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## How many dimensions?

Suppose you have some physical process you are want to describe, for example a simple situation where a few particles move around, possibly scatter from each other. Each particle has a few numbers that specify its particular state at a given time. It can have position (specified by 3 numbers), velocity (3 other numbers), electric charge, mass, energy etc. Once you specify those numbers you know precisely what particle you are discussing and what it is doing. If you have a complicated situation, with many particles moving around, you end up having to specify lots of numbers to completely describe the situation.

Let’s look at the inverse problem, suppose you are given a series of such measurements, can you reconstruct the corresponding situation? the trick is that those numbers don’t come labeled, no higher authority gives them names like charge or position, interpretation is completely up to you (as is always the case!). This is not that easy actually, and as we’ll see the results are sometimes ambiguous, so I’ll be generous and let you perform as many thought experiments as you want, even infinitely many. Why not? we are talking about theorist’s favorite experiments, the ones that cost no money.

## Sometimes we learn things in unexpected places

I attended a colloquium talk on Thursday at the Institute I’m visiting for a couple of days. It was a colloquium on interesting magnetic orderings. There I learned that there are many species of bacteria that produce single domain magnetic crystals of high purity.
Here is a link with some of that information. It seems that this helps the bacteria determine the up and down direction: a really difficult problem in aquatic media. Sometimes it would be really cool to be equipped with such an extra magnetic sense (especially if I get turned around in a city I don’t know). We people fake it by GPS devices or by having a compass, but who walks with a compass nowadays anyway? Also, your favorite GPS device might run out of batteries…

In any case, I often get amazed by such facts

Ask any academic and they will tell you all about the joy that is the grant proposal. In fact, I expect you may find it hard to stop them…There are so many research grants out there, and since success rates are low and terms are short, part of your job is to become an expert on those grant proposals. There are some grants that want you to be bold, and some that want you to blur the boundaries. Some grants require you to go all-American, then others are best used to employ Russian citizens of average height. Then there are those grants that prefer you conduct interesting interdisciplinary international inter-institutional research, leaving you scratching your head. Life is so much more interesting with the grant proposal, it’s almost as good as American politics.

You may expect that in Canada, a place where around any election time you’d find relatively competent politicians arguing endlessly about the same old boring “issues”, with not a word uttered about their wardrobes (or family dramas, or bizarre rituals), even the joyful process of the grant proposal is somehow made dull. You’d be correct, of course.

## Physics puzzle: candles and centrifuges.

Candle in an open container spinning in a centrifuge

So here we have a physics puzzle to tease you a little bit. The idea is simple. You put a candle in an open container (let us say a glass, like in the restaurants). You attach the container to a centrifuge, with a long arm, and you make the centrifuge spin at high speeds.

The big question is: what happens?

This is sometimes the flavor of big physics questions. One puts a couple of ingredients together, sometimes in a lab, and sometimes as a theoretical exercise, and one is supposed to come up with a detailed description of what happens. In this problem there are many ingredients, and it is not obvious what is important and what is not. This is what makes these problems fun. This problem was suggested to me by Nick Warner a couple of weeks ago. I thought it would be good to release it with some picture.

## String Gas Cosmology

This post in “Science After Sunclipse” reminded me of one of the most beautiful ideas to come out of the continuing attempts to combine String Theory and Cosmology. The idea of Robert Brandenberger and Cumrun Vafa, dubbed more recently as “String Gas Cosmology”, is a wonderfully creative attempt to explain why our world has three spatial directions. There is no other theory on the market where the dimensionality of space could be determined dynamically, or at least come out as a result of a calculation, rather than being put in as an input, so in String Theory this is a natural question to ask. The idea for an answer,  provided by String Gas Cosmology, could be stated naturally and simply, which is what I try doing in this post. As usual, one has to remember that the devil is in the details, and those at the moment provide a real challenge for the idea.

One of the beautiful aspects of this approach is even asking the question: why four dimensions?  famously, the dimension of spacetime in string theory is larger than the observed one, in the simplest scenarios 10 or 11 spacetime dimensions, depending on details. We of course only see 3 spatial directions to move in, and can only make sense out of one time direction. Isn’t that a clean Popperian falsification of String Theory?

## Where in the world is….

By the time you start reading this post, I will be squirming in my seat trying to get a little bit of extra leg room, or I will have landed already. In any case, just to add some meaningful content and to avoid complaining about cramped conditions in airplanes, I thought I would give you a picture I took from an airplane a few years ago. And all you have to do is figure out where in the world this picture was taken.

Hint: there are geographic features in the photo that are visible…

## Everyday Physics: suction cups.

You might have seen them in your shower, kitchen refrigerator or walls, or even you might have seen them hanging from the glass panel in the rear of a car you are following. They are suction cups, the product that really sucks. Erm, maybe that is not the best description of these objects, since they are very cool and useful and they don’t suck. Well, they do, but in the sense of suction, not in the other colloquial sense.

A cross section of a suction cup attached to a wall

So, how do these little suckers work? That’s the post for today. I have not explained any device recently, so I thought these would be a nice addition to the everyday physics series.