Saturday, July 28, 2007


I am guilty of frequently using physics speech in daily life, an annoying habit I also noticed among many of my colleagues [1]. You'll find me stating "My brain feels very Boltzmannian today", or "The customer density in this store is too high for my metastable mental balance". I have a friend who calls Chinese take out "the canonical choice" and another friend who, when asked whether he had made a decision, famously explained "I don't yet want my wave-function to collapse". My ex-boyfriend once called it "the physicist's Tourette-syndrome" [2].

One of my favourite physics-speech words is self-consistent. Self-consistency is tightly related to nothing. You know, that "nothing" that causes your wife to conclude her whole life is a disaster, we're all going to die in a nuclear accident, her glasses vanished (again!), and btw that's all your fault (obviously). But if you ask her what's the matter. Well, nothing.
    "There's nothing I hate more than nothing
    Nothing keeps me up at night
    I toss and turn over nothing
    Nothing could cause a great big fight
    Hey -- what's the matter?
    Don't tell me nothing."

~Edie Brickell, Nothing

1. Self-consistent

Science is our attempt to understand the world we live in. We observe and try to find reliable rules upon which to build our expectations. We search for explanations that are useful to make predictions, a framework to understand our environment and shape our future according to our needs. If our observations disagree with our rules, or observations seemingly disagree with each other (I swear I left my glasses in the kitchen), we are irritated and try to find a mistake. Something being in contradiction with itself [3] is what I mean with not self-consistent (What's the matter? - Nothing!).

On a mathematical basis this is very straight forward. E.g. If you assume my mood is given by a real valued continuous function f on the compact interval [now, then] with f(now)f(then) smaller than 0, this isn't self-consistent with the expectation it can do so without having a zero [4]. For more details on my mood, see sidebar.

Self-consistency is a very powerful concept in theoretical physics: if one talks about a probability, that probability better should not be larger than one. If one starts with the axioms of quantum mechanics, it's not self-consistent to talk about a particle's definite position and momentum. The speed of light being observer independent is not compatible with Galileo invariance and the standard addition law for velocities. Instead, self-consistency requires the addition law to be modified. This lead Einstein to develop Special Relativity.

A particularly nice example comes from multi-particle quantum mechanics, where an iterative approach can be used to find a 'self-consistent' solution for the electron distribution e.g. in a crystal or for an atom with many electrons (see self-consistent field method or Hartree-Fock method). A state of several charged particles will not be just a tensor product of the single particles, since the particles interact and influence each other. One starts with the tensor product as a 'guess' and applies the 'rules' of the theory. That is, by solving the Schrödinger equation with the mean- field potential which effectively describes the interaction, a new set of single particle wave functions can be computed. This result will however in general not agree with the initial guess: it is not self-consistent. In this case, one repeats the procedure with using the result as an improved guess. Given that the differential equations behave nicely, this iterative procedure leads one to find a fixed point with the properties that the initial distribution agrees with the resulting one: it is self-consistent.

A similar requirement holds for quantum corrections. A theory that is subject to quantum corrections but whose initial formulation does not take into account the existence of such extra terms is strictly speaking not self-consistent (see also the interesting discussion to our recent post on Phenomenological Quantum Gravity).

There are some subtleties one needs to consider, most importantly that our knowledge is limited in various regards. Self-consistency might only hold under certain assumptions or in certain limiting regimes, like small velocities (relative to the speed of light), large distances (relative to the Planck length) or at energies below a certain threshold. Likewise, not being self-consistent might be the result of having applied a theory outside these limits (typically, using an expansion outside a radius of convergence). In some cases (gravitational backreaction), violations of self-consistency can be negligible.

However, one might argue if it is possible at all to arrive at such a disagreement then at least one of the assumptions was unnecessary to begin with, and could have been replaced by requiring self-consistency. Unfortunately, this is often more easily said than done -- physics is not mathematics. We rarely start with writing down a set of axioms which one could check for self-consistency. Instead, in many cases one starts with little more than a patchwork of hints, and an idea how to connect them. Self-consistency in this case is somewhat more subtle to check. My friends and I often kill each others ideas by working out nonsensical consequences. Here, at least as important as self-consistency is that a theory in physics also has to be consistent with observation.

2. Consistent with Observation

The classical Maxwell-Lorentz theory is self-consistent. However, it is in disagreement with the stability of the atom. According to the classical theory, an electron circling around the nucleus should radiate off energy. The solution to this problem was the development of quantum mechanics. The inconsistency in this case was one with observation. Without quantizing the orbits of the electron, atoms would not be stable, and we would not exist.

This requirement is specific to sciences that describe the real world out there. Such a theory can be 'wrong' (not consistent with observation) even though it is mathematically sound. Sometimes however, these two issues get confused. E.g. in a recent Discover issue, Seth Lloyd wrote:

    "The vast majority of scientific ideas are (a) wrong and (b) useless. The briefest acquaintance with the real world shows that there are some forms of knowledge that will never be made scientific [...] I would bet that 99.8 percent of ideas put forth by scientists are wrong and will never be included in the body of scientific fact. Over the years, I have refereed many papers claiming to invalidate the laws of quantum mechanics. I’ve even written one or two of them myself. All of these papers are wrong. That is actually how it should be: What makes scientific ideas scientific is not that they are right but that they are capable of being proved wrong."

~Seth Lloyd, You know too much

The current issue now had a letter in reply to this article:

    "I was taken aback by Seth Lloyd's assertion that "99.8 percent of ideas put forth by scientists are [probably] wrong" and even more so by his statement that "of the 0.2 percent of ideas that turn out to be correct ... [t]he great majority of them are relatively useless." His thesis omits a basic trait of what we call science -- that it is a continuous fabric, weaving all provable knowledge together [...] we do science for a science sake, because a fundamental principle of science is that we never know when a discovery will be useful"

~Eric Fisher, Springfield, IL.

Well, the majority of my scientific ideas are definitely (a) wrong and (b) useless, but these usually don't end up in a peer review process. However, the reply letter apparently referred to the word 'correct' as 'provable knowledge', and to science as the 'weave' of all that knowledge. It might indeed be that the mathematical framework of a theory that is not consistent with observation turns out to be useful later but that doesn't change the fact that this idea is 'wrong' in the meaning that it does not describe nature. Peer review today seems to be mostly concerned with checking self-consistency, whereas being non-consistent with observation is ironically increasingly tolerated as a 'known problem'. Like, the CC being 120 orders of magnitude too large is a known problem. Oohm, actually the result is just infinity. But, hey, you've turned your integration contour the wrong way, the result is not infinity, but infinity + 2 Pi.

The requirement of consistency with observation was for me the main reason to chose theoretical physics over maths. The world of mathematics, so I found, is too large for me and I got lost in following runaway thoughts, or generalizing concepts just because it was possible. It is the connection to the real world, provided by our observations, that can guide physicists through these possibilities and lead the way. (And, speaking of observations and getting lost, I'd really like to know where my glasses are.)

3. Self-contained

Unlike maths, theoretical physics aims to describes the real world out there. This advantageous guiding principle can also be a weakness when it comes to the quantities we deal with. Mathematics deals with well defined quantities whose properties are examined. In physics one wants to describe nature, and the exact definitions of the quantities are in many cases subject of discussion as well. Consider how our understanding of space and time has changed over the last centuries!

