Fusion AGM Highlights

The meeting conferred Honorary Membership on Loretta Souza, OUSA's Societies Assistant, for her help with the AGM arrangements; she joins Julia Maddock, the former Student Liaison Officer at the Institute of Physics, who was made an honorary member in recognition of her help in setting up the Society.

We had to re-elect our officers for another year; or, to be precise, elect them for the first time, since this was the first such meeting. This resulted in a Committee which was not too dissimilar from the previous one. Eleanor Cowan continues to co-ordinate events, but will be assisted by Tina Heaton and by Jacqui Dodds, a new addition to the Committee. Eleanor's former "sideline" of looking after Merchandise passes to John Pollard, another "new boy". Paul Ruffle will run the Newsletter and the Web site, and Norrette Moore and Jim Grozier continue as Treasurer and Membership Secretary respectively. Jim and Eleanor also continue to occupy the purely formal posts of Chair and Secretary, to satisfy our Constitution and the Bank. We also persuaded Sybil Richardson to join the Committee as Officer without Portfolio. The five original Officers are very pleased to have Jacqui, John and Sybil on board to help us with the task of running an active and growing Society (at 320 members, we are the seventh biggest OU society).

We also agreed an amendment to the Constitution so that membership will be renewable on the anniversary of joining.

Jim gets his just deserts at the AGM!

QUANTA AND CONTINUUM

Physics and Astronomy Dept News

SMXR356 Electromagnetism, experiments, applications and simulations

For many years students have wished that the electromagnetism course, SMT356, had a summer school. We agreed, but there simply weren't the funds to support it. However, the OU in its wisdom recently decided to detach each residential school (RS) from its parent course, and boost the content so that it becomes an independent 10 point course. The OU system does not call them Summer Schools any more, even when they are!

This change has given us the opportunity to make good the deficiency. In 2002 students of SMT356 will, for the first time, have a chance to spend a simulating summer week doing experiments and computer simulations in electromagnetism. They will also have some lectures and problem sessions closely related to the 30 point Electromagnetism course. There is also the little matter of 10 points at third level, which, at the time of writing we believe will count towards an honours degree.

Both of the third level physics courses, SM355 - Quantum Mech-anics and SMT356 - Electromagnetism (and their successors when they are remade) will, from 2002, have associated 10 point residential schools. The RSs will alternate (EM in 2002, QM in 2003, etc) in phase with the associated 30 point courses. We expect most students to do the RS course in the same year as the associated course, although this is not compulsory. Indeed, in principle anyone can do the courses but we strongly advise students who are not doing the associated course to read 'Are you ready for SMXR356?' (or SMXR355 next year) very carefully, and take its message seriously.

The new arrangement marks a big change from the recent past when the SM355 summer school was compulsory for SM355 students. We don't know how many students doing both parent course will do both associated RS courses. I personally, and most of my colleagues too, have very mixed feelings about this since every survey tells us how very helpful the SM355 summer school was to students doing that course. We don't like to think that some students might no longer receive the benefit attendance brings. We hope SMXR356 will give electromagnetism students a big boost like that received by attendees at the SM355 summer school, perhaps helping them to visualize some of the more abstract field concepts. Of course, a key role of the course within the OU physics profile is to boost the amount of serious experimental activity within physics oriented degrees.

Any student who wants to know more about the RS courses can contact Gillian Knight the Course Manager.

Ray Mackintosh, Course Team Chair.

Ask-a-Boffin

Can anyone answer these questions?

Norrette Moore asks "Can any of you boffins explain Planck's constant h from first principles, without using any other constants?"

David Parker works for a Laser cutting company and asks "Why can we cut 15mm mild steel but only manage to cut 3 to 4mm aluminium? The wavelength is 10.64nm and the power is 3.5kW."

Award of the Napier Bequest

A new prize, the Napier Bequest, has been awarded to a FUSION member for achieving the best performance in 2001 in 3rd Level Physics and Astronomy courses.

Peter Napier was an OU student who took most of the physics and astronomy courses. He left a bequest which the Physics & Astronomy Department has decided to use to fund two annual prizes. One is for the best performance on S207 - The Physical World, awarded this year to Caroline Fowler-Wright. The other, for the best performance on the Level 3 courses that the Department hosts (currently S357, S381, SM355 and SMT356) goes to FUSION member Jackie Drew.

