Newtonian relativity

DaleSpam

I agree 100%.

A.T.

In what sense was Newton's model "based on" absolute motion? Is absolute (linear) motion a requirement for his model to work (give correct quantitative predictions)? Or is it just an irrelevant assumption that doesn't change the quantitative predictions of his model?

rbj

regarding String Theory or M-Theory, are they metaphysics, religion, or flubnubitz? is it experimentally testable (i think the right word might be "falsifiable")?

for (relative) speeds that are slow (w.r.t. c), isn't absolute time a reasonable axiom to draw from everyday experience? since Newton was a few centuries before Einstein or even Maxwell, he might consider a particle property of light traveling at a finite speed, but since this speed was sooo far faster than anything else he witnesses, i can totally understand why the absoluteness of time would be taken as axiomatic for that time. in fact, it continues to work for 99% of the mechanics we do today, including that of planetary and spacecraft motion.

so i don't get either yours or Dale's critique of Newton's axiomatic insights of the day. they sure seem reasonable to me, but given them we would expect an absolute frame of reference for Maxwell's equations and we always seemed to put that in our own frames, as observers. eventually physicists started to think about testable consequences of that notion and hence the Michaelson-Morley experiment was conceived. even though Einstein must have known about M-M, he did not use it in his SR thought experiment. it was more of this insight that, if all constant-velocity observers have equal claim to being "at rest" (and they are at rest from the POV of their own frame-of-reference), then their laws of nature must be equal and they both should measure and observe c to be the same. that is sorta a logical consequence of Galilean relativity, but it's subtle, at least for the 17th century. he came to that insight without drawing explicitly on the negative result of M-M, but i imagine that he used that to reinforce his thinking.

i wonder what Einstein would have done if there was some systemic mistake made in M-M and repeated experiments that caused them to conclude we were moving through the aether. he might have ignored it as non-sensical and proceeded with his development of SR.

still, i don't see how Newton would have been expected to use any other axiom for time than he did, given the physical world he observed. and this lesson should apply to us today regarding SR and GR, QM, Standard Model, ΛCDM, etc. what we take now as axioms might be refuted by our descendants. for instance, the accelerating expansion of the universe surely left me slack-jawed in the 90s. i did not believe it at first, but unlike superluminal neutrinos (which i also didn't believe), the accelerated expansion of the universe is an observation that is standing the test of time and repeatability.

D H

That's a different topic.


You misunderstand the concept of absolute time and space. You are thinking of time duration as being invariant. One second on the Earth = one second at Pluto's orbit = one second on a spacecraft going at 100c (there is no speed of light limit in Newtonian mechanics). That is not absolute time. Absolute time has duration as an invariant plus a fixed point in time, presumably the moment God began creation, designated as T=0. Absolute space similarly has deeply religious undertones. Per Newton, both absolute time and absolute space are for the most part hidden from us mortals. And yes, Newton did think that way. He was deeply, deeply religious, even by the standards of his time. In a sense, Newton wasn't the first scientist; he was the last magician.

harrylin

Evidently we disagree, as my summary description of what Newton did is IMHO a good example of the scientific method. Perhaps there are different opinions about what the scientific method is; and we should not deviate too much from the topic (or start it in a separate thread). However, the first description that I found as it is linked from Wikipedia, is the following:

http://teacher.nsrl.rochester.edu/ph...AppendixE.html

At first sight my opinion is consistent with that description of the scientific method; and it would be helpful if you can elaborate, consistent with that description, why you believe that Newton did something quite different.

harrylin

As always with such things (never sure to call them postulates, axioms, hypotheses or whatever) one can certainly replace one hypothesis by another one; but I suppose that you do not ask that. Merely deleting that hypothesis from his presentation results in lack of definition of such things as rotation and linear acceleration; and as you can verify for yourself, already his first law becomes then undefined so that it cannot predict anything. He assumed (don't forget when he lived and what he could know!) that the stars are fixed in space; a practical application of this was to measure absolute rotation as relative to those "fixed stars". You can also be in relative rotation without being in absolute rotation, and vice versa.

harrylin

That sounds like positivistic philosophy to me (or, to use your phrasing, the "positivistic philosophy trap"); using the word "unscientific" isn't helpful and on that issue I side with Popper - which is again another topic...

