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Marcos Santander | IceCube | USA

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Leaping and hopping on a moonshadow

Wednesday, August 31st, 2011

How do you align a telescope to make sure it’s pointing in the right direction? Piece of cake: you use any cheap sky mapping software that could give you the position of a star in the sky at a given time for your location, you point your telescope in that direction and make sure that you see the right star. Done!

Now, how do you align a neutrino telescope like IceCube to make sure it’s pointing in the right direction? Well, as you may imagine, this is a little bit harder. With no high energy neutrino sources detected so far, we can’t really use the same procedure as with an optical telescope, but there’s a way: using cosmic rays.

Every second, IceCube detects about 2000 muons coming from the interaction between cosmic rays (protons, most of the times) and the Earth’s upper atmosphere. These protons have energies of 20 TeV on average, about 6 times the energy of the protons going around the LHC ring. Although protons of this energy can’t be used for galactic astronomy since their trajectories are bent by the interstellar magnetic field over scales of tens or hundreds of parsecs, they can travel on pretty straight paths over shorter distances like those characteristic of the inner Solar System.

Just as there are no neutrino point sources that have been observed so far, due to these magnetic deflections there are also no point sources of cosmic rays in the sky, but in this case the idea is not to look for a source of cosmic rays, but for an antisource. The Moon, conveniently located only 360000 km away from our planet, is a very good absorber of cosmic rays. Because of that, the Moon should be casting a cosmic ray shadow, blocking all cosmic rays that come from its direction, and then preventing us from detecting the muons associated with those ill-fated protons in IceCube. Also, since the distance is small the shadow is not destroyed by magnetic field deflections for cosmic rays of TeV energies, so the effect should really be observable by IceCube.

If we map the cosmic rays detected around the location of the Moon, we should see a deficit in the number of cosmic rays that are coming from that part of the sky. If we don’t see such a deficit, then that means that there’s something wrong with the directional reconstruction algorithms that we use for cosmic rays and neutrinos. This is actually an old idea, first proposed by G.W. Clark in 1957 and used in many occasions by different experiments to characterize the angular resolution of their detectors.

In IceCube, this work was performed by Laura Gladstone, David Boersma and two undergraduate students: Jan Blumenthal and Hugo Stiebel. Here’s how IceCube “sees” the Moon. The map shown below corresponds to the Moon shadow observed with IceCube in its 59-string configuration that was operated between May 2009 and May 2010. During this period, the detector recorded 22 million events in an angular window of 8 x 8 degrees centered around the position of the Moon.

The map below shows one result from the analysis. The color scale indicates the total number of events shadowed by the Moon at each position in the map. The deepest deficit corresponds to a total of 8192 events blocked by the Moon, and the location of this deepest point agrees with the expected position of the Moon to within 0.1 degrees. The statistical significance of the detection is around 14 sigma, where the usual rule in particle physics is that anything above 3 sigma indicates “evidence” for something being observed, and 5 sigma indicates a “discovery.” Probably it is too late to claim discovery of the Moon given that people have even walked on it, but those 14 sigma tell us that we are very confident that the shadow we see is not a fluke.

We’re right now writing a paper that will include all the details of the Moon shadow observation.

The shadow of the Moon as was observed with the 59-string configuration of IceCube (preliminary plot).

Not only the fact that we see the Moon is important for IceCube, also its width tells us something about the precision of the reconstruction methods that we use to determine the arrival direction of the cosmic rays. The width of the shadow is of the order of 1 degree, which agrees with simulations that we have of the detector.

Besides its use as a sanity check for the pointing capabilities of the detector, the Moon shadow can also be useful for physics since it can provide a measurement of the antimatter content in cosmic rays, a value that has implications for dark matter since this antimatter could be generated in WIMP annihilations.

Due to the magnetic field of the Earth, the proton shadow of the Moon should be slightly shifted to the left of the expected position of the Moon in the map above. This effect is stronger at lower energies, where the offset can be as large as ~1 degree of offset for cosmic rays of ~1 TeV. For IceCube energies the effect may be too small to be measured, but other experiments working at lower energies actually see this deflection happening. What’s important about this deflection is that if there’s a small fraction of antiprotons in the cosmic ray flux they would produce a shadow that’s deflected by the same amount as the normal proton shadow but in the opposite direction from the Moon due to their opposite electrical charge. At the moment I’m writing a cosmic ray propagation code to see how much and in which direction we should see the shadow shifting when observed from the South Pole (see an example of the propagation code output in the image below).

3D view of beams of GeV protons (green lines) being propagated towards the Earth (light blue sphere in the middle) which are scattered away by the geomagnetic field.


