A typical Lorentz Contraction proof relies on the axiom that "the speed of light is constant" and goes as follows. Given:

  • Frame $F_1$ moves at speed $v$ relative to frame $F_0$. In frame $F_1$ sit 2 parallel mirrors.
  • Distance between mirrors is measured as $l_0$ in $F_1$ (at rest relative to mirrors).
  • Distance between mirrors is measured as $l$ in $F_0$ (while mirrors move past in $F_1$ at speed $v$).
  • Time for light to make "roundtrip" between mirrors measured as $t_0$ in $F_1$ (at rest relative to mirrors).
  • Time for light to make "roundtrip" between mirrors measured as $t$ in $F_0$ (while mirrors move past in $F_1$ at speed $v$).
  • Already proved $t = \frac{t_0}{\sqrt{1-\frac{v^2}{c^2}}}=\gamma t_0$ (time dilation).

A "roundtrip" of light passing between mirrors takes two trips; measured from $F_0$, those trips take times $t_1$ and $t_2$. During those trips, the ship travels $vt_1$ and $vt_2$, meaning light travels $l+vt_1$ and $l-vt_2$ when light moves in the same and opposite directions as $F_1$, respectively, all measured in $F_0$. The constancy of the speed of light gives:

  • Trip 1 (light moves same direction as $F_1$ relative to $F_0$): $c = \frac{l + vt_1}{t_1}$ $\Rightarrow$ $t_1 = \frac{l}{c-v}$
  • Trip 2 (light moves opposite direction as $F_1$ relative to $F_0$): $c = \frac{l - vt_2}{t_2}$ $\Rightarrow$ $t_2 = \frac{l}{c+v}$
  • So, $\color{red}{t = t_1 + t_2 = \frac{l}{c-v} + \frac{l}{c+v} =\frac{2lc}{c^2-v^2}= \frac{2l/c}{1-\frac{v^2}{c^2}} = \frac{2\gamma^2}{c} l}$.

Measured in $F_1$, the "roundtrip" distance is simply $2l_0$, and so $c=\frac{2l_0}{t_0} \Rightarrow t_0 = \frac{2l_0}{c}$.

Combining this with time dilation yields $t=\gamma t_0 = \gamma\frac{2l_0}{c} = \frac{2\gamma}{c}l_0$.

Putting it all together yields $$\frac{2\gamma^2}{c}l =t = \frac{2\gamma}{c}l_0 \Rightarrow l = \frac{l_0}{\gamma} \tag*{$\Box$}$$


Can I shorten this proof to just use "one trip" between the mirrors instead of a "round trip"? I have tried, and cannot! I $\color{red}{\text{have highlighted in red}}$ the part of the proof where the round trip yields some nice cancellation.

What am I missing?

  • There are proofs that rely on axioms other than "the speed of light is constant", but I'm looking for a proof that just relies on that.

  • The typical proof for time dilation $t=\frac{t_0}{\sqrt{1-\frac{v^2}{c^2}}}$ involves light bouncing between two mirrors that are PERPENDICULAR to the motion of the reference frames. I went through this proof, and it absolutely does NOT break down when only one trip between the mirrors is considered. The proof in this question involves mirrors separated by a distance PARALLEL to the motion of the reference frames.

  • If "roundtrip" is unclear, here are two animations, each depicting two "roundtrips":

Roundtrips with tracers

2 roundtrips

First image made by me. Second image from Help Me Gain an Intuitive Understanding of Lorentz Contraction , which goes through this same proof based on the speed of light being constant.

A proof I’ve seen for time dilation is as follows, and only seems to require a single trip of a light beam:

Suppose a pair of mirrors separated by distance $L$ is moving past at speed $v$, such that the displacement between the mirrors is perpendicular to the motion of the mirrors. In the reference frame of the mirrors, light bouncing between the mirrors travels distance $L$ in $t_0$ seconds at speed $c=\frac{L}{t_0}$. In the reference frame relative to which the mirrors are moving at speed $v$, however, light bouncing between the mirrors takes time $t$ to do so and travels $\sqrt{(vt)^2 + L^2}$. So, $c = \frac{\sqrt{(vt)^2 + L^2}}{t}$ as well because the speed of light is constant to all observers. Solving for $t$ and substituting $t_0=\frac{L}{c}$ yields $t=\frac{t_0}{\sqrt{1-\frac{v^2}{c^2}}}$.

light bouncing between mirrors separated by L

Image used is from what about doing the laser beam in a moving reference frame but with a ball

  • $\begingroup$ Hey Zach, in order to comply with our referencing guidelines you should really quote that image and indicate its source. If it's not clear how to do that, feel free to ask for help. Welcome to the site! $\endgroup$ – David Z Jun 6 '18 at 21:58
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    $\begingroup$ I wonder if this is related to the inability of measuring the one-way speed of light. $\endgroup$ – Mark H Jun 6 '18 at 23:21
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    $\begingroup$ A derivation of the Lorentz transformation doesn't have to talk about light at all. See, e.g., arxiv.org/abs/physics/0302045 $\endgroup$ – Ben Crowell Jun 7 '18 at 0:02
  • $\begingroup$ Zach, I note that your edit doesn't change the fact that you have an unattributed image. That still needs to be fixed. $\endgroup$ – David Z Jun 7 '18 at 0:17
  • $\begingroup$ @DavidZ spent some time making a similar gif in Mathematica (the red/blue one made by ME), but I actually like having them both here. In the Markdown I posted the link to the phsyics.stackexchange post from which I copied the other image. $\endgroup$ – Zach Siegel Jun 7 '18 at 0:21

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