Clap trap
Q: Near thunder is heard as a clap followed by a rumble, presumably from
sound waves that have travelled by a longer route. But why with distant
thunder is there often a short rumble building up to the clap?
A: The questioner is correct in assuming that a close lightning strike is
normally heard as a thunder clap followed by a rumble because the rest of the
lightning discharge is more distant. However, even if you observe just a
single lightning strike to the Earth there would be other parts of the
lightning discharge contained within the cloud which an observer cannot see.
These in-cloud discharges can be very extensive horizontally.
Earth strikes do tend to be much louder than the in-cloud parts of the
discharge, so they will often be heard as the loudest part of a peal of
thunder even if they do not come from the nearest part of the lightning
discharge.
And this is the answer to the query. Normally when you hear a more distant
Earth strike you hear some rumbling first because another part of the
discharge – which is possihly hidden within the cloud – was actually closer.
A: Lightning usually flashes between clouds carrying different charges. In
the right conditions the initial discharge triggers a cascade of discharges to
nearby clouds. This happens very quickly during the flash itself and creates a
plasma tube which can be up to several miles long. Strikes to the ground are
typically caused by leakage from the plasma tube to the Earth.
The thunder is the sound of the plasma tube forming and collapsing. This
happens almost instantaneously for a whole tube. The duration of the thunder
clap is the time difference between arrival of sound from the nearest part of
the tube and sound from its farthest point. The rumbles are caused by sound
arriving from a part of the tube that is end-on to the observer and the bangs
arise from a part that is side-on, when all the sound from that section arrives
at once.
American studies have used microphone arrays to determine the geometry of
the plasma tube from the sound of the thunder clap.
Striking problems
Q: Why does flint spark?
A: It isn’t the flint that sparks, unless you mean a cigarette or gas
lighter flint. When real flint (silicon dioxide) is struck against steel it is
hard enough to detach particles from the surface of the steel. The friction
energy of the strike is enough to heat these steel particles so that they burn
in atmospheric oxygen to form Fe3O4, the grey oxide
which used to be called “smithy scale”.
Lighter “flints” are different; they are composed of a mixture of rare-earth
and similar metals (mostly cerium), and are rubbed against a serrated hard
steel wheel or rasp. In this case, it is the particles of “flint” which
burn.
Strait not narrow
Q: In the Ordnance Survey National Atlas of Great Britain (1986) it states
that the English Channel widens at the Strait of Dover by 70 centimetres each
year. Even if the figure is much smaller, surely any widening in some way
affects the new tunnel. How is the widening allowed for?
A: In reference to tbe widening of the English Channel at the Strait of
Dover, Ordnance Survey confirms that the figure of 70 centimetres each year
represents coastal erosion, not the movement of Great Britain relative to
France.
There are very minor changes to the distance between England and France,
which amount to approximately 1 millimetre every ten years. This is caused by
the Southeast of England sinking at a very slow rate.
When Eurotunnel and TML (the contractors) contemplated the design of the
tunnel project, this fact was taken into account and 30 000 joints in the
tunnel can comfortably take up the movement.
This week's questions
Metal work: When a magnet picks up paperclips or repels/attracts another
magnet, work is being done. Working against gravity or friction must use
energy. Can someone please explain in words rather than mathematics where the
energy comes from and whether a magnet in use (such as one holding notes to a
refrigerator door) gradually becomes weaker as its magnetism is used by
supporting itself against gravity?
Changing period: How can the “memory effect” which affects the charge level
on nickel-cadmium batteries be explained in chemical terms?
Rising damp?: One of my local churches has a square tower with a small
spire on each corner of the tower. As I drove by the church one hot summer night
I saw what appeared to be smoke coming out of the tip of each of the
spires.
This could have been some form of condensation of water vapour caused by an
electrostatic process or some Bernoulli effect resulting in localised pressure
changes around the stonework, but I would like to know for certain.
Can anyone explain what it was?
The spinners: Why does a Frisbee need to spin in order to fly?
Brush mush: Why does fresh orange juice taste so awful after you have just
brushed your teeth?
Dreamy sleepers: If you are travelling in a train at about 80 kilometres per
hour and you gaze out of the window at the adjacent railway track about 6
metres to the side, the sleepers of the adjacent track can appear stationary.
If both sunlight and eyesight are continuous what causes this stroboscopic
effect?
Questions and Answers
Questions and answers should be kept as concise as possible; we reserve the
right to edit items for clarity and style. Please make sure that you always
include a daytime telephone number and a fax number if you have one. Questions
should be restricted to scientific queries about everyday phenomena. The
writers of all answers that are published will receive a 拢10 book token.
IPC Magazines Ltd reserves all rights to use question and answer material
submitted by readers. Send questions and answers to The Last Word, New
杏吧原创, King’s Reach Tower, Stamford Street, London SE1 9LS (fax 0171 261
6464) or by e-mail to newsc1@stirling.ac.uk (e-mail users must include their
postal address).
Heading for zero
Your editorial “Prime time to remember …” (22 October) was very well put.
