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Hot Topics


Many different perspectives on the LHC

The past few weeks have brought many different perspectives on the Large Hadron Collider and its host, the particle physics laboratory CERN. As excitement builds for the LHC’s restart this spring, everyone from physicists to storytellers are sharing their thoughts on the massive undertaking and what it may or may not discover in the coming years.

The ties that bind

By Steven Goldfarb | January 18, 2015
A few weeks ago, LHC physicist Steven Goldfarb found himself in what he calls “one of the most beautiful places on Earth”: wedged between a metallic cable tray and a row of dusty cooling pipes at the bottom of Sector 13 of the ATLAS detector at CERN.

Will Self’s CERN

By Seth Zenz | January 16, 2015
Listening to Will Self’s five-part radio program Self Orbits CERN, Seth Zenz has trouble recognizing the CERN that Self describes. But he can recognize elements of Self’s visit and how he might have put them together that way.

Physics and outreach: my social media adventure

By Nhan Tran | January 9, 2015
US LHC physicist Nhan Tran steps into the realm of social media for the first time with his Large Hadron Collider #hashtag spoof video. Nhan says he wants viewers to see the video and think “what the heck are those guys talking about?” and go and learn more.

Latest Posts

How to build your own particle detector

Wednesday, January 21st, 2015

This article ran in symmetry on Jan. 20, 2015

Make a cloud chamber and watch fundamental particles zip through your living room! Image: Sandbox Studio, Chicago

Make a cloud chamber and watch fundamental particles zip through your living room! Image: Sandbox Studio, Chicago

The scale of the detectors at the Large Hadron Collider is almost incomprehensible: They weigh thousands of tons, contain millions of detecting elements and support a research program for an international community of thousands of scientists.

But particle detectors aren’t always so complicated. In fact, some particle detectors are so simple that you can make (and operate) them in your own home.

The Continuously Sensitive Diffusion Cloud Chamber is one such detector. Originally developed at UC Berkeley in 1938, this type of detector uses evaporated alcohol to make a ‘cloud’ that is extremely sensitive to passing particles.

Cosmic rays are particles that are constantly crashing into the Earth from space. When they hit Earth’s atmosphere, they release a shower of less massive particles, many of which invisibly rain down to us.

When a cosmic ray zips through a cloud, it creates ghostly particle tracks that are visible to the naked eye.

Building a cloud chamber is easy and requires only a few simple materials and steps:


  • Clear plastic or glass tub (such as a fish tank) with a solid lid (plastic or metal)
  • Felt
  • Isopropyl alcohol (90% or more. You can find this at a pharmacy or special order from a chemical supply company. Wear safety goggles when handling the alcohol.)
  • Dry ice (frozen carbon dioxide. Often used at fish markets and grocery stores to keep products cool. Wear thick gloves when handling the dry ice.)


  1. Cut the felt so that it is the size of the bottom of the fish tank. Glue it down inside the tank (on the bottom where the sand and fake treasure chests would normally go).
  2. Once the felt is secured, soak it in the isopropyl alcohol until it is saturated. Drain off any excess alcohol.
  3. Place the lid on top of dry ice so that it lies flat. You might want to have the dry ice in a container or box so that it is more stable.
  4. Flip the tank upside down, so that the felt-covered bottom of the tank is on top, and place the mouth of the tank on top of the lid.
  5. Wait about 10 minutes… then turn off the lights and shine a flashlight into your tank.
Artwork by: Sandbox Studio, Chicago

What is happening inside your cloud chamber?

The alcohol absorbed by the felt is at room temperature and is slowly evaporating into the air. But as the evaporated alcohol sinks toward the dry ice, it cools down and wants to turn back into a liquid.

The air near the bottom of the tank is now supersaturated, which means that it is just below its atmospheric dew point. And just as water molecules cling to blades of grass on cool autumn mornings, the atmospheric alcohol will form cloud-like droplets on anything it can cling to.

Particles, coming through!

When a particle zips through your cloud chamber, it bumps into atmospheric molecules and knocks off some of their electrons, turning the molecules into charged ions. The atmospheric alcohol is attracted to these ions and clings to them, forming tiny droplets.

The resulting tracks left behind look like the contrails of airplane—long spindly lines marking the particle’s path through your cloud chamber.

What you can tell from your tracks?

Many different types of particles might pass through your cloud chamber. It might be hard to see, but you can actually differentiate between the types of particles based on the tracks they leave behind.

Short, fat tracks

Sorry—not a cosmic ray. When you see short, fat tracks, you’re seeing an atmospheric radon atom spitting out an alpha particle (a clump of two protons and two neutrons). Radon is a naturally occurring radioactive element, but it exists in such low concentrations in the air that it is less radioactive than peanut butter. Alpha particles spat out of radon atoms are bulky and low-energy, so they leave short, fat tracks.

Long, straight track

Congratulations! You’ve got muons! Muons are the heavier cousins of the electron and are produced when a cosmic ray bumps into an atmospheric molecule high up in the atmosphere. Because they are so massive, muons bludgeon their way through the air and leave clean, straight tracks.

Zig-zags and curly-cues

If your track looks like the path of a lost tourist in a foreign city, you’re looking at an electron or positron (the electron’s anti-matter twin). Electrons and positrons are created when a cosmic ray crashes into atmospheric molecules. Electrons and positrons are light particles and bounce around when they hit air molecules, leaving zig-zags and curly-cues.

Forked tracks

If your track splits, congratulations! You just saw a particle decay. Many particles are unstable and will decay into more stable particles. If your track suddenly forks, you are seeing physics in action!



Sarah Charley


Doubly charged Higgs bosons and lepton number violation are wickedly cool.

Hi Folks,

The Standard Model (SM) of particle physics is presently the best description of matter and its interactions at small distances and high energies. It is constructed based on observed conservation laws of nature. However, not all conservation laws found in the SM are intentional, for example lepton number conservation. New physics models, such as those that introduce singly and doubly charged Higgs bosons, are flexible enough to reproduce previously observed data but can either conserve or violate these accidental conservation laws. Therefore, some of the best ways of testing if these types of laws are much more fundamental may be with the help of new physics.

