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I didn't actually intend to visit the Synchrotron. I didn't actually know it was right next door (honestly - I don't exaggerate) to the Centre for Biomedical Imaging at Monash University in Melbourne until I arrived there on Monday. Somehow I managed to get myself tagged onto a tour with a group of students.

The synchrotron is a machine for producing broadband high energy X-rays (plus a lot of other lower energy electromagnetic radiation) for a variety of purposes - what they are used for depends on who has set up their experiment at the time. Electrons are accelerated to high energies, and then when they are bent they emit high-intensity broadband radiation in a very narrow beam- very useful stuff for physicists. We saw a bit of the gear needed to do the job - lots of magnets and electromagnetic cavities for accelerating the electrons with alternating electromagnetic fields. What I loved was the method for tuning the cavities. Someone would adjust them by punching tiny dents into the side of the metal (lots of copper here) at the right places - a low technology solution to a high technology problem.

What was I doing next door? I was in Melbourne last week visiting some potential collaborators. This coming week I'll be talking with some more - this time in Perth. I took the opportunity today to travel out to Rottnest Island and see the quokka (what's a quokka?) - a beautiful day trip from the city. But back to work on Tuesday.


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Seen on a notice at a Cambridge Cafe:

Waipa District Council. Permit to occupy pavement space. This is to certify that **** has been approved to occupy 15.000000 metres squared of pavement space. 

I might not have got the exact words right, but I certainly counted the number of zeros after the decimal point. 

The cafe will be delighted, I'm sure, to know they can occupy 15.000000 metres squared of space as opposed to 14.999999. That extra square millimetre will make all the difference to their before-tax annual profit. Perhaps they'll be disappointed they hadn't applied for 15.000001 metres squared. 

Presumably Waipa District Council, when they come round to check that this cafe is abiding by the rules of its various permits, will be measuring each linear dimension of the tables and chairs to the nearest millimetre. 


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There's been a lack of any activity from me for the last few weeks. This is mostly down to teaching overload at university. About 50% of my year's teaching comes in the first half of our first semester, which leaves me pretty-well no time to do anything else. Writing in a blog is the least of my priorities at the moment, I'm afraid.

That doesn't mean I'm not taking note of physics when it arises. On Monday night I was supervising a test - it was the loudest one I've ever experienced (save the 'test you can talk in'). That was down to 180 students shuffling pieces of paper about in a concrete-walled, concrete floored lecture theatre. With the sound reflected so nicely off the rigid walls (having a very different acoustic impedance from that of air) each turn-of-the page would have been heard several times before its energy finally decayed away. Couple that with a test script that in hindsight was badly formatted and required the students to keep turing to the back to look things up in a table, and there was a real hubbub of paper noise. The sign 'Quiet Please, Examination in Progress' on the door was quiet pointless.

Back some time later...

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...No, it isn't something everyone smokes... 

But it is common in machine mechanisms. The universal joint is a neat way of turning rotation in one plane into rotation in another. A common use is on driveshafts where you want the direction of the shaft to bend. There's a neat animation on Wikipedia of how the thing moves. Despite seeing them in action (including in our teaching lab) I'm always amazed that it works. Let's think about it. There are four pins on the joint - two for one shaft, and two for the other. They have to stay in the same geometry (namely at the four corners of a square) as both shafts rotate. That doesn't seem possible. Three points are what defines a plane - put in a fourth and surely it's not, in general, going to sit on that plane, let alone stay at an angle of 90 degrees in the plane from its two neighbours. The problem is 'solved' when you realize that the two shafts do not rotate at exactly the same rates. What I mean by that is that if you rotate the first shaft at constant rotation speed (angular speed) the second does not respond with a constant rotation. At some parts of its cycle it speeds up slightly, and at some parts it slows down. The extent of this speed-up and slow-down depends on the angle through which you bend your drive-shaft. A large angle of bend will cause a considerable fluctuation in rotation rate of the driven shaft as it goes through a cycle.  However, for small angles, this fluctuation is pretty small.

These fluctuations can be important and problematic, since a fluctuating rotation rate causes a fluctuating torque on the equipment. 

