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Cosmic Rays (Totally COSMIC, Man!)

Cosmic Rays (Totally COSMIC, Man!)

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Astronomy Picture of the Day (apod): "Little Dumbell Nebula" near Andromeda Galaxy in the sky:

Here's another thing I don't know enough about: cosmic rays!

I'm still reading Lee Smolin's The Trouble With Physics, a critique of string theory that suggests physicists instead of looking inward at the mathematics of superstring theory should look for experimental inspiration in astronomy observations that don't fit current theories.

So it's timely that yesterday a preprint was posted by Pasquale Blasi on "Open questions with ultra-high energy cosmic rays".

Here are the questions I'll try to answer:

  1. What is the general background knowledge?
  2. How is the current theory for the formation of cosmic rays holding up?
  3. What are the current state of the art cosmic ray detectors, and how generally do they work?
  4. What is this big problem with the "GZK cutoff"--the idea that we may be seeing such high energy cosmic rays that it would be impossible for them to cross the Universe for us to see?
  5. What is "doubly special relativity"? (or "deformed special relativity"?) (As an unconfirmed and somewhat vague hypothesis I don't expect many scientists to jump on its bandwagon)--I will have to go into more detail later!

It would be ironic if cosmic ray observations were to change our understanding from special relativity of how to change our view in different frames of reference! That's because one very basic and simple way to show that relativity works--that stuff moving near the speed of light has its "clock" made to run slow--is through observing muon particles in cosmic rays (here's a description of the standard experiment).

Basically, the muon is an unstable particle. When a cosmic ray (a particle coming from space onto our atmosphere--90% of the time a proton, 9% of the time a helium nucleus and about 1% of the time other) strikes a particle in our atmosphere, a "shower" of exotic particles like pions and muons are created. This schematic is from cosmicrays.org (although I don't know who runs that site!):

And here is a track in a bubble chamber of a pion moving to the right decaying into a muon--which then lives only a very short life--causing just a small blip!--and decays into an electron, which spirals around to the left:

I got that picture from this site.

I think the way it works is that a magnetic field causes a force on a particle with charge q (F=q vxB... for roughly circular motion this = q v B)... With a=v2/r for circular motion and F=ma (this is non-relativistic!), r=m v / q B = p / q B. So the cyclotron radius shouldn't change when one particle decays to another, as long as momentum is conserved...

(Heh, this checks out on Wikipedia though I forgot the charge q at first!)

So the radius changes slightly as you go from pion to electron because sometimes some momentum goes to another particle you don't see (because it doesn't have charge)... And when a charged particle accelerates it has to radiate too: so you don't get perfect circles, but decaying spirals.

(Another thing you'll notice from the r=p / q B equation is that it means that a cosmic ray with large momentum p will be curved in a larger circle by a magnetic field B. So even if most cosmic rays are isotropic, coming from all directions, maybe the higher energy ones will depend more on direction.)

Anyway, when I was a senior in college I tried an experiment similar to the one related above. Muons decay in only a very short time, so if it weren't for relativity, they wouldn't be able to make it down from the atmosphere to sea level. However because of relativity and because the muons move so close to the speed of light c, their lives are extended. Time for the fast muons slows down, and they don't die until they get down far enough for us to see them.

You can actually observe this by counting muons at the top of a mountain and at sea level.

For my senior experiment I tried counting muons in the top and bottom floors of the physics building... They came out equal... I wasn't that great at running experiments though I do remember carrying a bunch of lead bricks (to shield out non-muons from the detector!) up and down the building...

By the way, the great pioneer of cosmic ray physics was Bruno Rossi--after whom the Rossi X-ray Timing Explorer (RXTE), mentioned in a previous post, is named.

* * *

Cosmic ray astronomy is very different from telescope astronomy. We don't know where this stuff comes from--whereas astronomy based on light is all about knowing where the stuff comes from!

The problem is that cosmic rays--protons or helium nuelcii or heavier nucleii--are charged. So magnetic fields will deflect them as they pass through space. So we don't even find that cosmic rays come more strongly from the direction of our own galaxy. Instead they are "isotropic", coming from all directions of space pretty much equally.

What gives cosmic rays their energy?

The answer was theorized by Enrico Fermi, the great Italian physicist who worked on the Manhattan Project and directed the first controlled nuclear reaction.

Those of us who keep lab notes on yellow graph paper ruled notebooks can only pray that some day we will write a page of such enduring value!

Link to Symmetry magazine--which I think I will subscribe to!

The idea is that charged particles bounce back and force through magnetic fields in space--generated for example in supernova remnants--and along the way they pick up energy.

This has recently been more or less confirmed by observations! The idea is that the strong magnetic fields will not only accelerate charged particles but also those charged particles will radiate. So although we can't tell what direction cosmic rays come from, we can expect that supernova remnants will be bright in synchrotron radiation, which can be observed by X-ray telescopes.

For example, this paper shows from Chandra data that the supernova remnant Cas A--discussed here a couple of days ago because the Spitzer Infrared Space Telescope showed its explosion to be more symmetric than thought before--can indeed be responsible for many of the cosmic rays observed.

However, this preprint suggests that Cas A and other common supernova remnants couldn't cause the most energetic cosmic rays.

