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Posts Tagged ‘chemistry’

Grad School in the sciences is a life-changing endeavour, so do not be afraid to ask questions.

Hi Folks,

Quantum Diaries is not just a place to learn the latest news in particle physics; it is also a resource. It is a forum for sharing ideas and experiences.

In science, it is almost always necessary to have a PhD, but what is a PhD? It is a certification that the holder has demonstrated unambiguously her or his ability to thoroughly carry out an independent investigation addressing a well-defined question. Unsurprisingly, the journey to earning a PhD is never light work, but nor should it be. Scientists undertake painstaking work to learn about nature, its underpinnings, and all the wonderful phenomena that occur in everyday life. This journey, however, is also filled with unexpected consequences, disappointment, and sometimes even heartbreak.

It is also that time of year again when people start compiling their CVs, resumes, research statements, and personal statements, that time of year when people begin applying for graduate programs. For this post, I have asked a number of good friends and colleagues, from current graduate students to current post docs, what questions they wished they had asked when apply for graduate school, selecting a school, and selecting a research group.

However, if you are interested in applying to for PhD programs, you should always first yourself,  “Why do I want a research degree like a PhD?”

If you have an experience, question, or thought that you would like to share, comment below! A longer list only provides more information for applicants.

As Always, Happy Colliding

- Richard (@bravelittlemuon)

PS I would like to thank Adam, Amy, John, Josh, Lauren, Mike, Riti, and Sam for their contributions.

Applying to Graduate School:

“When scouting for grad schools, I investigated the top 40 schools in my program of interest.  For chemistry, research primarily occurs in one or two research labs, so for each school, I investigated the faculty list and group research pages.  I eliminated any school where there werre fewer than two faculty members whose fields I could see myself pursuing.  This narrowed down my list to about a dozen schools.  I then filtered based on location: I enjoy being near a big city, so I removed any school in a non-ideal location.  This let me with half a dozen schools, to which I applied.” - Adam Weingarten, Chemistry, Northwestern

“If there is faculty member you are interested in working for, ask both the professor and especially the students separately about the average length of time it takes students to graduate, and how long financial support might be available.” – Lauren Jarocha, Chemistry, UNC

“My university has a pretty small physics program that, presently, only specializes in a few areas. A great deal of the research from my lab happens in conjunction with other local institutes (such as NIST and NIH) or with members of the chemistry or biology departments. If you are interested in a smaller department, ask professors about Institutes and interdisciplinary studies that they might have some connection to, be it within academia or industry.” – Marguerite Brown, Physics, Georgetown

“If you can afford the application fees and the time, apply as broadly as you can.  It’s good to have options when it comes time to make final decisions about where to go. That said, don’t aim too high (you want to make sure you have realistic schools on your list, whatever “realistic” means given your grades and experience), and don’t aim too low (don’t waste time and money applying to a school that you wouldn’t go to even if it was the only school that accepted you, whether because of academics, location, or anything else).  Be as honest as possible with yourself on that front and get input from trusted older students and professors.  On the flip side, if you don’t get rejected from at least one or two schools, you didn’t aim high enough.  You want a blend of reach schools and realistic schools.” – Amy Lowitz, Physics, Wisconsin

Choosing a School

“One of the most common mistakes I see prospective graduate students make is choosing their institution based on wanting to work with a specific professor without getting a clear enough idea of the funding situation in that lab.  Don’t just ask the professor about funding.  Also ask their graduate students when the professor isn’t present.  Even then, you may have to read between the lines; funding can be a delicate subject, especially when it is lacking.” – Amy Lowitz, Physics, Wisconsin

“If you have a particular subfield/group you *know* you are interested in, check how many profs/postdocs/grads are in these groups, check if there are likely to be open slots, and if there are only 1 or 2 open slots make sure you know how to secure one. If they tell you there are currently no open slots, take this to mean that this group is probably closed for everything but the most exceptional circumstances, and do not take into account that group when making your decision.” – Samuel Ducatman, Physics, Wisconsin

“When choosing a school, I based my decision on how happy the grad students seemed, how energetic/curious the faculty appeared, and if the location would allow me to have extracurricular pursuits (such as writing, improv, playing games with people, going to the movies…basically a location where I could live in for 4-6 years).” – Adam Weingarten, Chemistry, Northwestern

