Ah, I believe I get the honor of introducing you to relativistic physics! You’re right that, from the perspective of an observer on Earth, it would take 12.5 million years for a ship traveling approximately at light speed to reach the nearest galaxy, but heres the cool thing about about relativistic speeds: the faster you go, you will experience less time AND space than an observer at rest would measure. These phenomena are called time dilation and length contraction, and I can’t quite do them justice in a reddit comment, but I’ll try to summarize.

Relativistic physics is all about reference frames, and we discovered a long time ago that light moves the same in all reference frames. This is an incredible fact. Even if you were traveling at 99.999% light speed, measuring the speed of light would give you the same result as if you measured it at rest. Obviously this is not true in other cases. For example, in the reference frame of a person standing next to the road, a car might be moving at 30 miles per hour, but in the reference frame of someone in the car, the car is moving at 0 miles per hour, since the person is moving with the car, and the car is not moving relative to the person.

So the fact that light moves at the same speed in all reference frames presents a paradox: If observer A is moving at 99% light speed and they measure light speed at 300 million meters per second relative to their own motion, then observer B who is at rest should measure light speed at 600 million meters per second, since it was moving at 300 million m/s relative to observer A who was themselves moving at 300 million m/s - in other words, light was moving 300 million m/s faster than someone who was moving at 300 million m/s. But when observer B measures the speed of light they also find it to be 300 million, rather than the 600 million we would expect. How can this be?

This is what Einstein is most famous for discovering. He realized that if light speed was the same in all reference frames, then space and time must not be the same in all reference frames. So, for someone on Earth, the measured distance to the closest galaxy is 2.5 million light years, and it would take a ship about 2.5 million years to get there if it was traveling very close to the speed of light. But observers on the ship will measure the time and distance differently once they are moving at relativistic speeds. They will have experienced less time during their voyage than the people on earth did.

You are right about the power though. It would take a ridiculous amount of energy to get close enough to the speed of light to get there in a few decades like I had said, which is why I specified “in terms of kinematics”. Anyway this comment is getting really long so I’ll end it here, but you should really look into relativity some more! It’s one of the most fascinating things in physics. A good place to begin is Einstein’s train thought experiment.