Antihydrogen, made from a single antiproton and single positron, is the simplest antiatom that can be created. As tests are constantly going on to try and find some asymmetry between matter and antimatter there is always a demand for antihydrogen to be experimented with. Of course the production is hard enough as it involves trying to create and then bind two significantly different particles but afterwards containment is also an issue as the antihydrogen threatens to annihilate any matter it touches.
Using electromagnetic radiation to probe matter is such an inherent part of physics that it comes up in every branch and every subject. Whether it’s the interaction between charges, understanding crystal structures, monitoring chemical reactions or logging the compounds found in foodstuff there is always spectroscopy. One of more common techniques is called pump-probe spectroscopy. A laser pulse of only a few hundred femtoseconds is split into two sections. The stronger is sent first and is called the pump and it is this pump which excites the matter being studied into a energetic state. Then along comes the probe pulse which records various optical properties of the matter. Normally the delay on the probe is varied in order to see how the relaxation occurs in the material.
Stopping power is the ability of a material to slow down charged particles (stopping power is only used for charged particles) passing through it by removing their energy through interactions with the particles in the material. In the early 1930s a physicist named Hans Bethe developed the Bethe formula for calculating the average energy loss over a distance within a material:
where me and e are the mass and charge of an electron; I is the average potential of the electrons in the material and n is their number density; and v and z both refer to the particle being retarded, being velocity and charge respectively (it should also be noted that this is a simplification for a non relativistic particle. Electrons and fast moving particles require corrections to the above formula). Continue reading Calculating Distance Over Which Charged Particles Pause
As is mentioned quite concisely in this page on waves, waves that meet each other undergo superposition and either constructive or destructive interference. This can be seen straight from Maxwell’s equations and so for a long time this was not questioned. It was eventually discovered that it is actually possible for photons to collide with each other in an effect called light by light scattering. According to particle physics such an interaction where two photons end up bouncing off each other is controlled by the W± bosons. Despite considerable effort being exerted from some very powerful lasers there has still not been direct observational evidence of elastic light by light scattering. Most of the information for its existence has come from parallel observations involving the anomalous values of the magnetic moment in electrons and muons.
The importance of the Sun in our solar system cannot be overstated. When considered, it is quickly realised that almost all sources of energy on this planet come from the Sun. For solar power it is obvious, but wind power is caused by the temperature gradient the Sun produces and fossil fuels originally started out as plants absorbing energy through photosynthesis. The two main exceptions are tidal power, which is a conversion of the Earth and Moon’s rotational kinetic energy, and geothermal power, which is gained from nuclear decay in the Earth’s core.
Ultra high energy cosmic rays (UHECR) are cosmic rays which are measured to have over 1018 eV of kinetic energy. Many of these particles exist beyond the Greisen–Zatsepin–Kuzmin limit, a theoretical limit based on the interaction of the cosmic rays above a certain energy threshold and the photons of the cosmic microwave background radiation. In essence if the particles were of too high energy they would have interacted and slowed down, but this restriction only applies over a certain distance. UHECRs are believed to be produced locally and so are not restricted by the limit. There is also a possibility that heavier nuclei may circumvent the limit also, but what particles make up UHECRs are still unknown. Despite this the mass compositions have been measured by the Pierre Auger Observatory in Argentina which is believed to show particles of higher mass than helium with an upper limit of about iron.
When a charge is placed in an alternating electric field it will oscillate according to the alternating force on it. If the time spent in each field direction is equal it should return to the same position after one cycle. This assumption is only true under the condition that the electric field is perfectly homogeneous and so there is no change of electric field strength as a function of distance. If the field is inhomogeneous, it will clearly have a gradient of field strength over the distance the charge oscillates. While the charge performs its half cycle within the stronger field it will experience greater returning force than the half of the cycle in the weaker section of field. This leads to, over one complete oscillation, an average force transporting the particle from strong to weak field. This is called the ponderomotive force and has the mathematical form:
Where e and m are the charge and mass of the particle, ω is the angular frequency of the field’s oscillations and the final term is the gradient of the electric field strength, E, squared (if E is small enough the action of the magnetic field can be ignored).