Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why there are no other kind of length contractions?

  1. Mar 12, 2013 #1


    User Avatar


    i have asked about this issue, but i think i havent got good answer to the question. can you help me, short answer would be good.

    its unclear to me still why there are length contractions or
    expansions other than lorenz contraction? Is there a theoretical principle or
    result that deny them?

    also, is it possible that some object could have length contraction or expansion
    when it is in the same inertial reference frame, or does some theoretical principle deny it? this is my question.

    the experimental results seem to point out that no other kind of length contractions or expansions exist in nature - they have never been observed in for example in particle experiments (all particles have same identical size), or in astronomy (all atoms in observed universe has exactly same size) . but this is not a result that would deny them of course. What is the true reason, why they do not exist?

    if i give you an equation (to be exact), an example of isotropical length expansion/contraction of object that has relative velocity v=0, that is, it is in the same inertial reference frame than its observer. can you say at first sight is there something wrong with it? does it violate some principle?

    transformation equation:

    let there be two regions of space a and a' and two observers o' and o who are in these regions.

    let relative velocity be zero v=0 and lorenz factor B=1, that is,
    both regions and observers are in the same rest reference frame.

    definition: if region a' has isotropical length expansion or -contraction,
    observer o' measures unit length of any object ds =[dx',dy',dz'] to be

    dx'= L dx
    dy' = L dy
    dz'= L dz

    where L is contraction factor; if L>1 region a' (and observer o')has length
    if 0<L<1 region a' has length expansion

    quantum mechanics seem also to demand that length contraction must have
    also different dynamical properties, if you think le broglie wavelength:

    if planck's constant h'=h both observers observe planck constanst to be same
    le broglie wavelength of any particle measured by o' is l'=h/p' = L (h/p)

    => p'= (1/L) p observer o' measures any momentum p to be (1/L) times larger

    => E'= (1/L) E

    m'= (1/L) m observer o' measures any object to have (1/L) times more energy
    and mass

    and if time dilation has always strict relation to momentum (as it has for
    relativistic particles: dt'=Bdt and p'=(1/B)p => dt'/dt = p/p')

    (dt'/dt) = (p/p')

    => dt'= L dt observer o' measures time dilation or expansion L
    for any object compared to observer o s observation

    if a free neutron would have isotropical length contraction L=1.05:

    -neutrons radius would be 1.05 times smaller
    -neutrons momentum, mass and rest energy would be 1.05 times larger
    -neutrons decay time would be 1.05 times faster (if L is constant)

    example 2:
    if L oscillates (or vibrates) L = (sin^2)(at) + L0 a is constant

    -particles mass, momentum and energy would oscillate
    -if two identical particles would collide, their trajectories after
    collision would depend on both particles momentum in the moment of
    collision. if you dont know the particles exact momentum at the moment of
    collision, or their phases, you would have to use propability function.

    but wait a minute, is this similar kind of propability function that the
    shrödingers wave equation is? in other words can you assume that particles
    mass ,energy, momentum, time rate and size oscillates, and its phase is
    random variable -and get out quantum statistics similar than what is based on schrödingers wave equation?
    (please do not concider this part a speculation. this is mathematical
    question: can you get shrödinger equation from these assumptions?)

    **i am asking a question, i dont speculate over existing theories, and please do
    not think that i am advocating any very stupid crackpot things, or that i am not
    satisfied with relativity theory - i am not stupid. i simply want to know theoretical reason why there are no other kind of length contractions - or is there such reason. i just dont have casual interest on physics and i am curious about this issue. a short answer will be good.**
  2. jcsd
  3. Mar 12, 2013 #2
    I'm not really sure I understand your question. Why are there not more kinds of length contraction? Well, why are there not more planets in our solar system? Why are there not more fundamental particles in the Standard Model?

    We don't see other types of length contraction, so why try to describe them?
  4. Mar 12, 2013 #3
    "...can you help me, short answer would be good."

    Based on your previous posts, shorter questions would be better, and I agree with the previous poster. Science is concerned with phenomena that are observed, and why they are there, not with phenomena that are not observed and why they are not.

    The value of unseen phenomena lies in their falsification of hypotheses entail by theories explaining phenomena that do exist.

    E.g. if hypothesis X should entail the appearance of (detectible) magnetic monopoles under condition(s) Y, where are the monopoles? The absence of monopoles then falsifies the theory that demands X.

    "Why don't we see anisotropic 'expansion'" can only serve as a rhetorical response to an experiment that should have shown anisotropic expansion under a hypothesis X necessary to a theory that would explain something that has us scratching our heads.
  5. Mar 12, 2013 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    What, exactly, do you mean by "length contractions or
    expansions other than lorenz contraction"?
  6. Mar 12, 2013 #5


    User Avatar

    **DanR please you dont have to remind me of my earlier post that are badly edited and frankly, piece of crap, and they should be deleted.

    why space dont ever contract other ways than in lorenz contraction? The lorenz contraction seem to be a weird single phenomenom of a relative particle or region of space that does not seem to exist elsewhere in physics (and it also exist always together with time dilation and increased momentum).
  7. Mar 12, 2013 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    smm, is your question along the lines of "time dilates with changes in gravitational potential, as shown by the Erza Pound experiments. Why doesn't length change with gravitational potential?"
  8. Mar 12, 2013 #7


    Staff: Mentor

    This is a perfectly valid transformation, and I have seen such a thing happen many times myself. I used to drive from Lansing, Michigan to Toronto, Ontario. Upon crossing the border into Canada I found that all lengths were expanded by a factor of 1.6. Luckily, since the expansion was isotropic my car's tires also expanded and I was still able to cover the expanded distances in the same amount of time as I would have if the expansion had not occured.

