# The speed of lights independence of the speed of the observer?

Simplex0
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I have red that originally Albert Einsteins just stated that the speed of light is independent of the movement of the light source. He did not included the movement of the observer, that was done years later and by someone else.
Why was there a need to include the movement of the observer in the statement and has that ben verified by experiments?

Odysseus
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### The speed of lights independence of the speed of the observer?

Quote:
I have red that originally Albert Einsteins just stated that the speed of light is independent of the movement of the light source. He did not included the movement of the observer, that was done years later and by someone else.

The key principle of relativity is that it makes no difference which of the observer and the source is movingâ€”indeed, the theories do away with the notion of an absolute rest frame, so nobodyâ€™s in a position to separate the two cases. The only significant datum in this regard is the relative speed, and since we usually look at a SR situation from the observerâ€™s (inertial) frame, itâ€™s typical for motion to be attributed to the source. I donâ€™t know what the importance of that â€œsomeone elseâ€? would be; AFAICT itâ€™s trivial to switch from one inertial frame to another, as the motions of each relative to the other, and the consequences thereof, are symmetrical.

Iâ€™m not sure itâ€™s what youâ€™re looking for, but Georges Sagnac performed an important early experiment involving a moving light-source, essentially an MMX apparatus mounted on a spinning turntable. IIRC his results disproved Ritzâ€™s â€œballisticâ€? theory of light propagation. Sagnac interferometers are used in inertial-guidance systems; modern one use fibre-optics to guide the light, instead of a polygonal set of mirrors.

Simplex0
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### RE: RE: I have red that

Message 59253 in response to message 59252

Quote:
Quote:
I have red that originally Albert Einsteins just stated that the speed of light is independent of the movement of the light source. He did not included the movement of the observer, that was done years later and by someone else.

The key principle of relativity is that it makes no difference which of the observer and the source is movingâ€”indeed, the theories do away with the notion of an absolute rest frame, so nobodyâ€™s in a position to separate the two cases. The only significant datum in this regard is the relative speed, and since we usually look at a SR situation from the observerâ€™s (inertial) frame, itâ€™s typical for motion to be attributed to the source. I donâ€™t know what the importance of that â€œsomeone elseâ€? would be; AFAICT itâ€™s trivial to switch from one inertial frame to another, as the motions of each relative to the other, and the consequences thereof, are symmetrical.

Iâ€™m not sure itâ€™s what youâ€™re looking for, but Georges Sagnac performed an important early experiment involving a moving light-source, essentially an MMX apparatus mounted on a spinning turntable. IIRC his results disproved Ritzâ€™s â€œballisticâ€? theory of light propagation. Sagnac interferometers are used in inertial-guidance systems; modern one use fibre-optics to guide the light, instead of a polygonal set of mirrors.

The fact that there is a time difference between the light beam going in the same direction and the beam going in opposite direction of the rotating table does that not imply that an observer in ring 'A' (clockwise rotation) would have a different opinion of the speed of light than a observer in ring 'B' (counterclockwise rotation)?

Mike Hewson
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Message 59254 in response to message 59253

Quote:

The fact that there is a time difference between the light beam going in the same direction and the beam going in opposite direction of the rotating table does that not imply that an observer in ring 'A' (clockwise rotation) would have a different opinion of the speed of light than a observer in ring 'B' (counterclockwise rotation)?

In the diagram from the page quoted there's only one observer, one travelling with the rotating apparatus. He just has one beam going clockwise, and the other going counterclockwise. By detecting a time difference in their traversal he can deduce that he is rotating. The faster he rotates the more the time gap separating arrivals. Or for that matter if the ring expands, radius increasing, the difference will grow too. As it's only a difference he's observing then he could attribute that to differential speeds each way. He hasn't made separate measurements of either though.

Now if you stop and reverse the sense of the rotation then you will note the above effects, but now one beam beats the other whence before it lagged.

A non-rotating observer nearby will note the rotation, and the time difference - but will account for that as different lengths to be travelled by one beam vs the other. He won't attribute that to light speed variation.

[aside]
It's important to appreciate that special relativity involves symmetric effects between inertial ( non-accelerating, non-rotating ) frames. By that is meant that if I transform co-ordinates/events between inertial frames I will arrive at the same overall form of description for some process. In that sense all the inertial frames are equivalent and their is no peculiar one to single out. There might be a relative velocity dependence in transforming between frames, but no result within a frame relies upon that relative velocity. So one cant tell 'how fast' an inertial frame is moving by measurements within it.

