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Comment by idlewords | Hacker News Reader

The big deal about these events is that, if you reasonably assume that it's a proton, then it violates a bound called the Greisen–Zatsepin–Kuzmin limit. Anything going that fast should be energetic enough to interact with the cosmic microwave background, which is hugely blueshifted in its frame of reference. The GZK is a kind of cosmic speed limit over long distances.

So anything this energetic would need have a nearby source (in astrophysical terms) so it doesn't have time to slow down. But when we trace these things back, we see bupkus in the direction it came from.

This means there is either (almost certainly) interesting new astrophysics, or (with tiny probability) new particle physics involved. Whatever is giving individual protons the energy of a thrown baseball is probably something worth studying.

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I wonder if it could be a background source that we don't understand. Maybe some freak coincidence where particles come in in exactly the right angles to fake a hyperenergetic photon. Or somebody messing with laser pointers. Or birds or whatever.

I used to work in particle physics, and never shared the confidence of my colleagues in rare events. If you just have 3-4 signal events, considering the expected number of events, you might statistically have a discovery. But you can't be sure how your detector is going to behave for those extreme events, because you have no benchmark. You have to assume a proton is a proton, an electron is an electron, and no weird things happen at high/low energies and angles (or that you understand how things change).

It is even worse if you are trying to disprove some specific model. You see no events of a certain kind. Does that mean you disproved the model, or that your setup (detector, triggers, event selection ...) is just blind in this very narrow part of the parameter space?

If you go out in space and look at what extrasolar stuff zaps you, you'll find a very standard power law distribution of charged particle energies, with no known upper limit. This is a better position to be in (from a detector confidence standpoint) than seeing a completely bananas event every few years. Here it's bananas (and progressively less bananas) all the way down.

It would certainly be convenient for me if it is a proton, given the experiments I work on (protons at the highest energies -> more ultrahigh energy neutrinos -> maybe my experiments will see something!), but, unfortunately for me, I don't think we know that. Nuclear species composition can typically only be done statistically, and it looks like this wasn't even measured in the fluorescence detector, which would give the best measurement of the penetration depth of the particle in the atmosphere (the best handle on particle id). In,the journal article, which admittedly I've just skimmed, they give direction reconstructions giving different species assumptions (and Milky Way models, which are important for heavier nuclei) and say that while they can exclude a photon (which... would be weird!), without data from the fluorescence detector, they can't tell the difference between a proton and a heavier nucleus.

Though given how TA mass-composition measurements turn light at the highest energies, they may perhaps privately argue it's likely a proton. But, alas, Pierre Auger Observatory would argue otherwise. I'll ask Toshihiro what he thinks next time I see him...

It is intriguing that the smaller TA has seen more high-energy events than the bigger PAO. Maybe there really are big differences between the northern and southern hemisphere at play here...

If Santa handed you a hundred billion dollars or so to study this stuff, what would be your dream detector?

Probably a integrated in-ice Cerenkov + radio neutrino detector with a giant cosmic ray detector (scintillators + water cerenkov tanks + fluorescence detectors + radio) on top, perhaps with some fluorescent/radio detectors deployed on tethered aerostats to look for upgoing air showers from taus. Sort of like IceCube Gen2 Radio on steroids, mishmashed with Auger and ANITA/PUEO/EUSO-SPB. Both in Antarctica and in Greenland to get both hemispheres. But there are arguments for other types of detectors too, and perhaps I'm not thinking big enough since 100 Billion is several orders of magnitude above what is probably reasonable :)

Here's to hoping that Santa reads your list. Better buy a very large pair of stockings, just in case...

> either interesting new astrophysics or new particle physics

Or measurement error which historically has been the most frequent explanation of these rule-breaking observations. Someone else will look at the data and notice an anomaly which once accounted for will make everything fit within expected models.

In this case though they've been observed over the past 30 years at at least 4 different detectors, which provides independent confirmation across multiple different research groups. It is very unlikely to be a "oops we didn't plug the cable in firmly" kind of situation. It would need to be something fundamental about the design of these detectors, which has been replicated four independent times, which has completely escaped the notice of physicists for >3 decades (which includes 3 decades worth of grad students incentivized to take a look at the problem with fresh eyes and make a name for themselves by explaining it all away).

