#Random thought of the day: "I wonder what the total mass of an Arthur C. Clarke-type space artefact made of solid osmium would be?" [1]
So, take a #massive cylinder - rotating, natch, you want to use it for time travel or other #causality violations, why else would you have one? - say, 1000m in diameter, and pi times as long (why not?)...
If I've done the math right, the answer is approximately 55.74 billion tonnes.
#Curiosity sated.
[1] #Osmium is the densest natural element, at a little over 22.5g / cm^2, vs. 1g/cm^2 for #water, or 7.8g/cm^2 for #iron.