# In a universe where the speed of light is infinite, are relativistic models and Newtonian models equivalent?

Take our universe. Observations are consistent with relativity, but not consistent with Newtonian mechanics. Assume that our current (relativistic) model of gravitation is correct. Now increase $c$ to infinity. Experimental observations in this altered universe are still consistent with relativity (by the assumption that relativity is correct). Are they now consistent with Newtonian mechanics as well?

## 3 Answers

In terms of special relativity versus Newtonian mechanics, the answer is yes. However in terms of general relativity this would be a much different universe. Instead of having curvature in a 4 dimensional Minkowki space-time, We would have a 1 dimensional time and a 3 dimensional space universe with no possible connections between space and time. So I don't think the equations of general relativity would result in the kind of gravitational forces we experience in our universe.

• Accepting this because the gallery appears to like it, not because I understand it. I shall go and learn about Minkowski spacetime now. – Rob N Jan 10 '12 at 21:34
• Maybe curvature and the Einstein Field Equations also existed, but in a different form with different consequences? – peterh Jul 16 '14 at 8:53

i think this is the same as asking, for values of v << c will newtonian mechanics apply? (since if c is very large, infinite if you will) if you word the question like that, the answer is obvious that yes, newtonian mechanics would be the only ones that ever apply.

A better definition of an 'infinite speed of light' is needed. If by that you mean that observers in relative motion will view the same space up to isometry and observe equal passages of time between events, then yes. However, there is no such thing as a 'relatavistic model' in Galilean space-time.

For example, electrodynamics is obviously Lorenz invariant. If you take as axioms Coulomb's law of electrostatic force and the Lorenz invariance of electric charge, you can derive the magnetic field (the Lorenz force may be derived from the Coulomb expression of force by Lorenz transformations since force is not a 4-vector).

However, if you replace Lorenz invariance with Galilean invariance in special relativity, all you have is Coulomb's law for electrostatics equipped with the fact that it also works for moving charges. This sort of physics, however, is really just like Newtonian gravity but without the principle of equivalence.