The PhD defenseThe defense: perhaps the most stressful, intimidating right of passage in the PhD process. It’s the culmination of years of work, all wrapped up in a single meeting. This week, Quantum Diarist Laura Gladstone gives us an overview of what it was like to finish her PhD. It’s something that quite a few of our diarists have survived!
It’s been a crazy 3 weeks since I officially finished my PhD. I’m in the transition from being a grad student slowly approaching insanity to a postdoc who has everything figured out, and it’s a rocky transition.
As of last Tuesday I am officially a PhD candidate… which doesn’t really change much, but it means that my department “officially” acknowledges that I’m on track for a doctorate degree some time in the future.
As a new US LHC blogger, I thought I would spend this first post talking a little about my background and how I decided what kind of postdoc position to look for. Lately, quite a few of my friends in their last year of graduate school have been asking about the latter.
Or, how do we mathematically describe the interaction of particles?
In my previous post, I addressed some questions concerning the nature of the wavefunction, the most truthful mathematical representation of a particle. Now let us make this simple idea more complete, getting closer to the deep mathematical structure of particle physics. This post is a bit more “mathematical” than the last, and will likely make the most sense to those who have taken a calculus course. But if you bear with me, you may also come to discover that this makes particle interactions even more attractive!
The field theory approach considers wavefunctions as fields. In the same way as the temperature field \(T(x,t)\) gives the value of the temperature in a room at space \(x\) and time \(t\), the wavefunction \(\phi (x,t)\) quantifies the probability of presence of a particle at space point \(x\) and time \(t\).
Cool! But if this sounds too abstract to you, then you should remember what Max Planck said concerning the rise of quantum physics: “The increasing distance between the image of the physical world and our common-sense perception of it simply indicates that we are gradually getting closer to reality”.
Almost all current studies in particle physics focus on interactions and decays of particles. How does the concept of interaction fit into the mathematical scheme?
The mother of all the properties of particles is called the Lagrangian function. Through this object a lot of properties of the theory can be computed. Here let’s consider the Lagrangian function for a complex scalar field without mass (one of the simplest available), representing particles with electric charge and no spin:
\(L(x) = \partial_\mu \phi(x)^* \partial^\mu \phi(x) \).
Mmm… Is it just a bunch of derivatives of fields? Not really. What do we mean when we read \(\phi(x)\)? Mathematically, we are considering \(\phi\) as a vector living in a vector space “attached” to the space-time point \(x\). For the nerds of geometry, we are dealing with fiber bundles, structures that can be represented pictorially in this way:
The important consequence is that, if \(x\) and \(y\) are two different space-time points, a field \(\phi(x)\) lives in a different vector space (fiber) with respect to \(\phi(y)\)! For this reason, we are not allowed to perform operations with them, like taking their sum or difference (it’s like comparing a pear with an apple… either sum two apples or two pears, please). This feature is highly non-trivial, because it changes the way we need to think about derivatives.
In the \(L\) function we have terms containing derivatives of the field \(\phi(x)\). Doing this, we are actually taking the difference of the value of the field at two different space-time points. But … we just outlined that we are not allowed to do it! How can we solve this issue?
If we want to compare fields pertaining to the same vector space, we need to slightly modify the notion of derivative introducing the covariant derivative \(D\):
\( D_\mu = \partial_\mu + ig A_\mu(x) \).
Here, on top of the derivative \(\partial\), there is the action of the “connection” \(A(x)\), a structure which takes care of “moving” all the fields in the same vector space, and eventually allows us to compare apples with apples and pears with pears.
So, a better way to write down the Lagrangian function is:
\(L(x) = D_\mu \phi(x)^* D^\mu \phi(x) \).
If we expand \(D\) in terms of the derivative and the connection, \(L\) reads:
\(L(x) = \partial_\mu \phi(x)^* \partial^\mu \phi(x) +ig A_\mu (\partial^\mu \phi^* \phi – \phi^* \partial^\mu \phi) + g^2 A^2 \phi^* \phi \).
Do you recognize the role of these three terms? The first one represents the propagation of the field \(\phi\). The last two are responsible for the interactions between the fields \(\phi, \phi^*\) and the \(A\) field, referred to as the “photon” in this context.
This slightly hand-waving argument involving fields and space-time is a simple handle to understand how the interactions among particles emerge as a geometric feature of the theory.
If we consider more sophisticated fields with spin and color charges, the argument doesn’t change. We need to consider a more refined “connection” \(A\), and we could see the physical interactions among quarks and gluons (namely QCD, Quantum Chromo Dynamics) emerging just from the mathematics.
