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Good points! In defense of my (fake) physics conjecture, let me make a couple of extrapolations from real relativistic behavior (with the caveat that I am kind of an armchair relativist) @FiveLate The increase in a particle's rest mass is a percentage of its "current" mass, which under relativistic acceleration can get big fast. The result of this compounding mass increase is where the "infinite mass" result comes from-- the integral diverges at light speed. However, if you stored just the "extra" mass increase from acceleration, the total mass increase wouldn't suffer this self-compounding effect and would therefore be much more manageable (non-infinite, anyway). Now if you further reduce your mass below whatever it is at rest, the "extra" amount you have to store can get pretty small. @Calderis To my understanding, the increase in mass comes from the acceleration, not actually being at the speed itself (this is of course can be seen as a relativistic effect, and is related to time slowing down for the traveler, etc.-- the changing of the rate of time results from the acceleration rather than the speed directly). Simply being at a certain speed doesn't confer extra mass to my understanding. Slowing down might also be view as an acceleration of course, and could have mass-gaining implications of its own, but it could be dealt with as in #1. Of course this is all hooey, the impossibility of FTL travel is the starting-point assumption of relativity, not its result. Therefore, we have to break the system in some way in order to have it (excepting capital-G Godlike powers to effect an Alcubierre drive). Really, my strongest point in believing FTL in the Cosmere arises from playing with mass is from the incredibly strong hint given in BoM, rather than in my hand-waving physical argument that even I know is full of holes. FTL has to be impossible if relativity holds. Therefore the question is: in what way is Brandon planning to violate relativity? I think the BoM scene between Wax and Khriss gives an indication on which way he's leaning.

I think a big hint might have been given in Bands of Mourning, when Khriss (I think?) asked Wax about how modulating his mass changed his speed. The example given then was of increasing his mass, which slowed him, but the obvious implication is that it works the other way too. The importance of the characters involved, and the obvious lesson in allomantic-affected physics, seems like a pretty big easter egg/foreshadow plant. The ability to control one's mass is especially important when approaching the the speed of light, as the object being accelerated actually gains mass (from a resting observer's viewpoint), which assymptotically retards further acceleration. Basically (to my understanding), the observation from particle accelerators is that an object's mass approaches infinite as its speed approaches that of light, which makes further acceleration more and more difficult. BUT... if you could somehow siphon off (or better, "store") the extra mass imparted by acceleration, then, yeah I suppose you could make the case that you could get faster than lightspeed. Potentially multiple times faster, as dividing your mass your mass by n would either n-times your speed (by conservation of momentum) or multiply it by the square root of n, depending on which conservation the author decides is more fundamental (I'm guessing momentum). And of course if you can reduce mass to near zero, you get near-instantaneous travel, which is a convenient thing to have in fiction. Plus near-infinite "stores" of mass, which could be useful in a number of ways. Note that in his comment, Peter didn't say that the Tor theory was wrong, only that the reasoning was a bit sloppy. With all respect to Peter, I wonder if it's possible he's obfuscating a bit? One last point-- accelerating by storing mass wouldn't be very useful for Wax in atmosphere, as the reduced mass would likely make him more susceptible to drag, so it's plausible to have them "discover" this rather obvious trick only later in the series...