© Left: CERN, Right: Claire Cramer.
In the collision shown on the left, electrons and positrons collide head-on in the ALEPH detector at CERN, producing a shower of other particles. The electrons and positrons are point particles, and the collision energy is the sum of the energies of the particles that hit one another. The collision of two strings sketched on the right is fundamentally different because the size of a string grows with its energy. A highly energetic string will be very long. When two energetic strings collide, the collision will only involve a small portion of each string, so the string collision is softer than the point particle collision. (Unit: 4)