## What is a Manifold?

August 30th, 2013 by ajb

Preamble
So I have decided to create a few posts about some of the important mathematical ideas that I regularly encounter. The idea is to be rather informal and try to give a general feeling about these objects rather than a proper mathematical definition. I am hoping these posts will be readable to anyone with a reasonable high school education in mathematics. I will assume basic algebra and elementary geometry in the plane for example.

Please email me if you have something you would like me to cover.

So on to manifolds…

Introduction
Manifolds are an important notion in geometry and topology. Basically they are spaces that locally look like the much more familiar Euclidean spaces. This allows the well-understood notions on Euclidean spaces to be generalised to manifolds rather directly.

Coordinates
Recall that the space $\mathbb{R}^{n}$ for some integer $n$, can be understood by assigning an n-tuple $x^{\mu}:=(x^{1}, x^{2}, \cdots , x^{n})$ of real numbers to each point; that is we can pick some coordinate system. The choice of coordinates is far from unique and a large part of geometry is related to the freedom in picking coordinates. We will not worry about changes of coordinates here.

Above we have a Cartesian coordinate system on the plane. Every point is assigned a pair of numbers.

It was Rene Descartes (1596-1650) who pioneered this analytic approach to geometry; today we honor him the term Cartesian when referring to coordinates. By employing coordinates, algebra and analysis can come to bare on question in geometry. This is in contrast to the axiomatic or synthetic approach that goes back to Euclid (circa 300 BC). The synthetic approach to geometry uses axioms, theorems and logical arguments to study spaces.

Manifolds
We can now describe the notion of a manifold, which is a higher dimensional analogue of a curve or a surface. A manifold of dimension $n$ is a space, you can think of a collection of points, that locally looks like $\mathbb{R}^{n}$ for some integer $n$. All curves in the two dimensional plane that do not intersect locally look like part of a line. All smooth surfaces in $\mathbb{R}^{3}$, that is surfaces that do not have sharp kinks, edges, or points all locally look like the two dimensional plane, see figure below. Thus such surfaces are two dimensional manifolds. One should imagine being able to tear any small piece off the smooth surface, then being able to stretch it, push down any “hills” and push up any “valleys”" to end up with a flat piece of the 2-plane.

From Wikipedia

A little more formally, a manifold of dimension $n$ can be thought of as being built up out of domains, which are open subsets of $\mathbb{R}^{n}$. Intuitively, an open set is a set of points, in this case a subset of points belonging to $\mathbb{R}^{n}$, such that any individual point can be displaced and still remain in the set. This gives a notion of points being “near” without the need for the notion of a distance between points. Such domains are commonly denoted as $U \subset \mathbb{R}^{n}$. One can then “build” a manifold by patching these domains together, a bit like how one would make a patchwork quilt. It is how these domains are patched together that really defines the manifold.

In essence, the domains allows us to employ local coordinates when dealing with manifolds. Thus, one can build the theory of manifolds based on our understanding of the space $\mathbb{R}^{n}$.

A quite familiar example to us all is the relation between a globe and a map. (I mean the common notion of a map, not the mathematical one!) The globe is a representation of the Earth (or any other planet) on the surface of the two sphere. Small pieces of the globe can always be represented as a map, which you think of as a piece of the two plane. Useful local coordinates on the maps can then be employed.

Image by Christian Fischer.

The picture to have in your mind when thinking about manifolds is the relation between a globe and a map. Small pieces of the globe can always be described by maps (pieces of the 2-plane). Moreover, the entire globe can be covered by a collection of maps: an atlas.

Etymology
The etymology of the word “manifold” is old English. The word literally means many folds. Today, generally the word has come to mean any object having many different parts or features. The use of manifold in the mathematical context is adapt, manifolds are generalisations of surfaces that are built up from many domains and have many features not seen on $\mathbb{R}^{n}$.

Not manifolds
It is also worth highlighting some spaces that are not manifolds. A very simple example would be a self-intersecting curve in the plane. Such a curve will have regions that look like “X”. At these intersection points we no longer have a manifold structure. Such spaces are known as manifolds with a singularity.

