Thread: My Relativity of Light Postulate

My postulate is this, that the velocity of light can only be gauged in the frame in which it was generated and that its apparent velocity in other frames in relative motion are illusory. The only frame which indicates the true behavior of light is the one which can be viewed by itself, with all others removed, and as if it is stationary, and the light will still be seen to exist. When any other relative frame is viewed in isolation, the light cannot exist because it has no source. You can't have a light beam existing by itself in space, therefore those other frames cannot be used to gauge the behavior of the light and do not require the true light frame to be adjusted to suit it by altering its time passage rate or physical dimensions

I do not believe that it is necessary to manipulate relative frames by modifying time and/or length to make it appear that light has the same velocity in all frames at the same time. I postulate that viewing a light beam in the frame in which is is generated from another frame in motion relative to it is like viewing a video recording of it on a portable player and moving the player around. The light beam's velocity is not really being sped up or slowed down by the motion of the video player, it is an illusion of sorts and therefore requires no correction. What say you of my postulate? Can you justify modifying the light generating frame to make it conform to a non generating frame so it appears to have the same velocity in both simultaneously?

How do you explain the Michelson–Morley experiment?

Originally Posted by KJW

How do you explain the Michelson–Morley experiment?

I didn't investigate it much actually, I just know they shot light beams through half mirrors back and forth in perpendicular directions to see if there would be a velocity different due to the earth's movement through the supposed aether. I don't see how it relates to my postulate because they were measuring the light speed in the same frame that it was generated in, the whole thing was moving as one unit, to my knowledge, so there was no Lorentz transformation type adjustments of time or length involved or would have been seen by Einstein as required, though his theories came after that anyway. It's consistent with my postulate, as far as I can see.

I devised a thought experiment to discuss my postulate. The text didn't come out well when I copy pasted it in the forum but I guess I can post it anyway, just some words will probably get joined together. It's kind of long but if you have time it may be interesting to read. I'll post it after this and try to fix the text problems.

Let there be a track 450,000 km long and a rocket 300,000 km long with a laser attached to the bottom of it's back end with a clock beside it,and a second synchronized clock attached to the bottom of its front end. Both clocks were also synchronized with a track clock while the rocket was parked there before the experiment, all observers agreed that the track was 1.5 light seconds long and the rocket 1 light second long by those three clocks.


The rocket travels left to right at a uniform velocity of 150,000 km/s along a path which is directly vertically above and parallel to the track, having previously accelerated to full speed before reaching the track, and when it reaches the point where its back end is at the left end of the track it fires the laser while recording the time on the back clock. When the rocket has traveled 150,000 km to the right, its front end will be directly above the right end of the track, its back end at the 150,000 km mark from the left end of the track, and the light beam will have reached the front clock, stopping it at that time and also triggering a device at the right end of the track to show what the time on that clock was and that the front end of the rocket had been there at the same time. The rocket then continues on it way in the same uniform motion, never having deviated from it while in the vicinity of the track.

From the track frame, it would seem as if the light beam had traveled 450,000 km, the full length of the track, in 1 second, while from the rocket frame it would seem that the same beam had only traveled 300,000 km, the length of the rocket,in 1 second. It would appear to an observer in the track frame that the light beam had traveled at 450,000 km/s, 50% faster than normal light speed, thus not conforming to Einstein's postulate that light travels at the same speed as viewed in all frames.

Plugging the numbers into a Lorentz transformation calculator indicates that to prevent that from being the perceived outcome the length of the rocket, with v= 0.5 c, should be contracted to 0.8660254037844386 of 300,000 km= 259807.62113533158 km in length, and that its time should be dilated to 1.1547005383792515 normal, meaning its clocks would run slower than the track clock. We know that the time of 1 second had been recorded on the rocket clock and it copied that figure to the right end of the track, so we know its time wasn't dilated. We also know that the rocket's front end had been traveling for 1 second at 150,000 km/s so we know it didn't travel farther than 150,000 km of track, so its length hadn't been contracted either, thus, a paradox apparently exists. My postulate to dispel the paradox is as follows.