If such an “antishadow” is observed, a direct measurement of the antiproton-to-proton ratio in the cosmic rays can be done by comparing directly the strengths of both shadows. If such a shadow is not observed, then a limit could be set on this ratio. This is what several experiments have done. Most recently by ARGO-YBJ, a cosmic ray detector located in Tibet. The current status of the antiproton/proton ratio measurements is summarized in the plot below, which I took from a conference proceeding by ARGO.

Most of the direct antiproton measurements have been performed only up to 100 GeV, with a ratio indicating that there’s only one antiproton for every 10000 protons in the cosmic ray flux. At higher energies only limits have been set so far with values of about 0.1 in the ratio, or equivalently that there’s less than 1 antiproton for every 10 protons.

Direct measurements and limits for the antiproton/proton ratio.

The recently launched AMS-2 cosmic ray detector that’s now taking data attached to the International Space Station should be able to extend the direct measurements up to 10^3 GeV.

I’ll keep you updated about any news on the IceCube moon shadow front. For the moment, I leave you with a song appropriately called ‘Moonshadow’ by Yusuf Islam (aka Cat Stevens.) I’m sure he was thinking of muons and antiprotons as he was writing it!





A postcard from the South Pole

Monday, August 15th, 2011

Some days ago, Freija Descamps, one of the “winter-overs” taking care of IceCube during the months-long South Pole night, sent to us in the North this great picture of the aurora australis, and I wanted to share it with you.

Serving as a background to the aurora is the pale glow of the Milky Way in the region of Sagittarius, with Scorpius hanging on top of it. Behind the curtain of stars, gas, and dust of the Sagittarius region sits the center of our home galaxy, a candidate source of neutrinos and energetic cosmic rays.


Checking IceCube’s pulse

Monday, July 25th, 2011

These last three weeks have been very intense and eventful in the home front. In the meanwhile, back at work, and besides the usual research work, I’ve been taking care of what it’s called “run monitoring.”

In a perfect world, once you’re done building a complex detector like IceCube your problems should be over, and you could take data without caring for how the apparatus is doing 1.5 km below the South Pole surface. Unfortunately, that’s not the world we live in.

On a daily basis, we have to make sure that each of the 5160 light sensors (aka DOMs, for Digital Optical Modules) deployed in the deep ice are working and doing fine, and for that we have a very nice monitoring system that can spot most problems automatically.

I’m showing below a screenshot of the monitoring system that shows the frequency with which each DOM saw light. Each little square is a DOM, blue indicating a lower rate of detection, and yellow-ish a higher one. Black DOMs were not taking data at that moment. Depth increases towards the bottom of the image. An interesting feature of this image is the blue horizontal band of DOMs in the middle of the image, which shows that DOMs at that depth record muon events systematically less often than the rest. This is not a detector issue, but a geological feature about 65000 years old that corresponds to a stadial (or cold period) during the last glacial period in the late Pleistocene where a lot of dust seems to have deposited in the then surface of the Antarctic ice. Weaker layers are seen in the upper part of the detector as well. Due to the high dust concentration light can’t propagate too far in the ice without being absorbed, causing the observed decrease in the DOM detection rate.

The detection rate for each DOM in a section of IceCube. Depth increases towards the bottom of the image, with DOMs beginning at 1.5 km deep at the top and ending at 2.5 km in the bottom.


A data-taking period (or “run”) is usually 8 hours long, during which IceCube records about 2000 muons going though the detector per second. It would be great if all these muons were associated with neutrinos of astrophysical origin, but most of them come from well-known cosmic rays hitting the Earth’s atmosphere.


Every second Mother Nature gives IceCube about 2000 more muons to chew on.

Most of the runs that I had to monitor were perfectly fine. With an uptime of 98% IceCube (now running on its final configuration) is doing great, and I’m sure that we’ll have more interesting results in the future with more data coming.


A bit of history

Tuesday, July 12th, 2011

I’ve been reading a bit lately about the history of the development of the first atomic bomb: the Manhattan Project. The topic is extremely fascinating for many reasons. It was not only a huge technological and scientific effort that required the direct implementation of theoretical ideas that emerged at most only 40 years before the construction of the bomb, but also a breaking point in world history. The human species had now been given the terrible and terrifying ability to annihilate itself.