Probably the professor of geometry in my undergrad course would call this explanation “Spaghetti Mathematics”, but I think it can give you a flavor of the mathematical subtleties involved in the theory of particle physics.
It’s been a crazy 3 weeks since I officially finished my PhD. I’m in the transition from being a grad student slowly approaching insanity to a postdoc who has everything figured out, and it’s a rocky transition.
The end of the PhD at Wisconsin has two steps. The first is the defense, which is a formal presentation of my research to the professors and committee, our colleagues, and very few friends and family. The second is actually turning the completed dissertation to the grad school, with the accompanying “margin check” appointment with the grad school. In between, the professors can send me comments about the thesis. I’ve heard so many stories of different universities setting up the end of a degree differently, it’s pretty much not worth going into the details. If you or someone you know is going through this process, you don’t need a comparison of how it works at different schools, you just need a lot of support and coping mechanisms. All the coping mechanisms you can think of, you need them. It’s ok, it’s a limited time, don’t feel guilty, just get through it. There is an end, and you will reach it.
The days surrounding the defense were planned out fairly carefully, including a practice talk with my colleagues, again with my parents (who visited for the defense), and delivery burritos. I ordered coffee and doughnuts for the defense from the places where you get those, and I realized why such an important day has such a surprisingly small variety of foods: because deviating from the traditional food is so very far down my list of priorities when there’s the physics to think about, and the committee, and the writing. The doughnuts just aren’t worth messing with. Plus, the traditional place to get doughnuts is already really good.
We even upheld a tradition the night before the defense. It’s not really a tradition per se, but I’ve seen it once and performed it once, so that makes it a tradition. If you find it useful, you can call it an even stronger tradition! We played an entire soundtrack and sung along, with laptops open working on defense slides. When my friend was defending, we watched “Chicago” the musical, and I was a little hoarse the next day. When I was defending, we listened to Leonard Bernstein’s version of Voltaire’s “Candide,” which has some wonderful wordplay and beautiful writing for choruses. The closing message was the comforting thought that it’s not going to be perfect, but life will go on.
“We’re neither wise nor pure nor good, we’ll do the best we know. We’ll build our house, and chop our wood, and make our garden grow.”
Hearing that at the apex of thesis stress, I think it will always make me cry. By contrast, there’s also a scene in Candide depicting the absurd juxtaposition of a fun-filled fair centered around a religious inquisition and hanging. Every time someone said they were looking forward to seeing my defense, I thought of this hanging-festival scene. I wonder if Pangloss had to provide his own doughnuts.
The defense itself went about as I expected it would. The arguments I presented had been polished over the last year, the slides over the last couple weeks, and the wording over a few days. My outfit was chosen well in advance to be comfortable, professional, and otherwise unremarkable (and keep my hair out my way). The seminar itself was scheduled for the time when we usually have lab group meetings, so the audience was the regular lab group albeit with a higher attendance-efficiency factor. The committee members were all present, even though one had to switch to a 6am flight into Madison to avoid impending flight cancellations. The questions from the committee mostly focused on understanding the implications of my results for other IceCube results, which I took to mean that my own work was presented well enough to not need further explanation.
It surprised me, in retrospect, how quickly the whole process went. The preparation took so long, but the defense itself went so quickly. From watching other people’s defenses, I knew to expect a few key moments: an introduction from my advisor, handshakes from many people at the end of the public session, the moment of walking out from the closed session to friends waiting in the hallway, and finally the first committee member coming out smiling to tell me they decided to pass me. I knew to look for these moments, and they went by so much faster in my own defense than I remember from my friends. Even though it went by so quickly, it still makes a difference having friends waiting in the hallway.
People asked me if it was a weight off my shoulders when I finally defended my thesis. It was, in a way, but even more it felt like cement shoes off my feet. Towards the end of the process, for the last year or so, a central part of myself felt professionally qualified, happy, and competent. I tried desperately to make that the main part. But until the PhD was finished, that part wasn’t the exterior truth. When I finished, I felt like the qualifications I had on paper matched how qualified I felt about myself. I’m still not an expert on many things, but I do know the dirty details of IceCube software and programing. I have my little corner of expertise, and no one can take that away. Degrees are different from job qualifications that way: if you stop working towards a PhD several years in, it doesn’t count as a fractional part of a degree; it’s just quitting. But if you work at almost any other job for a few years, you can more or less call it a few years of experience. A month before my defense, part of me knew I was so so so close to being done, but that didn’t mean I could take a break.
