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ZealousElixir

Newton - aware of relativity; who else?

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In Newton's Principia , Book One, he gives a definition of inertia containing the following line (assuming the translation is correct): "Resistance is usually ascribed to bodies at rest, and impulse to those in motion; but motion and rest, as commonly conceived, are only relatively distinguished; not are those bodies always truly at rest, which commonly are taken to be so." This pretty clearly indicates that Newton was aware of relativity between different reference frames, even if he didn't formulate tranformations between them (i.e. in terms of the Lorentz transform, as Einstein). The questions that naturally arise are: (1) Would anyone have been shocked by this seemingly trivial assertion? For while it seems intuitively obvious now, it also seems that many of Newton's contemporaries would not have intuitively thought of "relatively" at-rest bodies as in motion in another reference frame. Is that just because it complicates practical thought where such considerations are too tedious to account for (viz. high school-level physics)? (2) Who, if not Newton, first referred to the relativity of reference frames? (3) What was particularly radical about Einstein's conclusions in the special theory? Was he the first to formulate transforms between frames with Lorentz? It seems unreasonable, since he takes it as a given, and it seems to so precisely fit the situation (along with intuitively proving that the speed of light is constant in vacuo ). Later, ZE. //email me.//zealouselixir software.//msdn.//n00biez.//
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[edited by - zealouselixir on February 21, 2004 3:37:30 PM]

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Guest Anonymous Poster
quote:
Original post by ZealousElixir
(2) Who, if not Newton, first referred to the relativity of reference frames?

Galileo Galilei afaik.

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I'll try and answer some of these questions...
(1) The relativity of motion is not intuitive at first because we generally use the earth as a reference frame, and define it to be at rest, and everything moving in that reference frame to be in motion. However, there are examples such as a ship on the sea, for instance, where you have trouble deciding if the sea is "at rest" (or if there are currents), if the ship is "at rest" (no wind), or if both are moving in different directions (like sailing against the current). Such examples were already available in Newton's time and I wouldn't be surprised if some similar ideas had been used to explain the relativity of rest and motion.

(2) Dunno...

(3) Newton explained the relativity of rest and motion : your speed depends on the frame of reference. Einstein also introduced the relativity of length, duration, and simultaneity (maybe I'm forgetting some?). The length of an objects depends on your speed relative to it, the duration of an event depends on your speed relative to where this event is happening, and two events may be simultaneous to one observer and non-simultaneous to another if their speeds are different.

EDIT: I didn't know Einstein proved the speed of light to be constant in vacuo, I always thought it was one of his postulates, which came from Electromagnetics (where, due to maxwell's laws being true in any frame of reference, the speed of electromagnetic waves is the same in any frame of reference, and equal to sqrt(1/epsilon_0*mu_0)

Victor Nicollet, INT13 game programmer



[edited by - ToohrVyk on February 21, 2004 3:47:05 PM]

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1. The speed of light is not only constant in Vaccum, it is constant, RELATIVE to you, no matter how fast you''re travelling. i.e If you measure the speed of light from a spaceship travelling at 150,000 km/sec travelling towards a ray of light, the ray is still moving at 300,000 km/sec relative to you. This is the basic assumption of Einstein''s special relativity.

2. This was actually known BEFORE einstein formulated his theory, and was discovered in the Michaelson-Morrelly experiment... Einstein just made sense of everything around this (it leads to some weird things).

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"1. The speed of light is not only constant in Vaccum"

Oh, really. So light doesn''t slow down in water?? What radical idea is this?

--------------------------------------
I am the master of stories.....
If only I could just write them down...

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What I meant is that NOT ONLY is the speed of light constant in vaccum, BUT it is constant no matter how fast you are moving in that vaccum.

The fact that light slows down in different mediums (water, for example) is true, but not relevant as far as Relativity is concerned...

[edited by - DanielZ on February 21, 2004 4:20:47 PM]

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Guest Anonymous Poster
quote:
Original post by ToohrVyk
Einstein also introduced the relativity of length, duration, and simultaneity (maybe I''m forgetting some?).

Yeah, and the relativity of mass, which led him to the famous E=mc².

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quote:
Original post by DanielZ
What I meant is that NOT ONLY is the speed of light constant in vaccum, BUT it is constant no matter how fast you are moving in that vaccum.



Um, then can you give the definition of "constant" you're using?



[edited by - zealouselixir on February 21, 2004 5:58:21 PM]

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To ZealousElixir

My understanding is this. There are two forms of "relativity". Let's say I'm moving to the left at a speed v with respect to some reference point and you're moving to the right at a speed u with respect to the same reference point.

1) Galilean Relativity (i.e. Classical Relativity) - Relative to you, you are moving at a speed 0 and I'm moving toward you at a speed (v - u)

2) Einsteinian Relativity (i.e. Special Relativity) - Relative to you, you are moving at a speed 0 and I'm moving toward you at a speed (v - u)/(1 - vu/c^2)

Yeah, there's General Relativity too, but I haven't studied that, and that's just the general case of Special Relativity (or Special Relativity is just a special case of General Relativity, hence the names).

Basically, the difference is that Newton based his physics on a single assumption: the laws of physics are the same in all inertial reference frames. Einstein added a second assumption: the speed of light is constant in all inertial reference frames (Theoretically shown by Maxwell, Experimentally shown by Michelson and Morley).

Also, for what it's worth, we used (Galilean/Classical) relativity all the time in high school and in college. In fact, the only time we didn't was a short bit in Modern Physics when we covered Special Relativity. Even in Quantum Mechanics, we usually assume non-(Einsteinian/Special)relativistic particles.


To Nathaniel Hammen

Nope, light doesn't slow down in water. The speed of light is constant. The apparent slowing of light (or even the apparent acceleration of light (!)) has to do with the light rays being interrupted by atoms and the phase shift created by interacting with those atoms.


To the AP

I think the relativistic mass has been dropped in favor of relativistic momentum. One reason being that it creates some false parallels (e.g. F = ma and T = (1/2)mv^2 don't hold). Also, I think it's more properly E_0 = mc^2 (but that's a bit more nitpicky).


EDIT: Fixed a couple spelling errors.

[edited by - Way Walker on February 21, 2004 6:11:46 PM]

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Guest Anonymous Poster
quote:
Original post by Way Walker
I think the relativistic mass has been dropped in favor of relativistic momentum. One reason being that it creates some false parallels (e.g. F = ma and T = (1/2)mv^2 don''t hold). Also, I think it''s more properly E_0 = mc^2 (but that''s a bit more nitpicky).

It is true that when physicists speak about "mass" they usually mean rest mass rather than relativistic mass. However, we were discussing the relativistic concepts introduced by Einstein, and he did in fact introduce the concept of relativistic mass.

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