How different are those detectors? Is it possible they share a common or similar design?

> It would need to be something fundamental about the design of these detectors, which has been replicated four independent times, which has completely escaped the notice of physicists for >3 decades

Your parent wasn't suggesting you hadn't considered it, they were asking for more information about the designs. To what extent is / isn't common design mistake the probable cause?

I guarantee you we're not going to discover the problem here on HN by diving into it.

I promise you I am not remotely that ambitious.

How likely is it this Hacker News discussion is the place where a basic flaw in the experimental design is discovered?

maybe it could happen 4 or so times times in 30 years.

We've had many more events within an order of magnitude of these events' measured/reconstructed primary energy. The spectrum is a power law. While theoretically something completely different could be happening in the atmosphere of you add 2-10x the energy, there's really no indication thereof. Auger also has high energy event detected by multiple detection methods, not just the "standard" water-cherenkov or scintillator ground stations.

Several of them use multiple as-independent-as-possible detection methods. The most common are ground stations in a grid that measure high energy particles passing through, and then correlate them in time, followed by ultra fast (in terms of data acquisition - eg 20M samples/s) telescopes pointed at the atmosphere (not the sky), which can see the slight trace of fluorescence light left behind by the air shower from up to tens of kilometers away.

What's common to all of the detection methods I know of is the atmosphere. Back at Pierre Auger Observatory, we used a combination of LIDAR scans and weather balloons to constantly monitor it. The atmosphere is basically a calorimeter for these detectors.

The fluorescence detectors (FD) are generally considered the most direct measurement of both primary particle energy because the fluorescence mechanism is relatively simple (somebody is bound to criticize me for saying that), with a proportionality whose constant can be measured in the lab[1]. But there's still models and simulations that go into it. And these fluorescence telescopes can only be operated in moonless nights, so have a duty cycle of only about 10%.

Eg. Auger combines FD with surface detectors (SD) to use simultaneously measured events to calibrate the more indirect energy measurement of the SD and thus make use of their near 100% duty cycle.

[1] Other methods have been tried to measure this fluorescence yield. It's primarily done in the lab: https://arxiv.org/abs/1210.1319 For something completely different: my own masters thesis from a long time ago was an attempt to determine this yield indirectly because we know from detailed bottom up simulations that air shower shape is near universal given primary and energy. If we knew the event geometry (direction) well enough, we could use the ratio of Cherenkov to fluorescence light along the recorded track and fit our longitudinal shower size model using the fluorescence yield as the free parameter. In hindsight that was a lot of fun. I would enjoy that type of work a great deal more today, now that I feel that have less to prove. :)

Edit: formatting

I assume that limit accounts for the time dilation, too. At the speed the particle was traveling it wouldn't have had much time in its reference frame to interact with the background radiation, even blue-shifted as it would be, right?

Yeah, the model does account for it, and the time dilations are nuts. For the Oh-my-god particle (a similar early detection), I remember that it would have crossed a billion light years in what it perceived as a day.

Imagining that reference frame is a beautiful thought. Galaxies and stars coming into and out of existence in real time, supernovae exploding like fireworks constantly as the entire universe zips by.

Until that trip ends by being snagged by some puny detector on an even punier planet that just so happened to intersect your trajectory. "I came across a billion lightyears, and all I got was this scientific paper". Not even a t-shirt...

Probably a leaky warp reactor on a passing starship :)

Honestly, still mildly crazy. If:

- Whatever Star Trek says about warp reactors (matter / antimatter reaction) is valid. Reacts "some" amount of matter/antimatter every second.

- Klingon Bird of Prey can maintain an effectively infinite cloaking time from a human observation perspective, so it can be nearby.

- Warp reactor "occasionally" leaks (1% of 1% of 1% of reactions? I dunno...) so we might actually detect something. "Slightly" imperfect shielding.

- Problem: 5E1 J for OMG Particle (people say its a proton). 1.8E14 J for 1 gram of matter / antimatter annihilation. Except: 1 Proton = 1.6726231E-24 g. Proton rest mass energy is 1.503E−10 J. So the particle is more energetic than a Proton / Antiproton annihilation event (by a lot). It's been upshifted More than 100,000,000,000 from the rest mass energy.

