Mark Buchanan writes:
Most textbooks of quantum theory still say that random
and uncontrollable kicks foil any attempt to get “which way” information in the
two-slit experiment and still see an interference pattern. When Gerhard Rempe
says “Everyone believes”, he means that “most physicists believe”.
It is true, as Peter Grove points out, that measurement theory does not
invoke the uncertainty principle to explain why particles interfere like waves
until you look to see which way they go. Instead, it explains the loss of
interference in terms of the unavoidable entanglement that develops during a
measurement, a point that the experiment done by Rempe and his colleagues drives
home with some force.
No interpretation of the uncertainty principle鈥攚ith or without talk of
random kicks鈥攃an explain their results. In the apparatus, the atoms move
to the detecting screen along one of two paths. But unlike a conventional
two-slit experiment, these paths overlap.
Each beam is about half a millimetre in width, and they are displaced
relative to one another by a distance hundreds of times smaller. The experiment
does reveal “which path” information, but this isn’t information on the spatial
position of the particle. So the uncertainty in their momenta cannot, as
suggested by Simon Anderson, explain the loss of interference.
GM sanctions
I was disheartened to read the thinly veiled threats made by the heads
of leading biotechnology companies about future investment
in British bioscience in the light of the outcry about GM crops
(This Week, 27 February, p 7).
Comments such as “Britain is not the only place where you can do research”
and “It will harm investment in the UK” are doubtless meant to coerce scientists
into supporting one side, rather than tempering the alarming level of the debate
in the media.
These companies may say they do not want to work in countries that do not
support their research, but it is not the research that is at issue but the
commercial exploitation of the products.
People and governments have the right to say no to GM foods, wrongly or
rightly. The verdict on GM crops must be based on the actual and perceived risk
to public health and the environment. The issues should be resolved without the
threat by transnationals of what amounts to economic sanctions.
Letter
You make the point that it is the advances in farming techniques, not new
herbicide-resistant plants, that are doing the real damage to wildlife
(Editorial, 27 February, p 3).
Logically, we should control the introduction of new-generation pesticides if
they work too well, or ban the introduction of new machinery which allows
habitats to be destroyed. But halting progress is not possible. What is needed
are rules, probably tied to support payments, that mean that part of each farm
is worked not for food production but for the benefit of wildlife.
Like our historic buildings, wildlife needs statutory protection. It is too
important to be left to the whim of individuals.
Quantum certainty
Your article on quantum physics, suggesting that entanglement rather than the
uncertainty principle is the key to the quantum world, mangled facts
about an excellent experiment to a tabloid press level
(“An end to uncertainty”, 6 March, p 24).
The Heisenberg uncertainty principle is a direct consequence of the
fundamental axioms of quantum theory. As the experiment discussed in the article
has precisely the result predicted by quantum theory, it must verify the
uncertainty principle.
The uncertainty principle does not depend on “random kicks” in collisions but
on the impossibility of systems simultaneously possessing complementary
properties such as position and momentum. The statement by one of the
experimenters that “Everyone believes that when an interference pattern is lost,
it happens because of random kicks to the particles” is untrue.
No one who knows any quantum measurement theory has believed this for many a
year. The “random kicks” story is retained only as a picture for those who haven’t
studied quantum mechanics or measurement theory in detail.
Letter
Mark Buchanan writes about the unquestioning attitude of physicists towards
the rather irrelevant uncertainty principle. Richard Feynman, at least, was an
exception. In chapter two of his book QED, there is a footnote that
says: “If you get rid of all the old-fashioned ideas and instead use the ideas
that I’m explaining in these lectures, there is no need for an uncertainty
principle.”
Letter
It seems to me that this article is based on a misunderstanding of
Heisenberg’s uncertainty principle. It does not mean that we cannot know certain
quantities because the act of measuring certain properties changes those
properties.
Rather, it should be understood as meaning that no matter how
carefully you measure them, the product of the uncertainties in position and
momentum can never be less than a certain value.
Marking atoms by raising them to an excited state, as the experimenters did,
fixes their position to some degree of accuracy. It is the degree of accuracy to
which their position has been fixed that determines, using the uncertainty
principle, the uncertainty in their momenta, not any momentum transfer (“random
kicks”) from the exciting photons鈥攚hich is a very small effect.
I would expect this to explain the loss of the interference pattern, without
the need to invoke “quantum entanglement”.
True to tape
I was intrigued by Kate Charlesworth’s potted history of the hi-fi
(“Life, the Universe and (almost) everything”, 27 February, p 57).
