# Physics Stop

## Time travelMarcus WilsonAug 24

On Monday evening this week I managed to do a bit of time travelling while driving back home. I was driving back through one of those heavy showers that have been marauding around the place recently, with windscreen wipers full pelt on a rather wet road. However, these showers don’t last for very long, and the rain soon began to ease. As it did so, I noticed the road was getting drier.  Then the rain stopped all together, and I was left driving on an absolutely dry road.

That’s not what usually happens. Usually, the more rain there is, the wetter the road gets. I’m sure you’ve worked out what was happening.  I was heading in the same direction as the shower, but going faster than it. So I had overtaken it, as it were, and emerged from the rain ahead of the shower. There was a dry road, because the shower hadn’t got there yet.  Sure enough, I got home in the dry but within a few minutes it was raining – the same shower that I’d just driven through.

So I was experiencing the events of the rain shower backwards, because I was travelling faster than it. In one sense it was time travel. I was seeing events happen in a different order from what someone stationary on the ground would have seen.

Of course, it wasn’t really time travel. My clock was still going forward, as was everyone else’s. Now, if I’d been travelling faster than light, things might have been a little different. Special relativity says that time slows down for an observer travelling quickly  (from the point of view of someone who isn’t).   As this traveller approaches the speed of light, special relativity says that the passing of time for him becomes very slow indeed. In fact, at the speed of light, time wouldn’t pass at all for him. That’s one of the reasons that photons, light ‘particles’, behave very oddly.

What about beyond the speed of light? Physics as we know it doesn’t let us go there, not even with those neutrinos at Gran Sasso. If that result had been true, our understanding of physics would have been shaken up quite severely. The possibility of really travelling backwards in time might then have become a reality.

[ For those who are more mathematically inclined, the rain shower's also an example of why the partial derivative is not the same as the full derivative.  The full derivative for the rate of change of road wetness with respect to time was negative here - the road was getting dryer as I went alogn, but the partial derivative of road wetness with respect to time at constant position was still positive.  ]

## The wonders of concreteMarcus WilsonAug 21

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As I mentioned in an earlier post, one of the buildings near my office is currently being redeveloped. By redeveloped, I mean a mix of rebuilding, extending and renovation. In the last few weeks the building has been stripped back to its structural shell – pretty much all things that aren’t necessary to hold the place up have been removed. It’s being extended at the front, and will have a lovely glass frontage when finished and generally be much more pleasant to look at than when in its former state.

It’s been interesting to see just how little is required to hold up a four-storey building. The main concrete supports really don’t look much at all, and the that support each floor span an unnervingly long distance. One must assume that the original architects knew what they were doing and that a medium-sized earthquake isn’t going to send the thing falling.

The key is what is inside the concrete. On its own concrete is great at bearing compressive loads. The ‘Engineering Toolbox’ website gives me a compressive strength of 20-40 MPa. (One MPa is a million newtons force per metre square, or about 100 tonnes of weight per metre square – that is about 10 kg on a square centimetre)  But begin to stretch it (e.g. by trying to bend it) and it will fail. The tensile strength is only about a tenth of its compressive strength. A steel rod on the other hand does the opposite. It’s fantastic when stretched (a tensile strength of several hundred MPa), but a thin rod of it will buckle easily in compression.  Mix the two together, and you get the best of both. A reinforced concrete beam is a great example – as it curves under loading, the inside of the curve will be in compression (and the concrete can take that) but the outside in tension (and the steel can take that).  One simply hopes that there is enough steel to do the job.

It’s a cheap, effective way of building, which is why there is so much of the stuff about. Hide it behind something else, and it needn’t look ugly, as this building shouldn’t when finished.

So the next question is, when will they do my building?

## Wading recklessly into the water ownership debateMarcus WilsonAug 16

If you’re in New Zealand, you cannot have failed to be aware of the legal wranglings over the ownership of water. Who owns or has rights to the water in our rivers? The raising of this question is a not-so-subtle attempt from one half of the political spectrum to delay (or stop) the sale of state-owned-enterprises (notably hydroelectricity companies) by the other half of the political spectrum. Basically, if the water isn’t the government’s, then it can’t sell it, so it isn’t able to privatise the hydro companies – at least that’s the argument in hopelessly over-simplified terms.

There’s one issue regarding this that I haven’t heard at all in all the debate that is happening. What is it that the government is actually trying to sell?  It isn’t the water.  Hydro power does not involve the removal of water from the rivers. What flows into the station flows out again. So it isn’t water that is the asset that is being sold here. Rather, it’s electrical energy that the hydro-company sells. The energy is gravitational potential energy, as a result of gravity – a litre of water at a height is able to give up energy as it falls – e.g. into kinetic or movement energy. This kinetic energy is then transferred by the turbines in the power station into electrical energy. The water doesn’t vanish in the process; it simply loses height, and with it energy.

It’s similar to the way that electrical current isn’t ‘used up’ by a heater, light bulb or other device. You have two wires (discounting any earth wire) connected to the device. Current flows in one, and the same current flows out of the other. (Indeed, in a domestic supply, it reverses its direction 50 times a second.) How come then you get an electricity bill if what flows in flows out again? It’s because you are being billed for the energy that is transferred through this process. Putting that current through the device leads to transfer of energy from the power grid to the device, just like turning on the water flow through a turbine leads to energy being taken from the water.

