If I were to make a light clock, I would have some sort of laser pen attached to the top of one of two mirrors. That laser pen would be perpendicular to the surface of either/both mirrors. I would click a 'go' button and a beam of light would bounce between the mirrors. So far so good. Now, we introduce the very speedy observer to system. Lets say he/she gets to observe the moment I click the 'go' button. Which direction was the laser pen pointing for the observer? If perpendicular to the mirrors, why does the light beam strike the 2nd mirror at all?


closed as unclear what you're asking by Bill N, user36790, Daniel Griscom, Gert, Sebastian Riese Feb 18 '16 at 21:46

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ What is the orientation of the mirrors with each other in the lab reference frame? $\endgroup$ – Bill N Feb 13 '16 at 23:27
  • $\begingroup$ As I understand it, the light clock uses two mirrors that are parallel to each other so the light beam will bounce between the mirrors indefinitely. $\endgroup$ – S Bateman Feb 15 '16 at 3:08

The laser pen will point in the same direction for the moving observer, but the light ray will emerge at an angle. So the direction of the light ray won't be the same as the direction of the laser tube.

To see why this is imaging looking at a single pulse of light as it travels along the laser pen towards the aperture:

Moving laser pen

From your perspective, with the laser pen moving towards you, as the pulse of light travels horizontally along the pen it also travels vertically so it moves in a diagonal line. When the light emerges from the light pen it carries on moving in the same diagonal direction.

  • $\begingroup$ Does it make a difference if the observer is moving parallel versus perpendicular to the laser tube/normal of mirror surface? $\endgroup$ – Bill N Feb 15 '16 at 3:13
  • $\begingroup$ Looking at a typical laser pen, there doesn't appear to be more than maybe 3 degrees of deviation from perpendicular in the manner you describe. The tube is narrow and long compared to its diameter so I don't think there is that much room for the beam to exit side-ways. $\endgroup$ – S Bateman Feb 15 '16 at 3:25
  • 1
    $\begingroup$ @SBateman: relative to the pen the light moves straight along it (to the right as I've drawn the diagram). However because the pen is moving up relative to us, the light is also moving up relative to us. That's why for us the light moves both to the right and up therefore moving in a diagonal. $\endgroup$ – John Rennie Feb 15 '16 at 6:05
  • $\begingroup$ Awesome, I think I can visualize that now. This helped as well [link]galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/… Now, I have another question, of course! But I believe I am supposed to ask it in another question. $\endgroup$ – S Bateman Feb 16 '16 at 20:37

Not the answer you're looking for? Browse other questions tagged or ask your own question.