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Help with relativity

  1. May 19, 2010 #1
    I am having difficulty wrapping my mind around Einstein's relativity, and am wondering if someone can please help to enlighten me.

    My Physics teacher explained Einstein's relativity like this - say there were two spacecraft, one speeding forward and the other is standing still. If each spacecraft fired a photon beam forward, the measurement for the speed of the photon beam would be the same on each ship, because the speed of light is always constant. This is reconciled in the equation speed = distance over time, since time actually travels slower on the craft as the craft moves faster (though it's not experienced such on the craft). This makes sense - speed is distance over time, and if we know the speed of light, and we know the distance between the craft and the photon beam, then time is the variable.

    To get to the point, here is my question: What if you fired the photon beam off the BACK of the spaceship, so you're moving away from the photon beam? Time must still slow down, but if the speed of light is still constant, how can that be explained in terms of relative distance and time? That is - since you're moving away from the beam of light, intuitively one would think that the light beam would "stretch" relative to the craft and therefore appear to be moving faster than the speed of light - however we know that this isn't so. So what actually happens, from the perspective of the craft moving away from the light beam?
  2. jcsd
  3. May 19, 2010 #2
    from the craft shooting the beam the light moves away from the craft normally with no apparent abberations. To the stationary ship the light is moving away from them at the speed of light but its wave-length seems to be longer than it does on the moving ship.

    Paul, the 60 year old student
  4. May 19, 2010 #3
    Thanks Paul,

    What I still don't get though, is how can the moving craft experience the light beam at the normal speed of light?

    Speed is distance traveled over a period of time. I know that the speed of light is constant regardless of the frame of reference. I know that time slows down the faster one travels, but that's where it gets confusing for me. Since we're shooting the beam of light off the back of the craft, the beam should elongate insofar as the distance between the craft and the beam of light is greater than would be the case for a beam and the stationary craft. So if light has traveled a greater distance over a shorter time, how can we say the speed is constant?

    I know that it is, I just fail to understand why.
  5. May 20, 2010 #4
    No. The distance doesn't 'stretch'. You have to think of it as if the moving ship, from it's own perspective, is stationary and everything else is moving backwards relative to it (why it's called relativity). In this case the photon, after 1 year, will be 1 light year away.

    However, from the perspective of the other ship observing, the distance between the photon and the ship that fired it will be greater than 1 light year.

    Both are correct. This is so because each ship has a different experience of simultaneity. So when each ship measures the distance at the 'same time', in fact both ships idea of the 'same time' will differ.
  6. May 20, 2010 #5
    Look up 'relativity of simultaneity' on wikipedia for an simple enough explanation.
  7. May 23, 2010 #6
    I think you should start with the beginning. The beginning of special relativity is the derivation of the Lorentz transformation. You can find the derivation of the Lorentz transformation for the layman on


    but that text is in Dutch. If you want me to translate the text, please, let me know.

    My e-mail address: advdv@socsci.ru.nl
  8. May 23, 2010 #7
    Aaron is right. When you realise that you need at least two clocks on the rocket (one near you and one near the tail of the rocket) in order to measure the speed of the rear going light and when you know how are clocks are synchronised in relativity, it will all become clear. Look up "Einstein clock synchronisation" and "relativity of simultaneity".
  9. May 23, 2010 #8
    "..I am having difficulty wrapping my mind around Einstein's relativity, and am wondering if someone can please help to enlighten me..."

    I completely disagree that the reference will make it become clear. Give some insight, perhaps, but make it clear, that's asking to much. If anyone could really make it clear, they'd have a simple explanation.

    In fact, relativity does NOT make intuitive sense...you first have to accept that the concept you THINK is accurate based on everyday perceptions is WRONG: time and distance are NOT fixed. They "seem" to be absolute in everyday life, for example a ruler always seem to measure the same length, but we have discovered that they in fact vary according to relative observer motion.

    What does NOT vary is the speed of light. For reasons no one really understands, light is a constant to all observers. It still amazes me to say/write/ accept that. It is profoundly puzzling. I found it helpful to "memorize" that fact, after I reluctantly accepted it, and use it as a postulate for other applications.

    And so far lightspeed to set the upper bound on motion and information transfer, at least in this universe. But if time, distance (space) and lightspeed all emerge from the big bang, or even a small bang, why two are variable and one is fixed is not at all clear. It is just as mysterious as energy and mass apparently being two forms of some common entity.

    And don't let anyone claim that mass/energy/pressure bending space (gravity is a geometric phenomena rather than a real force] makes any sense in general relativity, either: it's an experimentally based finding and enables us to explain things we could not explain before, so it sure seems to be true.
    Last edited: May 23, 2010
  10. May 24, 2010 #9
    You do not understand relativity with words only. Maths is the key to the door.

    Crossing a river you can swim. Crossing the ocean you need a boat.
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