In contrast as soon as you invoke some acceleration ( rotation is acceleration ) then symmetry of description is lost. So all accelerated frames are not symmetric with one another, or with inertial ones. There will always be some component to observe ( purely within the frame ) that reveals the acceleration and depends on it's magnitude. Rotation in that sense is an absolute phenomenon. The light path time difference above is an example of that. Einstein's brilliant insight was to assign such absolute effects to spacetime distortion, or if you like gravity!
[\\aside]

Cheers, Mike.

( edit ) Actually I think it is this Sagnac effect that is used for the laser gyroscopes in the submarines. You start the trip with some platform orientation/state vector(s). The gyroscopes ( and other stuff ) give you moment to moment accelerations. This is integrated to velocity changes, which in turn is summated to displacements. One can correlate with other traditional measures, but in theory one can do this endlessly. In practice precision will be limited to some probably small degree hence eventual drift, but it would reduce the need to surface and re-initialise/orient. It's a good example of detecting acceleration without a window to look out of, despite no doubt other cues ( sounds etc ). A passive/furtive technique and quite useful for their role...

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

Simplex0
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### Would the same time

Would the same time difference occur if it was a linear movement instead of a rotating?

Simplex0
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### I't looks like it

debugas
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Message 59257 in response to message 59256

answering original question - the independence of light speed from observer is needed to eliminate the aether possibility - the medium in which light could be propogating (like sound in the air. if observer is moving relative to the air then the sound speed (as it carries with air) becomes dependent on observer relative motion to the air medium).

To eliminate aether medium for light propogation a Michelson Morley experiment was conducted

Some additional explanation on aether is probably needed:
the waves usually propogate in certain mediums so it was thought that light is propogating in some medium too. Then one would like to know what is earth's relative motion to that aether medium. It appeared however that light propogation is invariant for all inertial frames i.e. is independent of aether which would nesessarily be at rest in one paticular frame of reference and would be in relative motion to any other frame of reference.

think of the following thought experiment - you moving with speed v relative to air and you scream to send some sound forward - the speed of sound relative to you would be c - v (c sound speed in the air and v your speed relative to the air). If you instead try to through a rock (which speed is independent of the air) ahead then its speed relative to you will be c and no correction for v will be needed. So in this regard the light behaves like a rock and not like the sound.

But the light is not like the rock in the regard of rock emitter influencing the rock speed relative to let's say earth (rock speed would be c+u where u your speed relative to earth). Light speed is indifferent of its source as well and stays c (does not becomes c+u).

peanut
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### I was bored at work today so

I was bored at work today so I got thinking about my limited knowledge on relativity. If no absolute rest state exists, how does the Universe know that two reference frames can not move apart faster than the speed of light? Does that not mean that a two frames though millions of light years apart would have to know instantaneously that they can not move apart faster than light? That implies that time means nothing to the universe. The ole "spooky action at a distance".

2nd thought: If time is off no importance to the universe, might the search for changes in the structure of the universe that are time dependant (like passing gravity waves) be futile? What if the structure of the universe changes without us being able to detect it?

Einstein was a genius for sure. All my thoughts are ramblings of a lesser mind.

Chipper Q
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### Hi peanut, RE: If no

Hi peanut,

Quote:
If no absolute rest state exists, how does the Universe know that two reference frames can not move apart faster than the speed of light? Does that not mean that a two frames though millions of light years apart would have to know instantaneously that they can not move apart faster than light? That implies that time means nothing to the universe. The ole "spooky action at a distance".

It's not a question of one frame of reference knowing about another, it's more that nothing has ever been observed even matching the speed of light, but some particles can come close, depending on the event that accelerated (or created) them....

Quote:
2nd thought: If time is off no importance to the universe, might the search for changes in the structure of the universe that are time dependant (like passing gravity waves) be futile? What if the structure of the universe changes without us being able to detect it?

You measure the structure, and it is in fact changing (the universe is expanding and the expansion is accelerating). And the way to measure it properly involves more than just time and distance, so there are many things that are important, like matter and energy...

Chipper Q
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### I should have said nothing is

I should have said nothing is faster than light in vacuum. Interestingly, it's possible for a particle (usually an electron) to go faster than light in a material, and when that happens the particle gives off Cerenkov radiation...

peanut
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### So it is possible to exceed

So it is possible to exceed the speed of light? What happens to the mass of a particle that exceeds light speed? I thought it would get infinitely heavy before that happened. Perhaps that is more in the realm of quantum physics where things really get wierd.