# Physics Stop

## Bending lightMarcus WilsonMay 30

Here’s a nice picture I took a couple of years ago at Auckland Airport, looking over Manukau Harbour. Can you work out what’s happened to the Waitakere Ranges?

I’m in the middle of teaching a group of 3rd years about some of the ways that light doesn’t travel in straight lines, as part of their unit on computer modelling of electromagnetic effects.  It’s interesting that we teach them another paper (a whole 20 points) on electromagnetic waves, but implicitly assume that everything happens either in a vacuum, or in materials that have uniform properties.  In practice, that doesn’t happen.  Air isn’t at all homogeneous, its density, and therefore refractive index, changes significantly with height, and as a consequence, electromagnetic waves (e.g. light, radar, infra-red) don’t travel in straight lines.  Light gives mirages, and sometimes false images suspended upside down in mid-air, radio waves can ‘duct’ (follow the curve of the earth for long distances). Very different from the boring straight-line vacuum propagation (I’m ignoring General Relativity here).

Next time you’re at the beach, have a look at the horizon for some of these freakish effects.

## Why you need to proof readMarcus WilsonMay 28

I’ve just supervised a test for a group of second year students. On looking at their answers afterwards, it was rapidly clear that there was a problem with one of the questions. Specifically, I had given the value of Boltzmann’s constant as 1.38 times 10 to the power 23  Joules per Kelvin, instead of 1.38 times 10 to the power minus 23 Joules per Kelvin.   Just one little minus sign in an exponent – it had slipped my proof reading, but it makes a difference of 10 to the power 46.   Not exactly a trivial mistake, and using the wrong value (as all my students did) leads to answers that are clearly wrong.

Maybe it did confuse some students. But not a single one was prepared to comment that their answers didn’t look right.  Not a single one seemed to spot that the value given was wrong.   (Here’s a question – should a 2nd year physics or electronic engineering student be expected to know the values of constants like charge on electron, Boltzmann’s constant, Planck’s constant etc.?) Of course sticking your hand up in the middle of a test and telling the lecturer that he’s made a mistake takes a lot of courage, but writing a note on your manuscript that the answer doesn’t look right isn’t so difficult.  So I wonder whether it is a case that no-one could see there was a problem (I hope not) or that no-one was prepared to comment on it.  I’ll have to ask.

Mistakes like this do happen, quiet frequently.  Usually, like this one, they are trivial (I’m not penalising students for using the wrong value). Even after proof-reading things slip though the net. I remember proof-reading this test and checking the values of the constants.  But I think that often, when you look at your own work, you see what you intended to write, not what you actually wrote. Sometimes, mistakes can have really big consequences (e.g. in the case of the  Mars Climate Orbiter, about 300 million US dollars worth of consequence). As scientists and engineers, we need to have a culture of saying when we think others have made mistakes. And when that person is more senior than you,  it’s very difficult.

## Does the future affect the present?Marcus WilsonMay 25

Well, my comment on Naked Short Selling has certainly sparked a bit of discussion (for readers on sci.waikato.ac.nz/physicsstop you’ll need to look at the sciblogs hosting of the blog, http://sciblogs.co.nz/physics-stop/ ).  There’s the reasonable question asked as to whether letting people trade in things they don’t yet possess (basically anticipating the future – letting the future, or your interpretation of it, control the present) causes too much instability in the markets.

It’s a matter of common sense that the past influences the present  (like lingering animosity between two countries over something that happened hundreds of years ago, or a piece of baguette being dropped by a bird causes the Large Hadron Collider to be shut down briefly). In theoretical physics, we can describe this sort of effect in some systems with ‘Green’s Functions’.  Basically, if we ‘hit’ a system at a time T, what will the system be doing at a time T+t  (i.e. t seconds in the future). In realistic, common-experience systems, all Green’s functions share a couple of common properties:

First, as t gets large (we get to a time where the ‘hit’ was a long time in the past) the Green’s Function has decayed to zero.  In other words, if you leave it long enough, all past events are forgotten. Some systems have their Green’s functions decay much faster than others – ones describing the way your car suspension behaves when it hits a bump might be a few seconds, ones describing the movement of a tsunami following an earthquake might be a few hours.

Secondly, when t is negative (meaning at a time before the ‘hit’), the Green’s function is zero. This means that the car suspension doesn’t respond BEFORE you hit the pothole. In other words, the future does not influence the past.

