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    Speed and mass

    We know it from measuring the rest mass of a free electron - "free" means "unbound". In contrast to the photon, the electron does not move with a "natural velocity". Arbitrary speeds are possible - you should also remember that speed is relative, i.e. it also depends on the state of the...
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    Speed and mass

    The 511 KeV correspond to the rest mass of the electron. When you accelerate it, it gains additional kinetic energy (1/2)*m*v^2 You may accelerate a particle which is lighter than the electron to a higher speed such that the above term for the kinetic energy will yield the same number...
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    Speed and mass

    KeV is "kilo electron volt": it is the amount of energy which an electron gains through acceleration by a voltage of 1000 V; see e.g.
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    Speed and mass

    In principle, magnetic fields are helpful to measure the mass of a charged particle. A magnetic field exerts the Lorentz force on a moving charge; the resulting acceleration may be measured and is inverse to the mass of the electron. In practice, Penning traps are often used...
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    Speed and mass

    Unfortunately, in the history of physics, the notion of a "relativistic mass" had been introduced. Nowadays, this is rather regarded as a kind of energy which increases inertia. "Relativistic mass" is not a scalar number as we may expect but a tensor: "relativistic mass" would vary with the...
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    Faster than the speed of light

    Agreed ... in order to illustrate your comment, I refer to Gell-Mann's book: "The Quark and the Jaguar" (
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    Faster-than-light newbie question

    According to the principle of relativity, you can of course do this. The rest system of a ship travelling with a constant speed is an inertial coordinate system which can by no experiment be distinguished from any other inertial system. So, you can do your walk there just the same way as in...
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    The WHY of speed of light vs. the FACT thereof

    The value for the speed of light in vacuum is a historical artefact. It is the consequence of an unfortunate choice of units. In better chosen systems of units accounting for the equivalence of space and time, c is without dimension having the value 1.