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Movement relative to light

  1. Feb 19, 2013 #1
    This is partially a question and partially me explaining my understanding of c, frames of reference, and relative motion to see if I have it straight.

    So, c (the speed of light) is always the same when measured by an observer regardless of the intertial reference frame from which it is measured. This is due to relativistic effects (time dilation, length contraction, and the relativity of simultaneity). As an observer, I would never observe these effects occurring to me, but another observer would observe them happening to me, and these effects would explain my consistent measurements of c, regardless of my relative motion.

    I hope this is all correct so far. Now please allow me to state this with a practical example:

    If I move at 5mph relative to the ground I am standing on and I measure light hitting me as I walk, I will measure the light as c, regardless of my walking. This is because the relativistic effects occurring to me (but not noticed by me) compensate for this 5mph of movement exactly. This is the same regardless of whether I walk, run, or dodge, and regardless of the amount of lights sources and their positioning relative to me. Relativistic effect do occur between myself and the ground, but they are negligible at such insignificant fractions of c.

    My question is: Are the relativistic effects that occur to an observer, and compensate exactly for their movement so that c is always consistent due to the observer's movement, relative to the light itself, the light source, or what?

    Thank you for your time and energy.
    Last edited: Feb 19, 2013
  2. jcsd
  3. Feb 19, 2013 #2
    Firstly, I think you are on a contrary perspective to what is established, and I see this from your sentence "This is due to relativistic effects...". As their name indicates, they are effects, not causes. The cause is the invariance of the speed of light, to which the relativistic effects are, precisely, the effects. It is this invariance that is the SR postulate, and therefore you can not see the situation as if the relativistic effects were the reason for why light speed is invariant, but on the other way around, the invariance of the speed of light as a postulate has as consequence this effects on space and time measuring.
    So, think about this, I think your question doesn't make much sense in this perspective.
  4. Feb 19, 2013 #3


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    Yes, as long as you realize that in order to measure c, you have to perform a round trip experiment. Every inertial observer will get the same answer.
    Now you are talking about measuring the one-way speed of light which you cannot do. In Special Relativity, we define the one-way speed of light to be equal to the two-way speed of light and we synchronize our remote clocks so that when we use them to "measure" the one-way speed of light, we get the answer we programmed into the experiment.
    Prior to Einstein, scientists believed that light propagated at c relative to an absolute ether and our measurements, presumed to be done while traveling through the ether, yielded the same answer for the round-trip speed of light due to length contraction and time dilation. But Einstein showed that you could assume or define or postulate, without the need for any proof, that light propagated at c in any inertial frame you choose and everything would be consistent.
  5. Feb 19, 2013 #4


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    For a photon E = pc, so what if I measure E and p and define c as the ratio?
  6. Feb 19, 2013 #5
    Just elaborating a little more on the perfect answers that George gave:
    That is only correct for a very narrow definition of "observer": an observer in inertial motion, using a co-moving standard inertial reference system.
    If you use a GPS receiver, that apparatus takes the speed of radio signals as c wrt the ECI frame; consequently it measures the "closing" speed of radio signals relative to you as c-v (assuming that you're not at one of the poles, that v is much more than the 5km/h that you are walking).
    Somewhat yes: such things as Lorentz contraction and time dilation are taken care of by Nature. However clock re-synchronization must be taken care of by you. If you don't do that for every change of velocity, then not all effects will compensate.

    Some time ago I gave a calculation example, if I can find it back I'll add it here.*
    Your speed appears to be c relative to the light as the speed of light is independent of the light source.

    *found it: https://www.physicsforums.com/showpost.php?p=4117535&postcount=50
    Last edited: Feb 19, 2013
  7. Feb 19, 2013 #6
    Interesting. It makes sense to me that I'm moving at c from the light's perspective since I measure light at c, though my brain starts to break when I start to think about a photon's perpective. I have another thread that I started about that around here somewhere.

    I understand what's being said here about relativistic effects not causing c to be constant, but rather being a result of it. It seems to be me it can be thought of either way, though from an empirical standpoint all we can say is that we measure c as constant from any FoR.

    I also get what's being said about movement not being relative to some absolute ether, and that light is measured as c regardless of the FoR. Thank you for addressing that.

