Of Particular Significance

Author: Matt Strassler

Professors at research universities engage in many different activities, and one which is little known to the public involves teaching at short and focused “schools” for graduate students. These schools, which generally last one to four weeks, and are usually (but not always) held outside the main academic year in winter or summer, allow these students to learn advanced topics in short courses that their universities wouldn’t be able to offer.

For instance, at most universities in the United States, a course focused on the theory of quarks and gluons (the set of equations known as “QCD”) would be attended by just a few students. And many universities don’t even have a professor who is truly expert on this subject. But when interested students from many universities are brought together at one of these specially organized schools, a world’s expert on QCD can teach a group of students as large as fifty or more. Not only is there economy of scale in this arrangement, it also helps to foster a future community among the students who attend. I myself went to one such school when I was a graduate student, and the faculty and students I met there include a number who are my professional peers today.

Usually, professors are not paid to teach at these schools, even though preparing a course is often a huge amount of work. There are two inducements, other than the satisfaction derived from the teaching itself. The first is that travel and lodging are free for the teacher; they are paid for by the organizers of the school, who in turn get the required funds from their university and/or government organizations. The latter (wisely, in my opinion) see such schools as having national value, in that they help assure a strong national research community in the future. The second is that the schools are often held in places where a person would not regret spending a week. The schools at which I have taught over the years have occurred in Boulder, Colorado (USA); Vancouver, British Columbia (Canada); Fermilab National Lab in Aurora, Illinois (USA); Cambridge, England; Kyoto, Japan; and Varna, Bulgaria. I’ve also taught in Italy, previously in the towns of Trieste and Erice, and this month in Florence (i.e. Firenze). For the next ten days or so, I’ll be at the Galileo Galilei Institute for Theoretical Physics (GGI), which is named, of course, after Florence’s most famous scientist.

(Several of my previous short courses are available in written or video form, and most are still sufficiently up-to-date to be useful to future experts. All of them assume, at least in large part, that a student has had a beginning course in quantum field theory. I can provide some links later this week if there is interest, though most of them easily show up in a web search.)

This is my first visit to the GGI, which is associated with the University of Florence, and is located on a hill a couple of miles from downtown Florence, not very far from where Galileo himself lived for some years. It was founded around 2006 to host focused research workshops, as well as brief schools. The theoretical particle physics graduate students at this school have already learned about dark matter from Tomer Volansky (a collaborator of mine on a trigger-related project), and about supersymmetry from David Shih (a former colleague at Rutgers and a recent collaborator on a supersymmetry/LHC project.) They’ll also be learning about the Higgs phenomenon and its generalizations from Raman Sundrum (who’s been mentioned many times on this blog, and whom I visited last month); about the physics of “flavor” — including the issue of how the six different types quarks transition from one to another via the weak nuclear force — from Gino Isidori; and about the physics of quarks and gluons from one of the world’s great experts, Stefano Catani. (You may not recognize these names, as none of them have written books for the public or developed a popular website or blog; but any expert in the theoretical particle physics knows them very well.) And last and perhaps least, they’ll be learning various bits of particle physics that one ought to know in the context of particle colliders, and particularly of the Large Hadron Collider [LHC], from me.

One corollary of this news is that I’ll be pretty busy for the next ten days, so I’m not sure how active the blog will really be. But I can promise you at least one post on string theory!

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON January 15, 2014

Baloney.  Hogwash.  Garbage.

That’s what’s to be found in the phys.org news article claiming that “Scientists at Towson University in Towson, Maryland, have identified a practical, yet overlooked, test of string theory based on the motions of planets, moons and asteroids, reminiscent of Galileo’s famed test of gravity by dropping balls from the Tower of Pisa.”

Sounds too good to be true, right?  And it is.

What the scientists have done (or at least claim to have done, and I’ll be happy to take their claims at face value, since I can’t easily check them) is carry out  a technique to test the equivalence principle, a foundation stone of Einstein’s theory of gravity, which implies that all objects, no matter what material they are made of and no matter how heavy they are, will be pulled by gravity in the same way… with the same acceleration.  This principle, in Einstein’s theory, lies behind why all objects on earth fall with the same acceleration (when air resistance can be neglected), and behind why astronauts float in their space stations.

