The Oh My God Particle | Hacker News Reader

The Oh My God Particle

fourmilab.ch

100 pointsmnemonicsloth20 comments

Summary: A particle (probably a proton) was detected hitting the atmosphere at nearly the speed of light, which is kind of impossible or at least implausible, and they can't figure out how it happened or where it came from.

My question is: are they sure their equipment didn't just have a glitch?

http://en.wikipedia.org/wiki/Ultra-high-energy_cosmic_ray

Short answer: glitch isn't the right word. There was almost certainly a particle with an extremely high energy. Whether they can measure energies that high is questionable.

Long answer: I'm not familiar with this specific experiment, but I am familiar with ones that try to measure the same thing.

The thing about most of these devices is that they have a sweet spot in which they have the highest accuracy. These detectors have a sweet spot which is a few orders of magnitude below the energy measured here. So there is always the question of how well you can extrapolate.

You'll notice there is no uncertainty quoted on the energy. It would not be unreasonable to expect that uncertainty is at least 1 order of magnitude in each direction. Probably more.

While that might seem like a lot, it's also important to know that the relevant measurement is not the particle's energy, but actually the number of particles above a certain energy. So they don't care so much if the energy measurement is off by a lot, as long as it's really big.

> And thus, approximately: v = 0.9999999999999999999999951 c

Okay.

>So taking 3×108 metres per second as the speed of light, we find that the particle was traveling 2.9999999999999999999999853×108 metres per second,

Oh dear, that's a rather alarming piece of innumeracy. The speed of light is approximately 3x10^8 metres per second, All those 9's in 2.999 etc would imply the speed of light was exactly 3x10^8 metres per second which would be an extraordinarily weird coincidence.

The speed of light is exactly 299,792,458.0000000 (and so on) meters per second. This is because this is actually the definition of the meter now, so my string of "0"s that are significant digits is justified. 299,999,999.999... is therefore superluminal and you are correct.

> Sokolsky has calculated that at 3×1020 eV, even a single proton could travel no farther than 10 megaparsecs

I wonder how fast proton looses energy (due to interactions with photons of background radiation).

How much energy it would need to travel to us from edge of observable universe?

Particle have (very short) wavelegths associated with them. It might be interesting to observe larger amount of such high energy photons coming from same point in the sky especially if there is a black hole near their path that could bend it slightly. Maybe we could get some diffraction patterns. ... yeah I'm probably insane.

The usual energy loss mechanism for cosmic-rays (fast protons) is reverse-compton scattering off the cosmic-microwave background photons, which is quite amazing really. Some of the most energetic particles scattering off the ubiquitous but very low energy background.

Anyway, if you propose that this is the mechanism for energy loss and you know how dense the CMB photons are (which we do very well) then you can predict interesting things like the maximum distance a cosmic ray can travel before it runs out of steam entirely, see

http://en.wikipedia.org/wiki/Greisen–Zatsepin–Kuzmin_limit

Although i'm not sure how the OMG particle fits in with this scheme yet

I was looking at that too... the trick that I see is that that's a statistical limit, not a "travel X miles, lose X% energy, no matter what" limit. It's still entirely possible for a particle to be over the limit, it's just highly unlikely.

We detect super-energetic particles frequently, but only a couple "over the limit". Seems to fit the statistical model to me.

A pretty quick read about a super-fast proton detected in 1991, with a nice set of comparisons to get the point across. Such as that the proton had enough energy:

to light a 40 watt light bulb for more than a second.

Yowza.

That said, included in the article is this:

[Star Trek's best ship would take] a little more than 21 years [to reach the center of the galaxy]. By contrast, an observer on board the Oh-My-God particle would arrive at the nucleus of the Milky Way, according to his clock, just about 3 seconds after leaving Starbase Terra. That's more than 9,700,000 times faster than the starship.

* bzzzz * wrong. It's only perceived as faster to the riders, the particle takes ~32,000 years (according to their numbers) to do the same trip, which isn't pointed out in that section. At which point our whole civilization is probably dead or passed you by long ago. And they've apparently forgotten about suspended animation.

But that's what he said: "according to his [the rider's] clock" it would take about 3 seconds to reach the center of the Milky Way. So yes, he is talking about how fast the riders perceive it, which is all they care about, since they wrote off their entire families for dead before they left.

Now the strange thing about traveling at 1516c (i.e. far greater than the speed of light), is that the time dilation equation yields an imaginary time, in this example 1 / sqrt(1 - 1516^2). I'm not sure how that translates into a perceived travel time of 21 years.

Warp engines make a little stable bubble of space and shift what's around it, so onboard time matches galaxy time, and you can jaunt a significant way across the galaxy and still be home in a few years Earth time.

ahhhhh ... ok :)

Unfortunately, there's the small matter of them requiring orders of magnitude more energy than exists in the universe.

Link to the Alcubierre Drive - http://en.wikipedia.org/wiki/Alcubierre_drive

That sounds really cool. Where can I get one of those?

I think the imaginary time should be taken as an indication that it doesn't make sense to talk about travelling at v > c, in terms of a Special Relativistic framework at least :)

Yeah, I should've been clearer. But writing off your entire family as dead is only a consequence of riding the proton, so it has nearly no advantage. And again, there's suspended animation, so not only can you make the trip nigh-instantly (to you) on the ship, you can get back before people forget you exist.

It wasn't your clarity, it was your tone. If you're gonna go around "BZZZT WRONG!!!"-ing people, you really ought to be sure you understand the physics. As written, your post completely misses the boat about what relativity is all about.

Their statement that the scientists looked in the direction from whence the particle came, but did not see anything should be self-evident. Whatever caused the particle to be travelling at such speeds would still emit light at... the speed of light. They would have had to been looking BEFORE detecting the particle, even if only nanoseconds before.

Perhaps the source might still be emitting photons?

very interesting and also very well told.

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