Impossibility comes in degrees, not one absolute category

Kaku's central organizing device is a three-tier classification of impossible technologies. Class I impossibilities don't violate any known law of physics and could plausibly exist within a century or two, given sufficient engineering progress; these include force fields, certain forms of invisibility, and basic teleportation of small particles. Class II impossibilities sit at the very edge of current physical understanding and might take millennia to achieve, if they're achievable at all, including faster-than-light travel and time travel. Class III impossibilities genuinely contradict known physics as currently understood, such as perpetual motion machines, and would require overturning fundamental laws rather than merely advancing engineering. This taxonomy lets Kaku treat each idea seriously without pretending all speculative technologies are equally far-fetched.

Takeaway: Calling something impossible often just means we don't yet have the engineering, not that physics forbids it.