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I Special relativity and sameness

  1. Jan 30, 2017 #1
    • Moved from off topic thread
    It seems if the clocks show the same time, the rod is at rest in our frame. If clocks are out of sync, rod is in motion. The closer velocity is to that of light, the more clocks out of sync, Another question - are all these rods the SAME rod? For example you ask me - what is velocity of the rod? - Which rod? Could you please to concertize? Tell me how it appears (what are clocks readings) and I will tell you what it's velocity is. For example, beetle pest starts eating rod simultaneously from the both ends in the rod's rest frame. In different reference frames (that are in relative motion) the remainder of the rod will look differently. From the reference frame which moves with velocity close to c the rod will be eaten to the center from the one side, but from the other side it will be almost untouched. It seems like that.
  2. jcsd
  3. Jan 31, 2017 #2
    I'm pretty sure if you waited until the beetles stopped eating, and then brought the rod to each observer, they will measure the same amount of rod eaten, and that any given frame where it looks like more of the rod is eaten than in another (take note of the when) would just be some combination of the relativity of simultaneity and length contraction/time dilation. None of them are likely going to measure the exact same times for the events, because simultaneous events in one frame are not necessarily simultaneous in another. I don't understand how this is some sort of paradox.
  4. Jan 31, 2017 #3


    Staff: Mentor

    What physical experiment could you perform to measure the SAMEness of a rod?
  5. Jan 31, 2017 #4
    Let’s look on the rod at the different angle.

    Once upon a time there was a rod 10 miles long with marks every mile. It was like that:


    Then beetles started eating it from both sides. Let’s say in our frame K’ which moves at 0.9 c. Then at certain moment we see surplus of the rod like that:


    What is proper length of this rod? I don’t even know what to say.

    IMHO the problem is that we can see the rod (-1) ==(0)==(+1) only from a frame K’. From the frame K’’ which moves with another relative velocity V2 the rod may appear like that:


    From the frame K’’ which moves with another relative velocity V3 the rod may appear like that:


    It is simplification, sure I haven’t calculated it.

    But the rod (-1)==(0)==(+1) only “exists” in a frame K’ and has exact “absolute” velocity 0.9 c. We cannot see the rod (-1)==(0)==(+1) from any other frame.

    Note: part of the whole rod (-1)==(0)==(+1) has its proper length however.

    Heraclitus told that we cannot enter into the same river twice because it changes with time. The same river from the different reference frames will appear different. Was it the same river or not?

    I suppose that noetic rod has any velocity. But if we concertize material body to the smallest details we can say how fast and in which direction it moves.

    If we want to see the “same” river from all frames, observers in every frame have to take into account their own velocity and synchronize clocks accordingly.

    Maybe I am wrong,
  6. Jan 31, 2017 #5


    Staff: Mentor

    You didn't answer my question. Again: What physical experiment could you perform to measure the SAMEness of a rod? Please answer the question.
  7. Jan 31, 2017 #6


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    @Bartolomeo - The usual way to view it is that a supposedly one-dimensional rod is in fact a two-dimensional sheet in spacetime. Then you come in with your synchronisation convention and pick a one-dimensional slice to call "the rod now". So all the people with different synchronisation conventions are just taking different slices through the rod. They're viewing different parts of the same entity.

    Whether that constitutes "the same rod" or not is up to you.
  8. Jan 31, 2017 #7
    I think that I have to take readings of clocks on the end of the rod. At least. Or to take a photo.
  9. Jan 31, 2017 #8


    Staff: Mentor

    OK, so if you take a photo then how do you change that photo into a measurement of SAMEness?
  10. Jan 31, 2017 #9
    Pictures taken from different frames will be different I think. If the rod is covered with synchronized clocks (every spot) at each picture the readings will be different. If you will show me any chosen picture I may not agree with you. It was not the rod I wanted to see. But by appearance of the rod I can gess what frame it was photographed from.
  11. Jan 31, 2017 #10
    For example you ask me. What is velocity of the rod? Which one? Any rod has any velocity. Please describe in details or show me a picture. I will tell you what is velocity of exact rod.
  12. Jan 31, 2017 #11


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    No. If the cameras are co-located you will see the same clock readings regardless of frame (edit: note that this is not the same as taking identical pictures). If the cameras are not co-located you will see different clock readings regardless of frame. Remember the light speed delay.