- It "might" work if the one proton escaping represented a single proton gaining enough energy to overcome reactor core shielding confinement, which, in my opinion, seems somewhat plausible physics by Star Trek standards.

- PS: Personal guess is a Q-Clearance [3] experiment at a DOE lab we deny exists.

[1] https://en.wikipedia.org/wiki/Orders_of_magnitude_(energy)

[2] https://en.wikipedia.org/wiki/Proton

[3] https://en.wikipedia.org/wiki/Q_clearance

To the casual HN reader - my recommendation to you is to neithet laugh at nor dismiss out of hand analyses such as this (even as logic intrinsic to them merits as much picking apart as any other analysis one sees everyday on HN).

Robin Hanson's description of long-lived stars (usually strongest in infrared - i.e. the 3 stars our eyes lack the spectral response to see for every 1 star that we do see when looking into the night sky) significantly updated my priors on the likelihood of intelligent, non-human life roaming about the cosmos.

https://youtu.be/cQq2pKNDgIs?t=1210 (timestamped)

tl;dr: Red-dwarf stars have been around -- with all that goes with that.. -- for much, much longer than our sun.

Do those red dwarves have the heavier atomic elements necessary to make diverse biological interactions, on a planet geologically active enough to protect the biosphere with a magnetic field?

I recently encountered a description of how Earth relies on the remnants of some interactions between stellar remnants which would have been far rarer early on, even if red dwarf stars were present for so long.

You missing important part - the larger rest mass of particle, the less it will interfere with cosmic vacuum.

So, high energy electrons must be from near source (mostly, inside our galaxy), because otherwise they will slow down by magnetic field of our galaxy.

For high energy protons, this eased, but I could not remember approximations for possible distance.

And for heavy nucleus, may be possible to travel from very distant source, even from borders of Universe.

BTW, I remembered interest fact, that on some particle accelerators run experiments on accelerate macro-objects, like dust.

Latest accelerators are not very suited for such activity, but earlier linear accelerators works well on this, and give very interesting data.

I think the simplest explanation is that we're just not accurate enough at determining the trajectory of the particle in question. They tried matching against sources "just outside its arrival direction" but without access to the paper (the DOI link just doesn't work for me, loads an empty page) or knowledge about the instruments being used it's hard to say how much that rules out.

At least looking into the Telescope Array's workings, it seems plausible that it'd struggle to produce accurate trajectory data. It works by analyzing the light produced by cascading collisions of particles with the incoming one: https://en.wikipedia.org/wiki/Air_shower_(physics). At first glance this seems like it would be very noisy for determining trajectory.

My recollection from the Pierre Auger telescopes was 1-2 degrees uncertainty. TA is likely a similar order of magnitude. So matching a single event against candidate sources might not be the most reliable analysis. (Caveat: I didn't dig up papers to check my memory from 15 years ago.)

I'm relatively lay in astrophysics. Could the origin not be galactic magnetic field lines between Earth and the galactic nucleus? Like the magnetic mirror between Earth and the Sun but much larger.

To our best understanding, particles of such high energy do not originate in our own galaxy. We don't have a good understanding of what exact process might be putting this much energy into a single particle, but the commonly accepted acceleration mechanisms of, eg., supernovae don't reach anywhere near these energies.

GP also said "nearby source (in astrophysical terms)" which in this case is code for "could be over 100 million light years".

Could it be a small black hole orbiting the sun?

What initial speed must the proton have for consistency with our observed final “dragged down” speed (assuming some ballpark galactic distance)? Alternatively, could a “clump” of particles have smashed into our detector? Whatever the new physics winds up explaining this bonkers momentum, it’ll be fascinating.

Or a nearby emanation source we can’t readily detect?

I think that might fall under

> This means there is either (almost certainly) interesting new astrophysics,

possibly. An emanation source they can just barely pick up is a mystery they can puzzle out, right?

something akin to a standing wave, where an exceptional signal develops out of otherwise unremarkabley noisy surroundings?

Alien spacecraft?

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