And now, I can take a break.
This article appeared in Fermilab Today on Nov. 24, 2014.
On Nov. 21, for the third year in a row, the Fermilab Lecture Series invited five scientists to battle it out in an event called a physics slam. And for the third year in a row, the slam proved wildly popular, selling out Ramsey Auditorium more than a month in advance.
More than 800 people braved the cold to watch this year’s contest, in which the participants took on large and intricate concepts such as dark energy, exploding supernovae, neutrino detection and the overwhelming tide of big data. Each scientist was given 10 minutes to discuss a chosen topic in the most engaging and entertaining way possible, with the winner decided by audience applause.
Michael Hildreth of the University of Notre Dame kicked things off by humorously illustrating the importance of preserving data — not just the results of experiments, but the processes used to obtain those results. Marcelle Soares-Santos of the Fermilab Center for Particle Astrophysics took the stage dressed as the Doctor from “Doctor Who,” complete with a sonic screwdriver and a model TARDIS, to explore the effects of dark energy through time.
Joseph Zennamo of the University of Chicago brought the audience along on a high-energy journey through the “Weird and Wonderful World of Neutrinos,” as his talk was called. And Vic Gehman of Los Alamos National Laboratory blew minds with a presentation about supernova bursts and the creation of everything and everyone in the universe.
The winner was Fermilab’s Wes Ketchum, a member of the MicroBooNE collaboration. Ketchum’s work-intensive presentation used claymation to show how different particles interact inside a liquid-argon particle detector, depicting them as multicolored monsters bumping into one another and creating electrons for the detector’s sensors to pick up. Audience members won’t soon forget the sight of a large oxygen monster eating red-blob electrons.
After the slam, the five scientists took questions from the audience, including one about dark matter and neutrinos from an eight-year-old boy, sparking much discussion. Chris Miller, speech professor at the College of DuPage, made his third appearance as master of ceremonies for the Physics Slam, and thanked the audience — particularly the younger attendees — for making the trek to Fermilab on a Friday night to learn more about science.
Video of this year’s Physics Slam is available on Fermilab’s YouTube channel.
This article appeared in Fermilab Today on Nov. 18, 2014.
In late October, the American Physical Society Division of Particles and Fields announced that Stanford University professor emeritus of physics and Fermilab collaborator Stanley Wojcicki has been selected as the 2015 recipient of the W.K.H. Panofsky Prize in experimental particle physics. Panofsky, who died in 2007, was SLAC National Accelerator Laboratory’s first director, holding that position from 1961 to 1984.
“I knew Pief Panovsky for about 40 years, and I think he was a great man not only as a scientist, but also as a statesman and as a human being,” said Wojcicki, referring to Panofsky by his nickname. “So it doubles my pleasure and satisfaction in receiving an award that bears his name.”
Wojcicki was given the prestigious award “for his leadership and innovative contributions to experiments probing the flavor structure of quarks and leptons, in particular for his seminal role in the success of the MINOS long-baseline neutrino experiment.”
Wojcicki is a founding member of MINOS. He served as spokesperson from 1999 to 2004 and as co-spokesperson from 2004 to 2010.
“I feel a little embarrassed being singled out because, in high-energy physics, there is always a large number of individuals who have contributed and are absolutely essential to the success of the experiment,” he said. “This is certainly true of MINOS, where we had and have a number of excellent people.”
Wojcicki recalls the leadership of Caltech physicist Doug Michael, former MINOS co-spokesperson, who died in 2005.
“I always regret that Doug did not have a chance to see the results of an experiment that he very much contributed to,” Wojcicki said.
In 2006, MINOS measured an important parameter related to the mass difference between two neutrino types.
Fermilab physicist Doug Glenzinski chaired the Panofsky Prize review committee and says that the committee was impressed by Wojcicki’s work on flavor physics, which focuses on how particles change from one type to another, and his numerous contributions over decades of research.
“He is largely credited with making MINOS happen, with thinking about ways to advance neutrino measurements and with playing an active role in all aspects of the experiment from start to finish,” Glenzinski said.
More than 30 years ago, Wojcicki collaborated on charm quark research at Fermilab, later joining Fermilab’s neutrino explorations. Early on Wojcicki served on the Fermilab Users Executive Committee from 1969-71 and on the Program Advisory Committee from 1972-74. He has since been on many important committees, including serving as chair of the High-Energy Physics Advisory Panel for six years and as member of the P5 committee from 2005-08. He now continues his involvement in neutrino physics, participating in the NOvA and MINOS+ experiments.
