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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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Hmm.
Humans know relativity from beginning of civilisation.
Relativity = symmetry of continuum.

Maybe i missed something here ,
table of relativities:
As long as human concerned:
Position,absolute time, horisontal angle.
Pseudo-realivity: Scale (size,time,etc).
Mirror.(coordinate and angle sign relativity)

recently:
earth is sphere: Angles (full),

Scale (size) relativity proven to be wrong,probably Galileo.

Motion, Galileo/Newton.
Scale (time and size) are assumed as somewhere right.

Absolute zero: themperature shift relativity proven to be wrong,themperature scale relativity remains.

Time sign, don''t know who,related to thermodynamics.

Scale of several things (charge).

charge sign.

discovery of electron, absolute charge sign(no charge sign relativity).

Motion,length,time interval,and many other nice things, Einstein,special relativity.(e=mc^2 is not a relativity)

Scale of any kind(charge,masse,energy) relativity proven to be wrong (quanta).

Acceleration/Gravitation,even more nice things ,Einstein,general relativity.
Discovery of positron.Relative charge sign

Sign relativity(mirroring ,time sign,charge sign,antiparticle) revieved several times till today.

Heh.Probably i''ve missed something.
Really,here''s too many relativities...

Regards,

Dmytry Lavrov.

(Sorry for my crappy english )


...

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quote:
Original post by 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.



Ah, in my mind I was thinking more of concepts in Special Relativity than those specific concepts introduced by Einstein. I hope you can understand my misunderstanding and accept my apologies for the incorrect correction.

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quote:
Original post by Way Walker
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.



Isn''t that a lot like saying, you aren''t changing your speed as you run from point A to point B, you''re just running a longer path? In a macro sense, it does "slow down" in that it takes longer to travel a given linear distance. The breakdown is that on a micro scale, it''s travelling at exactly the same speed but taking a longer route (i.e. the distance cannot accurately be said to be along a strictly linear path, since you''ve put all these atoms in the way). Or is that just wrong?

Later,
ZE.

//email me.//zealouselixir software.//msdn.//n00biez.//
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quote:
Original post by ZealousElixir
Isn''t that a lot like saying, you aren''t changing your speed as you run from point A to point B, you''re just running a longer path? In a macro sense, it does "slow down" in that it takes longer to travel a given linear distance. The breakdown is that on a micro scale, it''s travelling at exactly the same speed but taking a longer route (i.e. the distance cannot accurately be said to be along a strictly linear path, since you''ve put all these atoms in the way). Or is that just wrong?



nope, youre right. its obvious when you think about...water isnt really an immutable object, its a vacuum filled with water molecules, but most of it is still vacuum. heres a somewhat stupid site (the first google gave me) that explains it:

http://www.physlink.com/Education/AskExperts/ae509.cfm

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quote:
Original post by ZealousElixir
Isn''t that a lot like saying, you aren''t changing your speed as you run from point A to point B, you''re just running a longer path? In a macro sense, it does "slow down" in that it takes longer to travel a given linear distance. The breakdown is that on a micro scale, it''s travelling at exactly the same speed but taking a longer route (i.e. the distance cannot accurately be said to be along a strictly linear path, since you''ve put all these atoms in the way). Or is that just wrong?



It doesn''t really travel a longer distance, although the idea of an "optical path length" does have its uses. Look into Fermat''s Principle and, if you can view java applets, check out

http://www.phy.ntnu.edu.tw/java/propagation/propagation.html

But I think the reality is closer to making stops along the way (see the site justo mentions). The photon interacts with an atom and then it takes some bit of time before it''s re-emitted.

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quote:
Original post by Way Walker
But I think the reality is closer to making stops along the way (see the site justo mentions). The photon interacts with an atom and then it takes some bit of time before it''s re-emitted.


In which case, it''s not the same photon... and so you cannot conclude that the speed of a single photon is altered as it passes through water... however, you can talk about the relative difference between the incident photon and the emergent photon, whereupon it becomes sensible to talk about a phase difference (as opposed to a change in phase). Extending this to, for example, a laser beam shone through a body of water, one can discuss the difference in properties of the emergent beam from the incident beam; for example, phase difference, scattering angle (measure of the loss of beam cohesion), etc.

On the E=mc2 thing... if you derive the total energy for a particle I believe it comes to E=mc2 + (m0c2)2 (it''s been more than a decade since I studied this though, so my memory could be a little hazy). You need to take into account rest mass and relativistic mass when considering the energy in a body. Unfortunately, common media has truncated this to being E=mc2, probably because it''s ''snappier''!

Cheers,

Timkin

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Guest Anonymous Poster
The total energy is mc^2

That is the rest mass + the kinetic energy

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quote:
Original post by Timkin
quote:
Original post by Way Walker
But I think the reality is closer to making stops along the way (see the site justo mentions). The photon interacts with an atom and then it takes some bit of time before it''s re-emitted.


