Light is one of the important natural ingredient responsible to make the life go at least on Earth and nature has devised its own engineering to convert light into matter by living species. Light has always been a hot topic of research for scientists from centuries. Production of light is an energy/radiation release in the visible region of electromagnetic spectrum. All atoms and molecules that make up matter typically emit light at characteristics energies. Stimulated emission occurs when matter in an excited state is perturbed
by a photon of light and gives rise to a further photon of light, typically at the same energy and phase as the perturbing photon. With the coming of Einstein famous mass-energy equation (E=mc2), everyone came to know about the interconversion of mass and energy. Turning matter into light, heat, and other forms of energy is nothing new, as nuclear fission and its applications have already spectacularly demonstrated. But the reverse i.e. conversion of energy into mass was observed through a pair production process. The process of pair production can be described as the conversion of light particles (i.e., photons) into one or more massive particles. The most common, natural and well-studied case is the one where two photons convert into an electron–positron pair which was supported with a theoretical explanation by Gregory Breit and John Wheeler process. Gregory Breit and John Wheeler proposed the mechanism in 1934 by using theory of the interaction between light and matter known as quantum electrodynamics (QED). Since the discovery of Einstein’s energy-mass equation, conversion of energy into mass has been a hot topic of both science fiction and research.
Theories describing light and matter interactions have always been at the fore front of physics research. However, pair production process with photons is not that simple because of momentum conservation laws, the creation of a pair of fermions (matter particles) out of a single photon cannot occur. However, matter creation is allowed by these laws when in the presence of another particle (another boson, or even a fermion) which can share the primary photon’s momentum. Thus, matter can be created out of two photons. The law of conservation of energy sets a minimum photon energy required for the creation of a pair of fermions: this threshold energy must be greater than the total rest energy of the fermions created. To create an electron-positron pair, the total energy of the photons, in the rest frame, must be at least 2mc2= 2×0.511MeV=1.022MeV(mis the mass of one electron and c is the speed of light in vacuum), an energy value that corresponds to soft gamma ray photons. The creation of a much more massive pair, like a proton and antiproton, requires photons with energy of more than 1.88 GeV (hard gamma ray photons).
Experiments to create matter from light using advanced laser facilities are going for the last few decades. The system involves two high-power laser beams, which are being used to create the photons of light to be smashed together. One of the photons has about 1000 times the energy of photons that produce visible light, and the other has 1,000,000,000 times the energy. The laser beams are focussed on two separate tiny targets inside a target chamber, which contains complex optics used to focus the laser beams and magnets used to deflect the charged particles. It is the charged positrons coming off the collision that the team will look for to confirm if the process was a success. The ATLAS experiment at the LHC observed two photons, particles of light, ricocheting off one another and producing two new photons. This year, they’ve taken that research a step further and discovered photons merging and transforming into something even more interesting: W bosons, particles that carry the weak force, which governs nuclear decay.
This research doesn’t just illustrate the central concept governing processes inside the LHC: that energy and matter are two sides of the same coin. It also confirms that at high enough energies, forces that seem separate in our everyday lives—electromagnetism and the weak force—are united.
Inside CERN’s accelerator complex, protons are accelerated close to the speed of light. Their normally rounded forms squish along the direction of motion as special relativity supersedes the classical laws of motion for processes taking place at the LHC. The two incoming protons see each other as compressed pancakes accompanied by an equally squeezed electromagnetic field (protons are charged, and all charged particles have an electromagnetic field). The energy of the LHC combined with the length contraction boosts the strength of the protons’ electromagnetic fields by a factor of 7500. When two protons graze each other, their squished electromagnetic fields intersect. These fields skip the classical “amplify” etiquette that applies at low energies and instead follow the rules outlined by quantum electrodynamics. Through these new laws, the two fields can merge and become the “E” in E=mc². Though a small amount of mass can produce a huge amount of energy (because of c2) but we will need to start with a huge amount of energy to produce even a tiny amount of mass. At extremely high energies, electromagnetism combines with the weak force and just as photons carry the electromagnetic force, the W and Z bosons carry the weak force. The reason photons can collide and produce W bosons in the LHC is that at the highest energies, those forces combine to make the electroweak force. Both photons and W bosons are force carriers, and they both carry the electroweak force and this phenomenon is really happening because nature is quantum mechanical.
If creating matter from light experiments become a success, then scientists will make it possible to convert light or energy into matter. According to first law of thermodynamics, energy can neither be created nor destroyed, that means any type of energy can be converted to mass back again. This will not only open new vista of applications and research but also help to understand the process that was important in the first 100 seconds of the universe and that is also seen in gamma ray bursts, which are the biggest explosions in the universe and one of physics greatest unsolved mysteries.
- Further reading
- Ferlic, Kenneth. “The Phenomenon of Pair Production.” 2006. (July 14, 2010) http://ryuc.info/creativityphysics/energy/pair_production.htm
- Lipson, Edward. “Lecture 17: Special Relativity.” Syracuse University. (July 14, 2010) http://physics.syr.edu/courses/PHY106/Slides/PPT/Lec17-Special-Relativity_2.pdf
- Nave, Rod. ” Relativistic Energy.” HyperPhysics. (July 14, 2010) http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/releng.html
- “Newton’s Dark Secrets: Magic or Mainstream Science?” NOVA. November 2005. (July 14, 2010) http://www.pbs.org/wgbh/nova/newton/alch-newman.html
- “Pair Production.” City Collegiate. (July 14, 2010)