Drill a small borehole a few miles towards the centre of the earth. Hollow a fibre optic strand to particle size. Hang the thread straight down the borehole. Insert a particle into the centre of the strand. Shine light into the strand. Will the photons accelerate the particle? Will the particle, because of relativity, no matter how fast it goes, even close to C, see the photons ahead of it as travelling at C, allowing the particle C amount of acceleration?
The strand is not laid horizontal to avoid the effects of gravity distorting the particle's path.