by Paul Gilster | Aug 10, 2011 | Antimatter |
Now that NASA’s Institute for Advanced Concepts (NIAC) is back in business, I’m reminded that it was through NIAC studies that both Gerald Jackson and James Bickford introduced the possibility of harvesting antimatter rather than producing it in huge particle accelerators. The idea resonates at a time when the worldwide output of antimatter is measured in nanograms per year, and the overall cost pegged at something like $100 trillion per gram. Find natural antimatter sources in space and you can think about collecting the ten micrograms that might power a 100-ton payload for a one-year round trip mission to Jupiter. Contrast that with Juno’s pace!
That assumes, of course, that we can gather enough antimatter to test the concept and develop propulsion systems — doubtless hybrids at first — that begin to draw on antimatter’s power. Bickford (Draper Laboratory, Cambridge MA) became interested in near-Earth antimatter when he realized that the bombardment of the upper atmosphere of the Earth by high-energy galactic cosmic rays should result in ‘pair production,’ creating an elementary particle and its antiparticle.
A planetary magnetic field can hold such particles in place, producing a localized source of antiprotons. The detection of antimatter in this configuration has now been confirmed by a team of researchers using data from the Pamela satellite (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics). In fact, Pamela picks up thousands of times more antiprotons in a region called the South Atlantic Anomaly than would be expected from normal particle decays.
Image: A cross-sectional view of the Van Allen radiation belts, noting the point where the South Atlantic Anomaly occurs. Credit: Wikimedia Commons.
We could go so far as to talk about an ‘antimatter belt’ around the Earth, as the paper on this work explains:
Antiprotons are… created in pair production processes in reactions of energetic CRs [cosmic rays] with Earth’s exosphere. Some of the antiparticles produced in the innermost region of the magnetosphere are captured by the geomagnetic field allowing the formation of an antiproton radiation belt around the Earth. The particles accumulate until they are removed due to annihilation or ionization losses. The trapped particles are characterized by a narrow pitch angle distribution centered around 90 deg and drift along geomagnetic field lines belonging to the same McIlwain L-shell where they were produced. Due to magnetospheric transport processes, the antiproton population is expected to be distributed over a wide range of radial distances.
The McIlwain L-shell referred to above describes the magnetic field lines under investigation. As to the South Atlantic Anomaly, it is here that the inner Van Allen radiation belt approaches the Earth’s surface most closely, which creates a higher degree of flux of energetic particles in the region. It turns out to be quite a lively place, as this Wikipedia article on the matter makes clear:
The South Atlantic Anomaly is of great significance to astronomical satellites and other spacecraft that orbit the Earth at several hundred kilometers altitude; these orbits take satellites through the anomaly periodically, exposing them to several minutes of strong radiation, caused by the trapped protons in the inner Van Allen belt, each time. The International Space Station, orbiting with an inclination of 51.6°, requires extra shielding to deal with this problem. The Hubble Space Telescope does not take observations while passing through the SAA. Astronauts are also affected by this region which is said to be the cause of peculiar ‘shooting stars’ (phosphenes) seen in the visual field of astronauts. Passing through the South Atlantic Anomaly is thought to be the reason for the early failures of the Globalstar network’s satellites.
What we’re seeing in the new work is that the Van Allen belt is indeed confining antiparticles in ways that the earlier NIAC work suggested. The antiprotons eventually encounter normal matter in the Earth’s atmosphere and are annihilated, but new antiparticles continue to be produced. The question is whether there may be enough antimatter here for hybrid missions like Steven Howe’s antimatter sail, which uses tiny amounts of antimatter to induce fission in a uranium-infused sail. James Bickford, in his Phase II study at NIAC, talked about a collection scheme that could collect 25 nanograms per day, using a plasma magnet to create a magnetic scoop that could be deployed in an equatorial Earth orbit, one that would trap incoming antiprotons.