Many books have been written about the subject (a particularly good one is “The Making of the Atomic Bomb”, by Richard Rhodes) and you can find a lot of information everywhere, but what has always interested me is what the physicists that were involved in the project thought at the moment. Especially since these were academic people who went from giving lectures and preparing homework assignments while working on basic research to developing the most terrible weapons of mass destruction known to date (without forgetting, of course, the historical context in which this happened)

A good window into the thoughts of some of these physicits is given in the 1981 documentary “The Day After Trinity“, which is centered on the figure of Robert Oppenheimer, the “father of the bomb”. Among the people interviewed for the documentary was Stan Ulam, a mathematician who helped in the development of the working concept for the first H-bomb (the Teller-Ulam design) and that on a brighter note contributed to the creation of a technique cherished by all physicists today: the Monte Carlo method. Ulam was working here in Madison at the time (1940) as an assistant professor when he volunteered for war work. He says that he received a letter from Hans Bethe, inviting him to join “an unidentified project that was doing important work, the physics having something to do with the interior of stars.” When he found out that he was going to New Mexico (where the development and ulterior testing of the bomb happened), he went to the library to borrow a tourist guide about the state. In an interview in 1987 for Los Alamos Science he says about the book:

At the back of the book, on the slip of paper on which borrowers signed their names, I read the names of Joan Hinton, David Frisch, Joseph McKibben, and all the other people who had been mysteriously disappearing [from Madison] to hush-hush war jobs without saying where. I had uncovered their destination in a simple and unexpected fashion. It is next to impossible to maintain absolute secrecy and security in war time.

So, I went to the library to see if the book was still available, and it was! Unfortunately, the slip is gone, probably due to the rebinding that it went through at some point when the library management system was switched to computers. A map of the state, showing a much smaller Los Alamos before the war, is included.

Tiny Los Alamos, before the US started the development of the bomb that later would give birth to the Los Alamos National Laboratory.


The Federal Writers' Project Guide to New Mexico from 1940.

On the topic of how hard it was to keep the location of the project secret, Richard Feynman mentions something similar in his book “Surely you’re joking, Mr Feynman!“, using his traditional sense of humor:

We were told to be very careful – not to buy our train ticket in Princeton, for example, because Princeton was a very small station, and if everybody bought train tickets to Albuquerque, New Mexico, in Princeton, there would be some suspicions that something was up. And so everybody bought their tickets somewhere else, except me, because I figured if everybody bought their tickets somewhere else…

So when I went to the train station and said, “I want to go to Albuquerque, New Mexico,” the man says, “Oh, so all this stuff is for you!” We had been shipping out crates full of counters for weeks and expecting that they didn’t notice the address was Albuquerque.

The whole enterprise raised many moral issues inside the physics community itself. It is particularly interesting to hear the testimony of Robert Wilson in the documentary. Feynman in his book recalls how everybody was excited in Los Alamos after the first successful test of the atomic bomb (the Trinity test). Everybody, except for Wilson, who he saw moping having realized about the impact of what they had accomplished and its terrible consequences. In the words of Oppenheimer himself: “Physicists have known sin, and this is a knowledge which they cannot lose.”



A gentle breeze of cosmic rays

Monday, May 30th, 2011

Quite a while ago I wrote a post talking about the IceCube neutrino telescope and its potential to become the first detector to observe sources of high energy neutrinos in the sky. IceCube, located at the South Pole, detects neutrinos not by observing them directly, but by detecting the particle that is created when a neutrino interacts with the ice that surrounds the telescope or the rock underneath it. This charged particle, a muon most of the times, emits a bluish light called Cherenkov radiation which can be detected by the array of light sensors that make up IceCube.

This is how neutrino detection goes, but it turns out that not only neutrinos are able to produce muons that reach the pristine ice that surrounds IceCube. Cosmic rays (charged particles coming from the cosmos), in fact, account for most of the events seen by IceCube. In general, for every million of cosmic ray events seen by IceCube there is only one neutrino in our data set (which is most of the times produced by another cosmic ray on the other side of the Earth!) This is my very convoluted way to say that besides being a very nice neutrino detector, IceCube is also an amazing cosmic ray detector.

So, is there anything interesting that we can do with these cosmic rays that light up the detector at a rate of about 2000 events per second? Maybe we could plot their arrival directions in a sky map and see if they’re pointing us back to their sources.

Well, there’s a problem. We know that permeating the vicinity of the Solar System there’s a magnetic field that bends the trajectories of these protons (that have energies of tens of TeVs) in pretty much the same way that the LHC magnets bend the trajectory of protons around the collider ring. The main difference here is that the magnetic field in the solar neighbourhood is not so organised and neat as that of the LHC, and these protons would follow pretty chaotic paths before they reach the Earth, at which point they would not be pointing back to the source that originated them. This is why we should expect to see a completely featureless sky if we were to just plot the incoming direction of these TeV cosmic rays.

