Was Newton partially wrong with his first law?

Question

Newton's First Law of Motion states:

"A stationary object remains at rest until you apply a force to it. Once you set it in motion, the object continues to move at a constant speed until it strikes another object."

However, the observable universe is described as expanding at a faster rate the farther away from Earth you get. For every megaparsec (~3.3 million light years), the matter is moving ~72 km/s faster.

Given that the expansion rate of the observable universe is increasing the farther away we get, and given that the universe has a set of measurable properties that make the universe as a whole, a single "object", I am interested to know how the "constant speed" is resolved, or if Newton's First Law of Motion needs to be revisited or if I'm missing something fairly basic.

We can't prove or disprove dark energy and dark matter at this time, so while they may play a factor and be an outside force which causes the expansion rate to increase, I am not including them when thinking about this question.

Thanks. Removed the cosmology tag because the question is not about the effects of gravity on the universal expansion, and therefore not related to General Relativity. — Speeddymon, Jun 20 '20 at 17:39
The expansion of the universe is understood using GR, not using Newtonian physics. — G. Smith, Jun 20 '20 at 17:45
Ok then please put that as an answer and I'll select it as the answer. — Speeddymon, Jun 20 '20 at 17:57

score 1 · Accepted Answer · answered Jun 20 '20 at 18:00

The short answer is yes.

The long answer is that Newton's First Law of Motion, as pretty much all of the Newtonian Mechanics concepts, are correct (meaning: they work) as long as you're in the right regime.

If you want to measure the motion of a small ball when thrown, Newtonian Mechanics gives you the right answer. What this actually means is that the measurements you make are similar enough to the ones that you theorize. If you want an exact answer, well, you'll probably not find one in physics (apart from a handful of constants that we decided to be exact). If you want the best answer we can get, then no, Newtonian mechanics is not right.

This may seem like a useless specification, but it's not. It's important to always remember that, even if the final aim of most theoretical physicists seems to be a Theory of Everything, for now, we only have models, that work well in the right regime.

So for the final answer, I'd say that Newton was right for the measurement in a suitable regime, but he was wrong for the deeper understanding.

An interesting (and newer) example of this is the Pion force. When first theorized, the pions were thought of as the exchange particles of the strong force. We now know that the gluons are the said particles, but in many nuclear physics studies, the measurements are still made using the pion force. In subnuclear experiments though, assuming the pion as a force carrier would be straight-up wrong.

score 0 · Answer 2 · answered Jun 20 '20 at 18:20

To try to answer your question let's think about an unspecified galaxy really far away from the earth. If the galaxy si sufficiently far away we can detect a redshift of the light coming from that galaxy. For this reason we can be tempted to assume that the galaxy is moving away from us, at a certain velocity that can be calculated with the amount of redshift we observe. However this reasoning is wrong.

The wrongness of this line of reasoning is due to the proprieties of spacetime, described in the theory of general relativity. In fact the concept of comparing velocities of two bodies in different position of space breaks down in general relativity. The concept of velocity with respect to an observer is simply not definable in GR. For this reason the correct interpretation of the redshift is that the metric of spacetime between us and the galaxy has changed. This is taken as proof of the expansion of the universe.

So in summary we can say that Newton's first law of dynamic simply can't be applied in this context. In fact the concepts of: stationary object or movement at constant speed simply have no place in a curved spacetime.

score 0 · Answer 3 · answered Jun 20 '20 at 21:21

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The first law itself is not wrong, but the application of it to the Newtonian concepts of Absolute Space and Absolute Time is wrong. In general relativity we can use Newton's first law to define an inertial reference frame, which replaces Absolute Space and Absolute Time. An inertial reference frame has only a local definition, and we can build spacetime as a composition of many locally defined inertial reference frames.

answered Jun 20 '20 at 21:21

Charles Francis

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So each inertial frame will have a differential size? – Jun 21 '20 at 00:04

Was Newton partially wrong with his first law?

3 Answers3