Antimatter trapped in Earth’s inner radiation belt offers us useful savings, if Bickford is right in thinking that space harvesting will prove five orders of magnitude more cost effective than antimatter creation here on Earth. I also noticed an interesting comment in his Phase II NIAC report: “Future enhanced systems would be able to collect from the GCR [galactic cosmic ray] flux en route to further supplement the fuel supply.” Obviously, exploiting antimatter trapped near the Earth and other Solar System worlds assumes a robust space-based infrastructure, but it may be one that will finally be able to bring antimatter propulsion into a new era of experimentation.
James Bickford’s Phase II report is titled “Extraction of Antiparticles Concentrated in Planetary Magnetic Fields” (online at the NIAC site). Back in 2007 I looked at this work in three connected posts, which may be useful in putting all this in context:
The Pamela work is found in Adriani et al., “The discovery of geomagnetically trapped cosmic ray antiprotons,” Astrophysical Journal Letters Vol. 37, No. 2, L29 (abstract / preprint). See also Gusev et al., “Antiparticle content in the magnetosphere,” Advances in Space Research, Volume 42, Issue 9, p. 1550-1555 (2008). Abstract available.
by Paul Gilster | Mar 11, 2011 | Antimatter |
Are we ever going to use antimatter to drive a starship? The question is tantalizing because while chemical reactions liberate about one part in a billion of the energy trapped inside matter — and even nuclear reactions spring only about one percent of that energy free — antimatter promises to release what Frank Close calls ‘the full mc2 latent within matter.’ But assuming you can make antimatter in large enough amounts (no mean task), the question of storage looms large. We know how to store antimatter in so-called Penning traps, using electric and magnetic fields to hold it, but thus far we’re talking about vanishingly small amounts of the stuff.
Moreover, such storage doesn’t scale well. An antimatter trap demands that you put charged particles into a small volume. The more antimatter you put in, the closer the particles are to each other, and we know that electrically charged particles with the same sign of charge repel each other. Keep pushing more and more antimatter particles into a container and it gets harder and harder to get them to co-exist. We know how to store about a million antiprotons at once, but Close points out in his book Antimatter (Oxford University Press, 2010) that a million antiprotons is a billion billion times smaller than what you would need to work with a single gram of antimatter.
Antihydrogen seems to offer a way out, because if you can make such an anti-atom (and it was accomplished eight years ago at CERN), the electric charges of positrons and antiprotons cancel each other out. But now the electric fields restraining our antimatter are useless, for atoms of antihydrogen are neutral. Antimatter that comes into contact with normal matter annihilates, so whatever state our antimatter is in, we have to find ways to keep it isolated.
A Novel Antihydrogen Trap
One solution for antihydrogen is being explored at CERN through the international effort known as the ALPHA collaboration, which reported its findings in a recent issue of Nature. Here positrons and antiprotons are cooled and held in the separate sections of what researchers are calling a Minimum Magnetic Field Trap by electric and magnetic fields before being nudged together by an oscillating electric field, forming low-energy antihydrogen. Keep the anti-atoms at low energy levels and although they are neutral in charge, they still have a magnetic moment that can be used to capture and hold them. Says ALPHA team member Joel Fajans (UC-Berkeley):
“Trapping antihydrogen proved to be much more difficult than creating antihydrogen. ALPHA routinely makes thousands of antihydrogen atoms in a single second, but most are too ‘hot'”—too energetic—”to be held in the trap. We have to be lucky to catch one.”
Image: Antiprotons and positrons are brought into the ALPHA trap from opposite ends and held there by electric and magnetic fields. Brought together, they form anti-atoms neutral in charge but with a magnetic moment. If their energy is low enough they can be held by the octupole and mirror fields of the Minimum Magnetic Field Trap. Credit: Lawrence Berkeley National Laboratory.