“At the visitor weekend, pay attention to how happy the [current] grads seem. Remember they are likely to be primarily 1st years, who generally are the most happy, but still check. Pay attention to the other students visiting, some of them will be in your incoming class. Make sure there is a good social vibe.” – Samuel Ducatman, Physics, Wisconsin

“When I was visiting a prospective grad student, there was a professor at a university I was visiting whose research I was really interested in, but the university would only allow tuition support for 5 years. When I asked his students about graduation rates and times, however, the answer I got was, ‘Anyone who graduates in 5 years hasn’t actually learned anything, it takes at least 7 or 8 years before people should really graduate anyway. Seven years is average for our group.’ In some fields, there is a stigma associated with longer graduation times and a financial burden that you may have to plan for in advance.” – Lauren Jarocha, Chemistry, UNC

Choosing a Group

“When considering a sub-field, look for what interests you of course, but bear in mind that many people change their focus, many don’t know exactly what they want to do immediately upon entering grad school, and your picture of the different areas of research may change over time. Ask around among your contemporaries and older students, especially when it comes to particular advisers.” – Joshua Sayre, PhD, Physics, Pittsburgh
“If you know that you’re interested in an academic career that is more teaching oriented or research oriented, ask about teaching or grant writing opportunities, respectively. I know plenty of fellow students who didn’t start asking about teaching opportunities their 4th or 5th year of their program, and often by then it was too late. If you know that finding funding will be a big part of your future, joining a group where the students take an active part in writing grants and grant renewals is invaluable experience.” -  Lauren Jarocha, Chemistry, UNC
“For choosing groups, I attended group and subgroup meetings, met with faculty to discuss research and ideas, and read several recent publications from each group of interest.  What I did not do (and wish I had) was talk with the graduate students, see how they and the group operated.  For example, I am very motivated and curious to try new ideas, so in my current research group my PI plays a minimal role in my life.  The most important aspect is how well one’s working style fits with the group mentality, followed by research interest.  There’s a ton of cool, exciting research going on, but finding a group with fun, happy, motivated people will make or break the PhD experience.” – Adam Weingarten, Chemistry, Northwestern
“I went into [Condensed Matter Theory] and not [X] because (1) In the summer of my first year I had no research, and I came close to having no income because of this. I realized I needed someone who could promise me research/funding and real advising. The [X] group was pretty filled up (and there were some politics), so it was impossible to get more than this. (2) I thought the professors in CMT treated me with more respect then the [X] profs I talked to.” - John Doe, Physics
“I believe that choosing which grad schools to apply to should primarily be about the research, so this question is more for after you’ve (hopefully) been accepted to a couple schools.  If you are going into theoretical physics, and if you don’t have some sort of fellowship from them or an outside agency, ask them how much their theory students [teach].  Do they have to TA every semester for their funding?  Do they at least get summers off?  Or do they only have to TA for the first one or two years?  This shouldn’t be the primary factor in deciding where to go – research always is – but it’s not something that should be ignored completely.  Teaching is usually somewhat rewarding in my experience, but it adds absolutely no benefit to your career if you are focused on a professorship at a research university.  Every hour you spend steaching is an hour someone else is researching and you aren’t.  And 10-20 hours a week of teaching adds up.” – Michael Saelim, Physics, Cornell
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Hi All,

Exciting news came out the Japanese physics lab KEK (@KEK_jp, @KEK_en) last week about some pretty exotic combinations of quarks and anti-quarks. And yes, “exotic” is the new “tantalizing.” At any rate, I generally like assuming that people do not know much about hadrons so here is a quick explanation of what they are. On the other hand, click to jump pass “Hadrons 101″ and straight to the news.

