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Why Time and Length Contraction/ Expansion

  1. May 18, 2012 #1
    I have been wondering why the universe should behave in such a way. There is no obvious reason why gravity or speed should change the rate you travel through time or the relative distances, 1 second should be 1 second and 1 metre 1 metre whatever the strength of gravity or speed.
    The only reason I can come up with is that it changes the geometry of spacetime from one form to another, it should be possible to make say spherical spactime appear square.
    If time can slow down in one direction then it must speed up in the other.
    At c is it mass or space that becomes infinite? I ask this because if space becomes infinite and time 0 at c then at a singularity i.e in a Black Hole the opposite should happen as speed becomes 0.
    This would give at one end infinite space being crossed in 0 time and at the other infinite time taken to cross 0 space and everything inbetween being the universe as we see it.
    Is it possible that gravity and time are the same thing, something travelling in time slightly faster than something else would appear as an attraction between the 2 things the greater the time difference the greater the attraction would seem.
  2. jcsd
  3. May 18, 2012 #2
    The reason this works is, well, light travels at c in all reference frames.


    Nope. Remember that (almost) everything's relative. If observers a and b are moving relative to each other, a observes b's clock as moving slowly and b observes a's clock as moving slowly.

    An object's mass approaches infinite relative to you as the object's speed approaches c relative to you. Again, note that the object will observe itself as having normal mass and you as having ridiculously high mass. Space doesn't become infinite at c in any way. Neither does time. There are no absolute velocities.

    Still can't understand this.

    Gravity isn't a thing. It's an interaction. Also, this isn't making any sense. "Something traveling in time faster than something else," again, is relative to one's frame of reference.
  4. May 18, 2012 #3


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    Nor is there any obvious reason why they should not! The only "obvious" things about physics are the experimental results- and those lead to the conclusion that time interval and distance depend upon the frame of reference from which they are observed.

    You state this as if it were logically necessary. Are you assuming some kind of "conservation of time" law? Do you have anything other than assumption to base that on?

    An object with non-zero rest mass cannot travel at c (relative to any frame of reference). Modern physics would not say that mass increases as speed increases. Rather it is the total "mass-energy" that increases.

    Relative to what frame of reference?

    No, it isn't! "Gravity", in the usual sense, is a force, while time is not.
  5. May 18, 2012 #4
    There is a good reason - the principle of relativity. The principle of relativity states that the laws of physics are the same for every inertial frame of reference. So, you should have no way to tell, if you are in a locked room, if you are truly moving or not.

    This is the reason why the speed of light is the same for everyone - if it wasn't, an observer in the locked room could shine a flashlight and determine that he is in motion, because the speed of light is different.

    And to maintain a constant speed of light, space and time must undergo transformations.

    Of course, you can look at it the other way around and say that because transformations of space and time maintain the speed of light, the principle of relativity holds.
  6. May 21, 2012 #5
    Mark you have the closest understanding to what I am trying to say, To maintain a constant speed of light space and time must undergo transformations.
    This infers that the universe is built around the speed of light and everything else or what we know as reality within the universe changes, so why do time and space change and not the speed of light?
  7. May 21, 2012 #6


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    This is wrong. Same Laws in every inertial frame does NOT imply the frame invariant light speed as stated in relativity. That is why SR has two postulates:

    1) Galilean Invariance
    2) Frame invariant light speed

    They are both needed. 2 doesn't follow from 1.

    Nope, not necessarily. You don't need frame invariant light speed to satisfy Galilean Invariance. If light behaved according to the emission theory where light speed depends on the source speed, the observer shining the light himself would always measure the same speed. He could not determine the movement of his closed lab. The emission theory satisfies Galilean Invariance but the light speed is not frame invariant there. This is the difference to Relativity and since observation agrees with Relativity the emission theory was dropped.

    To make it short: The 2nd postulate doesn't just mean:

    - Every observer sees the same speed for his own light, produced at rest in his frame.