All this is logical. BUT, there is a system in physics that has Green’s functions that are not zero for negative t.  The Klein Gordon equation is used to describe relativistic (spin zero) quantum mechanical particles. Let’s not worry about what it is for now.  But its Green’s function can be found without too much difficulty, if you’re a good maths undergraduate, and it has a surprising and worrying property. It isn’t zero at negative t. The implication is that the future can influence the past.

Ouch. An  interpretation of these backwards-moving-in-time solutions is that they represent antiparticles.  That is, an antiparticle (and antiparticles are very very real – ask anyone who’s had a PET scan) can be thought of as a particle moving backwards in time.

Sometimes mathematics overtakes common sense.

Is there any analogy to anti-particles in the financial markets? Maybe there are anti-traders lurking out there waiting to annihilate unsuspecting traders on their lunch breaks (over a baguette, of course).

## Going NakedMarcus WilsonMay 22

I’ve been reading about Naked Short Selling, following Germany’s decision this week to ban it. What the financial world gets up to is rather interesting, to say the least.  For those who don’t wish to read about it themselves, my summary is this:  Short-selling is where you borrow something, then sell it, buy it back later at a lower price (you hope) and return it to its owner, and you pocket the difference; Naked Short Selling is when you are so keen you sell something before you’ve even borrowed it.

Hmm.  Short selling kind of reminds me of what electrons can do in Quantum Electrodynamics (QED).  They can ‘borrow’ energy, for a short time, use it (e.g. for creating electron/positron pairs), so long as they give it back again later. I’m not sure whether there is an equivalent to the naked version – i.e. using the energy before they’ve borrowed it. It all gets rather complicated, which is why I steer well clear of QED.

## What spin would you like with that?Marcus WilsonMay 20

I’ve been trying to avoid commenting on the recent mobile phone and cancer study (since everyone else is) but something I’ve just read has rather irked me so I’m going to do it.

So we have a study that’s just been published in the International Journal of Epidemiology. As with other high interest scientific research, it gets reported in mainstream media, with a bit of journalistic / editorial interpretation on it. Or ‘spin’, if you’d rather.

So we see the rather calming and reasuring headline on www.bbc.co.uk "No proof of mobile cancer risk, major study concludes". In New Zealand, we are a bit more dithery about things, and www.stuff.co.nz presents it as "Mobile phones, cancer risk inconclusive". (Don’t you just hate the way a comma gets used as a substitue for the word ‘and’, as if it’s too much work to put in another two characters).  But go to the American-based www.foodconsumer.org we read "Cellphone use may increase brain cancer risk". Three different headlines, all of them reporting the same thing, and all of them true, but leading you in different directions in your thinking.  Read the BBC report, and you may be left thinking "Well, mobile phone use seems to be no more dangerous than anything else  - I’ll go on as I was".  Read the food consumer report, and you may start to worry about what you’ve done to your brain.

See how reporting can colour your thinking?

What has really irked me about  foodconsumer is that, after saying that the cellphone industry is reporting that there is no conclusive evidence of a link between use and cancer, they quote some unidentified health observer as saying "cell phone users need to make their own conclusion and they should not wait for others to tell them whether long term use of cell phones is safe."

Um, let’s have a think about this statement. On the one hand, we have a reasonably scientific study carried out over a huge sample over several years. (Yes, the study has its flaws – it is a really hard one to properly control – let’s face it, would you volunteer for a study where you had to hold your cellphone to your right ear for exactly hour a day for ten years?)  On the other we have Joe Bloggs who doesn’t have a degree in physics or epidemiology, but does have access to the internet and its rather dubious reporting. Now, if Joe Bloggs knew some science, I would support him making his own conclusion based upon the evidence out there, but I suspect Mr Bloggs doesn’t, and so any conclusion he makes is probably going to be a bit flaky. N.B. I’m not saying that’s Mr Bloggs’ fault – not everyone has or should have a degree in science – but what people write about science should consider this fact.

This is why we need some decent science reporting, that doesn’t have agendas to push (except for good science) not just in NZ (Well done you Science Media Centre people…) but everywhere.

## Physics Stop 2010-05-19 20:10:53Marcus WilsonMay 19

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Alison’s recent bioblog entry made interesting reading/listening for me – Dan Meyer talking about how traditionally-phrased physics and maths problems tend to hinder students from working things out and grasping what is important – instead it teaches ‘learned helplessness’. Real world problems don’t come in neat little packages that you can do in a few minutes in a tutorial class. Likewise, real world experiements don’t come with detailed written instructions and ‘right’ results that you can look up in a data book – in practice, we don’t do experiments that people have done before, because people have done them before, but that’s what we often load our practical classes with.

