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

Lorentz Transformation - Exponential factor, why not Proportional?

  1. Jan 4, 2005 #1
    Lorentz Transformation --- Exponential factor, why not Proportional?

    When using the Lorentz Transformation, the increase in mass, or the decrease in space-time is an Exponential, why can't it be Proportional? What is the logic behind that? For instance, if SR had a proportional decrease in time, then if you traveled .5c, time would be cut in half (t=(t/2)). But, according to the Lorentz Transformation, to cut time in half, you have to travel about .86c, why is that? Why can't v = .5c, then time = .5t? Is this just how nature works, or is there an actual reason?
  2. jcsd
  3. Jan 4, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper

    I believe that mother Nature has an awkward habit of following the principles of Relativity:1).The laws of physics are invariant to general coordinate transformations (change of noniertial reference frames).2)The speed of light in vacuo is constant and independent of the noniertial reference frame it is measured.
    Since LT are a consequence of a weaker version of the priciples stated,i guess the question should be equivelent to
    "Why Nature behaves according to the Principles of Relativity??"and that's a question for the phylosophers...

  4. Jan 4, 2005 #3
    So, the exponental factor is just something of nature, we cannot explaine why it operates that way? We just observe it, and say that it is true?
  5. Jan 5, 2005 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    The factor isn't really exponential, it's 1/sqrt(1-(v/c)^2).

    The interesting thing about this factor is that the Lorentz interval is conserved. This is

    (c * time interval)^2 - (distance interval)^2

    This quantity is the same for all observers. As a relatively simple formula, and as it's an invariant, this quantity is extremely interesting.

    The Lorentz transforms are the only linear transforms that leave this interval invariant. So you might say that the specific factors are "because" the Lorentz interval was invariant. Of course you can always ask "why" again ("Why is the Lorentz interval invariant) and wind up at a point where there is no answer. One can always do this - asking the question "why" more than 2-3 times in a row is going to reach a point where there is no answer.
  6. Jan 5, 2005 #5
    Yes, because physics is a science of nature which is based on the experiment and observation. All we can do is created the best theory to predict things and making the module which is suited the theory. If the result came from the observation is diferrent from the module then we will have to elliminate that module and the theory and replace them by the new one. I really don't like the idea of somebody that considering physics is just a bunch of algebra and theory. That will ruin the purpose of the whole subject.
  7. Jan 5, 2005 #6

    I can say that the lorenz transformation is an exponental factor, because you have to get extremely close to c for a significant change to take place. So, with that said, let me ask this. How did we discover the effects of the lorenz transformation? The scientist (Im assuming he goes by the name lorenz) must have observed it somehow, and developed a math equation to express it. But how did he observe it? :rolleyes:
  8. Jan 5, 2005 #7


    User Avatar
    Science Advisor
    Gold Member

    1) It's interesting to me as on one hand it seems sensible to me thta the fundamental theories of nature respect the principle of diffeomorphism invariance (i.e. general covariance), but on the otherhand I wonder is this an empricial requirement (whilsts Lornetz covariance woul most definitely seem to be an empriical requirement I'm not sure if we can confidentally assert the same for the general case) or is it a philosophical requirement (it seems to me that it was a philosphical wish for general covariance that drove Einstein to formualte genral relativity).

    2) Shurely you mean that to be true only in the local case? :confused:
  9. Jan 5, 2005 #8
    It seems that no one has a real answer, but how did we observe the effects of the lorenz transformation in the first place? Maybe if I know how we observed it, I can conclude why it may work that way.
  10. Jan 5, 2005 #9


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    The first experiment that was explained in terms of what were later understood to be Lorentz Transformations was the Michaelson-Morley experiment, which failed to find a relative motion between the earth in its orbit and the supposed ether. Fizgerald conjectured that if x' observed in a frame at motion with respect to the observer behaved as the observer's x multipled by [tex]\frac{1}{\sqrt{1 - (\frac{v}{c})^2}}[/tex] then the M-M null effect would result.
  11. Jan 5, 2005 #10
    selfAdjoint, can you please please explain this in lamen terms? And what's the M-M null effect?
  12. Jan 5, 2005 #11


    User Avatar

    Staff: Mentor

    The MM experiment was designed to measure the earth's motion through the "aether" - since earth is moving in several ways that could be measured (rotating about its axis and revolving around the sun, for starters), it stood to reason that this motion could be detected by measuring the speed of light in different directions through this motion.

    The way the experiment works is very much like a person swimming across a river: the person swims at a constant speed (or so he thinks) straight across the river, and can calculate his speed based on the distance across the river and the time it took to cross it. To a person on either bank, however, the swimmer swam at a diagonal because of the motion of the river. Thus, someone on the bank calculates a different speed for the swimmer. The MM experiment found that the swimmer (a beam of light) always went at the same speed regardless of how the speed was measured.

