While the speed of light is the "ultimate speed limit" for any particle.. Is there any upper limit to kinetic energy?
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2Mass of the universe. – CuriousOne May 10 '15 at 05:24
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@CuriousOne So, what's that limit for the mass of the universe? – Volker Siegel May 10 '15 at 06:57
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2Theoretically? None. Practically? Whatever it is. – CuriousOne May 10 '15 at 07:19
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1There is actually no theoretical limit for the mass of the universe. – Vamsi Gorugantula May 10 '15 at 16:56
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In principle, no: a particle may leave a reaction with any kinetic energy you can imagine.
However in practice, there's a limit known as the GZK cutoff. A baryon (proton or neutron) with energy above about $10^{19}\,\rm eV$ will see the cosmic microwave background blue-shifted so strongly that the baryon can scatter from the background photons to produce pions. The consequence is that any particle with energy above this limit must be "local", since it has not yet lost its energy. ("Local" is relative, however; the GZK horizon contains many galaxies.)
I am not sure whether there is a GZK-like limit on the kinetic energy of photons or electrons. For macroscopic objects, Randall Munroe discusses this sort of limit for baseballs and meteorites. But in general, whether there's a limit to a particle's kinetic energy depends on how close you are to the source of your accelerated particle: close, no limit; far off, limited.
rob
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it seems to me c is a limit to v, and kinetic energy is defined as 1/2mv^2, so it approaches a limit as v approaches c, quantum mechanics apart. – anna v May 10 '15 at 03:27
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10@annav Kinetic energy is not defined as $\frac12mv^2$; that's a useful approximation when $v\ll c$, which isn't the regime we're interested in here. – rob May 10 '15 at 03:30