Now, here's the really neat bit about the universal joint as far as I'm concerned. I don't care much for mechanical mechanisms (Hmm - maybe my third year mechanical engineering class shouldn't hear that...) but I do like astronomy. The maths governing the fluctuation in rotation rate of the driven driveshaft is exactly the same as the maths determining one of the two contributions to the Equation of Time. This equation is what determines how 'fast' or 'slow' solar time (that is, what a sundial would measure) is compared to clock time.

A day is only 24 hours long on average through the year. Sometimes it is about half a minute shorter, sometimes about half a minute longer. We can see the effects of this at the moment - as we come out of summer (sigh...) the long days are ending. The mornings are now considerably darker (sunrise is much later) than it used to be.  But the evenings are still pretty light. Sunset hasn't shifted a lot. This is because our days have actually been longer than 24 hours for a few weeks. Clock time has gradually got ahead of solar time. (But, fear not, it will reverse itself quickly.) There are two reasons for this effect, which is pretty noticeable around November (light mornings compared to evenings) and February (light evenings compared to mornings). First, there's a contribution due to the earth's orbit around the sun being elliptical, not circular. The earth moves quicker in December/January than it does in June/July. Secondly, there's an effect due to the fact that the plane of rotation of the earth on its axis (and its the earth's rotation that obviously controls clock time) is tilted compared to the plane of orbit of the earth around the sun. The two planes are not the same. The maths for this second contribution is pretty horrendous but it was done long ago and if you are interested you can go and look it up. 

Note that we are talking about two different planes of rotation, slightly tilted with respect to each other. That's exactly the same as for the universal joint. The same mathematics applies to both. Which makes the universal joint, after all, something worth looking at. 





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I shudder to think what it must have been like in the path of Cyclone Winston. It is hard to conceive of winds 230 km/h sustained for minutes at a time. I remember vividly what is now known as the Great Storm of 1987 (an extra-tropical cyclone) which pulverised south-east England on 15/16 October 1987. There were (according to Wikipedia - ahem!)  gusts close to 200 km/h recorded in Sussex (where I lived), but there were possibly higher ones than these - the anemometers failed.  I spent the night listening to trees falling one by one around our house. Opposite the house was (and still is) a very tall Wellingtonia - one of the earliest specimens of this tree planted in the UK - and if that had fallen on us there wouldn't have been much house left. It stood firm, thankfully. That is frightening stuff.  But that's probably small fry compared to what Cyclone Winston did. 

One thing that I didn't personally experience in 1987 was the storm surge. (Being about 40 km inland kind of protected us from that.) Storm surges are a major cause of deaths in cyclones. The sea level can rise substantially during a storm - and coupling that with a high tide can lead to widespread and sudden flooding. 

There are lots of ways that a storm can raise water level. Winds can blow water towards the shore, and the Coriolis force acting on moving water can cause a build up. One simple effect is that the low-pressure in the storm simply 'sucks' the water level upwards.

Atmospheric pressure (about 1000 millibars or about 100 kPa) can hold up about ten metres of water. If you had a thin tube, filled it with water, sealed one end,  put the other open end in a bucket of water, and lifted the closed end ten metres into the air, you'd see that you got to the point where the water in the tube couldn't be supported any more. A vacuum would form above this height. See it here! In fact, what you have is a barometer - the height of the water is proportional to the atmospheric pressure. A 1 millibar change in pressure corresponds to about a 10 mm of water. With Cyclone Winston, the pressure dropped to 915 millibar, meaning about an 85 cm increase in the height of the ocean to this effect alone. This may not sound much but the disturbance doesn't remain localized - it will propagate out in a similar way to a tsunami. A fairly small shift in sea level in the ocean can correspond to a much more considerable shift when the wave slows down close to the shore. Throw in the effects of wind and rainfall and so forth, and one can end up with a devastating and sudden increase in sea level.  

At a more gentle level, atmospheric pressure is what holds up the water in a pet water dispenser, like the one we use with our chickens. There would be no point having a dispenser more than 10 metres high (that would water a lot of chooks indeed) - there would be no water supported above this height. 





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Yesterday I was part of a very interesting workshop on Science in Society, in Auckland. There was a plethora of good examples of science communication discussed - including forest restoration on the East Coast, biological control of pests in vineyards in Canterbury and improvement of health outcomes for Native Americans in Montana. For me, it was clear that there were some resounding messages coming through about science communicaton. 

