Light Clock Example this is not HW

1. Oct 4, 2011

slam7211

Ok I am only a physics undergrad so if this is obvious im sorry.
I understand the concepts of SR pretty well but one thing has bugged me. In the most basic light clock example used to describe the effects of time dilation, where 2 mirrors are placed 1 ls apart on a moving boxcar with an observer on the car and not on the car. with each observer measuring the same event happening at different times. I know that the light has to hit the mirrors in both reference frames because events must always occur in all reference frames; however, the thing I dont get is in the reference frame of the observer outside the boxcar the light is moving at a diagonal in order to hit both mirrors because the mirrors are moving with respect to the observer. How do you make sense of light moving like that when bounced off of 2 flat mirrors

2. Oct 4, 2011

ghwellsjr

Suppose the observer on the ground had his own light clock, would it be easier for you to understand how from the reference frame of the observer on the moving car the light would be going on a diagonal?

3. Oct 4, 2011

slam7211

Not Really my point is simply light from a flat mirror should bounce straight back but it doesn't when you get to this example because events must occur, basically I'm asking if there is a better explanation than simply it happens because the event must occur

4. Oct 4, 2011

Staff: Mentor

What if someone riding in the boxcar dropped a ball straight down and bounced it off the floor. From his viewpoint, the ball just moves down and up in a vertical line. Would an observer on the tracks see the same trajectory?

5. Oct 4, 2011

slam7211

I mean that makes sense but the thing is that balls have mass and move with their reference frames. therefore it looks to the observer on the car that the person bouncing the ball is moving so the ball would have the same velocity as the observer before he released it. the speed and trajectory of a wave is independent of the motion of its source

Last edited: Oct 4, 2011
6. Oct 4, 2011

ghwellsjr

Suppose you had a laser beam shining on the bouncing ball, illuminating its trajectory, would the beam track the ball in the moving frame, or would it point behind?

7. Oct 4, 2011

Staff: Mentor

That's not how mirrors work. The angle of incidence equals the angle of reflection. It only bounces straight back if it bounces straight in. In all frames the light follows Maxwells equations and therefore all of the usual laws of optics.

8. Oct 4, 2011

slam7211

That makes sense for all future reflections after the initial diagonal bounce between 2 mirrors but it does not explain how it makes the first bounce from one mirror to the other because the angle of incidence is 90 for the first reflection

Can you be more specific about which reference frame we are talking about here I don't understand the example

9. Oct 4, 2011

Staff: Mentor

No, the angle of incidence is never 90 degrees in the moving frame. Otherwise the reflection would be at 90 degrees. The laws of optics are the same in all frames.

10. Oct 4, 2011

slam7211

how is this maybe im not visualizing it correctly but if you shoot a light wave at the mirror, and in the rest frame its angle of incidence is 90 then how can the mirrors moving laterally change the angle of incidence, it can change where on the mirror it hits but it wont change the angle of incidence (if I shine a laser pointer at a mirror and move the mirror off center the angle remains 90)

11. Oct 4, 2011

Staff: Mentor

The mirror moving laterally doesn't change anything, but the emitter moving laterally means that the initial wave is not moving at 90 deg.

12. Oct 4, 2011

slam7211

hold on I thought both the speed and trajectory of a wave were independent of its source's movement, am i wrong there?

13. Oct 4, 2011

Staff: Mentor

14. Oct 4, 2011

slam7211

Thanks, sorry if I was asking a stupid question it just always bugged me a bit

15. Oct 4, 2011

ghwellsjr

I'm asking if when you dropped the ball, you had a laser located at the place where you dropped the ball, shining down on the ball, would the laser beam remain focused on the ball all the way to the floor and back up again or would the light be diverted behind where the rail car used to be? I'm asking if you think it would stay on the ball when viewed by the observer in the rail car, then why wouldn't it stay on the ball when viewed by the observer on the ground?

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