Observed Conservation Laws of Nature and the Standard Model

Conservation laws, like the conservation of energy or the conservation of linear momentum, have the most remarkable impact on life and the universe. Conservation of energy, for example, tells us that cars need fuel to operate and perpetual motion machines can never exist. A football sailing across a pitch does not suddenly jerk to the left at 90º because conversation of linear momentum, unless acted upon by a player (a force). This is Newton’s First Law of Motion. In particle physics, conservation laws are not taken lightly; they dictate how particles are allowed to behave and forbid some processes from occurring. To see this in action, lets consider a top quark (t) decaying into a W boson and a bottom quark (b).



A top quark cannot radiate a W+ boson and remain a top quark because of conservation of electric charge. Top quarks have an electric charge of +2/3 e, whereas W+ bosons have an electric charge of +1e, and we know quite well that

(+2/3)e ≠ (+1)e + (+2/3)e.

For reference a proton has an electric charge of +1e and an electron has an electric charge of -1e. However, a top quark can radiate a W+ boson and become a bottom quark, which has electric charge of -1/3e. Since

(+2/3)e = (+1)e + (-1/3)e,

we see that electric charge is conserved.

Conservation of energy, angular momentum, electric charged, etc., are so well-established that the SM is constructed to automatically obey these laws. If we pick any mathematical term in the SM that describes how two or more particles interact (for example how the top quark, bottom quark, and W boson interact with each other) and then add up the electric charge of all the participating particles, we will find that the total electric charge is zero:

The top quark-bottom quark-W boson vertices in the Standard Model, and the net charge carried by each interaction.

The top quark-bottom quark-W boson interaction terms in the Standard Model. Bars above quarks indicate that the quark is an antiparticle and has opposite charges.


Accidental Conservation Laws

However, not all conservation laws that appear in the SM are intentional. Conservation of lepton number is an example of this. A lepton is any SM fermion that does not interact with the strong nuclear force. There are six leptons in total: the electron, muon, tau, electron-neutrino, muon-neutrino, and tau-neutrino. We assign lepton number

L=1 to all leptons (electron, muon, tau, and all three neutrinos),

L=-1 to all antileptons (positron, antimuon, antitau, and all three antineutrinos),

L=0 to all other particles.

With these quantum number assignments, we see that lepton number is a conserved in the SM. To clarify this important point: we get lepton number conservation for free due to our very rigid requirements when constructing the SM, namely the correct conservation laws (e.g., electric and color charge) and particle content. Since lepton number conservation was not intentional, we say that lepton number is accidentally conserved. Just as we counted the electric charge for the top-bottom-W interaction, we can count the net lepton number for the electron-neutrino-W interaction in the SM and see that lepton number really is zero:


The W boson-neutrino-electron interaction terms in the Standard Model. Bars above leptons indicate that the lepton is an antiparticle and has opposite charges.

However, lepton number conservation is not required to explain data. At no point in constructing the SM did we require that it be conserved. Because of this, many physicists question whether lepton number is actually conserved. It may be, but we do not know. This is indeed one topic that is actively researched. An interesting example of a scenario in which lepton number conservation could be tested is the class of theories with singly and doubly charged Higgs boson. That is right, there are theories containing additional Higgs bosons that an electric charged equal or double the electric charge of the proton.


Models with scalar SU(2) triplets contain additional neutral Higgs bosons as well as singly and doubly charged Higgs bosons.

Doubly charged Higgs bosons have an electric charge that is twice as large as a proton (2e), which leads to rather peculiar properties. As discussed above, every interaction between two or more particles must respect the SM conservation laws, such as conservation of electric charge. Because of this, a doubly charged Higgs (+2e) cannot decay into a top quark (+2/3 e) and an antibottom quark (+1/3 e),

(+2)e ≠ (+2/3)e + (+1/3)e.

However, a doubly charged Higgs (+2e) can decay into two W bosons (+1e) or two antileptons (+1e) with the same electric charge,

(+2)e = (+1)e + (+1)e.

but that is it. A doubly charged Higgs boson cannot decay into any other pair of SM particles because it would violate electric charge conservation. For these two types of interactions, we can also check whether or not lepton number is conserved:

For the decay into same-sign W boson pairs, the total lepton number is 0L + 0L + 0L = 0L. In this case, lepton number is conserved!

For the decay into same-sign leptons pairs, the total lepton number is 0L + (-1)L + (-1)L = -2L. In this case, lepton number is violated!


Doubly charged Higgs boson interactions for same-sign W boson pairs and same-sign electron pairs. Bars indicate antiparticles. C’s indicate charge flipping.

Therefore if we observe a doubly charged Higgs decaying into a pair of same-sign leptons, then we have evidence that lepton number is violated. If we only observe doubly charged Higgs decaying into same-sign W bosons, then one may speculate that lepton number is conserved in the SM.

Doubly Charged Higgs Factories

Doubly charged Higgs bosons do not interact with quarks (otherwise it would violate electric charge conservation), so we have to rely on vector boson fusion (VBF) to produce them. VBF is when two bosons from on-coming quarks are radiated and then scatter off each other, as seen in the diagram below.

Figure 2: Diagram depicting the process known as WW Scattering, where two quarks from two protons each radiate a W boson that then elastically interact with one another.

Diagram depicting the process known as WW Scattering, where two quarks from two protons each radiate a W boson that then elastically interact with one another.

If two down quarks, one from each oncoming proton, radiate a W- boson (-1e) and become up quarks, the two W- bosons can fuse into a negatively, doubly charged Higgs (-2e). If lepton number is violated, the Higgs boson can decay into a pair of same-sign electrons (2x -1e). Counting lepton number at the beginning of the process (L = 0 – 0 = 0) and at the end (L = 0 – 2 = -2!), we see that it changes by two units!

Same-sign W- pairs fusing into a doubly charged Higgs boson that decays into same-sign electrons.

Same-sign W- pairs fusing into a doubly charged Higgs boson that decays into same-sign electrons.

