Michael Murphy's Research
Variable fundamental constants?
If you're looking for information on my recent nuclear physics
measurement in a galaxy half way across the Universe see this
MEDIA RELEASE: Precise nuclear physics measurement in a galaxy far far away!
I am a lecturer and researcher at the Centre for Astrophysics &
Supercomputing, Swinburne University of Technology. A major part of my
work concerns the question "Do the constants of nature vary?". For the
past 7 years I have been working with John Webb and Victor Flambaum on a search for possible variations
in the fundamental constants of Nature, particularly the fine
structure constant, usually denoted by the Greek letter alpha.
The alpha project has received sustained media publicity and I have tried
to keep a scrap book of some of the major media releases here.
The following is an explanation of my research in layman's terms. See
also Chris Churchill's Fine Structure Constant Page for
more non-technical information. A slightly more technical overview
article has recently been published in Physics
World Magazine. If you want a more technical explanation then I
suggest you read some of my recent
publications.
Contents
The alpha team
The basics
The experiment
The results to date
The implications
The future
Aside: is it c or e that is varying?
The alpha team
This is an international collaborative effort involving mainly John K. Webb and Victor V. Flambaum at UNSW and myself at the University of
Cambridge. Other collaborators who have contributed to the work
presented in these pages are (in alphabetical order):
John
D. Barrow1,
Chris W. Churchill2,
Francois Combes3,
Stephen J. Curran4,
Michael J. Drinkwater5,
Vladimir A. Dzuba4,
Jason X. Prochaska6,
Panayiotis Tzanavaris4,
Tommy Wiklind7,
Arthur M. Wolfe8
at institutions
1DAMTP, Cambridge,
2Penn. State,
3Obs. Paris,
4UNSW,
5UQ,
6UCO-Lick Obs.,
7OSO and
8UCSD
The basics
Here are some links explaining the basics of fundamental constants
with particular emphasis on alpha.
What is a fundamental constant?: @ NIST
What is the fine structure constant?: @
NIST, @
Physlink
How are the constants measured?: @ Physics Today
Put briefly, a fundamental constant is a number that is central to a given
theory - that is, to calculate/predict the results of an experiment, you
need to know that number. But you can't use known theories to
calculate that number - it must be measured in an
experiment. In essence, these numbers are fundamental because we have
no idea where they come from! And since no-one knows how to calculate
their values, and because we do find the same value with different
experiments conducted at different times/places, we assume that
these numbers are, in fact, constants. The experiments we conduct tend
to be limited to laboratories on the Earth during the last 100 years
or so. But what if the constants are/were different in different
places in the universe or at different epochs in cosmic history? An
experiment should test the constancy of the constants in these extreme
cases.
Our experiment picks on one very well known constant: the fine
structure constant, alpha. This constant is the central parameter in
electromagnetism - the theory of how light and matter interact. Alpha
is a combination of other constants that you might be more familiar
with: alpha =
e2/hc where c is the speed of light,
e is the charge of an electron and h is Planck's
constant. Thus, alpha is important for a relativistic (i.e. c)
quantum mechanical (i.e. h) theory of electromagnetism
(i.e. e). But alpha is, in some sense, more fundamental the these
other constants (see Aside - is it c or e that
varies?). Alpha is known to exquisite precision from laboratory
measurements:
1/alpha = 137.03599958
with an
experimental uncertainty of just 0.00000052! But was alpha the same, say,
billions of years ago? Or was/is it the same in extremely distant regions
of the universe?
The experiment
Our investigation is astrophysical in nature. We look back in time
through most of the history of universe to compare the value of alpha
back then with what we measure in the laboratory on Earth today. To do
this we have used the largest optical telescope in the world - The Keck I 10-m
telescope on Mauna Kea, Hawaii - to record spectra of extremely
distant quasars.
