Relativistic compressor "paradox" Let's say we have a metro tunnel that is a circular loop with 1 m x 1 m rectangular cross section. The tunnel is 100 m long. There's a 50 m long train in the tunnel, the train fits snuggly in the tunnel. And there's air in the tunnel, at 1 atm pressure, and there are air channels connecting the tunnel to the athmosphere. The train accelerates from 0 to 0.86 c, and becomes 25 m long, and 25 cubic meters of air is sucked into the tunnel through the air channels. Now we plug the air channels and stop the train smoothly, the train becomes 50 m long, and air pressure increases in the tunnel, pressure will be 1.5 atm at the end. What we did there was: We accelerated a train and some air to 0.86 c, then we decelerated the train and the air to zero velocity. AND we produced some compressed air. So the problem is that compressed air that we got for free.