“I feel really fortunate to have been connected with Fermilab since its inception,” Wojcicki said. “I think Fermilab is a great lab, and I hope it will continue as such for many years to come.”
Hanging around a pool table might seem like an odd place to learn physics, but a couple of hours on our department’s slanted table could teach you a few things about asymmetry. The third time a pool ball flew off the table and hit the far wall I knew something was broken. The pool table’s refusal to obey the laws of physics gives aspiring physicists a healthy distrust of the simplified mechanics they learnt in undergrad. Whether in explaining why pool balls bounce sideways off lumpy cushions or why galaxies exist, asymmetries are vital to understanding the world around us. Looking at dark matter theories that interact asymmetrically with visible matter can give us new clues as to why matter exists.
Alternatives to the classic WIMP (weakly interacting massive particles) dark matter scenario are becoming increasingly important. Natural supersymmetry is looking less and less likely, and could be ruled out in 2015 by the Large Hadron Collider. Asymmetric dark matter theories provide new avenues to search for dark matter and help explain where the material in our universe comes from -baryogenesis. Baryogenesis is in some ways a more important cosmological problem than dark matter. The Standard Model of particle physics describes all the matter that you are familiar with, from trees to stars, but fails to explain how this matter came to be. In fact, the Standard Model predicts a sparsely populated universe, where most of the matter and antimatter has long since annihilated each another. In particle colliders, whenever a particle of matter is created, an opposing particle of antimatter is also created. Antimatter is matter with all its charges reversed, like a photo negative. While it is often said that opposites attract, in the particle physics world opposites annihilate. But when we look at the universe around us, all we see is matter. There are no antistars and antiplanets, no antihumans living on some distant world. So if matter and antimatter are always created together, how did this happen? If there were equal amounts of matter and antimatter, each would annihilate the other in the first fractions of a second and our universe would be stillborn. The creation of this asymmetry between matter and antimatter is known as baryogenesis, and is one of the strongest cosmological confirmations of physics beyond the Standard Model. The exact amount of asymmetry determines how much matter, and consequently how many stars and galaxies, exists now.
And what about the other 85% of matter in the universe? This dark matter has only shown itself through gravitational interactions, but it has shaped the evolution of the universe. Dark matter keeps galaxies from tearing themselves apart, and outnumbers visible matter five to one. Five to one is a curious ratio. If dark and visible matter were entirely different substances with a completely independent history, you would not expect almost the same amount of dark and normal matter. This is like counting the number of trees in the world and finding that it’s the same as the number of pebbles. While we know that dark and visible matter are not the same substance (the Standard Model does not include any dark matter candidates), this similarity cannot be ignored. The similarity in abundances between dark and visible matter implies that they were caused by the same mechanism, created in the same way. As the abundance of matter is determined by the asymmetry between antimatter and matter, this leads us to a relationship between baryogenesis and dark matter.
Asymmetric dark matter theories have attracted significant attention in the last few years, and are now studied by physicists across the world. This has give us a cornucopia of asymmetric dark matter theories. Despite this, there are several common threads and predictions that allow us to test many of them at once. In asymmetric dark matter theories baryogenesis is caused by interactions between dark and normal matter. By having dark matter interact differently with matter and antimatter, we can get marginally more matter in the universe then antimatter. After the matter and antimatter annihilate each other, there is some minuscule amount of matter left standing. These leftovers go on to become the universe you know. Typically, a similar asymmetry in dark matter and its antiparticle is also made, so there is a similar amount of dark matter left over as well. This promotes dark matter from being a necessary, yet boring spectator in the cosmic tango to an active participant, saving our universe from desolation. Asymmetric dark matter also provides new ways to search for dark matter, such as neutrinos generated from dark matter in the sun. As asymmetric dark matter interacts with normal matter, large bodies like the sun and the earth can capture a reservoir of dark matter, sitting at their core. This can generate ghostlike neutrinos, or provide an obstacle for dark matter in direct detection experiments. Asymmetric dark matter theories can also tell us where we do not expect to see dark matter. A large effort has been made to see tell-tale signs of dark matter annihilating with its antiparticle throughout the universe, but it is yet to meet with success. While experiments like the Fermi space telescope have found potential signals (such as a 130 GeV line in 2012), these signals are ambiguous or fail to survive the test of time. The majority of asymmetric dark matter theories predict that there is no signal, as all the anti dark matter has long since been destroyed.
