# Why does the universe require relativity? [closed]

Are there any known compelling reasons why our universe is governed by relativity instead of Newtonian physics?

I recognize that an infinite $$c$$ would be a problem, so I'm not asking about that; I figure that so long as you still conserve energy and momentum etc., everything would be limited to arbitrarily large but finite speeds, ensuring that causality remains intact and preserving our computationally tractable universe.

I suspect that there are lots of instances where relativity informs our understanding of the operation of stars or subatomic processes or what have you, but which if any of those kinds of things couldn't be more easily explained with a Newtonian outlook?

In case it helps, my motivation here is to try and understand whether relativity makes sense from an Occam's razor perspective. And I know that relativity fits all our observations beautifully, and that it's all implied once you accept that physics are invariant in any reference frame, but that all doesn't strike me as the simplest a priori arrangement we could have had for a universe, and I find that possibility unsettling.

## closed as off-topic by John Rennie, Kyle Kanos, Emilio Pisanty, GiorgioP, Jon CusterJun 12 at 16:24

• This question does not appear to be about physics within the scope defined in the help center.
If this question can be reworded to fit the rules in the help center, please edit the question.

• The problem with questions like this is that what you regard as simple or obvious is a matter of opinion. The valid in all reference frames criterion that you cite is called general covariance and most physicists would regard it as a remarkably simple principle and it makes a lot of sense that it applies to the universe. Now it turns out that Newtonian mechanics can also be formulated in a generally covariant way, but when you do this the result is messy and compicated while formulating GR is simple and clear. – John Rennie Jun 7 at 8:22
• Indeed, GR is arguably the simplest possible generally covariant theory that could describe the universe. So the fact that the universe requires GR (to use your phrase) is because GR is the simplest possible theory the universe could require. – John Rennie Jun 7 at 8:23
• Probably not exactly relevant, but there is a derivation in Rindler's book Relativity: Special, General, and Cosmological that shows, given a few symmetry assumptions, that Galilean transformations and Lorentz transformations are the only possible "relativity transformations." You still need some empirical fact to decide between the two types of transformations, but it is interesting nonetheless. Something like this seems to do the same thing. Imo, I don't find these derivations enlightening though, because they seem hard to follow. – SpiralRain Jun 7 at 9:02
• simplest a priori arrangement we could have had for a universe is no universe at all with no physics to worry about. From occams razors perspective mainstream relativity is the simplest theory we could think about that is consistent with the observed facts. Just read something about eather theory that predated relativity and tried to explain constancy of light using newtonian frame of thoughts.They basicly "explained" everthing relativity can, but the theory was very artificial and ugly and opened up new questions about why is the eather so strange. – Umaxo Jun 7 at 10:21
• I'm voting to close this question as off-topic because it's largely a question about philosophy instead of physics. – Kyle Kanos Jun 7 at 11:29

## 1 Answer

Guth gives the answer on p.296 of the 1997 ed. of his book entitled "The Inflationary Universe": "Draw the outline of two spheres that contain the same density, such that Sphere B has twice the radius of Sphere A. To compare the amount of time it will take each sphere to collapse, we can compare the gravitational acceleration of the points P and Q", which he draws with point P on the smaller sphere and point Q on the larger one, before going to say that, as per Euclidean geometry, "sphere B has 8 times the volume of sphere A, and therefore 8 times the mass....On the other hand, the point Q is twice as far away the center of its sphere as the point P" is from its own sphere. "Since the force of gravitation [in Newton's theory] falls off as the square of the distance, the extra distance of the point Q implies that the force is weakened by a factor of 4. Combining the effects of the extra mass and the extra distance, the force on Q is ...twice as great as the force on P. It follows that the acceleration of Q will be twice...that of P, so at any given time the velocity of Q will be twice...that of P. Since that distance that Q must move while collapsing to the center is also twice as large...both spheres will collapse in exactly the same amount of time!"

So, although Newtonian physics looks simpler, and is even used for the outline of some orbits by NASA, it would result in a universe that would collapse to a point, REGARDLESS OF ITS SIZE, as soon as it would reach ANY size at all! That's NOT what Olbers, Hubble, and everyone else have always been seeing! (I know it's surprising that this consideration would've been overlooked for a couple of centuries, but such oversights do happen: Olbers' Paradox, which is much simpler and is described on Wikipedia, was only formulated in writing 6 or 8 times before the 1929 discovery of the Hubble expansion! That's why physics is such a great field to work in!)