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What is the relationship of General Relativity and Newtonian Mechanics? Namely, which laws does GR replace of Newtonian Mechanics, and which laws of Newtonian Mechanics are incorporated into it. Or is GR a complete replacement and overhaul?

Qmechanic
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Christopher King
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6 Answers6

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Arguably, GR does not actually replace Newton's laws of motion, but extends their validity from a Newtonian universe with absolute time and Galilean invariance to a relativistic universe with curved space-time and local Lorentz invariance.

From this point of view, Newton's first law just states the existence of a connection on the tangent bundle and that in absence of forces, bodies move along auto-parallel trajectories. Here, the connection is basically the thing that allows us to compare velocities at different points along the trajectory so we can determine when a body 'perseveres in its state of motion'.

Newton's second law still applies in the modified form $$ F^\lambda = \frac{DP^\lambda}{d\tau} $$ in terms of four-vectors, covariant derivates and proper time.

Newton's third law is essentially a statement about momentum conservation, which in a way is trivial in general covariant theories and non-trivial under certain additional assumptions on space-time geometry.

What actually does get replaced is Newton's law of gravity, but it of course can be recovered in the appropriate limit.

Christoph
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  • I like to say that GTR is an "application note" for Newton's first law as another way of saying your second paragraph. GTR tells one the generalised conditions when Newton I can be applied. – Selene Routley May 14 '14 at 12:46
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This is a philosophy of science type of question, in my opinion. If we look at the history of science after the enlightenment, we will see new theories emerging, mainly to explain/clarify discrepancies in old theories.

At the limits where boundary values of the problems are compatible with both theories, one theory mergers naturally with the other as far as experimental predictions and observations go, but laws do not.

The incorporation of laws and theories in other theories of broader scope or different bounds is not a one to one process. Think of Quantum mechanics and classical mechanics, where certainties of classical become probabilities of quantum. Nevertheless both predict experimental behavior correctly within their bounds of applicability.

What is the relationship of General Relativity and Newtonian Mechanics? Namely, which laws does GR replace of Newtonian Mechanics, and which laws of Newtonian Mechanics are incorporated into it. Or is GR a complete replacement and overhaul?

There is no direct replacement of laws . As the other answers say that if one solves a general relativity problem, taking the limit where the variables are such that the effects of general relativity are unmeasurable, will give the values expected from Newtonian mechanics.

This article might help. It is projecting the behavior of matter under Newtons laws into geometrical space that gives an intuition for the development of General Relativity, which is an extended geometrical concept. Thus it is not a one to one correspondence of laws to laws but of experimental (thought experiment to start with) expectations that join Newtonian to GR solutions smoothly. GR solutions will distort expectations of experiments from Newtonian solutions. It is not the laws but the solutions of the differential equations predicated by the laws of each framework that merge into each other at the limits of accuracy for each framework.

anna v
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General Theory of Relativity extended the notion of Inertia to Spacetime (the scale which was established in Special Theory of Relativity). Plus, it turned down Newton's Gravitational Theories saying Gravity isn't real force. It's merely an inertial force similar to what we feel in decelerating car.

General Theory of Relativity is much more than this. But, this is where Newtonian Mechanics will not agree with General Theory of Relativity & vice versa.

In practice, however, Newtonian Mechanics would describe most of real world inertial and Gravitational phenomenon. But, it'd fail at many peculiar phenomenon like Mercury's motion.

Earth is a Spoon
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In Newtonean mechanics distances and times vary together according to velocity. But if distances varies at same rate with variance of velocity, time value would be 1(constant). So Newtonean mechanics can be explained by GR.

However, in special relativity time does not be calculated according to object's velocity but the speed of light. Then Newtonean mechanics could not be proved by SR. So if gravitation is considered with the conception of spacetime, NM could not be explained by GR.

Concerning the view of Newtonean time stated above, deep discussion should be necessary. But in NM space (distance) is considered relative in general, so there is no reason to think time as absolute, and to regard that distance and velocity change at different rate.

Wei Sungriun
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  • This isn't all clear to me what you're trying to say here or how it answers the question. – Kyle Kanos Mar 11 '15 at 12:39
  • I think that the great obstacle for the unification of NM and the theory of relativity is the import of outside observer in both theory. If we coincide observer with object, we can imagine the relative world where velocity and distance(space) changes simultaneously, then time has value of 1(one). In this case speed of light has relative property, then the conception of spacetime might be useless and also phenomenon like time dilation might not occur which could not be thought in NM. But my thinking might be wrong. Thank you. – Wei Sungriun Mar 12 '15 at 00:08
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Relativity supersedes classical mechanics. However at low speeds compared to $c$, classical mechanics is a very, very good approximation.

Many of the principles (such as action = - reaction and f = ma) of classical mechanics hold in relativity as well. What was added is 1) $c$ is the same to all observers and 2) The laws of physics are the same in every inertial reference frame.

Kvothe
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Also in Newtonian Physics time is treated as a variable which is independent of space coordinates, whereas in GR time may not be de-attached from space, being time part of the coordinate system (x,y,z,t) and thus the term space-time. The real relationship between Newtonian Physics and GR is that every case of Newtonian Physics can be proved with GR. In effect, you can arrive to the same Newtonian equations using GR, but not the other way around specially (as Qmechanic said) when you approach $c$ or under the effect of large gravitational bodies, like planets or stars.

Luis
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  • I know what you want to say in your first sentence, but what you write isn't information one can work with. And going further, it's not clear what "case ... can be proven means", but you are implying that at least any result obtained in Newtonian spacetime agrees with the corresponding one in general relativity. But that's not true and it's not clear in what sense you'd "arrive" at Newtonian equations. – Nikolaj-K Feb 11 '14 at 23:38
  • Not to confused things, that is not what I said. First of all, there is no concept of "spacetime" in Newton. What i said is the opposite: if you can arrive to a correct result in Newton, you should be able to arrive with the same result with GE, but the opposite is not necessarily true. I believe this is line with what the user is trying to find out when he asks "what laws of classical mechanics specifically "graduated" to general relativity"? Only as long as you can obtain the same result. – Luis Feb 12 '14 at 06:10