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July 2015 Archives

I love this one. Really, it's maths not physics, but there is a bit of experimental physics creeping in at the fringes when the experimenters realize that the first method is biased. The second method is much better designed. 

Regrettably, pi-day (March 14th, 2015, or 3.14.15) only works if you use the US system of recording dates.  But fear not,  e-day (2nd July 2018, or  February 7th, 2018 if you're American) isn't so far away...

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Here's a puzzling photograph that Hans Bachor showed me at the end of the NZ Institute of Physics conference last week. It comes from his public lecture on lasers a week ago. And we don't have the answer to it, so maybe you can enlighten us (pun intended). 


The photo is of a demonstration of total internal reflection with a laser. Hans is holding a container of water, which has a small hole at the bottom. Consequently there is a jet of water emerging. A laser is held up to the container, and with careful orientation it can be made to shine down the stream of water. The light follows the water, due to total internal reflection at the boundary between the water and the air (rather like a fibre-optic). Actually, it's not TOTAL internal reflection - if it were we wouldn't see the light escaping from the stream of water, but a great proportion of it is contained within the water stream. 

Now, in this case, Hans didn't quite get the hole the right size and shape. Consequently the stream breaks up into discrete droplets, which you can see in the photograph. Now, here's the puzzle. Look at the droplets and you can see that a couple of them are shining green - i.e. they appear to have laser light in them. 

But how does that work? Light moves so much faster than water one can consider the water to be 'frozen' in space as far as the light is concerned. While the laser light will happily travel along the water stream, when the stream breaks up into drops there is no total internal reflection anymore. The drops should not be glowing. Perhaps the light is jumping from drop to drop to drop. Unlikely - each drop will scatter the light considerably so that very little will jump from one drop to the next - let alone across many drops. 

As you think about this, you should bear in mind the conditions the photograph is taken over. It's a flash photograph, but it's likely that the shutter is open for longer than the flash illuminates the scence. This might (or might not) be significant, since the flash will capture the position of the water stream, but the shutter will still be letting in light from the laser even after the flash has stopped. So the capturing of the 'green' laser light in the photograph is not completely synchronized with the capturing of the rest of the image. 

Our best hypothesis is that the light that is that drops are illuminated directly by light that is emerging from the end of the stream - that is, the light leaves the stream, travels though the air, and hits a drop. In the spirit of Eugenia Etkina's ISLE approach then, are there other hypotheses and what experiments can we formulate to test them?

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So the NZ Institute of Physics conference is in full swing. I have a bit of a break between the end of the last session and tonight's conference dinner, so there's time to give some highlights so far. 

Well, first, the low-light: Like the rest of my family and half of Hamilton I've had a horrible cold. On Sunday morning I was wondering whether I'd be able to make any of the conference. But I've managed to hold things together and now I've stopped sneezing I'm rather less infectious than I was at the weekend. So I've been able to get to some of the sessions. 

So what's been going on? We've heard from Hans Bachor that after decades of international scientific research into getting lasers to work, the world's first funding application for using lasers was for a 'death ray'. Fortunately, applications have grown well beyond this one (which is still, thankfully, not in place) and far beyond the ideas of the original researchers (i.e. 'blue sky' research can have real value). We've seen edible fibre-optics (basically jelly), and heard from Jenni Adams about the ICE CUBE detector at the South Pole for detecting high-energy neutrinos. 

The speed talk session last night gave us a rapid-fire mix-and-match bag of physics research from across the country - from Kannan Ridings' simulations of the melting of metal nanowire's through to Inga Smith's (unanswered) question of why do so few women do physics?  

But the real highlight for me has been Eugenia Etkina's inspiring talk yesterday and workshop this morning, on physics laboratory experiments. The basic idea here is that experimental science is done by experts in a particular way (and she has evidence for this), including a cycle of observation, hypothesis, experimental design, prediction-making, experimental testing, then judgement. Experiments  by experts are done for particular reasons - either to observe, to test, or to apply. Give a group of scientists a practical problem and they will tackle it in a very systematic way, that usually allows them to get to the bottom of what's happening. Give the same problem to first-year university students, and it's a mess of hypothesis, tesing, judgement, observation all rolled into one. So it then makes sense for us to give students opportunities to carry out the same scientific processes as real scientists. Too often we give them a series of instructions to follow. This isn't how real science works. It simply doesn't help them learn science. 

At the end of her talk, Eugenia asked a very simple but really telling question. "How do you know that Newton's third law is true?" My initial answer, to be honest, was: "because the text-books say so". Not the answer of a scientist.  Thinking about it a bit more, I can say "because that's what I experience...if I hit something hard it hurts...i.e. if I exert a large force on something it exerts a large force on me". But here's (roughly) what one of Eugenia's students said when given the same question:

"I have carried out many independent tests of this law and have not found a single case where it is violated." Now, that is the response of a real scientist. 






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First the low-tech:  The conjunction of Venus (the brighter one) and Jupiter as recorded by my very lousy cellphone camera  just after sunset yesterday. 


Now the high-tech: A day before that Pluto occulted a star. It moved in front of the star, rather like an eclipse. The significance of the event was that it allowed Pluto's atmosphere to be studied - by looking at the way the light moved through and around the atmosphere, various properties of the atmosphere can be inferred. The SOFIA project was in action, capturing the event, at the other end of the cost spectrum to my mobile phone.

There'll be more Pluto excitement coming as the New Horizons probe flies closeby in just a couple of weeks.  


P.S. I should add in the conversation I had with my son (just turned 3) yesterday, after showing him the planets outside. 

Benjamin: "I don't like planets"

Me: "Why not?"

Benjamin: "Because they're quite noisy"

Me: "How are they noisy?"

Benjamin: "Because Grandad says they're quite loud, actually."

Umm.... Work that one out!

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