Okey then. How fast would a object be traveling in constantly accelerating at the speed UA suggests after 1 year? I'm very aware that a object with mass can reach the speed of light. But requires explinentianly more energy the closer you reach the speed of light. Where does this constantly increasing source of energy come from?

I thought you were good at maths?

You need to consider special relativity ...

The equation to calculate your speed is an asymptote. You never reach light speed a constant acceleration of 9.81m/s as counter intuitive as that may sound.

The relevant equation is v/c = tanh (at/c). Since tanh(at/c) is always less than 1, you can never reach the speed of light.

According to the theory of special relativity, earth accelerating at one standard gravity (9.80665 m/sē) will have the speeds shown below.

T (days), v/c

10, 0.028255

20, 0.056465

30, 0.084586

40, 0.112572

50, 0.140380

60, 0.167969

70, 0.195298

80, 0.222326

90, 0.249017

100, 0.275335

110, 0.301246

120, 0.326721

130, 0.351729

140, 0.376245

150, 0.400245

160, 0.423708

170, 0.446617

180, 0.468954

190, 0.490707

200, 0.511865

210, 0.532420

220, 0.552366

230, 0.571698

240, 0.590416

250, 0.608520

260, 0.626012

270, 0.642895

280, 0.659176

290, 0.674862

300, 0.689961

310, 0.704482

320, 0.718437

330, 0.731836

340, 0.744692

350, 0.757019

360, 0.768829

1 year, 0.774818

400, 0.811193

500, 0.888158

600, 0.934877

700, 0.962469

2 years, 0.968315

800, 0.978500

900, 0.987726

1000, 0.993007

3 years, 0.995924

4 years, 0.999482

5 years, 0.999934

So it will never get to the speed of light. It will tend towards light speed, but never ever ever get there.

So FET is sound in this respect too.