The ALPHA experiment plans to contribute to the clarification of this mystery by testing the behavior of neutral antimatter in the form of anti-hydrogen atoms. As all forces working between matter-matter, matter-antimatter and antimatter-antimatter are supposed to be the same, an anti-atom should be the perfect mirror image of its matter counterpart. I.e. the interaction between the anti-electron (positron) and the antiproton in antihydrogen should be indistinguishable from that between the electron and the proton in hydrogen.
This interaction can be probed using laser light with extremely high resolution - in principle down to one part in a million trillion. The antihydrogen atom could provide the best possible direct test of the difference between matter and anti-matter, as it is the only system so far available that allows precision tests of the interactions between anti-particles only.
Furthermore, since antihydrogen is a neutral system it offers the possibility of testing the influence of gravity on anti-matter. Theoretically this influence is expected to be the same as that for matter, but this postulate has never been investigated due to the non-availability of neutral anti-matter. If antimatter does not fall in a gravitational field, we will be able to detect such behavior easily, but in the more likely (depending on your feelings on the matter) situation where antimatter does fall, but perhaps with slightly different acceleration, lasers also offer powerful means for observing the behavior of the anti-atoms.
The explanation that Bohr proposed to explain this was that the electron(s) that were known to be a part of the atom (Thomson's experiment) and were suspected to be distributed as a cloud around the atom due to Rutherford's discovery of the nucleus, were in fact only able to have certain distinct energies. The energy of the electron is quantized - hence the later name of quantum physics. The famous (and wrong) secondary school image of the atom was thus created with the electron(s) orbiting the nucleus like planets orbiting the sun.
Each line in the Balmer series was thus found to correspond to specific energy changes of the electron in the hydrogen atom. In an energy diagram we can view it as in the following figure
The Balmer series corresponds to transitions between the level designated '2' and the levels 3,4,5 and 6. The energy difference from the ground level (n=1) to the first excited state (n=2) is also indicated and the corresponding wavelength is 121.5 nm - which is deep in the ultra-violet.
where 'E' is energy, 'm' is the mass and 'c' is the speed of light. This equation basically expresses that matter is a very concentrated form of energy. The equation express the amount of energy contained in a particle of mass 'm' at rest. If the particle moves the total energy of the particle can be expressed as
where 'p' is the momentum (mass times velocity) of the particle. As this is a quadratic equation it has two solutions for the energy E: a positive and a negative one. Dirac decided that the negative ones must correspond to some physical reality and thus proposed that a negative energy solution corresponded to a positive energy anti-particle. Furthermore he proposed that, like an ocean, all the negative energy states are filled up to zero with particles. This would be invisible to a standard observer, as it is supposed to be a homogeneous background. However, as shown on the figure below, if enough energy is available it is possible to excite a particle from the negative energy state into a positive energy state, thus leaving a 'hole' behind in the sea of negative states. This hole is like a mirror image of the particle that we excited away. The hole can move around like a particle, and as it corresponds to the lack of a negative energy particle it behaves like a positive energy hole or anti-particle. This mechanism tells us how we can create anti-particles by having enough energy and suitable conditions for momentum conservation. It also predicts that anti-particles are always created in a pair with a particle.
This of course is the heart of the problem. The currently most accepted theory for theorigin of the universe is the Big Bang model, that starts - as the name indicates - with a Big Bang, i.e. with a huge energy density that slowly expands. As the universe expands it cools like an expanding gas. But all this energy eventually manifests itself as particles. However, we just learned that particles are always created together with a twin anti-particle. Today we should thus have half the universe filled with anti-matter. When matter and anti-matter meet the matter can 'fall' into the anti-matter 'hole' and the energy difference is released. This energy difference is given by Einsteins famous equation and is enormous. One gram of (anti)matter corresponds to the same amount of energy as released by fission of 1 kg of Uranium 235 or as 15 kTons of TNT (the size of the Hiroshima bomb). Thus if half the universe was anti-matter we would expect the sky to be illuminated by matter- anti-matter annihilations as they are called. No evidence for such behavior or indeed any significant amount of anti-matter in the universe as a whole has so far been observed. Thus on this account half the universe is missing or there is some subtle difference between matter and anti-matter that can account for the difference.
One of the goals of anti-matter science is thus to set limits for the differences between matter and anti-matter, and thus provide insight into the Universe as a whole as well as the physical laws governing it.
This transition from the ground state of the atom (1s) to the first excited state (2s) may be bridged by two photons whose total energy matches the transition energy difference. This is a rather unlikely process and thus demands large numbers of photons (high light intensities), however it has the advantage that if we use to counter propagating laser beams of the same frequency (in our case the same beam retro-reflected on a mirror) the Doppler shift is compensated as the Doppler shift of one photon is exactly the opposite (to first order) of that of the other counter propagating photon. Recall that a photon source moving towards you is blue-shifted (towards higher frequencies). This is the same phenomenon as the sound emitted from a passing ambulance - when moving towards you the pitch is higher and when moving away lower.
As the velocity of the
atom thus
exits the equation all atoms, independent of velocity, will be resonant
with
the laser when the laser frequency matches the transition. In this was
the
laser light may interact with the atom.
An Krypton Ion laser (called Innova Sabre) is a basic type of laser which uses a DC current to population invert ions in a gas (Krypton). The laser has been equipped to emit light at three wavelengths only : 406 nm, 413 nm and 416 nm. The laser emits between 3 and 4 watts of power. These wavelengths are defined by some fixed electronic transitions in the gas used in the laser and can not be changed. In order to introduce tunability in the system this laser is used to pump a so-called dye laser (Coherent 899). In the dye laser the gain medium is a dye dissolved in a suitable solvent (Ethylene Glycol). The dye used is called Coumarin 102 and emits light in the range 450 - 520 nm. The molecules of the dye are large and have many many states and thus when excited the light emitted is for all practical purposes continuous in the wavelength range of the dye, and the dye laser can thus be made to laser on any wavelength in the range of the dye. The wavelength used in our system is 486 nm. The bandwidth of the dye laser as supplied by the manufacturer is about 500 kHz - thus far from the transition linewidth of 1.3 Hz of the 1s-2s transition in (anti)-hydrogen. By adding a more stable reference the dye laser line width can "easily" be improved 1000 fold - or more. The 486 nm light has two times the wavelength of the light needed for the two photon transition. One may double the frequency of the light by shining it into a non-linear crystal. The processes in the crystal do, with some feeble efficiency, 'merge' two photons into one and thus creates light of double the frequency (half the wavelength). The process is proportional to the square of the intensity of the light and it is thus very beneficial to place this crystal in a re-circulator (resonator) such that the fundamental beam (486 nm) passes many times through the crystal. Out of this chain is created the 243 nm light needed for the 1s-2s spectroscopy. About 50 mW of laser light at 243 nm can be generated.
In reality people often use another (so-called cloverleaf) configuration which avoids the infinite wires and has the same field. The criterion on the field is that it has a minimum in the center of the trap. The typical depth of these kinds of traps is a few Kelvin. The anti-hydrogen thus has to be produced at low temperatures for anything to be captured by the trap.
The previous ATHENA setup did not include a trap for neutrals, which means that precision spectroscopy is still a bit into the future. Trapping of the neutral are, however, one of the main points of the new ALPHA Project. Still, many other potential experiments awaits with the setup as it is now. Amongst possible simple experiments is the ionization of anti-hydrogen in the ground state. As no-one has ever studied directly the interaction between two anti-particles such an experiment already offers interesting possibilities as well as being a precursor for more precise measurements.
Comments to the pages on spectroscopy may be sent to Niels Madsen -Last modified February 4, 2005