Should you hold your breath for B-modes?

This morning (14th March 2014) I woke to a social media world abuzz with the exciting possibility that primordial B-modes have been detected; a result that would have a huge impact on cosmology. Let’s try to answer the question: what is the probability that such a detection has been made? As per Bayes’ theorem, the answer splits into two factors: the prior and the likelihood. The prior probabilities are relatively straight forward, but the likelihood is hard, since of course we don’t have access to the CMB data or the results! However, rather deviously, we do have access to the humans who have access to the data, and they form a biased tracer of the underlying information which we can tap into, since we understand some aspects of human behaviour fairly well.
 
 
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Lets look at the various factors:
 
 
 
(1) Using humans-who-know as biased tracers of the information we don’t know
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This worked very well for Planck, correctly predicting that N_eff would be closer to three than four, and that there would be no significant exciting new discoveries.  Let’s apply this technique here. Now if the team in question had found a clean, statistically significant detection (e.g. > 4-sigma) of primordial B-modes (or any other big new result), they would probably have had to delay publication to do a lot more checks of the result to make sure they weren’t missing any systematics (in order to protect their reputations against mistakes – remember the OPERA faster-than-light neutrinos?).
 
 
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This would slow down the whole publication/announcement process, frustrating the excited team. This extra checking is unlikely to have taken much less than a month and might take several months. Imagine walking around as a team member for several months with a >4 sigma primordial B-mode signal sitting on the tip of your tongue, waiting for final confirmation, and not being able to tell anyone!
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Given this, we now have to ask, what is the probability that those excited team members did not tell anyone about the big discovery during that month or more, but then did let the cat out the bag in the last few days? Not very big I would argue. Apart from the fact that humans are terrible at keeping secrets, most communication is non-verbal: you can often see when a person is keeping a secret even if they don’t say anything. The team’s work colleagues would have become suspicious… Now perhaps the team is very small and are trained in the 007 techniques of keeping secrets (unlikely, they are physicists after all!), but even if that were the case, why would they have failed at the last hurdle, just days before the formal announcement?

I think a much more likely possibility is that the team have found a hint of something interesting, but not more. A hint doesn’t need to be investigated so deeply, so it would not have slowed publication much. A hint would also mean the team were not so excited about the result, so they would be able to keep their “secret” more easily.

[Update 15/3/14: one way out of this conclusion would be if the analysis was done by a very small team (two or three people perhaps) who were able to keep the secret even from the wider team, while they were doing the cross-checks. The results may have only been disseminated to the team within the last week or two, leading to the sudden spread of rumours this week.]

Now lets look at the priors. These split into two:

(2) Experimental Priors. The probability that a ground-based experiment could make a clean, statistically-significant detection of primordial B-modes given the other ground-based experimental results so far, is arguably rather small [Update 15/3/14: discussions around the experimental capabilities of BICEP2 and KECK/SPUD suggest that it is significantly more powerful than my initial prior estimates, so like any good Bayesian we should update our priors as new info comes in! You can see presentations about the experiments here and here which show how good the projections are. Whether they have reached these levels at this point is speculation.]

(3) Physics Priors: what is the prior theoretical probability that primordial B-modes are large enough to be detected by this experiment? This is tricky since there are so many inflationary models and I won’t comment further other than to say there is no particular theoretical reason to expect the signal to be at the level detectable by this experiment (an argument that has been made against spending money on chasing inflationary B-modes).

When we combine our human-based estimate of the likelihood with these priors, we are unfortunately driven towards the boring end of the spectrum. For the sake of concreteness let me bet that any interesting, robust and unexpected results are below 3-sigma, and more likely around the 2-sigma level; i.e. not statistically significant (yet). Conversely, if there is a claim of a > 3-sigma detection, I suspect that it will be found to be an undiscovered systematic or else it is one that is not robust to changes in the model assumed or priors on parameters. Finally one can be cynical and conjecture that the rumour has been carefully injected into the bloodstream of the cosmology community as a way to create excitement. Nevertheless, even in this case it is hard to imagine them calling a press conference if they had found nothing new, so we can probably put a lower limit on the  “hint” of perhaps 1.5-1.8 sigma.

