It's a fantasy as long as you are still living under the tyranny of the rocket equation.

I don't think anybody thinks such a ship is possible with chemical rockets.

It's worse than that. It's not possible with any type of rocket. If you are accelerating by pushing mass in an opposite direction then you'll never build a ship that can accelerate at 1G for years at a time.

Unless we invent a reactionless drive the idea of traveling between solar systems remains a pipe dream.

Why? Can you expand?

Just need Astrophage /s

Ha, got the Hail Mary ref.

Yep. At 1g you could literally go to the edge of the visible universe in less than 50 years (assuming you are targeting the edge as defined at time of departure).

If this is true (I'm too stupid to do the math), then consider my mind blown!

It is technically true, but requires all kinds of miracles, not the least of which is avoiding every trace of matter/antimatter between here and there.

Oh, and all light from the universe is now gamma radiation focused intensely ahead of the spacecraft cooking the whole thing.

An atomic nucleus sitting in deep space becomes an apocalyptic collision at 99.99% light speed.

You need to spend equal amounts of energy and time on both acceleration and deceleration. It doesn’t help to go fast if you can’t stop at where you are going.

Check out Pohl Anderson's "Tau Zero" which examines precisely this scenario. A starship using bussard ramjet needs to find an "empty" section of space free of interstellar hydrogen so they can fix their engines and decelerate without getting fried. This requires them to continue accelerating so that time dilation effects will allow the trip within the crew's lifetime. That's the setup, not the plot.

Depends what the cargo is.

True - if it's a warhead, the acceleration is a bonus.

Well, if you just want to destroy a planet at near light speed, they would never see it coming and deceleration is unnecessary. Scary thought though.