Positron Emission Tomography – Sci-advent – Day 14


positron emmision tomographyOne may think that anti-matter features only in theoretical physics textbooks or in sci-fi devices, nonetheless it is very much in current use. Positrons are the anti-particle of electrons and their existence was proposed theoretically by Paul Dirac in 1928 and they were observed experimentally a year later. Nowadays positrons have a number of applications, including medical imaging.

Positron Emission Tomography (PET) is a three-dimensional imaging technique that works by detecting pairs of gamma rays emitted indirectly by a positron-emitting radionuclide introduced into the body. The radioactive tracer is usually injected into the subject, once inside the body it undergoes positron emission decay and it emits a positron. The positron travels in tissue for a short distance, losing kinetic energy until it is able to interact with an electron. The positron-electron interaction annihilates the pair generating gamma rays which are detected by the scanner.  Finally the images are built with the aid of computers.


Peltier Effect – Sci-advent – Day 13


peltier effectThe Peltier effect is named after Jean Charles Athanase Peltier who discovered it by accident while investigating electricity. In the eventful experiment, Peltier joined a copper and a bismuth wires together and connected them to each other, then to a battery. When he switched the battery on, one of the junctions of the two wires got hot, while the other junction got cold.

The Peltier effect is the heat exchange that results when electricity is passed across a junction of two conductors, and is a close relative of the Seebeck effect (effectively the same phenomenon in reverse, used in thermocouples used to measure temperature), and the Thomson effect (generation of electricity along a conductor with a temperature gradient). Sparing ourselves the maths, conduction electrons have different energies in different materials, and so when they are forced to move from one conductor to another, they either gain or lose energy. This difference is either released as heat, or absorbed from the surroundings.

When two conductors are arranged in a circuit, they form a heat pump, able to move heat from one junction to the other. Unfortunately, though, it’s not always this simple, as the Peltier effect is always up against the Joule effect – the ‘frictional’ heating that results from electrons bouncing off the atoms. In most systems, this swamps the Peltier effect, and means that all that you get is a bit more heating at one junction, and a bit less heating at the other. Nonetheless, the Peltier effect has a lot of technological potential. It is very reliable, and since it has no moving parts, it rarely needs maintenance while being mobile.

Magnetism – Sci-advent – Day 12

magnet iron filingsMaterials that respond to the application of a magnetic field are described as magnetic materials. Magnetism can be attractive (paramagnetism) or repulsive (diamagnetism). Some materials are permanent magnets, this mess that their magnetic fields are persistent and they are caused by ferromagnetism.

Magnetic phenomena are closely related to electricity: a magnetic field can be created by moving electric charges. Electromagnetic radiation, such as light, is a form of energy emitted and absorbed by charged particles. It can exhibit a wave-like behaviour as it propagated through space.

It is possible to map the magnetic field of an object by measuring the strength and direction of the field at various locations. By following the arrows drawn you end up with field lines for the field. A map of this sort can be visualised, for instance, by doing a very simple experiment involving a magnet bar and some iron filings (see image above).




Periodic Table (by abundance) – Sci-advent – Day 11


Periodic Table Relative AbundanceA 1970 periodic table by Prof. Wm. F. Sheehan of the University of Santa Clara that claims to show the elements according to relative abundance at the Earth’s surface.

Dmitri Mendeleev published a first version of the periodic table in 1869. The table was developed to illustrate periodic trends in the properties of the then-known elements, which are presented in order of increasing atomic number. This allowed Mendeleev to predict some properties of elements that were unknown at the time.

Mendeleev’s periodic table has since been expanded and refined with the discovery or synthesis of further new elements.


Zombie Spiders – Sci-advent – Day 10

A normal spider web on the left, compared to that built by a zombie spider (right).

Spiders and their webs are an excellent example of a predator, but can you enslave a spider? Well it seems that a species of wasp has mastered the art. The unsuspected spider is instructed by the parasite to leave its web behind and start building a new one with a very different architecture that will serve as a nest to nurse the larva of the wasp. The new web has a thick cover and a lower platform where a cocoon hangs. The cover protects the cocoon from rain for instance. Once the wasp hatches it then has the zombie spider as a first meal…

Ada Lovelace – Sci-advent – Day 9

Ada Lovelace

Ada Lovelace. Painting by Margaret Sarah Carpenter (1793–1872)

Ada Augusta Byron, Countess of Lovelace, was the daughter of the poet George Gordon, Lord Byron. She studied mathematics at the University of London with Charles Babbage, whose analytical engines were the precursors of the modern computer. Today 10th of December, it would have been her 197th birthday. That is why Google created a doodle for her (see image below).

Ada Lovelace is today known as a mathematician and computer pioneer; she created the concept of an operating system. Supplementing her translation of an Italian article on Babbage’s analytical engine with an encoded algorithm she published the first computer program, albeit for a machine that would not be built until more than 150 years later as a historical project.

The Ada computer language was named after her.

Lovelace Doodle

Sir Patrick Moore – Sci-advent – Day 8

Sir Patrick MooreBritish astronomer and broadcaster Sir Patrick Moore, died aged 89

Sir Patrick Moore was an inspiration to generations of astronomers and scientists in general. He presented the BBC programme The Sky At Night for over 50 years, making him the longest-running host of the same television show ever. The first programme was on April 24th, 1957. Sir Patrick’s last appearance was last Monday, December 3rd, 2012.
He wrote dozens of books on astronomy and his research was used by the US and the Russians in their space programmes.


Total Solar Eclipse – Sci-advent – Day 7

2009 Marislands Enewetak

Total solar eclipse over the Marshall Islands in 2009. Picture by Vojtech Rusin.

A solar eclipse happens when, as seen from the Earth, the Moon passes in front of the Sun and thus blocking it either fully or partially. This can happen only at new moon, when the Sun and the Moon are in conjunction as seen from Earth.


Mathematical Theorems – Sci-advent – Day 6


Maths Theorems Graph

Mathematical theorem network built from Walter Rudin’s Principles of Mathematical Analysis.

Scientific knowledge is built by building up on hypotheses and theories, repeatedly check them against observations of the natural world and continue to refine those explanations based on new ideas and observations. In the case of mathematics, that knowledge is organised in an incredibly structured manner. Starting up with properties of natural numbers, called axioms, and slowly working our way up, reaching the real numbers, calculus, and… well beyond. To prove new theorems, mathematicians make use of old theorems, creating a network of interconnected results—a mathematical house of cards.

Andy Reagan has recently published a blog post entitled “What’s the most important theorem?” where following Walter Rudin’s Principles of Mathematical Analysis, he displays them as nodes in a network.


Large Hadron Collider – Sci-advent – Day 5



The Large Hadron Collider (LHC) is the world’s largest and highest-energy particle accelerator. It was built by the European Organization for Nuclear Research (CERN). It has become a prominent facility due to the work that is being carried there to prove or disprove the existence of the Higgs boson and of the large family of new particles predicted by supersymmetric theories.

The LHC was built in collaboration with over 10,000 scientists and engineers from over 100 countries, as well as hundreds of universities and laboratories. It lies in a tunnel 27 kilometres in circumference, as deep as 175 metres (574 ft) beneath the Franco-Swiss border near Geneva, Switzerland.