In physics it has often happened that concepts of a theory's constituents only developed with the theory itself (e.g. the notion of a tensor or the Fock-space). As such it happens in physics that one can deal with quantities even though the framework does not itself define them. One might say in such a case the theory is incomplete, or not self-contained.

Due to this complication, I've known more than one mathematician who frowned upon approaches in theoretical physics as too vague, whereas physicists often find mathematical rigour too constraining, and instead prefer to rely on their intuition. Joe Polchinski expressed this as follows:

    "[A] chain of reasoning is only as strong as its weakest step. Rigor generally makes the strongest steps stronger still - to prove something it is necessary to understand the physics very well first - and so it is often not the critical point where the most effort should be applied. [A]nother problem with rigor [is]: it is hard to get it right. If one makes one error the whole thing breaks, whereas a good physical argument is more robust."

~Joe Polchinski, Guest Post at CV

When it comes to formulating an idea, physicists often set different priorities than mathematicians. In some cases it might just not be necessary to define a quantity because one can sit down and measure it (e.g. the PDFs). Or, one can just leave a question open (will be studied in a forthcoming publication) and get a useful theory nevertheless. All of our present theories leave questions open. Despite this being possible, it is unsatisfactory, and the attempt to make a theory self-contained has lead to many insights throughout the history of science.

Newton's dynamics deals with forces, yet there is nothing in this framework that explains the origin of a force. It contains masses, yet does not explain the origin of masses. Maxwell's theory provides an origin of a force (electromagnetic). It has a source term (J), yet it does not explain the dynamics of the source term. This system has to be closed, e.g. with minimal coupling to another field whose dynamics is known. The classical Maxwell-Lorentz theory does this, it is self-contained and self-consistent. However, as mentioned above, this theory is not consistent with observation. Today we know the sources for the electromagnetic field are fermions, they obey the Dirac equation and Fermi statistic. However, if you look at an atom close enough you'll notice that quantum electrodynamics alone also isn't able to describe it satisfactory...

Besides the existence of space and time per se, the number of space-time dimensions is one of these open questions that I find very interesting. It has most often been an additional assumption. An exception is string theory where self-consistency requires space-time to have a certain number of dimensions. However - if it also contains an explanation why we observe only three of them, nobody has yet found it. So again, we are left with open questions.

4. Simple and Natural [5]

The last guiding principle that I want to mention is simplicity, or the question whether one can reduce a messy system of axioms and principles to something more simple. Is there a way to derive the parameters of the standard model from a single unified approach? Is there a way to derive the axioms of quantization? Is there a way to derive that our spacetime has dimension three, or Lorentzian signature?

In my opinion, simplicity is often overrated compared to the first three points I listed. We tend to perceive simplicity as elegance or beauty, concepts we strive to achieve, but these guidelines can turn out to be false friends. If you can find your glasses, look around and you'll notice that the world has many facettes that are neither elegant nor simple (like my husband impatiently waiting for me to finish). Even if you'd expect the underlying laws of nature to be simple, you'll still have to make the case that a certain observable reflects the elementary theory rather than being a potentially very involved consequence of a complex dynamical system, or an emergent feature. A typical example are the average distances of planets from the sun, a Sacred Mystery of the Cosmos that today nobody would try to derive from a theory of first principles (restrictions apply).

Also, we tend to find things simpler the more familiar we are with them, up to the level of completely forgetting about them (did you say something?). E.g. we are so used to starting with a Lagrangian that we tend to forget that its usefulness rests on the validity of the action principle. It is also quite interesting to note that researchers who are familiar with a field often find it 'simple' and 'natural'... I therefore support Tommaso's suggestions to renormalize simplicity to the generalized grandmother.

In this regard I also want to highlight the argument that one can allegedly derive all the parameters in the standard model 'simply' from today's existence of intelligent life. Notwithstanding the additional complication of 'intelligent', could somebody please simply explain 'existence' and 'life'?


Much like classical electrodynamics, Einstein's field equations too have a source term whose dynamics one needs to know. The system can be closed with an equation of state for each component. This theory is self-consistent [6], and it is consistent with all available observations. It reaches its limits if one asks for the microscopic description of the constituents. The transition from the macro- to the microscopic regime can be made for the sources of the gravitational field, but not also for the coupled gravitational field (oh, and then there's the CC, but this is a known problem).

Two theories that yield the same predictions for all observables I'd call equivalent (if you don't like that, accept it as my definition of equivalence.) But our observations are limited, and unlike the case of classical electrodynamics not being consistent with the stability of the atom, there is presently no observational evidence in disagreement with classical gravity.

For me this then raises the question:
    Is there more than one theory that is self-consistent, self-contained and consistent with all present observations?

In a recent comment, Moshe remarked:"To paraphrase Ted Jacobson, you don't quantize the metric for the same reason you don't go about quantizing ocean waves." That sounds certainly reasonable, but if I look at water close enough I will find the spectral lines of the hydrogen atom and evidence for its constituents. And their quantization. To me, this just doesn't satisfactory solve the question what the microscopic structure of the 'medium', here space-time, is.

And what have we learned from all this...?

Let me go back to the start: If you ask a question and the answer is 'Nothing', you most likely asked the wrong question, or misunderstood the answer.

Ah... Stefan found my glasses (don't ask).

See also: Self-Consistency at The Reference Frame

[1]This habit is especially dominant -- and not entirely voluntarily -- among the not native English speakers, whose vocabulary naturally is most developed in the job related area.
[2] Unintentional cursing and uttering of obscenities, called Coprolalia, is actually only a specific feature of the Tourette syndrom.

[3] However, some years ago I was taught the word 'self-consistency' in psychology has a different meaning, it refers to a person accumulating knowledge from his/her own behaviour. A person whose thoughts and actions are in agreement and not in contradiction is called 'clear'. (At least in German. I couldn't find any reference to this online, and I'm not a psychologist, so better don't trust me on that.).
[4] See:
Bolzano's theorem.
[5] "Woman on Window", by F.L. Campello.
For more, see here.
[6] Note that this theory is self-consistent at arbitrary scales as long as you don't ask for the microscopic origin of the sources.

TAGS: , ,


The most spherical object ever made... used for the gyroscopes in NASA's Gravity Probe B. Launched in April 2004, Gravity Probe B tests two effects predicted by Einstein's theory: the geodetic effect and the frame-dragging (see here for a brief intro).

In order for Gravity Probe B to measure these tiny effects, it must use a gyroscope that is nearly perfect—one that will not wobble or drift more than 10-12 degrees per hour while it is spinning.

"A nearly-perfect gyroscope must be nearly perfect in two ways: sphericity and homogeneity. Every point on its surface must be exactly the same distance from the center (a perfect sphere), and its structure must be identical from one side to the other [...]

After years of research and development, Gravity Probe B produced just such a gyroscope. It is a 1.5-inch sphere of fused quartz, polished and “lapped” to within a few atomic layers of perfect sphericity. A scan of its surface shows that only .01 microns separate the highest point from the lowest point. Transform the gyroscope into the size of the Earth and its highest mountains and deepest ocean trenches would be a mere eight feet from sea level!"


Thursday, July 26, 2007

FIAS, the Frankfurt Institute for Advanced Studies

This week, I was again at the new campus of my old university. The science departments of the Johann Wolfgang Goethe University are all moving out of downtown Frankfurt into the fields of Niederursel, where new buildings keep springing up at an extraordinary rate. One of these new buildings is especially eye-catching with its bright-red finish.

