Plus Blog
November 6, 2013
How do you balance a cardboard cutout of a triangle on a pencil? Trial and error is one way, but maths can save you lots of bending down and picking it up. Take the pencil and a ruler and connect the midpoint of each side to the opposite corner. You'll find that the three lines intersect in a single point, which lies exactly a third of the way from the midpoint of each side to the opposite vertex. That point, called the centroid, is the centre of mass of the triangle. If the triangle is made from uniform material, so it's not more lumpy in some places than in others, then the centroid is the unique point on which you can balance it without it tipping over. Amazingly, the centroid would also be the centre of mass of the triangle if its mass was concentrated only at its corners, and evenly divided between them. The centroid of a triangle. Image created using an interactivity from Math Open Reference. Instead of drawing a line from the midpoint of a given side to the opposite corner, you could also draw the line which passes through the midpoint but forms a right angle with the side the midpoint is on. If you do this for each side you again get three lines, and again these meet at a single point, called the circumcentre of the triangle. If you now draw a circle with the circumcentre as its center passing through one of the triangle's corners, you will find that the other two corners of the triangle lie on the circle too! The circumcentre of the triangle is also the centre of the unique circle that contains the three corners of the triangle. But it doesn't need to lie inside the triangle — in fact, it only does if all the triangle's angles are less than 90 degrees (so the triangle is acute). If one angle is greater than 90 degrees (the triangle is obtuse) then the circumcenter lies outside the triangle, and if one angle is exactly equal to 90 degrees then it is the midpoint of the hypothenuse. The circumcentre of a triangle. Image created using an interactivity from Math Open Reference. But there's another point that qualifies as a centre of a triangle. You find it by drawing aline from each corner that is perpendicular to the opposite side. Amazingly, the three lines again meet in a single point, called the orthocentre. As for the circumcentre, the orthocentre lies inside the triangle if the triangle is acute and outside it if it is obtuse. If one of the angles is exactly equal to 90 degrees then the orthocenter will be one of the corners. The orthocentre of a triangle. Image created using an interactivity from Math Open Reference. And what links all these points together? A straight line! The beautiful fact that the centroid, circumcentre and orthocentre of a triangle all lie on a straight line was first noticed in the 18th century by Leonhard Euler, one of the most prolific mathematicians of all time. That line now carries his name: it's called the Euler line of a triangle. You can play around with the three different centres and the Euler line on Math Open Reference which has beautiful interactive demonstrations, from which we made the images illustrating these articles. 
October 21, 2013
Last year we asked Plus readers to send in their own popular maths articles, with winning entries to be published in a forthcoming book. The winners were chosen a while back and we're now proud to announce that 50: Visions of mathematics is with the printers (well, nearly) and will be out for the world to read in spring 2014. The book celebrates the 50th anniversary of the Institute of Mathematics and its Applications and will be published by Oxford University Press. Besides the winning contributions from Plus readers it contains articles from our favourite popular maths authors, including Ian Stewart, Marcus du Sautoy, Simon Singh, Chris Budd and John D. Barrow. There will be a foreword by Dara Ó Briain and also fifty images showcasing the beauty and diversity of maths. A big thank you for taking part in the competition and sending in image suggestions. The book was edited by Sam Parc with the help of a devoted editorial team including our humble selves. To keep up to date with its progress follow Sam on Twitter!
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October 15, 2013
It's Ada Lovelace day, celebrating the work of women in mathematics, science, technology and engineering. To celebrate we bring you a selection of articles by female authors that were published on Plus this year (including those written by the entirely female Plus editorial staff). To find our more about the pioneering work of Ada Lovelace herself read our article Ada Lovelace  visions of today. You can also check out our Ada Lovelace day blogs from 2012 and 2011 for more by and about women mathematicians. Do infinites exist in nature? — Marianne Freiberger and Rachel Thomas talk to physicists, philosophers and cosmologists in search of the unbounded. The lost mathematicians: Numbers in the (not so) dark ages — Charlotte Mulcare asks what medieval mathematicians got up to and whether they left a useful legacy. Let me take you down, cos we're going too ... quantum fields — In this fourpart series the Plus editors Marianne Freiberger and Rachel Thomas explore the dramatic history of quantum electrodynamics. Cognition, brains and Riemann — Joselle DiNunzio Kehoe explores our perception of space, Riemann's geometry and parallels between the two. Folding the future: From origami to engineering — Kim Krieger finds an unlikely connection between combinatorics, origami and engineering. What is space? — Francesca Vidotto tells us how the existence of tiny black holes make it impossible to measure lengths shorter than a shortest scale. Are there parallel universes? — Plus editor Marianne Freiberger finds parallel worlds in the puzzling mathematics of quantum mechanics. Is the Universe simple or complex? — Faye Kilburn asks biologists, physicists, mathematicians and philosophers for their idea of how the Universe is structured. 
