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Are the transformations just observed ones or real ones?

  1. Sep 10, 2013 #1
    Hello!
    Are the transformations such as time dilation, length contraction and relativistic mass just observed ones or real ones?
    Thank you.
     
  2. jcsd
  3. Sep 10, 2013 #2

    Dale

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    What is the difference? How are we supposed to learn about reality other than through observing it?

    More explicitly, is there an experiment which could tell the difference between an observed transform and a real one? If not, then the question is not scientific.
     
  4. Sep 10, 2013 #3

    Nugatory

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    Look for a pinned thread rgth at the top of this forum on experimental support for special relativity:https://www.physicsforums.com/showthread.php?t=229034

    Time dilation, relativistic mass increase, and length contraction have all been observed and measured.

    (And I'm somewhat unclear on what you mean by the difference between "observed" and "real". If by "observed" you mean some sort of optical illusion, they're not illusions, they're real).
     
  5. Sep 10, 2013 #4
    I mean we observe things through photons and because relative to us something is traveling then what we see will be different because of the transforms; So what if we could see reality, the object as it is, in other words we use the transforms to somewhat find the real value will it be correct?
    I hope you understood what I mean, it has nothing to do with philosophy.
     
  6. Sep 10, 2013 #5

    Dale

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    Then please describe the experiment that you are thinking of. If you can describe the experiment sufficiently then we should be able to figure out the predicted outcome or possibly the actual experimental outcome.
     
  7. Sep 10, 2013 #6

    phinds

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    I don't disagree with the previous answers but I think it might help to state the following: you are at this very moment traveling at .9999c from some frame of reference. From that frame of reference, you are severely time dilated and length contracted. This is a real and measurable fact from that frame of reference. Do you feel any different?

    From some other frame of reference, you are now traveling at .9c and are only mildly time dilated and length contracted from that frame of reference.

    Are the observations from your frame of reference any more valid than those from the other two frames of reference? No.

    You cannot really talk about "real" unless you specify the frame of reference from which you are defining "real".
     
  8. Sep 10, 2013 #7
    We do see reality, and reality looks different depending on your point of view, and yet, all the different points of view are consistent with each other. - That's relativity.
     
  9. Sep 10, 2013 #8
    Perhaps you are referring to the fact that light takes a finite amount of time to reach us from any event, and our view may be distorted by this delay? If so, the answer is that length contraction, time dilation and so on are effects that remain even after you account for the finite travel time of the light that you are using to see things with.

    Here's an example to make this clear: muons are particles that, when at rest, live for about 2 * 10^-6 seconds before they decay. Given that lifetime and the fact that nothing can go faster than c, it might seem like they shouldn't be able to travel more than about c * (2 * 10^-6 seconds) = 600 meters before they decay. But in fact muons that are travelling near the speed of light can travel much farther than 600 meters before they decay. The reason is that the "internal clock" of a fast muon is slowed down by time dilation, so they live much longer than 2 * 10^-6 seconds. Whatever you mean by "real," this seems to me to be a clear indication that time dilation is "real" and not just a deceptive appearance.
     
  10. Sep 10, 2013 #9

    Jano L.

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    The time dilation (see the example above) and increase in effective mass (Kaufmann's experiments and later ones) have been observed, and thus considered as real. The effect of length contraction has not been observed, as far as I know. The theory, by the way, predicts that the moving bodies would appear to stationary observer as if they were rotated, not contracted (due to high speed). The length contraction is basic result of the theory of relativity, so in light of other successes of that theory, most physicists believe it exists, but as I said we do not have direct evidence so we do not know for sure if it is real.
     
  11. Sep 11, 2013 #10
    Neither, if between inertial frames: they are not real in the sense that they are not "absolute", and they are not just observed in the sense that really something changed when an object changed speed.
    Non-inertial motion breaks the symmetry in observations: a clock that is moved fast around will be found to have lost time relatively to a clock that is kept steady (ignoring gravitational effects etc). One just can't say that a fast moving clock is "really slow", as that would imply the observation of absolute speed in the sense of "really going fast through space".

    [addendum] An important clue to understanding is relativity of simultaneity: depending on how you decide to synchronize your clocks, you "observe" that a relatively to you moving clock is ticking slow or fast. It all boils down to your free choice to pretend that you are "really in rest" or "really moving"; you cannot say that it's true. Consequently we cannot, as you put it, say that we see the object "as it really is". According to you, if you take yourself to be in rest, a relatively to you fast moving object is length contracted; but you may instead hold that object to be in rest and therefore not length contracted.
     
    Last edited: Sep 11, 2013
  12. Sep 11, 2013 #11

    ghwellsjr

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    This, to me, is a trick question because the coordinate effects as a result of transformation are not observable. What's real are all the observations and measurements that anyone makes and they don't change when a transformation is performed.
     