But you know that I would be writing about this if this were the end of the story. Last year, IceCube published its first map of the TeV cosmic ray sky, and we found that, actually, there are significant features in it. These features are very weak,  with the “hottest” spots in the sky differing from the number of events detected on the “coldest” spots by only parts in thousands. This is where a data set with a huge number of cosmic rays becomes handy; with IceCube gathering billions of cosmic rays events every year, we can measure these minute differences very accurately. This study, performed by fellow UW-Madison colleagues Rasha Abassi, Paolo Desiati, and Juan Carlos Diaz Velez with data taken when the detector was only one-quarter of its final size, revealed that the cosmic ray sky is anisotropic, and that the excess and deficit regions that are visible take about half the sky each.

Large scale anisotropy of cosmic rays as seen with the IceCube detector in its 22-string configuration. The red colour in this map indicates a deviation of 0.2% from a flat sky, while the blue indicates a deficit of the same strength.

The next question that we asked ourselves was: is that all that there is to it? Is this half-and-half feature the only remarkable thing about this cosmic ray sky? This is the question that we’ve been trying to answer for the past year with the group that I work with. Our group (Dr. Simona Toscano, Dr. Segev BenZvi, Prof Stefan Westerhoff, and myself) has focused its attention on the search for smaller structures in this cosmic ray sky, to see if there are features that are smaller in size than those previously reported. And here again the answer was yes!

Calculating the angular power spectrum of the sky map that we got for data taken with IceCube in its 59-string configuration (about 2/3 completed) we obtained the blue points shown in the graph below. The y axis shows a value that gives an idea of how strong the features in the sky are at a certain angular scale (given by the upper x-axis) We knew from the previous analysis that structures that are large (with sizes between 90 and 180 degrees in the sky) were present, but as you can see the blue points don’t go immediately into the grey bands which indicated what we should expect for a “featureless” sky but rather remain away from them up to angular sizes of 15 degrees.

This tells us that besides the large scale structure already reported by IceCube, there must be regions of excess and deficit of cosmic rays that have typical sizes of ~ 20 degrees in the sky (~40 times the size of the Full Moon.)

The power spectrum of the cosmic ray anisotropy detected by IceCube. The presence of a large scale structure is evidenced by the peak to the left, while smaller structure can be seen as a departure from the gray band regions for angular scales between 15 and 35 degrees. After the subtraction of the large scale structure, the small scale structure persists (seen in red dots), which indicates that the presence of smaller structures is not due to an artifact caused by the presence of the large scale anisotropy.


Using a technique that allows us to filter out the large scale structures to focus only on the smaller regions, we got the map shown below, where we can see localised regions of excess and deficit of cosmic rays coming across the Southern sky. We also see that, as we were expecting, these regions are about 20 degrees in size. The causes of these “hotspots” are still unknown, but we’re working to see if we can determine what’s causing them. Possible reasons include nearby pulsars, the configuration of the local interstellar magnetic field, or a combination of these two factors, but more information is needed to determine what the possible sources of this anisotropy may be.

This is how the Southern sky cosmic ray sky looks like at TeV energies once structures larger than ~60 degrees have been filtered out. Both regions with an excess (red) or deficit (blue) of cosmic rays when compared to an isotropic sky are clearly visible.

Similar excesses and deficits have been observed in the past by experiments located in the Northern hemisphere, but this is the first detection of this kind of structure in the Southern sky. You can take a look at the preprint of the paper we submitted to the Astrophysical Journal here. We’re right now trying to organise a workshop in October where we will discuss possible theories and the details of observations made in the North with colleagues from other experiments.

Interesting times ahead!




Men who stare at quantum

Saturday, April 9th, 2011

LSD trips, CIA agents, bank transactions, spoon-bending psychics, self-help books. All seemingly disconnected and far from common topics for a physics talk. However, Prof David Kaiser from MIT  managed to combine all these exotic ingredients into a very entertaining physics colloquium titled “How the hippies saved physics” that we had the opportunity to enjoy last week here in Madison. The talk revolved around the unlikely adventures of the members of the “Fundamental Fyziks Group” (FFG), an ensemble of about 10 physics PhD students based in Berkeley who got their degrees in the mid-seventies and are the main topic of Kaiser’s upcoming book.

In discussing the activities of the group, Kaiser warned us, one must understand the peculiar circumstances that physics in the US was going through at the time. Right around the end of the 60s and the beginning of the 70s, the physics community realized that it was producing far more physicists than the job market (both outside and inside academia) was able to absorb. The burst of this bubble left many highly qualified scientists without a place to continue research, forcing most of them to find jobs that were unrelated to their studies.

