[Harv72b]

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Why the limit on the speed? As long as you have enough fuel, you can keep accelerating forever.

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This is not thought to be true. As I understand it, most scientists believe that the speed of light is the universal "speed limit". There are theoretical ways around this, which I mentioned in my last post, but I don't believe it's possible to simply "accelerate forever", even in a vacuum. There's also the little matter of various objects which exist in space, and how to avoid them as you get closer and closer to the speed of light.

And then there's the logistical problem. Beyond a certain point, any added acceleration you gained by carrying extra fuel would be offset by the increased thrust required to get the added mass moving. This would be true even if the ship were built in orbit. You would either have to design a ship which was pretty much all fuel (and find enough fuel to fill it), or else come up with some form of propulsion which could refuel itself during travel. Since space is notoriously devoid of things like rocket fuel, this would be difficult to do (though not impossible; I've often wondered about a "space jet" which could collect various subatomic particles traveling freely through space and use them for propulsion, even using light itself as fuel. I think that's a ways off from being practctally possible, though).