  13. Sep 11, 2013 #12
    Your question is ambiguous, leading to unclear responses. I suggest a different wording which avoids the word “real”.

    I can use light rays and a clock to measure the length of a remote object which is at rest in respect to me. However the same experimental protocol will deliver a different numerical outcome if the object is moving in respect to me, all things equal. Does that mean:
    i) that the so-called “length of the object" varies due to its relative motion in respect to me? ... or does that mean that
    ii) the measurement protocol I used delivers a “biased value” for the length of the object due to its relative motion in respect to me?

    In the first case the length should not be considered as an attribute of the object, it is an attribute of my relationship to it (likewise the color) . In the second case, the length can be assigned as an attribute of the object, although its appearance may vary depending on experimental conditions (likewise the shape).

    Hopefully physicists will clarify the SR view on this alternative.
     
  14. Sep 12, 2013 #13
    I'm afraid that I don't understand your phrasing better than that of the OP; nevertheless I guess that I and others already answered it (in different phrasings). Note that SR describes not motion relative to people but motion relative to inertial reference systems.

    Rephrasing your questions I would say that:
    i) the so-called “length of the object" varies as function of variation of its motion as measured with any inertial reference system; and that
    ii) the measurement protocol that you use delivers a “biased value” for the length of the object as function of your free choice of clock synchronization.

    Probably it would be useful if we give an example of the effect of clock synchronization (ii) on the measurement (i).
     
  15. Sep 12, 2013 #14

    Dale

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    The mainstream SR philosophical view is clearly the first. The reason that the second doesn't work is that "bias" implies that one frame is right and the others are wrong. This is opposed to the principle of relativity.

    However, there is no experimental way to distinguish between the first and second, so it is a matter of philosophical preference. I tried to get the OP to recognize that by thinking about possible experiments.
     
  16. Sep 12, 2013 #15
    Thank you for your input. I do think the precise wording of question/answers is a key element for non-physicists like me grasping the about-ness of this non-intuitive theory. However I must say that in this particular case the expressions “ inertial reference system” and “free choice of clock synchronization” look inappropriate to me.
    To be precise I wish to add that the value returned by the measurement protocol in the specific case where the target object is at rest in respect to the inertial-system-from-which-the-measurement-is-exercised can be assigned as an attribute of the object: physicists refer to the “proper length” of the object.
    However, the value returned by the same measurement protocol (no change to the clock synchronization process) in the general case where the target object is in constant motion in respect to the inertial-system-from-which-the-measurement-is-exercised, being different from the proper-length, can be considered :
    1) either as tracing a “length contraction”, which seems to refer to something happening to the target object itself, not to the way it “appears” from a given perspective...
    2) or as an apparent-length of the target object, which means that in such experimental conditions the protocol delivers a biased value of the proper-length and therefore cannot be considered as a valid method for measuring it.

    Eventually the initial question relates to the actual meaning SR assigns to an expression like “length contraction”: does it refer to something happening to the target object or to the way the (unaffected) object is “perceived” from a different “perspective”?
    Hopefully the “OP” (as you say) will tell us whether this wording matches his/her concerns... but anyway I'm curious to learn about the answer.
     
  17. Sep 12, 2013 #16
    In my understanding the principle or relativity of motion tells that it does not make sense to state that a physical object is at rest better than in constant motion (from an absolute perspective) or the other way round. This is not what is at stake here.
    An experimental protocol may be appropriate to measure a physical quantity under certain experimental conditions and the same protocol may be inappropriate if these constraints are not met. Hence the “biased value”. As you can read from my previous input, I'm trying to understand whether “length contraction” refers to something which affects the target object itself or the way it is “perceived” through a non-invasive measurement process.
     
  18. Sep 12, 2013 #17

    ghwellsjr

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    Yes.
     
  19. Sep 12, 2013 #18

    Dale

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    I think that is exactly what is at stake here. If A and B are two inertial observers moving relative to each other and each performs the same experimental measurement but you say that A's is biased and B's is not then you are certainly violating the principle of relativity.
     
  20. Sep 12, 2013 #19

    Jano L.

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    No, in the theory of relativity the length contraction has nothing to do with perception. The inter-molecule distances and the electromagnetic field pattern around them actually contract. If that was no so, the molecules would not be in equilibrium positions and the material would be in a state of tension, which would manifest as length extension in the the co-moving frame and could lead to breaking of the body into pieces.

    An object moving fast with respect to the Earth actually contracts in the frame of the Earth, so for example, fast moving limousine of rest length 10m could fit into garage 5 m long and in principle you could close the door. The other thing is, what happens next... such car would have very high energy.
     
  21. Sep 12, 2013 #20

    ghwellsjr

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    If that's true, then don't you think that you should also point out that in the frame of the limousine, it's the garage that actually contracts? And if you agree, then how does this help resolve the OP's question?
     
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