Bright future in dark matter

August 25, 1995

There is ample evidence to suggest that physics as a scientific discipline has been in relative decline for several years. Although a problem not only in the United Kingdom - similar trends are evident in the United States, Japan and elsewhere - the possibility that decline could accelerate into collapse must be avoided.

The danger in the UK lies in the drift away from physics in both schools and universities, due in part to the perceived difficulty of the subject and to the increasing tendency for physics to be taught (at least to GCSE level) by non-physicists. Clearly if the numbers of physics graduates also continues to fall, the situation in schools will worsen, and so on.

Within our university physics departments other threats are apparent. Only a quarter of the top-rated 30 departments surveyed last year for the Institute of Physics were not operating at a substantial loss, a fact unlikely to make physics a favourite of vice chancellors for investment in new staff or facilities! Even the Government's ROPAs initiative brought little joy to hard-pressed physics research groups, only 2-3 per cent of this Pounds 70 million funding injection going to support basic physics research.

Does this apparent decline in UK physics matter? Certainly it would, if it were to continue. Although no longer having the commercial or political support of the life sciences, physics remains the language by which we seek to describe and understand the physical world, from subatomic to cosmic scales. Of course, it also provides the tools by which other sciences can progress, eg Nuclear Magnetic Resonance, X-rays, electron microscopy. In addition, there remains a substantial sector of physics-based industries which, according to an Insitute of Physics survey, outperformed the general manufacturing sector throughout the 1980s, in productivity, export growth and profitability. Other arguments for the continuing importance of a healthy physics community can be made in the contribution of physicists to interdisciplinary research, certain to become more important in the future as more and more complex systems are studied in increasing detail.

However, for a physicist like me the primary argument for maintaining a strong physics community remains the intrinsic importance and potential of the subject itself. In this respect, history offers an important lesson. Just a century ago, physics was also apparently in decline, with most major problems "solved" and little that was new. Then, in successive years, Rontgen discovered X-rays, Becquerel discovered radioactivity and Thomson identified the electron. Over the next 20 years, Einstein was to develop the theory of relativity and the development of quantum mechanics heralded a new industrial revolution that today has a commercial value well in excess of the UK's gross national product.

In 1995, if we have learned anything it must be that nature cannot be outdone in the Science Foresight stakes. Thus, when Laplace pointed out, in 1795, that Newtonian gravity should lead to the existence of a "black hole", and the discovery of the neutron in 1934 led to Landau and Zwicky's speculation on the possibility of "neutron stars", few took serious interest. Since then, of course, astronomers have found both these exotic objects to occur frequently within our own galaxy. Earlier this summer, newspaper headlines announced the discovery of the "superatom", a state of matter found at very low temperatures in which the concept of individual particles ceases to have meaning. In fact, the corresponding phenomenon, the Bose-Einstein condensation, was predicted by Einstein 70 years ago. Now that this elusive form of matter has been produced in the laboratory, we can be sure that sooner or later it will be put to practical use.

Of course, the latter examples are where theoretical prediction preceded discovery. Far more often the richness of nature delivers totally unexpected discoveries. Most often these are made when scientists struggle to explain some apparent anomaly or misfit of experiment and theory. By their very nature, research in particle physics and astronomy are especially rich in throwing up new insights into the nature of our physical world. That represents a prime motive for the community that the Particle Physics and Astronomy Research Council supports, and offers an intellectual (and technical) challenge shared by all concerned, from graduate students to chief executive.

One current topic that is exercising astronomers and particle physicists may serve to illustrate the point. There is now very good evidence that most of the matter in our universe is of unknown form and remains - as yet - undetected. This so-called "dark matter" may represent 90, or even 99, per cent of the material of the universe. Many would agree it is - at least - a most unsatisfactory state of affairs that we humans, our planet Earth, and all we survey apparently represent a minor element of the physical world.

The clearest evidence for dark matter comes from the observed rigidity with which spiral galaxies, like our own Milky Way, rotate. The inverse square law of gravity results in only matter inside the radial position of the stars and gas in a particular spiral arm having any effect on its speed of rotation. Thus, if only the visible matter (mainly stars) were effective, then the outer parts of a galaxy would rotate increasingly slowly. In fact, the rate of galaxy rotation is maintained well beyond the extremities of the visible stars, implying the presence of considerable unseen matter. On a still larger scale, individual galaxies are often seen to cluster like a swarm of bees in a loose association tens or hundreds strong. These galaxies must be held together by gravity, but again nothing like sufficient binding mass can be seen in the stars that make up the individual galaxies, nor in the hot intracluster gas discovered by X-ray observations to fill the space between the galaxies.

Finally, on the largest scales of cosmology, there is evidence of a similar shortfall of matter. Although the arguments are less direct than with galaxy rotation or the binding of galaxy clusters, most cosmologists believe our universe contains sufficient mass (some three atoms per cubic metre, on average) for the present expansion (the "Hubble flow") to eventually stop, and perhaps reverse. Either the so-called "critical density" or the "closure density" requires a ratio between dark and visible matter of at least ten to one.

