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The 'mechanism' of length contraction

  1. Apr 21, 2014 #1
    Let's suppose that we have a rod which is 2 meters long in its rest frame. Its rest length can be defined as a set of points which do not occupy the same place, measured in a frame which is at rest with the rod as a whole.

    Now if we travel relative to the rod, it gets length contracted, I understand this, but does this mean that some of its points somehow 'disappear'. By this I mean does length contraction imply the loss of some points which combined together make the original length of the rod? If the rod gets length contracted by 90% does it lose some of its parts, or are the spacetime points of the rod that exist measured in the rest frame still there? I really don't understand how can two points that are one next to another get contracted. I hope someone can explain the mechanism of this relativistic effect to me, and how to all the points of the initial rod get preserved in moving frames, if they do?
  2. jcsd
  3. Apr 21, 2014 #2


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    The rod clearly cannot "lose" some or any of its parts i.e. the constituent particles of the rod cannot "disappear". This obviously violates Baryon number conservation. Even if we were in a high-energy regime where particle number is not conserved, the creation and destruction of particles would be consistent amongst all Lorentz frames.

    Now your second question is a much harder and deeper one to answer in a satisfactory manner. Prior to Einstein, the fluid defined by the ether picked out a mean rest frame with respect to which electromagnetism played out. It was calculated that a rod moving relative to the ether would contract in length and the explanation given for this, along with a valid calculation based upon Maxwell's equations, was that the electrostatic interactions between the constituent particles of the rod get modified due to motion (relative to the ether) and hence the length of the rod contracts since the spacings between particles are of course determined by the electrostatic interactions between them. This was effectively a constructive explanation of length contraction relative to the ether.

    Special relativity does not offer such a constructive explanation for the symmetrical effect of length contraction between inertial frames that results from its basic postulates (I have italicized these terms because they characterize the crucial differences between length contraction in the ether theory and one form of length contraction in SR-note there is still an analogue of the former type of length contraction in SR wherein a rod initially at rest in an inertial frame once accelerated will begin length contracting in that same frame).

    Here are some articles for you to read on this:

    https://webspace.utexas.edu/aam829/1/m/Relativity_files/SHPMPFitzGerald.pdf [Broken]

    However it is not hard to understand the deductive reasoning behind length contraction within the framework of SR. It is simply due to the fact that the notion of the length of a (Born) rigid ruler (an inherently space-like notion in causal terms) depends on a notion of simultaneity. More precisely, each point of the rigid ruler is described by a world-line in space-time thus ascribing to the ruler a "world-tube". The length of the ruler would then be defined as the distance between simultaneous events on the extremities of the world-tube. So clearly the notion of length is simultaneity dependent (be it radar simultaneity or slow-clock transport or whatever you like) and this in turn implies, due to the relativity of simultaneity (which follows deductively from the fundamental postulates of SR), that observers in different frames in general ascribe different lengths to the rigid ruler.

    Geometrically speaking the length of the ruler, relative to a given frame, is given by the intersection of said world-tube with the simultaneity hypersurfaces of this frame and as you know these simultaneity hypersurfaces tilt at different angles from one frame to another relative to those of a given frame, this being an inherent property of SR. As a result we get length contraction. This does not mean that the particles making up the ruler "disappear" when going from one frame to another nor does it imply that the ruler loses its parts in such a transition (as already stated this is obviously impossible)-rather it is simply that length itself is a simultaneity dependent notion and as such is affected by the relativity of simultaneity.

