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Understanding Special Relativity (beginner questions)

  1. Mar 23, 2013 #1
    I have a large interest in physics; however, my only exposure to special relativity has been through a few books and youtube videos. Because of this I still have a few questions about how special relativity fits into reality.

    I believe that special relativity says that any observer will see all other observers that are in relative motion to move slower through time. If I see someone moving towards me, then I will see them moving slower through time. But because of the ever shortening distance that their light has to travel to reach me, I will actually see them in fast forward? In the same note, if someone is moving away from you, their time would be slowed by special relativity and by the fact that each light wave has to go farther than the last. So my perceived time of different things is sped up/slowed depending on if the object is heading closer to or farther from me, and all objects in motion then move a few notches slower through time purely based off of their speed relative to me.

    Is this how these two features overlap? If the person moving toward you had 2 parallel mirrors with a laser shooting between them, would you calculate that laser as moving faster than the speed of light?
  2. jcsd
  3. Mar 23, 2013 #2


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    Welcome to PF!

    SR effects like time dilation don't really describe what an observer would see with his eyes. Your description of what an observer would see with his eyes is absolutely right; it's dominated by the time taken for the light to propagate.

    Time dilation describes how one frame of reference compares with another. You can conceptualize a frame of reference as a grid of rigid rulers, with a clock at every intersection. The clocks are synchronized with one another by exchanging light signals.

    For an observer moving at 10% of the speed of light, the effects due to propagation of light are about 10%, whereas the time dilation effect is about 1% (0.1 squared).
  4. Mar 24, 2013 #3


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    You were doing so well up to this point, I wonder why you went astray in the rest of your post:
    I'm not even sure what you are describing, but surely you have read that nothing can go faster than the speed of light, haven't you?

    Maybe you could draw a picture or explain more precisely what you have in mind.

    Also, are the two features that you are concerned about, the Time Dilation and the transit time of light?
  5. Mar 24, 2013 #4
    Light moves at speed c in all reference frames, regardless of the relative motion of the light source. Therefore whether the person was moving toward you, away from you or stationary relative to you, you would measure the laser between the mirrors as traveling at c. Time dilation is simply a logical consequence of this fact, so understanding this will help a lot in understanding the rest of special relativity.
  6. Mar 24, 2013 #5
    Yes, I think I worded that poorly. Bcrowell helped explain it to me by saying that special relativity doesn't show what exactly an observer would see with his eyes.

    http://www.zamandayolculuk.com/cetinbal/PU/rel_light_clock_eb_2.jpg [Broken]

    I understand that because I must see the speed of light at value c, and because I see that it must cover an addition horizontal distance when it is in motion, then I must see it tick less often. And this would also affect everything else, as the clock couldn't disagree with the time in the same reference frame.

    So then if this clock were in motion toward me, because the transit time of light was shrinking, I would see everything in a slight fast forward. So then I would also see the beam of light in a slight fast forward? To calculate the "real" speed of light I would have to factor out the transit time of the light? I think Bcrowell helped answer this by saying that special relativity describes the universe as how it exists from your reference frame, but other things affect how you actually perceive it.

    So when people say that the speed of light is always c, they mean that it always IS c, but the changing transit time of that information can cause it to be perceived at higher or lower values. For a while I was trying to combine the transit time of light with special relativity to say that special relativity sped things up through time that were moving toward you and slowed things down that were moving away. Separating the ideas makes more sense.
    Last edited by a moderator: May 6, 2017
  7. Mar 24, 2013 #6


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    We cannot see the speed of light. We cannot observe light traveling. We cannot calculate the "real" speed of light. What we do in Special Relativity is define the propagation of light to be c in any Inertial Reference Frame and we use that definition to synchronize stationary clocks in the IRF with one another by exchanging light signals.

    What we can see are the events of the light bounding back and forth between the moving mirrors if we allow a portion of the light to reflect off in our direction. The rate at which we see the reflections arriving at our eyes is not dependent on our selected reference frame or even our definition of how light propagates but how we separate the transit time from the rate at which the light bounces back and forth is dependent on those factors.
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