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and for still greater velocities the square-root becomes

imaginary. From this we conclude that in the theory of relativity

the velocity c plays the part of a limiting velocity, which can

neither be reached nor exceeded by any real body.

Of course this feature of the velocity c as a limiting velocity

also clearly follows from the equations of the Lorentz

transformation, for these became meaningless if we choose values

of v greater than c.

If, on the contrary, we had considered a metre-rod at rest in the

x-axis with respect to K, then we should have found that the

length of the rod as judged from K′ would have been

image010

this is quite in accordance with the principle of relativity

which forms the basis of our considerations.