There is no doubt in the mind of Australia’s Chief Scientist, Professor Ian Chubb, the future will be shaped by science technology, engineering and mathematics. Unfortunately, he finds that at present the standing of science, as an expert authority, is being challenged. Furthermore, Ian Chubb finds that the science message is getting lost in the white noise of the mainstream media. I was heartened to hear his positive words about science communication, social media, science and technology education and innovative Australian workplaces.
It was refreshing to see Australia’s Chief Scientist out and about and addressing public forums such as this one. Although judging by the faces, the suits and the overheard conversations at the drinks and nibbles prior to the address, I think this was definitely a speech to the science and technology faithful. That is a pity, his words were worth exposure and considered comment in the mainstream Australian media.
Prof. Ian Chubb at the Climate congress, Copenhagen 2009, March 10-12. Opening session.
Professor Ian Chubb emphasises Mathematics, Engineering and Science provide the enabling skills and knowledge that underpin every aspect of modern life. They help us understand the natural world and enable us to respond as humans to this world with a constructed view aimed at improving the lot of human kind.
In Australia, as in many economies, we have observed a decline in the number of people choosing a career in these disciplines. Not only that, the STEM subjects (Science Technology Engineering and Mathematics), as he called them, are taken for granted or simply ignored. Although it is obvious without at least an appreciation of these subjects, a modern citizen is hampered in their ability to critically evaluate and make informed decisions about the issues that are shaping their future. Among his many roles as Australia’s Chief Scientist, Professor Ian Chubb has been charged with examining this decline and offering strategies to address it.
Professor Ian Chubb is eminently suited to this task. He was appointed to the position of Chief Scientist on 19 April 2011 and commenced the role on 23 May 2011. Prior to his appointment as Chief Scientist, Professor Ian Chubb was Vice-Chancellor of the Australian National University. Professor Chubb’s research focused on the neurosciences. Although he jokingly said on the night he would prefer not to be quizzed, on science specifics, by such an informed audience. He has co-authored some 70 full papers and co-edited one book all related to his research. In 1999 Professor Chubb was made an Officer of the Order of Australia (AO) for “service to the development of higher education policy and its implementation at state, national and international levels, as an administrator in the tertiary education sector, and to research particularly in the field of neuroscience
At just over two tonnes, the second stage of an Atlas V rocket, makes for an unusual ‘kinetic probe’. Nonetheless on October 9, 2009 NASA deliberately impacted a spent Centaur rocket into the lunar south polar crater Cabeus. The target area was a permanently shadowed region within this crater. The impact, not surprisingly, ejected a spectacular plume of debris, dust and vapour.
Science experiment, observe the system, perturb it, and measure what happens
The US scientists had thrown a heavy object at the Moon. They then threw all the instruments possible to monitor the impact. The prize was a decades-long search to directly find water on the Moon.
The impact would have been majestic to watch. Picture those slow motion images of Apollo astronauts on the Moon. Hold that thought and then imagine the impact. An observer could marvel at the slow motion, low gravity, return of the dust and debris cloud to the Moon’s surface. If you could see in the infra-red, the impact flash lasts for 10 seconds. There is a cloud of debris, dust, and vapour rising. At eight seconds the the ejecta cloud is 4.5km in diameter, in the ultra-violet spectrum, the plume is 10km in diameter. At 20 seconds after impact the ejecta cloud was is at its maximum diameter of 8.5km and the plume has reduced to little less than 10km.
The observer would be watching a science experiment on a grand scale.
The observer in this experiment was neither you nor I, it was a trailing “shepherding spacecraft”. The Centaur had propelled NASA’s Lunar Reconnaissance Orbiter and Lunar Crater Observation and Sensing Satellite to the Moon. Shortly after launch the Lunar Reconnaissance Orbiter had separated to go on its own mission. Once in lunar orbit the Centaur had vented its remaining fuel. Control was then assumed, for the next four months, by the Lunar Crater Observation and Sensing Satellite as the shepherding satellite. During this next period the shepherding satellite maneuvered the Centaur to allow the Sun to bake-out residual water and volatiles. This was to ensure that no contaminant chemicals were passengers to the lunar impact site. The Centaur’s fuel was a volatile combination of liquid hydrogen and liquid oxygen, both chemicals that were to be scanned for in the impact cloud. The Lunar Crater Observation and Sensing Satellite also calibrated its instruments, then targeted the Centaur to impact with the Moon. Four months of meticulous preparation.
LCROSS spacecraft with Centaur stage, image credit NASA
The Lunar Crater Observation and Sensing Satellite carried nine instruments, including cameras, spectrometers and a radiometer. The spectrometers measured the reflected light at different wavelengths. These enabled the identification of the chemicals present in the ejected cloud.