The Lorentz transformation isn't fooling us, we know what actually happened and that neither time nor length had really been altered in any way, therefore, we know that it is in fact possible to perceive light as traveling faster than 300,000 km/s but we also know that it's an illusion, it never really traveled that fast, we just had to measure the speed of light in the same frame that it was generated in and not from another frame in relative motion to it, because the result would not be the true velocity, it would be a trick of perspective, you might say.

From the perspective of the rocket, if we reverse the perception of relative motion such that we view the rocket as being stationary and the track moving instead, we saw the track move 150,000 km under us in 1 second, exactly as we should. Light velocity seemed normal, because we didn't measure it relative to the track, we measured it from inside the rocket, based on things also inside the rocket, and found that it traveled 300,000 km in 1 second, exactly as it should. We did the correct thing, we measured the velocity of light in the same frame which it had been generated in because we could not have measured it from the track frame, or based on things in the track frame, and obtained the correct reading. You could calculate the correct velocity from the track perspective but you would need to use only things located inside the rocket for the calculation, which would be complicated and you would need information about the things in the rocket.


My postulate is that whichever frame can be viewed on its own, with all other relative frames removed, and the light still be seen to exist is the only frame in which the velocity of light can be accurately gauged and that from all other frames the light in that frame should be considered as if it is a video recording of the light in the frame in which it was generated and seen to actually exist. You could move the video player around any way you want while playing the video but it won't change the actual velocity of the light, it will be an illusion of added motion. That true light frame can be considered as if it is stationary in space, as far as the behavior of the light init is concerned, no matter how that frame moves in relation to another or another moves in relation to it.


Now you might ask what about when one frame generates light and another moves in relation to it and the light then reaches an observer in that other frame? In that case you would consider the light to be moving from the generator to the observer in the other frame in accordance with normal light velocity as viewed from the perspective of the generating frame being stationary and the other frame moving in relation to it, even if the generating frame happens to be on the ground. It is then a matter of the angles and relative motions involved, as if the light waves were sound or water waves.


The thing which makes this postulate different from Einstein's is that no time or length alteration or perceived alteration is involved. There would still be “relativity of simultaneity” but it would be entirely due to the time it takes for the light waves to reach the observers' eyes from the generators, which was not involved in Einstein's thought experiments, like the train and lightning one.

But one can measure the speed of light from a moving light source and that speed is c. Also, it makes no difference whether the light source is moving relative to the observer, or the observer is moving relative to the light source... the speed the light is c. Once a pulse of light has left its source, how does it "know" what frame of reference the source is in?

How do you explain Maxwell's equations?

How do you explain the Fizeau experiment?

Originally Posted by KJW

But one can measure the speed of light from a moving light source and that speed is c. Also, it makes no difference whether the light source is moving relative to the observer, or the observer is moving relative to the light source... the speed the light is c. Once a pulse of light has left its source, how does it "know" what frame of reference the source is in?

How do you explain Maxwell's equations?

How do you explain the Fizeau experiment?

I don't explain Maxwell's equations, it's electrical engineering stuff to me. I don't see how it relates to my postulate or thought experiment paradox.

I found an article which explains the Fizeau experiment results as being attributable to the Doppler effect http://labs.plantbio.cornell.edu/wayne/pdfs/FIZEAU.pdf . It shows that it explains the results with greater than twice the accuracy of the Special Relativity explanation, pretty much proving it as being invalid. If it was invalid there, how valid can it be anywhere else?

To avoid people needing to download the cited article, here's the pertinent part, the results of their version of the experiment. Obviously, the Doppler effect explanation is vastly more in line with the actual results than the Special Relativity explanation, which was not even close, showing how deficient it really is.

