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Is there a way to understand the underlying "cause" for observing (special) relativity? What is the structure/geometry that makes these observations make sense? I noticed that if all objects are required to move at the speed of light through a normal four dimensional euclidean space, this has essentially the opposite of the intended effect. Things in motion relative to you would appear stretched out and their clocks would be sped up. Is there a way to explain the discrepancy?

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    $\begingroup$ The underlying structure is imbedded in the signature of the metric on Minkowski space. Take that as given and everything else follows. Or if you prefer start with invariance of the speed of light together with some symmetry assumptions; this comes down to the same thing. $\endgroup$ – WillO Apr 24 '17 at 20:51
  • $\begingroup$ If all objects are required to move at the speed of light through a normal four dimensional space-time environment, the result is the intended SR effect. youtube.com/… $\endgroup$ – Sean Apr 25 '17 at 21:33
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Let me try an analogy. Back in the early days of America people wanted to measure the exact distance between their flats in New York. As you know, New York has a very regular map, there are streets going in one direction and avenues going along the perpendicular direction. The distance between two flats was not easily measurable if the houses were not at the same street or avenue, so they decided to measure the distance $\Delta x$ along the avenues and the distance $\Delta y$ along the streets and compute the beeline distance $d$ with the formula $d^2=\Delta x^2+\Delta y^2$. This worked very well so each two New Yorkers could easily determine the distance between their flats.

However, time passed, new technology has been developed. Distances could now be determined directly using sound waves and exact clocks. Clocks got more and more precise and at some point somebody noticed that there are slight discrepancies between the distances calculated along the streets and avenues and the directly measured distances. Interestingly, the discrepancies were not always the same, they seemed to depend on the heights of the houses considered.

The first skyscrapers were built. They gave rise to further experiments. It turned out that if one flat is in a skyscraper and the other is near the ground, the discrepancies are biggest. So it seemed to depend on the height difference $\Delta z$ between the flats. After many experiments somebody proposed that the formula for calculating the distance is wrong and a better ansatz is $D^2=\Delta x^2+\Delta y^2+\Delta z^2$ (or, in term of the old length $d$, $D^2=d^2+\Delta z^2$) which puts the distances along the streets and avenues and the height on an equal footing. The new formula has been experimentally verified and turned out to be correct.

enter image description here

One interesting observation. The "old" length $d$ has a physical meaning. If you hang a rope between the two flats (the rope will have the correct length $D$), it will drop a shadow onto the ground. This shadow will have exactly the "wrong" length $d$. Actually, in reality, nobody is interested in the "correct" distance $D$ between the flats, the much more relevant quantities are the distance between the houses - which is exactly $d$ - and the heights of the houses $z_1$ and $z_2$.

This is Special Relativity. $d$ corresponds to the usual three-dimensional length, $z$ corresponds to time and $D$ corresponds to the space-time interval.

However, after even more time, the measurements got even more exact. Again discrepancies have been found between the direct measurement and the calculated result. After a lot of research it has been found that it was due to the fact that the curvature of the earth surface has been neglected. So the formula needed to be modified again to take account for the curvature of the earth surface. Obviously, the discrepancies grew with the distance but with the new formula scientists could in principle calculate distances between flats around the globe. But for practical purposes (within one city) the original formula was precise enough and remained a good approximation for the exact formula. This exact formula (which I didn't state) corresponds to the metric of curved space-time in General Relativity.

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  • $\begingroup$ By the way, you get all the fancy effects of SR if you freely rotate the rope which connects the two flats. You will see that the shadow of the rope gets shorter or longer depending on the rope's slope. But it can never get longer than the actual rope. The shadow is longest if the rope lays on the earth. This is similar to the length contraction. Of course, the height difference between the ropes' ends changes as well, if you rotate the rope. This is similar to time dilation. The analogy is not full due to different signs in the Minkowski metric, unfortunately but most of the things survive. $\endgroup$ – Photon Apr 28 '17 at 13:52
  • $\begingroup$ Thank you again Photon. I guess my question can be rephrased by asking why one would observe the "shadow" of a four dimensional object, rather than a "slice" of a four dimensional object. $\endgroup$ – Jeff Bass Apr 29 '17 at 1:23
  • $\begingroup$ Hmm, a bit hard to explain without a Minkowski diagram. I will write another answer (to embed a second graphic). $\endgroup$ – Photon Apr 29 '17 at 10:23
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You are right, the formula for length contraction is not as simply explained as the analogy in my first answer implied. If you have an extended object, its 4D representation is a strip through space-time:

enter image description here

In the picture we have a rod whose world strip is marked green. There are two coordinate systems: The lab frame $(t_S,x_S)$ and the comoving frame $(t_L,x_L)$ of the rod. The lines $P_4P_2$ and $P_1P_3$ are parallel to the $x_L$ axis and therefore are equal time snapshots of the rod at the times $t_{L,1}$ and $t_{L,2}$ in the rod's frame $(t_L,x_L)$. If you want, they are cuts/slices through the world strip at some given times. In the stationary frame $(t_S,x_S)$ the rod is seen as $P_1P_2$ at some time $t_{S,0}$.

To get the length of the rod in the lab frame $l=x_{P2,S}-x_{P1,S}=\Delta x_{12,S}$, we have to project $P_1P_2$ to the $x_S$ axis. We can transform this into the comoving system using Lorentz transformations and get $\Delta x_{12,L}$. This is the projection of $P_1P_2$ to the $x_L$ axis. Note that it is equal to the projection of $P_4P_2$ or $P_1P_3$ to the $x_L$ axis which is exactly the length $l'$ of the rod in the comoving frame.

Try to do the math and you will get exactly the formula for Lorentz length contraction!

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  • $\begingroup$ The diagram makes things very intuitive. Why does the x axis in the rod's frame not rotate to keep a 90 degree angle with its time axis? Is this essentially equivalent to saying that spacetime is not euclidean but a Minkowski space? $\endgroup$ – Jeff Bass Apr 29 '17 at 14:57
  • $\begingroup$ Yes, exactly. Just try transforming the basis vectors $(0;1)$ and $(1;0)$ with a Lorentz boost. You will get the basis vectors in the moving inertial frame. By the way, the Lorentz boosts are essentially pseudo-rotations which gives a nice intuition to them. $\endgroup$ – Photon Apr 29 '17 at 15:25
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Underlying structure of Special Relativity (SR) is 3 dimensional real Lobachevskian (hyperbolic) geometry. Minkowski R4 so called space-time is just 19 century useless and misleading concept which says that ,for instance , you need 3 dimensional Euclidean space to work with the two dimensional surface of a sphere ( globus). When you fly e.g. from SF to Berlin airplane auto pilot has no idea about the three dimensional space the Earth is embeded in. It uses only two internal coordinates on the sphere and it guides precisely plane to destination. Lobachevskian velocities space of SR is 3 dimensional ,non compact ( velocities of photons are at the Eboundary at infinity) , metric space of constant Gaussian curvature K=-1/c square ,distance in it is called relative velocity , and all geometric formulas of Lobachevskian geometry can be translated to physics which in considerable portion is SR. See e.g. "Expansion of Universe -mistake of Edwin Hubble.... " in Acta Physica Polonica B, or the same paper on researchgate, or under name of von Brzeski on Harvard astrophysics site..

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  • $\begingroup$ Dear Georg von Brzeski: For your information, Physics.SE has a policy that it is OK to cite oneself, but it should be stated clearly and explicitly in the answer itself, not in attached links or comments. $\endgroup$ – Qmechanic Apr 28 '17 at 16:43
  • $\begingroup$ Precisely and clearly, Acta Physica Polonica, B, vol39,2008, n6 Title: " Expansion of the Universe -Mistake of Edwin Hubble? Cosmological Redshiftand and Related Electromagnetic Phenomena in Static Lobachevskian (hyperbolic) Universe." author: J.Georg von Brzeski $\endgroup$ – Georg von Brzeski May 1 '17 at 20:32

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