Near-infrared absorbance attributeble to water vapour and ice, and ultraviolet emissions attributable to hydroxyl radicals (OH-) support the presence of water in the debris. The researchers determined from these observations that there was over 5%, by mass, of water ice in the lunar regolith of the impact site. Certainly this is small by terrestrial soil standards, but more substantial than most earlier estimates.
Over a year after the impact, in the October 22, 2010 issue of the journal Science, the results of this experiment were delivered to the world’s attention. This certainly marked a defining moment for lunar scientists, directly confirming the availability of water on the moon. It was however neither the first nor last word on this.
Cabeus crater LCROSS impact site, photo credit NASA
Early attempts
Since the first lunar sample were carried back to earth by Apollo astronauts in the late 1960s, scientists have operated under the presumption that the moon was entirely dry. In total 382kg of lunar material was bought to Earth by the Apollo missions astronauts and a further 0.32kg by the unmanned USSR Lunar missions. New analyses of these rocks with improved analytical techniques have made it possible to perform highly sensitive isotopic measurements on very small lunar grains. These analyses are revealing water in Apollo samples that were once thought to be dry.
Well before these new studies, scientists had been puzzling about why more water was not seen on the moon. It was thought that volatile materials, such as water, could be accumulating at the moon’s permanently shaded polar regions. Here they could be trapped for geological periods of time without significant loss. The in 1998, the orbiting Lunar Prospector spacecraft measured the the abundance of elements on the moon’s surface using neutron spectroscopy. This provided compelling evidence for enhanced hydrogen concentrations, and by inference water, at both of the lunar poles.
In 1999 the Cassini spacecraft flew by the moon on its way to Saturn. It turned its Visual and Infrared Mapping Spectrometer to the moon. By measuring the surface reflectance of light from the moon scientists found absorption attributed to hydroxyl and water on the sunlit surface of the moon. These results were not published until 10 years later, in October 2009. The reason was renewed interest in water on the moon.
On October 22, 2008 Chandrayaan-1 was launched on a lunar mission by the Indian Space Research Organisation. One of its major scientific missions was to look for water on the moon. It had three different instruments ready to make 2008-10 an interesting period for lunar water exploration.
Chandrayaan-1, India’s lunar water finder
The Chandrayaan-1 story is told in detail elsewhere. Here I intend to showcase the marvelous outcome of Chandrayaan-1’s water finding experiments. Perhaps the most exciting of all these was one of the simplest. This was the CHandra’s Altitudinal Composition Explorer (CHACE) on board the Moon Impact Probe.
On November 14 2008 (the birthday of the late Pandit Jawaharlal Nehru, India’s 1st Prime Minister) the Moon Impact Probe became the first Indian built object to reach the surface of the Moon. The probe was a 34kg box-shaped object containing a video image system, radar altimeter, and The CHACE mass spectrometer.
Symbolically the Indian tricolour was painted on three sides of the Moon Impact Probe. This enables India to also lay claim to having the “Indian tricolour placed on the Moon”. Needless to say that “placing” in this case was a hard landing in the Moon’s south polar region near the Shackleton crater, flying over the Malapert mountain en route.
The CHACE mass spectrometer took 650 spectra of the tenuous lunar atmosphere during its 1487 second, 98km, plunge to the lunar surface. Tenuous is right the atmosphere even on the sunlit side is only 7/10,000,000,000th of the Earth’s atmosphere.
The mass spectrometer was tuned to look find water and direct evidence of water it did find. The team leader of the experiment, Dr S M Ahmed, remembers, “We all were jumping when we saw water was literally pouring out of our instrument
A pulse of particles speeds into the vacuum chamber. Positrons, 20,000,000 antimatter particles, clumped in a pulse one nanosecond deep. Like a silent, angry swarm they are targeted into a porous silica target. The positrons are confined by a magnetic field, increasing their interaction with the silica. Some attract electrons and synthesize into positronium, a hybrid unstable ‘molecule’. Before it can decay the positronium is excited first by a burst of ultra-violet laser light. Then a second burst, this time in the infrared. With each excitation the positronium puffs-up. The still bound, positron and electron orbit further and further away from each other. Decreasing their opportunity to meet. Increasing the lifetime of the positronium. Eventually they meet. They annihilate. Mutually destructing in a soundless flash of energy. A pair of gamma rays to remind us of a nanosecond long courtship.
Producing positronium, binding the negatively charged electron with its antimatter equivalent, the positively charged positron, is not a normal occurrence in our world. If this experiment sounds a little like science fiction, the reason for doing it is part of an even more exotic tale. This laboratory experiment is to produce ‘long-lived’ positronium is so that the researchers can measure the effect of gravity on it. Published in January 2012, this success represents 10 years of hard work for Allen Mills and his team from the University of California, Riverside.