Some of the founding members of the Fundamental Fyziks Group: Jack Sarfatti, Saul-Paul Sirag, Fred Alan Wolfe, and Nick Herbert. Certainly not the usual physics lab crowd.

This was the case of most of the members of the FFG. With several of them having day jobs, they realized that they had enough time in their hands to discuss topics that they thought didn’t receive the attention they deserved in the physics education curricula of the day. The group would hold weekly meetings in a room in LBNL booked by those who were still enrolled as grad students there, and they would talk about their main topic of interest: the foundations of quantum mechanics.

Ever since the beginnings of quantum mechanics, the strange implications of the theory made some of its founding fathers (like Bohr, Heisenberg, or Pauli) wonder about its philosophical ramifications. This road was abandoned after those early days, probably because there was a lot of concrete work to be done with the new theory having so many predictions to test. This is precisely the road that the FFG wanted to retake, and to do that they focused on Bell’s theorem, fairly unknown at the time but directly related to very hot topics today, as are quantum entanglement and its applications.

Although today we have banks testing quantum encryption methods based in quantum entanglement, back in the day the topic was basically in the sidelines of physics, as it was seen by many as being “too philosophical” to be considered real science. The early discussions about Bell’s 1964 paper (today one of the most cited in history) happened during the weekly meetings of the FFG, and most of the early citations to the paper come from members of the group.

Using Bell’s theorem as a tool, the group tried to explain some “phenomena” that were even more bizarre than quantum mechanics itself, as you’ll see. You have to remember that these were crazy times to be living in the San Francisco Bay Area, with many things happening there: the hippie movement, the anti-war protests, the experimentation with LSD and other drugs, the practice of yoga and vegetarianism (mainstream today), and an overall search for spirituality. Not all scientists were immune to these phenomena, and some of them got involved in things like testing the psychic “powers” of  spoon-bender Uri Geller under not-so-strict laboratory conditions. This even led to an article in Nature where the scientists talked about the positive detection of a “Geller effect.”

This observation called for a theory to support it, and here’s where the FFG members enter the game claiming that these observations were related to Bell’s theorem. How the connection went I can’t say, you’ll probably have to read Kaiser’s book, but according to the group, there was a lot of room left in quantum theory to accommodate things that could not be explained, things like: telekinesis, mind reading, and so on. That, of course, if you assume that they could actually not be explained through much simpler means.

The members of the group, most of them outside academia, started looking for non-standard ways to fund their non-standard research in the connection between quantum mechanics and the paranormal, and surely enough they found it. They got funding from SRI (the same place where they tested Geller), they got funding from a branch of the CIA created to train “psychic spies”, and they even got a lot of money from self-help book author Werner Erhard.

Kaiser recounts some of the “remote viewing” experiments that were carried out by the CIA. One agent would go out on the field somewhere in the bay area and stare at an object while another agent back at the station would try to get their minds “entangled” and describe what the other person was seeing. Most of the times the agent back at the station would receive LSD as an “enhancer.” It is not hard to understand why these experiments involving a guy tripping on acid trying to read the thoughts of another guy miles away didn’t go too far.

Having secured the funding for their studies, they had to solve the problem of how to get the message out to the community, specially since most of the big journals had banned topics like Bell’s theorem and the foundations of quantum mechanics from appearing in their pages. Here too they had to rely on non-standard procedures to get things done. After some publications in an obscure Swedish journal, they found their communication channel in Ira Einhorn, a renown anti-war activist who was running a newsletter known as “the Unicorn preprint service.” These preprints were mainly hard-to-read photocopied copies of articles, many of them written by the FFG members, that were mailed to many prime physicists of the day (usually without asking about whether or not they wanted to receive these articles.) These publications came to a sudden stop when Einhorn fled the US while being tried for the the murder of her girlfriend (he’s still serving time for the crime.)

There’s no doubt that most of the discussion of the members of the group belonged to fringe science, but many of their points were extremely insightful, and that forced many “mainstream” physicists to take some of these topics seriously. According to Kaiser, some of the foundational papers of the today well-established field of quantum information science owe a lot to the activity of the FFG during those early days.

Today, Bell’s theorem is a mainstream topic again, and a discussion about it can be found in almost any grad level textbook on quantum mechanics. It has been a long and winding road back into academia but it certainly made it back. Even though this is a happy ending, an uneasy feeling remains: could we be discarding something vital as fringe activity today? In my opinion, even if we are skipping over some important idea, patiently evaluating and sorting through the overwhelming volume of crackpot theories that circulate today on the internet implies a price too high to pay.

