Any form of propulsion that obeys Newton has
hard limits to it's space travel potential. Even spitting out single particles at near the speed of light for the most efficient way to generate thrust per unit of expelled mass still constrains you to the tyranny of the rocket equation, which puts
hard physical limits on you.
https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation
The rocket equation also underestimates any craft that gets over a fraction of C
Currently, we have no evidence that reactionless propulsion is physically possible and one existing would directly contradict the conservation of momentum.
"technological development" isn't a magic word or force of reality. "Technological development" is the pay off of immense engineering investment and discovering new phenomenon, but every axis you can possibly put effort into engineering and optimizing has a finite limit at some point, and there are finite new phenomenons to discover.
The entire past 100 and some years of technological development has been basically down to mastering the electromagnetic force. But, we've basically used up the novelty that was there, and there is no new second electromagnetic force to discover. In fact, the nuclear force was also discovered and tapped out relatively quickly.
A great example of this is the elements. All evidence points to the outcome that the elements we can build stuff out of right now are the only elements you will ever be able to build anything out of. All artificial elements, even ones that are relatively "stable", have half lifes that preclude building stuff out of them, and there is no evidence that it is possible to modulate the rate that an unstable atom decays. So no "exotic" elements that could magically power space ships or anything will exist.
Intergalactic travel of humans is implausible unless you get into pretty radical transhumanism, or assume it's possible to perfectly maintain a biological human forever somehow, including brain functionality.
Brain uploads are another thing that people don't seem to recognize are radically more difficult and close to impossible. "Scanning" a brain is treated as an engineering problem, but it might not be. Every sensor relies on a physical interaction, most of them based on electromagnetic energy. How do you make an electron or photon or something interact in a measurable way with a cell deep inside someone's brain without that particle interacting with all the identical matter in the way or cutting open and taking apart that brain? Well, thanks to the mastery of the electromagnetic force, we have MRIs which kind of do in fact do that. But even if we had a magic MRI machine for example with infinite resolution (yet another thing that has fundamental limits), that would only let you look at molecules with with hydrogen, so you wouldn't be able to survey, say, the ion content of brain cells directly. If you are not aware, ion gradients are fairly important in human cell behavior.
Nevermind that scanning and uploading someone's brain, if it were possible, does not transfer the original conscious experience to the computer. A new copy may go on in a digital world but you still die.