October 4, 2013
Next week will be an exciting one for a handful of scientists with the announcement of the 2013 Nobel prizes. You'll be able to watch live at the Nobel site as the awards for physiology or medicine are announced on Monday, physics on Tuesday, chemistry on Wednesday and the peace prize on Friday. Economics will follow on Monday 14 October and the literature prize soon after. (The announcements start about 1111.30am Central European Time or 1010.30am British Summer Time. And yes, despite the rubbish weather, we are still on summer time!) With the recent experimental confirmation of the Higgs boson last year at the Large Hadron Collider, rumours are beginning to swirl that the physics prize might go to some of the physicists who predicted its existence and the mechanism that gives mass to matter in the Universe. Six physicists, Robert Brout, François Englert, Peter Higgs, Gerry Guralnik, Carl Richard Hagen and Tom Kibble, contributed to three revolutionary papers published in 1964 that explained this theory. The Nobel prize can be awarded to up to three people so it will be interesting to see who will be recognised and what the reaction of the physics community will be. We'll be watching the announcements of all the prizes with great interest. There may not be a Nobel prize specifically for mathematics, but you can be sure that maths will have played a vital role in the research of many of the 2013 Nobel Laureates. You can read more about the discovery of the Higgs boson and the previous Nobel prizes on Plus.
Secret symmetry and the Higgs boson
The Higgs boson: a massive discovery
A Nobel Prize for quantum optics
How to make a marriage stable
Shattering crystal symmetries
Exploding stars clinch Nobel Prize
And the Nobel Prize in Mathematics goes to... There might not be a Nobel Prize for mathematics, but you can read more about the winners of the highly prestigious Abel prize and Fields medals. 
October 1, 2013
If you walk around the twodimensional plane you can keep walking indefinitely in all directions. You could say, in a very handwavy and intuitive sense, that there is infinity all around the edge of the plane, only of course you can never get to or see that edge. But still, you could try to imagine what happens if you shrink that infinityedge to a point. Perhaps this would be a little like tightening the draw string on the rim of a fabric bag. Once you've tightened it, the bag is closed and resembles a deformed sphere. Stereographic projection. Image: JeanChristophe Benoist. There is a way of making this intuition precise. Imagine a sphere and the plane that contains its equator. For any point p on that equatorial plane, draw the straight line that connects it to the North pole of the sphere. That straight line is going to intersect the sphere at some point. If p is on the exterior of the sphere it will intersect the Northern hemisphere of the sphere. If p is in the interior of the sphere the line will intersect the Southern hemisphere of the sphere. And if p lies on the sphere, so it's actually on the equator, it will itself be the intersection point. This way of associating to every point in the plane exactly one point on the sphere is called the stereographic projection. It's easy to see that the further out on the plane your point p, the closer its projected image on the sphere is to the North pole. But no point on the plane projects to the North pole itself. The North pole is still available and as a sequence of points move out towards infinity on the plane, their projections move towards the North pole on the sphere. So you now declare that infinity is just a point (you draw the drawstring tight) and that its projection is the North pole of the sphere. What you get is a continuous onetoone correspondence between your plane together with infinity and the sphere. The two can be treated as one and the same thing. The plane with a point at infinity appended is called the Riemann sphere after the 18th century mathematician Bernhard Riemann (although strictly speaking the Riemann sphere is the complex plane with infinity appended — see here for more on complex numbers). This is incredibly useful. You are probably familiar with functions that take the number line into itself. An example is it takes a number from the number line as input and returns as output. Unfortunately, the function is not defined at because division by is not allowed. However, as gets closer and closer to gets closer and closer to plus infinity if you're coming from the positive side, or minus infinity if you're coming from the negative side. If you could treat plus and minus infinity as one and the same ordinary point, then the function could be defined at and would be perfectly well behaved there. You can also define functions that take the plane into itself (the complex function is an example) and again they may not be defined at every point because you have division by 0. However, by treating infinity as an extra point of the plane and looking at the whole thing as a sphere you may end up with a function that's perfectly tame and well behaved everywhere. A lot of complex analysis, the study of complex functions, is done on the Riemann sphere rather than the complex plane. 
September 16, 2013
Do you remember those pretty field lines that emerge when you scatter iron filings around a magnet? In the case of a simple magnet the field is static; it doesn't change with time. But magnetism is just one aspect of something bigger: electromagnetism. You are at this very moment immersed in electromagnetic fields, generated by the Earth, the Sun, and even your toaster. James Clerk Maxwell realised, in 1864, that electricity and magnetism were just two sides of the same coin and that light was made up of electromagnetic waves. He developed an elegant theory describing the unified force of electromagnetism and the equations that describe the dynamics of an electromagnetic field now carry his name. Today's Guardian has a great introduction to Maxwell's equations, written by their science correspondent Alok Jha. And if you'd like to venture further into the wonderful world of field theory you can read our series of articles about what happened next, starting with Let me take you down, cos we're going to ... quantum fields. 