Hadrons 101: Meeting the Folks: The Baryons & Mesons

Hadrons are pretty cool stuff and are magnitudes more quirky than those quarky quarks. The two most famous hadrons, the name for any stable combination of quarks and anti-quarks, are undoubtedly the proton and the neutron:

According to our best description of hadrons (Quantum Chromodynamics), the proton is effectively* made up two up-type quarks, each with an electric charge of +2/3 elementary charges**; one down-type quark, which has an electric charge of -1/3 elementary charges; and all three quarks are held together by gluons, which are electrically neutral. Similarly, the neutron is effectively composed of two down-type quarks, one up-type quark, and all the quarks are held strongly together by gluons. Specifically, any combination of three quarks or anti-quarks is called a baryon. Now just toss an electron around the proton and you have hydrogen, the most abundant element in the Universe! Bringing together two protons, two neutrons, and two electrons makes helium. As they say, the rest is Chemistry.

However, as the name implies, baryons are not the only type of hadrons in town. There also exists mesons, combinations of exactly one quark and one anti-quark. As an example, we have the pions (pronounced: pie-ons). The π+ (pronounced: pie-plus) has an electric charge of +1 elementary charges, and consists of an up-type quark & an anti-down-type quark. Its anti-particle partner, the π- (pronounced: pie-minus), has a charge of -1, and is made up of an anti-up-type quark & a down-type quark.

 

If we now include heavier quarks, like strange-type quarks and bottom-type quarks, then we can construct all kinds of baryons, mesons, anti-baryons, and anti-mesons. Interactive lists of all known mesons and all known baryons are available from the Particle Data Group (PDG)***. That is it. There is nothing more to know about hadrons, nor has there been any recent discovery of additional types of hadrons. Thanks for reading and have a great day!

 

* By “effectively,” I mean to ignore and gloss over the fact that there are tons more things in a proton, like photons and heavier quarks, but their aggregate influences cancel out.

** Here, an elementary charge is the magnitude of an electron’s electron charge. In other words, the electric charge of an electron is (-1) elementary charges (that is, “negative one elementary charges”). Sometimes an elementary charge is defined as the electric charge of a proton, but that is entirely tautological for our present purpose.

*** If you are unfamiliar with the PDG, it is arguably the most useful site to high energy physicists aside from CERN’s ROOT user guides and Wikipedia’s Standard Model articles.

The News: That’s Belle with an e

So KEK operates a super-high intensity electron-positron collider in order to study super-rare physics phenomena. It’s kind of super. Well, guess what. While analyzing collisions with the Belle detector experiment, researchers discovered the existence of two new hadrons, each made of four quarks! That’s right, count them: 1, 2, 3, 4 quarks! In each case, one of the four quarks is a bottom-type quark and another is an anti-bottom quark. (Cool bottom-quark stuff.) The remaining two quarks are believed to be an up-type quark and an anti-down type quark.

The two exotic hadrons have been named Zb(10610) and Zb(10650). Here, the “Z” implies that our hadrons are “exotic,” i.e., not a baryon or meson, the subscript “b” indicates that it contains a bottom-quark, and the 10610/10650 tell us that our hadrons weigh 10,610 MeV/c2 and 10,650 MeV/c2, respectively. A proton’s mass is about 938 MeV/c2, so both hadrons are about 11 times heavier than the proton (that is pretty heavy). The Belle Collaboration presser is really great, so I will not add much more.

Other Exotic Hadrons: When Barry met Sally.

For those keeping track, the Belle Collaboration’s recent finding of two new 4-quark hadrons makes it the twelfth-or-so “tetra-quark” discovery. What makes this so special, however, is that all previous tetra-quarks have been limited to include a charm-type quark and an anti-charm-type quark. This is definitely the first case to include bottom-type quarks, and therefore offer more evidence that the formation of such states is not a unique property of particularly charming quarks but rather a naturally occurring phenomenon affecting all quarks.

Furthermore, it suggests the possibility of 5-quark hadrons, called penta-quarks. Now these things take the cake. They are a sort of grand link between elementary particle physics and nuclear physics. To be exact, we know 6-quark systems exist: it is called deuterium, a radioactive stable isotope of hydrogen (Thanks to @incognitoman for pointing out that deuterium is, in fact, stable.). 9-quark systems definitely exist too, e.g., He-3 and tritium. Etc. You get the idea. Discovering the existence of five-quark hadrons empirically establishes a very elegant and fundamental principle: That in order to produce a new nuclear isotope, so long as all Standard Model symmetries are conserved, one must simply tack on quarks and anti-quarks. Surprisingly straightforward, right? Though sadly, history is not on the side of 5-quark systems.

