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Time Dilation Concept Confusion

  1. Jul 28, 2011 #1
    Recently I've watched a few videos and read over some explanations of time dilation and am quite confused by several aspects.

    In the video I watched, the clock tracked time by a beam of light bouncing between two metal plates, incrementing one second per touch of the bottom plate. The path of the light beam is vertical when the clock is stationary, which insures the shortest (and quickest) path between the two metal plates.

    Time dilation was shown as: if a spaceship with the clock moved close to the speed of light, the clock would slow down. Why? The light beam's path would become slanted and diagonal, which causes a longer path for the light particles. Since light always moves at the same speed, this causes the clock to accumulate seconds (and therefore time) at a slower rate due to the longer path of the light beam in hitting the bottom metal plate.

    Now, my confusion stems from a rather simple contradiction that I thought of. I'm not sure if the contradiction is valid, so I would like some explanation of why my contradiction cannot be valid as I'm sure my contradiction is not valid im some manner.

    If the slowing of time for the clock in the spaceship is due to the lengthier path that the light must travel, then would not the slowing of time truly be a consequence of the clock's mechanism? In other words, the fault lies with the design of the clock's mechanism. When establishing the definition of a "second," my thoughts are that it should be defined as something non-arbitrary, such as "the time taken for a beam of light to travel the shortest round-trip path i.e. a vertical path between two metal plates."

    With the above in mind, I am unable to rid the logic that time dilation is not truly a slowing of time but rather a fault on the clock's mechanism. Furthermore, in this case, though the clock may present a slower passage of time due to its design, the human body would continue to function in the same time intervals as in any other frame of reference. Thus the human body would NOT age any slower or quicker than a stationary human. It would simply be the clock showing a slower passage of time due to the increased speed's effect on the light particle's path between the two metal plates.

    I would greatly appreciate anyone who could explain to me how my logic is flawed :)
  2. jcsd
  3. Jul 28, 2011 #2


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    A light clock, as you described, would slow down but so would everything else that is dependent on time in any way, including biological clocks. However, none of these clocks moving together, in other words, all at rest in the same reference frame can tell that they are being slowed down. In fact, whose to say that they are actually running at the "correct" rate of time and everybody else, traveling with respect to them, is the one that is slowed down?
  4. Jul 28, 2011 #3


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    If that was the case, then we would get different amounts of time dilation for clocks that used different mechanisms, but in reality experiments show that all clocks obey the same rules.
  5. Jul 28, 2011 #4
    My confusion lies with HOW biological clocks would slow down if the slowed passage of time on a light clock is based on the fact that the light must take a longer path to mark off a second. A biological clock would not involve light traveling a longer path...and does not involve anything traveling a significantly longer path as space is quite limited.
  6. Jul 28, 2011 #5
    Bcranger, there's another way to look at this. You might consider what is going on with observers in relative motion from a geometric point of view. The sketch below depicts the motion of red and blue guys moving away from each other at relativistic speeds. The motions are charted as red moving to the left with respect to a black rest system and blue moving to the right at the same speed with respect to the black system. The situation is shown in the context of a 4-dimensional universe. The red and blue guy move along their respective X4 axes at light speed. Special relativity tells us that the X1 axis rotates in the 4-dimensional space such that a photon world line would bisect the angle between the X4 axis and the X1 axis for all observers. This results in different 3-D cross-section views of the universe for blue and red observers.

    Each observer lives in a different instantaneous 3-D universe. In the upper left sketch, when the red observer is at station 9, his view of the universe intersects the blue guy's X4 axis at station 8. And when the blue guy is at his blue station 9, his view of the universe intersects the red guy's X4 axis at station 8. This results in the time dilation effect for both observers. But clocks have not really objectively slowed down in terms of the big 4-D universe picture. The dilation effect just results from different cross-section views of the universe.

    The right side sketch below shows how you can derive the time dilation formula using the Pythagorean theorem for the right triangle formed by displacements along red X4, blue X1 and blue X4. Blue X4 is the hypotenuse.

    The lower left sketch shows why the red rocket seems contracted in the 3-D cross-section view of the blue guy's instantaneous 3-D space.

  7. Jul 28, 2011 #6


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    The same electromagnetic interactions that are characterized as light between remotedly located pieces of matter are also at work between pieces of matter in close proximity. Why would you think that biological matter is subject to different laws of physics than non-biological matter?
  8. Jul 29, 2011 #7


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    You have to go back an consider what the light speed postulate means. It means that everyone measures the speed of light as having the same value as measured relative to themselves. . If I am sitting next to a light clock which has the mirrors 114,891229 m apart it will take 1 sec for the light to make the round trip by my measurement. Light travels at 299,792,458m/s, and that's the distance of the round trip. It doesn't matter how I measure that second,( for instance, saying to myself "1 one thousand". Not highly accurate but works for this example)

    For someone watching the light clock and me traveling past them at 0.866 c with their own light clock will note that as their light clock ticks off one sec, The light for my light clock, having to travel a longer diagonal path, at the same speed as the light from my clock, has only made one leg of the round trip. His clock ticks off 2 secs for every 1 my clock ticks off.
    In addition, Since I am just finishing saying " 1 one thousand" at the end of my 1 tick (having started at the beginning of the tick). he must note the same, otherwise we would have a physical contradiction. Thus he also notes that I say it at half speed.

    Like this:

    Here the counters play the role of verbally counting off.

    We simply measure the duration of "one second" differently
    Last edited by a moderator: Apr 26, 2017
  9. Jul 29, 2011 #8
    Bc..welcome to physics forums....

    Since no one has said it explicitly I will: the passage of time does NOT change locally....in the frame of reference of the fast moving spaceship, inside the spacehsip, everything is "normal". Clocks tick and people age normally. It is only from the perspective of a distant viewer, say on earth, that time appears dilated on the spaceship.

    If observers on the spaceship looked back at the same type clock on earth, it too would appear to be running slower....only when they slow down and compare elapsed times together, in the same frame (same place and speed) can they tell whose clock is really running slower....It will be that clock that accelerated, moved fast, then decelerated. People ton the spaceship will have aged less and their clock compared to a "stationary" clock will display less elapsed time. All physical processes, biological, chemical, radioactive decay will ALL display reduced aging.

    For more technical details, read about how the global positioning system (GPS), as an example, has to correct for time dilation to remain accurate: satellite time is slower than earth time....due to both gravitational and velocity effects. It's not only relative speed that affects our perception of time, so does gravity!!!!!
  10. Jul 29, 2011 #9
    Very nice animation. Thanks.
    Last edited by a moderator: Apr 26, 2017
  11. Jul 29, 2011 #10
    Very Cool animation Janus!
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