    As for not being able to measure a one-way trip of light, is this saying that an observer cannot measure light without referencing another object (besides the light itself)? If so, that addresses my question as to what relative motion accounts for our consistent measurements of c. It would be the motion relative to whatever else we're referencing to measure c's round trip (e.g. a mirror and the distance between the observer and the mirror). I know I'm once again referring to relativistic effects as "causing" c to be constant, but it helps me to think of it that way. Then again, maybe thinking of it that way is not helping my cause...
  8. Feb 20, 2013 #7
    The light can't have a perspective. Light can't observe and no observer can reach the speed of light. Approaching the speed of light, the length of objects shrinks to zero and time "freezes".
    That's correct; it's a logical result if one starts with the assumption that c is invariant (note: too many people confound invariant with constant). Inversely, the invariance of c is a logical result if one starts with the assumptions that Maxwell's equations are valid as well as the conservation laws.
    That wasn't said as that cannot be shown; the point was that the measured speed of light is the same in all inertial frames (ignoring gravitation).
    I'm not sure what you mean (but it sounds wrong). We measure light coming from stars etc., and we don't measure anything but the light in such cases.
    Did you follow my calculation example?
  9. Feb 20, 2013 #8
    Frustratingly, I posses almost no knowledge of mathematics, and I only understand physics conceptually. I'm basically useless with numbers. I think I do follow all that you've said above, and thank you for correcting my erroneous understandings.

    I did also read through your post in the thread you linked to. I got most of it, except for the equations.

    What I'm trying to get at is that speed is a function of distance and time, so speed cannot be measured without referencing some distance and some time. So, say I am measuring light coming from a star, as in your example. If I move towards that star at 1-mph, the length between me and the star contracts, and time on the star dilates, all from my FoR, of course. Due to those relativistic effects, my measurements calculate the lights as c, not c-10.

    This is speaking talking about relativistic effects as though the cause c to be invariant (not constant?), but we've been over that already.

    What I'm getting at here is that it is my motion relative to the star that causes the relativistic effects, not my motion relative to the light itself. If there was just an observer and a photon in empty space, would relativistic effects occur due to the relative motion between the observer and the photon?

    I'm jumping around here a bit struggling to articulate my question(s) here, so it's at least somewhat clear. Apologies for the clutter.
  10. Feb 20, 2013 #9
    Particularly there I did my best to use extremely simple mathematics - the kind that you used at secondary school. So if you didn't "get" the calculations, it's likely because of lack of familiarity with the basics of SR, or simply because you assumed that it would be too difficult! The basic equation that I did not write out there is for calculating "gamma", the Lorentz factor: http://en.wikipedia.org/wiki/Lorentz_factor
    It's quite impossible to fully grasp how SR works without doing some calculation exercises; they are not for nothing part of courses. So, in order to get a good conceptual grip of how SR works, please try.
    That's wrong. If the star was moving at the same speed away from you, that would not affect the speed of light coming to you; it's irrelevant for your measurement of the speed of that light. It's the same with radio waves coming from GPS satellites: the speed of those signals is completely independent of the velocity of the satellite. That's the second postulate, which is based on Maxwell's theory of light propagation.
    Relativistic effects of light can only be detected due to change of motion of the reference system. If you use a reference system that is constantly in inertial motion, you would likely assume that it is always in rest; consequently you would not detect any relativistic effects of light. Light following the Maxwell equations is just "classical". However you would still be able to detect relativistic effects of other things such as electrons.
  11. Feb 20, 2013 #10
    I will. Thanks for the encouragement. I do have a mental block against mathematics.

    Hmm. That's actually what I'm trying to say. My FoR relative to a light source is irrelevant to my measurements of the speed of light. It's always going to come out to c. The fact that the speed of light does not change depending on my FoR can be thought of as being "caused" by relativistic effects between myself and the reference system witch which I'm using to measure the light. Is this still wrong?

    Yes! That what my original question was asking! You need a reference besides light itself to measure the velocity of light. I hope I've got this right now, but it's probable I don't :)

    Because if I am not moving relative to anything besides the light itself, there's no actual relative motion to cause relativistic effects?

    This flew over my head.

    Because there would be relative motion between myself and the electrons?

    Thank you so much. These forums and the people in them are an invaluable resource in my quest to understand this stuff. I deeply appreciate it.

    p.s. I learned how to use quotes :)
  12. Feb 20, 2013 #11
    That sounds quite OK to me; basically that is a "Lorentz transformation".
    I can't find anything wrong in that. :smile:
    In 19th century physics, light was modeled as a wave with a speed c relative to a rest frame (the ether). The constancy of the speed of light relative to a single reference system is considered "classical", not a special "effect".
    No. In classical physics, electrons were not thought to have limit speed - it should be possible to make them go at any speed. In contrast, during the development of SR it became clear that nothing material can reach the speed of light. This was first observed with electrons. There is much more to SR than light.
    Great :smile:
  13. Mar 8, 2013 #12
    Just realized I think I should have said that you need a reference besides light itself to detect the relativistic effects of relative motion to light, not the velocity of light. The velocity of light could be measured "classically" without another reference frame, I think.
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