> we'll expect people to live hundreds of years by the time we can send a ship to alpha centauri.
Even hundreds of years is not a realistic life expectancy to reach the nearest star and requires not just an enormous overabundance of energy, the fuel source has to be available at the midway point so that you can decelerate. Humans at another star without discovery of faster than light travel mechanisms (mainly wormholes) seems purely in the realm of science fiction.
For comparison, Voyager would need to travel 1666x further in the same time frame. It’s not nihilistic to suggest that 3 orders of magnitude improvement in our lifetime isn’t likely. Oh and Voyager is basically on life support having shut down most systems.
I didn’t say it’s outright impossible or not worth pursuing. Just that believing it’ll happen in our lifetime is unrealistic precisely because of the currently known laws of physics and the orders of magnitude of technological achievement necessary to attain more than 10% the speed of light for anything that has any mass (and again assuming all you want to accomplish is a flyby at relativistic speeds).
But that's only the start, any advancements in making sattelites on a chip could massively reduce the weight and therefore power requirements. Even with all losses accounted for. I believe this could be doable in the next 50 years. You just need to build the most powerful laser anyone has every built and power it for as long as you can.