"The fringes shift in a velocity-difference-dependent manner. Linear regression shows that the zeroth-order central fringe shifts with a rate of 0.0419 per m/s. If the fringe shift resulted from the linear addition of the velocity of light in water and the velocity of the water, as predicted by Newtonian theory, the observed fringe shift would be 0.0784 per m/s. If the fringe shift resulted from the nonlinear addition of the velocity of light in water and the velocity of water, as predicted by the Special Theory of Relativity, the observed fringe shift would be 0.0151 per m/s. If the fringe shift resulted from the Doppler-induced transformation of the wavelength of light flowing against the water, the observed fringe shift would be 0.0441 per m/s."


In regard to this thread, I invite readers to try to explain the paradox I presented, using whatever postulates or equations they choose. I gave my explanation, now I'm interested in hearing other peoples' explanation, by actually doing it right here so I don't have to go and read a bunch of other experiments.

I assume the only thing people will try to use to resolve it is the Lorentz transformation, so I invite them to do it right here. The problem I foresee them having is that we know that both observers saw 1 second of time pass on their synchronized clocks and we know where the ends of the rocket were in relation to the track at the start and finish of its trip, so how is anyone going to be able to modify them to match the Lorentz transformation requirements? You might say it's just a thought experiment and real world results might be different. I suppose that's true but we'll never know. Maybe it's possible to make the Lorentz transformation work with this thought experiment, I didn't try that hard, it doesn't look like it to me though. I'll think about it some more and see what I come up with.

I'll try to work the thought experiment out using Lorentz transformation again right here, tell me if I make a mistake.

We contract the length of the rocket to 0.8660254037844386 of its proper length, according to the calculator at Time dilation/length contraction when you put 0.5 in the "v" box, because the rocket travels at 150,000 km/s, 0.5 c. That gives us a length of 259,807.62 km instead the original 300.000 km.

Then we see how much track is left in front of it, 450,000 - 259,807.62= 190,192.38 km.

Now we dilate the rocket's time by a factor of 1.1547005383792515, so in track time we see how far it gets in that many seconds on the 190,192.38 km of track in front of the contracted rocket, at a speed of 150,000 km/s. So 150,000 km x 1.1547005383792515= 129,903.81056766579 km.

So the track observer would have seen the light beam travel a total of the contracted rocket length, 259,807.62 km, while its front end moved a distance of 129,903.81056766579 km along the track, so we add those together and get a total of 389,711.43056766579 km that the track observer saw the light travel in 1 second of rocket time and 1.1547005383792515 seconds of track time. Dividing 389,711.43056766579 by 1.1547005383792515 gives us 337,499.999016774 km he saw light travel in 1 second, and there was 60,288.56943233421 km of track left in front of the rocket at that time.

The observer on the rocket thought he got all the way to the end and marked the time of 1 second there as he passed over it but apparently that's not what the track observer saw. He still seems to have seen light traveling faster than normal though, by 12.5%. Maybe I did something wrong there, did anyone else did a different answer?

Originally Posted by Biff

It shows that it explains the results with greater than twice the accuracy of the Special Relativity explanation, pretty much proving it as being invalid.

Other similar experiments are in agreement with Special Relativity, so maybe this single experiment is in error somehow. And since Zeeman verified the existence of the dispersion term predicted by Lorentz, one can't say that these other experiments weren't accurate.

I looked up "Doppler Theory" in Wikipedia, and couldn't find anything other than the Doppler effect. I read in the article you posted that the predicted fringe shifts are independent of the refractive index. Did they test that independence?

Originally Posted by Biff

I don't explain Maxwell's equations, it's electrical engineering stuff to me. I don't see how it relates to my postulate or thought experiment paradox.

The reason I mentioned Maxwell's equations is because they lead to the wave equations for the electric and magnetic fields:



The thing to note is that these equations are invariant to Lorentz transformations. Thus, Special Relativity is built into Maxwell's equations.

