# Special Relativity, 2nd postulate, and a dilemma

• W.RonG
In summary, the conversation discusses the 2nd postulate of Einstein's "On the Electrodynamics of Moving Bodies" which states that light is propagated in empty space with a constant speed, independent of the motion of the emitter. The conversation then delves into thought experiments involving an emitter and receiver in motion relative to each other, and the effects of this motion on the position of the light waves. The conversation also brings up the idea of a fixed reference point in space and the concept of relativity of length and time.
W.RonG
Re-reading "On the Electrodynamics of Moving Bodies" by A. Einstein from 1905 (English translation I found on the internet). Contrary to much of what I've seen (the speed of light is the same for all observers or whatever) the 2nd postulate actually states that light is propagated in empty space with velocity c, and this velocity is independent of the state of motion of the emitter. I've been playing around with some thought experiments employing very basic emitters and receivers (observers). I have a point emitter of light "A". the wavefronts of light emanating from "A" will be a sequence of expanding spheres with "A" inside the spheres. is this concept agreeable to all? makes sense to me.
Now the question is, can "A" move away from the center of these spheres. The 2nd postulate seems to say yes, once the light energy is traveling in space its propagation is independent of anything "A" does after that particular wavefront was emitted. again, agreed?
However if I am "A", can I move toward the spheres in one direction causing in effect my own Doppler shift? I picture "A" as seeing itself always at the center of its propagating wavefront spheres. an independent observer "B" can receive light waves propagated from "A" and discern it as coming from a particular direction, that is where "A" was when that wave was emitted. "A" may have moved since then relative to "B" but the wave source direction doesn't change (see "Planetary Aberration" - the sun moves in the time it takes for the light to get here so what we really see is where it was). But does this effect define a point fixed in space where the sun was, where the present wavefront sphere is centered? I keep coming back to the idea that even SRT has, hidden within it, a point-of-view related to fixed space. in fact go back and re-read the 2nd postulate. light propagates with respect to "space", and the emitter can move with respect to "space", therefore "space" seems to be a fixed or defineable reference.
thoughts?
rg

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W.RonG said:
Re-reading "On the Electrodynamics of Moving Bodies" by A. Einstein from 1905 (English translation I found on the internet). Contrary to much of what I've seen (the speed of light is the same for all observers or whatever) the 2nd postulate actually states that light is propagated in empty space with velocity c, and this velocity is independent of the state of motion of the emitter.
Here's a quote from a translation of that paper:
1. The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of co-ordinates in uniform translatory motion.

2. Any ray of light moves in the "stationary'' system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body.​
Sounds pretty clear to me that the second postulate states that the speed of light is the same with respect to any observer (who is then the "stationary" system), regardless of the speed of the emitter.

Reference: http://www.fourmilab.ch/etexts/einstein/specrel/www/"

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That's the one I've been reading. Those are "reflexions" starting section 2 based on the introductory postulates. I am focused (sorry) mainly on the emitter and trying to sort out what its relationship is with the light waves it has been emitting. Are these spheres? for a point emitter in free space em energy must propagate equally in all directions. I see spheres. can the emitter then move after some spheres have been launched into space? of course. there could be acceleration, but there could also be constant translatory motion. What effect does this motion have on the positions in space of the spheres and the emitter? seems to depend on an observer. I arrive at two different effects on the waves of light, from POV of "A" and POV of "B" if they are moving relative to each other.
rg

W.RonG said:
That's the one I've been reading. Those are "reflexions" starting section 2 based on the introductory postulates.
Not exactly. While Einstein certainly refers to the postulates in the introduction, the paragraph introducing the two principles that I quoted above states:
The following reflexions are based on the principle of relativity and on the principle of the constancy of the velocity of light. These two principles we define as follows:​
The two restated postulates are the "principles" referred to; the "reflexions" are his conclusions about "the Relativity of Lengths and Times", the subject of that section.