This is the new building of FIAS, the Frankfurt Institute for Advanced Studies, and it's interesting not only because of its colour - it's one of the first public research institutes in Germany financed to a large extent by the money of private sponsors.

Universities in Germany have traditionally been financed by public money of the state and federal governments, and they usually don't have large funds at their own. Frankfurt University is a bit special in this respect, since it has been founded in 1914 by wealthy Frankfurt citizens. While today it is a publicly funded university as it is common in Germany, there is a strong tradition of private sponsoring of research and higher education.

So, a few years ago, theoretical physicist Walter Greiner and neuroscientist Wolf Singer started using their connections to raise private funds to establish a new kind of institute, which was supposed to be legally independent, but closely connected to the university and its science departments. It should bring together theorists from such diverse areas as biology, chemistry, neuroscience, physics, and computer science in order to address problems all revolving around a common theme: The study of structure formation and self-organization in complex systems.

This was the beginning of FIAS.

Today, there are more than 50 scientists, guests and students working together on cooperative phenomena on length scales ranging from quarks in colour superconductivity and heavy ion collisions over atoms in atomic clusters and macromolecules to cells in the immune system and the brain. Details and more links can be found on the pages of the FIAS scientists.

The training of graduate students is organized in a Graduate School. Last summer, I was involved in the compilation of a brochure presenting the FIAS, and I was fascinated by the really inspiring atmosphere among the students, who come from all over the world and form very diverse scientific backgrounds, but were always involved in interesting discussions.

In September, the FIAS is supposed to move into the new, red building, which was built for the institute by a private sponsor, the Giersch Foundation. There, FIAS scientist will have a place to work and think - it will be interesting to follow the outcome of this kind of "experiment".


Tuesday, July 24, 2007

Don't fart

Okay, it's unlikely you visit this blog to hear my opinion about farting, but I just read this article in New Scientist

How the obesity epidemic is aggravating global warming
(Issue June 30th - July6th, p. 21)

which is the most ridiculous fart line up of weak links designed to support a specific opinion that I've come across lately. The argumentation of the author, Ian Roberts (a professor of public health in London), is roughly: if you're fat you are wasting energy. Either by storing fat such that it can't even be used as bio fuel, or by moving it around with the help of gasoline powered transportation devices.

To begin with, despite of what the title says, the author does not actually talk about global warming, but about wasting energy. The connection between both is just assumed in the first sentence with 'we know humans are causing [global warming]', and not even once addressed after this. On the other hand, also the connection between wasting energy and obesity is constructed to make the point that you should loose weight to save the earth:

"[...] it is becoming clear that obese people are having a direct impact on the climate. This is happening through their lifestyles and the amount and type of food they eat, and the worse the obesity epidemic gets the greater its impact on global warming."

Well, if one wants to criticize a lifestyle, then one should criticise a lifestyle, but not add several associative leaps after that. Let us start with asking what exactly is a 'waste' of energy? Using energy for purposes that do not necessarily improve our well-being could generally be considered a waste. That goes for breaking a cellphone (consider all the energy needed to produce it), browsing the web the whole day (your home wireless doesn't run on vacuum energy) as well as for unnecessary consumption of food for whose production energy was needed.

However, whether that food is actually eaten or thrown away is completely irrelevant in this context. Also, on an equal footing one can argue that the mere presence of diet products damages the climate: it takes energy to produce and transport them, but the energy gain after consumption is lowered. Is there any reason to waste energy on producing diet coke when one can as well drink water? And while we're at it, is there any reason to go jogging every morning - isn't that just a waste of energy? Come to think about it, civilization itself seems to be a waste of energy.

The article goes on arguing

"[...] his greater bulk and higher metabolic rate will cause him to feel the heat more in the globally warmed summers, and he will be the first to turn on the energy intensive air conditioning."

If one argues that overweight people turn on the AC more often because they sweat more easily, one might want to take into account that underweight (or generally sickly) people tend to turn on the heating more often. People who suffer from back pain, arthritis and shortness of breath might use their car more often (as the article states), but this must not necessarily be a cause of obesity. The only thing one can state is that being healthy and well adapted to the part of the world you live in minimizes the additional energy needed to survive and feel comfortable (how 'needed' relates to 'actually used' is a completely different question).

I am definitely in favor of more sidewalks, of increased awareness for health risks caused by obesity, and I totally agree that we should save energy. But I would appreciate a scientific discussion of these issues, and not a mixed up mesh of several issues all drowned in politcal correctness.

In a similar spirit I read last week several articles claiming "Meat is murder on the environment" or likewise, a 'conclusion' based on a paper "Evaluating environmental impacts of the Japanese beef cow–calf system by the life cycle assessment method" (published in Animal Science Journal 78 (4), 424–432)

"a kilogram of beef is responsible for the equivalent of the amount of CO2 emitted by the average European car every 250 kilometres"

Being a vegetarian myself, I could give you a good number of reasons to drop the meat, but nothing you wouldn't find online in some thousand other places, so let me just focus on the issue at hand. If you want to save energy with the food you buy and eat, the most important factor to consider is origin and transportation.
  • Your apple from New-Zealand, labeled 'bio' or not, doesn't tunnel to you. In fact you could say since, unlike beef, vegetables and friuts consist mostly of water, the amount of gasoline needed per energy content (joule) of transported food is higher for greens. So, preferably buy stuff that was not transported all around the globe whenever you can.
  • If you buy products from countries where slash and burn is still practiced, you're damaging the environment more than if you support your local farmer - even if he's somewhat more expensive than Safeway.
  • And, needless to say, don't buy stuff you don't need. Each time you have to throw something away, you are throwing away all the energy that was necessary to produce it. That doesn't only go for food, but for everything else including wrappings.

I want to add that much like cows, human flatulence as well release methane, which is said to contribute to global warming. So maybe we should consider a national anti-fart campaign? Regarding the vegetarian factor, also please note that "The cellulose in vegetables cannot be digested, therefore vegetarians produce more gas than people with a mixed diet." [source]

The bottomline of this writing is: don't construct or publish ridiculous cross-relations that are scientifically doubtful for a catchy headline.

See also: Global Warming

Monday, July 23, 2007

This and That

  • I am very proud to report that I eventually managed to install a recent-comments-box in the sidebar!! Thanks go via several detours back to Clifford.

  • Flip has an excellent post on The Braneworld and the Hierarchy in the Randall Sundrum (I) model

  • Hey America, Germany is catching up.

  • Idea of the day: I suggest that journals which reject more than 70% of submitted manuscripts should offer a consolidation gift. What I have in mind is a shirt saying "My manuscript went to PRD and all I got was this lousy T-shirt".

  • Ever felt like your brain is too small? Think twice (if you have capacity left): Man with tiny brain shocks doctors

  • Coincidentally, I came across the German version of Lee Smolin's book Warum gibt es die Welt? (Life of the Cosmos), which I found somewhat disturbing (I mean, even more than the English version). Among other things (that concern Japanese surfer) I learned that New York is the largest city on the planet (such the re-translation). Apologies to the translator*, but should you consider buying that book, I strongly recommend the English version (to read the original sentence go to amazon, and search inside for "irrelevant content" - amazingly the result is only one hit).