Originally Posted by Biff

I invite readers to try to explain the paradox I presented

As I say in my signature below, "there are no paradoxes in relativity, just people's misunderstandings of it". The reason why there are no paradoxes is because all results of measurements carried out by observers in different frames of reference are fully consistent when the correct transformations in four-dimensional spacetime are taken into account. When it comes to notions of length and time for different observers, the most common error people make is to assume that reality is three-dimensional instead of four-dimensional. By assuming that reality is three-dimensional, they assume that observers in different frames of reference have different measurements of the same thing. But in four-dimensional spacetime, observers in different frames of reference are actually measuring different things when it comes to length contraction and time dilation.

Originally Posted by Biff

The problem I foresee them having is that we know that both observers saw 1 second of time pass on their synchronized clocks

That's not true. For the observer on the track, the rocket took 1 second to travel the 150000 km at 0.5c, and the light took 1.5 seconds to travel the 450000 km track at c. For the observer in the rocket, the clocks are not synchronised. For one thing, you accelerated the rocket from 0 to 0.5c in an instant, and thus you are not entirely within the scope of special relativity. Special relativity can deal quite easily with the problem you posed, and there is no paradox.

Originally Posted by Biff

I assume the only thing people will try to use to resolve it is the Lorentz transformation

For these types of problems, I prefer to use spacetime diagrams. One benefit of using spacetime diagrams is that it becomes clear why there are no paradoxes.

Originally Posted by KJW

I looked up "Doppler Theory" in Wikipedia, and couldn't find anything other than the Doppler effect.

That would be because it was the "Doppler theory" of the authors of the article, their theory being based on the Doppler effect. Their results proved beyond all doubt that Special Relativity is not valid, it's game over right there for Einstein because that's what he based it on, the Frizeau experiment and stellar aberration.

Originally Posted by KJW

That's not true. For the observer on the track, the rocket took 1 second to travel the 150000 km at 0.5c, and the light took 1.5 seconds to travel the 450000 km track at c. For the observer in the rocket, the clocks are not synchronised. For one thing, you accelerated the rocket from 0 to 0.5c in an instant, and thus you are not entirely within the scope of special relativity. Special relativity can deal quite easily with the problem you posed, and there is no paradox.

I didn't say it was parked at the left end of the track and then accelerated and then stopped at the right end. It traveled over the track after having reached full speed long before reaching it. It simply fired the laser at the exact time that the back end of the rocket was directly above the left end of the track and it continued on after its front end passed over the right end of the track, those were simply the points at which the rocket observers saw the laser fire and the beam reach the front end of the rocket. And how did the track observer see the light travel the length of the track in 1.5 seconds if he saw the rocket only take 1 second for its front to reach the end? Did the rocket leave the light beam behind in the air and continue past the right end of the track without it?

Originally Posted by KJW

For these types of problems, I prefer to use spacetime diagrams. One benefit of using spacetime diagrams is that it becomes clear why there are no paradoxes.

Okay, let's see your spacetime diagram of the scenario I described then.

Originally Posted by Biff

Originally Posted by KJW

I looked up "Doppler Theory" in Wikipedia, and couldn't find anything other than the Doppler effect.

That would be because it was the "Doppler theory" of the authors of the article, their theory being based on the Doppler effect.

The article was from 2013, so if it was of any significance (such as overthrowing Special Relativity), it would be in Wikipedia.

Originally Posted by Biff

Okay, well let's see your spacetime diagram of the scenario I described then

I am not able to submit diagrams to the forum. The best I can do is draw them and post the numerical results obtained from them. At this point it is not clear to me what the point of my solution to the scenario is, because the two observers are not going to agree about the various measurements other than the speed of light.

However, I should remark that both observers do agree on the measurements each observer makes. For example, if the observer in the rocket measures some time interval to be 1 second, then the observer next to the track agrees that the observer in the rocket measured that time interval to be 1 second.

Originally Posted by Biff

I didn't say it was parked at the left end of the track and then accelerated and then stopped at the right end. It travelled over the track after having reached full speed long before reaching it.