I am focused (sorry) mainly on the emitter and trying to sort out what its relationship is with the light waves it has been emitting. Are these spheres? for a point emitter in free space em energy must propagate equally in all directions. I see spheres. can the emitter then move after some spheres have been launched into space? of course. there could be acceleration, but there could also be constant translatory motion. What effect does this motion have on the positions in space of the spheres and the emitter? seems to depend on an observer. I arrive at two different effects on the waves of light, from POV of "A" and POV of "B" if they are moving relative to each other.
I suspect I know what's bugging you. Imagine an emitter in an inertial frame A moving at some speed with respect to observer B in another inertial frame. That emitter emits a spherical shell of light at the exact moment that observer B passes by the emitter. In frame A, the light forms a sphere about the emitter; in frame B, the light forms a sphere about observer B.

See this: "[URL constancy of the speed of light is paradoxical
[/URL]

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Thanks. I'll check that out.
The site looks familiar but I didn't have it bookmarked. Pictures should help (I'm basically a tech after all).
rg

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W.RonG said:
Now the question is, can "A" move away from the center of these spheres?

it depends on whos point-of-view. from B's POV, yes. from A's POV, no.
I keep coming back to the idea that even SRT has, hidden within it, a point-of-view related to fixed space. in fact go back and re-read the 2nd postulate. light propagates with respect to "space", and the emitter can move with respect to "space", therefore "space" seems to be a fixed or defineable reference.

see, but the thing is this "space" is empty. no nothing at all in it. not even a something that you can measure movement or velocity against (what they might call an "aether"). you can measure movement and velocity relative to yourself, the observer.

if there was an aether, like the wind we could sort of feel it breezing past our faces. knowing that sound needs a medium to propagate, if you can feel that wind blowing past your face from left to right, you will measure the speed of sound to be faster from your left than your right. but if you can't feel that aether blowing past your Michaelson-Morley inferometer, then you can't tell that your moving through it, and maybe you can't tell your moving through it because "it" isn't there.

so, if it ain't there, if there is no meaning to the concept of this vacuum of nothingness blasting past your face at 0.9 c, then either you at A or the other guy at B have equal claim to being "stationary" (and it's the other guy who's moving) and you have to measure identical physical quantities identically. one of those quantities is the speed of propagation of the EM field, which, for both you and the guy at B is $1/\sqrt{\epsilon_0 \mu_0}$.

Apparently my query is not an original thought (not that I believed it was . . . ).
Reading wiki entry for Lorentz Ether Theory, there is a reference under the heading Ether where H. Poincare' said in 1900 that the ether explains where the ray of light is after leaving the emitter and before it reaches the receiver/observer. that's the basic concept of what I was asking about. Is there more info on this point of view?
rg

## 1. What is the second postulate of special relativity?

The second postulate of special relativity states that the speed of light in a vacuum is the same for all observers, regardless of their relative motion. This means that the speed of light is a constant, and does not vary based on the observer's velocity.

## 2. How does the second postulate affect our understanding of space and time?

The second postulate of special relativity has led to the concept of spacetime, where space and time are interconnected and can be affected by an observer's velocity. It also forms the basis of Einstein's famous equation, E=mc^2, which explains the relationship between energy, mass, and the speed of light.

## 3. What is the dilemma posed by special relativity?

The dilemma posed by special relativity is known as the "twin paradox." It describes a scenario where one twin travels at high speeds in space while the other stays on Earth. When the traveling twin returns, they would have aged less than the twin who stayed on Earth, creating a paradox of time dilation.

## 4. How is special relativity different from classical mechanics?

Special relativity differs from classical mechanics in that it takes into account the effects of an observer's velocity on the measurement of time and space. In classical mechanics, time and space are considered absolute and do not change based on an observer's frame of reference.

## 5. What are some real-world applications of special relativity?

Special relativity has numerous practical applications, including GPS technology, particle accelerators, and nuclear power. It also plays a critical role in understanding the behavior of objects at high speeds, such as the movements of subatomic particles and stars in the universe.

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