How large would such a construction need to be to accelerate 100 tons at 1g? Maybe someone could do the math for us. I assume it's on the order of dozens/hundreds of miles long per dimension and would be completely infeasible compared to just using an engine with high thrust to begin with.

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Here's some rough math.

From wiki, assume a typical ion engine can produce 150mN of thrust from 4,000 W of power input.

Using a space station solar panel as an example of solar collection in space, each space station solar panel is 420 square meters in size and produces 31,000 W of power.

One space station solar panel would then provide (31,000 W / 4,000 W) * 150 mN = 1,162 mN, or .001162 N of force.

The force required to accelerate 100 tons at 1g requires 996,402 Newtons of force.

To generate that much force, you would then need 996,402 N / .001162 N = 857,488,812 space station solar panels worth of power.

As one space station solar panel is 420 square meters, then that requires 857,488,812 * 420 square meters = 360,145,301,040 square meters of solar panels.

Assuming square construction, each side would need to be 600,121 meters, or 373 miles long.

I assure you, just using high thrust engines makes infinitely more sense than building a pv-based ship scaled up so far that the ship's dimensions are nearly 400 miles long on each edge. At least for any time soon ..