So the question shouldn’t be over who owns the water. It should be over who has the right to do something useful with that water.

## Equipment failureMarcus WilsonAug 13

In the last couple of weeks, I’ve been working with one of our technicians tracking down what has been going wrong with one of the experiments we get our third year physics students to do. It’s on Brownian Motion. Specifically, analyze the movement of small particles suspended in water by scattering of laser light. By studying the way in which the scattered light varies in intensity with time, we can work out the size of the particles in the suspension.

So says the theory. However, in practice the pattern of scattered light is nothing like what we’d expect in this situation. There was clearly something going wrong, but working out what hasn’t been straightforward.

In the end, we just went through piece by piece through all the equipment, and the interfaces between the equipment, checking each was doing what it should have been. In the end Stewart worked it out – we had a dodgy oscilloscope. It’s rather easy to trust your instrumentation, especially that you’ve paid a lot of money for, but it is worth remembering that sometimes it breaks, and, when it breaks, it might not do so in a manner that is obvious. A piece of equipment that spits out the dummy and refuses to do anything is rather less dangerous than one that, on the face of it, is doing its job, but actually is getting it wrong. In this case the consequences of the fault are hardly serious – we’ve just had an experiment that was clearly giving puzzling and unbelievable results. In fact, for the last couple of years, I haven’t had the students even attempt it, because I’ve known something’s been amiss with it. However, that’s not always the case.

There are similarities I think with those faster-than-light neutrinos that hit the headlines last year. It was a crazy result – hence the attention – but on the face of it the experimental results appeared to be real. But, very careful checking of the apparatus highlighted a couple of glitches with the equipment. It wasn’t doing exactly what it was supposed to be doing. The problem was small, but it was big enough to produce a sensational result.

Fortunately, science has ways of correcting itself, and in due course the problem was tracked down by some careful investigation. It’s interesting that this is a skill we often overlook in teaching our students. In an effort to illustrate the theory, we present them with experiments that actually work (or at least try to). We never (certainly not here) deliberately give students dodgy equipment and then teach them how to find out what’s going wrong. Given that it is a skill that any experimentalist needs to have, it should be one we teach. Something to try in the future with another class of guinea pigs.

## Some thoughts from teaching appraisalsMarcus WilsonAug 08

First of all, sorry for the lack of entries. That’s what having a baby does. To write blog entries, or to try to extract a smile out of the bubba?  Baby wins every time!

Anyway, this morning I was going through some teaching appraisal forms. At the end of every paper, the students have the opportunity to give feedback to the teachers, through questionnaires. They are asked a series of questions on both the paper and the teachers, with a Likert-scale response. For example, there’s the statement "The paper was well organized and ran smoothly", to which they respond by choosing Always, Usually, Sometimes, Seldom or Never.

Always is given a score of 1, through to Never which is given a score of 5, then responses from students get averaged to give lecturers and papers an overall score. What is a good score? That’s not easy information to come by. Partly this is down to privacy – we are not obliged to reveal our scores to anyone – partly I think because people don’t want to be accused of ‘blowing their own trumpet’. I was talking to a colleague yesterday who admitted that he would typically get an average of about 1.1 in papers, and then apologized profusely for this in case I though he was being big-headed. Anyway, one thing we can know for sure is that we’d like to see scores go down from year to year.

So, I was disappointed when I looked at this year’s score for my third year mechanical engineering paper. It’s a pretty mediocre score, really, and, what’s more, it’s exactly the same as it was last year, when I taught the paper for the first time. This year I did a lot of things differently, but the score hasn’t budged.

So, I’ve looked carefully at the free comments the students write on the questionnaires. Hopefully these would reveal to me where I’ve been going wrong. But no. Rather perplexingly, the comments I’ve had are overwhelmingly positive. So why the low-ish scores on the Likert questions. I’m rather baffled. I thought that maybe it was a case that the annoyed students just couldn’t be bothered to write any useful comment on the questionnaires. But it’s not this – there are students who’ve given me some really positive comments and yet have given me lousy scores on the five-point scale. I can’t work it out. Maybe mechanical engineers are just hard to please. Perhaps its a bit of engineering pedantry: ‘Always’ means to them 100% of the time; doing it merely 99% of the time isn’t enough to get that response.

Since the scores are baffling, I won’t dwell on them too much. Much more interesting are some of the comments, particularly on the test, which the large majority students loved (so says the feedback). In this paper I trialled the idea of a ‘test you can talk in’. Here are a couple of comments:

Test format … was choice: learnt heaps

The test format was great. It seems like a good way to have discussion within groups, which helped people learn…

The ‘learning’ through peer-discussion was of course the whole point of the test.  Rather than it being merely a summative, stressful exercise, it was intended that the test would be a learning experience in itself. And, indeed, it appears that this has been the case. That’s great feedback and will encourage me to use this format more widely.

Also, I had a comment from a student who is clearly up-to-date with theories of learning:

MULTI-CHOICE Q’S AT START OF CLASS EVERYDAY ARE THE MOST EFFECTIVE TOOL AT ENGAGING CLASS EVER!!  :D

(Capitalization and the smiley are the student’s.)  Glad you think so. So do I.

It’s well worth trawling through the appraisal questionnaires and digesting comments. In this case they are far more positive and helpful that the scores alone suggest.

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