Now go discuss and ask questions! :)

Run-of-the-mill hadrons that are common to everyday interactions involving the Strong Nuclear Force (QCD) are colloquially called “standard hadrons.” They include mesons (quark-anti-quark pairs) and baryons (three-quark/anti-quark combinations). Quark combinations consisting of more than three quarks are called “exotic hadrons.”

 

 

 

 

Happy Colliding.

- richard (@bravelittlemuon)

 

PS, I am always happy to write about topics upon request. You know, QED, QCD, OED, etc.

http://en.wikipedia.org/wiki/Neutron
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Update I: Included Medicine Award (Oct 03)

Update II: Included Physics Award (Oct 04)

… it’s Nobel Week! October means three things: Halloween (duh), Fall, and Nobel Week, the week during which the famed prizes are awarded to those who have “conferred the greatest benefit on mankind” [1]. Okay, before I get comments about the subjectivity of those who award the prizes, I gladly admit that the history of the prize is not without controversy relating to those who have & have not won, in both the science and non-science categories.

I am just going to ignore all of that and talk about why everyone should be excited about this week. Though before I talk about this week’s Nobels, I feel I should probably give the SparkNotes version of the prizes’ history.

Figure 1: The 2008 Chemistry Prize was awarded for the discovery and development of green fluorescent protein (GFP), which when inserted into a soon-to-be parent is passed onto an offspring who can then glow green. Glowing cat!
(Image: The Nobel Foundation)

[1] http://www.nobelprize.org/alfred_nobel/will/will-full.html

A Brief History of Alfred Nobel

Figure 2: Alfred Nobel. (Image: The Nobel Foundation)

The year is 1866, the Second Industrial Revolution is raging, innovation is surging, and the US Civil War over.

Insert Alfred Nobel: A son of a successful engineer who developed controlled explosives for the demolition and mining industries. The younger Nobel, unsurprisingly, decided be a chemist after playing with nitroglycerin in a French laboratory. As a public service announcement, I should probably mention that nitroglycerin is very dangerous and is a principle ingredient in dynamite. In fact, Nobel was so convinced that nitroglycerine had useful application in construction that he decided to invent dynamite. Needless to say, dynamite made Nobel a very, very, very rich man. At the end of his life, he decided to endow, with the bulk of his fortune, a set of prizes to recognize those who have contributed greatest in the Fields of Medicine, Physics, Chemistry, Literature, and Peace. Economics, though not stipulated in the original will, was added later and is funded separately.

Figure 2: The chemical structure of nitroglycerin. This stuff is wicked; the physical chemistry behind its structure worth a gander. Consider this an advertisement to go earn a chemistry degree. (Image: Wikipedia)

What Makes a Prize

The Nobels has come a long way since they were first instituted. Most notably, they no longer are awarded for the greatest discovery or invention from the past year; the prizes now award those results with the most lasting influence and impact. Take last year for example. The 2011 award for Physiology or Medicine went solely to Sir Robert Edwards for having developed in vitro fertilization. You would think something that is, in every sense of the word, responsible for the existence of millions of people would have been awarded long, long ago. I mean, that is what went through my mind last October. Therein lies the novelty of the Nobel Prizes: These days, the awards are given to what seem like common knowledge, because in some sense they are. What one has to realize though is that prior a laureate’s discovery or invention, these ideas and concepts just did not exist. Imagine a world in which no one knew of insulin (Nobel 1923). Weird, no?