Jackie is currently a full-time carer looking after her two young children; nevertheless in 2001 she managed to juggle her maternal responsibilities with not only SM355 - Quantum Mechanics but also S357 - Space, Time and Cosmology and came out with top marks in both. Having had her appetite whetted she is planning to spend the £50 prize money on some books to further her studies of relativity.

In 2002, her final year, Jackie is studying S381 - The Energetic Universe and MS323 - Introduction to Non-Linear Dynamics. She currently lives in Fleet, Hampshire, but is moving to Hawaii soon. We wish her well for the move, and hope that, when she has read the books and got bored with sunbathing, she might write us the odd newsletter article on Einstein's Field Equations!

First Class FUSION

We are now on First Class! Join us on OUSA Fusion for information on short notice events, chat about physics or exchange information on what's going on in your area. You can find us in OUSA Societies or in the Science Room.

Postgraduate Information Plea

The following motion was passed at the recent OUSA conference:

This Association calls upon the University to provide appropriately timed workshops or day schools for graduating Open University students to ensure that students reaching the end of their studies can be well informed in terms of issues such as postgraduate study opportunities available, the opportunities and implications of becoming a research student and also to provide help and advice on compiling PhD proposals and applying for studentships.

T-shirt Design Competition

Can you think of a new idea for the next FUSION T-shirt? Send us your ideas and guess what you will win?

SciBAr

What is SciBAr ? - This is a series of conversation events held by the British Association, a registered charity formed to promote the public understanding of science - where the general public gets an opportunity to put questions to a scientist on the subject at hand.

The February SciBAr at the Maddox St Wine Bar in London Mission to Mars - Moving in on the Martians? was well attended with a mix of professionals, scientists, Sci-Fi aficionados and anyone else who wandered in.

The floor was held by a visiting scientist - in this case: Monica Grady and the event was facilitated by a member of the BA: Judith Willetts.

When I arrived, a little late, the discussion was already in full flow (probably in direct proportion to the wine flow!) The ongoing debate at that moment was the ability for NASA to get to Mars within nine months - disagreements arose - some chap thought it would take longer due to acclimatisation requirements - others thought that the journey should take no longer than the time taken by surveyor.

Topics then ranged from the feasibility of creating domes on Mars to use as stopovers for further space exploration, to the ALH 84001 meteorite discovered that may or may not contain microbes. One proposal that reached the floor: "...the visible canals are man-made", was quickly debated and demolished - with grace.

It was a wonderfully relaxed session - if you wanted to say something all you had to do was speak clearly or raise your hand for the microphone. No one individual monopolised the conversation and the evening ended when everyone had had his or her say - in this case it took about an hour and a half.

Check out www.britassoc.org.uk for a list of future events.

Norrette Moore

M.Sc.ellany

17th October 2001 - "Stop writing now, please remain seated until your paper has been collected."

That's it - my very last exam! Having exhausted all possible physics courses as an undergraduate, I am now embarking on the MSc in Science - and there are NO exams!

The first challenge for a post-grad is to choose a path through the module. There are two official strands: Frontiers of Medical Science or Science Studies, but you can mix and match between them. The first thing I noticed was the shortage of hard natural science courses in the program - I have been struggling over my decision to take the MSc - but the availability of physics material at the OU is on a par with most other universities, when it comes to distance learning, the public understanding of science or medical physics is the best one can hope for.

So I chose S804 - Communicating Science - this will help me with science writing and creating research papers; followed by S809 - Imaging in Medicine and finally S810: a research project (almost a mini dissertation). Each is worth 60 points.

Course material consists of a range of set books and the course material; a CD-ROM of articles and research papers, etc (about 300 in all); videos (some clips of the mighty Feynmann here!); audiotapes and of course First Class. Now, FC is an integral part of this MSc strand - science conferencing is a major part of Block 2, and tutorials will be given via these conferences - without use of FC and the Internet for research, one cannot obtain full marks.

As I write, I'm now a month into Communicating Science and must admit am finding it a radical change from hard physics. I'm now reading about science rather than doing it (well, doing the theory anyway). So at the moment it's 'come back Schrödinger, all is forgiven'. I'll give a more detailed view next time.