PS. thanks to this discussion I took Popper from the shelve, and I agree with Einstein's remark there, in agreement with Popper and against positivism:
"theory cannot be fabricated out of the results of observation, but it can only be invented".

harrylin

Well seen; people often put extremely unreasonable/unfair demands on others in the past, and even pretend that those others used "wrong" (or illogical or whatever) thinking simply because these used hypotheses that they dislike or because they don't understand the logical development of thought, based on the knowledge and way of thinking of that person at that time.

chrisbaird

Newton developed a theory that matched and correctly predicted all experimental results within the level experimental accuracy available at the time. I don't see what the problem is. Universal time is very testable. Take two synchronized clocks and put one on the top of the mountain and the other deep in the ocean and let them run for awhile then bring them back and compare. Using the types of clocks available at the time of Newton, you would find them to match and therefore time is universal. Absolute time is also observable. There is an absolute t = 0 and it is the Big Bang. We cannot observe before the Big Bang, therefore time before then has no meaning. Newton was right on both counts (within the experimental accuracy of his time), so he does not need to correct anything. His theory is still right at low speeds and in small gravitational fields. To insist a dead theorist correct his theory because of experimental data that is only available hundreds of years later is absurd.

harrylin

While I don't share your view of calling not being measurably wrong the same as being right, the imagined problem isn't exactly that; instead it is alleged that Newton didn't follow the scientific method, or that his logic was illogical. However I have the impression that such allegations don't cut wood; as yet nobody showed the error that Newton supposedly made.

A.T.

Sorry, I don't see your definition problems. Since Newton's laws work fine in all inertial frames, I don't see why the assumption of an absolute rest frame is necessary in his model.

harrylin

http://teacher.nsrl.rochester.edu/ph...AppendixE.html sounds quite OK to me.
Thus, step 2:
Formulation of an hypothesis to explain the phenomena. In physics, the hypothesis often takes the form of a causal mechanism or a mathematical relation.

harrylin

You read Newton from the perspective of post-Newton models. Just read it again, and tell me what in his theory could be the basis for defining such items as "inertial frames" (and note that no such term appears in his theory). Interestingly, it seems that as recent as Einstein's first SR paper the term "inertial frame" wasn't in use, for there he defined the frames to which SR relates as those in which Newton's laws [supposedly] hold.

DaleSpam

So you would agree in an absolute way with the premise "scientific assumptions should take the form of a causal mechanism or a mathematical relation". ?

russ_watters

I'm not sure how it could get any clearer, since we've been discussing the same point for the entire thread and in the other thread too! I'll do my best to state it as succinctly and clearly as possible:

Assume only that which is necessary.

That is an essential part of any activity [supposedly] grounded in logic. The danger in violating it is clear: if you throw in an irrelevant assumption, confirmation of your hypothesis' prediction will then erroneously appear to support that assumption. But fixing the problem is also simple: assume the opposite and see if the logic still works. If it does, then the assumption was unnecessary. That is not true now nor was it true then. As already stated, Newton's contemporaries even pointed that out to him. Rotation is not inertial motion, so showing that rotation and acceleration are absolute does not tell you anything useful about whether inertial motion is absolute. Newton knew the difference. From your link: Notice: "absolute motion" in this context refers only to translational motion, not rotational motion. There he says that the use of relative motion only is "without any inconvenience" and that the assumption of absolute motion is "philosophical" only. In other words, he really should know that inserting the assumption adds no value to the theory. Here he shows that rotation is absolute, while stating that this does not change the fact that translation is not. Again he reiterates that absolute linear motion is undetectable.

t_evans

I'm aware that this is going off topic from the very meritable discussion on the scientific method, but it's one of my favorite points and relates to the initial question. [/ end off topic apology]

A really nice thing is to look at this whole exercise the other way around.

Normally, you'd think of Galilean relativity as a consequence of Newton's laws - i.e. it only contains second derivatives wrt to time, so you can add any first or zero order constants of integration you like, given you apply them correctly.

However, you can turn this argument beautifully on it's head - you can derive the classical action, and therefore all of classical mechanics, from assuming just assuming Galilean relativity. I would recommend reading Landau-Lifshitz Mechanics I to anyone who hasn't yet. Deriving Newton's laws in this way is pretty much the first thing done.

Thus you can think of relativity as the axiom and mechanics as the consequence, rather than the other way around.

D H

Drink that parsimonious wine!


(source: http://www.flickr.com/photos/shishberg/2213562731/)

And he does so even more clearly in Corollary V to his laws of motion:

The motions of bodies included in a given space are the same among themselves, whether that space is at rest, or moves uniformly forwards in a right line without any circular motion.

For the differences of the motions tending towards the same parts, and the sums of those that tend towards contrary parts, are, at first (by supposition), in both cases the same; and it is from those sums and differences that the collisions and impulses do arise with which the bodies mutually impinge one upon another. Wherefore (by Law II), the effects of those collisions will be equal in both cases; and therefore the mutual motions of the bodies among themselves in the one case will remain equal to the mutual motions of the bodies among themselves in the other. A clear proof of which we have from the experiment of a ship; where all motions happen after the same manner, whether the ship is at rest, or is carried uniformly forwards in a right line.

Galilean relativity, right down to the ships.