Clearly we’re in the earliest stages of this work. In the team’s 335 experimental trials, 38 antihydrogen atoms were recorded that had been held in the trap for about two-tenths of a second. Thousands of antihydrogen atoms are created in each of the trials, but most turn out to be too energetic and wind up annihilating themselves against the walls of the trap. In this Lawrence Berkeley National Laboratory news release, Fajans adds a progress update:
“Our report in Nature describes ALPHA’s first successes at trapping antihydrogen atoms, but we’re constantly improving the number and length of time we can hold onto them. We’re getting close to the point where we can do some classes of experiments on antimatter atoms. The first attempts will be crude, but no one has ever done anything like them before.”
Taming the Positron
So we’re making progress, but it’s slow and infinitely painstaking. Further interesting news comes from the University of California at San Diego, where physicist Clifford Surko is constructing what may turn out to be the world’s largest antimatter container. Surko is working not with antihydrogen but positrons, the anti-electrons first predicted by Paul Dirac some eighty years ago. Again the trick is to slow the positrons to low energy levels and let them accumulate for storage in a ‘bottle’ that holds them with magnetic and electric fields, cooled to temperatures as low as liquid helium, to the point where they can be compressed to high densities.
One result is the possibility of creating beams of positrons that can be used to study how antiparticles react with normal matter. Surko is interested in using such beams to understand the properties of material surfaces, and his team is actively investigating what happens when positrons bind with normal matter. As you would guess, such ‘binding’ lasts no more than a billionth of a second, but as Surko says, “the ‘stickiness’ of the positron is an important facet of the chemistry of matter and antimatter.” The new trap in his San Diego laboratory should be capable of storing more than a trillion antimatter particles at a time. Let me quote him again (from a UC-SD news release):
“These developments are enabling many new studies of nature. Examples include the formation and study of antihydrogen, the antimatter counterpart of hydrogen; the investigation of electron-positron plasmas, similar to those believed to be present at the magnetic poles of neutron stars, using a device now being developed at Columbia University; and the creation of much larger bursts of positrons which could eventually enable the creation of an annihilation gamma ray laser.”
An interesting long-term goal is the creation of portable antimatter traps, which should allow us to find uses for antimatter in settings far removed from the huge scientific facilities in which it is now made. Robert Forward was fascinated with ‘mirror matter’ and its implications for propulsion, writing often on the topic and editing a newsletter on antimatter that he circulated among interested colleagues. But he was keenly aware of the problems of production and storage, issues we’ll have to solve before we can think about using antimatter stored in portable traps for actual space missions. Much painstaking work on the basics lies ahead.
The antihydrogen paper is Andresen et al., “Trapped antihydrogen,” Nature 468 (2 December 2010), pp. 673-676 (abstract). Clifford Surko described his work on positrons at the recent meeting of the American Association for the Advancement of Science in a talk called “Taming Dirac’s Particle.” The session he spoke in was aptly named “Through the Looking Glass: Recent Adventures in Antimatter.”
by Paul Gilster | Feb 22, 2010 | Antimatter |
Interstellar theorist Richard Obousy (Baylor University) has some thoughts about William and Arthur Edelstein’s ideas on flight near the speed of light. As discussed in these pages on Friday, the Edelsteins, in a presentation delivered at the American Physical Society, had argued that a relativistic rocket would encounter interstellar hydrogen in such compressed form that its crew would be exposed to huge radiation doses, up to 10,000 sieverts in the first second. Because even a 10-centimeter layer of aluminum shielding would stop only a tiny fraction of this energy, the Edelsteins concluded that travel near lightspeed would be all but impossible.
Obousy, who handles the Project Icarus Web site, has his own credentials related to high speed travel, authoring a number of papers like the recent “Casimir energy and the possibility of higher dimensional manipulation” (abstract) that press for continued work into breakthrough propulsion. And when he talked to astrophysicist Ian O’Neill about warp drive concepts last week, Obousy said that we are in the process of laying down “…a mathematical and physical framework for how such a device might function, given the convenient caveat of a ‘sufficiently advanced technology.'” The device, he said, is purely theoretical as of now and we have no evidence that it could be built.
