how was speed of light measured?

[J.C.MacSwell]

Not proposing anything but a thought experiment. But sure, "wall off half the universe." I'm walled off from at least half the of it right now, actually, just from sitting on a planet.

 

Of course, there is still radiation from the "wall" itself, which could never be completely eliminated. But in any case the point stands. It's not your velocity that is the problem (inasmuch as there is no such thing as absolute velocity), but the fact that something else is approaching you at a very high relative velocity and thus radiating especially nasty photons at you. And that seems practically unavoidable in our own neighborhood once we ourselves are moving at very high velocities relative to the Earth (and, consequently, the cmbr).

 

Or is it? You're moving at arbitrarily high velocity relative to cmbr. An arbitary number of shield walls (at arbitrary distance from one another) is moving ahead and with you, each slightly slower than the last, so each is approaching both its adjacent walls at an arbitrarily small velocity. Problem solved?

 

Well, maybe you break the World Record! There may be no CMBR left with all the energy consumed for the project.