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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Why does a particles mass increase as it moves?

Because this is the way that nature works. Despite the seemingly mysterious nature of this concept, mass is simply the resistance a body has to being accelerated. In Newton's equation F = mass x acceleration, mass is the 'coefficient of inertia' which you multiply with a bodies acceleration to determine the magnitude of the applied force. Ernst Mach, a famous german physicist of the late 19th century, proposed that inertia is simply the measure of the gravitational force that all other bodies in the universe have on the single particle experiencing inertia. The quantity M in the equation is then related to the gravitational influence of all the other bodies in the universe upon the body being accelerated. Today, physicists prefer to consider that mass is not produced by 'distant matter', but by a local phenomenon having to do with the so-called Higgs field. This field has yet to be detected, experimentally, in the guise of the new particle called the Higgs Boson, but if found it would be the capstone for our most modern theory of force called The Standard Model.

Experimentally, the increase of mass with velocity has been confirmed to great precision; better than 1 part per million. The fact that all particle accelerators built since the 1960's work at all is a testament to the existence of this special relativistic effect. The fundamental reason for why nature should behave this way is lost in the philosophical discussions about why the physical world appears the way it does, and not some other way. If the mass happens to be that of the electron, because 100 percent of the electrons mass is contained in the energy of its electromagnetic field, some physicists have thought of the electron's mass as caused by the tangeling of its own electromagnetic field lines as the electron moves. Also, massless particles exist, such as the light photon, but this condition goes hand-in-hand with the particle speed being that of light.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.