By looking at the precisely measured orbits of different objects in the solar system, which are made from different materials, the authors (James Overduin, Jack Mitcham and Zoey Warecki of little-known Towson University) claim in their July 2013 paper to have provided new tests that the equivalence principle applies to different materials.  That’s very nice work.  The principle works to the precision reached by their tests — which aren’t as precise as some other types of tests, but do explore some domains that haven’t previously been explored.

But what’s that got to do with string theory?  If you read their paper, you will notice that the word “String Theory” appears in only one obscure sentence in the introduction, referring to a very specific form of string theory [with an extremely light spin-zero field, called the dilaton], implying that their work might be relevant for string theory if we lived in a stringy universe that had such a field.  Not even the conclusion, much less the bulk of the paper, mentions strings or string theory.  That’s because the paper has nothing to do with testing string theory; it is merely testing Einstein’s theory of gravity. 

The reason it can’t test string theory is

  1. String theory doesn’t make a precise prediction for how the equivalence principle will be modified, and among the many possible universes string theory can lead to, many have no measurable modification of the equivalence principle;
  2. Even if a violation of the equivalence principle had been detected, or is detected in the future, it isn’t necessarily due to string theory.  It might be due to some other modification of Einstein’s gravity — in fact, the authors consider one such modification in their paper!

So here we have a nice little paper that tests Einstein’s theory of gravity and puts constraints on various alternatives to it — though none of those alternatives is unique to string theory nor is uniquely predicted by string theory.  How did this get billed as a practical test of string theory?

You’ll have to ask the author of the phys.org article, which appears to be a Towson University press release.  [“Provided by Towson University”, says the last line of the phys.org article.] Or you’ll have to ask the scientists involved (unless one of them is the author) — who ought to be pretty darned embarrassed that their work was billed in this way.  I hope they didn’t do this on purpose.  It’s certainly great free advertising for Towson University; who cares if the article’s right if people are willing to read it?  But a willingness to distort the facts to impress and mislead the public is not a worthy attribute.

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON January 13, 2014

Wow, it was unusually cold last week. In a small fraction of the globe. For a couple of days. And what does that cold snap, that big wiggle in the Polar Vortex that carries high-atmospheric winds around the North Pole, imply about “climate change”, also known as “global warming”, also known as “global weirding”?

The answer is very simple. Nothing.

If you heard anyone suggest otherwise — whether they said that the extreme cold implies that there is no global warming going on, or they said that the extreme cold implies that global warming is happening — you should seriously question anything that person says when it comes to climate change. Because that person does not respect (or perhaps even understand) the difference between anecdote and evidence; between weather and climate; between a large fluctuation and a small but long-term trend. Or between media hoopla and science.

In the interest of an imperfect analogy: Let me ask you this. Are you generally happier, or less happy, than you were five years ago? Answer this as best you can.

Now let me ask you another question. Did you, within the last month, have a really, really bad day, or a really, really good one?

Does the answer to the second question have much to do with the answer to the first one?

Barring an exceptional recent disaster in your personal or professional life, the fact that, say, last Thursday your car broke down, you locked yourself out of your house, your dog vomited on the carpet and you got caught in the rain without your umbrella does not have anything to do with whether you are a happier person than you were five years ago. Being a happier person has more to do with whether you have a better job, a happier family, a better sense of self-esteem, and things like that. And even if you love your job, you know there are going to be really bad days in the office sometimes. That’s just the way it goes. We all know that.

It’s the same with daily and monthly and yearly fluctuations in the stock market compared to the slow but fairly steady century-long growth of the U.S. economy (both curves corrected for inflation.)