  • Quotation of the day:

    "The days come and go like muffled and veiled figures sent from a distant friendly party, but they say nothing, and if we do not use the gifts they bring, they carry them as silently away."

    Ralph W. Emerson, in Society and Solitude [Vol 7], Chapter VII: Works and Days

* It turned out my husband knows him personally. It's a small world...

Sunday, July 22, 2007

GZK cutoff confirmed

In an earlier post, Bee explained the physics behind the GZK (Greisen, Zatsepin and Kuzmin) cutoff: protons traveling through outer space will - when their energy crosses a certain threshold - no longer experience the universe as transparent. If their energy is high enough, the protons can scatter with the omnipresent photons of the Cosmic Microwave Background, and create pions. As a result, their mean free paths drops considerably and only very little of them are expected to reach earth. This threshold for photopion production for ultra high energetic protons is known as the GZK cutoff.

The presence of this cutoff had been observed by the HiRes cosmic ray array (Observation of the GZK Cutoff by the HiRes Experiment, arXiv:astro-ph/0703099), but had been disputed by the results from the Japanese detector AGASA (Akeno Giant Air Shower Array) which caused excitement when it failed to see the cut-off in data obtained up to 2004. A third experiment, the Pierre Auger Observatory on the plains of the Pampa Amarilla in western Argentina, which started taking data last year, now settled the question:

"If the AGASA had been correct, then we should have seen 30 events [at or above 1020 eV], and we see two," says Alan Watson, a physicist from the University of Leeds, U.K., and spokesperson for the Auger collaboration [source]. According to Watson, the data also suggests that these highest energy rays comprise protons and heavier nuclei, the latter of which don't feel the GZK drag.

The results were announced on the 30th International Cosmic Ray Conference in Merida, Yucatan, Mexico, and had a brief mentioning in Nature. The Nature article also points out that there is prospect of identifying the regions of the sources of the highest energetic particles, but these data are preliminary. "Unless I talk in my sleep, even my wife doesn't know what these regions are", as Watson was quoted in Nature.

And of course, now that there is new data, somebody is around to claim one needs an even larger experiment to understand it: "Now we understand that above the GZK cutoff there are ten times less cosmic rays than we thought 10 years ago, so we may need a detector ten times as big as Auger," says Masahiro Teshima of the Max Planck Institute for Physics in Munich, Germany, who worked on AGASA and is working on the Telescope Array [source].

The recent paper by the Pierre Auger collaboration with more details was on the arxiv last week:
    The UHECR spectrum measured at the Pierre Auger Observatory and its astrophysical implications
    T.Yamamoto, for the Pierre Auger Collaboration, arXiv:0707.2638

    Abstract: The Southern part of the Pierre Auger Observatory is nearing completion, and has been in stable operation since January 2004 while it has grown in size. The large sample of data collected so far has led to a significant improvement in the measurement of the energy spectrum of UHE cosmic rays over that previously reported by the Pierre Auger Observatory, both in statistics and in systematic uncertainties. We summarize two measurements of the energy spectrum, one based on the high-statistics surface detize. The large sample of data collected so far has led to a significant improvement in the measurement of the energy spectrum of UHE cosmic rays over that previously reported by the Pierre Auger Observatory, both in statistics and in systematic uncertainties. We summarize two measurements of the energy spectrum, one based on the high-statistics surface detector data, and the other of the hybrid data, where the precision of the fluorescence measurements is enhanced by additional information from the surface array. The complementarity of the two approaches is emphasized and results are compared. Possible astrophysical implications of our measurements, and in particular the presence of spectral features, are discussed.

The upper end of the cosmic ray energy spectrum as measured by the Pierre Auger Observatory: The black dots represent data points, the blue and red curves are expectations derived from different models for the composition and energy distribution of the cosmic ray particles, all based on well-established physics including the GZK cutoff mechanism. Two events cannot be understood as stemming from protons, but may well be explained by heavier nuclei. (Figure from T. Yamamoto, The UHECR spectrum measured at the Pierre Auger Observatory and its astrophysical implications, ICRC'07; Credits: Auger Collaboration, technical information)

More plots and data can be found on the websites of the Pierre Auger Observatory.


Saturday, July 21, 2007

The LHC at Nature Insight

With less than a year to go before the start of the Large Hadron Collider at CERN, there has been a lot of media coverage about this huge collider lately - see e.g. at NYT, The New Yorker, and of course Bee's post The World's Largest Microscope.

Much more in-depth information on the physics, the history, and the engineering aspects of the LHC can be found in this week's Nature Insight: The Large Hadron Collider. Unfortunately, a subscription is required for the full content, but two interesting articles are freely available:

How the LHC came to be, by former CERN Director-General Chris Llewellyn Smith, on the political and organisational struggles involved with the building such an international, multi-billion euro machine, and Beyond the standard model with the LHC, by CERN theorist John Ellis (the guy with the penguins - see page 5), on the different options on possible new physics that might be discovered at the LHC.

Have a nice weekend!


Wednesday, July 18, 2007

Phenomenological Quantum Gravity

[This is the promised brief write-up of my talk at the Loops '07 in Morelia, slides can be found here, some more info about the conference here and here.

When I submitted the title for this talk, I actually expected a reply saying "Look. This is THE international conference on Quantum Gravity. We already have ten people speaking about phenomonelogy - could you be a bit more precise here?". But instead, I found myself joking I am the phenomenology of the conference. Therefore, I added a somewhat extended motivation to my talk which I found blog-suitable, so here it is.]

The standard model (SM) of particle physics [1] is an extremely precise theory and has demonstrated its predictive power over the last decades. But it has also left us with several unsolved problems, question that can not be answered - that can not even be addressed within the SM. There are the mysterious whys: why three families, three generations, three interactions, three spatial dimensions? Why these interactions, why these masses, and these couplings? There are the cosmological puzzles, there is dark matter and dark energy. And then there is the holy grail of quantum gravity (see also: my top ten unsolved physics problems).

There are two ways to attack these problems. The one is a top-down approach. Stating with a promising fundamental theory one tries to reach common ground and to connect to the standard model from a reductionist approach. The difficulty with this approach is that not only one needs that 'promising candidate for the fundamental theory', but most often one also has to come up with a whole new mathematical framework to deal with it. Most of the talks on the conference [2] were top down approaches. The other way is to start from what we know and extend the SM in a constructivist approach. Examples for that might be to take the SM Lagrangian and just add all kinds of higher order operators, thereby potentially giving up symmetries we know and like. The difficulty with this approach is to figure out what to do with all these potential extensions, and how to extract sensible knowledge about the fundamental theory from it.

I like it simple. Indeed, the most difficult thing about my work is how to pronounce 'phenomenology' (and I've practiced several years to manage that). So I picture myself somewhere in the middle. People have called that 'effective models' or 'test theories'. Others have called it 'cute' or 'nonsense'. I like to call it 'top-down inspired bottom-up approaches'. That is to say, I take some specific features that promising candidates for fundamental theories have, add them to the standard model and examine the phenomenology. Typical examples are e.g. just asking what the presence of extra dimensions lead to. Or the presence of a minimal length. Or a preferred reference frame. You might also examine what consequences it would have if the holographic principle or entropy bounds would hold. Or whether stochastic fluctuations of the background geometry would have observable consequences.