You did say:

Let there be a track 450,000 km long and a rocket 300,000 km long with a laser attached to the bottom of it's back end with a clock beside it, and a second synchronized clock attached to the bottom of its front end. Both clocks were also synchronized with a track clock while the rocket was parked there before the experiment, all observers agreed that the track was 1.5 light seconds long and the rocket 1 light second long by those three clocks.

That is, the rocket did accelerate between the synchronisation of the clocks and the run of the experiment. The clocks will not remain synchronised during the run of the experiment. However, your description does tell me that the rocket is 300000 km long in its own frame of reference and not in the track frame of reference, which changes what I said in my previous post. Due to length contraction of the rocket in the track frame of reference, the next paragraph of your description:

The rocket travels left to right at a velocity of 150,000 km/s along a path which is directly vertically above and parallel to the track and when it reaches the point where its back end is at the left end of the track it fires the laser while recording the time on the back clock. When the rocket has travelled 150,000 km to the right, its front end will be at the right end of the track, its back end at the 150,000 km mark from the left end of the track, and the light beam will have reached the front clock, stopping it at that time and also triggering a device at the right end of the track to show what the time on that clock was and that the front end of the rocket had been there at the same time.

is invalid. Because your scenario is not valid, I can't provide a solution to it. You need to rephrase the scenario in a way that does not make any assumptions.

The article was from 2013, so if it was of any significance (such as overthrowing Special Relativity), it would be in Wikipedia.

That is a very naive assumption. Wikipedia is like first physics forum that comes up when you Google "physics forum", meaning that unless something conforms with the Einstein Fan Club's views people will kick up a stink and you'll get shut out one way or another.

Anyway, I've lost interest in this thread and it's true that the thought experiment isn't that great and somebody could probably Einstein it given sufficient time, so I'm going to think up some better stuff and post a new thread when I do, might be soon, might be a while, I don't know yet. Thanks for participating in this one though.

Here's my finalized solution to the rocket and track paradox I described, it occurred to me after further consideration. The solution requires a slightly different version of relativity, one which makes more sense, I guess you could call it the Theory of Relativity of Time and Velocity in Relation to Light, just a working title.

If clocks on the rocket and at the track were both visible to the people in both frames and each considered themselves to be stationary and the other in motion then here is what each would think, given that 1 second would be seen by both to pass on the rocket clock and 1.5 seconds to pass on the track clock. As in Einstein's theory, the passage of time would be at different rates in the two frames, and the dilation based on the relative motion but, unlike in his theory, both would see the same times on both clocks regardless of which one was considered by either to be moving. The frame with the light beam in it would be seen by both parties to have a slower rate of time passage, but they would perceive each other's velocities to be reciprocally contracted. This is less simplistic than Einstein's theory and may take some time to grasp. There is no length contraction involved in this version, but velocity contraction instead. Since velocity is distance traveled in a certain time, the variation in time rate causes a corresponding variation in perceived velocity.

The track people would consider the rocket's velocity to be 100,000 km/s in track time and 150,000 km/s in rocket time.

The rocket people would consider the track's velocity to be 100,000 km/s in track time and 150,000 km/s in rocket time.

I tried the above theory with two way lasers but it didn't work, so I guess that's a flaw. It would only work the way I said in that last post if there's a single beam. I don't think there's any way around having to just consider the frame with the light generators and receptors in it as stationary and the difference in light speed as seen from another frame in relative motion to it as an illusion of sorts.

How can Einstein's theory make it work when there's two lasers in opposite directions? Time dilation and length contraction is not going to compensate in both directions at once, same as the modified version I just tried couldn't. It brings the forward beam into normal speed but then the rearward beam is going too slow. What the above post actually showed was how it's possible to formulate a theory in which you manipulate various factors to make light appear to travel at the same speed in two frames in relative motion. The problem is, as I just described, it falls apart when two beams are moving in opposite directions in it. There is no manipulation that can resolve that, as far as I know.

