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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?
     
  22. Sep 13, 2013 #21
    They are essential for a good understanding. When I have time I'll give a numerical example that may clarify it better than 100 words (I think that I gave one in the past, if I can find it back that will save time).

    Yes, that's what I said. Whatever changes state is affected; what does not change state cannot be affected by the non-change. And the example to-be-given will clarify how the measured value depends on your free of choice of clock synchronization.
    Once more, see above! Length is "relative" and proper length is "absolute".
     
  23. Sep 13, 2013 #22
    Your statement would be correct if it were true that the relative motion between observers can cause a divergence of measurement results, but this cannot be. If A and B obtain different outcomes when running analogue measurement protocols (i.e. the phenomena they observe are different), it must be due to an objective difference in their respective experimental conditions (otherwise we would be debating on non-determinist patterns, which is not the case here). One cannot attribute this difference in results to observers being “moving relative to each other” since this would indeed breech the principle of relativity of motion. The relative motion between observers cannot, of its own, cause a difference in observed phenomena since it would mean that one of both observers goes objectively faster than the other one, and this is precisely what the above principle forbids: in spite of their relative motion in respect to each other, none of them can be said “moving faster” than the other one, none of them can be said “in absolute rest”.
    In the debate at stake, the rationale for the difference in the outcome of the observations is that one observer is at rest in respect to the target object whereas the other one is in motion in respect to the same object. Their relative speed in respect to the target object is different, and that is an objective difference in their respective experimental conditions. It is the cause for the divergence of their experimental results. And there is no breech to the principle of relativity.
    Indeed the relative motion between observers is a possible consequence of that fact, but there is no equivalence: it could be that both observers move away from the target object with the same relative speed but in opposite directions. This counter-example shows that in spite of being in relative motion between each other, both observers could have the same speed in respect to the target object, in which case it is impossible that they obtain different results: this would breech the principle of isotropy of the propagation of light in space (may I recall that the scenario at stake assumes that observers measure the propagation time of light rays and then convert it into distances using the factor c).
    One must trace the difference in experimental results to a proper cause, to an objective difference in experimental conditions, to something which holds for an observer but not for the other one, to an asymmetry between their respective experimental conditions. The “motion between observers” is a symmetrical and reciprocal clause which cannot make it.
    Very appropriately the generic title given to this forum reads: “Special & General Relativity - Dependence of various physical phenomena on relative motion of the observer and the observed objects. Exp. & theo. theories of relativity”.
     
  24. Sep 13, 2013 #23
    So the “length” is “relative” to what? To the measurement process? This is precisely what we would normally call the “apparent length”, a biased value of the “proper length”. What is the “apparent diameter” of the moon if not an effect of the observation conditions, a biased value of its “proper diameter”? Certainly the moon itself is not affected.
    You seem to indicate that the observed object “changes state” (changes length?) as a consequence of being observed … how long is this change effective? Is the “length contraction” of the object caused / provoked / induced by the incoming light ray from the measurement device? How is the information transmitted which triggers the magnitude of the contraction? ... I'm afraid your statement will induce more questions than answers.
    Obviously you make a difference between relativistic and non-relativistic patterns. We are looking for a proper wording to make the nature of this difference understandable by non-experts. You don't need to invent anything, just tell us what the SR theory says.
     
  25. Sep 13, 2013 #24

    Dale

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    Ah, OK, I had misunderstood what you were saying previously. However, your concern is not relevant here. Length is a defined quantity, specifically length is defined as the distance between the front and back of an object at one instant in time. The velocity of the object is not relevant to the definition of length.

    You are thinking about the distinction between proper length and length. Proper length is defined as the length of an object in its rest frame, so the speed of an object is relevant for the definition of proper length. It is a different concept than length.

    An object's length in some frame may differ from its proper length, but that does not imply bias: they are both unbiased measurements of different quantities. Similarly, the object's length may differ in different frames, but that also does not imply any bias because of the prinicple of relativity.
     
    Last edited: Sep 13, 2013
  26. Sep 14, 2013 #25
    No and no. As I said before, I believe that sound bites are insufficient to explain these things; physics requires calculation examples to clarify the meaning of sentences. So here's my (totally unrealistic) numerical example:

    1. A space shuttle with a 10m long ruler (let's call it the "Moving Ruler") and two linear CCD arrays with clocks at its ends is accelerated to 0.1c relative to the launch site. The detectors function thus as combined position and time detector arrays. After one turn around the Earth it passes at that speed very close to CCD detectors that are situated on a 10m long ruler on the launch site; let's call that the Stationary Ruler.
    0.1c -> γ ≈ 1.005
    The length of such an object is defined as the distance between two extremities as determined at the same time.
    Let's suppose that you are standing there doing the measurements. You had synchronized your clocks to the launch frame shortly before the shuttle took off and that synchronization is still valid (let's assume negligible rotation of the Earth during the mesurements). According to your measurements, the length of the Moving Ruler was 9.95m at fly-by.

    Now, suppose that the astronauts synchronized their clocks only before departure; then their clocks should be nearly* synchronous with each other according to the launch pad frame. Consequently they will measure that the stationary ruler is 10.05m long. Now they scratch their heads and wonder if they should announce that they disproved relativity theory (remember CERN). But then one of them suddenly realizes that they had forgotten to synchronize their on-board clocks to the moving frame. They quickly do so, and at the next fly-by they measure that - as expected - the Stationary Ruler "has" a length of 9.95m.
    Of course, they always measure the length of their own 10m long ruler to be 10m long, as measured with a co-moving ruler of 1m. And also you agree with them that the proper length of the Moving Ruler is and was 10m.

    [addendum:] Note also that according to the inertial frame in which the flying shuttle is in rest, the "Moving Ruler" expanded from 9.95m to 10.00m.

    I hope that with the understanding that follows from the above example, my earlier comments will be clear upon a second reading.

    *only nearly, due to slightly different v(t) profiles
     
    Last edited: Sep 14, 2013
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