Quasars are extremely massive (about a billion solar mass) black holes
that rip apart and suck in any surrounding material. They reside at the
centre of some galaxies and pull in the surrounding stars and gas. The
inspiralling material heats up as it collides with other material causing
the quasar to shine extremely brightly. In fact most quasars outshine their
host galaxies which contain more than 100 billion stars. A massive black
hole also resides at the centre of our own galaxy - The Milky Way - but it
is not heavy enough to begin to shine as brightly as a quasar. Quasars are
so bright and so compact that they can be seen even when the quasar is many
billions of light years away (to compare, the age of the universe is about
14 billion years).
The study of Quasars is very interesting in itself but, in our experiment,
we use the quasars only as background sources of light to reveal our real
targets of interest, intervening absorption clouds. These clouds are mostly
made up of hydrogen (the most abundant element in the universe) but also
contain ionized traces of heavier atoms such as Magnesium (Mg), Aluminium
(Al), Silicon (Si), Chromium (Cr), Iron (Fe), Nickel (Ni) and Zinc (Zn) -
metallic atoms familiar to us here on Earth.
The metallic atoms absorb some of the quasar light on its 10 billion year
journey to the Earth. This absorption is very characteristic of each atom,
much like a fingerprint is characteristic of each person. As we receive the
light at the telescope, we spread it out into its colours - we form the
spectrum of the quasar light - to reveal each atom's fingerprint. See
Fig. 1 for a schematic explanation of this. Other snazzy figures exist at
The New York Times and The Washington Post.
Figure 1: We look back from Earth to a distant quasar and spread the
light out into a spectrum. As the light travels to Earth it traverses an
intervening absorption cloud. The atoms in the cloud imprint the quasar
spectrum with their "fingerprint" absorption lines (labelled as "'Metal'
absorption lines"). We compare this fingerprint with that found in
laboratories here on Earth to infer any possible changes in the fine
structure constant.
The results to date
We have now analysed the spectra of 128 absorption clouds observed
toward 68 different quasars with the HIgh
Resolution Echelle Spectrograph (HIRES) on the Keck I 10-m telescope and
compared the atomic fingerprints with those measured in laboratory
experiments here on Earth. On average, we see very slight differences
between the quasar and laboratory spectra: the lines making up the
fingerprints appear at slightly different wavelengths/colours than
expected! The separation between the absorption lines is controlled by
the fine structure constant, alpha, and so we interpret these
differences as changes in alpha. See Fig. 2 for a recent plot of our
results.
Figure 2: Our measured fractional change in the fine structure
constant, alpha=e2/hc, is plotted as a function of
fractional look-back time (0=present day, 1=beginning of time, about 13.4
billion years ago). The corresponding cosmological redshift is shown along
the bottom axis. Each point represents the average value of about 10
absorption systems. There is now evidence that alpha has evolved one part
in 100,000 over the last 10 billion (or so) years. Another possibility is
that alpha is simply different in distant regions of the universe from what
we measure on Earth.
The implications
Since alpha fixes the strength of the electromagnetic interaction, it
plays an absolutely fundamental role in the working of atoms and how those
atoms interact with light. In short, alpha controls most of the phenomena
you see around you in everyday life. If the results above are confirmed by
future experiments then does a change in alpha of 1 part in 100 000 over 10
billion years mean anything for everyday life? There are several answers to
this question.
The first and most naive answer is "No". The change in alpha we observe
means very little for everyday life because alpha won't have changed by
very much over the lifetime of humanity. This is not enough to cause atoms
to fall to bits or for the Sun to burn its fuel at a terribly different
rate.
But we should learn a lesson from history here. When Faraday discovered
electricity in the 19th century, people often questioned him as to the
significance of his discovery. At that time, Faraday could not possibly
have imagined how society would become so completely dependent on
electricity today. When Chancellor of the Exchequer, William Gladstone was
invited to a demonstration of Faraday's electrical equipment, Gladstone is
reputed to have remarked "It is very interesting, Mr Faraday, but what
practical worth is it?" Faraday is supposed to have replied "One day, sir,
you may tax it.". One doesn't know at the present what everyday
implications our discovery may have if it is shown to be correct but I'd be
willing to bet that one day, you might have to pay tax on it.
