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A fellow engineering student told me many years ago, that $E = mc^2$ means is that as an object of mass $m$ approaches $c$, the speed of light, it's mass increases and, at the speed of light, becomes infinite. If I solve this equation for $m$,

$m = E/c^2$, how in the world can its mass increase? - the denominator is extremely large(186,000 mi/sec)2, making m a very SMALL number, which makes sense, since the smaller the mass of an object becomes as c is approached, the more it would tend to change it's form into pure energy, E. That makes sense to me. But an object increasing in mass as it speeds up does not make sense to me...

Thanks for anyone's explanation(as simple as possible, please)

DilithiumMatrix
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Charles R.
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1 Answers1

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First of all, the speed of the object (relative to you, or relative to anything else) does not appear in the equation $E=mc^2$, so the equation cannot possibly say anything about what happens as the speed of the object increases.

Second, the equation in fact applies only to an object at rest. The correct equation for a moving object is $E=mc^2+(mv^2/2)+hot$, where $v$ is the particle's velocity and $hot$ are higher order terms that can be ignored except at extremely high velocities. Call it $E=mc^2+KE$. ($KE$, the kinetic energy, depends on the velocity of the particle and is therefore observer-dependent).

The significance of this equation is that $E$ is conserved in particle interactions (for any observer!). Therefore any loss in kinetic energy must be compensated for by an increase in rest mass, and vice versa.

WillO
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