kanyalsingh

I don't forsee it being too difficult to get such a package relatively close to light speed, with a rather loose definition of "relatively" (and no pun intended). Half light speed? 75% light speed? It's certainly better than what we could do with a spaceship containing a large and fragile human.

you supplied that energy in creating that pressure. If you increase the pressure, work is being done somewhere, or some other conversion is taking place. Energy can be stored in a number of forms, and you have to account for all of them.

I think a sensible definition of the amount of matter (if you wanted to count it) is the number of fermions. Since anti-particles are given negative fermion numbers and fermion number is conserved, I think that matter is indeed conserved, then. Alternative measures of the amount of matter already mentioned are the amount of energy which is also conserved, or the amount of mass