This site uses cookies. By continuing to use this site you agree to our use of cookies. To find out more, see our Privacy and Cookies policy.
Skip to the content

Free weekly newswire

Sign up to receive all our latest news direct to your inbox.

Physics on film's multimedia channel features exclusive video interviews with leading figures in the physics community.

Visit our multimedia channel to see the latest video.

Margaret Harris: September 2009 Archives

1987 esa calendar small.jpg
Images from the European Orbiter on the 1987 ESA calendar. Credit: ESA

By Margaret Harris

The question of what to do with old calendars is (literally) a perennial one, but the European Space Agency has an interesting solution: post them on the web.

The agency has created an online gallery of calendars and posters depicting missions from the last 30 years. The images are drawn from archives at the European Space Operations Centre in Darmstadt, Germany, and from a retired employee’s private collection. They include both satellite photos like the one on this calendar — which would have made a great Christmas gift back in 1986 — and artists’ impressions of missions.

It’s not clear why ESOC has chosen to post these images now. There’s no information on the website about any special exhibition, for example, and the nominal 40th-anniversary tie-in seems a little odd, given that ESOC is now 42 years old.

But whatever the excuse, leafing through the various posters is both a nice reminder of the agency’s successes and an interesting glimpse of how it has advertised itself over the years. I particularly liked the city of Darmstadt’s poster, which used a picture of a rocket to promote a week of extended shop-openings back in the 1980s. Unlike the others, it’s not an official ESA image, but I can see why they like it — it neatly captures the public’s enthusiasm for space, and the eagerness to appropriate “cool” space imagery for utterly unrelated purposes. Space-age shopping hours — whatever will they think of next?

The official kilogram. Credit: BIPM

By Margaret Harris

Pick the correct definition of a kilogram:

a) the mass of a body with a de Broglie wavelength of 6.626069311 × 10^-34 m at a velocity of 1 m/s

b) a mass of a body at rest such that Planck’s constant h is 6.626069311 × 10^-34 Js

c) a mass of exactly 5.0184512725 × 10^25 unbound carbon-12 atoms at rest in their ground state

d) the mass of a lump of platinum-iridium sitting under three vacuum jars in a French laboratory

Readers with an interest in metrology will know that the answer is d) — and anyone who didn’t know it could probably have guessed from the photo. But why is the kilogram, alone of all SI units, defined by something so un-fundamental as a lump of metal?

The difficulty, as Bryan Kibble explained this afternoon in a talk at the QuAMP conference in Leeds, is that several of the alternatives have problems of their own. Options a) and b) both rely on pinning down a value for Planck’s constant, and thus might seem like the best way to go; indeed, one of them may actually become the new SI definition, perhaps as early as 2011. However, Kibble argued, both options are somewhat circular, swapping uncertainty in the kilogram for uncertainty in other Planck-derived units, and there’s not really any new science involved in them.

A definition in terms of carbon-12 atoms — or indeed, any kind of atoms — would be more satisfying, Kibble says, but as efforts like the Avogadro project at the UK’s National Physical Laboratory have shown, counting atoms isn’t a trivial task.

Nobody offered any solutions during the question period after the talk, but we did manage to pin down one thing: any fluctuations in fundamental constants (like the fine structure constant, for example) will not affect the kilogram problem — at least not for around 1000 years. So that’s all right then.

Tunneling in action

By Margaret Harris

How long does an electron take to tunnel out of an atom exposed to a strong laser field?

Given the somewhat esoteric nature of the question, you might assume that the answer would lie firmly in the realm of theory. But Ursula Keller, whose talk opened this year’s International Conference on Quantum, Atomic, Molecular and Plasma Physics (otherwise known as QuAMP), is an experimentalist, and she and her group at ETH Zurich have made some interesting progress towards pinning down just how long this fundamental quantum-mechanical process takes.

Using a technique called attosecond angular streaking, Keller’s team found an upper bound for the tunneling time of 34 attoseconds. That’s quick — in fact, Keller claims it is the fastest process ever measured, although some might quibble with that distinction. I’m afraid I only grasped her group’s methodology in small chunks — that’s the trouble with talks sometimes — but you can read more in a paper published in Nature Physics last year.

One development that isn’t addressed in the paper, but which Keller touched on in her talk, is just how controversial their result has been among theorists. The idea that tunneling takes a tiny but finite time makes some intuitive sense, but this is quantum mechanics — intuitive sense doesn’t always come into it. Indeed, some theorists have predicted that the electron’s escape literally takes no time at all, while others suggest that tunneling isn’t even the right way to look at the process.

The arguments on this have become so heated, Keller says — half-jokingly I think — that a few of the people involved aren’t on speaking terms anymore. One thing is clear: the debate on electron tunneling is sure to carry on much longer than the process itself.