As on the pool table, even little asymmetries can have a profound effect on what we see. While much progress is made from finding new symmetries, we can’t forget the importance of imperfections in science. Asymmetric dark matter can explain where the matter in our universe came from, and gives dark and normal matter a common origin. Dark matter is no longer a passive observer in the evolution of our universe; it plays a pivotal role in the world around us.
This article appeared in DOE Pulse on Nov. 10, 2014.
Since he was a graduate student in Germany, Oliver Gutsche wanted to combine research in particle physics with computing for the large experiments that probe the building blocks of matter.
“When I started working on the physics data coming from one of the experiments at DESY, I was equally interested in everything that had to do with large-scale computing,” said Gutsche of his time at the German laboratory. Gutsche now works at DOE’s Fermi National Accelerator Laboratory. “So I also began working on the computing side of particle physics. For me that was always the combination I wanted to do.”
Gutsche’s desire to merge the two focuses has paid off. For the past four years Gutsche has been in charge of worldwide computing operations of the Large Hadron Collider’s CMS experiment, one of two experiments credited with the 2012 Higgs boson discovery. In December he was awarded the CMS Collaboration Award for his contributions to the global CMS computing system. And more recently, he has been promoted to assistant head of the Scientific Computing Division at Fermilab.
As head of CMS Computing Operations, Gutsche orchestrates data processing, simulations, data analysis and transfers and manages infrastructure and many more central tasks. Monte Carlo simulations of particle interactions, for example, are a key deliverable of the CMS Computing Operations group. Monte Carlo simulations employ randomness to simulate the collisions of the LHC and their products in a statistical way.
“You have to simulate the randomness of nature,” explained Gutsche. “We need Monte Carlo collisions to make sure we understand the data recorded by the CMS experiment and to compare them to the theory.”
When Gutsche received his Ph.D. from the University of Hamburg in 2005, he was looking for a job to combine LHC work, large-scale computing and a U.S. postdoc experience.
“Fermilab was an ideal place to do LHC physics research and LHC computing at the same time,” he said. His postdoc work led to his appointment as an application physicist at Fermilab and as the CMS Computing Operations lead.
Today Gutsche interacts regularly with people at universities and laboratories across the United States and at CERN, host laboratory of the LHC, often starting the day at 7 a.m. for transatlantic or transcontinental meetings.
“I try to talk physics and computing with everyone involved, even those in different time zones, from CERN to the west coast,” he said. Late afternoon in the United States is a good time for writing code. “That’s when everything quiets down and Europe is asleep.”
Gutsche expects to further enhance the cooperation between U.S. particle physicists and their international colleagues, mostly in Europe, by using the new premier U.S. Department of Energy’s Energy Sciences Network recently announced in anticipation of the LHC’s restart in spring 2015 at higher energy.
Helping connect the research done by particle physicists around the world, Gutsche finds excitement in all the work he does.
“Of course the Higgs boson discovery was very exciting,” Gutsche said. “But in CMS Computing Operations everything is exciting because we prepare the basis for hundreds of physics analyses so far and many more to come, not only for the major discoveries.”
On Saturday, I went to see Interstellar at the London BFI IMAX. I wouldn’t usually be so extravagant; my usual cinema trips are on 2-for-1 Orange Wednesdays. But I felt Interstellar was worth seeing in all its IMAX high resolution glory – and it definitely was. The film, directed by Christopher Nolan, is an epic masterpiece describing the journey of Cooper (Matthew McConaughey), a pilot-turned farmer-turned pilot again out of our galaxy in search of a new home for humans. The Earth is blighted by, well, blight, and the human race is starving. It paints a grim picture of our potential future here on Earth, and it seems entirely plausible.
Interstellar did an excellent job of using physics. This should be expected – the filmmakers worked with prominent theorist Kip Thorne. Thorne has worked on all those delicious-sounding areas of cosmology that I wish I had the brains for – black holes, wormholes, quantum gravity, gravitational waves, relativistic stars, etc. Genuine equations were used to visualise the black hole and wormhole featured in Interstellar – a fact that the majority of the audience would not know or particularly care about, but satisfies physicists. There is something to be said for not straying too far from real science in films when we live in a world plagued by quack-science that has tarred the word quantum (for examples, just search for quantum healing, or note that searching for quantum crystals brings up a site for buying “quantum balance crystals” before it brings up anything related to quantum mechanics and solid-state physics).
I was surprised to find posts on the internet “explaining” Interstellar. The film wrapped everything up nicely in my opinion. But most people haven’t done a course in General Relativity! I don’t pretend to be any sort of expert, it was 2 years ago and I can barely remember the maths but I do know the basic concept of gravitational time dilation. It is this, it seems, that was confusing people – how time was passing at a different rate for those on Earth and those in space.