1. It needs to be driven by the community. Here, community could mean a town or village, a marae, an industry group, a school - any group of people with an interest in achieving something. The participation of the scientist is as a partner, often as a junior partner. In other words, the community takes the lead. The scientist(s) doesn't go out and say "Right, now I am going to do some science communication." If she does, no-one will listen. Instead, she needs to be listening and responsive to the (scientific) needs of others. 

2. Communication is about relationships. Richard  Faull gave a very humbling talk about his work on Huntington's Disease, done in partnership with several Maori families across the country for whom Huntington's is tragically real.  It is a true partnership. To achieve what he has done has taken decades of building relationships. Listening to people's stories, spending weeks on Maraes, being available Christmas Day for someone to offload their fears for the future.  

3. There is a difference between outputs and outcomes: It is easy(ish) to write journal articles about science communication projects. That's an output. An outcome is a lasting impact for the people concerned:

Communication isn't complete until it is put into practice for the people for whom it makes an impact  - Polly Atatoa-Carr

Now here's the problem for the scientist (i.e., me). We are all tasked to be science communicators. (Yes, we are - if you're a member of a professional organization you'll probably find it's part of your responsibilities as a member - and, if nothing else, it is your duty as a professional to talk about your profession.) But it isn't something we can do (as in "Right, I need to do some science communication in the next few months - what shall I do?") Soana Pamaka, of Tamaki College in Auckland, summed it up "Schools are sick and tired of being 'done to'." Instead, we need to build relationships with community groups and be open to respond to opportunities that arise. Almost certainly, those opportunities will not be in our specialist areas. I mean, how many community groups have an interest in neural field models? But if we have good relationships, then groups will come to us because they know us. And we have to respond to that. For example, at Tamaki College, which has a fantastic science programme, the science communication is driven by the school, which means the children, with guidance from teachers. The scientists work in partnership with them. 

Better science communication needs better relationships with communities, and be community-driven. The scientists need to be open to respond to those opportunities. How ironic then, for a workshop on 'Science in Society' nearly all participants were scientists or educators.



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The big breaking physics news is the detection of gravitational waves. These waves are distortions in space-time, caused by a large mass doing something spectacular (two colliding black holes in this case)  that propagate across the universe and create tiny changes in space when they reach us. The commentary here describes what goes on. Essentially, things change their length/width. When a gravitational wave passes through my office (say ceiling to floor) one can imagine the length of the office increasing slightly, coupled with a decrease in the width of the office, followed by the reverse - a decrease in the length and an increase in the width.  But its not just that the bricks that make up the room vibrate (e.g. as in a seismic wave) - its the whole of space that does it. 

These waves were predicted by Einstein in 1916, just after the publication of his theory of General Relativity. Their discovery is further evidence for the theory. But it's not just about Einstein. Gravitational waves provide another way of observing the universe - 'seeing' what's going on. Up to now, we've been stuck with light-based observations (be it visible light, infra-red, microwave - they all are electromagnetic waves). There are neutrino observations too, but these aren't exactly easy. But gravitational waves are something else - it's like seeing AND hearing something, rather than just seeing it. 

So how are they detected? The concept is rather simple, as explained in the commentary. Build a (large, meaning 4 km in the case of LIGO) interferometer with two arms. Pass light up and down each arm. The light from the two different paths will interfere - such interference could be constructive (if a peak from one arm comes at the same time as a peak from the other) or destructive (if a peak comes with a trough). If everything is stable, the interference is stable. But when a gravitational wave passes, the arms change their lengths. Not by much. The light takes longer to pass up and down one arm, and shorter to pass up and down the other. Now the timings of the peaks and the troughs change, and the interference signal changes. We detect a gravitational wave. 

The difficulty to now has been detecting the tiny signals amongst larger 'noise' signals, but a recent upgrade to the LIGO detector has done its job. Well done LIGO team!





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Alison Campbell alerted me to the following: Physicist Andreas Wahl shoots himself with a gun underwater - and proves a point about drag force. 

For the record - I won't be repeating this. Physics or no physics, the guy is crazy. 