    EDIT: In case the lack of a constructive explanation (in this case a dynamical explanation in terms of Maxwell's equations) disturbs you, note that this kind of deductive reasoning is quite pervasive in physics. In statistical mechanics we start with the postulate that in thermal equilibrium, all possible accessible microstates of a system are equally likely and deduce the macroscopic properties (pressure, temperature, chemical potential etc.) through derivatives of the partition function without any fundamental dynamical explanation of the microscopic physics (of course the situation in statistical mechanics is a bit different in that such an explanation is readily available to us through Newton's 2nd law whereas in SR the situation is more subtle).
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  4. Apr 21, 2014 #3


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    No, that's not how the rod's rest length is defined; it's how the rod itself is defined. More precisely, the rod is a set of *worldlines* of distinct points, i.e., a set of non-intersecting timelike curves which occupy a continuous "world tube" in spacetime. These curves exist in all frames, obviously, since they are curves in spacetime.

    The rest length of the rod is defined as the length along a particular spacelike slice through the set of worldlines that defines the rod, namely, the slice which is orthogonal to every worldline in the set.

    No, it means that, if we travel relative to the rod, the rod's length in our rest frame is defined by a *different* spacelike slice through the set of worldlines that defines the rod--namely, the slice that is orthogonal to *our* worldline, rather than to the rod worldlines. This length is shorter than the rod's rest length, so we say the rod is "length contracted" in our rest frame. But all the worldlines are still there, so all the rod's points are still there (since each point of the rod corresponds to a distinct worldline).

    On the spacetime viewpoint, there is no "mechanism"; length contraction is just geometry, i.e., it's a consequence of taking a different spacelike slice through the rod. Nothing about the rod itself changes; all that changes is which spacelike slice we use to define its "length" relative to us.
  5. Apr 21, 2014 #4


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    How many points can you fit into 2m? And when some of them disappear, how many do you have?
  6. Apr 21, 2014 #5
    Rather than disappear, don't they get squashed closer?

    If that is what "geometrically" means, does it mean that what we might locally beleive are spherical (protons, neutrons, s-orbitals, equal potential radius from point charge, etc...) are thought to be existentially flattened in the direction of travel for length contracted objects?

    If so, how is this understood with respect to the physical laws holding good in all IRFs? If the shape of the s-orbital, for example, is observed to be a serverly contracted to an oblate spheroid, yet the observed c is constant in all directions from the center of that spheroid, doesn't that envoke a change to the laws describing the shape of the orbital, and likewise other flattened things that need accounting for... if c got "contracted" as well it might all work out fine in proportion, but with c invarient, don't the laws have to be changed when the local spherical things are observed at speed to be contracted along one axis?
  7. Apr 21, 2014 #6


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    Not in any invariant sense, no. See below.

    No, because the geometry in question is spacetime geometry, not spatial geometry. When we call an object "spherical" we mean that a spacelike slice of it in our rest frame is spherical; but that's obviously frame-dependent, i.e., it depends on how you take a spacelike slice of the object.

    The *spacetime* geometry of the object, OTOH, is invariant, and there are invariant ways of determining whether or not the individual atoms in the object are being "squashed closer". The general method is called the "kinematic decomposition", and is described on Wikipedia here:


    The expansion scalar is the particular piece of the kinematic decomposition that tells whether the object is being "squashed" in an invariant sense; in the case we have been discussing (a rigid rod moving inertially), the expansion scalar is zero, indicating that the rod is not being squashed or stretched at all.

    Physical laws must always be expressible in terms of invariants, like the expansion scalar, not frame-dependent quantities like the spatial geometry of an object. That's why they hold good in all IRFs.

    Atomic orbitals are a bad choice of example here, because the theory that is used to derive their shapes is non-relativistic; AFAIK there is no relativistic version of it.

    But I think what you're really focusing on doesn't depend on any particular feature of atomic orbitals, so let's use a simpler example: a light source and a "spherical" detector, where "spherical" means "in a frame in which the light source and the detector are both at rest, the detector is perfectly spherical, and the light source is at its exact center". See further development of this example below.

    No. If we look at an invariant description of the light source and detector in my example above, what we will have is a set of worldlines describing a certain geometric object in spacetime (not space!). What invariant properties do these worldlines have?