[Update 15/3/14: several sources now put Alan Guth and Andrei Linde at the press conference on Monday, and Andrei is scheduled to give a talk at the joint Tufts/MIT cosmology colloquium on Tuesday.  This ups the ante in terms of the above lower limit. How many sigmas do we think would be needed for Alan and Andrei to be invited to, and agree to attend, the press conference? I am not sure, but presumably at least 2-sigma. ]

Of course, even a 2-sigma hint of something exciting would be wonderful (though as Roberto reminded me, a significant fraction of 2-sigma results turn out to be wrong) and it could be that everything I have written is wrong: I certainly hope this is the case…the Universe has proven to be rather boring since 2000 and we desperately need some new excitement. All will be revealed come Monday, so stay tuned!

 
 
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* Update: 15/3/14 – Since writing my post yesterday morning a lot more details have emerged about the specifics of the rumour. In particular, the rumour suggests that a value of r=0.2 has been “detected”, which would enable them to perhaps get a 4-5 sigma detection of primordial gravitational waves. However, this would be in tension with existing results from the WMAP and Planck constraints (see fig below) which would be fascinating, but would have put the team in a serious quandary about whether to publish their results.
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The fact that Andrei Linda is giving a seminar the day after the press conference suggests that he is particularly happy about the results and wants to emphasise the wider implications. Since he is primarily associated with chaotic inflation, that was looking rather bad after Planck, we may speculate that the new results will be more consistent with chaotic inflation and a larger value of r, and perhaps as large as the rumoured value of 0.2? Time will tell!
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planckfigurer
 
 
 
 
 
 
 
 
 
 
 
 

The Wheel Turns

In the old days, academic journals would typically give you 25 or more free reprints of any article you published. They were exact replicas of your article as it appeared in print, complete with volume and page numbers, and could be sent to interested colleagues as a “post-print” advertising option, or given to family and friends (because your loved ones deserve  a boring, jargon-filled derivative piece of research material they can’t understand for Christmas!)

This practise has long since disappeared for most or perhaps all journals covering subjects with large overlap with the arxiv (one of the few downsides of the arxiv, as I will argue below). Reprints ceased to make sense: the free immediacy of the arxiv changed how people felt about preprints. The diets of researcher’s rapidly changed from a staple of hard-copy published articles to electronic preprints from the arxiv which could trivially be emailed to researchers who might be interested (but actually aren’t). Reprints and old-school printed preprints simply piled up on desks and filing cabinets only to thrown out en masse when people moved office (the dreaded clear-out!)

I remember very clearly how my PhD my supervisor, the wonderful late Dennis Sciama, would come into my office in the morning with a hard copy of a pre-print. By then I was reading the arxiv in the morning far more regularly than I ate breakfast, so I had almost always seen the preprint and had either printed my own copy or saved it to file, so didn’t want another copy. After a couple of months Dennis simply stopped bringing them to me and so, sadly, I lost a channel of chatting to him. Fortunately there was always lunch. Let’s hope Soylent doesn’t kill this avenue for chatting between academics!

Anyway, fast forward to 2013. Now if you want an official copy of your paper, you have to download it from the journal webpage. If your institution doesn’t subscribe then you have to pay, typically $25-$35 for a paper – even if you are the author, which I think is disgraceful. Now you might think this is irrelevant – typically the arxiv version has all the info. That is true, but in the last few years we have seen the rise of funding linked closely to bibliometrics and in South Africa at least, the government is now demanding hard copies of journal articles as a cheap way (for them) to be sure of the validity of authorship claims, to circumvent bibliometric fraud. Hence the arxiv version is not good enough.

Ironically now those free journal reprints would come in really handy. In the case of a book, the funding agency wants a copy of the book (for free)! In some cases, the online version of a journal is one or two years behind the print edition to encourage libraries to subscribe to the excessively expensive hard-copy version, and if your university library doesn’t subscribe to the hard copy, then you may have no option but to buy a copy of your own paper, just to prove you wrote it!

This was the amazing time-wasting rabbit hole into I went down a year or so ago when I tried to get a copy of our paper on Fisher4cast, our Fisher matrix code, which we published in IJMP since there are very journals that accept papers about codes. Not only did none of my three institutions have the print version, neither apparently did anyone else. Eventually our librarian, through some quantum process which appeared to involve tunneling briefly into a parallel universe in which someone actually subscribed to the print version, was able to get a copy. But it was tortuous in the extreme and I came very close to simply buying the article, which would have killed me inside.