Image courtesy of Wolfram

The double cone above looks very much like a manifold apart from the point at which the two cones meet. Everywhere not near this apex locally looks like the plane. This is also an example of a manifold with a singularity.

Manifolds with a boundary are not manifold!. For example the finite cylinder has two circles which are one dimensional manifolds as it’s boundaries.

Applications
Manifolds can have a variety of extra structures on them. Indeed I have been very loose with the class of manifolds I have discussed here. Anyway, manifolds can have a differentiable structure on them meaning that we can do calculus on them. This is great for physics and indeed smooth manifolds are important in theoretical physics.

For example, smooth manifolds that come with a metric, that is a notion of distance on them, are at the heart of Einstein’s general relativity. Also, smooth manifolds also appear as the phase spaces in classical mechanics and these carry another interesting structure, that of a Poisson bracket.

I will say more about Poisson brackets in another post.

## Paul Frampton: The Motion Picture

August 30th, 2013 by ajb
 Hollywood seems to have taken an interest in the tale of Prof. Paul Frampton, who in November 2012, was convicted and sentenced to 56 months for drug smuggling in Argentina. You can read an earlier post about this case here.

Described as a modern take on Lolita, the story follows Paul Frampton, a divorced theoretical particle physicist, who meets Denise Milani, a Czech bikini model, on the online dating site Mate1.com. Milani’s pictures on the site show a dark-haired, dark-eyed beauty with a supposedly natural DDD breast size. The two begin to correspond and plan their perfect life together, but first, the woman asks the British professor if he would deliver a special package to her, setting him on a course of danger.

Maxine Swann in the Hollywood Reporter

I really hope that Brian Cox gets to play Frampton. Not that one, this one

Fox Searchlight Nabs ‘Professor and Bikini Model’ for Steve Zaillian to Produce (Exclusive), The Hollywood reporter.

Paul Frampton: The Movie, IOP Blog.

## A level physics on the up

August 21st, 2013 by ajb
 For the seventh year running the number of students choosing to study A level physics has risen; from 27,368 in 2006 to 35,569 this year. However, students sitting A level exams for all subjects is down by 1.1% as compared to last year.

The government set up the Stimulating Physics Network (SPN) with the aim of getting at least 35,000 students sitting A level physics by 2014. The Institute of Physics has also been very active in promoting physics and physics teaching.

As you can see, the target set by the government has now been reached a year earlier.

Our Stimulating Physics Network is addressing the chronic problem of too few specialist physics teachers in the UK by offering free, bespoke, in-school training to non-specialist science teachers.

Professor Sir Peter Knight, President of IOP

In figures

A level physics – 35,000 target reached IOP news

Stimulating Physics Network

## A tale of two pizzas

August 10th, 2013 by ajb
 Image Credit: Pizza Slice Clip Art from vector.me (by johnny_automatic) My wife and I were in a pizza shop near Warsaw old town. We were looking at the menu and saw that 31cm pizza costs about 30 PLN and a 61cm pizza costs about 70 PLN. There is of course a little variation with the exact toppings. Based on this my wife said that we would be better off getting two 31cm pizzas rather than one 61cm pizza.

So I bought the one 61cm pizza.

But why?
In all fairness, my wife did immediately realise her mistake…

The pizzas, which are assumed to be circular, are specified by their (approximate) diameter $d$, which is of course twice the radius $r$. Thus surface area of the pizza is given by

$A = \pi r^{2}$.

Importantly the area varies as $r^{2}$ and not simply as $r$. Thus a pizza with twice the diameter, or equivalently twice the radius of a given pizza, has four times the surface area, not twice the surface area.

Simply

$\frac{A_{2}}{A_{1}} = \frac{\pi (2r)^{2}}{\pi r^{2}} =4$.

Roughly, given that the pizza was not a perfect circle and that 61 is not quite twice 31, I get about twice as much pizza buying the single 61cm as compared with buying two 31cm pizza.