    It actually means:

    - Every observer sees the same speed for all light, even if produced by sources moving relative to him.
  8. May 21, 2012 #7


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    We don't know why c is invariant. All we can do is observe that it is invariant with all of the most precise and careful experiments that we can perform. We then use that observed invariance to make all sorts of other experimental predictions, and when they are confirmed we take that as further evidence of the fact that c is invariant.

    Btw, I agree with A.T.'s objection to Mark M's comments above.
  9. May 21, 2012 #8

    Some physicists think they know why c is invariant. They feel that it is because for every observer (regardless of velocity relative to other observers) the instantaneous 3-D cross-section view corresponds to a coordinate axis (say X1) that is rotated such that the world line of a photon always bisects the angle between the X4 world line and the X1 axis. So, in this view, the more fundamental question is: "Why has nature organized X4 world lines and X1 axes in this manner for all observers?"

    Other physicists recoil from this view, feeling that it implies a 4-dimensional universe, which would not be consistent with their Lorentz Ether Theory or other favorite interpretation of special relativity. Others reject the idea, believing that it is not possible to know the underlying explanation for invariant c.
    Last edited: May 21, 2012
  10. May 22, 2012 #9


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    I would not use this term for the first postulate. You appear to mean by it what is usually called the "Principle of Relativity", which was, AFAIK, first enunciated by Galileo. But "Galilean Invariance", as I've seen that term used, means something more specific: not just that physical laws must be invariant under *some* set of transformations, but that in particular they must be invariant under *Galilean* transformations (as distinct from Lorentz transformations, for example). The latter is most definitely *not* a postulate of SR.
  11. May 22, 2012 #10
    Lots of discussion about relativity which explains the phenomena but am still looking for why the universe should behave in such a way.
    Also regarding c logically the only thing whose speed would be the same for everyone would be something that is stationary which would make the observers the things that are moving.
  12. May 22, 2012 #11


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    Yes, that is the better more general term.
  13. May 22, 2012 #12
    Because that's what experiments confirm. And please don't try to just add velocities by adding the vectors representing them, that's now how you do it.

    No ... why would something that's stationary in one reference frame be stationary in all reference frames?

    No one is truly moving. There is no definition for moving that is agreed upon by all observers. Please try to wrap your head around this.
  14. May 22, 2012 #13
    For part one, maybe understand how these measurements are defined.

    That is, how Length & Time are defined. It is derived from the photon, which itself has a specific invariant "Interval" (combination of length/time measurements that adds to c). From that Interval we "invent" a nice 50/50 split for equal parts of time/length. Better termed as Proper time & proper length.

    We all measure the same interval for a photon, it's c. But depending on your comparative motion to someone else, the two independent measures of the photons speed in comparison will not have equal time / length measures. But they both will calculate c as the photons speed.

    So it is because of how we define the measure of length & time, i.e. from an invariant Speed/interval.

    Guess I'm saying the interval is what's fundamental, from it comes length / time. Trying to reconcile length/time with time dilation & length contraction is tough, given all our experience at such low comparative velocities, where those measurements are assumed to be proper. (apples to apples)

    I find reconciling the interval concept with time dilation & length contraction is easier.

    As to why it "behaves" that way, well it was poorly disciplined.
    Last edited: May 22, 2012
  15. May 22, 2012 #14


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    I'm not sure physics can answer this question. Maybe philosophy or religion can say why we live in this universe rather than some other with different "laws".
  16. May 22, 2012 #15


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    i might have a little different spin than Dale. my spin is more like what i think that string theorist Michael Duff or author John Barrow might have: variance of dimensionful physical constants, like c, G, ħ, ε0, or kB is "operationally meaning[less]" (Duff) or "observationally indistinguishable" (Barrow). we can choose to make c invariant by choosing units to make it so. Nature doesn't give a rat's @ss what units human beings choose to use, but physically, light (or any other instantaneous interaction) always propagates at a rate of 1 Planck length per 1 Planck time. and the values of three of those other four physical constants are 1 unit ( ε0 is 1/(4π)) in terms of Planck units.