    Well, we know one property at the outset: light rays emitted at some instant by the light source in all directions will strike the detector simultaneously in the frame in which the source and the detector are all at rest (call this frame O). As I've just stated the property, it doesn't sound invariant, but we can remedy that easily: we simply pick out the events at which the light rays strike each individual piece of the detector, and call that set of events set D. Set D is then a geometric object in spacetime, most easily described as the intersection of two other geometric objects: the future light cone of the emission event at the light source (call this event E), and a particular spacelike slice which is orthogonal to the light source's worldline (and all the detector worldlines too, of course). And if we look at the spatial geometry of set D (i.e., its geometry as seen in the spacelike slice that picks it out), it will be spherical.

    Now, suppose we are moving relative to the light source and detector. What does the detector "look like" in our rest frame? Well, first of all, we realize that that question requires the use of a *different* spacelike slice, one that's orthogonal to our worldline, not the light source/detector worldlines. And the intersection of this slice with the family of worldlines describing the detector will give us a *different* set of points in spacetime, set D'. And the spatial geometry of *this* set of points, in the spacelike slice we are using now, will *not* be spherical; it will be an ellipsoid with its shorter axis in the direction of motion.

    But also, notice that set D' does *not* describe a set of events at which light rays from the source strike the detector! It can't, because that set of events is set D, *not* set D'. In other words, in the moving frame, the detector is ellipsoidal, not spherical, but also light rays from the source do not all strike the detector at the same time. *That* is how c can be the same in the moving frame even though the detector is length contracted.

    This is a good illustration of the fact that length contraction is not a fundamental concept in relativity; i.e., you can't use length contraction, by itself, to analyze a scenario, or you will make mistakes. Whenever there is length contraction present, you also have to take into account relativity of simultaneity (and possibly time dilation as well) in order to do a correct analysis. This is a big reason why I prefer the spacetime approach: the correct analysis is just geometry, but *spacetime* geometry, not spatial geometry.
  8. Apr 21, 2014 #7
    From the perspective of the traveling frame, all the individual material points of the rod would still be visible, although they appear to be closer together. Also, the people in the rest frame would claim that the people in the traveling frame are not seeing the material points of the rod all at the same time, although the people in the traveling frame claim that they are.
    Yes. This is what would be reckoned from the moving frame.
    Yes. We've already seen how conventional mechanics is modified to be consistent with SR for particle momentum, etc. I don't know how this is accounted for in the theory that you have described, but it undoubtedly has been worked out. Another area where modifications would have to be made is in deformational mechanics, particularly solid mechanics, where conventional deformational kinematics evaluates strains based on the configurations of bodies at constant time (as reckoned from their rest frame). As reckoned from another inertial frame, the same bodies would be distorted (even without developing stress), and calculation of strains and stresses would be problematic.

  9. Apr 21, 2014 #8


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    I am not sure what you mean. There are an infinite number of points between any two points. Points are not countable.
  10. Apr 21, 2014 #9


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    Consider a one-meter stick and a two-meter stick. There is a one-to-one correspondence between points on the two sticks. You can match every point on the one-meter stick with a point on the two-meter stick, and you can match every point on the two-meter stick with a point on the one-meter stick.
  11. Apr 21, 2014 #10


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    Short answer: No, they just move closer together. (In the coordinate system in which the rod is originally at rest, the rear will have a higher acceleration than the front).

    If I were to write a longer answer, I would emphasize the same things as PeterDonis. Let's say that Alice is comoving with the rod before the boost, and Bob is comoving with the rod after the bost. I think it's very important to understand that after the boost, what Alice thinks of as the length of the rod is a number that she associates with a certain line segment in spacetime, and what Bob thinks of as the length of the rod is a number that he associates with a different line segment in spacetime. They're not measuring the same thing.
  12. Apr 21, 2014 #11


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    Read the links I posted. This topic enshrines an age-old debate amongst philosophers of physics.
  13. Apr 21, 2014 #12


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    Just to clarify, the example I have had in mind does not involve any acceleration of the rod; the rod moves inertially the whole time. Only the observer changes his state of motion--i.e., he starts out comoving with Alice (and the rod), and ends up comoving with Bob (who is moving relative to the rod).