I have seen thought experiments in they try to resolve it by saying that there are different times at each end of the moving rocket, or three rockets at the same speed. They'll say the stationary observer sees lasers fired from the middle rocket's clock in both directions to clocks on the two outer rockets and then they bounce back to the center rocket, reaching it at the same time though having reached the outer rockets at different times. They'll say the rear rocket's clock was showing a later time than the front rocket's clock, and the middle rocket's clock showed a time midway between them.

The flaw in their reasoning is easy to point out. What happens if you flip the two frames and consider the rockets to be stationary and the observer moving? Obviously they would see the same time on all three rocket clocks, because why would they be varied when they're all in the same stationary frame? They would also see the beams hot both outer clocks simultaneously, they wouldn't see one beam going slower than the other on their way to the outer clocks, and then reversing which one was faster and which one slower after bouncing off the clocks and heading toward the center clock. None of that would make sense with the frames flipped. It obviously has to look the same no matter which frame we arbitrarily decide to be stationary, so the theory falls apart.

The error they, and Einstein, made was in thinking the beams had a particular velocity in the observer's frame when, in reality, it only had a certain velocity in the rocket's frame. Velocity is distance and time in an inertial frame. Both the light source and the light targets were in the rocket frame, neither were in the observer's "stationary" frame, thus, no velocity could be attributed to the beams in the observer's frame. They were simply watching velocity in another frame and their own motion relative to that frame had no effect on the velocity of the light beams whatsoever, neither adding to it nor subtracting from it. No manipulations of time or length had to be applied to the rocket or the beams to make the velocity of the beams appear to have the same velocity in the stationary frame, because it was just the "appearance" of velocity, there would be no way to actually measure the velocity of the beams in the stationary frame because they never went from anything or to anything in that frame. Appearance of velocity is not velocity, it's a completely different thing. Like I said, it's an "illusion", the illusion of velocity to the naive observer.

There is no "correct" frame of reference in which measurements are valid and measurements from other frames of reference are invalid. A measurement from any frame of reference is just as valid as a measurement from any other frame of reference. When I drive down the road, the world outside my car doesn't disappear due to being the wrong frame of reference. That is an absurd notion.


Special Relativity is the consequence of two postulates:

1. The laws of physics are the same in all inertial frames of reference.
2. The speed of light in vacuum is the same for all observers, regardless of the motion of the light source or observer.

The first postulate says that there are no preferred inertial frames of reference, which is quite natural because if there was a preferred inertial frame of reference, then there would have to be a basis for that preference. The second postulate is the result of the Minkowskian metric of spacetime. The speed of light separates time and space. Although time dilation and length contraction exist between different frames of reference, the notions of time and space themselves are the same in all frames of reference. That is, everyone agrees about what is time and what is space. And this requires that the speed of light be the same for all observers.


Do not confuse time dilation with the Doppler effect. Time dilation depends only on the speed relative to the observer's frame of reference, whereas the Doppler effect depends on the velocity relative to the observer. The Doppler effect is actually a combination of two things: 1, time dilation, and 2, the time it take light to travel the change in distance. In other words, time dilation is the result of the Doppler effect after compensating for the change in the distance to the observer. In the case of the transverse Doppler effect, where the motion is perpendicular to the line of site and therefore no change in distance to the observer, the effect is purely due to time dilation.

Interestingly, the twin paradox thought experiment can be resolved by applying the Doppler effect. This has the benefit of using a phenomenon that is directly observable by both twins, and makes very clear the asymmetry between the two twins. In particular, it makes very clear the distinction between true acceleration and relative acceleration.

Originally Posted by KJW

There is no "correct" frame of reference in which measurements are valid and measurements from other frames of reference are invalid. A measurement from any frame of reference is just as valid as a measurement from any other frame of reference. When I drive down the road, the world outside my car doesn't disappear due to being the wrong frame of reference. That is an absurd notion.


Special Relativity is the consequence of two postulates:

1. The laws of physics are the same in all inertial frames of reference.
2. The speed of light in vacuum is the same for all observers, regardless of the motion of the light source or observer.