These approaches do not claim to be a fundamental theory of their own. Instead, they are simplified scenarios, suitable to examine certain features as to whether their realization would be compatible with reality. These models have their limitations, they are only approximations to a full theory. But to me, in a certain sense physics is the art of approximation. It is the art of figuring out what can be neglected, it is the art of building models, and the art of simplification.

    "Science may be described as the art of systematic over-simplification."

~Karl Popper


One can imagine more beyond the standard model than just QG! So, if we are talking about phenomenology of quantum gravity we'll have to ask what we actually mean with that. To me, quantum gravity is the question how we can reconcile the apparent disagreements between classical General Relativity (GR) and QFT. And I say 'apparent' because nature knows how quantum objects fall, so there has to be a solution to that problem [3]. To be honest though, we don't even know that gravity is quantized at all.

I carefully state we don't 'know' because we've no observational evidence for gravity to be quantized whatsoever. (The fact that we don't understand how a quantized field can be coupled to an unquantized gravitational field doesn't mean it's impossible.) Indeed one can be sceptical about whether it's observable at all. This is reflected very aptly in the below quotation from Freeman Dyson, which I think is deliberately provocative and basically says my whole field of work doesn't exist:

    "According to my hypothesis, the gravitational field described by Einstein's theory of general relativity is a purely classical field without any quantum behavior [...] If this hypothesis is true, we have two separate worlds, the classical world of gravitation and the quantum world of atoms, described by separate theories. The two theories are mathematically different and cannot be applied simultaneously. But no inconsistency can arise from using both theories, because any differences between their predictions are physically undetectable."

~Freeman Dyson [Source]

Well. Needless to say, I do think there there is phenomenology of QG that is in principle observable, even though we might not yet be able to observe it. And I do think that observing it will lead us a way to QG.

However, there are various scenarios that could be realized at Planckian energies. Gravity could be quantized within one or the other approach. Also, higher order terms in classical gravity could become important. Or, there could be semi-classical effects coming into the game. Now one tries to take some insights from these approaches, leading to the above mentioned phenomenological models. Already here one most often has a redundancy. That is, various scenarios can lead to the same effect. E.g. modified dispersion relations, or the Planck scale being a fundamental limit to our resolution are effects that show up in more than one approach. In addition, there's a second step in which these models are then used to make predictions. Again, various models, even though different, could yield the same predictions. That's what I like to call the 'inverse problem': how can we learn something about the underlying theory of quantum gravity from potential signatures?

In the figure below I stress 'new and old' phenomenology because a sensible model shouldn't only be useful to make new predictions, it should also reproduce all that stuff we know and like. I have a really hard time to take seriously a model that doesn't reproduce the standard model and GR in suitable limits.

Now here are some approaches in this category of 'top down inspired buttom up approaches' that I find very interesting (for some literature, see e.g. this list):

(And possibly we can maybe soon add macroscopic non-locality to that list, an interesting scenario that Fotini, Lee and Chanda are presently looking into.)

However, whenever one works within such a model one has to be aware of its limitations. E.g. the models with large extra dimensions are in my opinion such a case in which has been done what sensibly could be done. And now we'll have to turn on the LHC and see. After the original ideas had been outlined, many people began to build more and more specific models with a lot of extra features. It's not that I don't find that interesting, but it's somewhat besides the point. To me it's like building a house and worrying about the color of the curtains before the first brick has been laid.

Now, all of the approaches I've mentioned above are attempts to get definitive signatures of QG, but so far none of these predictions on its own would be really conclusive. Take e.g. a possible modification of the GZK cutoff - could have been 'new' physics, but not clear which, or maybe just some ununderstood 'old' physics, like the showers not being created by protons from outside our galaxy as generally assumed?

So, my suggestion to make progress in this regard is to construct models that are suitable to investigate observables in varios different areas. In such a way, we could be able to combine predictions and make them more conclusive. Think about the situation with GR at the beginning of the last century: It predicted a perihelion precession of Mercury, but there were other explanations like an additional planet, a quadrupole moment of the sun, or maybe a modification of Newtonian gravity. It took another observable - in this case light deflection by the sun - that was predicted within the same framework, and confirmed GR was the correct description of nature [4]. And please note, a factor 2 mattered here [5].

I personally am very optimistic about the future progress in quantum gravity - and that not only because it's hard to beat Dyson's pessimism. I think it doesn't matter where we start from, may it be a top-down, a buttom-up approach or somewhere in the middle. I also think it doesn't matter which direction each of us starts into. The history of science tells us that there often are various different ways to arrive at the same conclusion. A particularly nice example is how Schrödinger's wave formulation and Heisenberg's matrix approach turned eventually out to be part of the same theory.

I think as long as we listen to what our theories tell us, if we take into account what nature has to say, are willing to redirect our research according to this - and if we don't get lost in distractions along the way, then I think we have good chances to find a way to quantum gravity. And this finally solves the mystery of the quotation on the last slide of my talk:

    'The problem is all inside your head' she said to me
    The answer is easy if you take it logically
    I’d like to help you in your struggle to be free
    There must be fifty ways to [quantum gravity]

[1] In my notation the SM includes General Relativity.
[2] The exception being the very recommendable talk on
Effective Quantum Gravity by John F. Donoghue.
[3] Though 3 years living in the US have tought me there's actually no such thing as a 'problem' - it's called a challenge. One just has to like them, eh?
[4] Admittedly, what the measurement actually said was not as straight forward as one would have wished. I leave it to my husband to elaborate on this interesting part of the history of science.
[5] The resulting deviation can be reproduced in the Newtonian approach up to a factor 1/2.

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PS on Zeitgeist...

More at

Tuesday, July 17, 2007

AvH's 10 point plan

The Alexander von Humboldt Foundation is the master of science networking among the German non-profit foundations. If you've managed to get one of their scholarships you become part of their brotherhood for a lifetime, including a membership card - Unfortunately I don't know about the secret handshake, since I've never even applied. The largest drawback of their scholarships is that one can only apply to a host who is also a member (Humboldtianer!), which was the reason for me to choose the German Academic Exchange Service (DAAD) instead.

However, I've just found that AvH came up with a ten point plan of recommendations "for making Germany more attractive for international cutting-edge researchers". Their suggestions make a lot of sense to me and I find the press release worth mentioning. Even though some of it (2./7.) addresses specifically German problems, especially the points 9. and 10. apply to many other countries as well, so does 4., and 3. is generally a good idea (that I too have mentioned repeatedly, and in my opinion an issue that will become more important the more complex and global the scientific community becomes). Let us hope that all these pretty word-ideas will have concrete consequences in the not to far future.

For the full text, see here. In brief the points are:

1. More jobs for scientists and scholars

On average, German professors supervise 63 students. This is more than twice as many as the average at top-rank international universities.

2. Academic careers need planning certainty: establishing tenure track as an option for junior researchers

German universities must take measures to plan the career stage between a doctorate and a secure professorship and make it compatible internationally. On the pattern of the Anglo-Saxon tenure track, clear, qualifying steps should be defined at which decisions are made about remaining at an institution.

3. Career support as an advisory and supervisory task of academic managers

Senior academics as well as university and/or institute directors must play an active role in human resources development for their junior researchers. Young scientists and scholars need careers advice.

4. Promoting early independence by taking risks in financing research

By international comparison, young academics in Germany have less scope for decision-making and action. Funding programmes for early, independent research must be strengthened. Especially for researchers at an early stage in their careers, procedures should be profiled for research work involving an unknown risk factor.