In fact, the “staggering total of 拢620 million a year less [spent] on
research than … 10 years ago” applies only to the civil departments. Adding
defence research and development the government’s expenditure has fallen by a
doubly staggering 拢1400 million a year; and by 1996/7 the drop will top
拢1800 million. Not a penny of the decrease in defence expenditure has
reached the civil R&D sector.
The biggest fall in civil R&D has occurred in areas for which the
Department of Trade and Industry (DTI) has responsibility. The one arm of
government which should be doing most at the interface between the science
base and industry, helping the introduction of new technologies, has been
withdrawing as fast as it can – more than 拢40 million has been lost from
joint DTI-SERC programmes alone. From 拢900 million per annum in 1984/5,
at the present steady rate of decline, DTI spending on R&D will be zero by
1998.
David Hunt must show he can reverse the government’s flight from investment
in research.
Born bright?
Charles Goodhart bemoans the lack of weight given by educational
psychologists to the evidence for a strong genetic component in IQ scores
(Forum, 22 October). This he puts down to their lack of formal training in
genetics. But his arguments will do little to help advance their
knowledge.
The tendency of IQ scores to regress to the mean is indeed consistent with
genetic explanations, but it is equally predictable from those based entirely
on environmental factors. The tendency of children’s scores to regress to the
mean could be accounted for by those aspects of social environment that we all
share (education system, popular culture, etc.).
Goodhart concludes that an environmentally based explanation would require
that “unintelligent parents provide better conditions for their children’s
mental development”. This is totally illogical; the children’s scores may
increase towards the mean, but those from the intelligent families are still
higher. Regression to the mean provides no evidence for distinguishing between
any of the hypotheses.
Even less creditable is his comparison of human and ape intelligence. No
one denies that human intelligence is only possible because of our genetic
make-up, and that this separates us from the apes. But this tells us nothing
about the reasons for differences between members of human populations. To
take an extreme case, if everyone had the same genes for a characteristic, any
variation in that characteristic must be environmentally determined, despite
the key role of genes in producing that characteristic in the first place.
Goodhart is concerned not to put off ideas just because they are
“politically incorrect”. But in view of the important social and political
consequences of the arguments he raises, it is a pity that he did not pay more
attention to “scientific correctness”.
* * *
Goodhart argues that: “If the
differences really are inborn, there is no use forcing children beyond their
natural capacities.” This statement assumes that any innate differences
between children lie in a predetermined “capacity” for learning which cannot
be exceeded. What reason is there to believe this? Why must we assume that
children with a lower IQ have a lower capacity for learning, or that all
efforts to assist their intellectual development are doomed to fail?
Jogging junkies
Methadone, a “mild” opiate drug was interestingly discussed in your articles
on drug addiction (1 October) as the most widely used though controversial
therapeutic substitute.
Non-drug treatments such as psychotherapy, behaviour therapy, and cash
incentives were also considered, but one potentially valuable treatment was
not mentioned at all.
This is offering addicts the possibility of creating internal substitute
drugs by their own efforts.
Around 16 years ago “endorphins”, the brain’s own opiate substance, were
first described. Their effects resemble those of morphine and they are
released into the circulation by, inter alia, vigorous physical exercise, such
as running and jogging. They have been held at least partly responsible for
the favourable mood – at its extreme the “runner’s high” – which such exercise
can produce in ordinary people and in mildly depressed or anxious psychiatric
patients. Exercise can also produce analgesia, reduced sensitivity to pain,
which is probably the most important medical effect of externally administered
opiate drugs.
Although the detailed mechanisms by which physical exercise is translated
into these psychological benefits are not clear, there is at least a prima
facie case for examining whether the endorphins released through exercise
might act as substitute drugs and so help addicts. To produce endorphins in
this way would be cheap and relatively pleasant.
Since physical exercise is increasingly being prescribed by GPs, sometimes
in collaboration with sport psychologists, in their day-to-day practice, the
time for systematic trials with addicts could not be more propitious.
Open secrets
I am afraid that your editorial (“Secrets that should be told”, 1 October)
reflects neither the government’s position nor what has been happening in
Europe.
The government has made it clear that making more information available
about medicines would be beneficial. That is why we actively supported the
provision in the new European licensing legislation for summary assessment
reports to be made available to the public for applications using the new
centralised procedure. We have also made it perfectly clear that in Britain,
Section 118 of the Medicines Act by no means excludes proper implementation of
the general duty to safeguard the public health.
The article is specifically inaccurate in suggesting that the government
and its advisers were unable to provide information explaining why the
sleeping pill Halcion was banned. When the drug was withdrawn in 1991, the
chairman of the Committee on Safety of Medicines wrote to all doctors
explaining the reasons for the withdrawal and he made himself fully available
to the press for further explanation. Furthermore, when the licences were
revoked in 1993, the Department of Health issued a press release explaining
the basis for the decision.
It is still not clear what information will be made available to the public
from the new European centralised procedure. The details are still being
considered by the European Commission, which will be responsible for issuing
Eurolicences. It will in any event apply only to the products which must go
through the centralised procedure (such as certain biotechnology ones) and
others which choose the centralised route.
