# Is the direction of a pulse from a laser pen dependent on the observer?

1. Jan 2, 2008

Suppose I am in an inertial non-accelerated reference frame, and I observe a laser pen moving in a direction perpendicular to its length at a uniform constant velocity of one meter per second. If the laser pen were to fire very briefly, say for 1 millisecond, while in this state of uniform motion, what direction would the pulse of light take? You can assume, for this thought experiment, that this laser pen emits a perfectly straight beam. Now, after you have answered this question, tell me what direction the same pulse of light would take if observed from the reference frame of the moving pen. Compare.

-Mike Parus

2. Jan 2, 2008

### JesseM

Yes, if the laser pen is oriented perpendicular to its motion, the laser will appear to be at an angle, in such a way that a photon from the pen always remains on the pen's axis. Naturally this means that in the pen's own rest frame, the laser always appears to go straight out, regardless of the pen's velocity relative to any other observer (if this wasn't true, it would violate the postulate that says the laws of physics are the same in all inertial frames, and then an observer in the pen's rest frame could use the angle of the laser to determine her absolute velocity relative to some preferred frame).

3. Jan 2, 2008

### lightarrow

Stellar aberration:
http://en.wikipedia.org/wiki/Aberration_of_light

4. Jan 2, 2008

### Staff: Mentor

Angles of motion are trivially frame variant, even in Newtonian mechanics.

5. Jan 4, 2008

### YellowTaxi

the question posed is just an example of the light-clock that's used as a quick graphical method to derive the time dilation. the photon carries the perpendicular velocity of the pen with it and so it moves along the diagonal, rather than in the instaneous direction that the pen is pointing at th instant of emission.

I think the Relativistic Aberration effect is the only way to convince yourself it IS the same photon though.

6. Jan 4, 2008

### phyti

The second postulate states (indirectly) that the speed of light is independent of the source. It will not assume any speed component of the pen. If it's a single photon or a coherent beam it will move at 90 deg to the motion. The clock works on the basis of multiple photon dispersion with an adequate intensity.

7. Jan 4, 2008

### YellowTaxi

I think that's completely wrong. The intensity of the photon 'cloud' is dragged forward in the direction of motion. The headlight effect - aka relativistic aberration.
Assuming relativity isn't flawed.

Treating the photon in the way phyti suggests would give the correct result for the time dilation factor. But you would then have to conclude that the photon in the stationary frame was not the same photon emitted from the moving frame. And that would contradict the laws of physics (and relativistic aberration).

8. Jan 5, 2008

### phyti

Check the internet for definitions of the second postulate regarding the speed of light.
That principle alone will remove many problems without any calculations.

When multiple photons leave an event at the same speed c, but in all directions (a sphere), only the one that has an x vector component matching the one for the observer will become part of the light clock. That's the reason different observers have different time rates, they are each timing different photons. If the light intensity is not sufficient,
there won't be a photon to match, and your clock won't work. Consider if the source of light emitted say 10 photons per second in random directions. Your clock would occasionally detect one, but it too would be random.

9. Jan 5, 2008

### Staff: Mentor

There are an infinite number of possible reference frames, so this idea would require that an infinite number of photons be emitted. This would therefore require an infinite amount of energy.

In general, the number and identity of particles is frame invariant. It wouldn't make sense to talk about transformations between frames otherwise.

Last edited: Jan 5, 2008
10. Jan 6, 2008

### phyti

The number of photons emitted depends on the event, not the possible number of observers.

11. Jan 6, 2008

### JesseM

I think phyti was just assuming a source which emits photons in all different directions, like a light bulb...only some photons will go in the right direction to hit the top mirror (if the mirror has a finite size there will be a certain range of angles), and observers in all frames will agree which photons do that, although they'll measure the angle of each differently.
There is no need for a disagreement on the number or identity of photons. Again, all observers will agree on which of the photons go at the correct angle to reach the top mirror, although they will disagree on what that angle is.

12. Jan 6, 2008

### Staff: Mentor

I hope you are right, that makes sense. But this comment:
Seems to say that the identity of the photons is frame variant. On another site I wasted a lot of time with a crackpot that seriously proposed the idea of the identity of photons being frame-variant, so this may be a common idea.

phyti, can you clarify?

13. Jan 6, 2008

### YellowTaxi

video

I distinctly remember asking this question of my physics lecturer and not receiving a satisfactory answer (in fact he didn't answer the question). So I don't think that its at all an uncommon misconception, even with university lecturers. I don't think he was crackpot or a dim-wit either, just his comprehension of the subject may have been a bit shaky in some areas.

Last edited by a moderator: Sep 25, 2014
14. Jan 7, 2008

### phyti

A diagram will clarify it better than words, I'm working on it.

15. Jan 7, 2008

### yuiop

Say the laser pen is moving in the x direction relative to you. The photon leaving the lazer pen acquires a boost in the x direction but this compensated for the reduction in transverse y velocity component of the photon so that the speed of the photon is still c relative to you. The only thing that has changed is the direction of the photon. If there was a glass tube from the pen to the target, the photon would remain in the tube from your point of view and from the point of view of an observer co-moving with the pen a. If light did not change direction like this when emitted, then the photon would miss the target co-moving with pen and you would be able to deduce your absolute velocity. That of course, would completely invalidate the theory of special relativity and does not happen. Multiple photons are not required.

16. Jan 8, 2008

### phyti

kev:
This violates the 2nd postulate, c is independent of the source.

This is true and what is shown in the attachment.

#### Attached Files:

• ###### wzdm-sr-time.doc
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172.5 KB
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17. Jan 8, 2008

### JesseM

Not if you don't snip out the context of kev's statement:
Do you disagree? If the laser pen is moving in the x-direction, the photon will have the same velocity in the x-direction as the pen, but the velocity in the y-direction will be reduced (relative to a photon from a laser pen at rest) so that the total velocity is still c.

18. Jan 8, 2008

### YellowTaxi

the 2nd postulate only really says the speed is always c.
Einstein was human and often confused the term 'velocity' when he almost certainly meant 'the speed is never changed'. (ie he really meant magnitude of the velocity)

So he just meant that |c| is never increased by motion of the source. ;-)
[or likewise by motion of the observer if you're really pedantic]

The relativistic boosts that kev mentions have nothing to do with it whatsoever. It's really not that complicated...
Likewise the Aberration effect I mentioned would be due to time of flight considerations and have nothing to do with it either - I was wrong.

DaleSpam has the right idea in my honest opinion. It's that simple. Just like the video I linked to.

And you can blame Dr Einstein for the confusion if you wish...

19. Jan 9, 2008

### yogi

To embellish on the experiment - if a light pen is mounted orthogonal to the floor of a train car traveling at velocity v along the X axis, a single photon emitted from the light pen in the Y direction will strike the top of the car directly above the pen as per
post #2. Now if an opening is cut in the floor and a single photon enters the car from beneath, at a direction orthogonal to the floor - it will not strike the top of the car directly above the opening (a la aberration). In the frame of the car, the photon emitted by the light pen has a velocity c in the Y direction relative to the frame of the car and the light pen. The photon that enters from the exterior will have a velocity c at an angle wrt to the frame of the moving car

20. Jan 9, 2008

### Ich

Einstein was not only human, but also german, and talked about Geschwindigkeit, not speed or velocity. You could hardly blame him for the confusion.