We must also remember that the members of the FFG, regardless of their unorthodox study interests, were very smart and very well trained people,  with great control over the technical details of their research, no matter how far-fetched the implications of their claims were.

In the 2009 feature film “Men who stare at goats”, George Clooney plays a soldier who has been trained in the fine arts of “remote viewing”, telekinesis, and “phasing” the particles in his body into waves by a specially created branch of the army to be, well, a psychic spy. A legend at the beginning of the film reads: “More of this is true that you would believe.” Who knows if that also holds for some crazy ideas about physics out there.

PS: The video of Kaiser’s talk is not yet available on the UW website, but here’s a link to a video of a similar talk he gave some time ago.


DOMs away!

Thursday, January 6th, 2011

2010 has been a very fruitful year for IceCube, and the grand finale for the year could not have been better with the completion of the detector on December 18th.

Many of us were following the events unfolding at the South Pole through a live video feed, and although it was not HD video, it wasn’t hard to notice the smiles in the faces of the people working on the last string of light sensors to go down into the ice. At around 6 pm New Zealand time the 86th and final IceCube string was secured in place, a great ending for a story that had started 7 years before, with the drilling of the first IceCube hole.

The successful completion of the telescope owes much to the development of the hot water drill that is able to make its way through 2.5 km of solid Antarctic ice, making room for the strings of 60 DOMs (Digital Optical Modules) that go into the ice. The first step in the drilling process is to penetrate the ~50 m thick firn layer (i.e. compacted snow). The drill in charge of this part has a copper coil through which hot water is pumped, melting the snow around it.

As soon as the actual ice is reached, the firn drill is replaced by the hot water drill. The drill creates a hole in the ice by sending a highly pressurized stream of hot water that is able to melt more than 750 metric tons of ice (200000 gallons) in about 48 hours, reaching a depth of 2.5 km (1.5 mi). Almost 18000 liters (4800 gallons) of airplane fuel are used to drill each hole.

After that, it is time for the deployment crew to attach the DOMs to the cable string and send them down the hole, in an operation that takes about 11 hours.

Here you have a video of the last DOM, named “Moa”, being lowered into the hole.

And a short video of the same DOM disappearing into the pond of water in the last hole

Even though IceCube has been producing science for a while, having the entire detector will increase the amount and quality of the data we take.

Interesting times ahead!


All stat mech and no play…

Thursday, November 11th, 2010

It’s pretty late in the night, and I’m still working my way through several statistical mechanics homework problems. This reminded me of one of the funniest (and most depressing) paragraphs that I’ve ever found in a physics textbook.

Without further ado, I give you the first paragraph of page 1 of chapter 1 of David Goodstein’s book “States of matter”

Ludwig Boltzmann, who spent much of his life studying statistical
mechanics, died in 1906, by his own hand. Paul Ehrenfest, carrying on the
work, died similarly in 1933. Now it is our turn to study statistical
mechanics. Perhaps it will be wise to approach the subject cautiously…

David Goodstein (States of Matter)

Isn’t that a great intro? I’ll be back soon with something more enthusiastic to say about physics!


The skies of Wisconsin

Sunday, October 10th, 2010

When I heard the learn’d astronomer;
When the proofs, the figures, were ranged in columns before me;
When I was shown the charts and the diagrams, to add, divide, and
measure them;
When I, sitting, heard the astronomer, where he lectured with much
applause in the lecture-room,
How soon, unaccountable, I became tired and sick;
Till rising and gliding out, I wander’d off by myself,
In the mystical moist night-air, and from time to time,
Look’d up in perfect silence at the stars.

“When I heard the learn’d astronomer”, from Leaves of Grass, by Walt Whitman

If you make a living out of looking at data gathered by a huge telescope (I know, a weird telescope, but a telescope in the end) there’s no better way to take a break from that than by… looking at the sky through another telescope.

Some time ago, with the excuse of showing the skies of Wisconsin to my daughter, I bought a small reflecting telescope. This happened after walking back home many nights and seeing this beautiful, crystal clear sky above my head of which I knew nothing about. As it was to be expected, the moment I got the telescope shipped to me, and just to respect a long-standing astronomical tradition, clouds showed up everywhere and the crystal skies were gone.

However, things have been nicer lately, and I’ve had a couple of good, clear nights, so I’ve been able to venture outside to take a first glimpse at the sky, and I wanted to share my 3-am excitement with you.