The vast majority of products will continue to be handled by national
regulatory authorities through a decentralised procedure. Thanks to the
British government taking a very active lead, the majority, if not all, member
states now accept the need for similar assessment reports for the
decentralised procedure. But the other member states are making it quite clear
that they want to see how the central arrangements will operate in practice
before committing themselves.
They are also adamant that whatever arrangements are agreed for the
decentralised applications should be set out in European legislation so that
everyone will follow the same rules. That seems eminently sensible to me.
Far from being out of step with developments in Europe, Britain is playing
a leading role and will continue to do so as developments
unfold.
Neglected?
Further to the recent correspondence concerning stipend levels for
postgraduate students (Letters, 10 September and 8 October), as ex-Science and
Engineering Research Council students now being funded by the Natural
Environment Research Council, our grant this year is 拢4920, some
拢210 less than for all existing NERC students. This disparity between
stipend levels, coupled with the fact that the NERC did not bother to inform
us of the level of our grants for the present academic year, leaves us with
the dispiriting impression that the NERC does not place value upon either us
or, by implication, our research.
The already low level of our income is further eroded by the high cost of
living we, together with many other students in the south of England,
experience. It is no longer realistic to assume that in all areas of the
country outside London rents are at the same level. We believe that it is time
the research councils introduced additional allowances for all students
working in areas with a high cost of living to reflect this situation.
Together with raising stipend levels of “low-paying” research councils
(such as the NERC and the Earth and Physical Sciences Research Council) to
those of the more generous research councils (such as the Biotechnology and
Biological Sciences Research Council and the Medical Research Council) this
would go some way to proving to research students that they and their work are
recognised as valuable assets.
Delicious dish
Wonderful, delicious, simple, we will certainly make it again.
I refer to Ajiaco de Papas (“First catch your goanna”, 15 October).
I thought my husband was joking when he suggested a New 杏吧原创 recipe
for our visiting vegetarian son and his wife. They were impressed, and I was
amazed at how tasty and filling it was.
Next on the menu for us is the Groundnut Stew, but I don’t think the Roast
Goanna or Fried Bee Larvae will appear on the menu here – perhaps when we
visit Australian relations.
* * *
Your recipe for Groundnut Stew is somewhat different from what our cook
used to produce; he never used ginger or coriander. And he never used mutton
or pork; sheep do not thrive anywhere in Ghana, and pigs are very scarce even
in the south of the country. The north is almost entirely Muslim, and pigs do
not thrive there.
For those interested in this very nutritious and delicious dish, the meat
used is chicken or beef (or goat, if you can get it). It usually contains
carrots and chopped tomatoes, and is often garnished with quartered hard-
boiled eggs. We serve it to vegetarian guests without the meat but with
increased quantities of hard-boiled eggs.
Incidentally, kenkey is a kind of porridge made with fermented maize.
Fascinating photons
I think that Tim Shelton-Jones (Letters, 8 October) has rather missed the
point of my work featured by John Gribbin (New 杏吧原创, Science, 20
August).
I was advancing the following modest proposition. We have an entity which
we call an “electron”, which has an easily measured mass and charge, and a
meaningful size apparently less than 10鈭�18 metres. On the
other hand, we have an entity usually referred to as a “photon” which has no
measurable mass, no electric charge and a “size” which can only be defined in
terms of the macroscopic boundaries of the apparatus in which it is
propagated.
To encompass two such different entities under the single title of
“particle” is merely to propagate confusion and I believed that the attempt
fosters scientific sloppiness. Shelton-Jones seems to want tidiness above all
things. To me, the possibility that electron diffraction is slightly different
from optical diffraction is not “untidy” but very interesting, as it could
suggest new experimental tests of theories.
I must add that trying to discuss the propagation of a photon in terms of
the propagation of a probability wave involves considerable difficulties which
I cannot go into here, but which were touched on in my original paper.
There is an important lesson here. Over the fifty or so years since quantum
mechanics was first formulated, many philosophical superstructures have been
built on top of its practical experimental structure.
Only in the past few years, with the development of techniques which enable
detailed examination of optical fields containing low photon numbers and the
interactions of single atoms, can such superstructures begin to be put to the
test.
I believe we are now entering a fascinating new period in the examination
of quantum mechanics and its microscopic predictions, but we are not looking
for tidiness and symmetry. We are comparing the predictions of different
theories and testing them.
Internet ads
Just think about the consequences if advertising on the Internet developed
(This Week, 1 October and Letters, 22 October).
Individuals’ e-mail addresses could be assessed for ABC categories and sold
by the usual mailing list agents. I think there would be many eager buyers in
view of the low cost of mailing thousands of potential customers.
Even if this were not the case, and assuming that Internet advertising
really took off, what kind of volume would we finish up with? How would those
who already read 400 Usenet messages a day like an additional 400 “buy”
exhortations?
Just how much mail could result from Internet advertising?
Letters to the Editor
Write to: Letters to the Editor, New 杏吧原创, King’s Reach Tower;
Stamford Street, London SE1 9LS, or fax to 0171 261 6464. Please include a
daytime telephone number, and cite the date of the articles mentioned. We
reserve the right to edit longer letters. Your letters may also be published
in New 杏吧原创 newsletters.