Things are pretty complicated because not only does the supernova remnant magnetic field accelerate the cosmic rays, but the cosmic rays themselves then change the energy and magnetic field of the supernova remnant.

* * *

Ok, What is the current state of the art of cosmic ray detectors, and how do they work?

I'm going to focus on the highest energy cosmic rays. That's where things are most exciting: we don't know where they come from, and these energies (1020 electron volts) are greater than those that can be produced in the fanciest and most expensive particle accelerators on Earth (the Large Hadron Collider should go up to scales of 1012 eV, in a more controlled and observable way than cosmic ray collisions!)

The latest and greatest cosmic ray detector is/will be the Pierre Auger Observatory, which is actually a pair of observatories, one in the Northern Hemisphere (Colorado) and the other in the Southern Hemisphere (Argentina). That way they can observe the entire sky. Otherwise, if there were a slight difference in cosmic rays from different directions, that would mess up the observation.

Each observatory will detect cosmic rays in two different ways.

By one method, 1,600 tanks of water are placed on the ground--each tank capable of holding 3,000 gallons. Here they are all together:

Haha. That so totally looks like a scene out of one of the Aliens movies! Like each tank is going to hold some kind of Alien reptile that's going to be shipped out to be a parasitic host and burst through someone's rib cage! Well, it's not like that actually.

Actually each tank is placed out in the rural middle of nowhere:

Then when a cosmic ray passes through the tank of water it produces Cherenkov radiation, which is what happens when a particle moves faster than the speed of light.

Ok, wait a minute--are we ready to throw out relativity yet? No, not yet!

You can go faster than light goes within some material. Relativity just says you can't go faster than light in a vacuum and light in a vacuum moves at c. In water, light is slowed down slightly, so a cosmic ray passing through water can go faster than the speed of light in water. And if it does, it'll give off this radiation, which is a little like a sonic boom given off by a supersonic airplane.

The other method the Auger Observatory uses is to look at the UV light emitted when a cosmic ray strikes the atmosphere (this is fluorescence). So the two methods are a check on each other. They expect a real cosmic ray to cause both UV light, seen by several telescopes, and also Cherenkov radiation seen inside these huge tubs of water.

The total cost of the Auger Observatory was $50 million, to be compared with other costs mentioned here (and in the NASA budget on the sidebar).

The Auger Observatories aren't ready to go public yet with their data--they are still calibrating.

Previous high energy cosmic ray observatories that have gone public with their data:

KASKADE looked at what the cosmic rays are made of for energies up to 1018 eV, where there maybe be a turnover (a "dip" or "ankle") in the graph of cosmic rays vs. energy. The number of cosmic rays from galactic supernova remnants may drop off at high energies, but that may be taken over by some unknown (perhaps extragalactic) source of high energy cosmic rays.

AGASA, a Japanese array

HiRes, the High Resolution Fly's Eye, run by U of Utah

Actually these last two observatories disagree with each other on the most important point!

Here's a debate on which experiment has it wrong!

That's why the Auger Observatory results are so eagerly anticipated.

* * *

So what? So two observatories disagree, one made a mistake, and the new telescope should set things right.

Well, if AGASA is right, maybe something funny is going on. (Funny-strange, not funny-haha.)

The thing is that high energy cosmic rays passing through empty space could get messed up by the radiation left over from the Big Bang (see earlier entries!) that fills all space. This should limit how far cosmic rays with such large energy can travel, and so we shouldn't see that many cosmic rays with energies like 5x1018 eV. This is called the GZK limit, after Greisen, Zatsepin, and Kuzmin who first theorized it. HiRes seemed to show that really high energy cosmic rays became rare as expected by this theory, but not AGASA.

Here is a paper by Floyd Stecker. I used to work down the hall from him at NASA. He was a fun guy--loved California (though he lived in Maryland), loved to drive fast (though he always got speeding tickets!)

Anyway, Floyd works on cosmic rays (though I never talked with him about them much!) and in this paper he mentions speculations that violations of special relativity could cause the GZK limit not to apply--could allow high energy cosmic rays to pass through the Big Bang radiation freely.

He suggests a new kind of cosmic ray detector--one that will look down on Earth from space for the ultraviolet fluorescence radiation when cosmic rays crash against the atmosphere.

Why would anyone expect special relativity to break down? Well, there's a suggestion that the Planck length (10-34 cm or so) is fundamental. It's the only length you can calculate based only on the fundamental constants of nature, like the speed of light c, Planck's constant h, the gravitational constant G. It's where quantum theory and gravity (General Relativity) collide to make space and time meaningless--requiring a new unknown theory of Quantum Gravity. So the new Doubly Special Relativity (or Deformed Special Relativity--DSR in either case) hypotheses (there are at least two versions) say that as in normal special relativity, everyone will observe the same speed of light c, but also that everyone will observe the same special Planck scale--there will be a special energy or momentum (related to the Planck length) that different observers will also agree on. The math is here but I haven't gone over it yet. Also, there's this paper.

DSR doesn't neccessarily mean the GZK cutoff will be changed, but it is possible according to some versions of DSR.

For more discussion on this, see this blog entry by Christine Dantas, with communication from Lee Smolin.
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