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Are relativistic effects real ?

  1. Jun 3, 2008 #1
    Are relativistic effects "real"?

    At this point i'd like to detour the topic and ask why time is so special that it doesn't come back to its original measurement. After all, once all moving observers are stopped wrt to each other, and they compare their measuring rods, they measure equal. so why not the clocks?

    If one goes by saying they do measure similarly afterwards, it makes sense to assume all the intermediate slowdown was only "virtual".

    since clock is a "cumulative" device, if one were cycling on the spaceship, a stationary observer might calculate the cumulative distance travelled as different than what the astronaut believes. but when they actually come down, shake hands, and compare the logs, both will read same

    [edit: I split off this topic from the original https://www.physicsforums.com/showthread.php?t=236885" thread--Doc Al]
    Last edited by a moderator: Apr 23, 2017
  2. jcsd
  3. Jun 3, 2008 #2


    Staff: Mentor

    I don't understand your point. Clocks do "come back" to their original measurement. Once two observers are stopped wrt each other if they compare their clocks they find that each measures 1s to be the same duration.
  4. Jun 3, 2008 #3
    the point being, the clock appearing slower is exactly that, "it appears".
    a person walking from front to back of the spaceship thats 100m long, would say "i walked 100m today". a stationary observer would be incorrect in asserting "no you walked 50m, since the spaceship was travelling at so-and-so speed" and your meter shrunk to half of mine. The stationary observer may have illusion that his meter is the bigger one, but if he puts the two rods side by side (hence moving it at same speed as rocket), he can assure himself both are same length. this uncanny ability given to stationary observer to "eyeball" the moving observers instruments is highly suspicious
  5. Jun 3, 2008 #4
    Hello stillwonder.

    First of all remember there is no observer in the privileged position of being stationary. All movement is relative so when two objects or observers move relative to each other, each can consider himself still and the other moving or vice versa.

    As regards the contraction being an illusion, it is not. Once again remember that each sees the others measuring stick contract.

    If when at rest relative to each other, in an inertial ( non-accelerating ) system, two rods of identical length are placed side by side they will, by direct comparison, be of the same length. If we could engineer a situation whereby these same two rods could pass each other in relative, inertial motion, as close as we please in space so as to remove any optical effects, and thus compare their lengths as before, by direct comparison, then Relativity predicts that each would be shorter than the other from the other observer’s frame. As far as I am aware the technical problems in doing such an experiment have not yet been overcome and so it is still a prediction but it is all part and parcel of the theory.

    This does not fit our common sense ideas and is hard to imagine,but that is how it is.

  6. Jun 3, 2008 #5


    Staff: Mentor

    And would you say that relativistically moving muons just "appear" to reach the earth?
  7. Jun 4, 2008 #6
    Ok this is not an answer but a new question. Who is moving????? if two observers are 10 lightyears apart, and i will not say how but their clocks both start at 0... Then observer B blasts of at .5c toward observer A. whos clock slows?? Do they both see the the others clock run slow?? Who knows which frame is stationary?? maybe they both began at .5c and when observer B blasted off toward observer A closing the gap at .5c he really stoped and became the stationary frame. if so then observer A,s clock would seem to be the one that suffers time dialation... Or maybe when they started they were both moving at .25c at which point when Observer B blasts of toward observer A closing the gap at .5c they are indeed both moving at .25c toward each other . and when they meet they will have the same reading on their clocks.
  8. Jun 4, 2008 #7
    Hello ne4pres


    ---Do they both see the the others clock run slow?? ----

    Yes under all conditions of relative inertial motion.

  9. Jun 4, 2008 #8
    Hello ne4pres

    Yes under all conditions of relative inertial motion

    !!!I should have added after allowing for any optical effects of their relative motion.

  10. Jun 4, 2008 #9
    a basketball looking like a football instead of a baseball to a moving observer shows whether there is some wool being pulled. taking "instantaneous" snapshot while two rods are arbitrarily close is again suspicious. how about allowing arbitrarily large amount of time to make the measurement instead of "hurrying it up".

    I have no problems with the predictions and measurements under relativity. But they are just that, measurements. To attribute these measurements to underlying reality, if any, is a jump of faith.
  11. Jun 4, 2008 #10
    Hello stillwonder.

    I'm afraid i can't help you any more. I have given you the facts and any textbook will confirm what i have said. If you cannot accept that then that is your choice.

  12. Jun 4, 2008 #11


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    How about telling us how this could be done, given two rods in relative motion?
    I think that we get to the point now.
    You seem to assume that an (idealized) measuring rod really is some 1-dimensional entity, which has a given length, and that it is impossible that this length really changes depending on the observer.
    So far so good, but relativity tells us that we must not ignore the rod's extension in time. It existed some time before and will exist some time in the future.
    Hence the rod really is a 2-dimensional entity, extended in one space- and one time dimension.
    The 1-dimensional entity that you regard as the rod is merely a hypothetic slice throug the real rod, what remains if you observe it at a definite time t and forget about its extension in time.
    The point is, every observer in relative motion to another will regard a different slice through the real rod as "the rod". Different slices of course have different lengths, but that does not change the real, 2-dimensional rod.
    It's up to you whether you call length contraction real or not, but it misses the point.

