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Given length invariance in Euclidean 3D space between two inertial frames:$$ds^2=ds'^2$$ Can Galilean transformation be derived like Lorentz transformation derived from space-time interval invariance?

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  • $\begingroup$ I am not sure, but perhaps one would have to add time to the Galilean "spacetime interval"? $\endgroup$
    – jng224
    Feb 16, 2021 at 14:09
  • $\begingroup$ Since the Galilei group is not a true isometry group for 3 dimensions, I don't think you will be able to do this. The best one can do is obtain the Galilei group as an Inonu-Wigner contraction of the Poincare group (this is more or less what would mathematically correspond to @Jonas his comment). $\endgroup$
    – NDewolf
    Feb 16, 2021 at 14:19

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The answer is negative, because that invariance property is also valid regarding two reference systems such that one is inertial and the other is not inertial and thus the coordinate transformations are not Galileian.

If we also impose $\Delta t=\Delta t'$, we find that the admitted transformations are more general than the Galileian ones: $$t'=t+c$$ $$x'= R(t) x + c(t)$$ where $R(t)$ is an orthogonal matrix depending on time in general and $c(t)$ a tridimensional translation depending on time in general.

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I think what we should start from is not invariance of a scalar $ds^2$, but co(ntra)variance of a $3$-vector. Note that in terms of split-complex numbers the $2$-dimensional Lorentz transformation$$dt^\prime=\gamma(dt-\beta dx),\,dx^\prime=\gamma(-\beta dt+dx)$$satisfies$$dt^\prime+jdx^\prime=\gamma(1-j\beta)(dt+jdx).$$The Galilean equivalent using dual numbers is$$dt^\prime=dt,\,dx^\prime=-\beta dt+dx\iff dt^\prime+\epsilon dx^\prime=(1-\epsilon\beta)(dt+\epsilon dx).$$Note that $\gamma^2(1-j\beta)(1+j\beta)=(1-\epsilon\beta)(1+\epsilon\beta)=1$ is the reason we can invert these transformations, i.e. swap the roles of the two coordinate systems. It also establishes the theories' respective invariances of$$(dt+jdx)(dt-jdx)=dt^2-dx^2,\,(dt+\epsilon dx)(dt-\epsilon dx)=dt^2.$$That $dx$ isn't even in that last expression is a hint of what's about to go wrong.

I'm tidying things up a bit by nondimensionalising a velocity (though this needn't commit us to an invariant speed in the latter transformation family) and making space $1$-dimensional, but you can fix both of those. The important issue is the invariance of quadratic quantities needn't be equivalent to a specific transformation of linear ones in the $\epsilon$-based case, because $\epsilon^2=0\ne1=j^2$. The linear transformation implies the quadratic invariance but, as others have noted, the converse is false, ultimately because dual numbers don't allow division due to their zero divisors. This can be seen as a motive for expecting nature to be Lorentzian rather than Galilean.

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