More seriously, our results have the potential to revolutionise the way we
understand the universe on all scales, from the subatomic to the
universal. All of modern physical theory is based on the assumption that
the laws of physics remain the same no matter where or when you happen to
be. Physicists have what is called a "standard model" of the universe which
allows them to explain all observed phenomena. This standard model cannot
explain variations in the constants and so, if our results are correct, the
standard model would need a complete overhaul: we will have discovered the
first hint of a completely new set of physical laws, hitherto unseen and
not to be understood for some time.
But perhaps we are closer to understanding varying constants than we
think. Some modern theories which step outside the "standard model" have
room for varying constants. In particular, theories such as string/M-theory
allow for changes in the constants. These theories attempt to unify all the
known forces of Nature (gravitation, electromagnetism, the strong and weak
nuclear forces) by postulating the existence of extra dimensions in
space. The extra dimensions (sometimes more than 20 in number!) are
"compactified" or rolled up onto themselves so we don't experience them in
our usual (3+1)-dimensional universe. If the size of the extra dimensions
changes (as do our three dimensions - ever heard the expanding universe?)
then this could manifest itself as changes in the fundamental
constants.
The future
Our recent results are by no means conclusive. We have thoroughly searched
for other possible explanations for our results. Research science is
constantly plagued by the problem of "systematic errors". These errors
mimic your result or somehow destroy it. They are notoriously hard to
identify. And this is what we've been looking for in our results. But we
still can't find anything that explains our results besides a varying
alpha!
The best approach in science is to always check (and re-check if necessary)
your results using different equipment and analyses. We are therefore
currently analysing a new, large dataset observed with the aptly named Very Large Telescope in
Chile. But there's a famous saying in science: "Extraordinary claims
require extraordinary evidence.". Though we are claiming something quite
extraordinary here, the evidence, though very strong, is not extraordinary
enough. Even if we confirmed the Keck results with the VLT, no one should
really believe that constants are varying until another type of
experiment confirmed the results. Possibilities for other types of
experiments include making very precise measurements of the fluctuations
seen on the Cosmic Microwave Background sky - the radiation left over from
the big bang. Another possibility is to measure very accurately the
abundances of the elements that were produced in the big bang. But these
methods have their own problems and systematic errors and their current
precision is not quite high enough. But we're hoping this will improve
soon!
Aside: is it c or e that is varying?
Above I mentioned that alpha is made up of three other constants: alpha =
e2/hc where c is the speed of light,
e is the charge of an electron and h is Planck's
constant. Both laymen and scientists alike always ask whether we have any
idea whether it's c, e or h that varies. This
frequently asked question has a subtle and often misunderstood answer. But
it's interesting, so read on!
In fact, one can never experimentally distinguish between a varying
c or e because these quantities are always measured in some
arbitrary units like meters, kilograms, seconds etc. Consider measuring the
time it takes light to travel between you and me on Monday and then again
on Tuesday. Imagine that the two answers were different. What does this
tell you? You might conclude that the speed of light, c, has changed
between Monday and Tuesday or, equally well, you could conclude that time
has slowed/accelerated or that your measuring rods (i.e. meter rules) have
changed length. These three conclusions are all equally valid and can not
be distinguished by an experiment! But alpha is special because it is a
dimensionless combination of other constants: alpha is just a
number, i.e. no units! We can therefore measure changes in alpha
unambiguously.
Some confusion has arisen recently in the literature about this
question. The problem is that there exist well defined theories called
"Varying Speed of Light" (VSL) and "Varying Electric Charge" (VEC)
theories. For example, in VSL theories, it is indeed the speed of light
that is considered to vary. But this is just a mathematical convenience:
one could easily convert any VSL theory into a VEC theory! The only reason
one chooses to label one particular theory a VSL or VEC theory is because
that theory might look simpler (mathematically and intuitively) when
considering a varying c or e. Essentially, the confusion is
that the (arbitrary) names given to these theories mask their inherent
duality (or triality if you include h!).
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