In which case, it''s not the same photon... and so you cannot conclude that the speed of a single photon is altered as it passes through water... however, you can talk about the relative difference between the incident photon and the emergent photon, whereupon it becomes sensible to talk about a phase difference (as opposed to a change in phase). Extending this to, for example, a laser beam shone through a body of water, one can discuss the difference in properties of the emergent beam from the incident beam; for example, phase difference, scattering angle (measure of the loss of beam cohesion), etc.



Yeah, I was a little sloppy with the terminology, thanks for the correction

quote:

On the E=mc2 thing... if you derive the total energy for a particle I believe it comes to E=mc2 + (m0c2)2 (it''s been more than a decade since I studied this though, so my memory could be a little hazy). You need to take into account rest mass and relativistic mass when considering the energy in a body. Unfortunately, common media has truncated this to being E=mc2, probably because it''s ''snappier''!



As the AP pointed out, if m is the relativistic mass, then

E=mc2

is indeed the total energy of the body. Perhaps you were thinking of

E2=(pc)2+(m0c2)2

where p is the momentum and m0 is the rest mass of the object? (The units in your equation "don''t add up" so to speak )

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quote:
Original post by Way Walker
As the AP pointed out, if m is the relativistic mass, then

E=mc2

is indeed the total energy of the body. Perhaps you were thinking of

E2=(pc)2+(m0c2)2

where p is the momentum and m0 is the rest mass of the object? (The units in your equation "don''t add up" so to speak )



Quite probably. As I said when I wrote it, it''s been a very long time since I studied this at Uni and I''ve had no reason to use it again since... I don''t often encounter relativistic effects in my daily life! Thanks for the correction.

Cheers,

Timkin

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Firstly, I must confess that I did not fully read all the posts. So forgive me if I step on any toes.

Now, here's a scientific history lesson. I am sorry for the length of the post but I feel it will enlighten all of you.

It was Galileo who first discovered the concept of relative motion. Consider the simple example of a swimmer doing laps in a river. When he swims upstream, he moves slower relative to the shore and when he swims downstream he moves faster relative to the shore. If he swims equally fast in both directions it is clear that he will not move equally fast in both directions relative to the shore due to the influence of the moving water. In this case it's simple vector arithmetic to determine how fast he's moving relative to the shore.

Quite some time passed and Newton came along. Now, Newton was particularly mad, but obviously brilliant in a way most of us will never understand. If you ever pick up Bill Bryson's "A Short History of Nearly Everything" he mentions Newton in several places. Apparently Edmund Halley had to beg Newton to formally write his laws of motion (p. 46). He mentions that Newton was mostly concerned with alchemy. He then notes that an analysis of a strand of Newton's hair revealed that the mercury level was over 40 times as high as that found in the hair of the average person. (If you don't know, mercury makes you quite mad.) Anyway Newton then formulated his laws of motion in roughly two years, although I am unsure how much he was influenced by Galileo. However, at the core of Newton's dynamics was the concept that time is a dimension that extends to infinity in both directions and is absolute for all observers.

Then a few hundred years later Maxwell formulated his laws of electromagnetism. It was noted that if you combined all four of the equations in a logical manner, the theory predicts the existence of an "electromagnetic wave." Now one problem was that traditionally, waves need a medium in which to propagate. A sound wave travels as a compression wave through conglomerates of matter, a wave in the ocean travels through water, etc. So scientists postulated the existence of a "luminiferous ether" through which these electromagnetic waves could propagate. It was then further postulated that this ether was flowing relative to the earth.

Now came Micelson and Morley. Michelson came up with an experiement to test the validity of the theory of the ether. Using an interferometer (a simple arragement of mirrors that can precisely measure the speed of light) he could determine the existence of the ether. Essentially at one point in the earth's orbit, the earth will be traveling into the ether and at the opposite point on the orbit, the earth will be traveling out of the ether. So like a swimmer in a river, electromagnetic waves will slow at the first point in the orbit, and gain a speed boost in the other. So they conducted their experiment.

They found that in these two measurements the speed of light varied only by a tiny, tiny percentage, incredibly less than what was predicted from the theory of the ether.

Now instead of abandoning the ether, many scientists started introducing fudge factors. The most famous of which is Henri Poincare's (read: Pwan-care-ray) explanation that as the interferometer travels through the ether, the electrons that make up the device are actually shortened in the direction of motion. An inredible claim! This became the prevailing explanation for awhile. His transformations that accounted for the foreshortening of the electrons in the device became known as the Lorentz Transformations.

But the ether was still there in the minds of many scientists!

