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As far as I know, the classical approach to special relativity is to take Einstein's postulates as the starting point of the logical sequence, then to derive the Lorentz transformations from them, and finally to derive the invariance of the interval and other consequences from the latter.

I'm curious to know if it would be sufficient to take the frame-independence of the interval alone as the initial hypothesis to derive the other results (including the Lorentz transformations and the postulates). Is this possible, or do we have to prove that the interval is Lorentz-invariant first? If this approach is consistent, would it offer any technical advantage w/r/t the classical one?

Qmechanic
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dahemar
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    Most of Einstein's methods are based on the invariance of light-like intervals, no? – dmckee --- ex-moderator kitten Apr 09 '16 at 23:48
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    possible duplicate: http://physics.stackexchange.com/q/80511/ – Wolpertinger Apr 10 '16 at 07:41
  • Didn't the Lorentz transformations come first? – jim Apr 10 '16 at 10:16
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    Related: http://physics.stackexchange.com/q/12664/2451 and links therein. Note that the Lorentz group $O(3,1):= { \Lambda\in {\rm Mat}_{4\times 4}(\mathbb{R}) \mid \Lambda^T\eta \Lambda = \eta }$ is defined as the $4\times 4$ matrices that preserve the Minkowski metric $\eta$. – Qmechanic Apr 10 '16 at 11:01
  • I doubt that it can. Invariance of the interval surely can't imply all the laws of physics have the same forms in all inertial frames? I don't know if it could even imply invariance of speed of light. – innisfree Apr 10 '16 at 11:14
  • @dmckee: But this invariance was stated as a consequence of the principle of relativity and the invariance of c, right? – dahemar Apr 11 '16 at 12:21
  • @jim: I think not. Historically, Einstein took his two postulates as the basis for the derivation of the Lorentz transformations (see On the electrodynamics of the moving bodies [1905]). I do remember reading that similar results had been obtained previously by Lorentz (I guess they’re called Lorentz transformations for a reason), although he admittedly didn’t prove them. Please correct me if I'm wrong. – dahemar Apr 11 '16 at 12:22
  • @Numrok: Why is this a duplicate? I think my question is a clearly different one. – dahemar Apr 11 '16 at 12:22
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    @DavidHerreroMartí i guess "related" would express it better, you're right it's not really a duplicate – Wolpertinger Apr 11 '16 at 12:34
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    @David Getting the general invariance of the interval takes some work, but the invariance of light-like intervals is the invariance of $c$. Like this: $c = (\Delta x)/(\Delta t)$ so $c \Delta t = \Delta x$ so $(c\Delta t)^2 = (\Delta x)^2$ so $(c\Delta t)^2 - (\Delta x)^2 = 0$. All those games Einstein plays with light clocks and so on rely on the invariance of light-like intervals. – dmckee --- ex-moderator kitten Apr 11 '16 at 13:47
  • @DavidHerreroMartí Taken from wiki: "Historically, the transformations were the result of attempts by Lorentz and others to explain how the speed of light was observed to be independent of the reference frame, and to understand the symmetries of the laws of electromagnetism. The Lorentz transformation is in accordance with special relativity, but was derived before special relativity. The transformations are named after the Dutch physicist Hendrik Lorentz." – jim Apr 11 '16 at 13:51
  • @dmckee Then... we could say the general invariance of the interval does, indeed, imply the invariance of c? – dahemar Apr 11 '16 at 13:53
  • @jim But, as far as I know, Lorentz's results were part of his former aether theory, which made several incorrect assumptions, and were derived by Einstein in a totally different manner, based only in his two postulates. Also from Wiki: "A little later in the same year Albert Einstein published his original paper on special relativity in which, again based on the relativity principle, he independently derived and radically reinterpreted the Lorentz transformations by changing the fundamental definitions of space and time intervals [...]." – dahemar Apr 11 '16 at 15:05
  • I am not debating that Einstein didn't have great physical intuition, but only that the Lorentz transformation equations existed first. – jim Apr 11 '16 at 15:43
  • Well, I perhaps misunderstood your first comment. I thought you were saying that Lorentz transformations came first in Einstein's logical sequence in the framework of special relativity (which is what the question is about). Indeed, you're right in saying that they already existed, although that's another story. – dahemar Apr 11 '16 at 15:57
  • My guess is that it is not sufficient. I think it is similar to how the "differential" process gives a single answer, whereas the "integral" (the reverse process) gives a "family" of answers. But please go ahead and postulate the frame-independent theory and derive the rest. – Guill Apr 16 '16 at 20:51
  • These articles, which discuss alternative axiomatic basis for special relativity, might be relevant: – dahemar Apr 19 '16 at 23:50

3 Answers3

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Yes.

