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

  1. Jan 2, 2008 #1
    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. jcsd
  3. Jan 2, 2008 #2


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    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).
  4. Jan 2, 2008 #3
    Stellar aberration:
  5. Jan 2, 2008 #4


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    Angles of motion are trivially frame variant, even in Newtonian mechanics.
  6. Jan 4, 2008 #5
    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.
  7. Jan 4, 2008 #6
    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.
  8. Jan 4, 2008 #7
    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).
  9. Jan 5, 2008 #8
    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.
  10. Jan 5, 2008 #9


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    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
  11. Jan 6, 2008 #10
    The number of photons emitted depends on the event, not the possible number of observers.
  12. Jan 6, 2008 #11


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    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.
  13. Jan 6, 2008 #12


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    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?
  14. Jan 6, 2008 #13

    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
  15. Jan 7, 2008 #14
    A diagram will clarify it better than words, I'm working on it.
  16. Jan 7, 2008 #15
    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.
  17. Jan 8, 2008 #16
    This violates the 2nd postulate, c is independent of the source.

    This is true and what is shown in the attachment.

    Attached Files:

  18. Jan 8, 2008 #17


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    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.
  19. Jan 8, 2008 #18
    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...
  20. Jan 9, 2008 #19
    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
  21. Jan 9, 2008 #20


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    Einstein was not only human, but also german, and talked about Geschwindigkeit, not speed or velocity. You could hardly blame him for the confusion.
  22. Jan 9, 2008 #21
    Both phyti and JesseM’s arguments will produce similar results if an experiment was carried out.
    JesseM simply means moving observers see the same photons differently, because spacetime is different for each moving observer. Observers would not agree that the photons would be in the same place at the same time and so a line of photons would be able to form different angles (aberration effect) depending on their velocity to the laser pen.
    Phyti’s suggests that the many photons leave the laser pen forming an arc of a photon sphere. Depending on how the observers move they would see different lines of photons for different angles. The known fact that the motion of a light source does not affect the speed of light give this argument some credibility, but it is not the view taken by relativity.
    If light does propagate through a preferred frame and the motion of the laser pen were significant. Then there is a possibility that the intensity of light coming from the laser would vary, depending on its orientation and velocity to the preferred frame.
    It may be possible to detect an increase in the intensity of light slightly to one side of the laser, as this light would also lie on the light sphere.
    I read that a research group are doing an experiment to measure laser light after it travels 60m to see if it’s affected by the rotation of the earth.
  23. Jan 10, 2008 #22
    I found this simple amplified Doppler shift laser diffraction experiment that gives clear support to the existence of a preferred reference frame. It shows that the speed of light depends on the propagation direction, which follows on from phyti’s argument. In that the relative velocity of light coming from the laser (called relative speed s in phyti’s post #18 attached word document) depends on the laser’s velocity.
    I’m not concerned about the authors take on the Ether Gauge Theory (section 2) – that’s another issue. The experimental results are what’s important and support phyti’s argument.


    They’ve carried out a second more sensitive experiment (using highly sensitive segmented photo-diodes to measure the position of diffracted light spots) to validate their findings, see


    I’m impressed by their work.
  24. Jan 12, 2008 #23
    Wisp - If I remember correctly, the Mar 31 article was discussed at length in another PF thread some months ago - with considerable negative comment being made.

    Is there any other interpretation to be accorded to the result - if it is confirmed independently it will upset the establishment significantly
  25. Jan 12, 2008 #24

    I did a search and found some comments from an earlier discussion. But having now given the experiment more thought I can see that comments that rubbished the paper are false.
    Example: The comment about the authors SR line being stuck on 0, being wrong, but should vary as a diurnal (1-day) sine wave.
    No it doesn’t, SRT does predict a flat line. The sine variation is sidereal and ffocuses on the CMBR frame.
    I found many other comments criticize the paper without good reason.
    What should be discussed is the opportunity to expand on the idea and look for evidence that would follow on from their findings.
    For example: Are there any research groups testing for the variation in laser light (frequency, position, or intensity) as the earth rotates?
    If these findings are correct, ignoring them would be the worst option.
  26. Jan 12, 2008 #25


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    There's nothing at all unusual about measuring the dipole moment of the cosmic microwave background radiation.

    To use this to support any sort of "ether theory" is, however is basically wrongheaded.

    Note that the paper in question does not yet meet PF guidelines for discussion, as it has not yet been accepted for publication in a peer reviewed journal, and appears to have a definte speculative bent.

    Nature Physics does seem to be listed in thomson's guide, http://scientific.thomson.com/links/journals/n/ [Broken], though I'm not terribly familiar with it.

    The main point to be made is that dipole anisotropies in the cosmic microwave background (CMB) are in now way in conflict with Special Relativity (SR) - any claim otherwise is based on a fundamental misunderstanding SR.

    It would be very helpful to one of the main goals of PF, explaining the mainstream view of science to a wide audience, if non-mainstream theories like "ether drift" were *not*dragged into the discussion, and if the mainstream explanation of stellar aberration given already by other posters was heeded (and possibly explained in more depth if needed).

    If this discussion drifts too far off track, I may have to lock it, because we are not here to debate non-mainstream theories.
    Last edited by a moderator: May 3, 2017
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