But should we keep investigating? On that score, Obousy has no doubts. With regard to shielding, he argues that metamaterials that bend radiation around objects are a place to begin, offering a conceivable barrier against the kind of radiation the Edelsteins are talking about. All of which makes for lively reading, as does Obousy’s continuing work on the Project Icarus team. Icarus is the descendant of Project Daedalus, the 1970’s era starship design created by the British Interplanetary Society. And while the Icarus guidelines focus on fusion as the propulsion method of choice, Obousy’s interests extend not just to warp drive but also to antimatter possibilities.
The latter is of interest because of its huge energy density, drawing on the abundant energy available within all matter. A single kilogram of matter contains 9×1016 J of energy. “[I]n simpler terms,” says Obousy, “about five tonnes of antimatter would theoretically be enough to fuel all the world’s energy consumption for a single year.” But as he notes in this entry on the Project Icarus site, storage is a huge problem. Positively charged positrons exert a Coulombic force of repulsion against each other, one of the reasons we can store only tiny amounts with current technology.
Ideal storage involves neutral antimatter — antimatter with no net charge — which points to antihydrogen (a stable atom containing a single positron and an antiproton) as a solution. Storing antihydrogen in the form of a Bose Einstein Condensate is one possibility for packing more of the stuff in less space.
As to the vast cost of antimatter production, Obousy has this to say:
With regards to the question of production, current methods utilized at CERN are prohibitively expensive and generation of antihydrogen in quantities that would be valuable to spaceflight would cost trillions of dollars. Despite this, it’s important to recognize that CERN is not a dedicated antimatter production facility and that antihydrogen production is a remarkable, yet tertiary goal of the facility. According to recent research, a low-energy antiproton source could be constructed in the USA at a cost of around $500M over a five year period, and would be an important first step for mass production of antimatter. However the overall roadmap for antimatter propulsion would involve timescales closer to 50 years.
If we start talking near-future uses of antimatter, though, tiny quantities could be put to work in projects like Steven Howe’s antimatter sail, which would use antihydrogen to initiate a fission reaction in a small, uranium-coated sail. Howe developed this idea for NASA’s now defunct Institute for Advanced Concepts. The antimatter, which drifts from storage unit to sail, causes fission as it encounters the uranium, producing neutrons and fission fragments that leave the sail at enormous speeds. NASA’s John Cole, who studied the antimatter sail idea while at Marshall Space Flight Center, told me in 2003 that the sail could develop specific power on the order of 2000 kilowatts per kilogram, enough to drastically shorten human missions to the outer planets even if Howe’s estimates are an order of magnitude off.
Or could antimatter be used as a trigger for fusion? Obousy is interested in the prospect:
Although a spacecraft propelled by antimatter may be many decades away, it maybe possible to use antimatter in the near future to catalyze nuclear fusion reactions using antimatter. Only very small quantities would be required and this might provide an alternative method for liberating energy from fusion. Because Icarus must use current, or near technology, it is possible that Icarus will utilize this form of propulsion…
And he adds:
Clearly a multitude of technological hurdles must be overcome before antimatter use becomes routine in space exploration. However, the fundamental theoretical issues have been proved. Antimatter exists, antihydrogen can be created technologically, antihydrogen can be stored. The rest is progress.
For more on the potential uses of antihydrogen in propulsion, see Nieto et al., “Controlled antihyrogen propulsion for NASA’s future in very deep space,” NASA/JPL Workshop on Physics for Planetary Exploration, 2004 (available online).
by Paul Gilster | Jun 3, 2009 | Antimatter |
Antimatter’s allure for deep space propulsion is obvious. If matter is congealed energy, we need to find the best way to extract that energy, and our existing rockets are grossly inefficient. Even the best chemical rocket pulls only a billionth of the energy available in the atoms of its fuel, while a fission reaction, powerful as it seems, is tapping one part in a thousand of what is available. Fusion reactions like those in a hydrogen bomb use up something on the order of one percent of the total energy within matter. But antimatter can theoretically unlock all of it.