What is the alternative? Well, journals could give authors a pdf of the final journal article with some legal proviso that it is to be used only for personal use, a bit like what happens when you buy a song or e-book online. As usual the complex issues around this, set within a much broader context of creativity and the impact of the internet, are encapsulated in a wonderful TED talk by Larry Lessig,  which I highly recommend you watch. When most people are breaking the law, its time to change the law…

The Nobel Prize Randomness

This morning they announced the award of the 2013 Nobel prize in physics to François Englert and Peter Higgs for the theoretical work on the Higgs boson and mass-generation mechanism. My first reaction was one of sadness. The Nobel committee are restricted to awarding the prize to at most three people and in this case, perhaps much more clearly than in most other cases, this forced them to exclude deserving people.

On the theory side there were at least seven other strong theoretical contenders, namely Gerald Guralnik, Carl Hagen and Tom Kibble, who wrote an important paper independently of the others. Then there were the early achievements of Phillip Anderson and Jeffrey Goldstone, whose work was very relevant and the two Russians Sacha Migdal and Sacha Polyakov, whose paper appeared later after having being initially rejected. Finally, Robert Brout (who died the year before the discovery of the Higgs) co-wrote the paper with Englert, but the Nobel cannot be made posthumously.

On the observational side there were the thousands of scientists who built, ran and analysed the data from the two detectors: ATLAS and CMS,  who actually made the discovery of the Higgs at the LHC. They get no love from the Nobel committee. It isn’t hard to figure out why. They had only one slot left. Who should get it? There are many interesting discussions of who should get credit, e.g. herehere and here. Many would argue that Tom Kibble should have got it, but in the end the committee chose the conservative route.

My sadness comes from thinking of those who have been excluded from recognition, but much more I think this highlights an unfortunate aspect of the way we anoint with royal jelly certain people as “being worthy” and leave out others, in Nobel prizes and much more broadly when credit is given in physics.

The difficulties in awarding a prize and the randomness involved in who did the original research is beautifully described in the Nobel lecture of David Politzer who shared the 2004 Nobel prize in physics for the discovery of asymptotic freedom. The main theme of his entire lecture is the dilemma of attribution and I highly recommend it, especially to young physicists.

Tom Kibble himself commented that the work just didn’t seem that important at the time. This same idea is reflected beautifully in Politzer’s lecture were he says:

           “A key first step was to know the Yang-Mills beta function…By the way, Erick Weinberg was supposed to compute it for an appendix of his thesis, to carry out a generalization of a renormalization group flow argument that appears in the Coleman-Weinberg paper, except for a realistic, non-Abelian weak interaction theory. But, in the end, I guess he figured he had enough stuff to get his degree, and it was time for him to move on to something new. I had actually hoped we’d compare notes, but he never attempted the calculation.”

Yes, the Nobel prize for asymptotic freedom ending up going to something that was originally going to be an appendix in someone else’s thesis.

In this regard I like this parody that came out a week or so before the announcement of the prize in which it was “announced” that the Nobel prize had instead been awarded to the Higgs Boson itself. In many ways that seems fitting because we are marking a step forward in human knowledge, not who happened to submit their paper first.

You can read the original Nobel prize-winning papers by here.

My TEDGlobal 2013 experience

The past week I have been slightly depressed and in several different ways, in mourning. When I arrived at the TEDGlobal conference two weeks ago, I met old hands who joked about the “TED-ache” that would follow the end of the conference. They dropped a trail of aphorisms like bread crumbs about the sense of loss and exhaustion that would follow the conference, as if to guide us through the enormity of the experience that we were about to have, and to help us find our way out of the loss when it was all over. 

The main TED stage

The main TED stage

TED is insanely intense. It is a hyper-stimulated environment fueled by massive dopamine shots ingested from swimming in a sea of amazing ideas and people. It is fueled by oxytocin: 100 times a day you shake hands, hug and touch people who are genuinely keen to meet and know you. It is the biggest amusement park for grown-ups I have ever seen. I had less than fours hours sleep every night for a week and yet did not feel tired. As someone who needs lots of sleep this is clearly not normal, which is why it is not surprising that many people – including me – report crashing afterwards. And not surprisingly many come back year after year to get their fix.

