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# starship-design: Doppler trouble?

```Hi Antonio,

>A few doubts regarding radiation shielding and fractional lightspeed.
>Would there be any considerable effect from doppler-shifting of impinging
>radiation - taking into consideration relativistic effects?.

If we're talking about a carry-the-fuel-with-us design, then the maximum
cruise velocity is less than 0.3c (unless you want to carry thousants or
million times more fuel then ship). For such "low" velocities the doppler
shift is so small that blue still looks blue, except that it might look just
a little bit greener. Even for high velocities like 0.9c you don't have to
worry a lot. Only beyond 0.99c things may become interesting.
But in space there are X-rays, on the Earth's we aren't bothered by these
since the atmosphere absorpts most of it. In space we have to make our own
shielding against it.

>Ditto for particles?

Particles are more or less floating in space with no huge velocities
relative to the frame where the Sun is at rest. However if we're going to
meet them at 0.3c these particles will look similar to the particles that
are radiated from radioactive material. In fact if we go faster than 0.3c
the radiation will be more energetic than that of normal radiactive material.

>Would that make the ships hull look "harder" to the rad than to itself -
due >to mass increase?

Your "harder" relates only to the frame of the particle. It sees a big
"wall" coming at it. It observes the wall being thinner due to length
contraction. The number of atoms in the shield stays the same. So the
particle observes the shield as having the same number of atoms in a smaller
volume which is equivalent to the shield having a bigger density.

Don't make the mistake to using this denser/harder in the ship's frame. It
likely would make you conclude that you can make the shield thinner, which
isn't true.
Actually from the ship's frame, the particles look harder. (Therefore we
should not use E=0.5mv^2)

Timothy

```