Norrette Moore

Fusion: One Year On

What they* said about Fusion Day: "The AGM itself was well organised, well chaired and seemed to achieve most of its objectives" ... "There were good discussions and contributions from the floor" ... "All the talks were of a very high quality and most informative" ... "I was pleasantly surprised ... to find that ... the sessions were generally pitched at a level that everyone could understand" (* the delegates, that is, NOT the Committee!) Fusion's first Annual General Meeting took place on the OU campus at Milton Keynes on 2nd February and was attended by 20 members. We also had talks from Andy Greentree of the OU's Physics and Astronomy Department on Quantum Computing, Andrew Ball and Mark Bentley of the Planetary and Space Sciences Research Institute on various planetary missions, and Ray Mackintosh of the Physics and Astronomy Department on the background to his new book, Nucleus: A Trip to the Heart of Matter. After Andy's talk, a small party visited his quantum optics lab.

The five of us who set up the Society last January and kept it running for the first year all gave reports on what we had been doing, and ideas for the future; these tended to evolve into round-the-table discussions, and we think covered some useful ground. It was particularly helpful to have Ray Mackintosh and John Barker (a research student) in attendance from the Department, and Ray agreed to take away a number of ideas and progress them with his colleagues. This included a plea for more physics to be incorporated into the OU's Science MSc course.

We agreed to introduce a 3 years for £10 membership renewal scheme; to try to increase our profile amongst foundation-level students who may not have yet discovered that they are interested in physics; to bring in two new regular newsletter features, Ask-A-Boffin and Departmental News; and to extend membership of the Society to all Physics Associate Lecturers (aka Course Tutors). We also decided to try out a Fusion Conference on First Class, and to produce a new Schrödinger's Cat design for the next Fusion T-shirt.

Ray's entertaining and informative talk ended with an invitation to "do Nuclear Physics and see the world" - a reference to the fact that there are no major nuclear research facilities in the UK. Well, we didn't see the world, but we certainly saw Milton Keynes that night, as our convoy of cars snaked its way around the city's fine collection of roundabouts, flyovers and cul-de-sacs, trying to find the Swan at Woughton-on-the-Green. Inspired by Andy's talk, we had decided to adopt a strategy of going simultaneously left, right and straight ahead at each roundabout we came to, so that we would end up in a superposition of states, at least one of which ought to include the Swan. Sadly, it didn't work. Perhaps Planck's Constant has a different value in Milton Keynes, or maybe it was just that Eleanor's driving is too uncertain even for the Uncertainty Principle. Anyway, we eventually found it, and rounded the day off with drinks and some lovely pub food.

See our Web site for the full AGM report.

Nothing Can Go Faster Than Light?

It's probably the most well-known consequence of the Special Theory of Relativity, with the possible exception of Einstein's famous equation, E = mc². But is it true? Jim Grozier attended a talk by Bob Lambourne, OU Head of Physics and Astronomy, and sends this report.

Einstein showed that the total energy of a body with mass m, moving at a speed v relative to the observer, is mc² divided by a factor which approaches zero as v approaches c, the speed of light. So as the body speeds up, you have to give it more and more energy to accelerate it by a given amount, and in order to accelerate it to the speed of light you would have to give it an infinite amount of energy! So nothing with mass can travel at the speed of light, but photons (which are massless) travel at exactly c. Nothing can exceed this speed.

Or can it? Consider a searchlight beam which makes a complete revolution in just over six seconds, so that it has an angular velocity of 1 radian per second. If the beam is very narrow - a laser beam, say - it will produce a spot of light which sweeps across any object in its path. The speed at which the spot moves is given by the product of this angular velocity and the distance of the object - so if the object is 1 kilometre away, the speed of the spot will be 1 kms^-1; if it is 10km away, assuming the light is bright enough to reach it, the spot will be moving at 10 kms^-1. Now suppose the laser is really bright, and can be angled so as to sweep across the surface of the Moon, 400,000 km away; the spot's speed at the nearest point on the Moon's surface will be approximately 400,000 kms^-1 - faster than light!

A similar thing happens when two flat pieces of wood, at an angle θ, pass each other. If the speed of one piece of wood in its direction of travel is v, the scissors point, where the two intersect, will move at a speed v/sinθ. So if v is nearly the speed of light and θ is small, this speed too can exceed that of light.