Did paper tape coated with iron powder really come into being in 1922?
Up to the 1940s, the BBC made its everyday recordings on half-inch steel-tape
Marconi-Stille machines. This afforded about 15 minutes per 12-inch reel, and
the operators needed muscles. The quality was good enough but music programmes
were always recorded on film. This was cut mechanically but replayed
optically.
A German machine captured in the 1944 was sent back from the front and
arrived in our lab for evaluation. Everyone was astonished that it used a
quarter-inch paper tape coated with a magnetic medium and provided an unheard-of
playing time per small-sized reel.
One interesting feature was that the replay head could be rotated against the
direction of tape transport. This enabled a recording to be played back slowly
without lowering the frequency of the voice (although it sounded like a bathroom
echo). Presumably, it was intended for use by translators working on enemy radio
traffic.
Ken Green is right to question whether paper tape coated with steel powder
was invented as early as 1922. In fact, the idea was patented by Fritz Pfleumer,
a German chemist, only in 1928. The German manufacturer AEG bought the rights to
this patent in 1932 and by 1935 had developed the “Magnetophon” tape
谤别肠辞谤诲别谤鈥抬诲.
Nickel odium
While applauding the European Union’s moves to limit the nickel content of
jewellery, and researchers’ attempts to develop an accurate test
(This Week, 30 January, p 22),
I feel that an important fact has been overlooked.
As Alison Motluk pointed out at the beginning of the article, not all nickel
exposure occurs through jewellery. People can be “sensitised” to nickel by any
skin contact鈥攂y belt buckles or watches, for instance.
I had never worn nickel-based jewellery when I developed mild dermatitis on
my nose. It turned out that my glasses were brass-coated nickel, the brass
having been worn away after some years. After I purchased nickel-free frames,
the affected skin soon cleared up.
It's all Greek . . .
St John does indeed seem to suggest that material objects may be secondary
manifestations of primal information (“In the beginning was the word . . .”) in
the first three verses of his gospel
(Letters, 27 February, p 55).
However, “the word” is a translation of the Greek word logos and this concept
predates the gospel of St John.
Logos can be traced back to the Greek philosopher Heraclitus. It was
in Stoic thought, however, that it became important. The Stoics believed that
the logos described the inherent principle of rationality, the underlying
reality in the Universe.
For the Stoics, the logos was divine. It was the active,
all-pervading principle, holding all things together and directing their
development in a purposive harmony.
The logos was also used in Platonic philosophy, where the
logos helped to provide a link between the spiritual world and the world of
ordinary sense experience.
Using these ideas, it is not difficult to appreciate that some people also
translate the logos as the mind of God.
Letter
In Aristotle’s summary of the teachings of the Pythagoreans, he reports that
they supposed the elements of numbers to be the elements of all things, that
number is the principle of all things in respect of their matter and their
modifications, and that they attached special significance to a list of ten
pairs of binary opposites.
The Pythagorean belief that mathematical information was the key to
understanding the world was vividly supported by their discovery of the
mathematical basis of musical harmony.
All tied up
Just a short note concerning the mathematics of knotting a tie
(This Week, 6 March, p 6).
During a dull moment, I tried out the new version and noticed
where the problem lies. When you undo the tie, you end up with a fat knot on one
end of the tie. A proper knot should unfold leaving the tie flat.
Please note, I do realise this is a sad observation, but I was bored at
work.
Quantum rules
The glitch in Novell software, which shows the date you are checking when you
look to see when a file was last accessed
(Feedback, 6 March),
led me to wonder if it was designed to be run on quantum computers.
The fact that it is impossible to observe something without affecting its
state, therefore changing the attribute that you are trying to determine, is a
perfect example of quantum theory.
Letter
The offer by Australian Skeptics to pay A$80 000 for proof of ESP,
telepathy or telekinesis has never been taken up
(This Week, 27 February, p 19)
and never will be.
You see, it is well known that paranormal phenomena are large-scale quantum
effects. The act of observation by a sceptic immediately causes the system to
collapse to a “classical” or normal state.
Correction:
The article on ultrasound and tumour surgery
(This Week, 13 February 1999, p 23)
neglected to mention the contribution to the research
made by Sunita Chauhan, a PhD student at Imperial College, London, supervised by
Brian Davies and Mike Lowe.
Correction:
In “A theory of everything”
(Inside Science no 118, 20 February),
it was stated that the Americans C. N. Yang and R. Mills developed the Yang-Mills
theory. In fact, this work was published in 1954, whereas Yang was a Chinese
citizen until 1964.