The sky in October was very helpful in offering me a full suite of celestial objects to look at. I started with the easy part: Jupiter, high in the sky through most of the night, is an absolute marvel. You can see it very well, even with your naked eye, if you go outside after dusk and face southeast; it’s by far the brightest “star” on that part of the sky. Just 2 weeks ago Jupiter reached its closest approach to the Earth for the next 50 years, so if you have never looked at it, now it’s a good time to do it. The view through the telescope (even a small one) will reveal the 4 biggest moons that circle the planet, and also some structure in the clouds (which usually show up as belts of clouds of different color.)

I also looked at some very nice open clusters (groups of sister stars that formed in the same gas cocoon) that were unknown to me. The best one I saw was the Double Cluster, in the constellation Perseus. Even a good pair of binoculars could reveal this one as a patch of light with some of the brighter stars sticking out.

With the obvious, brightest objects in the sky covered, I started to look at dimmer stuff. Now I was venturing in uncharted waters for me, I’d never looked at the northern sky through a telescope, so I was completely lost. I got my laptop running a standard sky map software, and I went outside.

By the way, if you’re looking for this kind of software, the one I like the most is called Stellarium, which is not only nice looking and easy to use, but also free!

I was lucky to find, even in my not-so-dark suburban location, many galaxies that looked pretty nice. Of course, if you look through a modest telescope at one of these galaxies you may feel utterly disappointed, but you have to keep in mind that the fuzzy, colorless patch of light that you’re seeing is after all, a GALAXY! It’s all there, no Hubble between you and that huge, spinning, hungry monster. Not to mention the fact that the light hitting your retina directly has been traveling through intergalactic space for several million years. For me that’s just amazing.


Here's a picture of Jupiter I took a couple of nights ago. The original view through the telescope was much sharper than this, but I got the image using an old digital camera attached directly to the eyepiece. After a tiny amount of post-processing, the Galilean moons can be easily seen, from upper left to lower right: Ganymede, Europa, Jupiter, Io, and Callisto. Probably not the nicest picture of Jupiter you'll find on the web, but anyway...

But why am I telling you all this? Because you may have never looked through a telescope, and I would like to encourage you to do just that. Take your friends, your family, your significant other to one of the many astronomy clubs that exist all over the world and that will be happy to share with you their equipment, and I assure you that it will be a great experience.

I still have to meet the person that will look at the Moon, Saturn, or Jupiter through a telescope for the first time without being moved by the experience. For me, even after taking courses on related subjects, and having looked at the skies for as long as I can remember, I can’t help standing in awe when I look through my telescope, as Whitman said, “in perfect silence at the stars.”


Cosmic rays and everyday life

Saturday, January 16th, 2010

Every physicist, and probably every scientist, is faced at least once in a lifetime with the dreaded frase: “That’s very interesting, but what is that good for?” It doesn’t really matter how much your eyes are sparkling while you talk about your subject of research, it will always come, and you better have an answer for that in your pocket and ready to be used. And it’d better be good, since it can come from anybody: from the person seated right next to you in a long-haul flight, to most of your relatives, to somebody that you’ve just met in a party and that will fire away an innocent “what do you do for a living?”, unleashing the unavoidable chain reaction.

At the beginning, I always aimed high: I started to talk about our adventure as a species in discovering the ultimate mysteries of the cosmos, and how we have evolved from matter to conscience in this vast and beautiful universe. That did not always work. When talking about the Great Purposes of Science I get the feeling that people think we suffer from illusions of grandeur, and that we think that we’ll save humanity by counting light flashes in a block of ice. Getting “real” doesn’t help either, try convincing somebody that investing (their) millions to build an astrophysical observatory is the shortest path to improve the next generation of CAT scanners.

Frankly speaking, spinoffs are very important, and they may be used to highlight some of our collective work, but the reason why we do this is closer to my first attempt to an explanation than the second one. Anyway, let me put my vocational propaganda aside, and present you with this list of things where something as bizarre as cosmic radiation actually plays a role in our daily life, or where it has been found to have some, er, “practical” applications.

National security

Picture 1

A muon tomography scanner (orange structure) inspecting a truck (Photo: Decision Sciences Corp)

It seems like in today’s world everything is linked to this topic, and cosmic rays are no exception. The smuggling of nuclear material has always been an important concern to many countries, and there are some ways in which cosmic radiation can help us in preventing it. Cosmic rays are just plain protons and atomic nuclei that get their extravagant name only from their astrophysical origin. As they get to the upper layers of the Earth’s atmosphere, they interact with the nuclei of atoms that make up air  producing mesons (pions and kaons.) As these mesons decay they produce secondary particles, among them a huge number of muons, which can get to the ground.