So perhaps it is not the case that nothing can travel faster than light; perhaps we should say that no physical entity (which the spot of light, and the scissors point, are patently not) can do so. Well, yes. What Einstein actually said was that two observers who are moving relative to each other will in general disagree on the position and time co-ordinates of an event, so that they may actually disagree on which of two events happened first. This leads to the absurd possibility of one person observing an event - e.g. someone pushing a plunger which then causes an explosion - while another observer, in relative motion to the first, sees the explosion before the plunger is operated, contradicting our basic notion of causality. However, if the speed at which information travels is restricted to values less than or equal to the speed of light, order is restored and causality preserved. The spot of light on the Moon, and the scissors point, do not carry information, and cannot play a causal role between two events. Another example of a superluminal speed is the phase velocity of a microwave packet in a waveguide; it is the group velocity which determines the transfer of information, and this is always less than c.

Charged particles can certainly carry information, and the Whipple Telescope in Arizona is dedicated to observing the radiation given off by such particles travelling faster than light! Ah, but the speed I am referring to here is the speed in the Earth's upper atmosphere. Light travels slower in a medium than in a vacuum, and it is the speed in a vacuum that counts. It is this latter figure - now defined as 299,792,458 ms^-1 - which is the fundamental physical constant given by Special Relativity as the ultimate cosmic speed limit. Cerenkov radiation is given off by any charged particle moving at a speed intermediate between the values for the medium and for a vacuum, and even though the difference between the speeds in air and in vacuo is small, interactions between cosmic gamma rays and air molecules can give rise to particles travelling within this range; the Whipple Telescope detects the Cerenkov radiation given off by these particles, and is therefore able to detect the presence of the original gamma rays.

Astronomers observing Nova Persei in 1901 might have claimed to have discovered superluminal motion if they had had an accurate galactic distance scale. A few months after the explosion, they saw bright arcs centred on the nova, which appeared to be moving outwards at the speed of 11 arc minutes per year. Using the currently accepted value of approx. 1600 light years for the distance of this nova, an angular movement of 11 arc minutes corresponds to about 5 light years. If, therefore, the arcs were assumed to be rings or shells of stellar material thrown out by the explosion, as was the prevailing view at the time, they appeared to be travelling at five times the speed of light! However, because, even in those pre-relativistic times, such speeds seemed rather improbable, it was simply assumed that the arcs were travelling at or below light speed, and that the nova was actually much closer to us.

We now know that the arcs are not produced by moving material at all, but are in fact a phenomenon known as a light echo. There is thought to be an approximately planar dust cloud, about 50 light years in front of the site of the nova. This dust reflected the light from the nova in all directions, but at any one time an observer on Earth would see only those rays with a given travel time, i.e. all those points such as B in the diagram, which lie on a circle, centre A, radius AB. The three rays shown would have arrived on Earth at approximately one year intervals - first NA, the light received directly from the nova; then NB, then NC, with path lengths of 50, 51 and 52 light years respectively from the nova to the dust cloud; but BC = 5 light years, and that explains the apparent motion of 5 light years per year between B and C.

Consideration of such vast distances leads to another type of faster-than-light expansion that is perhaps more real. The distances between galaxies increase with the expansion of the universe, and for sufficiently distant galaxies, the rate of this increase can be numerically greater than the speed of light. (Indeed, according to modern cosmology, there was a period in the early universe (the inflationary period) when the size of the universe increased by a factor of 10⁵⁰ in 10^-32 seconds!). But the galaxies are not moving through space at this speed; it is rather, space itself that is expanding.

So Einstein has the last laugh after all; none of the phenomena described would allow faster-than-light travel in the way that most of us probably imagine it. So even if we eventually discover life somewhere in the universe, it is not likely to be near enough for us to visit or even communicate with. The Starship Enterprise gets round this problem by using warp speed, of course, but that is likely to remain in the realm of science fiction.

Or is it? The American scientist, Kip Thorne, has shown that wormholes, which could connect together far-flung corners of the universe, are acceptable solutions of Einstein's equations of General Relativity. Whether this means they could actually exist, or merely point to a failure of the theory, however, is open to debate. Finally, there is of course the quantum-mechanical phenomenon of entanglement, which predicts some form of communication between pairs of particles that have become separated by arbitrarily large distances; but that is another story...