As part of my Postgrad Certificate in Tertiary Teaching work, I’m going to be stirring up my second year Experimental Physics course a bit. The idea will be to have fewer sessions where students are given instructions, and more (well, one would be a start) where they are presented with a task (e.g. verify or otherwise Child’s law for a vacuum diode), given access to the apparatus they need (and a whole lot they don’t need, just like the real world) and have to work out their own method.  No lab instructions for them to blindly follow without a clue what they are doing. It’s entirely up to them to choose their experimental method.

The research literature on this says I will be in for a tough time, but the results will be that students will be better equipped for doing real-world science. And that, after all, is why they are studying for a BSc.  Isn’t it?

## How obvious is technology?Marcus WilsonMay 17

In order to help pass the time on my long-haul flights of the last couple of weeks, I bought a copy of H. G. Wells’  War of the WorldsI’ve read a lot of his work, but somehow this, arguably his most famous book, had previously escaped me.

I should emphasize the obvious point that this book is a work of fiction.  That means that the author doesn’t need to get the science completely right. Also, given that it was published in 1898, quite a lot has moved on in our knowledge of science and astronomy, so there are lots of ‘mistakes’ in there.

One point that really got my attention was the observation made by the central character that the Martian invaders did not appear to have invented the wheel. They had, however, clearly invented telescopes, space flight, the ‘heat ray’ (sounds rather like an intense beam of microwaves that cooks everything in its path) and some pretty viscious chemical weapons. To move about on the higher earth gravity, which causes problems for the poor Martians who evolved in the lesser gravitational field of Mars, they built themselves giant spider-like walking machines, when something with wheels might have proved far more energy efficient.

So, I was left thinking whether that was realistic.  Could a race develop space flight before developing the wheel?  Seems a bit unlikely to me. In order to land ten ‘pods’ at intervals of about a day, within just a few miles of each other just west of London,  by firing them out of a large gun on Mars, would surely have taken a fair bit of mathematical calculation on the part of the Martians, involving knowledge of the Earth and Mars orbits and rotation rates. I’m sure they’d have to have grasped angular momentum, amongst other things.  The way we teach angular momentum at school and university involves lots of examples of rotating circles (e.g. wheels) so surely they’d have been familiar with rotating discs? Evidently not.

Anyway, as I said, this is fiction, so it really doesn’t matter.

For those interested in speculating about future technology on Earth, get yourself a copy of Wells’ book "When the Sleeper Wakes". Essentially the scenario is that someone, in about 1900, falls asleep for 200 years and wakes up in 2100, surrounded by fantastic new technology including things like newspapers with moving pictures…

## Stunning PhysicsMarcus WilsonMay 12

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Sorry about the gap in activities – I had something unexpected come up which was rather more important and urgent than writing blog posts. More or less back online now.

A couple of weeks ago we had our University Open Day, at the University of Waikato. This covered the whole university, and, of course, our department put together some displays and activities on physics and engineering. Perhaps the highlight was a demonstration of a range-imager. As people walked into the lab, where the main displays were, they were confronted by a large screen with their silhouette projected onto it, ‘live’. And, next to their silhouette, were a couple of numbers, one giving their height, and one their distance from the camera. All this updated in real time, and able to handle several people in the field of view at once, all moving about.  Pretty impressive.

Except, that as I watched visitors to the lab, they didn’t really dwell on it. Maybe this kind of technology isn’t all that far removed from what everyone is used to nowadays – mobile phones that do far more than just phone are used by people without them batting an eyelid as to what is going on inside them.  Instead, what people most loved, were the old favourites – the Van Der Graaf generator (it wasn’t a good day for it though – too much moisture in the air) – the superconductor floating above a magnet, and the liquid nitrogen rocket (Put a bit of liquid nitrogen in a plastic bottle, ram in a cork, and retreat to a safe distance).  These are all demonstrations of fairly straightforward physical phenomena, explicitly electrical charge, magnetism and pressure / conservation of momentum.  These simple phenomena (especially charge) underly so much of modern technology, but not necessarily in a readily visible form. Possibly this is why their demonstration, in a simple manner, is so gripping.

Just speculation.