This continuous rain of muons from the sky can be used to scan through the contents of, say, a container crossing a border checkpoint. Muons penetrate deeply into matter, but as the absorption length changes with the atomic number of the material, they can be used to spot the presence of heavy elements, something that may indicate the use of some radiation shielding, or fissionable material itself. This technique is called “muon tomography”, and there are companies that are already manufacturing cosmic ray detectors with this purpose.


Picture 8

Prof Luis Alvarez (left) with Egyptologist Ahmed Fakhry (center) and Team Leader Jerry Anderson (right) in 1967 (Photo: LBNL)

The idea of muon tomography is actually more than 40 years old, and it was proposed by Nobel laureate physicist Luis W Alvarez in 1965 for a very different purpose: the identification of hidden chambers inside the Egyptian pyramids. The experiment involved installing spark chambers beneath the pyramids and then, by measuring the cosmic ray flux in different directions, it would be possible to determine if in any part of the the pyramid there was less matter than there should be, revealing the presence of a then unknown chamber. The experiment, unfortunately, got null results.

Muon tomography is still being used today as an archaeological tool to explore other pyramids: a group from the University of Texas at Austin is building detectors to explore Mayan pyramids, while another group from Mexico’s National Autonomous University is using this technique to probe the pyramids in Teotihuacan.

Global warming


The CLOUD experiment chamber (Photo CERN)

What are the hottest buzzwords today after “war on terror”? Oh yes, global warming. And here too cosmic rays may be playing a role. Now, I don’t want to start yet another flame war about global warming causes here, but just point out an interesting possibility.

The charged secondary particles produced by cosmic ray interactions in the atmosphere affect cloud formation, and hence may play an important role in the overall radiation balance of the Earth. Physicists behind the CLOUD experiment at CERN are trying to determine how important this effect is by emulating cosmic rays using the historic Proton Synchrotron accelerator. The accelerated particles are then sent into a reaction chamber (see image) where the pressure and temperature conditions of the atmosphere are recreated. The interaction between these particles and the atmosphere inside the chamber is analyzed to study the significance of cosmic radiation in the formation of clouds and its impact on global warming scenarios.


Your computer just froze? Well, wait before blaming it entirely on Bill Gates, as the the origin of the failure could be thousands of light-years away from Earth. Computers store information in RAM memory chips by charging and uncharging billions of tiny capacitors. The interaction of cosmic rays with these capacitors could turn a binary 0 into a 1 or vice versa with unpredictable consequences. According to Intel:

“Cosmic ray induced computer crashes have occurred and are expected to increase with frequency as devices (for example, transistors) decrease in size in chips. This problem is projected to become a major limiter of computer reliability in the next decade. “

This is not a minor concern for them, and they have a US patent for a cosmic ray detector that goes into every chip!

Most of the times, the glitch produced by a cosmic ray event will go unnoticed, but sometimes they may have drastic consequences. You can check with the passengers of a Qantas flight from 2008 for an example. Apparently, the airplane computers experienced a cosmic ray event causing a malfunction that sent the plane into a series of dives that left several passengers injured.

Solar weather

A huge Coronal Mass Ejection event as seen by the SOHO satellite in October of 2003. The dots and white tracks that you see as noise around 0:44s are the results of the interaction between the charged particles from the Sun and the camera on board the spacecraft. Several instruments had to be turned off as a safety measure.

The closest source of cosmic radiation is a good friend of ours: the Sun. Our star constantly sends into space a stream of charged particles known as the “solar wind”, whose intensity fluctuates according to the 11-year-long solar cycle. During a solar “storm”, satellite electronic boards can be efectively fried due to the high flux of charged particles going trough them, and sometimes satellites are turned off to protect them against these events. The consequences? Communications are interrupted and navigation systems such as GPS devices won’t work. To avoid the loss of expensive satellites, a good “space weather” forecast is necessary, and websites like SpaceWeather.com provide updated information about solar activity. As of now, the solar wind is gently blowing at 479 km/second, with a density of 1.7 protons/cm3

In the most violent of these storms, even entire power grids can be affected. The interaction of the solar wind with the Earth’s magnetic field generates rapidly changing magnetic fields that can induce high electric currents in long power lines. Just as an example, in 1989 a geomagnetic storm (which is how these events are called) caused a huge blackout across Quebec.


Cosmic radiation amounts to about 13% of the total radiation that your body receives throughout the year, although it changes according to the height and latitude of the place where you live. In principle, the higher you get, the more radiation you receive, so the most exposed group of people to this kind of radiation is the crew of long-haul flights. Even for these cases, the exposure is not high enough to have specific risks associated with it. In this respect, the World Health Organization doesn’t link any specific health effects associated with the exposure to cosmic radiation, although in an info sheet it advises frequent flyers to limit travels during pregnancy.