I don’t want to give away any spoilers, but Cooper ends up close to a black hole (aptly named “Gargantua”), where time runs much more slowly for him than anyone further away. This is a strange and frightening thought for us humans who spend our lives moving consistently forwards in time at the same rate (at least within our perception). In reality, you are ageing slightly faster at the top of a skyscraper than you are at the Earth’s surface – but the effect is too small to notice. It is, however, definitely there. General and special relativity are used to correct the time given by GPS satellites – they are in a weaker gravitational field than we are down on Earth, and so their clocks run slightly faster. Without this correction, GPS would not work.
To understand relativity, you need to remember that time is just another dimension, a fact that becomes important later in the film. Like our 3D space is warped around a massive object, so is time. The foundations of general relativity lie in something called the “equivalence principle”. Einstein himself wrote this as:
“A little reflection will show that the law of the equality of the inertial and gravitational mass is equivalent to the assertion that the acceleration imparted to a body by a gravitational field is independent of the nature of the body.“
What this means is that under gravity, all things will accelerate at the same rate, independent of their mass. We see this on Earth, where that rate of acceleration is 9.81m/s. A feather and a rock dropped together reach the ground at the same moment (ignoring air resistance!).
Next, Einstein deduced that an object in “free-fall”, i.e. an object with only gravity acting upon it, is not actually accelerating - there is no force of gravity. This was actually one of those rare “my mind is blown” moments I had during my degree. An object in free-fall is not accelerating – it is simply following a geodesic in curved space-time. A geodesic is the analogy to a straight line within curved space – think if it as the shortest path between two points on a sphere.
I’m going a bit off tangent here, but general relativity is a fascinating subject! What I wanted to get to is the time dilation part. Why does time run slower for someone in a strong gravitational field? It actually comes back to special relativity, general relativity’s less scary little brother. I was taught special relativity in the first year of my degree, and it was the first time I felt like I was learning real exciting physics. The postulates of SR are:
- The laws of physics are the same in all inertial frames of reference.
- The speed of light in free space has the same value c in all inertial frames of reference.
Combining the principle of a freely-falling (i.e. travelling on a geodesic in a gravitational field) laboratory and applying special relativity introduces time dilation. The Pound and Rebka experiment is helpful to read up on for understanding this. By the laws of SR, both an observer inside the laboratory and one outside should measure the speed of light as c. Imagine a beam of light in the laboratory, the observer outside sees the path of light bend as the laboratory falls, whilst the observer inside sees a straight line as they are in an inertial frame. This means that for the outside observer, the light has travelled a longer (curved) path. As light always travels at c, the observer will deduce that more time has passed inside the laboratory than the person inside will measure. The stronger the gravitational field, the faster the free-fall, and the more the light will appear curved to the outside observer – so the time dilation factor increases with the field strength.
The strength of the black hole’s field in Interstellar means that minutes for Cooper become years for those outside. He is “free-falling” at an incredible rate, so his clock is running thousands of times slower than the ones on Earth, but it feels totally normal to him. He sees his own clock running at a normal speed, but he knows that the ones on Earth are running much faster. Emotions run high as every second he spends on his mission could be years he is missing of his children’s’ lives. I came out of the film an emotional wreck – I’d shed many tears and my chest felt tight, it was genuinely traumatic. Don’t get me wrong, I cry at a lot of films (I even cried when Gandalf died in Lord of the Rings, even though I’d read the books and so knew he was fine.) but I’ve never left one still feeling so upset. But that helps make it a brilliant film – not just the special effects, the beautiful images of space and stars and black holes, but the human reality; at the end of the day, it is just a father fighting to save his children.
I strongly recommend you go see this film, whether you are a physicist or not. I could go on for a lot longer and discuss the paradoxes some of the wormhole travel introduces as well as some other puzzles, but that would reveal spoilers, so instead I’ll just stop here! Interstellar is heartbreaking, but also breathtaking, and also warns us to take care of our planet. It’s not so easy to find another, and for god’s sake don’t stop investing in science! Make sure you bring tissues.
This blog is all about particle physics and particle physicists. We can all agree, I suppose, on the notion of the particle physicist, right? There are even plenty of nice pictures up here! But do we know or are we aware of what a particle really is? This fundamental question tantalized me from the very beginning of my studies and before addressing more involved topics I think it is worth spending some time on this concept. Through the years I probably changed my opinion several times, according to the philosophy underlying the topic that I was investigating. Moreover, there’s probably not a single answer to this question.