Freeing Trapped Energy
The numbers are startling. A kilogram of antimatter, annihilating with ordinary matter, can produce ten billion times the amount of energy released when a kilogram of TNT explodes. Heck, a single gram of antimatter, which is about 1/25th of an ounce, would get you as much energy as you could produce from the fuel tanks of two dozen Space Shuttles. This is the ultimate kick if we can figure out a way to harvest all this energy, but as particle physicist and author Frank Close (Oxford University) shows in his new book Antimatter (Oxford University Press, 2009), we’re a long way from knowing how to go about this.
Close is a good, clear writer. Even the most abstruse parts of Antimatter — and that includes a thorny section on Paul Dirac’s use of the mathematical tools called ‘matrices’ to plumb the depths of antimatter’s role in the universe — are rendered forthrightly and understandably. And the conundrum of antimatter storage receives considerable attention. We can store the stuff in magnetic bottles but if we store positrons or antiprotons alone, we face the problem that like charges repel, which means we can’t put in large quantities (even if we had them) due to the repulsive forces that inevitably cause leakage. Neutral anti-hydrogen is also tricky because it is not responsive to the electric and magnetic fields we were hoping to use to keep matter and antimatter apart.
Current Storage and Proposed Options
You can see what this does to our thinking about antimatter in spacecraft. We’ve got to find ways to store antimatter in quantity that aren’t themselves so heavy that they become a huge factor in total mass. Of Gerald Smith’s work at Penn State and, later, Positronics Research, Close is skeptical. In one Smith paper, the authors outlined the basics of a trap that would carry a billion antiprotons for ten days. This was meant to be a prototype of a trap that would carry 1014 antiprotons for up to 120 days, sufficient for a round trip Mars mission. There is much more in the Smith proposal, but Close sees little to recommend it, at least so far:
This appears to have been more a management plan of how one would approach such a challenge rather than any tested proven route to a new technology. Ten years later, nothing like this has been achieved, nor was any of the work at CERN devoted to such endeavours. The maximum number of antiprotons ever stored in a trap is a million, and the focus of current research is on containing small numbers for precision measurements.
Antimatter in Quantity?
And we also have to reckon with ways to produce antimatter in sufficient quantity. Right now the energy inefficiency is enormous. Says Close:
…since the discovery of the antiproton in 1955, with LEAR at CERN and similar technology at Fermilab, the total amounts to less than a millionth of a gram. If we could collect together all that antimatter and then annihilate it with matter, we would only have enough energy to light a single electric light bulb for a few minutes. By contrast the energy expended in making it could have illuminated Times Square or Piccadilly Circus.
At the current rate (maybe a nanogram a year costing tens of millions of dollars), it would take hundreds of millions of years and over $1,000 trillion to produce one gram of antimatter. Or try this out:
To make a gram of antiprotons you will need 6 x 1023 of them, while a gram of positrons would require 1026. The most intense source of antiprotons is at Fermilab, USA. Their record production over a month in June 2007 produced 1014 antiprotons. Were they able to do this every month for a year they could produce about 1015, which equates to 1.5 billionths of a gram, or nanograms. Were we able to retain all of these antiprotons and annihilate them with 1.5 nanograms of matter, the total energy released would be about 270 Joules, which is like five seconds illumination by a feeble light bulb.
A Sail Concept Using Antimatter
No easy solutions in Close’s book. The antimatter rocket idea — annihilate antimatter with matter to produce gamma rays that heat a propellant before expelling it out the back of the rocket — sounds good until we reckon in the impracticality of storage and the current inability to produce antimatter in quantity. Antimatter is excellent at showing you the state of the art and where we may be heading in the near future, but it also reminds us of the need to modify our space concepts. Steve Howe’s fission-based ‘antimatter sail,’ for example, is built around the idea that we have huge constraints on antimatter production.
Think of a sail coated with a layer of uranium-235. A tiny amount of antimatter released from the spacecraft creates fission which kicks the payload to 116 kilometers per second, in Howe’s formulation of a mission to 250 AU. The key is the storage of antihydrogen, an antimatter atom consisting of an antiproton orbited by a positron, in the form of frozen pellets that evaporate as they drift toward the sail. We’re talking about a sail a mere fifteen feet in diameter, relying on antimatter for its punch.