So why, when there’s a big fluctuation in the daily, monthly or even seasonal weather, do people jump up and down about what the implications are for the long-term trends in climate? (more…)

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON January 13, 2014

Welcome 2014! And quite a start to the year, with a cold snap that rivals anything we’ve seen in two decades. I don’t remember cold like this since the horrid winter of 1994, when the Northeastern U.S. saw snowstorms and extreme cold that alternated back and forth for weeks. Of course, when I was a child in the 1970s, such chills happened a lot more often; I remember a number of New England mornings where I awoke to a thermometer reading of -20ºFahrenheit (-29ºCelsius) [244 Kelvin].

The scariest negative temperature numbers that one hears about from the media are associated with the “wind chill”, which is a number that is supposed to measure how cold the air “feels” to your skin.  But “wind chill” is a rather subjective and controversial measure — there’s no unique way to define it, since you’ll feel differently depending on how much exposed skin you have, on your body weight, on your age and conditioning, etc.  By contrast, the temperature measured by a thermometer is defined independent of how humans feel, and experts agree on what it is and means. Oh sure, people use different scales to measure it: Fahrenheit (F), Centigrade or Celsius (C), and Kelvin (K).  But the differences are no more than the distinction between meters and feet, or between kilograms and pounds; it’s straightforward, if a bit annoying, to convert from one to the other.

So everyone agrees the temperature is and feels extremely cold, But is it, from the point of nature, really that much colder than usual? To say it another way: it was 84ºF (29ºC) in southern Florida yesterday.  How much warmer is that than the -40ºF (-40ºC) that was registered in the cold Minnesota morning?

Well, you might first think: wow, it’s a difference of 124ºF (69ºC), which sounds like a huge difference.  But is it really so huge? (more…)

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON January 7, 2014

I’m still kind of exhausted from the effort (see yesterday’s post) of completing our survey of some of the many unexpected ways that the newly discovered Higgs particle might decay. But I would be remiss if, before heading off into the holiday break, I didn’t issue some well-deserved congratulations.

The Jade Rabbit rover on the surface of the Moon, 15 December. Credit:Xinhua

Congratulations, first, to China — to the scientists and engineers who’ve managed to put a lander and a rover on the Moon. If you think that’s easy… think again! And they succeeded on their first attempt, a real coup. Now let’s see what science they can do with it, exploring a region of the Moon that apparently may offer answers to important questions about the Moon’s history. Specifically, by accident or by design, the rover is going to be able to explore an area of considerable geological importance, involving one of the Moon’s giant lava flows, a relatively young one (1-2.5 billion years rather than 3 billion or more).

Soyuz VS06, with Gaia, lifted off from Europe’s Spaceport, French Guiana, on 19 December 2013. Copyright: ESA – S. Corvaja, 2013

Congratulations, next, to the scientists and engineers of the European Union, who’ve put a fantastic telescope into space, destined to orbit the sun. The Gaia mission is aimed at doing the extraordinary: mapping, with ultra-high precision, the locations and motions of no less than 1 billion stars within our galaxy — nearly 1% of the total number. The distance to each of these stars will be determined by parallax — looking at how the positions of stars wobble, from the perspective of the spacecraft as it orbits the sun — and the real motions of the stars will be determined by how they drift across the sky, and by the Doppler effect for light.  This wealth of information will help scientists figure out the shape and history of the galaxy to a degree never previously possible.  Meanwhile, Gaia will also be able to do a lot of other science, picking up distant supernovas outside our galaxy, nearby asteroids orbiting our sun, and signs of planets around other stars, as well as brown dwarfs (small failed stars) that may be floating around between the stars. Gaia can even check some aspects of Einstein’s theory of gravity! Read here about all the wonderful things this mission can do.

Congratulations also to the scientists and engineers in Iran, who’ve apparently moved their rocketry program, and its potential application to human space flight, among other things, another step forward. A second monkey has made the trip to the edge of space, a suborbital trip. (Did the first survive? it’s not clear, and admittedly Iran is known for photo-shopping reality into supporting the story it wants to tell. Not that it matters; it took the US several tries, back over 60 years ago, before a monkey survived the trip, and the survival rate continued to be poor for a while. )  Anyway, it puts Iran well on its way toward its goal of a human in space by 2018.