- The Standard Model: from geometry to detectors
The human mind conceived the Standard Model of Particle Physics to give a shape on the blackboard to the basic ingredients of particle physics: it is a field theory, with quantization rules, namely a quantum field theory and its roots go deep down to differential geometry.
But we know that “particles” like the Higgs boson have been discovered through complex detectors, relying on sophisticated electronic systems, tons of Monte Carlo simulations and data analysis. Quite far away from geometry, isn’t it?
So the question is: how do we fill this gap between theory and experiment? What do theoreticians think about and experimentalists see through the detectors? Furthermore, does a particle’s essence change from its creation to its detection?
- Essence and representation: the wavefunction
Let’s start with simple objects, like an electron. Can we imagine it as a tiny thing floating here and there? Mmm. Quantum mechanics already taught us that it is something more: it does not rotate around an atomic nucleus like the Earth around the Sun (see, e.g., Bohr’s model). The electron is more like a delocalized “presence” around the nucleus quantified by its “wavefunction”, a mathematical function that gives the probability of finding the electron at a certain place and time.
Let’s think about it: I just wrote that the electron is not a localized entity but it is spread in space and time through its wavefunction. Fine, but I still did not say what an electron is.
I have had long and intensive discussions about this question. In particular I remember one with my housemate (another theoretical physicist) that was about to end badly, with the waving of frying pans at each other. It’s not still clear to me if we agreed or not, but we still live together, at least.
Back to the electron, we could agree on considering its essence as its abstract definition, namely being one of the leptons in the Standard Model. But the impossibility of directly accessing it forces me to identify it with its most trustful representation, namely the wavefunction. I know its essence, but I cannot directly (i.e. with my senses) experience it. My human powers stop to the physical manifestation of its mathematical representation: I cannot go further.
Renè Magritte represented the difference between the representation of an object and the object itself in a famous painting “The treachery of images”:
“Ceci n’est pas une pipe”, it says, namely “This is not a pipe”. He is right, the picture is its representation. The pipe is defined as “A device for smoking, consisting of a tube of wood, clay, or other material with a small bowl at one end” and we can directly experience it. So its representation is not the pipe itself.
As I explained, this is somehow different in the case of the electron or other particles, where experience stops to the representation. So, according to my “humanity”, the electron is its wavefunction. But, to be consistent with what I just claimed: can we directly feel its wavefunction? Yes, we can. For example we can see its trace in a cloud chamber, or more elaborate detectors. Moreover, electricity and magnetism are (partly) manifestations of electron clouds in matter, and we experience those in everyday life.
You may wonder why I go through all these mental wanderings: just write down your formulas, calculate and be happy with (hopefully!) discoveries.
I do it because philosophy matters. And is nice. And now that we are a bit more aware of the essence of things that we are investigating, we can move a step forward and start addressing Quantum Chromo Dynamics (QCD), from its basic foundations to the latest results released by the community. I hope to have sufficiently stimulated your curiosity to follow me during the next steps!
Again, I want to stress that this is my own perspective, and maybe someone else would answer these questions in a different way. For example, what do you think?
A lot of us working in experimental neutrino physics think that these strange and tiny particles are pretty cool. Here are some fun facts about them from Fermilab physicist, and old classmate of mine, Tia Miceli. (Originally from Fermilab Today)
We don’t know much about neutrinos, but what we do know points to renegade particles that, despite their prevalence, are hard to pin down. Here are, in a nutshell, nine neutrino nuggets that scientists have figured out so far.
1. Neutrinos are super abundant. The shining sun sends 65 billion neutrinos per second per square centimeter to Earth. Neutrinos are the second most abundant particle in the universe. If we were to take a snapshot, we’d see that every cubic centimeter has approximately 1,000 photons and 300 neutrinos.
2. Neutrinos are almost massless. No one yet knows the mass of neutrinos, but it is at least a million times less massive than the lightest particle we know, the electron. We do know that each is so lightweight and so abundant that the total mass of all neutrinos in the universe is estimated to be equal to the total mass of all of the visible stars.
3. Neutrinos are perfect probes for the weak force. All other fundamental particles interact through the strong, electromagnetic or weak force or through some combination of the three. Neutrinos are the only particles that interact solely though the weak force. This makes neutrinos important for nailing down the details of the weak force.
4. Neutrinos are really hard to detect.On average, only one neutrino from the sun will interact with a person’s body during his or her lifetime. Since neutrino interactions are so rare, neutrino detectors must be huge. Super Kamiokande in Japan is as tall as Wilson Hall and holds 50,000 tons of ultrapure water. IceCube is buried between 1.5 and 2.5 kilometers under pure and clear ice in Antarctica, instrumenting a full cubic kilometer of ice.
