[gizmo686]

Your equation is correct in a Newtonian universe, but does not consider relativistic effects.

Generally when we talk about an object's acceleration, we are talking about its acceleration in its own reference frame. By definition, an object's velocity in its own reference frame is always 0, so acceleration does not become more difficult (ignoring the fact that the surrounding matter will be impacting you at higher speeds). From the perspective of an outside observer who is not accelerating, time would appear to slow down on the accelerating object, and the outside observer would record a decreasing acceleration.

[readerrrr]

Fun fact, from the perspective of the observer on the ship, time outside the ship would also appear to slow down.

[ikeboy]

So why is it difficult to maintain a constant acceleration? Is 1g really that impractical right now?

[gizmo686]

1) Maintaining an acceleration requires fuel.

2) Even though you don't observe yourself moving at increasing speeds, you do observe the rest of the universe moving at increasing speeds. This means that small particles impacting your ship will hit it with increasing energy.

[snowwrestler]

I'm being accelerated at 1g right now and I'm about 40...so in another decade or so I should be able to get to anywhere in the universe!

Jokes aside, there's no fuel source we know of that would allow a ship to accelerate continuously for 50 years at 1g.