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Do objects actually shrink?

  1. Apr 11, 2008 #1
    What's the reason that objects shrink when they move?
    Is Lorentz-contraction an illusion or is it real?
    Is there any experiment that verify moving objects really shrink?
    Last edited: Apr 11, 2008
  2. jcsd
  3. Apr 11, 2008 #2
    There are all kinds of experiments verifying the relativistic predictions. To date, there is no experiment for length contraction. Probably this is due to the extreme difficulties of staging such an experiment :-(
  4. Apr 11, 2008 #3


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    There is no definite scientific answer to philosophical questions like "what is real, unfortunately.

    If you focus on the measurements, however, there is no question that when you measure the length of a moving object, it gets shorter.
  5. Apr 11, 2008 #4
    gets shorter?

    I think that depending on the measurement procedure (radar detection, photographic detection) the moving object gets longer or even does not change its measured length.
  6. Apr 12, 2008 #5


    Staff: Mentor

    I disagree. Researchers designing experiments in particle accelerators always have to factor in length contraction when determining the behavior of a bunch of interacting particles. What better experimental evidence is there than successfully using a theory to design and operate a real device?
  7. Apr 12, 2008 #6
    Shinking is not an optical illusion, it really happens: if equal sized objects move fast, then you could fit more object into a certain space than in case of stationary objects. However this contraction does not cause any tension in the object, since the object did not shrank in it's own coordinate system (in the system where it moves the equilibrium distances between atoms and molecules have changed because the electromagnetic field of a charge depends on the speed).

    Note: i would still not recommend solving the housing problems with Lorentz contraction, since the time dilation would cancel the benefit of Lorentz contraction.
    Last edited: Apr 12, 2008
  8. Apr 12, 2008 #7


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    That's not length contraction though, that's time contraction. The lives of the particles before they decay is what's getting shorter. That translates to shorter trajectories, true, but that's not the same thing as contraction of a non-zero-length object's dimension.
  9. Apr 12, 2008 #8
    See DaveC426913' s answer. There is no test for length contraction (to date)
  10. Apr 12, 2008 #9
    There are enormous consistency requirements, though. You could argue that a lot of experiments test Lorentz contraction indirectly.

    As for a test of Lorentz contraction... If particle accelerators verify "time dilation", and the Michelson-Morely experiment verifies the constancy of c, then we had better have Lorentz contraction or else we'd be in pickle.
  11. Apr 12, 2008 #10
    Hi , everyone
    I think that the length contraction is due to the Relativity of simultaneity .
    To measure the length of an object we must know the space-time coordinates of the point of it's beginning and the point of it's end at the same moment and form this information we can measure the length .
    In the case of a moving object the idea of at the same moment is not valid due to the Relativity of simultaneity .
    So , the observed length is depending on the obsever himself .
    For an observer who is in the frame of the moving object the idea of at the same moment is valid so , he measures the length of the object as the same length he'd measured before moving . But for an observer who is in a stationary frame outside the moving object the idea of at the same moment is not valid so , he measures the length of the object as shorter than the length he'd measured before moving .
    Last edited: Apr 12, 2008
  12. Apr 12, 2008 #11

    Ken G

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    I think the problem with length contraction is that there really is no way to measure length, we only measure time. That's because a true length would connect two acausal events, so such a measurement would also have to be causally impossible. But we can measure proper times between events along the same world line, so we do that, and reconstruct an indirect concept of length. I feel that the concept of length in relativity (not classically) is entirely an arbitrary construct, but one with great unifying and explanatory power. If we make that construct, it has to Lorentz contract, but like pervect said, we can't tell if it's real. Even if we adopt the operational definition of "real" that says anything we measure directly is real, that still only applies to time intervals along our world line, not length intervals connecting world lines.