The first postulate says that there are no preferred inertial frames of reference, which is quite natural because if there was a preferred inertial frame of reference, then there would have to be a basis for that preference. The second postulate is the result of the Minkowskian metric of spacetime. The speed of light separates time and space. Although time dilation and length contraction exist between different frames of reference, the notions of time and space themselves are the same in all frames of reference. That is, everyone agrees about what is time and what is space. And this requires that the speed of light be the same for all observers.

My point is that seeing something moving at a certain speed, or velocity, between two points in a moving frame does not equate to that thing moving in your frame at the speed or velocity of the thing in that frame plus the speed of that frame in relation to yours, the two frames are not linked in respect to velocity. You can't measure velocity in one frame from another frame when the thing you want to know the velocity of is not interacting with anything in your frame, it's not going from or to anything in your frame. This is just my theory, maybe it can be proven wrong, I don't know at this point, I'm just expressing how things seem to me.

The point could be made that a moving train is linked to the ground, so if something moves inside the train it is also moving in the ground frame because the train is physically connected to the ground. What about in space though? A space station and a passing rocket have no physical linkage whatsoever so the point could be made that an object moving inside the rocket has no actual velocity in the space station's frame. A meaningless hypothetical perhaps but maybe relevant in regard to the speed of light as perceived by people in the two frames.

Originally Posted by Biff

Originally Posted by KJW

There is no "correct" frame of reference in which measurements are valid and measurements from other frames of reference are invalid. A measurement from any frame of reference is just as valid as a measurement from any other frame of reference. When I drive down the road, the world outside my car doesn't disappear due to being the wrong frame of reference. That is an absurd notion.


Special Relativity is the consequence of two postulates:

1. The laws of physics are the same in all inertial frames of reference.
2. The speed of light in vacuum is the same for all observers, regardless of the motion of the light source or observer.

The first postulate says that there are no preferred inertial frames of reference, which is quite natural because if there was a preferred inertial frame of reference, then there would have to be a basis for that preference. The second postulate is the result of the Minkowskian metric of spacetime. The speed of light separates time and space. Although time dilation and length contraction exist between different frames of reference, the notions of time and space themselves are the same in all frames of reference. That is, everyone agrees about what is time and what is space. And this requires that the speed of light be the same for all observers.

My point is that seeing something moving at a certain speed, or velocity, between two points in a moving frame does not equate to that thing moving in your frame at the speed or velocity of the thing in that frame plus the speed of that frame in relation to yours, the two frames are not linked in respect to velocity. You can't measure velocity in one frame from another frame when the thing you want to know the velocity of is not interacting with anything in your frame, it's not going from or to anything in your frame. This is just my theory, maybe it can be proven wrong, I don't know at this point, I'm just expressing how things seem to me.

The point could be made that a moving train is linked to the ground, so if something moves inside the train it is also moving in the ground frame because the train is physically connected to the ground. What about in space though? A space station and a passing rocket have no physical linkage whatsoever so the point could be made that an object moving inside the rocket has no actual velocity in the space station's frame. A meaningless hypothetical perhaps but maybe relevant in regard to the speed of light as perceived by people in the two frames.

If an object is moving at some velocity relative to some frame of reference, and the frame of reference is moving at some velocity relative to the observer, then the object is also moving at some velocity relative to the observer. The relativistic velocity-addition formula provides the velocity of the object relative to the observer given the other two velocities.

A frame of reference is a notion that is used as a basis for comparison, objects are generally described in terms of a frame of reference. In principle, frames of reference are not physical notions, although physical notions are often chosen. Relativity makes use of the concept of the set of all possible frames of reference, which is implicitly defined, and includes very few physical frames of reference. Relativity provides a way to transform a description in terms of one frame of reference to a description in terms of some other frame of reference. Although some frames of reference may be preferred for practical reasons, in principle there are no preferred frames of reference. This means that one can use any frame of reference to describe an object, and this description will be equally as valid as the description in any other frame of reference, including one that is physically based.