    In other words, I was interpreting the OP's words "if we travel relative to the rod" to mean that *we* accelerate, not that the rod does.
  14. Apr 22, 2014 #13


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    I guess I should clarify something too then. (It's probably already clear to you, but perhaps not to other readers, including the OP). My first paragraph is about the scenario where the rod is accelerated to a new velocity gently enough to allow internal forces to keep distances between "this atom and the next" approximately the same in all the comoving inertial coordinate systems (a different coordinate system at each event on "this" atom's world line).

    My second paragraph (the one where I said I agree with you) was about what happens if we just describe the same unaccelerated rod using two different coordinate systems.

    I didn't see the OP's words "if we travel relative to the rod". If I had, I might have skipped the first paragraph. I thought that it would be more tempting to ask if "points disappear" when the rod accelerates, so I assumed that he was talking about that.
  15. Apr 22, 2014 #14


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    Interesting posts here, Peterdonis etc is spot on - SR is about geometry.

    In SR nothing happens physically to the rod - its proper length is unchanged - all you are doing is a hyperbolic rotation in space-time so its apparent length is different.

    It's the exact analogue to getting a long stick through a door. You rotate it and it fits through. You haven't done anything physically to the length of the rod, but the geometry of the situation means it now fits through. Same with GR - you haven't done anything to the length of the rod, but you have geometrically rotated it so it now fits through smaller openings.

    Last edited: Apr 22, 2014
  16. Apr 23, 2014 #15
    What about two adjacent points, so point basically doesn't have a length and cannot contract?
    Does this imply that any worldtube is made of an infinite number of wordlines, since there are infinite points that 'make it up'
  17. Apr 23, 2014 #16
    The differential segment of rod between the two adjacent points length contracts.
    Yes. You can look at it that way. In Materials Science (solid and fluid mechanics), we talk about bodies being represented as continuua, and being comprised of an infinite array of differentially separated material points. When a body deforms, the material points can get closer together or farther apart, and we use this to quantify the local strains that the body experiences.

  18. Apr 23, 2014 #17


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    There is no such thing as two adjacent points. Between any two points there are an uncountably infinite number of points.

    Yes, and it is an uncountably infinite number of worldlines.
  19. Apr 24, 2014 #18

    But why does it get contracted in the sense of different simultaneity use? Imagine that we take a space-like slice while traveling with 0.5 c relative to the rod, we still take each worldline, just their different time stages from the initial rest frame and it this case it can be stated that the distance between the end and the front will remain the same, the rod just won't be made of the same 'parts'. Does the gamma factor regarding simultaneity have to do something with this, because it seems to me that simultaneity itself isn't sufficient to explain why a different spacelike slice has a new length that is shorter than the original one.
  20. Apr 24, 2014 #19


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    Do you see that "the rod" is a line segment in spacetime to the observer who's comoving with the rod, and a different line segment to the observer who's moving at 0.5c relative to the rod? If you draw a spacetime diagram, the latter line segment will be longer (in the diagram). You're right that the gamma factor (more accurately: the Lorentz transformation) is involved in explaining why the coordinate distance along that line segment is actually shorter.
  21. Apr 24, 2014 #20


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    We're talking about two different space-like slices so there's no reason why they must have the same length, any more than two different people must have the same weight or height or shoe size.

    The relativity of simultaneity comes in because it's the reason why we're talking about two different space-like slices. The length of an object is, by definition, the distance between where one end of the object is and where the other end is at the same time. Relativity of simultaneity means that "where the other end is at the same time" is different for the different observers; for one the length is the distance between point A and point B and for the other it is the distance between point A and point C. There's nothing surprising about these being different.
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