This was published in Proceedings of the Royal Society. The lack of any experimentally verifiable predictions is a problem. That aside, I wonder what would motivate the assumption that the product of velocities is limited to c^2? Maybe they just used something that would result in a "nice" mathematical solution?
“Einstein started working from information where the relative velocity is zero – what we knew about, such as rest mass, kinetic energy and so on – and then extrapolated what is known in the Newtonian world for velocities lower than c.
“Our thinking was: how do we make use of the essential essence of Einstein’s theory for velocities above c?”
Mathematically, what the mathematicians assumed is that for infinite relative velocity, there is a fixed relationship between the velocities of the two observers: where u is the first observer’s velocity, v is the second, the product of the two velocities is always c2.
“What we have is an equivalent theory [to Special Relativity] that applies for velocities beyond the speed of light. That theory is different from Special Relativity, but it has many of the same characteristics.
And readers with an interest in either physics or maths will be delighted with the vital assumptions: there has to be one, and only one, speed of light; and in all cases, a mathematical singularity occurs at the speed of light.
“If you believe what we’ve done,” Professor Hill said, “there can only be one speed of light in a universe. If there was a second speed of light, our mathematics wouldn’t work.
To get from the theory to any practical test is another matter entirely, and Professor Hill freely admits he doesn’t know how that might be achieved.