    This may be a good place to introduce the "two rockets" puzzle. I apologize if that's been explored already, but the puzzle is, if you have two identical rockets one in front of the other (never mind the exhaust!), connected by a taut but weak flimsy rope, and if both rockets take off in the exact same way at the same time for a stationary observer left behind, does relativity say the rope will break? This speaks to the issue of under what conditions the concept of length contraction does or does not have physical consequences.
  13. Apr 12, 2008 #12
    Lorentz got the right equations, but his interpretations are very different from Einstein's. He believed atoms and molecules have changed because the electromagnetic field of a charge depends on the speed, but he could not explain time dilation using this scheme while Einstein could explain both effects without this nonsense
    Last edited: Apr 12, 2008
  14. Apr 12, 2008 #13
    I believe the total mass (or length of mass along the axis of movement, of which I honestly forget the technical term) of both rockets AND the rope will shrink keeping the rope intact if you remove the rigors of spaceflight from the problem ^^

    Just my opinion :P
  15. Apr 12, 2008 #14
    Basically it has to do with the non-synchronization of clocks between two different inertial frames and thus the non-simultaneous measurements of the position of the end points of an object.
    If by "real" you mean "can be measured" then yes, it is real.
    Yes. Observe the electric field on a current carrying wire as measured from an inertial frame S' which is moving parallel to the wire relative to S, the frame in which the charge density is zero. In S' the charge density is non-zero. The charge density in that frame is thus non-zero and this is an indication of the relagive contractions of positive and negative charge densities.

  16. Apr 12, 2008 #15


    Staff: Mentor

    I am not talking about the lifetimes of unstable particles (which get longer, not shorter). I am talking about the distance between the front and back of a bunch of particles. This is called the "bunch length". If you know how many electrons are in a bunch and you know how close together they are in their rest frame then you know how much force they are exerting on each other due to electrostatic repulsion. You then know how strong your external field needs to be to keep them together. If you don't use length contraction you get the wrong answer and your bunch falls apart.

    PS I am not a particle physicist, so I may be wrong about the bunch falling apart. It may be some other problem, but the basic point remains.
    Last edited: Apr 12, 2008
  17. Apr 12, 2008 #16
    Perhaps this is simpler than we think... could it just be gravity? As the particles accellerate they compress due to the gravitational force being exerted upon the object? This would explain for any "shinking" and would also fit in with inertia in that the larger it is the harder it is for it to move... and shrink...

    Just a thought.
  18. Apr 12, 2008 #17


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    I have already read many stuff here, about the question of Lorentz contraction. My concern is not if it's real or not, but I'm still confused about contraction versus forces.

    To be precise, imagine two parallel plates of (perfect) conductors. Then there's an attractive Casimir force between them scaling as (1/distance between plates)^4. Imagine now there are springs or anything that prevent the plates to schrink. And consider now these plates in a boosted frame (with velocity orthogonal to the plates). In that moving frame the plates are closer to each other by a factor gamma. So the force between them should be increased by a factor gamma^4, which can be enormous. Then, the spring should break, seen in this frame?

    (I agree, obsviously, that in the proper frame the system is still at equilibrium, so nothing special should be seen also in the moving frame). But how, precisely, does it come, that's the question? How to solve the paradox?

    Ok maybe you can say, redo the computation of Casimir force in a moving frame, which maybe is not an obvious task. But the argument (or more properly the paradox) also apply force any kind of forces. (for instance the gravitational force betwenn the plates, scaling as 1/L^2, "newtonianly" speaking).

    Any ideas? Thanks a lot.
  19. Apr 12, 2008 #18
    Common question. AFAIK there is now way to divorce length contraction from time dilation in an experiment. Yes we can see that certain particles at high velocity should decay before they reach the Earth's surface, but they don't, thus time dilation is somewhat proven. But how do you say that it's also due to length contraction or matter contraction or however you want to spin it, and how do you measure that independently of time dilation?

    And the above question is only answerable in field theory I think, and I'm afraid I'm not qualified to say why. But I can say that Newtonian mechanics are not really useful in that particular experiment, you would want if anything general relativity to be precise, and then you get into all sorts of problems with the equations, because of the infinities involved I think, if I'm not too far off the beaten track. Just chiming in to see if any experts have an opinion.
    Last edited: Apr 12, 2008
  20. Apr 12, 2008 #19


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    Hi Jip,

    your question about the Casimir force is interesting, but I believe your conclusion is incorrect. It is true that from a moving frame the gap between the plates will appear different, but this will be compensated by a change in the spectrum of the cancelled vacuum modes. So all observers will measure the same force.

    I could be wrong about this - perhaps someone will check.

    Last edited: Apr 12, 2008
  21. Apr 12, 2008 #20
    It is not just objects.

    Suppose A and B are 1 light year away and at rest with respect to each other. Then a traveler from A to B will always travel less than one light year. If he travels very slowly it will be only very slightly less than one light year but if he travels very fast the distance will be much shorter. The more the traveler accelerates towards B the more the distance will shrink.
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