Ultimate physics is Sun.
There are no objects in entire universe more powerful than Sund.
Suns work with nuclear fusion.
Humans have not been able to achieve sustained fusion.
After 40 years of trying hard.
Everything has limits.
Space travel is limited because of distance.
It takes 9 minutes for light to travel from our sun to earth.
The next nearest sun takes months for light to travel to earth
That is the problem.
Universe is spread out sparsely.
Spacecraft needs sun light to generate power.
There is no sunlight between our sun and next nearest sun.
There is unimaginable vast cold dark space between adjacent suns.
Stop thinking science fiction is real.
Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world.
--Albert Einstein--