And finally, congratulations to my own country, the United States, for having passed a budget deal. Not out of the woods yet, but at least it was bipartisan, and we’re not yet talking about another damaging government shutdown, or worse, default. Politics isn’t rocket science. We’ll have to hope our politicians can learn something from China: that it’s good to find some common and worthy goals to work toward together, rather than to fight about absolutely everything and bring the nation’s operations to a halt.

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON December 20, 2013

After many months gestation and a difficult labor, a behemoth is born!  Yes, it’s done, finally: our 200 page tome entitled “Exotic Decays of the 125 GeV Higgs Boson“.  Written by thirteen hard-working theoretical particle physicists, this is a paper that examines a wide class of possible decays that our newly found Higgs particle might exhibit, but that would not occur if the Standard Model of particle physics (the equations we use to describe the known elementary particles and forces plus the simplest possible type of Higgs particle) were all there was to see at the Large Hadron Collider [LHC], the giant proton-proton collider outside of Geneva, Switzerland.  

[Non-experts; sorry, but this paper was written for experts, and probably has a minimum of two words of jargon per sentence. I promise you a summary soon.]

Why is looking for unusual and unexpected decays of the Higgs particle so important?  [I’ve written about the possibility of these “exotic” decays before on this website (see herehere,  hereherehereherehere and here).]  Because Higgs particles are sensitive creatures, easily altered, possibly in subtle ways, by interactions with new types of particles that we wouldn’t yet know about from the LHC or our other experiments. (This sensitivity of the Higgs was noted as far back to the early 1980s, though its generality was perhaps only emphasized in the last decade.)  The Higgs particle is very interesting not only on its own, for what it might reveal about the Higgs field (on which our very existence depends), but also as a potential opportunity for the discovery of currently unknown, lightweight particles, to which it might decay.  Such particles might be the keys to unlocking secrets of nature, such as what dark matter is, or maybe even (extreme speculation alert) the naturalness puzzle — very roughly, the puzzle of why the mass of the Higgs particle can be so small compared to the masses of the smallest possible black holes.

The goal of our paper, which is extensive in its coverage (though still not comprehensive — this is a very big subject) is to help our experimental colleagues at ATLAS and CMS, the general purpose experiments at the LHC, decide what to search for in their current (2011-2012) and future (2015-) data, and perhaps assist in their decisions on triggering strategies for the data collecting run that will begin in 2015.  (Sorry, LHCb folks, we haven’t yet looked at decays where you’d have an advantage.) And we hope it will guide theorists too, by highlighting important unanswered questions about how to look for certain types of exotic decays.  Of course the paper has to go through peer review before it is published, but we hope it will be useful to our colleagues immediately. Time is short; 2015 is not very far away.

Although our paper contains some review of the literature, a number of its results are entirely new.  I’ll tell you more about them after I’ve recovered, and probably after most people are back from break in January.  (Maybe for now, as a teaser, I’ll just say that one of the strongest limits we obtained, as an estimate based on reinterpreting published ATLAS and CMS data, is that no more than a few × 10-4 of Higgs particles decay to a pair of neutral spin-one particles with mass in the 20 – 62 GeV/c2 range… and the experimentalists themselves, by re-analyzing their data, could surely do better than we did!)  But for the moment, I’d simply like to encourage my fellow experts, both from the theory side and the experimental side, to take a look… comments are welcome.

Finally, I’d like to congratulate and thank my young colleagues, all of whom are pre-tenure and several of whom are still not professors yet, on their excellent work… it has been a pleasure to collaborate with them.  They led the way, not me.  They are (in alphabetical order): David Curtin, Rouven Essig, Stefania Gori, Prerit Jaiswal, Andrey Katz, Tao Liu, Zhen Liu, David McKeen, Jessie Shelton, Ze’ev Surujon, Brock Tweedie, and Yi-Ming Zhong. They hail from around the world, but they’ve worked together like family… a great example of how our international effort to understand nature’s deep mysteries brings unity of purpose from a diversity of origins.

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON December 19, 2013

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