# Modifying Newton's First Law of Motion

Discussion in 'Alternative Theories' started by hansda, Jun 8, 2017.

1. ### James RJust this guy, you know?Staff Member

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Have you ever heard of the electromagnetic tensor?

I agree, and I made no such claim.

3. ### exchemistValued Senior Member

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Exactly. So the concept of force still exists in GR, but the Newtonian concept of it has to be changed, to allow for the fact that velocities (and therefore momenta and kinetic energies) are frame-dependent. Is that right?

5. ### James RJust this guy, you know?Staff Member

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Well, velocities are obviously frame dependent even in Newtonian/Galilean relativity.

But, yes, in relativity (special or general), it is most useful to work with quantities that are not frame dependent, wherever possible. That's why we use rest mass rather than relativistic mass, and why most time-dependent quantities are expressed in terms of proper time rather than in terms of some other random time variable.

One more thing about force (and I warn that I'm by no means an expert on this): it turns out that the spatial direction of the acceleration of an object is not always the same as the spatial direction of the net force, even in special relativity. To describe that, we need something more general than the three-dimensional $\vec{F}=m\vec{a}$. Hence 4-force.

7. ### The GodValued Senior Member

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So if your understanding is that just because some qty in electromagnetism can be expressed in tensor form, it is in GR. Pl note that tensor like scalar and vector is a math tool, GR does not have copyrights over it. And by the way fluid mechanics also has tensors, so that is also 'in GR'. Yeah?

8. ### James RJust this guy, you know?Staff Member

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Correct.

Thanks for letting me know.

That's right. GR can cope with fluid mechanics.

9. ### geordiefRegistered Senior Member

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Can it be said that GR "sub contracts" areas of physics to EM and shows up to arrange the overall configuration subsequently?

10. ### exchemistValued Senior Member

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Yes indeed, of course. Is this something to do with the apparent rotation of an object moving fast relative to an observer?

11. ### sweetpeaRegistered Senior Member

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It seems you have a choice of how to work GR, force or curved spacetime...Remember these:

Last edited: Sep 15, 2017
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12. ### The GodValued Senior Member

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So James R says anything which can be mathematically expressed in Tensor form is "in GR".

And he also feels that GR can cope with fluid mechanics?

James R, it is apparent that few impressionable members follow you blindly, so you have a higher responsibility, please do not misteach. Seek help from those who know that there need not be any correlation between areas of physics (which require tensors for mathematical formulation) with GR.

13. ### The GodValued Senior Member

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you can make kind of that statement for Kaluza Klein theory, but not for GR.

14. ### geordiefRegistered Senior Member

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Not wishing (or being able) to be argumentative but just to dig a little deeper perhaps into my layers of ignorance, some EM effects do seem (on the surface and to my level of understanding ) to be subsumed into the spacetime curvature model (ie GR?) .

eg A ray of light is subject to a gravitational field . Are there other (macro) EM or other manifestations that are not subject to GR in the same way?

Again ,referring to my "second hand" understanding of this topic I was under the impression that GR only broke down in situations described as "singularities".

Maybe that was what you were getting at with the point you made about the KK model (which I am not familiar with) ?

15. ### James RJust this guy, you know?Staff Member

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The God:

GR is a model of space and time. Almost everything else we know about physics can sit happily in that model. There are some residual issues with the merger of quantum physics and GR, of course, but that's another topic.

Put it this way: the rest of physics works OK in the Newtonian world of absolute time and space, most of the time. It works even better in the Einsteinian world of GR, because GR is a more accurate model of time and space. This is not to say that GR is the perfect or final model.

Of course. What's special about fluid mechanics that means it no longer works in curved spacetime? Do you think it doesn't work or can't be formulated against a GR background? On what possible basis?

Get off your high horse and make an argument, if you have one. Empty criticism is pointless. It's like you're just trying to pick a fight with me for no reason other than your own amusement. Grow up.

From you, you mean? What help have you to give? All you provide is empty criticism. I await your "help".

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16. ### sweetpeaRegistered Senior Member

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The realm of relativistic hydrodynamics
http://www.einstein-online.info/spotlights/hydrodynamics_realm

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17. ### James RJust this guy, you know?Staff Member

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It seems that The God has abandoned this thread. Or, more accurately, run away.

18. ### hansdaValued Senior Member

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When three other forces are added vectorially for the resultant force, time is considered as it is considered in Newtonian Model. If this resultant force is considered against the background of local spacetime, time is simultaneously considered as in Newtonian Model and as in spacetime. Do you think, physics can be done this way, with two different models of time?

What we feel as weight is basically the "reaction force" as per Newton's Third Law of Motion.

The forces of nature, which are yet to be discovered; can be considered as hidden forces of nature.

Newtonian Model is still considered when satellites are launched.

Length contraction is real/physical or just an optical effect?

This is only your prediction but not a confirmation.

Do you imply that length contraction is an optical effect.

19. ### DaveC426913Valued Senior Member

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No! The math does not imply, nor permit, any 'compressive' force. You cannot argue against GR just because you want to, you have to produce a compelling case for reexamination of what has been demonstrated time & again. You cannot say "Well, overall it's a good 'theory' but in this particular case it's wrong..."

21. ### James RJust this guy, you know?Staff Member

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hansda:

When you're talking about adding two force vectors at a particular point in space at a particular time, there's no problem with relativistic time effects. They don't come into it.

Yes.

We don't seem to need any hidden forces, as things stand. (Having said that, we do have some problems with dark energy etc. that remain open.)

Yes. It works well enough for many purposes. That doesn't mean it's 100% correct. It doesn't explain the precession of the equinoxes completely, and it gets the bending of light by gravity wrong, too.

It's an effect of viewing something from a different frame of reference than the rest frame of the something. I wouldn't exactly describe it as an optical effect, since it's separate from anything to do with the propagation of light from one place to another.

Correct. But if things turned out differently we'd have to throw out lots of other results from relativity that have been confirmed experimentally.

22. ### hansdaValued Senior Member

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Did I say that, GR is wrong?

23. ### hansdaValued Senior Member

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Consider the same case. Do you observe time dilation? Do you observe a clock slowing down?

Last edited: Oct 4, 2017