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The inertialess drive is a fictional means of accelerating to close to the speed of light or faster-than-light travel, originally used in ''Triplanetary'' and the ''Lensman'' series by E.E. "Doc" Smith, and later by Robert A. Heinlein, Larry Niven, and Alastair Reynolds. == The concept of inertia == Inertia is the measure of the resistance of a material body to a change in state of motion (acceleration) under the effect of an applied force. This resistance is proportional to the mass of the body, and is usually expressed (in simplest form) as ''F = ma''. For a body to be rendered inertialess, in principle, its mass should be reduced to zero. In classical special and general relativity, massless bodies are constrained to always move at exactly the speed of light〔(【引用サイトリンク】title=Massless Particles Traveling at the Speed of Light )〕 (the speed of photons in a vacuum), and the term relativity in this context in fact implies that light is always measured to move at the same speed by an observer, no matter how rapidly the observer is moving relative to any body defined as fixed. This aspect of inertialessness is explored by the more exacting authors who have adopted the term after Smith's groundbreaking use. In classical special relativity, in order to move faster than the speed of light, a particle must have not zero mass—and inertia—but mathematically imaginary mass. This is, of course, as viewed by an external observer, and one can postulate that Smith's inertialess field is thus similar in form to a space warp that acts as a tachyon when viewed by an observer in the external universe. Whether this formulation can be expressed self-sufficiently in the context of Smith's fiction is, of course, irrelevant to the enjoyment of it. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「inertialess drive」の詳細全文を読む スポンサード リンク
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