Calculating Spacetime Interval: Alpha & Beta Supernovae

AI Thread Summary
The discussion centers on calculating the time interval between two supernova explosions, Alpha and Beta, using the spacetime interval equation. The user initially struggles with the equation due to the distances involved, leading to incorrect results. They later realize that since light-years already account for the speed of light, they do not need to multiply by c² in their calculations. Ultimately, they find that the time between the explosions, as perceived by the alien spacecraft, is 67.1 years. The conversation highlights the importance of unit consistency in relativistic calculations.
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Homework Statement
The star Alpha goes supernova. Ten years later and 100 ly away, as measured by astronomers in the galaxy, star Beta explodes.

An alien spacecraft passing through the galaxy finds that the distance between the two explosions is 120 ly. According to the aliens, what is the time between the explosions?

Relevant equations
c^2*t^2 - x^2 = c^2(t')^2 - (x')^2 The attempt at a solution
I tried using the spacetime interval equation but the distances are too small to see a difference and gives me a wrong answer. Then i did not include c^2 with the equation and i got the correct answer. Which is 67.1 years. I also tried to multiply x^2 and (x')^2 with c^2 and got the same answer. Is it under specific units, like light years, where we can neglect the c^2 or did I just used the wrong equation in the first place?
 
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Lightyear is the dist light travels in 1 year, so it's equal to c*1 year, and already the c is inside it, if you measure time in years. That's why you didn't have to multiply by c^2.
 
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