If lepton number is not violated, we will never see this decay and only see decays to two very, very energetic W- boson (-1e). Searching for vector boson fusion as well as lepton number violation are important components of the overarching Large Hadron Collider (LHC) research program at CERN. Unfortunately, there is no evidence for the existence of doubly charged scalars. On the other hand, we do have evidence for vector boson scattering (VBS) of the same-sign W bosons! Additional plots can be found on ATLAS’ website.  Reaching this tremendous milestone is a triumph for the LHC experiments. Vector boson fusion is a very, very, very, very, very rare process in the Standard Model and difficult to separate from other SM processes. Finding evidence for it is a first step in using the VBF process as a probe of new physics.

Words. Credit: Junjie Zhu (Michigan)

Same-sign W boson scattering candidate event at the LHC ATLAS experiment. Slide credit: Junjie Zhu (Michigan)

We have observed that some quantities, like momentum and electric charge, are conserved in nature. Conservation laws are few and far between, but are powerful. The modern framework of particle physics has these laws built into them, but has also been found to accidentally conserve other quantities, like lepton number. However, as lepton number is not required to reproduce data, it may be the case that these accidental laws are not, in fact, conserved. Theories that introduce charged Higgs bosons can reproduce data but also predict new interactions, such as doubly charged Higgs bosons decaying to same-sign W boson pairs and, if lepton number is violated, to same-sign charged lepton pairs. These new, exotic particles can be produced through vector boson fusion of two same-sign W boson pairs. VBF is a rare process in the SM and can greatly increase if new particles exist. At last, there is evidence for vector boson scattering of same-sign W bosons, and may be the next step to discovering new particles and new laws of nature!

Happy Colliding

- Richard (@BraveLittleMuon)


The Ties That Bind

Sunday, January 18th, 2015
Cleaning the ATLAS Experiment

Beneath the ATLAS detector – note the well-placed cable ties. IMAGE: Claudia Marcelloni, ATLAS Experiment © 2014 CERN.

A few weeks ago, I found myself in one of the most beautiful places on earth: wedged between a metallic cable tray and a row of dusty cooling pipes at the bottom of Sector 13 of the ATLAS Detector at CERN. My wrists were scratched from hard plastic cable ties, I had an industrial vacuum strapped to my back, and my only light came from a battery powered LED fastened to the front of my helmet. It was beautiful.

The ATLAS Detector is one of the largest, most complex scientific instruments ever constructed. It is 46 meters long, 26 meters high, and sits 80 metres underground, completely surrounding one of four points on the Large Hadron Collider (LHC), where proton beams are brought together to collide at high energies.  It is designed to capture remnants of the collisions, which appear in the form of particle tracks and energy deposits in its active components. Information from these remnants allows us to reconstruct properties of the collisions and, in doing so, to improve our understanding of the basic building blocks and forces of nature.

On that particular day, a few dozen of my colleagues and I were weaving our way through the detector, removing dirt and stray objects that had accumulated during the previous two years. The LHC had been shut down during that time, in order to upgrade the accelerator and prepare its detectors for proton collisions at higher energy. ATLAS is constructed around a set of very large, powerful magnets, designed to curve charged particles coming from the collisions, allowing us to precisely measure their momenta. Any metallic objects left in the detector risk turning into fast-moving projectiles when the magnets are powered up, so it was important for us to do a good job.

ATLAS Big Wheel

ATLAS is divided into 16 phi sectors with #13 at the bottom. IMAGE: Steven Goldfarb, ATLAS Experiment © 2014 CERN

The significance of the task, however, did not prevent my eyes from taking in the wonder of the beauty around me. ATLAS is shaped somewhat like a large barrel. For reference in construction, software, and physics analysis, we divide the angle around the beam axis, phi, into 16 sectors. Sector 13 is the lucky sector at the very bottom of the detector, which is where I found myself that morning. And I was right at ground zero, directly under the point of collision.

To get to that spot, I had to pass through a myriad of detector hardware, electronics, cables, and cooling pipes. One of the most striking aspects of the scenery is the ironic juxtaposition of construction-grade machinery, including built-in ladders and scaffolding, with delicate, highly sensitive detector components, some of which make positional measurements to micron (thousandth of a millimetre) precision. All of this is held in place by kilometres of cable trays, fixings, and what appear to be millions of plastic (sometimes sharp) cable ties.

Inside the ATLAS Detector

Scaffolding and ladder mounted inside the precision muon spectrometer. IMAGE: Steven Goldfarb, ATLAS Experiment © 2014 CERN.

The real beauty lies not in the parts themselves, but rather in the magnificent stories of international cooperation and collaboration that they tell. The cable tie that scratched my wrist secures a cable that was installed by an Iranian student from a Canadian university. Its purpose is to carry data from electronics designed in Germany, attached to a detector built in the USA and installed by a Russian technician.  On the other end, a Japanese readout system brings the data to a trigger designed in Australia, following the plans of a Moroccan scientist. The filtered data is processed by software written in Sweden following the plans of a French physicist at a Dutch laboratory, and then distributed by grid middleware designed by a Brazilian student at CERN. This allows the data to be analyzed by a Chinese physicist in Argentina working in a group chaired by an Israeli researcher and overseen by a British coordinator.  And what about the cable tie?  No idea, but that doesn’t take away from its beauty.

There are 178 institutions from 38 different countries participating in the ATLAS Experiment, which is only the beginning.  When one considers the international make-up of each of the institutions, it would be safe to claim that well over 100 countries from all corners of the globe are represented in the collaboration.  While this rich diversity is a wonderful story, the real beauty lies in the commonality.

All of the scientists, with their diverse social, cultural and linguistic backgrounds, share a common goal: a commitment to the success of the experiment. The plastic cable tie might scratch, but it is tight and well placed; its cable is held correctly and the data are delivered, as expected. This enormous, complex enterprise works because the researchers who built it are driven by the essential nature of the mission: to improve our understanding of the world we live in. We share a common dedication to the future, we know it depends on research like this, and we are thrilled to be a part of it.

ATLAS Collaboration Members

ATLAS Collaboration members in discussion. What discoveries are in store this year? IMAGE: Claudia Marcelloni, ATLAS Experiment © 2008 CERN.

This spring, the LHC will restart at an energy level higher than any accelerator has ever achieved before. This will allow the researchers from ATLAS, as well as the thousands of other physicists from partner experiments sharing the accelerator, to explore the fundamental components of our universe in more detail than ever before. These scientists share a common dream of discovery that will manifest itself in the excitement of the coming months. Whether or not that discovery comes this year or some time in the future, Sector 13 of the ATLAS detector reflects all the beauty of that dream.