Image: The Howe concept, a sail using antimatter to trigger a fission reaction. Credit: NIAC/Steve Howe.
Rather than thinking in terms of large storage tanks of the stuff, we’ll have to learn to work with what we’ve got or what we can harvest in the Solar System. That doesn’t mean that there won’t be future breakthroughs in production — at least, we can’t rule these out — but realistic antimatter work for the near term will have to involve ways to store tiny amounts in efficient containers and use them to catalyze other reactions. Steve Howe’s NIAC paper on the antimatter sail not only discusses a propulsion method but NIAC also has his report on ingenious storage options. Despite NIAC’s closure, we can still get the benefit of reports like Howe’s online.
by Paul Gilster | Nov 18, 2008 | Antimatter |
Irradiate a millimeter-thick gold target with the right kind of laser and you might get a surprise in the form of 100 billion positrons, the antimatter equivalent of electrons. Researchers had been studying the process at Lawrence Livermore National Laboratory, where they used thin targets that produced far fewer positrons. The new laser method came about through simulations that showed a thicker target was more effective.
And suddenly lasers and antimatter are again making news. Hui Chen is the Livermore scientist behind this work:
“We’ve detected far more anti-matter than anyone else has ever measured in a laser experiment. We’ve demonstrated the creation of a significant number of positrons using a short-pulse laser.”
Image: Physicist Hui Chen sets up targets for the antimatter experiment at the Jupiter laser facility. Credit: Lawrence Livermore National Laboratory.
What’s happening here is that ionized electrons are interacting with gold nuclei, giving off energy that decays into matter and antimatter. It’s a method that has been studied before at Livermore, using the laboratory’s petawatt laser. An article by physicist Michael Perry explains the interactions that laser achieved, first produced about a decade ago:
The intense beam of Livermore’s Petawatt laser was powerful enough to break up atoms by causing reactions in their nuclei. Accelerated by the laser, electrons traveling at nearly the speed of light collided with nuclei in a gold foil target, producing gamma rays that knocked out some of the neutrons from other gold nuclei and caused the gold to decay into elements such as platinum. Gamma rays also zoomed in on a layer of uranium sitting behind the gold and split uranium nuclei into lighter elements. Before the Petawatt, all of these effects had been solely in the domain of particle accelerators or nuclear reactors.
Accelerated to energies exceeding 100 megaelectronvolts, the electrons in the gold targets produced high-energy x rays. These in turn decayed into pairs of electrons and their antimatter counterparts, positrons, in such large numbers as to possibly generate an electron-positron plasma, never before created in the laboratory. An intense beam of protons also turned up. Not only was the Petawatt the most powerful laser in the world, but, unexpectedly, it also was a powerful ion accelerator.
The petawatt laser was decommissioned in mid-1999. Its original development centered on the study of inertial confinement fusion, in which a pellet of fuel could be ignited through intense laser bombardment. Variations on inertial confinement fusion play interestingly through interstellar propulsion studies, including the massive Daedalus probe designed by the British Interplanetary Society, which would have used deuterium and helium-3 as its fodder for a trip to Barnard’s Star.
But the petawatt laser also opened up the possibility of using a laser to do things particle accelerators had been called upon to do in the past. And now we’ve moved significantly beyond the earlier results — the petawatt experimenters detected roughly 100 antimatter particles compared to Chen’s one million. And note this: Chen’s number refers to particles that were directly detected, a result that produces an overall estimate of 100 billion positrons produced in the entire experiment.
We’ve looked in these pages at the possibility of harvesting naturally forming antimatter found in our own Solar System, even near the Earth, where cosmic ray interactions with the upper atmosphere produce small quantities. And the presence of antimatter near the center of our galaxy has been established, detectable because matter/antimatter annihilation produces gamma rays. The trick has always been that harvesting antimatter — or producing it in accelerators — yields small amounts at great expense. The latest work at least offers hope for more robust laboratory study of a material whose propulsive properties have long attracted interstellar theorists.
