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Victor you have made the same subjective observer mistake that is so common when discussing Special Relativity, you focused on the outside observer, when the subjective ship board observer is what matters.
The distance and C as a limit do not change, in other words the trip to Alpha Centauri takes roughly 5 years each way assuming we overcome the problem of hyper-acceleration, inertial dampening and an energy supply, that makes possible getting to near light speed so quickly. The inhabitants of the ship cannot perceive themselves as exceeding the speed of light, which means that they are the ones that take 5 years (actually much more with acceleration and deceleration as well as the fact that under the best of circumstances we are talking about .99 C, not C itself or some very strange things happen in the transition from matter to what constitutes a soliton energy particle/wave problem.
The perspective of outside observers is still many times greater than the minimal decade for a round trip because of the
Paradox of Simultaneity and
Time Dilation effect begins at the moment of acceleration and is protracting the observers time scale of the relative perspective to the trip at a rate the increases the faster the velocity of the ship. One of those weird effects predicted by the math of what happens as the ship would actually achieve light speed is that its density would become infinite and on board relative time would stop with respect to the outside observer.
Sound familiar?
This is essentially an event horizon phenomenon similar in principle to what happens around a Black Hole. That is why CERN is able to create mini black holes with its proposed use of the Particle Accelerator.
I understand the principle of 1 G acceleration AND deceleration but those two combined transition years alone, at each book-end of the trip would be observed on Earth as multiple decades at the very least and the math describing the acceleration/deceleration period of a one year experience on board the ship is not a trip distance of 1 L/Y but considerably less. In other words total on board trip time is even more than 5 years and closer to a decade.
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It has long been argued that a mini biosphere generational ship, preferably with a viable cryonic or other form of suspended animation, is the only practical initial approach to this endeavor. Rejuvenation tech would only be an added advantage, as well as a few other tricks I am saving for a novel.
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As for particle impacts I agree that is essentially a moot point because of the combined ability to generate a magnetic bottle field effect as well as for the need to shape the bow wave effect a hyper ballistic ship would generate to improve its efficiency. The bow wave front the ship generates simply as a consequence of velocity would then become akin to the prow of a ship slicing through a sea of star dust.
As for the amount of energy required for this kind of endeavor it still exceeds even our current ventures into fusion technology but might be possible with an antimatter drive or through a still unrecognized manipulation of gravity, which may be become more apparent as we resolve the physics of gravity more precisely over the coming decades.
(Victor I quoted you multiple times and I apologize for leaving them blank but as it was the result of a glitch and I want the engineers to see what happened before I repair the quoted text)