Will Self’s CERN

Friday, January 16th, 2015

“It doesn’t look to me like the rose window of Notre Dame. It looks like a filthy big machine down a hole.” — Will Self

Like any documentary, biography, or other educational program on the radio, Will Self’s five-part radio program Self Orbits CERN is partially a work of fiction. It is based, to be sure, on a real walk through the French countryside along the route of the Large Hadron Collider, on the quest for a promised “sense of wonder”. And it is based on real tours at CERN and real conversations. But editorial and narrative choices have to be made in producing a radio program, and in that sense it is exactly the story that Will Self wants to tell. He is, after all, a storyteller.

It is a story of a vast scientific bureaucracy that promises “to steal fire from the gods” through an over-polished public relations team, with day-to-day work done by narrow, technically-minded savants who dodge the big philosophical questions suggested by their work. It is a story of big ugly new machines whose function is incomprehensible. It is the story of a walk through thunderstorms and countryside punctuated by awkward meetings with a cast of characters who are always asked the same questions, and apparently never give a satisfactory answer.

Self’s CERN is not the CERN I recognize, but I can recognize the elements of his visit and how he might have put them together that way. Yes, CERN has secretariats and human resources and procurement, all the boring things that any big employer that builds on a vast scale has to have. And yes, many people working at CERN are specialists in the technical problems that define their jobs. Some of us are interested in the wider philosophical questions implied by trying to understand what the universe is made of and how it works, but some of us are simply really excited about the challenges of a tiny part of the overall project.

“I think you understand more than you let on.”Professor Akram Khan

The central conflict of the program feels a bit like it was engineered by Self, or at least made inevitable by his deliberately-cultivated ignorance. Why, for example, does he wait until halfway through the walk to ask for the basic overview of particle physics that he feels he’s missing, unless it adds to the drama he wants to create? By the end of the program, he admits that asking for explanations when he hasn’t learned much background is a bit unfair. But the trouble is not whether he knows the mathematics. The trouble, rather, is that he’s listened to a typical, very short summary of why we care about particle physics, and taken it literally. He has decided in advance that CERN is a quasi-religious entity that’s somehow prepared to answer big philosophical questions, and never quite reconsiders the discussion based on what’s actually on offer.

If his point is that particle physicists who speak to the public are sometimes careless, he’s absolutely right. We might say we are looking for how or why the universe was created, when really we mean we are learning what it’s made of and the rules for how that stuff interacts, which in turn lets us trace what happened in the past almost (but not quite) back to the moment of the Big Bang. When we say we’re replicating the conditions at that moment, we mean we’re creating particles so massive that they require the energy density that was present back then. We might say that the Higgs boson explains mass, when more precisely it’s part of the model that gives a mechanism for mass to exist in models whose symmetries forbid it. Usually a visit to CERN involves several different explanations from different people, from the high-level and media-savvy down to the technical details of particular systems. Most science journalists would put this information together to present the perspective they wanted, but Self apparently takes everything at face value, and asks everyone he meets for the big picture connections. His narrative is edited to literally cut off technical explanations, because he wants to hear about beauty and philosophy.

Will Self wants the people searching for facts about the universe to also interpret them in the broadest sense, but this is much harder than he implies. As part of a meeting of the UK CMS Collaboration at the University of Bristol last week, I had the opportunity to attend a seminar by Professor James Ladyman, who discussed the philosophy of science and the relationship of working scientists to it. One of the major points he drove home was just how specialized the philosophy of science can be: that the tremendous existing body of work on, for example, interpreting Quantum Mechanics requires years of research and thought which is distinct from learning to do calculations. Very few people have had time to learn both, and their work is important, but great scientific or great philosophical work is usually done by people who have specialized in only one or the other. In fact, we usually specialize a great deal more, into specific kinds of quantum mechanical interactions (e.g. LHC collisions) and specific ways of studying them (particular detectors and interactions).

Toward the end of the final episode, Self finds himself at Voltaire’s chateau near Ferney, France. Here, at last, is what he is looking for: a place where a polymath mused in beautiful surroundings on both philosophy and the natural world. Why have we lost that holistic approach to science? It turns out there are two very good reasons. First, we know an awful lot more than Voltaire did, which requires tremendous specialization discussed above. But second, science and philosophy are no longer the monopoly of rich European men with leisure time. It’s easy to do a bit of everything when you have very few peers and no obligation to complete any specific task. Scientists now have jobs that give them specific roles, working together as a part of a much wider task, in the case of CERN a literally global project. I might dabble in philosophy as an individual, but I recognize that my expertise is limited, and I really enjoy collaborating with my colleagues to cover together all the details we need to learn about the universe.

In Self’s world, physicists should be able to explain their work to writers, artists, and philosophers, and I agree: we should be able to explain it to everyone. But he — or at least, the character he plays in his own story — goes further, implying that scientific work whose goals and methods have not been explained well, or that cannot be recast in aesthetic and moral terms, is intrinsically suspect and potentially valueless. This is a false dichotomy: it’s perfectly possible, even likely, to have important research that is often explained poorly! Ultimately, Self Orbits CERN asks the right questions, but it is too busy musing about what the answers should be to pay attention to what they really are.

For all that, I recommend listening to the five 15-minute episodes. The music is lovely, the story engaging, and the description of the French countryside invigorating. The jokes were great, according to Miranda Sawyer (and you should probably trust her sense of humour rather than the woefully miscalibrated sense of humor that I brought from America). If you agree with me that Self has gone wrong in how he asks questions about science and which answers he expects, well, perhaps you will find some answers or new ideas for yourself.


The Theory of Everything

Thursday, January 15th, 2015

Last night I went to see The Theory of Everything, the biographical film about Stephen Hawking, adapted from the memoir of his ex-wife, Jane Wilde Hawking. News literally just in – it has been nominated for the Best Picture and Adapted Screenplay Oscars, and there are Best Actor and Best Actress nominations for Eddie Redmayne (Stephen) and Felicity Jones (Jane). Arguably today’s most famous scientist, Stephen Hawking is a theoretical physicist and cosmologist, now holding the position of Director of Research at Cambridge’s Centre for Theoretical Cosmology. He suffers from motor neurone disease; a degenerative disease that has left him unable to move most of the muscles in his body. He now communicates by selecting letters and words on a computer screen using one muscle in his cheek. His computerised voice is world famous and instantly recognisable. He is responsible for ground-breaking work on black holes and general relativity.


