INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS B: ATOMIC, MOLECULAR AND OPTICAL PHYSICS J. Phys. B: At. Mol. Opt. Phys. 38 (2005) S741–S752 doi:10.1088/0953-4075/38/9/020 The impact of Einstein’s theory of special relativity on particle accelerators Tomas Plettner, Robert L Byer and Robert H Siemann E L Ginzton Laboratory, 445 Via Palou, Stanford, CA 94305, USA Received 27 January 2005, in final form 1 March 2005 Published 25 April 2005 Online at stacks.iop.org/JPhysB/38/S741 Abstract We describe the consequences of the theory of special relativity on particle accelerators and present a historical overview of their evolution and contributions to science and the present limitations of existing accelerator technology. We report recent results of our experiment where we succeeded in accelerating relativistic electrons with visible light in vacuum. The experimental demonstration is the first of its kind and is the proof of principle for future linear laser-driven particle acceleration schemes in vacuum that may lead to the realization of electron–positron colliders beyond the TeV scale. Introduction Einstein’s initial motivation for postulating the theory of special relativity was to explain the observed and, at the time, puzzling behaviour of electromagnetic phenomena under transformation of moving coordinate systems. But he succeeded in formulating his theory in a far more general manner and thus revolutionized the entire body of physics. As a consequence, he modified the basic classical laws of mechanics formulated by Newton and for the first time equated mass to energy. The predictions made by this new theory are counterintuitive to our natural thinking of space and time and are at first difficult to accept as real physical phenomena. It is difficult to abandon the so-appealing concept of Galilean relativity and the simple addition of velocities. However, one century since its inception numerous physical phenomena are known to us that can only be explained by Einstein’s theory. One very striking example of special relativity is the observed kinematics of accelerating particles in particle accelerators. Einstein already predicted that accelerating electrons gain arbitrarily large kinetic energies as they approach the speed of light, which he argued they can never reach [1]. Thus, once a particle’s speed is close to c, its kinetic energy increases by its apparent increase of mass and not by a change in speed. In terms of its total energy the 0953-4075/05/090741+12$30.00 © 2005 IOP Publishing Ltd Printed in the UK S741