Let's define that the interval needs to be the quantity constructed by adding the square of the spatial interval to the negative of the square of the temporal interval. Now, we postulate that this is frame-invariant.

To be elegant and fast, I define $\eta_{\mu\nu} = +1$ for $\mu=\nu=1,2,3$ and $\eta_{\mu\nu}=-1$ for $\mu=\nu=0$ and $\eta_{\mu\nu}=0$ for the rest of the cases. $\mu, \nu$ run from $0$ to $3$. Now, the interval is $ds^2=\eta_{\mu\nu}dx^\mu dx^\nu$ where the Einstein summation convention is in use.

Now, in a different frame, the interval will be $\eta_{\alpha \beta}dx'^{\alpha}dx'^{\beta}$. Notice that the matrix $\eta$ will remain the same because we have taken it as a definition that the quantity we call the interval needs to be constructed by adding up the square of the spatial interval to the negative of the square of the temporal interval. And now, we invoke our postulate of the invariance of this interval via demanding that $$\eta_{\mu\nu}dx^{\mu}dx^{\nu}=\eta_{\alpha\beta}dx'^{\alpha}dx'^{\beta} \tag{1}$$

Or, $$\eta_{\mu\nu}dx^{\mu}dx^{\nu}=\eta_{\alpha\beta}\dfrac{\partial x'^{\alpha}}{\partial x^\sigma}\dfrac{\partial x'^{\beta}}{\partial x^\rho}dx^\sigma dx^\rho \tag{2}$$

But since the indices $\sigma$ and $\rho$ are dummy indices, we can change them to $\mu$ and $\nu$ without any harm. So, we re-write $(2)$ as

$$\eta_{\mu\nu}dx^{\mu}dx^{\nu}=\eta_{\alpha\beta}\dfrac{\partial x'^{\alpha}}{\partial x^\mu}\dfrac{\partial x'^{\beta}}{\partial x^\nu}dx^\mu dx^\nu \tag{3}$$

The only way $(3)$ can be generically true is if the following equality holds: $$\eta_{\mu\nu}=\eta_{\alpha\beta}\dfrac{\partial x'^{\alpha}}{\partial x^\mu}\dfrac{\partial x'^{\beta}}{\partial x^\nu} \tag{4}$$ Now, as the matrix $\eta$ doesn't change with place, its differentiation with respect to any of the coordinates will be zero. Thus, $$0=\eta_{\alpha\beta}\bigg[\dfrac{\partial^2 x'^\alpha}{\partial x^{\mu}\partial {x^\kappa}}\dfrac{\partial x'^\beta}{\partial x^\nu}+\dfrac{\partial^2 x'^\beta}{\partial x^{\nu}\partial {x^\kappa}}\dfrac{\partial x'^\alpha}{\partial x^\mu}\bigg] \tag{5}$$ Since the right hand side of $(5)$ is identically zero for all the values of the independent indices, we can interchange them. So, we interchange $\mu$ and $\kappa$ and add the resulting expression to $(5)$. Similarly, we interchange $\kappa$ and $\nu$ and subtract the resulting expression from $(5)$.

The resultant is

$$0=\eta_{\alpha\beta}\bigg[\dfrac{\partial^2 x'^\alpha}{\partial x^{\mu}\partial {x^\kappa}}\dfrac{\partial x'^\beta}{\partial x^\nu}+\dfrac{\partial^2 x'^\beta}{\partial x^{\nu}\partial {x^\kappa}}\dfrac{\partial x'^\alpha}{\partial x^\mu} + \dfrac{\partial^2 x'^\alpha}{\partial x^{\kappa}\partial {x^\mu}}\dfrac{\partial x'^\beta}{\partial x^\nu}+\dfrac{\partial^2 x'^\beta}{\partial x^{\nu}\partial {x^\mu}}\dfrac{\partial x'^\alpha}{\partial x^\kappa} - \dfrac{\partial^2 x'^\alpha}{\partial x^{\mu}\partial {x^\nu}}\dfrac{\partial x'^\beta}{\partial x^\kappa}-\dfrac{\partial^2 x'^\beta}{\partial x^{\kappa}\partial {x^\nu}}\dfrac{\partial x'^\alpha}{\partial x^\mu}\bigg] \tag{6}$$