I thought the film was fantastically made and the acting incredible; Redmayne’s portrayal of Hawking’s physical condition was uncanny. I shed a few tears at the plight of this man surviving against all the odds whilst doing incredible theoretical physics, and his wife, ever patient and loving, taking care of him and bearing his children despite his health getting only worse.
There have been some complaints about the lack of focus in the film on Hawking’s scientific work; the film instead focuses mainly on his relationship with Jane and their struggle as his condition deteriorates. This should not be a surprise when the film was adapted from Jane’s own writing. If you want to know more about Hawking’s work in physics, then I strongly recommend his physics books. I first attempted to read A Brief History of Time, his most famous publication, age 11. This was obviously optimistic of me, and I gave up after the first couple of chapters. I tried again during my A-levels but never got round to finishing it, but having now studied cosmology and general relativity in much more detail I fully intend to give it another try! I have however read The Universe in a Nutshell, a more accessible book on the history of modern physics and cosmology, as well as discussions on that holy grail of physics, and the title of the film, a ‘theory of everything’.

But what is a theory of everything? Also known as a ‘final theory’, an ‘ultimate theory’, and a ‘master theory’, it sounds rather grand. A ToE would elegantly explain our universe, maybe even in just one equation, linking all the aspects that we can not currently reconcile with each other. It would allow a deep understanding of the universe we live in, as Hawking himself professed despite being an atheist:

If we do discover a complete theory, it should in time be understandable in broad principle by everyone, not just a few scientists. Then we shall all, philosophers, scientists, and just ordinary people, be able to take part in the discussion of the question of why it is that we and the universe exist. If we find the answer to that, it would be the ultimate triumph of human reason — for then we would know the mind of God.

Sounds good, right? The ultimate triumph. Unfortunately, so far, attempts at developing a ToE have not delivered. Why not? First we need to understand a little more about the physics we know and understand.
Our universe has four forces governing everything that happens within it:

  • Electromagnetism – the interaction of photons and charged particles that we are familiar with in electricity, magnets, etc.
  • Weak force – the interaction responsible for radioactive decay.
  • Strong force – the interaction that binds together the protons and neutrons in a nucleus
  • Gravity – the attraction of bodies with mass to each other, the reason we don’t fly away from the Earth and why the Earth orbits the sun (and also why we know about dark matter!)

Why four? No one knows. It has been shown that at electromagnetism and the weak force can be combined into an ‘electroweak’ force at high energies. This means that in our everyday low energy universe (as opposed to the hot dense universe shortly after the big bang) that electromagnetism and the weak force are just two faces of the same force.

If electromagnetism and the weak force can be combined, can we do the same with the strong force and gravity? Combining the electroweak and the strong force results in a “GUT” – a Grand Unified Theory, (NB despite being grand, this does not yet include gravity). The energy required to see the joining of the strong and the electroweak would be beyond the levels we could reach with particle colliders. We do not currently have a generally accepted GUT, but there are lots of complicated theories in the works.
The final step to a ToE would be the joining of gravity with a GUT theory. This is the real sticking point. As Jane illustrates with a pea and a potato over dinner in the film, the unification of quantum field theory (the pea) on the tiny scales with general relativity (the potato) on large scales has so far proven undoable.
Quantum field theory is what we particle physicists deal with, the standard model of particle physics, tiny things like photons and quarks and electrons, all interacting via electromagnetism, the weak force and the strong force. General relativity is far in the other direction; stars, galaxies, galaxy clusters. Big things with lots of mass, causing curvatures in space-time that manifests as gravity. Both quantum field theory and general relativity have been tested to extreme precision – they both work perfectly on their relative scales. So where does the problem in joining them lie?

Hawking’s greatest work is on black holes; the infinitely small and dense aftermath of the collapse of an enormous star. Once a star greater than about 23 solar masses runs out of fuel to produce energy, its core collapses under its own weight, expelling its outer layers in an explosion called a supernova that outshines its own galaxy. If the core is big enough, it will continue collapsing until it becomes a ‘space-time singularity’ – a point in space infinitely small and dense, where not even light can escape.
When we try to understand the physics inside that point, we start encountering problems. We need both quantum field theory and general relativity – we have a tiny tiny space but a huge mass, and infinities start popping up all over the place. The maths just doesn’t work.

The evolution of stars, showing how a sufficiently large star can end its life as a black hole

The evolution of stars, showing how a sufficiently large star can end its life as a black hole

Stephen Hawking, with the computerised speech system that has allowed him to communicate after losing his ability to speak

Stephen Hawking, with the computerised speech system that has allowed him to communicate and continue his physics work after losing his ability to speak

Hawking has dedicated much of his life to trying to unify these two pillars of modern physics, so far with no luck. This begs the question, if his incredible mind cannot do it, what hope do we have? Currently, a popular approach is string theory – the theory that everything is made of tiny strings, vibrating in many (up to 26!) dimensions. This may sound silly, but it’s actually quite elegant – each different particle is made of a string vibrating in a different mode. An issue with string theory currently is it offers no easily testable predictions. Some of the best minds of today are working on this, so there is still hope!

Stephen Hawking is clearly an incredible man. He has a level of intelligence and a talent in mathematics and physics most of us physicists can only dream of. However, I believe Jane also deserves a huge amount of credit. The diagnosis of motor neurone disease came only shortly after they began dating, but she embarked on a life with him, marrying him and having his children, taking on the mammoth task of caring for him mostly alone, despite his prognosis of only 2 years to live.

Of course, Hawking has far exceeded those two years. He is now 73, reaching what is basically a normal life expectancy despite having a disease that has an average survival from onset of only 3-4 years. He was diagnosed aged only 21. Diseases such as his are tragic, leaving a person’s mind totally intact but trapped inside a failing body. Many would just give up, but Hawking’s love for both Jane and physics drove him to persevere and become the esteemed professor he is today.

