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Alternative MRI limits
Denis Le Bihan’s article on magnetic
resonance imaging (MRI) clearly shows
the incompatibility of the International
Commission on Non-Ionizing Radiation
Protection (ICNIRP) guidelines with MRI
practice (August pp16–17). Although the
ICNIRP recently relaxed its limits on static
magnetic fields, the stringent limits on
low-frequency magnetic fields remain
incompatible with certain MRI
procedures. Moreover, the ICNIRP limits
contrast sharply with those of the Institute
of Electrical and Electronics Engineers
(IEEE), even though both are derived
from the same scientific database.
The ICNIRP’s extreme precaution was
criticized in a series of letters to the journal
Health Physics as long ago as 1998, and the
incompatibility with MRI procedures was
foreseen. However, this excessive
precaution – which at frequencies in the
kilohertz range amounts to exposure limits
that are lower than the IEEE limits by
considerably more than an order of
magnitude – only became a practical
problem when the European Commission
mandated the ICNIRP limits in its 2004
directive restricting occupational
exposures to electromagnetic fields.
As of July, the ICNIRP is inviting
comments on a draft revision of its
low-frequency guidelines (covering the
0–100 kHz range). This draft includes a
partial relaxation of limits towards the
IEEE values (although it does not refer
to these specifically). But why stop there?
Why not eliminate all the excessive caution
in its guidelines and adopt the IEEE values
even up to 100 MHz? The IEEE standards
are based on the same science used by the
ICNIRP, and differ only in the magnitude
of their “safety factor”. As past chairmen
of IEEE who have been involved in
standards-setting for the safe use of
electromagnetic energy for more than
40 years, we feel that there is a scientific
case for international harmonization.
John M Osepchuk
Ronald C Petersen
Concord, Massachusetts, US
Bedminster, New Jersey, US
ron.petersen@verizon.net
S TFC
cosmology. I agree with Susskind that the
idea of a “cosmic watchmaker” is not
science – it is faith and belief expressed via
religion. However, it seems to me that a
multiverse of mutating universes is not
strictly science either, because it is, and
probably always will be, untestable.
The author rightly refers to theories, but
often these straddle what can be tested
(“science”) and what cannot (“philosophy”
or “belief”). The fact that scientists do not
all believe the same thing about the
existence of a cosmic watchmaker, let
alone what that cosmic watchmaker might
be like, shows there are areas of human
enquiry that cannot be scientifically proved
or disproved. That should not, however,
stop us asking the questions, nor seeking to
provide models and hypotheses based on
what we can test.
Accepting the reasonable hypothesis of a
multiverse as described in the article only
leads to more questions, such as “Why does
the multiverse exist at all?”. Surely, when
science leads to questions beyond its means
to answer, it is revealed – alongside religion
and art – as a basic part of human enquiry,
experience and expression.
Elaine Pierpont
Chipping Sodbury, South Gloucestershire, UK
elaine.pierpont@btinternet.com
Neutron scattering
gets short-changed
In your excellent coverage of the latest
round of funding cuts by the UK’s Science
and Technology Facilities Council (STFC),
you highlight the problems that will be
caused by the severely reduced operations
of the ISIS neutron source (August p7 and
p15). As a frequent neutron scatterer, I can
only echo your sentiment that it makes no
sense at all for the UK to build a superb
new facility like the £148m ISIS second
target station if it can be switched on for
only 120 days per year.
However, I cannot agree with your
analysis that this crisis has been triggered
by a fall in the value of the pound.
According to STFC’s own press release, its
budget includes additional funding to
compensate for the effect of international
exchange-rate movements on its large
international subscriptions. This extra
funding kicks in once the cost due to
currency fluctuations exceeds a £3m annual
cap. The STFC annual budget of £491m is
therefore well above its baseline allocation
of £429m from the 2007 comprehensive
spending review.
Next year will mark the 75th anniversary
of James Chadwick’s physics Nobel prize
for the discovery of the neutron. As things
stand, we could end up marking this event
by switching off ISIS, the UK’s flagship
neutron-scattering facility.
Neal Skipper
University College London
n.skipper@ucl.ac.uk
I very much appreciated Lee Smolin’s
article on “The unique universe”
(June p21–26), which argued against the
idea of a timeless multiverse. Perhaps
people who remain keenly interested in
physics well beyond retirement tend to
become more “middle of the road”
when the subject becomes linked with
philosophy, but, whatever the reason,
most thinking people I know appear to
accept that time must exist.
Even if its existence can be regarded as
a series of timeless “snapshots”, to be
interested in the progress of existence
requires the concept of “rate”, as in
“rate of change of temperature” etc, which
is easier to conceive and to quantify if the
participation of time is accepted. Because
of this, I have come to regard time as the
dimension in which change takes place.
If this view is taken, a timeless universe
would also be a changeless one. At my
age, I cannot believe that I live in a
changeless universe!
D P Donegan
Chorley, Lancashire, UK
Time, the multiverse
and belief
I read with interest Leonard Susskind’s
article on “Darwin’s legacy” (July pp42–
45) and how these ideas relate to
I would like to thank Robert Crease
for writing about the responses to the
question on religion in Physics World’s
20th anniversary survey (August p18), and
for highlighting the deep ignorance that
prevails over “religion” even in our well-educated community. To equate religious
belief with the tooth fairy, as one survey
respondent did, is to be unaware of our