Then came along Einstein who in 1905 produced The Special Theory of Relativity, and two articles explaning mysterious Brownian motion and the photoelectric effect (which really was the start of QM along with Bohr's quantization of the energy field in a blackbox). In Special Relativity, Einstein reinterpreted the Lorentz Transformation and finally abolished the luminiferous ether. Einstein won the Nobel Prize for his work with the photoelectric effect in 1921, which was quite some time after the article had been written (mostly due to WWI).

Special Relativity has only two postulates. First, the speed of light is constant for all observers. Second, the laws of physics are the same for all observers.

Now the most radical aspect of Einstein's Special Relativity was that space and time are not absolute. Also, the effects that a single observer sees are quite astonishing.

If your friend shoots off in a rocket traveling away from the earth at some appreciable percentage of the speed of light you (on earth) will see dramatic effects. First the rocket will be foreshortened in the direction of motion, given by the gamma term in the Lorentz Transforms (One over the square-root of one minus the ratio of the square of the velocity and the square of the speed of light.) And you, on earth, if you were to take out a telescope and peer into the window on the side of your friend's rocket, you will also observe that your friend appears to be moving around inside incredibly slowly (although his rocket is still traveling quite fast). This is the effect of time dilation. The rate at which time is slowed again by a factor of gamma. The astonishing thing is that your friend in the rocket will see you on earth moving at an incredibly slow rate, not a faster rate. So time passes at different rates for observers in relative motion to one another. If your friend were to travel out, then stop and turn around and come back, you would both find that you had aged many years and he would have only aged a few.

Then there is simultaneity, which essentially says that two observers in relative motion will not agree on the simultaneity of two events. I will allow you to develop this one for yourself.

So much for Newton's notion of absolute time.

So Einstein abolished the ether and gave physics a new "observer" perspective. He also discredited Newton's idea of absolute time.

For correctness of the posts:

The rest mass energy of a particle is E = mc^2
The total energy of a relativistic system is E^2 = (pc)^2 + mc^2.
(You can imagine this as a right triangle, i.e. the Pythagorean Theorem.)

Special Relativity is restricted to intertial frames (those that do not accelerate). General Relativity extends Special to include reference frams that do accelerate. If you want me to derive Einstein's initial thoughts that lead him to GR, I would be happy to oblige.

Or we can talk about Quantum Mechanics if you like. =)

If you guys want a good source for explaining relativity, check Gamow's Mr. Tompkins books. They're excellent.

This was all off the top of my head. So correct my if I err.

TT

[EDIT - Fixed miscellaneous grammatical errors. ]

[edited by - TThirion on February 25, 2004 5:21:00 PM]

[EDIT - Added: "... if you were to take out a telescope ..."

[edited by - TThirion on February 25, 2004 6:00:21 PM]

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>> This was all off the top of my head. So correct my if I err.

I´ll gladly do even if it´s just to be pedantic (have to kill some time be4 I get back to learning that GR you mentioned...)


>> If your friend shoots off in a rocket traveling away from
>> the earth at some appreciable percentage of the speed of
>> light you (on earth) will see dramatic effects. [...]. And
>> you, on earth, will also observe that your friend appears to
>> be moving incredibly slowly.

He´ll appear to be moving at an "appreciable percentage of the speed of light". I wouldn´t call that "incredibly slowly" .

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I apologize.

The rocket will be traveling an appreciable percentage of the speed of light. However if you were to take out a telescope and look through the window on the rocket, your friend will appear to be moving quite slowly.

Hope that clarifies somewhat.

TT

P.S. - I'm still an undergrad, so be merciful!

[edited by - TThirion on February 25, 2004 5:57:35 PM]

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quote:
Original post by Atheist
>> This was all off the top of my head. So correct my if I err.

I´ll gladly do even if it´s just to be pedantic (have to kill some time be4 I get back to learning that GR you mentioned...)
...



What text are you using to learn GR?
What level are you? (Undergrad, grad ...)
I just recently bought the Wald text but I haven't had time to crack it open.

TT



[edited by - TThirion on February 25, 2004 6:11:19 PM]

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I use several texts (2 books, a script, my notes from the lecture, some online-ressources and my TeXed shortscript to look things up, that I understood before). None of them are in english so naming them probably wouldn´t help you much.
Dunno about the grad, undergrad, ... terminoligy. I just finished my 9th semester and hope that I can start my diploma thesis (final work which takes one year) this summer.

@speed of the rocket: I initially thought this was a typo of yours but now it seems I don´t get your point. I´d say there are only two reasonable systems of reference. Yours and the SOR of the rocked. In either of them the relative velocity will be the same (as are other effects like the rocked seeing the earth flattened). And both are "the speed" since there is no other SOR to measure the speed against.
Maybe you mixed things up or maybe I did or maybe I just didn´t get your point. Mine was: You define the speed of the rocket by the speed you observe (so it´s relative to you).

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