Physics

News from the LHC: the integrated proton-proton luminosity at 7 TeV centre-of-mass energy has generously passed the mark of 40 inverse picobarns yesterday. The CMS experiment alone has integrated over 42 inverse picobarns, as shown in the graph below (the blue curve shows the data collected by CMS, the red one the data produced by the LHC).


In one description, an observer falls freely through empty space, in another one, she hits a surface smack on, yet both descriptions are completely equivalent. This example for a duality in modern physics was explained the last time in this series. There we saw that a black hole can also be described by a string theoretical membrane at the event horizon. The observer cannot escape the black hole because she literally gets stuck to the black hole’s event horizon, glued to it via strings.
Physical reality is composed of properties like distance, duration, velocity, area, volume, mass, energy, and temperature. To quantify these properties you need to measure them. And the act of measuring boils down to comparing against an agreed yardstick, a unit of measurement such as a foot, a gram, etc. 

Do you need a dedicated yardstick for each quantifiable property?

Would the answer to this question be 'yes', then physics as we know it, would not be possible. We would not be able to relate the various properties to each other, physics laws would not exist. Fortunately, the answer to the question is a clear 'no'. We need far fewer units than one might expect based on the number of physical properties. 
The CMS experiment has just released a new result which excludes the possibility that quarks have a substructure at energy scales below 4 TeV. The result comes from the analysis of just a handful of inverse picobarns of collision data -2.9 to be precise- and shows excellently just how well suited are the LHC collisions for this business. The limit is extended by over one TeV above the former result of the Tevatron experiments, and some 600 GeV above the results of the ATLAS collaboration, who also recently reported on their search for of quark compositeness in 7 TeV collisions, finding a limit at 3.4 TeV.
Week number one of my course on Subnuclear Gauge Physics is over. I think that in the first five hours of lesson I have given to my students a reasonable picture of the early experimental attempts and theoretical developments aimed at understanding the structure of atomic nuclei and individual nucleons with electron scattering. So I thought I might try and simplify the picture further, to reach a wider audience here. Of course, the topic is not terribly entertaining, unless one understands fully just how important these studies are for fundamental physics even nowadays -despite having started over 60 years back.
Swamped by my course of Subnuclear Gauge Physics, I have little time left to surf the web and keep an eye on what happens in the blogs I usually visit. Nevertheless, today is Saturday and I have allowed myself a short tour. Below is a list of the most interesting things I have read.


  • First of all, there's an interesting new blog out there, with experimental particle physics explained to laymen. The language is not English, but it is a language you should learn, too.
As 2010 nears its end, the Tevatron experiments feel the monopoly of top quark physics being taken from their hands, due to the good news on the running of the Large Hadron Collider. The ATLAS and CMS experiments there have started to mine their datasets, now amounting to over 20 inverse picobarns and growing significantly by the day. These datasets contain as many top quark pairs as half an inverse femtobarn worth of Tevatron collisions, due to the 20-fold higher cross section of top pairs at the LHC.
Energy (E) is a useful quantity. It has certain properties and connections with other such measures, like momentum p. E is in a certain sense paired with time t, much like momentum p and distance x are such a pair. This pairing is known from classical mechanics and it shows up in relativity: The merging of t and x into one entity, space-time, leads straight to such a melting also of E and p; they mingle up correspondingly. In quantum physics, the uncertainty Δx ~ 1/Δp is a very important formula, and it translates into Δt ~ 1/ΔE, too. Meaning: Their properties all correlate.
"Since two fermions cannot turn into three fermions, the experimental observation of three-jet events in e+e- annihilation, first accomplished by the TASSO collaboration in June 1979 and confirmed by the other collaborations at PETRA two months later, implies the discovery of a new particle. Similar to the quarks, this new particle hadronizes into a jet, and therefore cannot be a color singlet. These three-jet events are most naturally explained by a hard noncollinear bremsstrahlung . [...] Thus the 1979 discovery of the second gauge particle, the gluon, occurred more than fifty years after that of the photon. This particle is also the first [...] gauge particle with self-interactions.
Is energy conserved? "Of course it is!" anyone with just a rudimentary knowledge of physics will answer. A more pertinent answer would be: "if you can't show me a working perpetual motion machine, shut up and stop wasting my time!" 

The conservation of energy is an insight that stood the test of time. It was Julius von Mayer who first worded it in its clearest form: "Energy can be neither created nor destroyed". That was nearly 170 years ago. 

So why question energy conservation?

The interesting thing about physics is that the deeper you dig, the more you are forced to doubt existing principles. Dig deep into the universe, allow gravity to become a dominant feature, and the conservation of energy becomes much less obvious.