PW_mar13_sample_issue

Stephen Morris, University of Toronto
physicsworld.com Frontiers
How many dominoes will topple a cathedral tower?
When a science quiz on Dutch TV last year asked its participants how many dominoes would be needed to tip
over a domino as tall as the Domtoren – a 112 m-tall cathedral tower in Utrecht – mathematical physicist
J M J van Leeuwen of Leiden University in the Netherlands got his thinking cap on. Starting with a standardsized
domino 4.8 cm tall, he calculated the upper limit on how much larger each successive domino could be
in this game of “domino multiplication”, suggesting that the maximum ratio of successive domino heights is
30% larger than the widely accepted value of 1.5. Assuming a growth factor of 1.5, the answer is 20 dominoes,
but pushing the growth factor to 2, it should easily be done with just 12. Inspired by Van Leeuwen’s research,
the images above show Stephen Morris at the University of Toronto as he knocks over a series of 13 dominoes
with a growth factor of 1.5. He claims that the energy needed to tip the first fingernail-sized domino is
amplified two billion times by the end of the chain reaction – when a 45 kg block crashes to the floor. “If I had
29 dominoes,” says Morris, “the last domino would be as tall as the Empire State Building.” (arXiv:1301.0615)
Mobiles map the rain
Cellular communication networks can
be used to accurately predict large-scale
rainfall distribution patterns in real time,
according to researchers in the Netherlands.
The team created rainfall maps for
the whole of the country using data gathered
by telecom firms of the attenuation
of microwave signals across 2400 network
links over a four-month period. The resulting
maps are largely similar to measurements
taken by conventional weather-radar
and rain-gauge techniques.
The research was carried out by Aart
Overeem and colleagues from Wageningen
University and the Royal Netherlands
Meteorological Institute. The team looked
at the minimum and maximum received
signal power at each telephone tower in a
network over 15 min periods, as microwaves
are sent from one tower to the next. Signals
passing through falling raindrops are partly
absorbed by the water molecules and also
get scattered slightly, lowering the power
that reaches the receiving tower. The more
raindrops in the beam’s path – or the larger
the drops are – the more signal power is lost.
By comparing received powers for
each network link with reference values
for known dry periods – and factoring
in humidity and the water films that can
develop on the communications antennae
– the researchers were able to calculate the
rainfall densities along each path. These
values were then treated as point measurements
at the centre of each network link and
used to extrapolate the larger rain-distribution
maps. In the frequencies employed
in these links, attenuation caused by raindrops
are the only main source of power
reductions, apart from free space losses
(PNAS 10.1073/pnas.1217961110).
Innovation
Digital files stored and
retrieved using DNA
Scientists in the UK have stored about a
megabyte’s worth of text, images and speech into
a speck of DNA and then retrieved that data back
almost faultlessly. The research was carried out
by Nick Goldman and colleagues at the European
Bioinformatics Institute, who have stored digital
information by encoding it in the four different
bases that make up DNA. While the technique
does not offer the convenience of random
access or being rewriteable, its advantages
include being highly durable and also offering an
extremely high-density storage method.
The group used DNA that was produced in the
lab rather than from living organisms, since the
latter is vulnerable to mutation and data loss.
Unfortunately, it is only possible to synthesize
DNA in short strings and the shorter a string is,
the lower its data storage capacity. So the team
devised a coding scheme in which a fraction of
each string is reserved for indexing purposes,
specifying which file the string belongs to and
at what point in the file it is located, allowing a
single file to be made up of many strings.
To avoid errors that occur during both writing
and reading the team encoded data in trits –
digits with the values 0, 1 or 2 – and stipulated
that a given trit is represented by one of the
three bases not used to code the trit immediately
preceding it. The researchers tested their
scheme by encoding five data files into single
DNA sequences and then split those sequences
up into roughly 150 000 individual strings, all
117 bases long. They encoded a PDF of Watson
and Crick’s famous double-helix paper, a
Shakespearian sonnet and an audio recording
of 30 s of Martin Luther King’s “I have a dream”
speech in MP3 format. The team then uploaded
the encoded files to a private webpage to enable
a company in California to synthesize the DNA.
The DNA was then sent as a tiny quantity
of powder at room temperature and without
specialized packaging to the European Molecular
Biology Laboratory in Germany, where all five
files were sequenced and decoded. Four of the
files were identical copies of the originals, while
the fifth required some minor adjustment to
recover its full set of data.
The researchers claim to have achieved a
density of 2 petabytes per gram of DNA, which
could, in principle, allow at least 100 million
hours of high-definition video to be stored in a
teacup. Currently the technology is too expensive
to be competitive for all but the most long-term
archiving, but Goldman is confident that prices
will come down (Nature 494 77).
Physics World March 2013 5