    The Lorentz transformations accurately model this phenomena.
    Last edited: Jan 5, 2005
  13. Jan 5, 2005 #12


    User Avatar
    Science Advisor
    Homework Helper

    He's referring to the result of the Michelson and Morley experiment.
    It was an (extremely accurate) experiment conducted in 1887 which tried to determine the speed of light relative to the ether, which was assumed to be the medium in which light propagates.
    You can find lots of it on the internet. e.g. here:

    The null result meant that no difference in the speed of two lightbeams was found when the sources had different velcoties with respect to this ether.
    Lorentz and Fitzgerald showed that they could explain this null-result by saying that an object moving with respect to the ether got shorter by a factor of [itex]1/\sqrt{1-(v/c)^2}[/itex]. That's where the Lorentz-Fitzgerald contraction comes from. It's just called Lorentz contraction now.
    The interpretation was wrong, but the equation was still correct. Later, with Einstein's theory of relativity the same factor occurred.

    From a theoretical standpoint, Lorentz contraction is a result of the 2 postulates of special relativity. Like dextercioby said. You can derive it from these 2 principles.
  14. Jan 5, 2005 #13
    Thanks for all your replies, I understand it more. But, I still don't get one thing. They were able to observer that c is constant, so why didn't they make the lorenz transformation a proportional increase in mass, or decrease in length/time? They must have observed somehow, that the mass must get extreamly close to c for a significant relavistic effect to take place.

    I think it is, the math just expresses the nature of it, and the nature is that you must get extremely close to c for the relavistic effects to kick in.
    Last edited: Jan 5, 2005
  15. Jan 5, 2005 #14


    User Avatar
    Science Advisor

    The Lorentz factor is not exponential. "Exponential" is a mathematical term. It means that as the independent variable is incremented, for each increment, the functional effect is to multiply by some factor. Or in mathematical symbols, [itex]f(x)\propto e^{kx}[/itex]
  16. Jan 5, 2005 #15


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    I said before that Michaelson and Morley did NOT find a relative speed between the earth and the ether. This is therefore called a null effect. M-M is just an abbreviation for Michealson and Morley, pretty common on this forum.

    What Fitzgerald said (I have not read his paper, I am quoting what I have read about him) was that if every length in the ether, which the earth was moving through, was seen from the earth as multiplied by that particular nonlinear factor, then when you plugged these changed lengths into the math used by the experimenters, the relative velocity would be cancelled out. Even though it was "really there", because of the nonlinear length change, it would appear to be zero. He had worked out the math to show that was so. I do not know if Fitzgerald thought the length was really shortened or if it only appeared to be.
  17. Jan 5, 2005 #16
    OK, for those who do not understand my question, I will rephrase it.

    The decrease in space and time, and the increase in mass, are not proportional to the percentage of the speed of light which you are traveling, v/c. So, it is not porportional, I just used the phrase "exopnental", because it is in a way, not getting too technical. So, can someone explain why if I travel .5c, my mass does not increase to .5/c, and my time does not slow to .5l, and my length does not contracT to 5t? Why is it not proportional to the increase in velocity? We all know what the lorenz transformation tells us, but can someone explaine why it works that way, and not porportional to v/c? :rolleyes:

    I hope I explained it simple enough
    Last edited: Jan 5, 2005
  18. Jan 5, 2005 #17
    Maybe you can get same inkling as to why the transformation formulas involve a second order term
    If you consider that in nature we are dealing with things that can be expressed best on a graph with time at right angles to space. - and this geometry involves the theorom of Pythagoras - when we put time on a graph it is usually in the direction of the y axis - and space for example may be along the x axis - now when you are combining time and space measurment in what is called the interval which is constant for every frame- then you say that x squared plus y squared equals some constant squared for every reference frame - the constant is the velocity of light c appropriately scaled to make the units come out right. Does this make any sense - propably not!
    Last edited: Jan 5, 2005
  19. Jan 6, 2005 #18
    yogi, I will think on your reply over night (relavent to locationon earth, lol), maybe while I am lucid dreaming, I will travel near c, and figure it all out. Just kidding. Maybe I will make a graph in MS word, and try to see some sort of a relationship, which will yield why the lorenz transformation is not porportional to v. I will reply in about 9 hours.
  20. Jan 6, 2005 #19


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    We do not choose the laws of nature, they are what they are. It is our task to figure them out. One of Einsteins accomplishments was to show that the Lorentz transforms followed logically from his 2 postulates. In his 1905 paper he used the fact that the speed of light is constant to derive a difference equation he took a limit to arrive at a partial differential equation. The solution to this equation is the Lorentz transforms.
  21. Jan 6, 2005 #20

    OK, I have developed the module equation which will illistraight the lorenz transformation, keeping the speed of light constant, and keeping all the relavistic effects in SR, but it will be porportional to v. Look...


    If we want to do time dilation, just do


    For length contraction


    For mass increasment


    So, why can't this equation work. I am in no wat purposing that this equation will actually work, but how did they figure out exactly how the relavistic effects took place at speeds near c, they could have been guessing, although I'm sure they had a reason.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook