Making matter out of energy 1711 Collisions make particles

The equation E = mc2 gives the rate of exchange when matter is converted into energy. It is well known that such conversion can occur, and the term 'nuclear energy' is now part of everyday language. What is less well known is that the opposite also occurs. Basically, if enough energy is concentrated at one point, matter may be created out of this energy. In practice this only happens at the nuclear or sub-nuclear level — when, for example, a proton of high energy collides with another proton or neutron.

In the everyday 'household' world, we know that if an object collides with another object, its energy must go somewhere. For example, if two cars travelling at 100 km/h collide, both cars will be broken into pieces, and fragments may be thrown in all directions. The kinetic energy of the cars is converted into the kinetic energy of flying debris, work is done in distorting metal, heat is generated, and the cars may even go on fire. For a plane which crashes at 1000 km/h the devastation is even greater. But what happens if a proton travelling at almost 300,000 km/s crashes into a stationary proton? Protons cannot be distorted or broken into fragments, so where does the energy go?

In this case matter appears, as if 'out of nothing', in the form of a jet of particles, mostly in the forward direction. These newly born particles may be the familiar constituents of matter such as protons and neutrons (accompanied by their anti-particles), or 'new' particles which are unstable and decay within a tiny fraction of a second.

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