The positron is the antimatter version of the electron. It has the identical mass to the electron, but a positive charge. The researchers are trying to find whether matter and antimatter behave the same way under gravity. That is, do they weigh differently? If they find such behaviour it would truly rock the physics world.
Antimatter. The idea of it is weird. The idea of weighing it is stranger still.
The weirdness of antimatter
Antimatter has been one of the most fascinating fields of research ever since the prediction of its existence by Paul Dirac in 1929. Paul Dirac solved the basic equations of quantum mechanics. In doing so he found their solution implied the existence of antimatter particles. At the time most physicists, Dirac included, thought the idea was preposterous.
Within a short time, experimenters looking at cosmic rays bombarding the Earth found particles that behaved like electrons, but with a positive charge. In 1932 Carl Anderson carried out the definitive experiments demonstrating the existence of the anti-electron, naming it the positron, and being awarded the Nobel Prize in 1936 for his discovery.
3D image of Antihydrogen. Source: NSF
Dirac had postulated in 1931 that each of the fundamental particles has an equivalent antimatter partner. Furthermore, when matter and its antimatter opposite come into contact they annihilate each other, releasing energy into two equal energy photons. Their energy is given by Einstein’s famous equation E=mc². This latter relationship has been much exploited by science fiction writers. Matter – antimatter reactions powered the star-ship USS Enterprise in the 60s hit TV show Star Trek.
Work with high-energy antiparticles is now commonplace in physics and materials science. Anti-electrons are used regularly in the medical technique of positron emission scanning tomography. Perhaps more importantly, the equivalence model of matter and antimatter has been fully incorporated into the Standard Model of Particle Physics. Matter particles and their anti-matter pairs have the same mass and equal, but opposite charge.
The most important point in all of this focusses on when our universe came into being, the Big Bang. Models of the Big Bang predict that equal amounts of matter and antimatter were formed initially. Ordinary matter is clearly what our observable Universe is made of today. Where is the antimatter? A glaring, niggling, imbalance that begs for an explanation.
Timeline of the Universe: Big bang to present time. Source: NASA/WMAP Science Team
This imbalance could be explained by a slight difference in one of the fundamental properties of particle-antiparticle pairs (such as charge or mass). There isn’t yet any experimental evidence for such a difference. Another alternative might be a difference in their gravitational attractiveness. It is widely expected that the gravitational interaction of matter and antimatter should be identical. If they were not, then this could explain the preponderance of matter in our universe.
Elementary antimatter particles naturally occur in radioactive decays and in cosmic radiation. Some of them, such as the positron and the antiproton, have been studied extensively and even compared to their matter equivalents. Measurements with charged antiparticles are difficult because gravity is a far weaker force than the electromagnetic force. The first experiments to measure the gravitational attraction of antimatter were at the University of Stanford and CERN in 1974 and 1993 respectively. Both used charged antimatter particles. The experiments were marred by stray electric fields and did not produce satisfactory results.
Trying to do experiments like this requires precision and reproducible backgrounds. The world is a rather messy place to measure these relationships. Neutral positronium, such as in Mill’s experiments, or an anti-atom could be used to test the effect of gravity on antimatter for the first time, because it is immune to stray electromagnetic fields that have hampered the previous studies with charged antimatter particles.
Does antimatter fall down?
Understanding gravity has proven to be a little more complicated than falling apples. To the Greek philosopher and polymath Aristotle, the concept that heavy objects fell faster than light objects was obvious. His elegant, but fanciful, notion persisted well into the Middle Ages. The Italian physicist, mathematician, and astronomer Galileo Galilei successfully challenged Aristotle’s impractical theories of motion. Experiments using the swing of pendulums proved him correct. Observing their swing rates was more practical than the dropping of objects from the Leaning Tower of Pisa, as Galileo had originally proposed.
Galileo also put forward the basic principle of relativity, that the laws of physics are the same in any system that is moving at a constant speed in a straight line, regardless of its particular speed or direction. Hence, there is no absolute motion or absolute rest. This principle is central to Einstein’s special theory of relativity.
The key point under consideration here is the correlation between inertial mass and gravitational mass. This is the correlation between the forces measured on a mass when it is falling under gravity or being accelerated. The earliest experiments were done by English natural philosopher Isaac Newton and improved upon by the German mathematician and astronomer Friedrich Wilhelm Bessel in the 1820s.
The problem is that Newton’s theories and his mathematical formulae did not and do not explain the equivalence of the behavior of various masses under the influence of gravity, independent of the quantities of matter involved. The observation that the gravitational mass and the inertial mass is the same for all objects is unexplained within Newton’s Theories. The experiments of Galileo Galilei, decades before Newton, established that objects that have the same air or fluid resistance are accelerated by the force of the Earth’s gravity equally, regardless of their different inertial masses. Yet, the forces and energies that are required to accelerate various masses is completely dependent upon their different inertial masses, as can be seen from Newton’s Second Law of Motion, F = ma.