5. Making recruitment and appointments more professional

Appointment procedures must have an open outcome and be transparent. To this end, commissions charged with appointments must include external or independent expert reviewers. Good academics should be appointed quickly. Internationally respected universities can no longer afford to take years over appointments, particularly as universities and research establishments now actively have to recruit junior researchers internationally to a much greater extent than they did in the past.

6. Dissolve staff appointment schemes and adapt management structures

Rigid staff appointment schemes must make way for flexible appointment options, or be dissolved. Independent junior research group leaders must be put on a par with junior professors within the universities and in collaborations between universities and non-university research establishments.

7. Creating special regulations for collective wage agreements in the academic sector

According to many of those involved, the new wage agreement for the public service sector is not commensurate with appropriate remuneration for academic and non-academic staff at non-university and university research establishments. By comparison with other pay-scales, it is not competitive, either nationally or internationally, it restricts mobility, and its rigid conditions do not take account of the special features of academic life.

8. Internationally competitive remuneration

It must be ensured that cutting-edge researchers can be offered internationally competitive remuneration. The framework for allocating remuneration to professors currently valid at universities leaves too little scope for this.

9. Internationalising social security benefits

Internationally mobile researchers often have to accept major disadvantages or financial losses with regard to pension rights.

10. Increasing transparency and creating an attractive working environment

  • Academic employers in Germany must be put in a position to offer organisational and financial support for removal and relocation which is already the norm in other countries, especially when top-rank academic personnel are appointed.
  • Child-care facilities for internationally mobile researchers at universities and non-university research establishments must be expanded quickly and extensively. International appointments in Germany still often fail because there is a lack of child-care facilities.
  • Careers advice and support for (marital) partners seeking employment as well as so-called dual career advice or support for academic couples are required to attract internationally mobile researchers. Examples from abroad indicate that this does not necessarily mean concrete job offers ( which are often difficult to find), rather, intelligent counselling can satisfy many people's needs.
Related: See also The LHC Theory Initiative, The Terrascale Alliance, Temporary Display, and Temporary Display - Contd.


... is not only a German word that I've never heard a German actually using [1], but also the title of the new Smashing Pumpkins album. By coincidence, I've been wearing my ancient ZERO shirt last week, so I felt like it was my duty to pick up the CD.

It is an interesting album, but overall very disappointing. To begin with, I never liked Billy Corgan's voice, but if there's no way around it, it definitly goes better with melancholy and infinite sadness than with revolution. I mean, come on, he's composing a song in 2006 titled United States with lyrics saying "fight! I wanna fight! I wanna fight! revolution tonight!" and manages to sing such that it could as well have been about, say, compactification on Calabi Yau manifolds [2].

There are more politically flavored tracks on the album: For God and Country ("it's too late for some, it's too late for everyone") and Doomsday Clock ("it takes an unknown truth to get out, I'm guessing I'm born free, silly me") but the only thing worth mentioning about them is the fact there presently is a market for this. This tells a lot more about the 'Zeitgeist' than the music itself [3].

Most of the tracks on the CD sound extremely similar, drowned in an ever present electric guitar soup and exchangeable melodies. Billy Corgan is at his best with the slower and more thoughtful titles like e.g. Neverlost ("If you think just right, if you'll love you'll find, certain truths left behind").

Favourite tracks from previous albums: Disarm, To Sheila, Bullet with Butterfly Wings, 1979

[1] My husband proudly reports he can testify at least one incident in which one of his uncles, a Prof. for theology and philosophy, successfully used the word.
[2] That's why I call it a science blog.
[3] And while I am at it: the German 'ei' is pronounced like the English 'I' (or the beginning of the word 'aisle') in both places (whereas the German 'i' is pronounced like the English 'ee'). The German 'Z' is pronounced close to 'ts'. That is with 'Tsaitgaist', you'll make yourself understood better than with 'seetgeest'.

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Monday, July 16, 2007

What's new?

Nothing. Well, almost nothing.

  • I dyed my hair. The color is galled 'ginger'. I'd have called it pumpkin. It actually looks like foul apricots. Say of the day so far 'What happened to your hair?' - 'It's an allergic reaction.' - 'To what?' - 'Stupid questions.' (As one can easily deduce, my conversation partner in this case obviously was not Canadian.)

  • Though the plan was this year it would not be necessary to pack my household into boxes and drag them around, I will actually be moving twice before the end of the year. Don't ask. At least I am staying in town.

  • My last plant, which suffered significantly during my previous trip, has surprisingly recovered (well, at least half of it), and is so not looking forward to my upcoming trip. This is to warn you that I'll be flying to Europe on Thursday, and be off and away for a while.

  • I've found six degrees of freedom.

  • I just saw this paper on the arxiv:

    Search for Future Influence from L.H.C
    By Holger B. Nielsen, Masao Ninomiya

    Abstract: We propose an experiment which consists of pulling a card and use it to decide restrictions on the running of L.H.C. at CERN, such as luminosity, beam energy, or total shut down. The purpose of such an experiment is to look for influence from the future, backward causation. Since L.H.C. shall produce particles of a mathematically new type of fundamental scalars, i.e. the Higgs particles, there is potentially a chance to find hitherto unseen effects such as influence going from future to past, which we suggest in the present paper.

    which features the idea that the nature of the Higgs field is such that it attempts to avoid its own production: "When the Higgs particle shall be produced, we shall retest if there could be influence from the future so that, for instance, the potential production of a large number of Higgs particles in a certain time development would cause a pre-arrangement so that the large number of Higgs productions, should be avoided."

    Therefore - if this hypothesis is true - the LHC is likely to suffer an accident and has to be shut down. The argument is supported by the cancellation of the Superconducting Supercollider: "Thus it is really not unrealistic that precisely at the first a large number of Higgs production also our model-expectations that is influence from the future would show up. Very interestingly in this connection is that the S.S.C. in Texas accidentally would have been the first machine to produce Higgs on a large scale. However it were actually stopped after a quarter of the tunnel were built, almost a remarkable piece of bad luck."

    The authors therefore propose to give backwards causation an economically less damaging possibility to avoid Higgs production by means of a card game that settles runs for the LHC, and permits for the possibility to shut down completely in a quiet and undesastrous way.

    One should take this very seriously: "It must be warned that if our model were true and no such game about restricting strongly L.H.C. were played [...] then a “normal” (seemingly accidental) closure should occur. This could be potentially more damaging than just the loss of L.H.C. itself. Therefore not performing [...] our card game proposal could - if our model were correct - cause considerable danger."

    I find this interesting as it gives a completely new spin to postdiction. See, we now can have a theory that disables its own observability by backward causation. So, one can actually post-dict something before it has happened, and then go back into the future. Makes me wonder though why the universe hasn't disabled itself even before nucleosynthesis. Maybe God doesn't playing dice with the universe, but instead card games?

  • Have a good start into the week!