As you can see, the last term cancels the second and the fifth cancels the fourth. Notice that this argument is based on the symmetry of $\eta_{\alpha\beta}$ which is inherent in its definition. The remaining terms simplify to $$0=2\eta_{\alpha\beta} \dfrac{\partial^2x'^\alpha}{\partial x^\mu \partial x^\kappa} \dfrac{\partial x'^\beta}{\partial x^\nu} \tag{7}$$

This can be generically true only if $$ \dfrac{\partial^2x'^\alpha}{\partial x^\mu \partial x^\kappa} = 0 \tag{8} $$

The generic solution of this equation would then be $$x'^\alpha = \Lambda^\alpha _\mu x^\mu + c^\alpha \tag{9}$$ Here, $\Lambda$ and $c$ are constants in the sense they do not depend on the position in spacetime.

Therefore, $(4)$ translates to $$\eta_{\mu\nu} = \eta_{\alpha\beta}\Lambda^\alpha _\mu \Lambda^\beta _\nu \tag{10}$$ Or, in the matrix form, $$\eta = \Lambda^{T} \eta \Lambda \tag{11}$$

Thus, the allowed the set of proper homogeneous transformations is isomorphic to $SO(3, 1)$ and the set of all transformations is isomorphic to the Poincare group. All the results of special relativity can now be derived as we know the exact symmetry group of its transformations.

Edit

I gathered the arguments presented above from the wonderful GR book of Steven Weinberg, Gravitation and Cosmology.

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I'm curious to know if it would be sufficient to take the frame-independence of the interval alone as the initial hypothesis to derive the other results (including the Lorentz transformations and the postulates). Is this possible, or do we have to prove that the interval is Lorentz-invariant first?

Yes, this is certainly possible. A nice way to develop this is to use a coordinate-free approach, and an exposition along these lines is given in Bertel Laurent, Introduction to spacetime: a first course on relativity. Laurent gives two postulates: (1) there is a metric, and (2) inertial motion gives maximal proper time. Postulate #1 inherently assumes frame-independence, since it refers to an inherent property of vectors, a bilinear inner product, which exists without our ever even having mentioned frames of reference or coordinates.

If this approach is consistent, would it offer any technical advantage w/r/t the classical one?

It typically makes very little difference in the end what axiomatization you use for SR, because you end up with the same theory in the end. But IMO Einstein's 1905 postulates are not the way anyone would express them today based on how we think about physics, e.g., we don't conceive of optics and mechanics as separate sciences, and we don't think of photons as being any more special than any other massless particle.

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My understanding (which is based somewhat on Jackson's chapter on SR in Classical Elecrodynamics) is that the invariance of the interval is not enough to derive the Lorentz transformations - you also need the second postulate (that the speed of light is constant in all frames).

The invariance of the interval follows from the fact that spherical light waves obey the equation

$c^2t^2-(x^2+y^2+z^2)=0$

in one frame and

$c'^2t'^2-(x'^2+y'^2+z'^2)=0$

in another. If space is homogeneous and isotropic, as implied by the first postulate (that the laws of physics are invariant wrt the translational motion of frames moving in rectilinear paths), then the above equations can be related by a scale-factor only, which is independent of space and time:

$c^2t^2-(x^2+y^2+z^2)=\lambda(c'^2t'^2-(x'^2+y'^2+z'^2))$

You can show in order for the inverse transforms to be symmetric, $\lambda$ has to $=1$.

Then you need the second postulate ( $c=c'$ ) before you can find the frame transformations.

czechmea
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    This doesn't seem right to me. If you've already established what the metric is, then applying the metric to the energy-momentum four-vector gives $E^2=p^2+m^2$ (in units with c=1). This implies that massless particles have to travel at c, for otherwise you'd have a frame of reference in which the particle was at rest, but there would be no preferred direction for its momentum vector. –  Jun 16 '17 at 21:54