TEDGlobal is so intense that I found myself very emotional: laughing, crying and smiling most of the time. But you know what was perhaps the best of all? There was almost no negativity. Imagine that: a week without negativity, just a euphoric celebration of humanity surrounded by great, can-do people.

Attending TEDGlobal reminded me of Joseph Campbell’s commentary about life:

             “People say that what we’re all seeking is a meaning for life. I don’t think that’s     what we’re really seeking. I think what we’re seeking is an experience of being alive, so that our life experiences on the purely physical plane will have resonance within our own innermost being and reality, so that we actually feel the rapture of being alive. That’s what it’s all finally about.”

TED gives an intense experience of being alive but it is also about the meaning of your life. As I paged through the notes I took during the talks I found “TED gives you a healthy sense of shame” scrawled in big letters at the bottom of one page. Surrounded by so many amazing people there is a very real opportunity to feel small. Fortunately arrogance is in short supply and that is lucky because no matter what you base your sense of self-worth on, there is someone there who is doing it better than you. But instead of depressing me, it showed me what is possible. The participants as much as the speakers are role models of a life well-lived, a life infused with meaning. 

The audience at TED University

The audience at TED University

The intense sense of community and shared aspirations that exists at TEDGlobal in a brief island in space and time is like being inside a Star Trek movie in which humanity has reached above pettiness and war, and found a higher mission based on exploration and federation. At TED you have the chance to be the best version of yourself. Which is one of the reasons why leaving and returning to the “real world” is painful. Just like with Burning Man, decompression after the event is difficult. 

The emotions felt after TED are varied. Apart from coming down from the potent natural cocktail of adrenaline, dopamine and oxytocin and the loss of the best-version-of-yourself, you must deal with the loss of people with whom you have just met but with whom you share a genuine deep connection. We have not evolved skills to deal with making deep connections only to have them ripped away a week later. Then there is the distortion of time…

I remember bumping into a new TED friend whom I hadn’t seen for fifteen minutes, or perhaps fifteen hours. We hugged and joked that we hadn’t seen each other for ages and in TED-time, it was true. Since we had last seen each other we had laughed, cried, given standing ovations and partied. In the normal course of life that much emotional hiking up and down might have required six months. At TED it happens in a matter of hours. After TED one returns to a slow-motion world filled with slowly drying paint and a world with no empathy for what is an unashamedly first-world problem. How does one explain all this experience to people who have never attended TED?

Ironically the only people who can understand what you are going through are other TED participants. But they are no longer directly part of your life. And since it is your separation from them that is a major cause of your pain you are left with a strange quandary…reaching out for support intensifies your suffering!

But don’t get me wrong. These pangs of growth are totally worth it. Father’s Day was two days after TED closed but because of a cancelled flight I spent it alone in a Holiday Inn in Slough, near Heathrow, instead of spending time with my father. But in a strange twist of fate I was with him, because he was born in Slough; in a totally different world; one that had the shadow of Hitler looming over Europe. As I sat there I mused about the different roles of a Father: to love, to provide a safe haven to explore the world, to educate, to facilitate the learning of life lessons, to encourage, to excite, to provide growth experiences, to teach how to be happy…  Seen through this lens, TEDGlobal is very much like a father; it provides its participants with all these things and more, carried lovingly through a beautifully choreographed and orchestrated living art work.

800 people have gone out into the world temporarily a little down and depressed perhaps, but ultimately equipped and inspired to make the world a much better place. If you ever get the chance to go attend one of the TED conferences, go for it. It will change your life.

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I would like to express my sincerest thanks to the TEDx and Bill and Melinda Gates Foundation teams for the opportunity to attend the 2013 TEDGlobal conference. You can learn more about hosting your own TEDx event here and more about TEDxChange here. Locally we run the TEDxAIMS event.

For views of TEDGlobal 2013 that focus on what was actually presented I recommend the official blog and this bit of the internet. The official photostream is here.

Update: 14 July 2013. Henrik has a personal and much more useful summary of the conference here which I recommend!

