How can both observers measure the other's clock running slow?

[anantchowdhary]

Suppose A travels in a spaceship and B is on earth. At the time of launch they both synchronise their clocks. When 30 secs have passed by in A's clock he reads the time in B's clock to be 40 secs.But when 30 secs have passed in B's clock A's clock shows 40 seconds. Isn't this a contradiction to the first statement itself ?

 

[DaveC426913]

Suppose A travels in a spaceship and B is on earth. At the time of launch they both synchronise their clocks. When 30 secs have passed by in A's clock he reads the time in B's clock to be 40 secs.But when 30 secs have passed in B's clock A's clock shows 40 seconds. Isn't this a contradiction to the first statement itself ?

It's not nearly as simple as that.

1] In order to travel anywhere, A has to accelerate, this changes his frame of reference to non-inertial.
2] In order to read each others' clocks, A must return to B, changing his frame of reference again.

You want to Google The Twin Paradox.

 

[bcrowell]

When 30 secs have passed by in A's clock he reads the time in B's clock to be 40 secs.

He can't just instantaneously read B's clock. The best he can do is to have B send him a signal at the speed of light.