    Now, if the instantaneously co-located camera users calculate backwards from what they see then the answer will be frame dependent. But, again, they're just defining different slices of the same extended-in-space-and-time entity as the extended-in-space-only thing they call "the rod now".
    Last edited: Jan 31, 2017
  13. Jan 31, 2017 #12


    Staff: Mentor

    Yes. And pictures from different locations in one frame will also be different. Do you want a measure of SAMEness that behaves that way?
  14. Jan 31, 2017 #13

    Mister T

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    If everything you've stated so far refers to frame K' then the statement that the frame moves at speed 0.9 c is meaningless because you haven't stated that speed relative to anything. If that's the case, the frame's speed is not relevant. The rod is at rest in frame K', its proper length was 10 miles, now it's 2 miles.

    Note that the phrase "beetles begin eating" has meaning in only one frame. Other frames will observe that the beetles do not begin at the same time.
  15. Jan 31, 2017 #14
    Yes it is the same rod, but its "proper" age changes along its length in your rest frame. But none of this is what you see, as I said earlier. I am not a fan of "time distraction" and "length confusion" as a teaching tool. It always ends up like this thread . . .
    That is why I gave a link to a demonstration of what you see, which is how you would actually measure things.
  16. Feb 2, 2017 #15
    As far as I know in there are different techniques of taking pictures in Special Relativity with different outcome.

    In this case an observer must take picture this way: the observer places many cameras in his reference frame close to each other. The cameras should be in immediate vicinity to the rod. Then all these cameras make click simultaneously in observer’s frame. Then this observer glues pictures into one.

    It is like taking picture of long relativistic train. There are many cars in the train and each car has a clock. These clocks show the same time in train‘s frame. Observer on the platform places cameras along the platform and these cameras take picture simultaneously in his own frame. Then he glues picture into one. Due to relativity of simultaneity different clocks in different cars will show different time on the picture.

    Small note about pinhole camera.

    If you wish to take a picture by pinhole camera you can run into troubles, because rays from the different parts of the rod will come into aperture at different moment. If the object shines by reflected light the picture can be spoiled by optical effects, like Terrell – Penrose rotation.

    However, it is possible to take a picture by pinhole camera so as to see two clocks on the ends of the rod. In this case very close to every clock we have to place a flash. Then cameraman releases these flashes simultaneously in his frame. These flashes highlight these clocks and reflected beams simultaneously come to aperture, since their patches are hypotenuses of equal triangles. The rod will appear contracted and the clocks will show different time due to relativity of simultaneity.

    For example we can evaluate distortions of a square (magnitude of length contraction) by it‘s picture. In this case we place a flash at each corner of the square. Then cameraman releases flashes simultaneously in his reference frame. Since the square moves in the reference frame of a cameraman, the picture should be taken when the cameraman and the center of the square at the points of the closest approach. The square will appear distorted – it will be contracted in direction of its motion. Cameraman can measure vertical sides and horizontal ones and will measure magnitude of Lorentz contraction of the square.

    But, in this case the only spatial position at which the camera can make such a picture (so as all the lamps were visible in the picture) is the point of closest approach to the square or rod.
  17. Feb 2, 2017 #16


    Staff: Mentor

    With this definition of "same" then the rod in one frame is not the "same" as the rod in another frame. As you have defined it sameness is frame variant.
  18. Feb 2, 2017 #17

    The same is the same for any observer – I mean the object, which has exact clock readings and consequently appearance. For example, objects (-1)==(0)==(+1) and objects (-1)==(0)==(+1)==(+2) are not the same, thought we call the both „THE rod“.

    Are the log and a nesting box made of him the "same" tree? An artisan begins to cut it out starting from one end.

    It seems to me, from different reference frames we see „different“ rods. Thus „sameness“ does not depend on frame. One million different frames – one million different „appearances“.

    If we give a clear description of the physical object, this „momentarily concertized object“ has not only speed but also direction of motion. There is one and only one frame K‘ in which the rod (-1)==(0)==(+1) exists. We CANNOT make picture of THIS object from any other frame.
  19. Feb 2, 2017 #18


    Staff: Mentor

    Note, that your concept of "same" has very little (if any) physical value. It is not conserved, it is not invariant, it doesn't predict anything else. It is just a thing you have defined for the sole purpose of saying that the object isn't the same in different frames. There is no physical relevance to any other physical quantity. It is just a convention based on the Einstein synchronization convention and the relativity of simultaneity.
  20. Feb 2, 2017 #19


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    I would stamp a unique serial number on the rod. Then everyone would know it's the same rod when they see it.

    I'm not sure what this has to do with SR.
  21. Feb 2, 2017 #20

    Mister T

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    No one I know would do that. If THE contracted rod is (-1)==(0)==(+1) then THE same rod, less contracted, is (-1)===(0)===(+1).

    It certainly wouldn't be (-1)==(0)==(+1)==(+2).
    Last edited: Feb 2, 2017
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