Demystifying the Higgs Boson

The announcement of the potential discovery of the much-anticipated Higgs boson at the Large Hadron Collider (LHC) sent waves of excitement around the globe this month. The last – and probably the most important – missing piece of the standard model of particle physics appears to have fallen into place.

But, as the media dust settles, many readers will be left wondering: “what exactly is the Higgs?” – known colloquially as the `God particle’ – and “why should I care?

First off, the Higgs boson is a sub-atomic particle. That means it is much, much smaller than an atom in size. It was detected by smashing known particles together at very high energies and using the resulting energy to produce, we believe, the Higgs particles out of thin air. These Higgs particles then decay to known particles in very characteristic patterns that we see as a signature of the Higgs’ fleeting existence, much like a criminal leaving fingerprints at a crime scene.

Candidate Higgs collision in the Large Hadron Collider

Physicists have been quick to emphasise that they haven’t yet fully confirmed that what they have seen is actually the Higgs. So what is going on? Have they seen it or haven’t they?

This is best answered by analogy. Imagine your pet theory says that every night a new type of dark, giant rat, about half a meter long, will sneak into your garage at 9pm under cover of darkness. Your theory makes four definite predictions – it is a new type of rat, it is huge, it will come at 9pm and further, it is dark and hard to see. That night at 9pm sees you waiting expectantly in the garage. Suddenly you hear scurrying and you switch on the light only to see something big and dark disappear through a hole, but you don’t get a really good look at it.

Now there was definitely something there and both the size and shape were consistent with your giant rat theory. Moreover, whatever it was came exactly when your theory predicted, 9pm. But are you sure you have discovered this new type of rat? No, not really. It could be something else that looks like a giant rat. But for sure it couldn’t have been a glowing gnome, so you have ruled out some exotic possibilities.

That is the situation we are in now. The LHC has seen something and it is consistent with the predictions of our theory, the standard model, but it might be something much more exotic than the Higgs. Instead of a giant rat it might be a small dragon or something even more interesting.

So how does the LHC make new particles where none existed before?

Colliding particles together to make new particles is a bit like selling several ordinary cars and using the cash to buy a Ferrari. You convert mass (the cars) into cash (energy) and then back into mass (the Ferrari).  So how much does a Higgs cost?

The key is Einstein’s famous equation E = mc2. This tells you how much energy you need to make your Higgs, which weighs about 100 times more than a proton. If you don’t have enough money, you can’t buy a Ferrari and if you don’t have enough energy, you can’t make a Higgs. This was the situation before the LHC. Humanity couldn’t put enough energy into the particles we were smashing together to make Higgs particles. Now we think we do.

This analogy fails however, to capture the true excitement of fundamental science. We can pop down to our local prestige car showroom or look at http://www.ferrari.com, but we don’t know what Nature’s range of luxury cars is. All we can do is make new theories with the knowledge that if we scrape enough money together, we get to create cars no one has ever seen before and thereby test our theories of the Universe. And that is exactly the story that has unfolded over the past 48 years.

In 1964 theorists speculated that the Higgs particle must exist and the LHC appears to have finally confirmed it. On the one hand, given everything we know about the standard model of particle physics, which has been proven right for thirty years, the Higgs had to be there. But checking that it actually is there is important. It’s like you awaken after a serious car crash and wiggle your toes. It feels like your feet are still there, but do you crane your neck to look at them just to make sure? Of course!

The particle is named for Professor Peter Higgs, one of the six people who can plausibly be called proud parents of the Higgs. Since the Nobel Prize cannot be given to more than three people, there are going to be two very disappointed people (unfortunately one passed away before the recent discovery). Science, like other parts of life, is not always fair in assigning credit.

So apart from retail therapy on a grand scale, why should you care about the Higgs?

The Higgs is the centerpiece of our current understanding of nature at the smallest scales and it claims to explain the answer to a simple yet profound question:

“what is mass?

From Einstein’s theory of relativity we know that if a particle doesn’t have any rest mass it must travel at the speed of light. That would be bad for us because there would be no atoms and no life. Particles would pass like high speed ships in the night. But what is this rest mass and where does it come from? Why does an electron, for example, have any mass at all and why does it weigh nearly 200,000 times less than the top quark, the most massive known particle?