Saturday, July 14, 2007

First Light for the Gran Telescopio Canarias

Last night, the Gran Telescopio Canarias (GTC) at the Observatorio del Roque de los Muchachos of the European Northern Observatory (ENO) in La Palma, Canary Islands, Spain, saw its "First Light". The first star observed was Tycho 1205081, close to Polaris - a bit more photogenic is this shot of the pair of interacting galaxies UGC 10923 with extended star formation regions, taken with an exposure time of 50 seconds:

Interacting galaxies UGC 10923 seen with the eyes of the World's largest telescope (Credits: Gran Telescopio Canarias, Instituto de Astrofisica de Canarias)

The primary mirror of the new telescope consists is made up of 36 separate, hexagonal segments, fabricated at the Glaswerke Schott in Mainz, just around the corner from Frankfurt. Taken together, the segments have a light-collecting surface of 75.7 m2, which corresponds the a circular mirror with a diameter of 10.4 metres. At this size, it is the currently largest telescope for optical and near-infrared light!

The Gran Telescopio Canarias in La Palma, Canary Isles, in September 2006 (Credits: GTC project webcam)

This was in the news these days here (see e.g.,, or Le Monde), but the European Northern Observatory somehow has managed to issue a press release only in Spanish, so I am a bit at loss to find more details. Actually, the report in the FAZ is very good, and recalls the developments that lead to the construction of these huge telescopes:

I remember from the popular astronomy book I read as a kid that at that time the 5-metre mirror of the Mount Palomar telescope was thought to be the endpoint of the growth of telescope mirror size: Larger solid mirrors are to heavy and deform when the telescope is moved, and moreover, the image gets blurred anyway by the distortions caused to the light as it passes through the atmosphere. As a case in point, a 6-metre telescope in the Soviet Union was mentioned, which produced pictures of not as high a quality as expected from its size. I was quite disappointed when I read that.

Fortunately, both obstacles could be overcome with new technologies first realised in the 1990s: Active Optics, which means that the mirror is always kept in perfect shape by an array of motors and can therefore be lightweight, and large, and Adaptive Optics, which manages to compensate for the fluctuations of the density of air and allows for a seeing nearly as good as in space.

Among the big optical telescopes using these techniques - the Keck, Subaru and Gemini-North telescopes in Hawaii, the four mirrors of the Very Large Telescope and the Gemini-South telescope in Chile, the Large Binocular Telescope in Arizona, the Hobby-Eberly-telescope in Texas, and the South African Large Telescope in the South African Karoo - the Gran Telescopio Canarias is currently the largest one.

The good news is that all these telescopes will continue to take great shots of the Universe for the professionals and for armchair astronomers like me, even when the Hubble Space Telescope will once have stopped working.


Potentially Insane

If you have a look at the sidebar, you'll see that even the internet is presently bored! Here is what PI residents do when they go bonkers.

PI stands for... Probably Improbable, Politically Incorrect, Potentially Insane, Preon Infected, Problems Included, Proudly Ignorant, Promising Insults, Positively Irrational, Presently Insignificant, Philosophical Illusions, Physics Inside

Contributed submissions:

Promoting Ideas, Prain Included, Pump It, Plotting Infinity, Position Independent, Pissing Ion, Perfectly Intolerant, Protecting Insanity, Post Inflation, Plutonium Injection, Pain Intensifier, Premature Interruption, Positive Impact, Private Intrusion

And here is what Wikipedia had to add, see PI (disambiguation):

Primitive Instinct (sometimes), Public Intoxication (definitly), People's Initiative (more than useful), Principal Investigator (haven't seen one), Primary Immunodeficiency (not yet), Predictive Index (none), Provider Independent (that's what I dream of), Pass Interference (my job), Programmed Instruction (absent)

My apologies to the whole public outreach department. I expect a sentence of 4 months snow.

See also: 3.141592653589793238462...

Thursday, July 12, 2007


I once read a science fiction about the not-too far future. Our planet's flora became fed up with mankind, and decided to strike back. It began with plumbing problems - tree's roots destroying pipes, went on to grass breaking through the pavement and ivy growing over houses. I have to think about this each time when I see a tree causing cracks in a walkway, or grass growing in every possible and impossible place.

Tuesday, July 10, 2007

Shrinking Earth

No, this is not about a resuscitation of old ideas about the history of planet Earth, but these days I could learn that the Earth Is Smaller Than Assumed, according to geodesist from the University of Bonn who have discovered that the blue planet is really smaller than originally thought. Well - not really, I would say: these guys are talking about 5 millimetre, or 0.2 inch.

Anyway, this accurate result is really impressive! It results from the combined analysis of radio signals from distant quasars, observed by a worldwide net of more than 70 radio telescopes. Characteristic features in the radio signals from quasars are received at slightly different times at different places on Earth, and the combination of these measurements using the technique of Very Long Baseline Interferometry allows a very precise determination of the relative distance of the radio telescopes: These relative distances can be deduced up to 2 millimetre on 1000 km, or up to 2 parts per billion (ppb). From the network of radio telescopes distributed all around the globe, it is possible to calculate its dimension very precisely. This analysis, accomplished with improved precision over previous similar work by the Bonn geodesist, yields a diameter of the Earth 5 millimetre smaller than supposed so far. According to a report in the New Scientist about this result, the total diameter of the Earth at the equator is around 12,756.274 kilometres (7,926.3812 miles).

Axel Nothnagel of the University of Bonn, who heads the team that provided new and more accurate data about the diameter of the Earth. (Credits: University of Bonn Press Release, July 5, 2007, Frank Luerweg)

A propos shrinking Earth: Earth was shrinking by a huge step, in a metaphorical way, 45 years ago today, as I heard this morning on the radio: On July 10, 1962, TELSTAR was launched from Cape Canaveral, the first communications satellite which allowed live TV broadcast between Europe and North America, bridging by the speed of light a distance that is steadily growing by 18 millimetre per year...

The TELSTAR communications satellite, launched 45 years ago today (Source: Wikipedia on Telstar)

PS: The paper by the Axel Nothnagel team is: The contribution of Very Long Baseline Interferometry to ITRF2005, by Markus Vennebusch, Sarah Böckmann and Axel Nothnagel, Journal of Geodesy 81 (2007) 553-564, DOI: 10.1007/s00190-006-0117-x. If someone can tell me where I can find the 5 millimetre in that paper, I am very grateful ;-)

Today on the Arxiv

Today I came across this very entertaining paper
    Hollywood Blockbusters: Unlimited Fun but Limited Science Literacy
    By C.J. Efthimiou, R.A. Llewellyn

    Abstract: In this article, we examine specific scenes from popular action and sci-fi movies and show how they blatantly break the laws of physics, all in the name of entertainment, but coincidentally contributing to science illiteracy.

I didn't even know there is an arxiv for Physics and Society. The authors conclude with
    "Hollywood is reinforcing (or even creating) incorrect scientific attitudes that can have negative results for the society. This is a good reason to recommend that all citizens be taught critical thinking and be required to develop basic science and quantitative literacy."

It's hard to disagree with that recommendation, even without reading the paper. Though I have to say if somebody has the scientific attitude he might survive a jump from the 15th floor, I guess natural selection will take care of that. For most cases I think we've all been taught from earliest childhood on not to mix up fiction with reality... That is, except for those of us who end up in theoretical physics, involuntarily or on purpose bending and breaking the laws of nature on our notebooks.

Update: See also The Physics of Nonphysical Systems.

