Hi Graham, DrB, I did say _if_ there's no limit... I agree that there is probably not infinite energy in the universe, so it would be impossible in practice to simply keep pumping in more energy. Heh, in practical terms, I think the fact that there isn't infinite energy to draw upon would be the least of our worries, but the problem with what DrB is thinking about, as I see it, is the discontinuity in the curve that must happen when a hypothetical upper sub-lightspeed limit is reached.

It does seem intuitive that (c - one of the small planck units) should give a speed that is the maximum sub-lightspeed possible but because we're dealing with speed we have to use both the Planck length and time units, which gives us c. If you try to use only one of them i.e. length, the time factor can be anything, and likewise, if you try to use the Planck time the length can be any value.

I'm afraid that c-(c*1.6 x 10-35) won't work because you're trying to subtract a distance from a speed - it's like asking how fast is 10 m.p.h - 3 feet. Similarly, you can't ask how fast is 10 m.p.h - 3 seconds.