5. Neutrinos are like chameleons.There are three flavors of neutrinos: electron, muon and tau. As a neutrino travels along, it may switch back and forth between the flavors. These flavor “oscillations” confounded physicists for decades.
6. Neutrinos of electron flavor linger around electrons. When neutrinos travel through matter, they see dense clouds of electrons. Electron neutrinos will have trouble traversing these dense clouds, effectively slowing down while muon and tau flavors travel through unimpeded. The NOvA experiment is using this phenomenon to deduce more information about the neutrino masses.
7. Neutrinos let us see inside the sun.The light that reaches Earth takes 10,000 to 100,000 years to escape the thick plasma of the sun’s core. When light reaches the solar surface, it freely streams through open space to our planet in only 8 minutes. Neutrinos provide us a penetrating view into the core, where nuclear fusion powers the sun. They take only 3.2 seconds to escape to the solar surface and 8 minutes to reach Earth.
8. Neutrinos may have altered the course of the universe. Why is everything in the universe made predominantly of matter and not antimatter? Cosmologists think that at the start of the universe there were equal parts of matter and antimatter. Neutrino interactions may have tipped this delicate balance, enabling the formation of galaxies, stars and planets like our own Earth.
9. Neutrinos dissipate more than 99 percent of a supernova’s energy.Certain types of stellar explosions lose nearly all of their energy through neutrinos. These “core collapse” supernovae end as either a black hole or a neutron star. Neutrinos are used to understand how supernovae explode and tell us more about other astronomical objects like active galactic nuclei.
This article appeared in Fermilab Today on Nov. 3, 2014.
If you own a magnetron, you probably use it to cook frozen burritos. The device powers microwave ovens by converting electricity into electromagnetic radiation. But Fermilab engineers believe they’ve found an even better use. They’ve developed a new technique to use a magnetron to power a superconducting radio-frequency (SRF) cavity, potentially saving hundreds of millions of dollars in the construction and operating costs of future linear accelerators.
The technique is far from market-ready, but recent tests with Accelerator Division RF Department-developed components at the Fermilab AZero test facility have proven that the idea works. Team leaders Brian Chase and Ralph Pasquinelli have, with Fermilab’s Office of Partnerships and Technology Transfer, applied for a patent and are looking for industrial partners to help scale up the process.
Both high-energy physics and industrial applications could benefit from the development of a high-power, magnetron-based RF station. The SRF cavity power source is a major cost of accelerators, but thanks to a long manufacturing history, accelerator-scale magnetrons could be mass-produced at a fraction of the cost of klystrons and other technologies typically used to generate and control radio waves in accelerators.
“Instead of paying $10 to $15 per watt of continuous-wave RF power, we believe that we can deliver that for about $3 per watt,” Pasquinelli said.
That adds up quickly for modern projects like Fermilab’s Proton Improvement Plan II, with more than 100 cavities, or the proposed International Linear Collider, which will call for about 15,000 cavities requiring more than 3 billion watts of pulsed RF power. The magnetron design is also far more efficient than klystrons, further driving down long-term costs.
But the straightforward idea wasn’t without obstacles.
“For an accelerator, you need very precise control of the amplitude and the phase of the signal,” Chase said. That’s on the order of 0.01 percent accuracy. Magnetrons don’t normally allow this kind of control.
One solution, Chase realized, is to apply a well-known mathematical expression known as a Bessel function, developed in the 19th century for astronomical calculations. Chase repurposed the function for the magnetron’s phase modulation scheme, which allowed for a high degree of control over the signal’s amplitude. Similar possible solutions to the amplitude problem use two magnetrons, but doubling most of the hardware would mean negating potential savings.
“Our technique uses one magnetron, and we use this modulation scheme, which has been known for almost a hundred years. It’s just never been put together,” Pasquinelli said. “And we came in thinking, ‘Why didn’t anyone else think of that?'”
Chase and Pasquinelli are now working with Bob Kephart, director of the Illinois Accelerator Research Center, to find an industry partner to help them develop their idea. Inexpensive, controlled RF power is already needed in certain medical equipment, and according to Kephart, driving down the costs will allow new applications to surface, such as using accelerators to clean up flue gas or sterilizing municipal waste.
“The reason I’m not retired is that I want to build this prototype,” Pasquinelli said. “It’s a solution to a real-world problem, and it will be a lot of fun to build the first one.”