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knzhou
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IndeedThis is a classic paradox. Let's set $c = 1$ and start by working in the lab frame, where the muon will think that our clock is not only running slowcreated at $x = 0$ and $t = 0$ and travels to the right with speed $v$. It decays at $x = d$ at the moment it crashes into a clock, but running aheadwhich reads $t = t_{c,f}$. Then of course, bywe have $$d = v t_{c,f}, \qquad t_{c,f} = \gamma \tau$$ by the usual relativisticdefinition of speed and time dilation, where $dv/c^2$ delay$\tau$ is the lifetime of the muon in its rest frame.

Now let's work in the muon's frame, described by primed coordinates. The finalmuon is created at $x' = 0$ and $t' = 0$. At this moment in the muon's frame, $t' = 0$, the clock is at $x' = d/\gamma$ but its reading we take down$t_{c,i}$ is not zero. Instead, accordingperforming a Lorentz boost back to the lab frame, $$t_{c,i} = t = \gamma (t' + v x') = \frac{\gamma v d}{\gamma} = v d.$$ Of course, the clock will crash into the muon at $t' = (d/\gamma) / v = \tau$, which is precisely when the muon decays. At this moment, the reading on the clock is $$\frac{t}{\gamma} + \frac{dv}{c^2} = \frac{t}{\gamma} + \frac{v^2}{c^2} \frac{t}{\gamma} = \frac{t}{\gamma} \gamma^2 = \gamma t$$$$t_{c,i} + \frac{\tau}{\gamma} = v d + \frac{d}{v \gamma^2} = \frac{d}{v} \left(v^2 + \frac{1}{\gamma^2} \right) = \frac{d}{v}$$ which is perfectly consistent with whatprecisely the final reading $t_{c,f}$ we calculatefound in ourthe lab frame. So the paradox is resolved by remembering that $t_{c,i}$ isn't zero, due to the relativistic loss of simultaneity effect.

Of course, this answer depends on the machinery of Lorentz transformations, reference frames, and synchronized clocks. AXensen's answer gives a different explanation directly in terms of what the muon and Earth observers "see".

Indeed, the muon will think that our clock is not only running slow, but running ahead, by the usual relativistic $dv/c^2$ delay. The final reading we take down, according to the muon, is $$\frac{t}{\gamma} + \frac{dv}{c^2} = \frac{t}{\gamma} + \frac{v^2}{c^2} \frac{t}{\gamma} = \frac{t}{\gamma} \gamma^2 = \gamma t$$ which is perfectly consistent with what we calculate in our frame.

This is a classic paradox. Let's set $c = 1$ and start by working in the lab frame, where the muon is created at $x = 0$ and $t = 0$ and travels to the right with speed $v$. It decays at $x = d$ at the moment it crashes into a clock, which reads $t = t_{c,f}$. Then of course, we have $$d = v t_{c,f}, \qquad t_{c,f} = \gamma \tau$$ by the definition of speed and time dilation, where $\tau$ is the lifetime of the muon in its rest frame.

Now let's work in the muon's frame, described by primed coordinates. The muon is created at $x' = 0$ and $t' = 0$. At this moment in the muon's frame, $t' = 0$, the clock is at $x' = d/\gamma$ but its reading $t_{c,i}$ is not zero. Instead, performing a Lorentz boost back to the lab frame, $$t_{c,i} = t = \gamma (t' + v x') = \frac{\gamma v d}{\gamma} = v d.$$ Of course, the clock will crash into the muon at $t' = (d/\gamma) / v = \tau$, which is precisely when the muon decays. At this moment, the reading on the clock is $$t_{c,i} + \frac{\tau}{\gamma} = v d + \frac{d}{v \gamma^2} = \frac{d}{v} \left(v^2 + \frac{1}{\gamma^2} \right) = \frac{d}{v}$$ which is precisely the final reading $t_{c,f}$ we found in the lab frame. So the paradox is resolved by remembering that $t_{c,i}$ isn't zero, due to the relativistic loss of simultaneity effect.

Of course, this answer depends on the machinery of Lorentz transformations, reference frames, and synchronized clocks. AXensen's answer gives a different explanation directly in terms of what the muon and Earth observers "see".

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knzhou
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Indeed, the muon will think that our clock is not only running slow, but running ahead, by the usual relativistic $dv/c^2$ delay. The final reading we take down, according to the muon, is $$\frac{t}{\gamma} + \frac{dv}{c^2} = \frac{t}{\gamma} + \frac{v^2}{c^2} \frac{t}{\gamma} = \frac{t}{\gamma} \gamma^2 = \gamma t$$ which is perfectly consistent with what we calculate in our frame.