Talk:Inertial frame of reference

Should merge with Inertial reference frame ?

I think this has been done now

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Following edit by 62.254.128.4 was removed from article to this talk page, as it seems like discussion. --Zigger 18:32, 2004 Apr 27 (UTC)

Correction:

It is not satisfactory to define an inertial frame as above: rotate or accelerate with respect to what? In the context of Newtonian mechanics we can define an inertial frame as one in which Newton's laws hold. (Note that the question of whether a frame is inertial requires us to decide what forces are acting; strictly speaking there is no absolute means of determining whether or not a frame is inertial, for additional forces can always be postulated to explain behaviour that seems to show the frame is non-inertial. In practice, we are agreed on when to say a body is free of external influence because we are largely agreed on the conditions for the action of the various forces that enter our physical picture of the world.) It is a simple consequence of this definition and the laws that all members of the family of inertial frames move with constant velocity with respect to each other. An object stationary in one inertial frame need not appear stationary in another but will move at constant velocity in that frame. (NB: the claim above that such frames are 'purely theoretical' is rather confusing as a reference frame is evidently an abstract entity in any case. Presumably what is meant is that few moving bodies (eg the earth) yield an inertial coordinate system when we locate our origin on, and orient our axes with respect to, them.)

(William M. Connolley 20:30, 27 Sep 2004 (UTC)) What fun. I too was just going to complain about the same text "The inertial frame is a space-time coordinate system that neither rotates nor accelerates." Because unless you say accelerate-with-respect-to-something, this is meaningless.

I have written an new article that addresses precisely that issue. How does a rotating planet "know" how much to bulge at the equator? That is: how does a rotating planet know it is rotating at all?

In the newtonian framework it is assumed that there is absolute space that acts upon matter but is not acted upon. In general relativity the untenable concept of absolute space is replaced by a field concept, a pervasive field. This pervasive field is perfectly transparent to velocity, but when there is acceleration there is interaction with this field. --Cleon Teunissen 11:25, 5 Mar 2005 (UTC)

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In my search of the definition of inertial system I came across this page. It seems to me there is no good definition. At least is a system in free motion, e.g. a free toward the earth falling spaceship an inertial system. Another ship, falling at the opposite side of the earth all the same. These two systems don't move uniformly with respect to each other. There seems to be a circular argument in stating that no force exerted indicates inertia. Am I wrong?130.89.220.52 21:39, 6 Mar 2005 (UTC)

A good definition is a definition that does not lead to self-inconsistency in the framework of thought in which it is formulated. In general relativity the assumption that space is Euclidian is relinquished. Einstein showed that it is nonetheless possible to formulate a consistent and rich theory of motion. Indeed, relativistic physics is richer and deeper and more versatile than newtonian physics. In general relativity, there is a dependency on the scale of the perspective. If a volume of space is considered that is about as large as the size of a spaceship, and that spaceship is free-falling, then in that volume of space the co-moving frame of reference is an inertial frame of reference.

However, if a volume of space is considered that is large enough to contain a planet and its moon(s), then the center of mass of that planet and its moon(s) is the local inertial frame of reference. And so on for an entire solar system, an entire Galaxy, until the astronomer has arrived at the size of the observable part of the Universe.

Methinks you feel a definition can only be "good" if it can somehow be reconciled with Euclidean, absolute space. Gravity alters the very fabric of space-time geometry, the gravitational influence of a gravitating body changes the rate of time in its neighbourhood, and that changes everything . However, although very counterintuitive, the universe appears to be perfectly self-consistent, judging from the fact that our self-consistent theories describe the universe with uncanny accuracy. To understand current physics theories you must be prepared to relinquish all the trusted newtonian and euclidean assumptions but one: the demand of self-consistency. And no, the definiton is not circular, because it is also a statement about how gravitational interaction is mediated, so it departs fundamentally from the newtonian concept of gravity. --Cleon Teunissen 22:59, 6 Mar 2005 (UTC)

Dear mr. Teunissen, thank you for your elaborate answer. Allow me some remarks. I want to restrict my attention to special rel. th., because it is there where I see problems with the definition of inertial systems. In general rel. th. as far as I know the concept of an inertial system has only locally meaning. In SRT however forces don't enter the theory, but inertial systems do and their definition is dependent on the concept of force. It seems to me that for the main part of the SRT only the concept of mutually inertial systems is relevant, a rather overdone term for systems at constant speed difference.130.89.220.52 08:54, 7 Mar 2005 (UTC)

Special relativity is universally valid in a universe without gravitation. Formally, universes with matter in it fall outside the scope of special relativity. (But experience tells us (and general relativity confirms) that within a sufficiently local volume of space, for example the 27 kilometer diameter of the CERN particle accelerator ring, the laws of special relativity will hold good.) --Cleon Teunissen 10:39, 7 Mar 2005 (UTC)

(Not a good example, on second thoughts: CERN is, like everything on the surface of Earth, accelerating, but that acceleraton is negligable for the particle physics being conducted.) --Cleon Teunissen 10:49, 7 Mar 2005 (UTC)

Special relativity and acceleration

It seems to me that for the main part of the SRT only the concept of mutually inertial systems is relevant, a rather overdone term for systems at constant speed difference.130.89.220.52 08:54, 7 Mar 2005 (UTC)

Well, if you take two space-ships, both firing their thrusters, accelerating equally hard in the same direction, sending radio-signals to each other, and they adopt a co-moving coordinate system for performing calculations, then the laws of Special Relativity won't hold.

If they want to use Special Relativity, they must read their onboard accelerometers in order to establish their relation to an inertial coordinate system. Then they can correct for the artifacts of their acceleration. But in order to transform the measurements of their radio-recievers to what they would have measured if they would have moved inertially they need general relativity mathematics. --Cleon Teunissen 11:06, 7 Mar 2005 (UTC)