I strongly recommend watching The Theory of Everything, even to those uninterested in cosmology. It’s a beautiful, romantic drama set in picturesque Cambridge, emotionally powerful and moving, and certainly does not require you to understand the physics!


Someone once told me that if you read just one paper a week, you will become a world-class expert in your chosen topic after seven years. I’m not sure if this is true or not, but this strikes me as a terribly useful New Year’s resolution, certainly worthy of inclusion alongside getting a gym membership and learning to balance my checkbook!

Whether you are already an expert or a layperson looking to become an expert, the ability to read and digest scientific papers is an excellent skill to add to your repertoire. There are a number of great resources out there. For example, this Violent Metaphors blog post is wonderful in that it gives well-defined, step-by-step instructions for critically reading a primary source article – this is extremely useful if you really want to understand a scientific paper in depth.

Sometimes, though, you just need to read a lot of papers and assimilate a lot of new information in a short amount of time.   I’ve managed to streamline this process and would like to share with you some of my tips and tricks for reading papers as quickly as possible.

Dipping Your Toes

The first part of a paper that you will ever lay eyes upon is its abstract. This is, perhaps, the most crucial part of a paper, because it determines whether or not you should read the rest of the paper. A good abstract is short, to the point, and contains the following five ingredients.

  • - Brief background information
  • - Question/problem statement
  • - Experimental approach
  • - Results
  • - Impacts and implications of the work

I’ve found that the clearest and most easily understood abstracts are often only five sentences long: one sentence per point. Once you’ve identified these five key points, you’ll be able to make a decision as to whether or not the paper is interesting enough and useful enough to continue.

Down the Rabbit Hole

Now that you’ve decided that a certain paper is indeed worth your time, read the introduction first. If you don’t understand the background information contained in the introduction, you won’t understand the rest of the paper. This is not an indictment on your lack of knowledge; even the introduction of a paper will most likely need to be reread several times. In fact, reread the abstract at the same time. The abstract often contains the same material as the introduction, albeit in a more condensed form, and the more ways that you are exposed to the same information, the more likely it is to sink in.

While you are reading, take copious notes – even if you never read those notes again, the amount of processing that your brain has to do to in order regurgitate information back onto paper is often enough to make an idea “stick”.   In fact, my office is littered with filled-up legal pads that I will never read again for this very reason. First and foremost, while you are looking through the introduction, try to highlight and write down the background, the problem statement, the approach, the result, and the implication of the work, even if you have already identified these five points in the abstract. If you can identify these, you’re well on your way to understanding the rest of the paper. You should also write down any unfamiliar terms or jargon so that you can look them up later.

Reading a Paper is Never Just Reading ‘a’ Paper

As you read, I suggest you have available two copies of the paper in question. Chances are, this paper will cite other papers, so keep one copy open to the ‘References’ section as you read – that way you can quickly look up other papers as they are cited in the text. Reading a paper rarely if ever consists of reading only ONE paper. As you are reading, you may (actually, will) want to keep some of these new references open beside you as well.

As you begin to check out other papers, you’ll start to discover some patterns: there are generally a few key works in the field that almost everything else refers to and these are the papers that are crucial to read (or at the very least to identify so that you can skim them later). Usually there will be a couple of big review articles in the field, and a paper describing a big experimental result will often refer back to a design paper – these types of papers can be helpful to skim. And of course, knowing which papers to read will also help you figure out which papers to skip over, which is a crucial part of extracting information efficiently.

Spoiler Alert

Once you are reasonably comfortable with the introduction, skip to the conclusion. Most scientific papers have an hourglass shape. An introduction typically starts off broad in scope and hones in on a specific problem statement; a conclusion on the other hand will usually start with a concise, focused summary of the results and then zoom out to provide some broader context for the work and/or future prospects for the experiment in question. As you read the conclusion, first identify where the authors have summarized the highlights of the paper. You should now know exactly what the paper is about. Pay attention, too, if the conclusion discusses any further work to be done, especially if the paper is a couple years old. You might be able to find an updated version or more recent result.

If it’s difficult to understand the highlights of the paper as stated in the conclusion, there are a couple of things you can try. You can flip back and reread the problem statement from the introduction to see how it matches up with the conclusion.  You can do a literature search for similar works or review articles that might state the same thing but in a way that is easier to understand. Or, you can go through the results in more detail in the body of the paper.

The Meat in the Sandwich: Methods, Data, Analysis, Results

After reading the abstract, the introduction, and the conclusion, you should have the big idea down fairly well. If you still want to dig into the meat of a paper, start with the results. Results are typically self-contained with no references to other papers. Methods sections can be very technical and may or may not contain references to other papers, so consider yourself forewarned. If you absolutely have to dive into the main body of a paper, I recommend looking at the figures and tables in the results section first. A picture really is worth a thousand words. And don’t neglect figure captions – these can be very informative.


There you have it. I’ve found this outside-in method of reading papers to be as effective as it is quick. Hopefully this was helpful, and I wish you the best of luck in your next paper-reading endeavor!


United for peace

Monday, January 12th, 2015

The past week saw extremely sad events in Paris, reminding us that our society relies on a fragile equilibrium. This is just the most recent episode over the last years in a long list of events around the world – and also in Amsterdam, the city where I now live.

We have been flooded through the mass media by analyses, considerations, speeches and public actions. I don’t think it is necessary to add more here, because what we mostly need is time to think: about us as individuals and as active parts of a complex society.

Nevertheless, I would like to remind myself – and everyone who will read these thoughts – about what we can do as men and women of science. Even though fear and anger may knock at our doors, we need to find what could keep us united across different countries, cultures, religions and faiths. And fight for it.

As scientists, we are privileged: our job is to generate knowledge, the common heritage of mankind. Science is a universal endeavor involving people from every country, social background and culture. No matter what we think and believe, we collaborate daily to reach a high goal. Science, like any other intercultural enterprise, is a training for peace, and we are in extreme need of it and anything else that keeps us united in purity of interests, freedom and friendship.

The "tree of peace" in The Hague, which carries people's wishes for a better and peaceful world.

The “tree of peace” in The Hague (NL), which carries people’s wishes for a better and peaceful world.