The mantra is that the Higgs gives mass to all particles, hence the joke that the ‘God Particle’ must be Catholic.  But if the Higgs is the puppet master that determines how much each particle weighs, how does it pull the strings?

Perhaps it is easiest to think of mass as something that resists an object from accelerating. It is far easier for Rodger Federer to serve a tennis ball at 160 km/h than it would be to do the same with a 7 kg shot put.

So to explain mass, we need only to find a very natural way to resist acceleration. The Higgs field, which permeates all of space, does this.  It connects to particles via a force that is described mathematically in a very similar way to the action of a rubber band. If you try to stretch a rubber band you have to accelerate the ends of the band and it resists you. That resistance is very much like mass.

The strength of the virtual rubber band connecting the Higgs to each type of particle is different and voila! each different type of particle has a unqiue mass. Electrons are lighter than quarks because their connection to the Higgs is weaker. So in a sense, in the standard model, “mass” is an illusion perpetuated by the Higgs field which is pulling all the strings.

One of the amazing implications of this view is that masses of particles can change if the value of the Higgs field changes. Indeed, the standard model suggests that in the first second or so after the Big Bang, the Higgs field was zero, all its rubber band connections to other particles were slack and hence all particles were massless and traveled at the speed of light.

Let’s try an analogy to tease out some intuition for this radical idea. Imagine you join a new startup company. They don’t have much money so they pay you in shares instead, with the restriction that you cannot sell the shares until they say you can. This makes the shares basically worthless in the meantime. For a long time the company carries on like this, and each one of the new employees gets a different number of shares. Some have 42, others 6400 and so on. It doesn’t really matter how many shares everyone has at this stage, they don’t have any value in the sense that they can’t be sold to someone on the street.

But one day the company is listed on the stock market, just like Facebook did recently, and suddenly you can sell each of your shares at a set value. Suddenly everyone’s portfolio has made money but the value of each portfolio is different. People with a lot of shares are rich, others are not, but everyone has made some money. In this analogy the stock price plays the role of the Higgs field and the value of each person’s stock portfolio plays the role of each particle’s mass, which is unique to it.

Confirming the radical view of mass that goes with the Higgs is obviously important and exciting from a fundamental point of view. But there are other reasons the Higgs is important too. Apart from mass, one of the fundamental quantum properties of a particle is its “spin”. This doesn’t really have a nice everyday interpretation so just think of it as a label for now. But it is key to understanding why it is called the Higgs boson.

What is a boson? Bosons, named after Indian physicist Satyendra Bose, are fundamental particles with spin equal to 0, 1, 2 etc… It turns out all the forces of nature are described by such particles. On the flip side, all matter – like the electron, quarks, protons and neutrons making up our bodies and cream cheese bagels – is made of particles with spin ½. The Higgs is a boson because we believe it has spin 0. If the Higgs is confirmed, it will be the first spin 0 particle ever discovered. All other known bosons have higher spin.

Before now we had never seen any spin-0 particles, which made people justifiably skeptical about their existence. It turns out that spin-0 particles are very useful for something completely different: making the universe expand faster and faster, something that is needed both in the very early universe and today. Indeed, the 2011 Nobel Prize in physics went to the discovery that the cosmos is expanding in such an accelerating fashion, something which surprised everyone. Confirming the spin-0 nature of the Higgs would give strong circumstantial support to the idea that cosmic acceleration is driven by bosons.

So will the LHC shut down once it confirms the properties of the Higgs? Definitely not. In fact, it is still only running at half power and the real hope is that when it ramps up fully we will find new physics that will guide us in building a better, sexier model of reality at the smallest possible scales. If the Higgs is the Ferrari we were expecting, what are the as yet unknown exotics waiting to be predicted and discovered? It is the joy of physics that the human mind can use creativity and mathematics as lantern and guide to predict some truth about the Universe that is perhaps only found to be true 50 years later.

In 1973 Jacob Bronowski wrote

Physics in the 20th century is an immortal work. The human imagination working communally has produced no monuments to equal it, not the pyramids, not the Iliad, not the ballads, not the cathedrals. The men who made these conceptions one after another are the pioneering heroes of our age.

It is now truer than ever.

 

 

Some lovely animations of the Higgs collisions are available at the CMS experiment site.

1929 words

Copyright Bruce Bassett