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If a future astronaut travelled to Alpha Centauri at a significant percentage of light-speed? Apart from increased blue shifted radiation from their direction of travel - how would they experience other cosmic radiation?

If the trip took a subject several times faster than an observer on earth. Would the astronaut get less or more time / exposure to cosmic radiation?

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Note that if your astronauts have a relativistic velocity then random protons floating in space are ionizing radiation when they hit them, and you don't want to ask about dust grains. So, it depends on the density of gas and dust in the way, how fast you are going, how much mass you can afford to put and the front of the vessel and if you have any of the spiffy magnetic field doodads that SF authors like to assume so that every expedition doesn't have to travel slowly in a asteroid or comment nucleus. –  dmckee Aug 11 '12 at 2:52
    
@dmckee not "comment", but "comet" :), the ones that have tails . –  anna v Aug 11 '12 at 5:01
    
So - if I understand this correctly - their internal cross space/time section of proton exposure stays proportional to the external space/time volume they are moving through rather than the internal relativistic one which is slower relative to the outside space. –  velniukas Aug 11 '12 at 7:58

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While special relativity undermines our intuition of simultaneity, shifting frames of reference does not alter whether or not an event happens. In fact, this is the basis of many thought experiments. So the number of collisions with particles, massive or massless, experienced by the astronaut is the same to all observers. True, the time elapsed during the journey can be arbitrarily short from the astronaut's point of view; this just means the average flux of cosmic radiation hitting him can be arbitrarily high. You can think of this as length contraction increasing the density of the radiation field in front of him.

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