    Now, if you don't mean a unit conversion, then such a transformation would not be inertial. E.g. Newton's second law is not invariant under the transformation but goes from f=ma to f=Lma.
    Last edited: Mar 12, 2013
  9. Mar 12, 2013 #8


    User Avatar

    to put the question in one hitline:

    "why the space or particle in its own rest reference frame DO NOT have contractions ,ever?"

    -what is the reason why space or particle do not have contractions or expansions in rest? is there a deep principle or fundamental reason why it does not have any contractions or expansions in rest?
  10. Mar 12, 2013 #9
    The contractions are a matter of perspective. It is a similar reason why an object at rest with regard to you has no momentum, while someone moving with respect to you will measure it to have momentum.
  11. Mar 12, 2013 #10


    User Avatar
    Gold Member
    2016 Award

    Uh ... you really are beating a dead horse here. You need to come up with a reason why they SHOULD have contraction or expansion if you think they should, not keep asking why something that doesn't exist doesn't exist.
  12. Mar 12, 2013 #11


    Staff: Mentor

    I already answered this.
  13. Mar 13, 2013 #12


    User Avatar

    let me write here my motivation for the question:

    -it is a known fact that space or a particle can contract when it has
    a relative motion. therefore it is reasonable to ask is it possible that
    the space or particle can contract under other conditions, for example: can
    it contract when it is at rest? this is a phenomenon that has never been
    observed, but quantum mechanics, electromagnetism, special relativity and
    classical dynamics do not either deny its existence.

    it seems that it is additional postulate : space do not contract at rest. There are no known reasons for this, but it is very comfortable to postulate it since experimental results in high energy particle physics has never encountered such phenomenom. it is almost comes without saying.

    but if some experiment in the future would encounter a particle that is at
    rest and has some kind of length contraction, no theoretical physicist would
    either say directly that its impossible phenomenom.

    but if such length contraction can exist, that has very "wild" concequence:
    in theory, you can then contract any particle or region of space into
    a very small volume or expand it into a very large volume. for example
    in theory, you can expand neutrino to a size of a sun. there is not deep
    reason why this would not be possible but also it is true that there are no known methods that can do this, no known process in the world can contract or expand particles.

    dalespam wrote:
    Now, if you don't mean a unit conversion, then such a transformation would not be inertial. E.g. Newton's second law is not invariant under the transformation but goes from f=ma to f=Lma.

    Dalespam, you dont take into account a possibility that length contracted particle or region of space may have also different dynamical properties: different rest mass, momentum, energy, time dilation and strengthness of 4 principal interactions.
    Last edited: Mar 13, 2013
  14. Mar 13, 2013 #13
    You're not going to get a different answer. Yes, there may be "other types of length contraction," just like there may be a teapot orbiting the sun and just like there may be a flying spaghetti monster. However, since we do not see any of these phenomena, we do not build our theories to reflect them.
  15. Mar 13, 2013 #14


    User Avatar
    Gold Member

    This is not very clear. It takes at least two things to have relative motion - so how can one thing possess this, as if it were a property ?
  16. Mar 13, 2013 #15


    User Avatar

    NanakixIII i think my hitline question above is valid. "what is the theoretical basis of the assumption or fact that space do not contract in rest? does any accepted theory have anything to say to it? or is it a postulate?"

    am i right NanakixIII that You do a "categorical mistake" if you say that its worthless to think possibilities and restrictions of mathematically well described physical theories in the basis of argument that its worthless to think every kind of possibilities imaginable?
  17. Mar 13, 2013 #16
    "relative motion" is not an objective property which a space or particle "has" that causes it to objectively "contract." relative motion is a description of the situation which exists between the observer and the object - not completely dissimilar to the case when a moving object appears to shrink when it is moving away, or the fact that from Earth we only see one half of the Moon's surface. Physics works whichever inertial reference frame you use. You can use the frame of the object rather than your own rest frame, and in that case, you will not find any contraction.

    You understand that for an observer traveling along with that "space or particle" there is no contraction, right?
    Last edited: Mar 13, 2013
  18. Mar 13, 2013 #17


    Staff: Mentor

    Yes. I took it into account. I specifically said that the dynamical properties would necessarily change so that Newtons law would change from f=ma to f=Lma.

    There are only two possibilities here: either you are talking about a change in units, or you are talking about a non-inertial transform.

    You have asked about the theoretical basis. That would be the principle of relativity. The principle of relativity, when reduced to math, requires that inertial transforms be symmetric under spatial translations, temporal translations, spatial rotations, and boosts.
  19. Mar 13, 2013 #18
    Length contraction / time dilation arise because of the differences in spacetime geometry between two moving objects (their relative motion). Once the geometry side of SR is understood, I don't think your question will stand. It really doesn't make sense...from a geometric perspective.

    Interesting sci-fi perspective though, perhaps we will manipulate the temporal dimension to accelerate our flying saucers of the future. :smile:
  20. Mar 13, 2013 #19


    User Avatar

    if the contracted space or particle has also time dilation or "fastness",
    that would mean all its phenomena have slowed down or accelerated by some
    factor- because the time rate effects to all physical processes.

    that would mean that observer o' can measure different F' than what observer o
  21. Mar 13, 2013 #20


    Staff: Mentor

    I think that is what is called a scale transformation. The laws of physics are not invariant under scale transformations. You should work this out for yourself. What would Newtons laws and Maxwells equations look like under a scale transformation?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Why there are no other kind of length contractions?
  1. Length contraction (Replies: 17)