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Sunday, 24 July 2011

Space Shuttle Retirements: The End of an Era

The last of NASA's space shuttles, Atlantis, has gone into retirement. The shuttle touched down on July 21st at the Kennedy Space Centre, delivering its four crew members safely back to Earth.

The space shuttle program has faithfully delivered astronauts, supplies and maintenance to the International Space Station since 1981. The space shuttle Discovery launched the Hubble Space Telescope and the shuttles have since made several missions to repair and install new instruments in the telescope.

The fleet of shuttles that operated in the program – Enterprise, Columbia, Challenger, Discovery, Atlantis and Endeavour – were revolutionary in that they were reusable. Earlier spacecraft such as Apollo, which carried Neil Armstrong, Buzz Aldrin and Michael Collins to the Moon in 1969, returned to Earth by splashing down in the ocean, but the winged space shuttles could glide majestically down to land on a runway.

Atlantis lands for the final time.  Source: NASA

The decision to retire the fleet of shuttles was made in 2004, following the deaths of the crew on board the space shuttle Columbia, which broke apart on re-entry into the atmosphere. A previous accident, in which the Challenger shuttle broke apart just 73 seconds after take-off in 1986, had already claimed the lives of 7 NASA astronauts.

The average cost of launching a space shuttle into orbit is around $450 million. In 2005, NASA spent almost 30% of its total budget on the space shuttle program. The decision to close down the program will mean job losses for around 3,000 NASA employees.

Until plans for a replacement for the space shuttle program are drawn up, astronauts will be transported to and from the International Space Station by Russian spacecraft. There are reports that a private company - either Orbital Sciences, Lockheed Martin or Boeing - will step in to fill the gap of providing a reusable spacecraft to replace the retired shuttles.

For more information, don't miss this program airing tonight at 9 pm on BBC2.

Saturday, 23 July 2011

Breaking News: First Hints of Higgs Boson?

On 22 July, results indicating the presence of a Higgs boson with a mass in the range 120-140 GeV were reported at the EPS-HEP11 conference in Grenoble, by two teams of researchers working independently at the LHC.

The first results to be reported were from the ATLAS experiment, followed by (weaker) results from CMS.  Matt Strassler's blog appears to have been one of the first places to break the news following the conference proceedings.  The ATLAS team state the significance of their result as being 2.8 sigma, although this is before the "look elsewhere" effect is taken into account.  Once that is included, the probability of the peak in ATLAS's data being due to random statistical fluctuation rises to around 8%.

For a discussion of uncertainties and significance in the results, see:
How Certain is Certain?

The CMS experiment also sees a signal in the 120-145 GeV, albeit a smaller one than ATLAS.

Click here for more information about how the Higgs search is carried out at LHC

The Higgs Boson (also known as the "God Particle") was proposed by Peter Higgs in 1964 as part of the Higgs mechanism, which attempts to explain why particles have mass.  Its discovery is one of the primary goals of the Large Hadron Collider, the 7.5 billion euro particle collider that lies in a 27 km circular tunnel underground near Geneva.

To find out more about the Higgs boson and the new results, please see:

These results are exciting because they both show evidence of a Higgs boson in the expected mass range.  However, more data is required before we can say for certain that we have seen the Higgs particle.


CMS results available here: (technical)

Saturday, 16 July 2011

BBC series: The Story of Maths

The BBC is re-airing its 2008 series, "The Story of Maths", presented by Marcus du Sautoy.

Catch the first episode "The Language of the Universe" on iplayer here (UK only)
The second episode, "The Genius of the East" will air on BBC 4 on Tuesday 19th July at 8pm.

The first hour-long documentary traces the origins of mathematics, detailing the contributions made by the Egyptians, Babylonians and Greeks.  It tells the story of how numbers such as pi, zero, irrational numbers and the Golden Ratio came to play such an important role in mathematics.  Episode Two will move east to China, and the final two episodes will trace the story of maths up to the present day.

I often feel patronised by science documentaries, but I'm actually learning things from this series, which would seems like it would also be very accessible to non-mathematicians.  Even though I'm already familiar with the theorems, learning about their origins and how ancient people came to realise them gives fresh food for thought.  One of the things I love most about maths is how every problem can be approached using a range of different methods, so it's interesting to see how ancient cultures arrived at the same mathematical truths as modern mathematicians, often while seeking solutions to very different problems.

The Story of Math on DVD

Tuesday, 5 July 2011

Tevatron finds CP-violation at 3.9 sigma: could this be mechanism for matter-antimatter asymmetry?

Matter-antimatter asymmetry, as I wrote in this blog post and in this introductory article, is one of the great unsolved mysteries of the Universe. The problem in simple terms is that there is an excess of matter over antimatter in the Universe, and scientists and astronomers are baffled as to why. One would naively expect the Universe to behave symmetrically, treating matter and antimatter exactly the same rather than preferring one to the other. After all, symmetry and conservation laws play a huge part in physics.

However, physicists have increasingly found that this is not the case: some processes in particle physics produce more matter than antimatter, an effect known as baryogenesis. These processes violate CP symmetry: a combination of charge conjugation symmetry and parity symmetry.

Click here for an introduction to CP violation as a mechanism for baryogenesis.

Following the discovery of interactions that violate CP symmetry in 1964, the Standard Model of particle physics was modified to take account of CP violation, by adding a complex phase into the CKM matrix describing quark mixing. However, the maximum amount of CP violation that can be included in the Standard Model in this way is still much much too small to account for the observed preponderance of matter over antimatter in the Universe. To put it in perspective, even if the parameters of the Standard Model were adjusted so as to give the maximum possible amount of CP violation, it would still only account for an excess of matter roughly equal in size to one galaxy - and there are millions of galaxies in the Universe!

Recent experiments provide evidence that the amount of CP violation observed in nature is greater than the amount that is allowed by the Standard Model. Researchers at the Tevatron* have measured the dimuon charge asymmetry – the number of muons compared to antimuons that are produced in a particular reaction – and found that more muons are produced than antimuons. The amount by which muon production exceeds antimuon production is larger than that predicted by the standard model.

The researchers have quoted the disagreement between their results and the standard model as 3.9 sigma – this means that if the Standard Model prediction is correct, there is a 0.005% probability of obtaining the result they did. When an experiment gives a result that the standard theory says should only occur 0.005% of the time, it becomes sensible to ask whether the standard theory might be wrong. However, with thousands of particle physics experiments currently taking place around the world, one would expect to see a few anomalous results occurring, even if there was nothing wrong with the underlying theory. For this reason, the convention in particle physics is to disbelieve the current theory only if the level of disagreement between the theory and the results is 5 sigma – i.e. if the theory predicts that the observed result will occur 0.00003% of the time.

In conclusion, this experiment has provided strong evidence that there may be more CP-violation occurring in the Universe than current Standard Model particle physics can explain. This CP-violation could be the mechanism by which the early Universe produced more matter than antimatter. If these results can be confirmed at the desired 5 sigma level of accuracy, they provide further motivation to develop a theory of particle physics that goes beyond the Standard Model.