This brings me to why Nobel Week is so much fun. Sometimes you know quite a bit about the award-winning discovery and so you get to spend the day reading news articles and science blogs learning all about the topic’s history. Werner Forssmann’s invention of the cardiac catheter (Nobel 1953) has a hysterical history that is well worth a read. At other times, you have no idea what the award citation even means, but you just know it is worth spending a few minutes or even a few hours learning. I mean, why else would a Nobel be awarded? Take, as another example, 2008′s Physics prize. The award citation reads:

“… for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics,” [2]

and

“for the discovery of the origin of the broken symmetry
which predicts the existence of at least three families of quarks in nature
.” [2]

Yup, it is a mouthful and probably seems a bit obtuse. That is, until you start looking up Wikipedia or news articles (or Quantum Diaries!), and realize how amazingly awesome these discoveries are. I mean, sure discovering spontaneous symmetry breaking (SSB) sounds nice and fancy but did you know that is why the bosons in the Standard Model of Physics have the masses they do?!? SSB, when applied specifically to the Electroweak bosons (photon, W, & Z) is the Higgs Mechanism, and when applied to fermions, is what generates the higgs boson. SSB is an established scientific fact and is also the driving force behind superconductivity (Nobel 1972) Whether or not the higgs boson exists, however, is completely different story.

Figure 3: The quark sector of the Standard Model of Particle Physics and their discovery dates. (Image: Nobel Foundation)

So back in 1977 a Fermilab team, led by Leon Lederman, discovered the bottom quark (Nobel 1988), and in 1995, the CDF & DZero Tevatron experiments discovered the top quark. Ever wonder how we knew to look for them in the first place? It was because of something called the CKM matrix. It was introduced as a way of organizing the the different ways particles in the Standard Model could interact and decay. However, as gorgeous as this new organization was, in order to work the CKM matrix required the existence of two new quarks. Well guess what, Fermilab found those two quarks and set the Standard Model in stone.

The 2009 Nobel Prizes are equally impressive. Half the prize was awarded for the development of fiber optics, which is the foundation of modern telecommunications, and something called Charged-Coupled Devices (CCD). What took me a few hours to learn is that if you take this sensor, attach a flashbulb, a battery, and maybe a memory card, you get a digital camera. In other words, half the 2009 prize was awarded for inventing the digital camera. The prize winners were simply trying to develop a better way of storing data and inadvertently created an entire industry. A fun fact: the first transistor (Nobel 1967) was made of paperclips. If you are curious about what makes transistors so important, take apart your computer and take a peek. (Please, make sure the computer is unplugged before opening it.)

[2] http://www.nobelprize.org/nobel_prizes/physics/laureates/2008/

Does Every Major Discovery/Invention Get a Prize?

No. First off, Nobel Prizes are no longer awarded posthumously. Secondly, from my discussions about this issue, there seems to be a consensus there may be a limit to what is & is not awarded when it comes to the sciences. Now the Swedish Academies always reserve the right to set a new precedent, however, it is unlikely that any organizations will be awarded a Nobel in science categories anytime soon. (This is the complete opposite for the Peace Prize, of course.) What does this all mean? Well, the top quark was a pretty heavy discovery and is well worth its weight in gold, at least in my opinion. However, to whom would you award the prize? No single person at the CDF experiment can justly say she or he discovered the quark; it was a team effort and all CDF personnel can proudly state she or he helped discover the quark.

“Which of the Gang of Six, if the higgs boson is discovered, should get the Nobel, if at all?” is an honest, open question and is well above my pay grade. A similar statement could be made about Supersymmetry.

Turning Nobel Week into Fun-bel Week

Now for the fun part. So during this week, pick your favorite subject, which of course is physics, and go figure out what the whole big hubbub is. Depending on your timezone, this may either be with your morning coffee or afternoon tea. In any case, it is an excuse to learn something new! :)

Alternatively, you can check back here Tuesday afternoon (Madison/Chicago time) because I am sure many of us will be commenting on the latest news.

This Week’s Schedule

Live Video Player here.

Physiology or Medicine – Awarded for the discovery of the innate and adaptive immune systems! Okay, really this is great. The human body has evolved to be inherently immune to certain pathogens. The human body, in its resourcefulness, can also adapt and become immune to pathogens. The end result is that when the two are combined and wait a few hundred thousand years,  you get us!

Physics – Awarded for discovering that expansion rate of the universe, is itself increasing. The universe expands, Edwin Hubble discovered that decades ago. Today’s award winners discovered that the universe expands at an accelerating rate! Bravo!

Chemistry – The prize will be announced on Wednesday 5 October, 11:45 a.m. CET [5:45 am  CDT/Chicago].