Monday, July 09, 2007

Monday Links

In case you're just sitting at breakfast looking for a good read:

Sunday, July 08, 2007

The LHC Theory Initiative

Want proof that the grass is always greener on the other side? I just read this article

Refilling the Physicist Pool

about the LHC theory initiative:

"We are behind the Europeans, and we believe very strongly that we shouldn't just leave this work to the Europeans," Baur said in a UB statement. [...]
Funding in the US for particle physics as a whole and theoretical particle physics in particular has declined significantly over the past 15 years, Baur said. In addition, physics departments in US universities tend to hire faculty members who develop innovative ideas, whereas in Europe, the physics culture puts equal emphasis on novel research and solid calculations that help advance the field as a whole. But with the Large Hadron Collider -- the world's largest particle accelerator -- coming online in the next year or sooner, Baur said, the US cannot afford to fall behind."

It's interesting that in the US ideas are 'innovative' whereas in Europe they are 'novel' (especially since both refers to a field that is several decades old, and hasn't seen very much novelty lately). Admittedly, I find the perspective of a 'physics culture' that produces 'solid' Next-to-next-to-next-to-next-to leading order calculations somewhat depressing.

For German counterpart, see also the Terrascale Alliance.

Saturday, July 07, 2007


I spent half of the day trying to sort through all that stuff which has accumulated on my desk while I was away. My efforts where impressively unsuccessful. The only thing that came out of this was the poem below. I think I'll go for a walk, buy a lighter and then give it a second try.


      Cardboard boxes, paper piles,
      Unread books, and many files,
      Coffee cups and empty cans,
      Post-its, trash and broken pens.

      Unpaid bills, forgotten friends,
      Pieces, broken in my hands,
      Wedding photos in between
      Notebooks and a magazine.

      Plastic plants, a moving box,
      And a pair of unmatched socks,
      Unfinished, and missing pieces,
      Leave me wondering where peace is.

[For more, check my website]

... I actually think I have a lighter... if only I could find it... what a mess!

Friday, July 06, 2007

It's all about sex...

... yes, we already knew that. Men are intelligent to impress women, and women are intelligent to find the best men. That's why you're sitting on your desk, chewing a pen, trying to quantize gravity.

Here's what Psychology tells us today (Source: Ten Politically Incorrect Truths About Human Nature, by Alan S. Miller and Satoshi Kanazawa):

"Women often say no to men. Men have had to conquer foreign lands, win battles and wars, compose symphonies, author books, write sonnets, paint cathedral ceilings, make scientific discoveries, play in rock bands, and write new computer software in order to impress women so that they will agree to have sex with them. Men have built (and destroyed) civilization in order to impress women, so that they might say yes."

Well, and once you've destroyed a civilization and sufficiently impressed every women that was 'fit' enough to survive, keep in mind that by your human nature you are actually polygamous because it's an evolutionary advantage:

"Relative to monogamy, polygyny creates greater fitness variance (the distance between the "winners" and the "losers" in the reproductive game) among males than among females because it allows a few males to monopolize all the females in the group. The greater fitness variance among males creates greater pressure for men to compete with each other for mates. Only big and tall males can win mating opportunities. Among pair-bonding species like humans, in which males and females stay together to raise their children, females also prefer to mate with big and tall males because they can provide better physical protection against predators and other males."

And I'm sure 6 feet 4 also come in handy for changing light-bulbs. On the other hand, there are certain natural selection mechanism in societies which tolerate polygamy. As you'll also learn from the above article, suicide terrorists are dominantly Muslim because a) polygamy increases competition among men and b) because they are promised 72 virgins in heaven. (If only things were that simple. I still think airline passengers should stroke pigs before boarding, definitly preferable to throwing away my Coke each time I go through security.)

Also, sorry to report, but having children is statistically seen a bad idea for men when it comes to the peak of the crime-and-creativity curve:

"These calculations have been performed by natural and sexual selection, so to speak, which then equips male brains with a psychological mechanism to incline them to be increasingly competitive immediately after puberty and make them less competitive right after the birth of their first child. Men simply do not feel like acting violently, stealing, or conducting additional scientific experiments, or they just want to settle down after the birth of their child but they do not know exactly why."

I especially like the part with 'they don't know why'. And finally, a Harvard professor solved the puzzle why men prefer D-cups:

"Until very recently, it was a mystery to evolutionary psychology why men prefer women with large breasts, since the size of a woman's breasts has no relationship to her ability to lactate. But Harvard anthropologist Frank Marlowe contends that larger, and hence heavier, breasts sag more conspicuously with age than do smaller breasts. Thus they make it easier for men to judge a woman's age (and her reproductive value) by sight—suggesting why men find women with large breasts more attractive."

Well, I think there's truth in it, as my age seems to be incredibly hard to judge. Related, you'll be interested to hear that a recent study shows Women Don't Talk More Than Guys:

"The researchers placed microphones on 396 college students for periods ranging from two to 10 days, sampled their conversations and calculated how many words they used in the course of a day. The score: Women, 16,215. Men, 15,669.The difference: 546 words: "Not statistically significant," say the researchers."

Have a nice weekend. Have fun. Reproduce. Go, discover a new country or write a sonnet.

Thursday, July 05, 2007

The Planck Scale

The Planck scales - a length and a mass* - indicate the limits in which we expect quantum gravitational effects to become important

Gravity coupled to matter requires a coupling constant G that has units of length over mass. One finds the Planck scale if one lets quantum mechanics come into the game. For this, let us consider a quantum particle of a (so far unknown) mass mp with a Compton wavelength lp, the relation between both given by the Planck constant

This is the quantum input. Now consider that particle to be as localized as it is possible taking into account its quantum properties. That is, the mass mp is localized within a space-time region with extensions given by the particle's own Compton wavelength. The higher the mass of that particle, the smaller the wavelength. However, we know that General Relativity says if we push a fixed amount of mass together in a smaller and smaller region, it will eventually form a black hole. More general, one can ask when the perturbation of the metric that this particle causes will be of order one:

which then can be solved for the mass, and subsequently for the length scale we were looking for. If one puts in some numbers one finds

These Planck scales thus indicate the limit in which the quantum properties of our particle will cause a non-negligible perturbation of the space-time metric, and we really have to worry about how to reconcile the classical with the quantum regime. Compared to energies that can be reached at the collider (the LHC will have a center of mass energy of the order 10 TeV), the Planck mass is huge. This reflects the fact that the gravitational force between elementary particles is very weak compared to the the other forces that we know, and this is what makes it so hard to experimentally observe quantum gravitational effect.

Max Planck introduced these quantities in 1899, the paper (it's in German) is available online

(Credits to Stefan for finding it). You'll find the natural mass scales introduced on page 479ff. He didn't call them 'Planck' scales then, and it is also interesting why he found them useful to introduce, namely because the aliens would also use them

    "It is interesting to note that with the help of the [above constants] it is possible to introduce units [...] which [...] remain meaningful for all times and also for extraterrestrial and non-human cultures, and therefore can be understood as 'natural units'."

Coincidentally, yesterday I saw a paper on the arxiv
    What is Special About the Planck Mass?
    By C. Sivaram
    Abstract: Planck introduced his famous units of mass, length and time a hundred years ago. The many interesting facets of the Planck mass and length are explored. The Planck mass ubiquitously occurs in astrophysics, cosmology, quantum gravity, string theory, etc. Current aspects of its implications for unification of fundamental interactions, energy dependence of coupling constants, dark energy, etc. are discussed.

which gives a nice introduction into the appearances of various mass scales in physics, with some historical notes.

* With the speed of light set to be equal 1, in which case a length is the same as a time. It you find that confusing, just define a Planck time by dividing the length through the speed of light.