How much time does a moving observer measure me measuring?

Question

suppose I and a friend are moving relative to each other at 87% the speed of light. My friend has a large pendulum with a time period of 10 seconds. Since the pendulum is moving relative to me too, due to length contraction, I measure a time of 20 seconds on my stop watch. Now, how much time would the friend see me measuring? I feel like it should be 40, but im having an argument with someone that repeating this would then cause the time measured by each observer for the time measured by the other will increase exponentially, "which doesnt make sense"

user253751 · Accepted Answer

Imagine your stopwatch ticks (audibly) every second.
Your stopwatch ticks 20 times. Your friend won't see extra ticks or missing ticks. So he will see 20 ticks, too. And the screen will say 20 seconds because it changed 20 times.
But each tick will appear to take 2 seconds. So if your friend uses their own stopwatch to see how long it takes you to do your measurement with your stopwatch, they will measure 40 seconds.

How much time does a moving observer measure me measuring?

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