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Relativity 23

according to our previous results. Although observers in both frames see the same

event, they disagree about the length of time the particle thrown from the other frame

requires to make the collision and return.

Replacing Tin Eq. (1.12) with its equivalent in terms of T 0 , we have

VB

Y 1 ^2 c^2



T 0

A

B

A

B

Collision as seen from frame S:

Collision as seen from frame S′:

S′

x′

z′

y′

v

S

y

z

x

Y

B

A

V′B

VA

Figure 1.13An elastic collision as observed in two different frames of reference. The balls are initially

Yapart, which is the same distance in both frames since S moves only in the xdirection.

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