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You can only accelerate your spaceship in the otherwise empty universe by ejecting either matter or radiation then it would not be the only thing in the universe and the normal laws would apply
Imagine a universe devoid of matter in which we introduce a spacecraft. We accelerate it.What is its 'potential energy' or momentum, or 'relative mass'How will you measure it?And if you can't, does it exist?==Now try to define a 'speed'.Can you?=
There is one point to be made though, you're importing reference frames from this universe into the 'empty one' when you use those definitions, and I'm not sure they are the same? But that's my fault not thinking it through before posting..