BUT, what I have done, is a quick post-hoc analysis from the safety of my own office. There's a little bit of maths involved, but the gist of it is this. The drag force on an object (in turbulent conditions - which this most certainly is), is given by the equation c rho A v2 where c is the 'drag coefficient', rho is the density of the fluid in which the object moves, A its cross-sectional area and v its speed. If we equate this to the objects mass times acceleration (Newton's second law) we get an expression for the acceleration of the object in terms of some physical parameters. Solving the equation (integrate it!) gives an exponential decay relationship between the velocity of the object and the distance it travels. Thismeans there's a characteristic length-scale, d, given by:

d = rho_bullet x b / (c rho_liquid)  

where b is the length of the bullet.  Broadly speaking, d gives you an indication over what distance the speed will decay over. We can now stuff some numbers in. Let's assume the density of the bullet is about five times the density of the water (note how it's only the ratio of the two that matters) and that the bullet is about 2 cm long. The drag coefficient will be quite low, given its a streamlined object; say about 0.1. That gives a distance scale of around a metre. How far does the bullet travel? From the video, I'd say something around a metre. 

So, what's the difference between water and air? It's the density of the fluid. Air has a density around 1 kg per metre cubed (rather than water's 1000 kg per metre cubed). Fire the bullet in air, and the length scale goes to one thousand metres (1 km). I'm not a gun expert, but that figure seems about right. 

I also think he was very wise to have his head out of the water. Sound travels rather well underwater.

And it's very easy to say all that without a gun pointing at your chest. 


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Don't miss the BBC poll on what is the world's most beautiful equation. Are you a fan of Einstein's field equation, or does the Riemann zeta-function hold you in raptures? There's some great commentary on the twelve candidates here

How did I vote? Well, that would be telling, but the fact that my very first publication is titled Auxiliary-field quantum Monte Carlo calculations for the relativistic electron gas [read it here! - at least if you have access to the Journal of Physics: Condensed Matter] might give the physicist readers a few clues. 


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Going back to my last post, our fancy balance proclaims that it weighs objects from 0 to 200 g with a precision of 0.001 g (that's one milligram).   And it does - put an object on and the balance gives you an attractive-looking number on its prominent display reading 184.139 g, or something similar. It is precise to 1 milligram. It's not reading 184.138 g, neither is it reading 184.140 g, it is reading 184.139 g. 

So does that mean our test object has a mass of 184.139 g? Unfortunately not. Just because the balance gives us that number that precisely, it doesn't mean that it is that accurate. University lecturers always have a good giggle when some poor unsuspecting first year student records an answer to a wildly inappropriate number of significant figures - for example she might measure a speed in the lab of 1.48392348837 m/s.   Precise, yes. Accurate, no. However, when a third year student does the same time (I've usually got the message across by then, aided by deducting marks on assignments for stupid use of significant figures), the humour turns into despair. 

So what does our test object weigh? (What is it's mass? I mean). Well, I can weigh it several times and see how the results are spread. I've done that. It's a few milligrams. On taking the object off, and putting it on again, I don't see the recorded mass change by more than three of four milligrams. If I take a lot of measurements, and work out the mean mass and its standard uncertainty (with a bit of statistics) I can get something that has a random uncertainty of only a milligram or so. 

However, that still doesn't mean our test object has a mass of 184.139 g (or whatever our calculation says). I may have accounted for random uncertainty, by weighing it multiple times, but there's certainly other systematic sources of uncertainty. These are significant. Just look at the graph. This is the mass of the test object (as recorded by the balance) over the period starting Tuesday morning this week. Our object is getting lighter! Quite a lot lighter, too. It's moved about 40 mg over three days! That's about one part in 5000. 


What's happening? One interesting thing to note is that we initially calibrated the balance with a nominal 200.000 g mass (sent with the equipment) and a completely empty pan (0.000 g). I've also been weighing the calibration mass and the empty pan over the course of the three days too. They have shown no drift at all - just a couple of milligrams of random uncertainty, as far as I can see. 

The manual suggests that the equipment is affected by temperature and humidity. Now, Monday was one of those horrible Waikato days with a warm, damp atmosphere and lots of rain. One of those days where, if the humidity got any higher, it would be raining in your office. And there was a lot of rain Monday night, before I made the measurements. On Tuesday morning, everything 'felt' damp - but we've been drying out ever since. Is it a long-term drop in humidity in the lab that's caused the change? And is it because the test object was actually heavier (maybe there was some condensation on it) or is it because the humidity has made the balance 'stick' or affected the electronics in some way. I'm not sure?

But what I am sure about is that saying I have a 184.139 g test mass is, at present, unjustified. 





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