    it is true that "that the universe is built around the speed of light", but it's not just light or the EM interaction. imagine you are i are holding charges of opposite polarity (that attract each other) and i perturb my charge (essentially a transmitting antenna) and that causes your charge to be perturbed (essentially it's a receiving antenna). if i do this a million times a second, it can be tuned in with an AM radio; 100 million times a second, in your FM radio; or 500 trillion times a second, you would see it as a blur of orange light. or, i can send you Morse code by perturbing my charge and letting your charge react. but the point is, if a third party who is equal distant from both you and me, when this observer sees me perturb my charge and sees your charge react, it will be delayed by an interval of time equal to the distance between us divided by c.

    now, instead of holding two charges, suppose we're as big as gods and we are each holding planets and i start sending you Morse code by perturbing the position of my planet. that "instantaneous" interaction also propagates from my perturbed planet to yours, at the same speed c. c is not just the speed of light or EM, but it is the speed of all other instantaneous interactions. it is in the fundamental nature of space and time that effect is delayed after cause in direct proportion to the distance between cause and effect. that is how "the universe is built around the speed of light". that's why information cannot be transmitted at a faster speed than c.

    if all the dimensionless fundamental constants remain unchanged (a change in the fine-structure constant would be noticed), then there is no way we could know if c has changed or not. we don't measure dimensionful values without reference to some other like-dimensioned standard (which is eventually referred back to the basic unit definitions). when we measure a length, we count tick marks on a ruler. when we measure a time, we count ticks of a clock. the reading always a dimensionless number.

    so c could be any real, positive, and finite value (and it's not dimensionless) and we will always perceive it as 299792458 m/s as long as we continue to define the meter as we have.

    even if we were to revert the definition of the meter back to what it was in 1959 (the distance between two scratch marks on a prototype platinum/iridium bar just outside of Paris), this speed of propagation (of all things "instantaneous") is always 1 Planck length per Planck time and if the number of Planck lengths per meter (or the number of Planck times per second) changes, then we notice something dimensionless has changed, and that it is a meaningful change in reality. a change of our perception of c might have changed as a consequence, but that is a secondary effect. the real issue is why the atoms of that prototype bar have changed their size (with respect to the Planck length), assuming that the prototype is a "good" prototype and doesn't gain or lose any atoms between the two scratch marks.

    the speed of light is invariant, because any hypothesized variance of it, in and of itself, is meaningless. perhaps God might notice, but we mortals can't know the difference.
    Last edited: May 22, 2012
  17. May 22, 2012 #16
    DrGreg, I think you have given us one of the best answers for the probing John15 seems to be doing.

    I think perhaps, even though constant c was established experimentally, the other invariances of special relativity (associated with Lorentz transformations on the coordinates) logically come first--then a special unique speed (the same for all observers) is logically implied. I think your comments apply particularly to "the laws of physics are the same for all observers." Once that is established, then the same speed, c, for all observers is easy to understand.

    Richard Feynman: "Why nature is mathematical is a mystery...The fact that there are rules at all is a kind of miracle."
    Last edited: May 22, 2012
  18. May 28, 2012 #17
    From nitsui combination of length/time that adds to c. This seems to make sense. I think it would be possible to create a universe just using time and space.
    After further thought what happens at t=0. The effect of time is 0 so eveything happens instantly, this means that you would exist eveywhere in the universe at the same time, so the space you take up would effectively be infinite, it would take 0 time to get anywhere but at the same time the length of your existance would also = 0. This offers a possibly new solution to the twin paradox, they would both live the same amount of time but one would appear to die younger than the other.
    At the other end of the scale with t=infinity space would appear to shrink to 0, it would take an infinite amount of time to do anything so you would effectively be fixed to a small point in space for eternity. To observers in normal time the first would appear to move at infinite speed for an infinitely small time and the second would just appear to be fixed in space.
    Perhaps time/ length dilation could be linked to wavelength, I am trying to work out why blue/short wave should appear closer than red/longer wavelength, that is literal, this seems to mean they are made of different amounts of time and space but with the total adding up to c.
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