Either A) your brother is wrong, B) Mach-9 does not exist in whatever he was talking about, or C) in whatever he was talking about, mach-9 had some kind of disadvantage, such as requiring more fuel, or less control, or something else. Or mach-9 was using a slower speed of sound. (the speed of sound depends on what you are going through. Mach-6 underwater is faster than mach-6 in the upper atmosphere. I don't know whether it is faster than mach-9 though, but there are probably 2 different things through which mach-6 in one is faster than mach-9 in the other)
Assuming that the speed of sound remains the same, the higher the mach, the higher the speed.
< Message edited by stealthwings -- 12/27/2010 18:42:04 >