Peace – The prize will be announced on Friday 7 October, 11:00 a.m. CET [5:00 am  CDT/Chicago].

Economics – The prize will be announced on Monday 10 October, 1:00 p.m. CET [7:00 am  CDT/Chicago].

Literature – To Be Announced

 

 

 

 

Regardless of the outcome, I would love to read everyone’s thoughts and speculations before and after the awards!

Happy Colliding

- richard (@bravelittlemuon)

 

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Update: I accidentally miscalculated the decay rate of K40 in a banana. There are 12 decays, per second, per banana, not 18.

Wimps, they are everywhere! They pervade the Universe to its furthest reaches; they help make this little galaxy of ours spin right round like a record (we think); and they can even be found with all the fruit in your local grocery store.

Figure 1: ( L) Two colliding galaxies galaxy clusters (Image: NASA’s Chandra X-Ray Observatory). (R) Bananas, what else? (Image: Google)

WIMPs: Weakly-Interactive Massive Particles, is an all-encompassing term used to describe any particle that has (1) mass, and (2) is unlikely to interactive with other particles. This term is amazing; it describes particles we know exist and is a generic, blanket-term that adequately describes many hypothetical particles.

Neutrinos: The Prototypical WIMP

Back in 1930, there was a bit of a crisis in the freshly established field of particle physics. The primary mechanism that mediates most nuclear reactions, known as β-decay (beta-decay), violated (at the time) one of the great pillars of experimental physics: The Law of Conservation of Energy. This law says that energy can NEVER be created or destroyed, ever. Period. Sure, energy can be converted from one type, like vibrational energy, to another type, like heat, but it can never just magically (dis)appear.

Figure 2: In β-decay, before 1930, neutrons were (erroneously) believed to decay into a high speed electron (β-) and a proton (p+).

Before 1930, physicists thought that when an atom’s nucleus decayed via β-decay a very energetic electron (at the time called a β particle) would be emitted from the nucleus. From the Conservation of Energy, the energy of an electron is exactly predicted. The experimental result was pretty much as far off from the prediction as possible and implied the terrifying notion that perhaps energy was not conserved for Quantum Mechanics. Then, in 1930, the Nobel Prize-Winning physicist Wolfgang Pauli noticed that the experimental measurements of β-decay looked a bit like what one would expect if instead of one particle being emitted by a radioactive nucleus, two particles were emitted.

Prof. Pauli thought the idea of a radioactive nucleus emitting two particles, one visible (the electron) and one invisible, was horrible, silly, and unprofessional. Consequentially, he decided to pen a letter to the physics community suggesting there existed such a particle. :) Using this idea and what could only be described as a level of intuition beyond that of genius, Nobel Laureate Enrico Fermi suggested that perhaps nuclear decay was actually the manifestation of a new, weak force and aptly named it the Weak Nuclear Force (note the capitalization).

To recap: 1 hypothetical particle mediated by 1 hypothetical force.

Figure 3: Prof. Pauli proposed that β-decay actually included an electrically neutral particle with little mass (χ0), in addition to the final-state electron (β-) & proton (p+). This once-hypothetical particle is now known as the anti-neutrino (ν).

30 years later, in 1962, Prof. Pauli’s invisible particles (by then called neutrinos) were discovered; 20 years after that, the Weak Force was definitively confirmed; and after another 20 years, neutrinos were found to have mass.

Since 1930, hundreds of theories have invoked the existence of new particles that (1) have mass, and (2) interact weakly (note lack of capitalization) with other particles, which may/may not involve the Weak Nuclear Force (note capitalization, again). At some point in the 1980s, it was finally decided to coin a generic term that described these particles from other large classes of particles that are, say, massless or readily interact with other particles, e.g., with photons or gluons.

Dark Matter: The Elephant in the Galaxy

Kepler’s Laws of Motion & General Relativity are phenomenal at predicting the orbits of planets and solar systems around immense sources of gravity, like stars & black holes. However, there are two known astronomical observations where our predictions do not readily match the experimental results.