STAFF PROFILE - Dr Carole Haswell

CAROLE HASWELL is a Lecturer and Postgraduate Tutor at the OU Department of Physics and Astronomy, and a member of the Astronomy Research Group.

Having gained a BA in Physics at Oxford University, Carole spent 11 years in the United States, commencing with a PhD in Astronomy at the University of Texas, then postdoctoral appointments at the Space Telescope Science Institute, "home" of the Hubble Space Telescope, and Columbia University in New York. While at Columbia she gave her first lecture course, entitled Introductory Astronomy, at Barnard College. Carole's first permanent academic position was at the University of Sussex, from which she joined the OU in April 1999. She says 'Being a member of the Physics and Astronomy department at the OU is a real joy. The atmosphere is more friendly and collegial than anywhere else I've worked, and the energy and dedication that OU undergraduate students display is truly inspiring.'

Carole's teaching activities at the OU have included writing part of the course units for S207 - The Physical World and having 'lots of fun designing multimedia questions for two of the books' (so now you know who to blame!). She has co-written the new 3rd level astrophysics course, S381 - The Energetic Universe and is also tutoring it.

Carole is a member of the Interacting Binaries Gang at Milton Keynes, and currently supervises four PhD students and a postdoctoral fellow. Her main research interest is in accretion-powered compact binary star systems, where one star accretes material from the other (the donor star). These include X-ray transients - perhaps the most dramatically variable objects in our galaxy, which rise from obscurity in a matter of days to become among the brightest objects in the X-ray sky. (There is strong evidence that the majority of these systems contain black holes).

She is also interested in cataclysmic variables - interacting binary stars in which a white dwarf accretes material from a main sequence companion star - particularly those which exhibit superhumps, where the accretion disc becomes eccentric, and precesses at a different angular speed to that of the donor star. (More information on these topics, including animations, is available at http://physics.open.ac.uk/research/astro/astro_work/research.html.)

Having grown up on the incomparable North Yorkshire coast, Carole bemoans the fact that she lives 'just about as far from the sea as is possible in the UK' but finds solace in tennis, skiing, and snowboarding in Milton Keynes, and the 'glorious English countryside'. She hones her ability to deal with adversity and disappointment by supporting Middlesborough Football Club.

Simulation of superhump binary.

QUANTUM COMPUTERS - Part 1

In the first of a series of articles about quantum computers, Andy Greentree explains the genesis of modern computing. > Part 2 > Part 3

Quantum computers promise to revolutionise modern computing, and therefore make a profound impact on our lives. In this series of articles I want to try and give you a flavour of quantum computing, the physics behind quantum computing, how it differs from conventional computing and illustrate some of the technologies being used.

Classical Computers and Moore's Law

Before we start on quantum computers, it is probably best to start with the computers we are familiar with today. Physicists term these classical computers to distinguish them from quantum computers. Although the first electronic computer was built in 1943 (Colossus, at Bletchley Park) the history of adding machines goes back much further, past slide rules, Babbage's engines and abacuses to ticks on clay tablets.

From their wartime beginning, the power of electronic computers was clear. Initially being custom built, the first mass produced computer was the IBM 650, built using valves with switches and friendly flashing lights (and an optional punch card reader) to interface with the programmer. From these beginnings, computers have grown in importance (and shrunk in size) to be one of the most important tools we have at our disposal. To give a feel for the growth of computing power, consider a mobile phone. If mobile phones were built using the valve technology of the IBM 650, they would be the size of the Empire State Building.

Back in 1943, few people would have doubted that computers would improve in performance, but the speed of that growth and the penetration of computers into our lives would have been difficult to imagine. To quantify the computer revolutions, we consider system performance of a top of the range computer (measured in millions of operations per second or MIPS), as a function of time from 1943 to the present. If you plot this out, you would notice a fairly steady increase in (log of) performance as a function of time. This steady increase from Colossus with one operation every ten seconds (10^-7 MIPS) to some of today's machines working at 10⁶ MIPS is surprising. Gordon Moore (former head of IBM) first noticed the trend in 1965. He noted that the speed of computers was doubling every 18 months. This is now called Moore's Law.

