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I'm new to this (wonderful) forum, and to SR too...

I've a general question about the space time interval invariance.

Say we have two points A and B, at rest each other, at distance AB.

Now A and B simultaneously in their reference frame emit a flash of light.

The space time interval between these two events is spacelike and equals -(AB)

^{2}.

Now a fast spaceship C travels from A toward B and is reached by the two flashes at the middle point between A and B. For C the two flashes are not emitted simultaneously, and I want to use the invariance of the space time interval to compute T, that is the difference in time (measured by C) between the two flashes. The distance between A and B measured by C is shortened by the Lorentz contraction: AB

_{c}< AB. The space time interval measured by C should be:

(CT)

^{2}- (AB

_{c})

^{2}

and this number should equal -(AB)

^{2}. It results

(CT)

^{2}= (AB

_{c})

^{2}- (AB)

^{2}< 0.

So, how can I find T, as a square root of a negative number?

Sorry if the question is stupid....

er