The first has to do with how our galaxy spins like a top. Theoretically, the more distant you are from a galaxy’s center, the slower you orbit around the center; vice versa, the closer you are to the center of the galaxy’s center, the faster you orbit around it. Experimentally, astronomers have found that after a certain distance from the galaxy’s center an object’s speed becomes roughly constant. In other words, if Earth were half as close to the galactic center as it is now, its speed will not have appreciably changed. See figure 4 (below) for nice little graph that compares what is observed (solid line) and what is predicted (dotted line). Furthermore, this is not just our galaxy; this is common to all galaxies. Weird, right?

Figure 4: (A) The theoretical prediction of how fast an object travels (velocity) around the galactic center, as a function of (radial) distance from the center. (B) The experimental observation. (Image: Penn State)

The second disagreement between theory and experiment comes from watching galaxies collide with one another. Yes, I literally mean watching galaxies collide into one another (and you thought the LHC was wicked). This is how it looks:

Figure 5: Chandra X-Ray Image of two galaxies galaxy clusters colliding. The pink regions represent the visible portions of the galaxies; the blue regions represent the invisible (dark matter) portions, as calculated from gravitational lensing. (Image: NASA)

Astronomers & astrophysicists can usually determine how massive galaxies & stars are by how bright they are; however, the mass can also be determined by a phenomenon called gravitational lensing (a triumph of General Relativity). When NASA’s Chandra X-Ray telescope took this little snapshot of two galaxies (pink) passing right through each other it was discovered, rather surprisingly, that the mass deduced from the brightness of the galaxies was only a fraction of the mass deduced from gravitational lensings (blue). You can think of this as physically feeling more matter than what can visibly be seen.

What is fascinating is that these problems (of cosmic proportion) wonderfully disappear if there exists in the universe a very stable (read: does not decay), massive, weakly-interacting particle. Sounds familiar? It better because this type of WIMP is commonly known as Dark Matter! Normally, if a theory does not work, then it is just thrown out. What makes General Relativity different is that we know it works; it has made a whole slew of correct predictions that are pretty unique. Predicting the precession of the perihelion of the planet Mercury is not as easy as it sounds. I am probably a bit biased but personally I think it is a very simple solution to two “non-trivial” problems.

Bananas: A Daily Source of K-40

Since I bought a bunch of bananas this morning, I thought I would add a WIMP-related fact about bananas. Like I mentioned earlier, β-decay occurs when a proton neutron decays into a neutron proton by emitting an electron and an anti-neutrino. From a particle physics perspective, this occurs when a down-type quark emits a W- boson (via the Weak Force) and becomes an up-type quark. The W- boson, which by our definition is a WIMP itself, then decays into an electron (e-) and an anti-neutrino (ν – a WIMP). This is how a neutron, which has two down-type & one up-type quark, becomes a proton, which has one-down type & two up-type quarks.

Figure 6: The fully understood mechanism of β-decay in which a neutron (n0) can decay into a proton (p+) when a d-type quark (d) in a neutron emits a W- boson (W-) and becomes an u-type quark (u). The W- boson consequentially decays into an electron (e-) and an anti-neutrino (νe).

This type of nuclear transmutation often occurs when a light atom, like potassium (K), has too many neutrons. Potassium-40, which has 19 protons & 21 neutrons, makes up about 0.01% of all naturally forming potassium. Bananas are an exceptionally great source of this vital element, about 450 mg worth, and consequentially have about 45 μg (or ~6.8·1017 atoms) of the radioactive K-40 isotope. This translates to roughly 18 12 nuclear decays (or 18 12 neutrinos), per second, per banana. Considering humans and bananas have coexisted for quite a while in peaceful harmony, minus the whole humans-eat-banans thing, it is my professional opinion that bananas are perfectly safe. :)

Dark Matter Detection: CRESST

Okay, I have to be honest: I have a secret agenda in writing about WIMPs. The Cryogenic Rare Event Search with Superconducting Thermometers (CRESST) Experiment Collaboration will be announcing some, uh… interesting results at a press conference tomorrow, as a part of the Topics in Astroparticle & Underground Physics Conference (TAUP 2011). I have no idea what will be said or shown aside from this press release that states the “latest results from the CRESST Experiment provide an indication of dark matter.”

 

With that, I bid you adieu & Happy Colliding.

- richard (@bravelittlemuon)

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