Moore's law in action for number of transistors and size of transistors.

That Moore's Law should hold at all is remarkable, that it should still be valid today, with all of the changes in technology that have occurred since 1943 is truly astonishing. There are several other manifestations of Moore's Law. Not only has the speed been doubling, but the number of transistors per chip has been doubling and the size of each transistor has been halving, every 18 months. (An interesting aside is that the cost of the latest computer has remained more or less constant, look back over old computer adverts and see for yourself!).

Do we really expect Moore's Law to be a universal law of the universe, on a par with relativity, quantum mechanics and Murphy's Law? To investigate we have to look at some limits. If Moore's Law is to be believed then by 2030, the transistors powering every computer will be the size of Hydrogen atoms. We don't know how to build anything like that yet (the real problem not being the size of the transistors but the packing between them). What we do know is that at this scale, the classical physics that describes the operation of the transistors of today¹ breaks down. The physics of the very small is quantum physics and so this point represents a barrier beyond which no classical computer can go.

Is that really it? In 2030 do we have to stop building new computers? In order to progress further we need to understand the quantum barrier and learn how this problem can be turned into an opportunity to build some of the most powerful devices imaginable².

¹Although transistors perform classical logic and work with classical particles doing (largely) classical things, a full, consistent understanding of transistors requires a knowledge of quantum physics and barrier tunnelling. ²Historically, physicists have usually been much happier tackling the problems of 30 years in the future than the problems of today.

In the next installment, Andy Greentree explains Heisenberg's Uncertainty Principle and wave-particle duality. > Part 2

How to Determine the Height of a Skyscraper with a Barometer

Magnus Magnusson's favorite after-dinner story originally came from the Engineers Weekly of Denmark. It illustrates the virtues and pitfalls of thinking for oneself.

It concerned the above question in a physics degree exam at the University of Copenhagen. One student answered as follows:

'You tie a long piece of string to the neck of the barometer, and then lower the barometer from the roof of the skyscraper to the ground. The length of the string, plus the length of the barometer, will equal the height of the skyscraper.'

This highly original answer so incensed the examiner that he failed the student immediately. The student appealed to the university authorities on the ground that his answer was indisputably correct, and that he should have been given full marks. The university appointed an impartial arbiter to decide the case, a visiting professor from the University of Washington, called Dr. Alexander Calandra. Dr. Calandra ruled that although the answer was technically correct, it did not display any noticeable knowledge of physics; and to resolve the matter, he called the student in and gave him six minutes in which to answer the question verbally in a way that showed at least a minimal familiarity with the basic principles of physics.

For five minutes there was complete silence. The student sat there frowning heavily, deep in thought. Dr. Calandra reminded him that time was running out, to which the student replied that he had several extremely relevant answers to the question, but could not make up his mind which one of them was best. 'You had better hurry up' said Dr. Calandra.

'All right then' said the student. 'You take the barometer up to the roof of the skyscraper, drop it over the edge, and measure the time it takes to reach the ground. The height of the building can then be worked out in terms of the formula H=½gt². But bad luck on the barometer.'

'Or if the sun happens to be shining, you could measure the height of the barometer, then set it up on end and measure the length of its shadow. Then you measure the shadow of the skyscraper, and thereafter it is a simple matter of proportional arithmetic to work out the height of the skyscraper.'

'But if you wanted to be highly scientific about it, you could tie a short piece of string to the neck of the barometer and swing it like a pendulum, first at ground level and then on the roof of the skyscraper, and work out the building's height by the effect of the difference in the res-toring force on the period, T=2¼√(H/g).'

'Or if the skyscraper had an outside emergency staircase, it would be easier simply to walk up it and mark off the height of the skyscraper with a pencil, in barometer-lengths, and then add them up.'

'If you merely wanted to be boring and orthodox about it, of course, you could use the barometer to measure the air-pressure on the roof of the skyscraper, compare it with standard air-pressure on the ground, and convert the difference in millibars into feet to give you the height of the skyscraper.'

'But since we are constantly being exhorted to exercise independence of mind and apply scientific methods, undoubtedly the best way would be to knock on the janitor's door and say to him, "If you would like a nice new barometer, I will give you this one if you tell me the height of this skyscraper".'