So what can this dominant matter be? That is a current hot topic for physicists, particularly those working in the PPARC area. Indeed the "best bets" fall nicely across the fields of particle physics and astronomy. The most conservative explanation for the mass binding a large spiral galaxy is some form of baryonic (ie ordinary) matter which for some reason is very under-luminous. Two known astronomical objects that satisfy this criterion are planets and brown dwarf stars. The latter are stars of perhaps 1 per cent of the Sun's mass, that did not quite achieve the core temperature and pressure to ignite hydrogen burning; hence they remain very cool and faint. Since brown dwarfs and planets are so faint they could exist in large numbers but remain unseen. However, recent deep infrared surveys (more sensitive to such cool objects) have failed to reveal nearly enough dwarf stars to explain the rigid rotation of our galaxy. A new technique is now being tried, sensitive to any object over the wide mass range from 0.01 to 100 solar masses. That is the technique of gravitational lensing, by which means an unseen object happening to lie in the line of sight to a normal, bright star will focus that light, causing a temporary increase in the visible star's brightness for a few days or weeks as the nearer object passes across the line of sight. Although an increasing number of good sightings of this general relativistic effect are being reported, again the numbers of unseen "dark matter" candidates are falling well short of the required binding mass.

The diagnostic power of the gravitational lensing searches is underlined by the fact that the technique is sensitive to concentrations of baryonic matter in any form, whether stars, planets or in more exotic forms such as neutron stars or black holes. It is therefore more surprising that - even in the nearest astronomical site of substantial dark matter, namely the outer reaches of our own Milky Way galaxy - the required amount of baryonic matter is nowhere to be seen.

Can the particle physicists help? They can if the elusive matter that is apparently so abundant in our universe is in a form that can hide away by interacting with normal matter extremely weakly (other than, of course, via its gravitational pull). Again we begin with a candidate whose existence is well established, the neutrino. This tiny particle does indeed interact so weakly that neutrinos produced by the Sun's nuclear furnace escape from the centre of the Sun unhindered; in contrast the radiation we see emerging as light and heat takes several million years to diffuse to the surface of the Sun. However, to date, the mass of the neutrino has defied measurement. Current limits show it to be no more than 1 per cent of the electron's mass but - given the large number of neutrinos predicted to be flooding the universe - this small mass would suffice to solve the dark matter problem. Experiments to measure this crucial quantity are in progress in CERN and elsewhere. If not the neutrino, what else? Several candidates are suggested by particle physics theorists, including the so-called WIMPS (weakly interacting massive particles). Leading a number of attempts to detect and measure the WIMPS is one presently under way, 1,000 metres underground, in a former salt mine in Boulby, Yorkshire. The technical challenge is to detect the few WIMPS that are predicted to interact with the Boulby detector each day, against a background rate, caused by cosmic radiation and terrestrial radioactivity, in the order of a million times larger.

The intense efforts by astronomers and particle physicists must surely detect the dark matter within the next ten years. If not, we may need to contemplate the more radical explanation that gravity is not the uniform and universal force we have always believed. Whatever the answer, a solution to the dark matter problem will be a further significant step forward in understanding the nature of the physical world. What practical benefits may follow only time will tell.

Identifying the nature of the dark matter is not the only challenge on the agenda for particle physicists and astronomers, of course. Indeed, it is exciting to contemplate the progress that will surely result from the powerful new facilities and techniques coming on stream in the years ahead. The Large Hadron Collider at CERN will create energies not seen since one million-millionth of a second after the Big Bang. The first gravitational wave detectors will open up a whole new way of observing the universe, providing a sensitive means of detecting fluctuations in space-time, caused by distant stellar explosions, or merging black holes. Supercomputers will model the large-scale structure of the universe and probe the strong force holding the atomic nucleus together, while new techniques of adaptive optics will compensate ground-based telescopes for atmospheric blurring, yielding images as sharp as the present Hubble Space Telescope. In space, the next year will see the launch of three major new missions, in each of which UK groups are heavily involved. SOHO will probe the Sun's hidden interior by monitoring the way its surface oscillates coherently on timescales of minutes to hours. Four identical spacecraft will fly in formation through the Earth's outermost atmosphere to study, in concert with SOHO, the effects of "solar weather". Finally, a new infrared telescope will be launched on the ISO spacecraft, where the whole telescope is held in a sealed container of liquid helium, the extremely low temperature allowing observations of the infrared sky with unprecedented sensitivity.

Given the richness of nature, there is no doubt that these technical advances, across the broad front of PPARC's research remit, will lead to a stream of new knowledge on the nature of our physical world.

But, can we look for a solution to the declining interest in physics in the schools from this active programme of research and discovery? In PPARC we are determined to try. A coordinated effort is being made, using our network of university researchers, to reach out to local communities and - using Internet and related media facilities, such as the World Wide Web (invented by particle physicists) - making exciting and up-to-date curriculum-related material available to physics teachers across the country.

I am optimistic that the new focus the PPARC brings to "cosmic physics" can be a major factor in ensuring the bright future for UK physics, vital for both wealth creation and the quality of life.

Kenneth Pounds

Chief executive of the Particle Physics and Astronomy Research Council.

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