# Special relativity, light direction and wave source.

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1. Jan 27, 2014

### Banana Joe

If the speed at which light waves propagate in vacuum is independent both of the motion of the wave source and of the inertial frame of reference of the observer, why isn't the direction? Shouldn't the light travel directly upward from the point of emission and not diagonally, following the rocket?

I was under the impression that "time" doesn't exist... Isn't it just something we created to measure the succession of events? Past and future exist only in our minds, the only thing that actually exists is the present, the now, and that is true simultaneously anywhere in the Universe. If we decide that one second is one 86400th of Earth's rotation on its axis at current speed, isn't that measure absolute and unchangeable? How can a rocket slow it down?

How does speed slow down the oscillation of the atoms, cellular reproduction and decay (and everything else)? And even if it does, does it necessarily mean that "time" has slowed down? If "time" did slow down, shouldn't everything be affected, including light?

Thanks for the clarifications!

Last edited by a moderator: Sep 25, 2014
2. Jan 27, 2014

### Mentz114

For a spherical wavefront the emission/absorption looks something like this. The light is the sphere emitted by the botton dot and absorbed by the top dot which moves wrt to the emission point. The light appears in the stationary frame to have travelled diagonally.

http://www.blatword.co.uk/space-time/wavemove.mpeg

3. Jan 27, 2014

### Banana Joe

The light reached the moving top dot after it reached the stationary one, so it covered more space in more time, where is it that "the time slows down"? If I were in the rocket why wouldn't I be able to tell that it took longer to complete one period?

Last edited: Jan 27, 2014
4. Jan 27, 2014

### Mentz114

I thought you were asking why the light appears to be diagonal in the moving frame. In the clip, the line joining the bottom dot with the (upper) moving dot is the light vector.

Please look up 'Light-clock' to see how we can deduce the slowing down of moving clocks.

The wiki article is OK. Scroll down fro the light clock.

http://en.wikipedia.org/wiki/Time_dilation

5. Jan 27, 2014

### Banana Joe

I get why for the moving clock it takes longer to complete one period, the light has to travel more space at the same speed, but why wouldn't I notice that if I was on the rocket?

6. Jan 27, 2014

### Mentz114

On the rocket the light is moving vertically. There's nothing to notice.

7. Jan 27, 2014

### Banana Joe

The light wave is traveling diagonally at c from the point of origin, but from my perspective on the rocket wouldn't it seem to go up and down slower than c?

Last edited: Jan 27, 2014
8. Jan 27, 2014

### Ibix

No. You, in the rocket, explain that by saying the light is going straight up and down (1ms for a round trip, say). I, on Earth, say the light is traveling on a longer diagonal trajectory, so takes longer (1.1ms, say). But I also notice that your wristwatch is ticking slower than mine. That means I'm fine with the notion that you claim that the light is only taking 1ms, because I can see that your idea of time is slower than mine.

That's a bit of a mess. The clearer explanation is this:

Einstein asserted that the speed of light is constant for all inertial observers. He worked out (Mentz already suggested googling the light clock) that this implies length contraction and time dilation. Lots of people went out to see if reality matched his predictions - and it does so far.

9. Jan 27, 2014

### WannabeNewton

10. Jan 27, 2014

### Mentz114

Why would you think that ?

11. Jan 27, 2014

### Banana Joe

From Earth I see the light travel diagonally and measure that it takes 1ms to complete a period. On the rocket I see it going up and down in what I know is 1ms, so from the rocket's perspective it seems that the light traveled a shorter up/down path at less than c, while it actually traveled the longer diagonal path at c.

12. Jan 27, 2014

### Ibix

There is no "actually traveled". Either perspective is valid - the light goes up and down according to one observer and it follows a zig-zag path according to the other. Neither is less "actual" than the other.

This is an extension of the fact that you can pour a drink on a moving train without having to correct for the 120mph you are moving along the ground. You can consider yourself to be stationary, and the Earth to be passing backwards at 120mph. Thus you can pour the drink without thinking. Seen from the platform, however, you did really well to hit a cup that was traveling at 120mph - I couldn't do it.

13. Jan 27, 2014

### Banana Joe

I can pour a drink in a train without problem thanks to inertia, the drink and the cup are moving with the train.

The rocket did travel away from the emission point, or the light source and detector would have stayed at the center of the spherical wavefront and would have picked up the light that traveled straight up and down in less than 1ms, not the one that traveled diagonally in 1ms.

14. Jan 27, 2014

### Mentz114

That is the difference that results in the different tick rate. It is only a kind of analogy though, because the exact nature of the light source is irrelevant.

The fact is that the light hits the mirror and is reflected, and this must be seen from any POV because it happened.

15. Jan 27, 2014

### Banana Joe

If in the rocket I see the light travel up and down in 1ms, while it actually traveled the longer diagonal path, shouldn't I see the light travel at less than c?

16. Jan 27, 2014

### Mentz114

No, light travels at the same speed for everyone, irrespective of the relative velocity between the emitter and receiver.

17. Jan 27, 2014

### bahamagreen

It is confusing.

Looking at the link with the animation of the spherical wavefront, it looks like the part of it that goes straight up the screen would miss a moving mirror, but the part of it that goes in the right upward direction could hit a moving mirror.

That explanation seems to suggest that the mover and observer might identify the light hitting the mirror as originating from two different parts of the light's wavefront, depending on reference frame; but I'm sure that is not meant to be thought as true, is it?.

If that were true, then a detector on the mirror would be detecting one part of the wavefront as detected by the mover, but the same mirror and detector would be detecting a different part of the wavefront for the stationary observer.

I'm assuming there are wavefronts that can have, or have imposed on them, some variable attribute with respect to direction. If the spherical wavefront could be made to have variable phase with respect to direction of propagation, or some other measurable feature, the mover and observer would be able to tell which angle the light traveled by measuring this attribute at the mirror... What prevents that?

Is there something like the Doppler correction in SR that eliminates or "hides" this so that both observers' detections are always identical?

18. Jan 27, 2014

### Banana Joe

Light does travel at c from the point of emission, to the observer on the rocket it would just seem to go slower, because he sees only the vertical movement.

19. Jan 27, 2014

### Mentz114

Every point on the wavefront is moving at c. I can't follow your logic. The light seems to travel further when the clock is moving wrt you.

I'm signing off now because it is late in my timezone. Try to understand the Wiki article, or do a Google search.

20. Jan 27, 2014

### Banana Joe

Every point on the wavefront is moving at c away from the point of emission. In the video you linked, if the stationary observer looking from the side follows the point of the wavefront that will intercept the mirror, he will see that point traveling at c along the hypotenuse of the triangle, but another stationary observer placed on the route of the rocket (and the one on the rocket) would see that point going straight up, like it was traveling along h at a slower speed than c, because they would see it cover less space in the same time.