Jekyll and Hyde Particles May Hold a Clue to the Fate of the Universe

Recent results from the Sudbury Neutrino Observatory (SNO) in Canada suggest that neutrinos - originally thought to be massless - may have some mass after all. The SNO experiment, in which neutrinos emitted by nuclear reactions in the Sun are detected as a result of collisions with deuterium nuclei in a 1000-tonne tank of heavy water, has confirmed that the neutrinos oscillate en route to the Earth. There are three kinds or flavours of neutrino: electron-neutrinos, tau-neutrinos and muon-neutrinos. Oscillation means changing from one of these flavours into another, and, according to theory, this is only possible if neutrinos have mass. Amongst other implications, this means that they could account for some (but not all) of the dark matter which has been postulated to explain the rotation of galaxies, and to satisfy modern cosmological theories, which require a closed universe which will eventually stop expanding and start to contract. In fact, neutrinos could make up as much as 18% of the total mass in the Universe.

Sun continues to shine!

Solar scientists are also relieved at the results, as they are consistent with the standard solar model, which predicts a neutrino flux in excess of what had hitherto been observed. But the model is safe now that it is understood that, by the time they arrive at the Earth, some of the electron-neutrinos produced in the Sun have changed into other flavours, which previous experiments could not detect. The subject has certainly caught the public imagination, and has even inspired a play on Radio 4, No Future In Eternity.

Next steps: a 730km long laboratory!

So, you may be thinking, that just about wraps it up for the neutrino then? You might be forgiven for thinking so if you read the July issue of Physics World, whose bold headline proclaimed Solar Neutrino Puzzle Is Solved (page 5). In fact, there are still many things we don't know about neutrinos. MINOS will help to fill some of the gaps.

MINOS (Main Injector Neutrino Oscillation Search) will send a beam of neutrinos from Fermilab, in Chicago, to a disused mine in Soudan, Minnesota, 730km away. Because of the curvature of the Earth, the beam will need to be aimed into the Earth at an angle of 3.3 degrees (see diagram) and will actually be 10km below the surface at its mid-point.

But solid rock is not much of an obstacle for the neutrino, which interacts with other matter only via the weak nuclear force, and will happily travel right through the Earth and out the other side without incident. Detectors placed at the beginning and end of this trajectory will compare the make-up of the beam before and after its journey, and will therefore be able to produce direct evidence of any oscillations.

Why such a long baseline? Well, according to theory, the probability of transition from one flavour to another is the product of two factors, one of which increases, and one of which decreases, with baseline length. The distance from the neutrino source at Fermilab to Soudan, which is already being used for other experiments, and is far enough underground to shield out cosmic rays which would reduce the detector's sensitivity, is a good compromise between these two factors.

Down to Earth

Another neutrino project? What has MINOS got that its predecessors (which include SNO, Japan's Kamio-kande and Super-Kamiokande, and the long-running Homestake Goldmine experiment which will be familiar (!) to students of S281 - Astronomy & Planetary Science) lacked? Well, first of all, one of the big unknowns in solar neutrino experiments is the Sun itself, whose neutrino output can only be quantified by means of theoretical models; this uncertainty can be eliminated by using a terrestrial source, with a degree of control that solar scientists can only dream of. MINOS is not the only terrestrial long-baseline neutrino experiment - there is CNGS (CERN - Gran Sasso) in Europe, which will monitor the appearance of tau neutrinos, and K2K in Japan; but it is the only one that will actually measure the 'mixing parameters' - the factors that determine the transition probabilities - one of which is directly related to mass. MINOS will weigh what was previously thought unweighable, and will consequently make a major contribution to our understanding of how the Universe works.

Dr Philip Harris of Sussex University, a member of the project, told FUSION: 'Now that we know that neutrinos oscillate, it is all the more important to pin down the mixing parameters precisely, and this is what we aim to do by studying the neutrino beam at its point of departure as well as at the end of its journey. This is a very exciting time in this field, and the results will open a window into physics beyond the Standard Model.'

Watch this space!

So, when will all this happen? Well, the detectors are already under construction, but are such complex structures that it will be another two years before they are complete, with the big switch-on expected in 2004-5.

Jim Grozier