Travelling at the speed of light?[Question]

[Alex K]

And people tend to forget the braking power required - negative acceleration... what would that do to the time reference?

If my relativity is correct, that changes nothing if the acceleration and deceleration are near instantaneous. Only the velocity during the trip is important for the calculation of the proper time - if you could hypothetically brake instantaneously just before Andromeda, you would have exactly the same respective times there and on your ship upon arrival as if you kept going and read off the clocks as you passed Andromeda at full speed.

But, in a "realistic" scenario with finite acceleration and deceleration, it makes a difference: namely if you e.g. want to have speed 0 at your destination, you realistically have to start braking quite some time before. You will then not spend the entire trip at maximum speed; This will reduce the effect of the time dilation and the deceleration and acceleration process might cost disproportionaltely more on-board time because of the low dilation factor. It depends on how quickly your hypothetical cannon and brakes get you up to speed and back down. The better they are, the closer you will get to the ideal sqrt(1-v^2/c^2) factor that you have if you start out passing Earth at full speed and never slow down until you're past your destination.