The quest for peace is not just a hand-waving argument, nor fantasy of hopeful people: it is clearly stated even in the original documents of CERN – the European Center for Nuclear Research – signed by the founding members and shared by every single scientist working and studying there.

I. I. Rabi, an American scientist among the first supporters of CERN, greeted the 30th anniversary of CERN foundation with these words(*): “I hope all the scientists at CERN will remember to have more duties than just doing research in particle physics. They represent the results of centuries of research and study, showing the powers of the human mind. I hope they will not consider themselves technicians, but guardians of the European unity, so that Europe can protect peace in the world.”

Let’s build together a future of peace: we can do it.

(*) translated from the Italian version available here.


2015: The LHC returns

Saturday, January 10th, 2015

I’m really not one for New Year’s resolutions, but one that I ought to make is to do more writing for the US LHC blog.  Fortunately, this is the right year to be making that resolution, as we will have quite a lot to say in 2015 — the year that the Large Hadron Collider returns!  After two years of maintenance and improvements, everyone is raring to go for the restart of the machine this coming March.  There is still a lot to do to get ready.  But once we get going, we will be entering a dramatic period for particle physics — one that could make the discovery of the Higgs seem humdrum.

The most important physics consideration for the new run is the increase of the proton collision energy from 8 TeV to 13 TeV.  Remember that the original design energy of the LHC is 14 TeV — 8 TeV was just an opening step.  As we near the 14 TeV point, we will be able to do the physics that the LHC was meant to do all along.  And it is important to remember that we have no feasible plan to build an accelerator that can reach a higher energy on any near time horizon.  While we will continue to learn more as we record more and more data, through pursuits like precision measurements of the properties of the Higgs boson, it is increases in energy that open the door to the discovery of heavy particles, and there is no major energy increase coming any time soon.  If there is any major breakthrough to be made in the next decade, it will probably come within the first few years of it, as we get our first look at 13 TeV proton collisions.

How much is our reach for new physics extended with the increase in energy?  One interesting way to look at it is through a tool called Collider Reach that was devised by theorists Gavin Salam and Andreas Weiler.  (My apologies to them if I make any errors in my description of their work.)  This tool makes a rough estimate of the mass scale of new physics that we could have access to at a new LHC energy given previous studies at an old LHC energy, based on our understanding of how the momentum distributions of the quarks and gluons inside the proton evolve to the new beam energy.  There are many assumptions made for this estimate — in particular, that the old data analysis will work just as well under new conditions.  This might not be the case, as the LHC will be running not just at a higher energy, but also a higher collision rate (luminosity), which will make the collisions more complicated and harder to interpret.  But the tool at least gives us an estimate of the improved reach for new physics.

During the 2008 LHC run at 8 TeV, each experiment collected about 20 fb-1 of proton collision data.  In the upcoming “Run 2″ of the LHC at 13 TeV, which starts this year and is expected to run through the middle of 2018, we expect to record about 100 fb-1 of data, a factor of five increase.  (This is still a fairly rough estimate of the future total dataset size.)  Imagine that in 2008, you were looking for a particular model of physics that predicted a new particle, and you found that if that particle actually existed, it would have to have a mass of at least 3 TeV — a mass 24 times that of the Higgs boson.  How far in mass reach could your same analysis go with the Run 2 data?  The Collider Reach tool tells you:


Using the horizontal axis to find the 3 TeV point, we then look at the height of the green curve to tell us what to expect in Run 2.  That’s a bit more than 5 TeV — a 70% increase in the mass scale that your data analysis would have sensitivity to.

But you are impatient — how well could we do in 2015, the first year of the run?  We hope to get about 10 fb-1 this year. Here’s the revised plot:


The reach of the analysis is about 4 TeV. That is, with only 10% of the data, you get 50% of the increase in sensitivity that you would hope to achieve in the entire run.  So this first year counts!  One year from now, we will know a lot about what physics we have an opportunity to look at in the next few years — and if nature is kind to us, it will be something new and unexpected.

So what might this new physics be?  What are the challenges that we face in getting there?  How are physicists preparing to meet them?  You’ll be hearing a lot more about this in the year to come — and if I can keep to my New Year’s resolution, some of it you’ll hear from me.


It seems some disagreements are interminable: the Anabaptists versus the Calvinists, capitalism versus communism, the Hatfields versus the McCoys, or string theorists versus their detractors. It is the latter I will discuss here although the former may be more interesting. This essay is motivated[1] by a comment in the December 16, 2014 issue of Nature by George Ellis and Joe Silk. The comment takes issue with attempts by some string theorists and cosmologists to redefine the scientific method by eliminating the need for experimental testing and relying on elegance or similar criteria instead. I have a lot of sympathy with Ellis and Silk’s point of view but believe that it is up to scientists to define what science is and that hoping for deliverance by outside people, like philosophers, is doomed to failure

To understand what science is and what science is not, we need a well-defined model for how science behaves. Providing that well-defined model is the motivation behind each of my essays. The scientific method is quite simple: build models of how the universe works based on observation and simplicity. Then test them by comparing their predictions against new observation. Simplicity is needed since observations underdetermine the models (see for example: Willard Quine’s (1908 –2000) essay: The Two Dogmas of Empiricism).  Note also that what we do is build models: the standard model of particle physics, the nuclear shell model, string theory, etc. Quine refers to the internals of the models as myths and fictions. Henri Poincaré (1854 – 1912) talks of conventions and Hans Vaihinger (1852 –1933) of the philosophy of as if otherwise known as fictionalism. Thus it is important to remember that our models, even the so-called theory of everything, are only models and not reality.

It is the feedback loop of observation, model building and testing against new observation that define science and give it its successes. Let me repeat: The feedback loop is essential. To see why, consider example of astrology and why scientists reject it. Its practitioners consider it to be the very essence of elegance. Astrology uses careful measurements of current planetary locations and mathematics to predict their future locations, but it is based on an epistemology that places more reliance on the eloquence of ancient wisdom than on observation. Hence there is no attempt to test astrological predictions against observations. That would go against their fundamental principles of eloquence and the superiority of received knowledge to observation. Just as well, since astrological predictions routinely fail. Astrology’s failures provide a warning to those who wish to replace prediction and simplicity with other criteria. The testing of predictions against observation and simplicity are hard taskmasters and it would be nice to escape their tyranny but that path is fraught with danger, as astrology illustrates. The feedback loop from science has even been picked up by the business management community and has been built into the very structure of the management standards (see ISO Annex SL for example). It would be shame if management became more scientific than physics.

