Saturday, November 10, 2018

special relativity - Why don't two observers' clocks measure the same time between the same events?


Person A in reference frame A watches person B travel from Star 1 to Star 2 (a distance of d). Of course, from person B's reference frame, he is at rest and is watching Star 2 traveling to him.


Now we know from the principle of relativity, each one will measure the other one’s clock as running slower than his own.


Let’s say that Person A measures Person B’s speed to be v, and that Person A measures 10 years for person B to make it to Star 2. Let’s also say that person B is moving at the speed so that the Gamma Factor is 2. This means person A observes person’s B’s clock to have elapsed a time of 5 years.


Now let’s look at this from Person B’s perspective:


Person B observes Star 2 approaching (and Star 1 receding from) him also at speed v. Since the two stars are moving, the distance between them is length contracted (after all, if there were a ruler in between the stars, the moving ruler would be contracted) by a factor of 2. Since person B measures the initial distance to Star 2 to be d/2 and its speed v, he calculates the time to Star 2’s arrival to be 5 years. Since he observes person A’s clock as running slow (since Person A is moving also at speed v), when Star 2 arrives, he measures Person A’s clock to have elapsed a time of 2.5 years.


Do you see why I’m confused? Person A measures Person B’s elapsed time to be the same as Person B measures Person B’s elapsed time (both 5 years), but Person B does not measure Person A’s elapsed time to be the same as Person A measures Person A’s elapsed time (Person B get’s a measurement of 2.5 years while Person A measured 10 years). This is asymmetrical, which probably means it is wrong. But I’m not sure what the error is.


I suspect if I had done this correctly, each person should measure his own elapsed time to be 10 years and measure the other’s elapsed time to be 5 years. This would be symmetrical and would make the most sense, but again, I can’t seem to justify how person B wouldn’t measure his trip time to be 5 years.


What's my mistake?



Answer




Everything you have said describing the situation in your question is correct; Person A and Person B disagree about how much time elapses on person A's clock between the two events. (The first event is Person B leaving Star 1 and the second event is Person B arriving at Star 2) This is not a logical contradiction. It stems from the relativity of simultaneity and the fact that the time between two events is different in different reference frames.


The time between two events is minimized when the spatial separation between them is zero because the interval


$$\Delta s^2 = \Delta t^2 - \Delta x^2$$


is invariant (the same for everyone). Person B therefore perceives the minimum possible time between the two events, which is 5 years. Person A perceives some spatial separation between the events, and so perceives a longer time between them (10 years).


We can use this information to work out the speed $v$. For Person A, $\Delta t^2 - \Delta x^2 = 5^2$ because that's the answer for Person B, and it must be the same for A. We know $\Delta t^2 = 100$, so


$$100 - \Delta x^2 = 25$$


or


$$ \Delta x = \sqrt{75} = 5\sqrt{3}$$


$v$ is then


$$v = \frac{\Delta x}{\Delta t} = \frac{5\sqrt{3}}{10} = \frac{\sqrt{3}}{2}$$



The situation is not symmetric with respect to A and B because A is not moving relative to the stars, but B is. The existence of the stars breaks the symmetry. A symmetric situation would be if A and B start at their own stars, then meet in the middle.


Another symmetric scenario would be to let B start moving away from A. When A's clock reads 10 years, ask her what B's clock reads. When B's clock reads 10 years, ask him what A's clock reads. In that case, both would say that the other's clock reads 5 years.


So, if the setup of the problem is symmetric with respect to A and B, their answers should be, also. Because this problem does not have that symmetry, the answers A and B give do not have the symmetry.


Finally, you might be concerned that Person A thinks the time between the two events is 10 years, but according to Person B, Person A's clock reads only 2.5 years elapsed. This is due to the relativity of simultaneity. According to Person B, he is arriving at Star 2 and checking Person A's clock simultaneously. Those events have a big spatial separation, though. According to Person A, they are not simultaneous. Person A thinks Person B has checked her clock too soon.


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