Just how significant a step this is remains to be seen, but I note what Peter Beiersdorfer, who works with Chen at Livermore, has to say:
“We’ve entered a new era. Now, that we’ve looked for it, it’s almost like it hit us right on the head. We envision a center for antimatter research, using lasers as cheaper antimatter factories.”
We’ll know more shortly, for Chen is presenting her work at the American Physical Society’s Division of Plasma Physics meeting that runs through Friday this week. A Livermore news release is available. Thanks to Centauri Dreams reader Leith for the heads-up on this work.
by Paul Gilster | Nov 3, 2008 | Antimatter |
Antimatter’s great attraction from a propulsion standpoint is the ability to convert 100 percent of its mass into energy, a reaction impossible with fission or fusion methods. The trick, of course, is to find enough antimatter to use. We can produce it in particle accelerators but only in amounts that are vanishingly small. There is evidence that it is produced naturally, at least in trace amounts, in the relativistic jets produced by black holes and pulsars. Indeed, a cloud of antimatter 10,000 light years across has been described around our own galaxy’s center.
And at least one scientist, James Bickford (Draper Laboratory), has worked out ways to extract antimatter produced here in the Solar System, a method that he believes would be five orders of magnitude more cost effective than creating the stuff on Earth. But what about early antimatter, particles left over from the earliest days of the universe? According to prevalent theory, the universe may have been awash with the stuff shortly after the Big Bang, but most of it is assumed to have annihilated with ordinary matter, leaving only the slightly more numerous remnants of matter behind. Could any antimatter have survived?
Image: Antimatter is made up of elementary particles that have the same masses as their corresponding matter counterparts but the opposite charges and magnetic properties. This illustration shows what happens when a particle of antimatter collides with one of matter. The particles annihilate each other and produce energy according to Einstein’s famous equation, E=mc2, mostly in the form of gamma rays, which scientists are looking for using the Compton observatory. Secondary particles are also produced. Credit: CXC/M. Weiss.
Gary Steigman, who has been studying these matters at Ohio State, lays out current thinking on matter/antimatter asymmetry in the early universe in a new paper. A primordial imbalance between matter and antimatter seems to have been essential to the emergence of the universe we see. And the key to the possibility of antimatter surviving from the Big Bang era may be inflation, when spacetime itself seems to have expanded exponentially:
“If clumps of matter and antimatter existed next to each other before inflation, they may now be separated by more than the scale of the observable Universe, so we would never see them meet. But, they might be separated on smaller scales, such as those of superclusters or clusters, which is a much more interesting possibility.”
Usefully, we might be able to observe evidence for such antimatter in collisions between two galactic clusters. That signature would be marked by X-rays from the hot gases involved in the collision and the gamma rays associated with antimatter annihilation. And as we’ve seen in recent times, the Bullet Cluster is an excellent laboratory for such study, being the relatively nearby result of cluster collision. The data thus far garnered from the Chandra X-ray Observatory and the Compton Gamma Ray Observatory are setting strict limits on possible antimatter stores.
At least that’s true in the Bullet Cluster, where the antimatter signature simply does not appear. If antimatter is present, these results mean it amounts to less than three parts per million in this system. The search continues, with Steigman hoping to learn whether other colliding galaxy clusters show a similar paucity of antimatter. It would be helpful if an antimatter signature could tell us about the mysterious period of inflation — how long, for example, did it last? — but even Steigman calls this a long shot. “The collision of matter and antimatter is the most efficient process for generating energy in the Universe, but it just may not happen on very large scales,” says the scientist. “But I’m not giving up yet…”
The paper is Steigman, “When clusters collide: constraints on antimatter on the largest scales,” Journal of Cosmology and Astroparticle Physics 10 (2 October 2008). Available online. Also available at the arXiv site. See as well this Chandra news release.