Precision measurements by the Hungarian physicist Loránd Eötvös originally in 1885, and then again with improved instruments and precision between 1906 and 1909, established the universality of Newton’s law of gravitation. These were followed with a series of similar but more accurate experiments, these included experiments with different types of materials, on moving ships and in different locations around the Earth. These experiments demonstrated the equivalence of gravitational and inertial mass for ordinary matter. In turn, these experiments led to the modern understanding of the equivalence principle encoded in general relativity, which states that the gravitational and inertial masses are the same.
So far, so good, for understanding the behaviour of ordinary matter. Rather than disembodied logic alone, the combination of experiment, measurement, and sound reasoning proving to be the correct way to discern the laws that represent reality.
Gravity is now best described by general relativity. General relativity is a classical theory that does not imply the existence of antimatter. In the 1980s a quantum-mechanical formulation of gravity allowed for non-Newtonian contributions to the force which might lead to a difference in the gravitational force on matter and antimatter.
A number of theories propose how differential interactions between matter and antimatter may be explained. It must also be pointed out that numerous models and experiments with matter have been used to derive upper limits on the possible differences in the nature of such gravitational attractions.
Direct investigation of antimatter, experimentally, is characterised by an almost complete lack of data.
The experimenters
Allen Mills is not alone in his quest to measure the weight of antimatter.
In 2011 the AEgIS collaboration at CERN, had funding approved to use antihydrogen to measure any difference in the gravitational force on matter and antimatter. CERN is Europe’s particle-physics research lab located near Geneva in Switzerland. Perhaps currently best known for its search for the Higgs boson, the co-called God particle. The funding scale of the AEgIS experiment (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) is more modest. It’s goal, to create a horizontal beam of antihydrogen and to study its free fall in the Earth’s gravitational field with a matter wave interferometry apparatus, is scientifically equally far-reaching.
The quest to create, trap and study antihydrogen is now entering its third decade at CERN.
In 2002, the ATHENA experiment at CERN’s Antiproton Decelerator was the first to produce copious amounts of cold antihydrogen, the simplest atomic antimatter system. The ATHENA (AnTiHydrogEN Apparatus) experiment had the objective to produce, to store and to study antihydrogen at extremely low temperatures, at less than 1 Kelvin temperature. The goal was to compare the energy levels of antihydrogen and hydrogen with extreme accuracy. The ATHENA set-up was used as a proof of concept in this case for the successor experiment AEgIS.
Experimental area of CERN's Antiproton Decelerator Hall, showing ALPHA experiment. Source: CERN/Mikkel D. Lund
The antiprotons supplied by the Antiproton Decelerator were trapped and cooled, and brought into overlap with positrons from a radioactive sodium source in a cylindrical Penning trap. The produced anti-atoms, no longer confined in the charged-particle trap, drifted radially outward and annihilated on the electrodes. ATHENA’s sophisticated detector allowed the temporally and spatially resolved reconstruction of these annihilation events.
During the data taking periods in 2003 and 2004, the experimental parameters were optimized in order to maximize the antihydrogen production rate, and the temperature and internal quantum states of the anti-atoms were determined. ATHENA was not configured to measure the gravitational attraction of antihydrogen. Data taking with ATHENA has now ended.
It was collaborators from the ATHENA experiment, along with new groups from other institutes, that have designed the successor experiment, AEgIS, with the aim of performing gravitational studies with antimatter. The AEgIS proposal was submitted in January 2008 and approved by the CERN Research Board in December 2008. Construction began in early 2010.
Meanwhile the CERN group has been building on its expertise for the production and trapping of antihydrogen. A new experimental collaboration called ALPHA (Antihydrogen Laser PHysics Apparatus) is another successor to ATHENA. In late 2010 the ALPHA group managed, 38 times to confine single antihydrogen atoms for 172 milliseconds. At the time the spokesperson Jeffrey Hangst said, “We’re ecstatic. This is five years of hard work.”
By July 2011 they had confined seven antihydrogen anti-atoms for 1,000 seconds, extending their earlier results by nearly four orders of magnitude. To compare with these CERN successes, Mills, in his late 2011 experiment, produced 12 positronium atoms that did not annihilate until they hit the chamber wall. This journey of a few centimetres takes about a microsecond.
Based on these results Mills believes he can produce a collimated, long-lived beam for the direct measurement of the gravitational free fall of positronium atoms.
Kudos and plaudits
We have in 2012 then, two experiments, both different in their experimental make-up. Both are trying to measure the gravitational free-fall of antimatter: one using antihydrogen, one using positronium. Assuming that both will be successful, then one will be used as a confirmation of the results of the other.
This is how great science is done. Great scientists are nonetheless people. People are competitive. In years to come, the science textbooks will record, and perhaps laud, who was first to measure the weight of antimatter.
