So I had one idea. Let's say that you have a cube, it's mass is 1kg and you travel forward in time 10 years and bring it back. Now you have two cubes. Then you go forward again, this time 10 years - 1 Planck time into future. Then bring that cube back again. You can repeat this infinite number of times. You just have to go 1 Planck time before last time. You can create infinite mass. I know this is just stupid because time travel does not exist and discussing this doesn't make sense, but I wanted to share this. PS - maybe it's not that stupid, people discuss the grandfather's paradox and that is also not at this time possible.