detectors, probably from the same Japanese manufacturer, that were lowered on strings into wells melted into ice.

Cherenkov radiation is also a key mechanism behind ultra high energy cosmic ray (UHECR) detectors. UHECR are understood to be nuclei, not gamma rays. For ground based detectors, light tight water tanks are fitted with photo multipliers to capture the flash of cherenkov light even secondary particles pass through the water.

Additionally, they can use telescopes pointed at the atmosphere, using the atmosphere as a calorimeter, basically. The primary signal those telescopes look for is fluorescence, but when the direction of travel points at the telescope, cherenkov light far outstrips it in brightness, so it has to be included in the event reconstruction.

Most prominent contemporary example is the Pierre Auger Observatory https://auger.org

Not just gamma rays, cosmic rays too, although they filter the UV and are targeting nitrogen fluorescence and catch the cerenkov overlap. I’m sure there’s many people here who have worked on them.

I worked on HiRes/Telescope Array in Utah (Not CTA).

I don't think the title is that misleading.

We talk about the sonic boom as something that happens when we travel faster than the speed of sound. We don’t specify “in the same medium you’re traveling in”, and noone reasonable thinks “it’s really hard to travel faster than the sound in carbon”.

You’ve added “speed of light” in your reading and concluded that’s what the OP wanted to imply.

I think that was unfair, and distracted from the rest of your comment.

I, for one, didn’t know what the cherenkov radiation is, and when reading the title thought “oh, so it’s a sonic boom for light? That’s so cool!”

One difference is sound can only travel through a medium, so that bit is always implied. Not so with light.

Nothing can travel faster than light through vacuum, so that bit is implied as well.

I disagree with their quibble about the title being intentionally misleading but, meh, I've picked stranger technically correct hills to die on, the rest of their comment is interesting, it's fine.

Arguably there's a lot of people who don't know nothing is faster than light in a vacuum.

Because of science fiction, and lack of education in much of the world (looking at you USA), the title can easily make certain people think we discovered new science that allows us to travel faster than light. Especially for folks who only look at headlines.

The whole thing is a silly quibble. But sensationalized headlines get better engagement, which we can all see here.

Yeah, but what is a medium? Every material is basically empty space with an atom here and there. So there is not a hard boundary for a particle to instantaneously slow down from speed of light in the air to speed of light in water. Some of them will travel a bit deep before hitting their first H or O or electron, or some salt ion. Anyway, it won't travel faster than c.

It's funny because I was reading about Cherenkov radiation just yesterday and the same statement was made (although with a proper clarification afterwards).

-1 for Pedantic, but +1 for cool links to telescopes

Why do you believe that like if you are measuring it maybe you are doing it wrong. Seems to me that we need this light fastest X to hold up a load of physics even when there has been recently numerous experiments which challenge this premise.

>numerous experiments which challenge this premise.

Citation needed

> there has been recently numerous experiments which challenge this premise

I've seen Sabine Hossenfelder mentioning some papers but without the gory details.

But she also notably said:

>> A lot of research [in] the foundations of physics is now pseudocience. It hasn’t followed the scientific method for decades.

What experiments you're talking about?

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