But back to string theory. Gravity has always been a tough nut to crack. Isaac Newton (1643 – 1727) proposed the decidedly inelegant idea of instantaneous action at a distance and it served well until 1905 and the development of special theory of relativity. Newton’s theory of gravity and special relativity are inconsistent since the latter rules out instantaneous action at a distance. In 1916, Albert Einstein (1879 – 1955) with an honorable mention to David Hilbert (1862 – 1943) proposed the general theory of relativity to solve the problem. In 1919, the prediction of the general theory of relativity for the bending of light by the sun was confirmed by an observation by Arthur Eddington (1882 – 1944). Notice the progression: conflict between two models, proposed solution, confirmed prediction, and then acceptance.

Like special relativity and Newtonian gravity, general relativity and quantum mechanics are incompatible with one another. This has led to attempts to generate a combined theory. Currently string theory is the most popular candidate, but it seems to be stuck at the stage general relativity was in 1917 or maybe even 1915: a complicated (some would say elegant, others messy) mathematical theory but unconfirmed by experiment. Although progress is definitely being made, string theory may stay where it is for a long time. The problem is that the natural scale of quantum gravity is the Planck mass and this scale is beyond what we can explore directly by experiment. However, there is one place that quantum gravity may have left observable traces and that is in its role in the early Universe. There are experimental hints that may indicate a signature in the cosmic microwave background radiation but we must await further experimental results. In the meantime, we must accept that current theories of quantum gravity are doubly uncertain. Uncertain, in the first instance, because, like all scientific models, they may be rendered obsolete by new a understanding and uncertain, in the second instance, because they have not been experimentally verified through testable predictions.

Let’s now turn to the question of multiverses. This is an even worse dog’s breakfast than quantum gravity. The underlying problem is the fine tuning of the fundamental constants needed in order for life as we know it to exist. What is needed for life, as we do not know it, to exist is unknown. There are two popular ideas for why the Universe is fined tuned. One is that the constants were fine-tuned by an intelligent designer to allow for life as we know it. This explanation has the problem that by itself it can explain anything but predict nothing. An alternate is that there are many possible universes, all existing, and we are simply in the one where we can exist. This explanation has the problem that by itself it can explain anything but predict nothing.  It is ironic that to avoid an intelligent designer, a solution based on an equally dubious just so story is proposed. Since we are into just so stories, perhaps we can compromise by having the intelligent designer choosing one of the multiverses as the one true Universe. This leaves the question of who the one true intelligent designer is. As an old farm boy, I find the idea that Audhumbla, the cow of the Norse creation myth, is the intelligent designer to be the most elegant. Besides the idea of elegance, as a deciding criterion in science, has a certain bovine aspect to it. Who decides what constitutes elegance? Everyone thinks their own creation is the most elegant. This is only possible in Lake Wobegon, where all the women are strong, all the men are good-looking, and all the children are above average (A PRAIRIE HOME COMPANION – Garrison Keillor (b. 1942)). Not being in Lake Wobegon, we need objective criteria for what constitutes elegance. Good luck with that one.

Some may think the discussion in the last paragraph is frivolous, and quite by design it is.  This is to illustrate the point that once we allow the quest for knowledge to escape from the rigors of the scientific method’s feedback loop all bets are off and we have no objective reason to rule out astrology or even the very elegant Audhumbla. However, the idea of an intelligent designer or multiverses can still be saved if they are an essential part of a model with a track record of successful predictions. For example, if that animal I see in my lane is Fenrir, the great gray wolf, and not just a passing coyote, then the odds swing in favor of Audhumbla as the intelligent designer and Ragnarok is not far off. More likely, evidence will eventually be found in the cosmic microwave background or elsewhere for some variant of quantum gravity. Until then, patience (on both sides) is a virtue.

Though the mills of science grind slowly;
Yet they grind exceeding small;
Though with patience they stand waiting,
With exactness grind they all.[2]

[1] I have already broken my new year’s resolution to post no more philosophy of science blogs but this is the last, I promise.

[2] With apologies to Henry Wadsworth Longfellow (1807 – 1882)


Hi Quantum Diaries readers!  This is my first post.  Here goes nothing…

Physics is awesome. Of that I’m sure. We do it because we want to answer the BIG questions. But, in my mind, equally important to the work itself is explaining our work to the public. Without their support, we wouldn’t have the funds to do what we do. But it’s challenging to explain highly complex ideas – which often require a lot of background knowledge – and convey our enthusiasm for the subject and our work in a way that will motivate and interest others. That said, I generally find that non-experts are fascinated by the subject matter and interested in knowing more.

While there is a very dedicated and knowledgeable following, I believe it is very important to draw in new enthusiasts, especially the younger crowd. I don’t want to just reach the people who are already coming online to read this blog (my first post and I’m already insulting the readership!).  Instead, I want to get the interest of those who would not normally go out of their way to visit the site, but then see something that piques their interest and inspires them to learn more.

Maybe social media is the answer. Unfortunately, it’s not my forte. I’m terrible at Facebook. Up until a month or so ago, I barely knew what Twitter was. In my in-depth internet sleuthing I came across #hashtag by Jimmy Fallon and Justin Timberlake (Side note: I find the concept of hashtags fascinating). From there, with my fellow postdoc compadre Jim Dolen, the idea to do a Large Hadron Collider spoof of the video was born.

For people who don’t know what the LHC is, we want them to see the video and to think “what the heck are those guys talking about?” and go and learn more. It’s short (maybe catchy?). I think the other goal of the video is for people to see a side of physicists they maybe don’t usually see. We like to have fun. We have a (weird) sense of humor. Take a look — and let us know what you think. Is it something you’d share with your friends and family? Do you have any other ideas or comments? Thanks for reading/watching!

Follow us on Twitter because we have no idea what we’re doing and it’s funny! I hope to break into double-digit followers soon.

@_nhanvtran @jimmydolen