Furthermore, $70/60 \approx 1.2$. So I get near enough twice the pizza for my money by getting one 61cm pizza as compared with two 31cm.

The pizza

The result of my mathematics!

Above is the said 61cm pizza. It was very good.

In conclusion

## Even more IFS fractals

August 8th, 2013 by ajb

My wife and I have created a few more iterated function system fractals for you to enjoy.

You can find other examples of our collaboration here, here and here.

## Does mathematics really “exist”?

August 6th, 2013 by ajb
 Mike Rugnetta asks “Is Math a Feature of the Universe or a Feature of Human Creation?”.

Math is invisible. Unlike physics, chemistry, and biology we can’t see it, smell it, or even directly observe it in the universe. And so that has made a lot of really smart people ask, does it actually even EXIST?!?! Similar to the tree falling in the forest, there are people who believe that if no person existed to count, math wouldn’t be around . .at ALL!!!! But is this true? Do we live in a mathless universe? Or if math is a real entity that exists, are there formulas and mathematical concepts out there in the universe that are undiscovered? Or is it all fiction? Whew!! So many questions, so many theories… watch the episode and let us know what you think!

Mike Rugnetta of PBS’ Idea Channel

The discussion is rather philosophical…

August 6th, 2013 by ajb

 The Institute of Mathematics of the Polish Academy of Sciences at Warsaw is located on ul. Śniadeckich. I decided to have a quick look into the naming of the street. I discovered that it is named after two brothers, both of whom were outstanding Polish scholars end of the eighteenth and the first half of the nineteenth century.

 Jan Śniadecki (August 29, 1756– November 9, 1830) was a Polish mathematician, philosopher and astronomer. He published works on his observations of the then recently discovered planetoids. His O rachunku losów (On the Calculation of Chance, 1817) was a pioneering work in probability theory. (1823 painting by Jan Rustem)

 (1843 painting by Aleksander Sleńdziński) Jędrzej Śniadecki (30 November 1768 – 12 May 1838) was a Polish physician, chemist and biologist. He is best known for important book Początki chemii (The Beginnings of Chemistry), the first Polish-language chemistry textbook.

## Opportunity Physics; fundraising campaign

August 5th, 2013 by ajb
 The Institute of Physics will start a fundraising campaign Opportunity Physics which aims to raise £10m over 5 years. The money will be used to continue and extend the activities of the Institute both in the UK and abroad.

The official launch of the campaign, will be hosted by Professor Brian Cox and IOP president Professor Sir Peter Knight, on Monday 23 September 2013.

Why raise funds?

At the point where increased funding is needed to scale up our work, there are uncertainties about the sustainability of our main revenue stream which has come from IOP Publishing. It is prudent to plan for the change in funding landscape by raising funds now.

IOP

The campaign aims
In short there are four areas the IOP wants to focus on; discovery, society, economy and education.

The campaign aims can be found here (opens pdf)

Fundraising campaign, IOP website.

## Professor Jeff Forshaw wins Kelvin medal and prize

July 31st, 2013 by ajb
 Professor Jeff Forshaw of the University of Manchester has won the 2013 Kelvin medal and prize awarded by the Institute of Physics. The medal is for Prof. Forshaw’s wide-reaching work aimed at helping the general public to understand complex ideas in physics.

Prof. Forshaw has written two popular science books; “The quantum universe” and “Why does E=mc^2″, both with Brian Cox.

2013 Kelvin medal and prize, IOP website

Jeff Forshaw’s homepage

## Professor Copeland wins Rayleigh Medal and Prize

July 31st, 2013 by ajb
 Professor Edmund Copeland of the University of Nottingham has won the 2013 Rayleigh Medal and Prize as awarded by the Institute of Physics.

Professor Copeland was awarded the prize for his work on particle/string cosmology from the evolution of cosmic superstrings, to the determination of the nature of Inflation in string cosmology and to constraining dynamical models of dark energy and modified gravity.

A personal note
I first met Professor Copeland back in 2005 when was at the University of Sussex. I was there studying for my masters degree. He then, in the same year moved to Nottingham to establish the Particle Theory Group.