    It's also not uncommon that people reject this point of view as "absurd" or something like that. If you do so, that's ok with me, but I can assure you that then you will never understand relativty.
  13. Jun 4, 2008 #12

    if i read you correct, you are assuming two things

    1. The rod was formed by streching a ball of iron
    2. and the various stages of streching are what the observers observe when they move

    i'll have to disagree with both
    1. the ball was compressed from a larger rod
    2. there is no "time travel" involved. otehrwise the moving observer should see the other rod increasing in length
  14. Jun 4, 2008 #13
    thanks Matheinste.
    As i said i have no dispute with what the textbook says.
    Infact, all definitions in physics are "operational". ie what you measure is what it IS.
    and the measurement is consistent with the theory.
  15. Jun 4, 2008 #14


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    No, I assume that the rod was there before you start the experiment and ist still there when you finish.
    In a standard spacetime diagram (where the rod is at rest), the rod's endpoints are drawn as two vertical lines, and the rod itself is drawn as the area between these lines. The intersection of that "real" rod with the horizontal t=0 line is what you would call "the rod at time t=0".
    Every relatively moving observer will have a different opinion about the t'=0 line. All those t'=0 lines intersect the real (2-dimensional) rod at different angles, therefore every observer will see a different "rod at time t'=0".
  16. Jun 4, 2008 #15
    maybe i am being paranoid, but i feel being categorized along with the relativity buster gang!
    i have already stated i have no issues with the theory per se but only the personal baggage that people are loading it with.

    two rods (one stationary another moving) appearing to be same length doesn't mean they are similar. the fact that one is moving is riveted to its identity in that frame. even if it appears to be same length, it cannot be same. that is what i meant by "appears".


    lets say a 1m (at rest) rod looks like a 0.5m rod to a moving (speed v) observer. he places a 0.5m rod besides it and sees they appear to be same length (assuming plenty of these rods are encountered on his path). he tells another person sitting at the corner of the ship, "hey these are exactly the same length so they must be similar" (he already did spectral analysis to find they are made up of same material). so the corner guy says, why dont you jump over the rod at speed v in the same direction its moving and see if they appear similar. he jumps and boom one contracts another expands.

    a rod is measured longest in the rest frame. thats sort of special frame. directly or indirectly all calculations in intertial frames are wrt to this at rest frame. if you agree so far, there is something to be seen there, and i dont want to spoil it for anyone (assuming you havent already come upon it), but the key word is "measurement".
    Last edited: Jun 4, 2008
  17. Jun 5, 2008 #16


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    You basically answered your own question here:
    Yes, clocks are cumulative devices. And they function like a written log. But if a clock is keeping a record that is different throughout the flight, the clock won't suddenly spin to synchronize when he decelerates. Nor will the numbers written in a log magically change when you land to show the same elapsed time/distance. It will still have a different cumulative time elapsed. What becomes synchronized again when they land is the clock rate.

    GPS satellites show exactly this effect (though we've never brought one back that I know if - we just record and adjust to the cumulative difference).

    Similarly, if you happen to get hired to take a quick lap around Alpha Centuari at near light speed, you'll want to make sure you get paid by Earth hour or mile, not what's recorded in your log. You'll find your log shows much less than the 8.8 years/light years that Earth-based observers record.
    Last edited: Jun 5, 2008
  18. Jun 5, 2008 #17
    thats what this thread was about:
  19. Jun 5, 2008 #18

    allow me to rephrase my last line "both will read same" to "both will agree on the results since the ground observer understands relativity and will make concessions accordingly".

    although introducing accelaration changes the game, but in principle it shouldn't affect the point being made.

    Assumption: Ground crew g can make two identical spaceships take off from opposite directions equidistant such that they pass each other with equal but opposite velocity at point g. Obviously the acceleration phase is taken care of so they achieve the velocity v by the time they are at their posts '|' and there will not be acceleration after that. This can be done by using all the physics they have learnt in college :-)

    S syncs up with O when he passes him and stops the clock when he reaches the other post '|'. Similarly O stops the clock when S1 reaches him. Now if S is brought to O, whose clock shows less time? throughout the experiment they thought the other was slower.

    I understand O and S will not agree they stopped the clocks at the same time and therein lies the illusion. The ground crew g has calculated everything to arbitrary precision. The spaceships can be made identical. The distances |g and g| are precise. The light used to signal S and S1 to start is precise. So what gives?

    Last edited: Jun 5, 2008
  20. Jun 5, 2008 #19


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    What illusion? Therein lies the solution.
  21. Jun 5, 2008 #20


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    Neither, both will show the same time.
    Actually, no. Because as noted below, there will be a period where their clock has stopped and the other has continued running.
    No illusion. for O, S really does stop after O. and for S, O stops after S.
    Thus for O, when it stops at time T, clock S reads something less than T, but then continues to run until it reads T then stops. The reverse happens from S's perspective.
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