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Am a bit confused about the speed of light being constant, does this mean that whatever speed I am going at, up to and including the speed of light, I will always measure it as going 300000000 mts/sec faster than myself?
Welcome to PF!Am a bit confused about the speed of light being constant, does this mean that whatever speed I am going at, up to and including the speed of light, I will always measure it as going 300000000 mts/sec faster than myself?
Are you here talking about the car as an accelerated frame of reference?Now, if you do what you seem to suggest - take your physical system out of your lab and in your car, and measure the one-way speed of entering light rays while you are driving - then you may not find the same value.
Not necessarily so: "clock synchronization" is for each velocity different, so that it isn't a standard reference system anymore (it is not auto-correcting). Therefore I stressed the importance of synchronization at that velocity.[..] if the car is moving with a constant velocity wrt the lab system, then, certainly, the speed of light will be measured to c inside the car.
Do you mean that if we put two synchronized clocks at different places in a car at rest, and then accelerate the car up to a constant velocity, then, after the acceleration, the clocks are no longer synchronized, wrt an observer inside the car?Not necessarily so: "clock synchronization" is for each velocity different, so that it isn't a standard reference system anymore (it is not auto-correcting). Therefore I stressed the importance of synchronization at that velocity.
Do you mean that if we put two synchronized clocks at different places in a car at rest, say one clock in the front seat and one in the back seat, and then accelerate the car up to a constant velocity, then, after the acceleration, the clocks are no longer synchronized, wrt an observer inside the car?Not necessarily so: "clock synchronization" is for each velocity different, so that it isn't a standard reference system anymore (it is not auto-correcting). Therefore I stressed the importance of synchronization at that velocity.
Exactly.Do you mean that if we put two synchronized clocks at different places in a car at rest, say one clock in the front seat and one in the back seat, and then accelerate the car up to a constant velocity, then, after the acceleration, the clocks are no longer synchronized, wrt an observer inside the car?
Yes, just like sound: the speed of the emitter does not affect the speed of propagation.So does this mean if I am emitting the light it will always leave me at c whatever speed I am doing?
I think that the question should be phrased the other way round (often a misunderstanding already exists in the question). Someone who is not moving will still measure that light at c, independent of the motion of the source; that is the second postulate, based on a well established theory of electromagnetism and radiation.If I am moving at say 1/2 c and measure the light to be leaving me at c how can someone who is not moving still measure that light at c?
Probably you mean "time dilation". However, for one-way light speed, the first (and main!) change is a man-made change, as explained in the first section of the paper to which I gave you a link. He explains clock synchronisation. Did you understand it?You mention clocks and I think I read somewhere that einstein said something about time changing how does that work?
Yes, exactly.I am having problems with the fact that the speed of light is not dependant on the speed of the source, whereas normally the speed of something is dependant on its source i.e. if I fire a gun the speed of the bullet depends on the speed and direction of the gun although the speed is always the same relative to the gun. Someone standing still will measure the bullets speed as being different to that of the moving person holding the gun. With light I get the impression that both would measure it as moving at c, is this the case?
The speed of sound waves in the air, or water waves on the ocean don't have anything to do with the source. Generally, waves travel at a speed in the medium that depends on the characteristics of that medium, not on anything having to do with the source of the waves.I am having problems with the fact that the speed of light is not dependant on the speed of the source, whereas normally the speed of something is dependant on its source
At first sight, that's a nice introduction of the basics.Yes, exactly.
You might find the following discussion of special relativity easier to follow: Special Relativity. That's the first lecture in a series that covers all the usual relativistic effects, such as time dilation, length contraction, and the relativity of simultaneity.
One-way speed is the focus of the OP and I usually try to answer the question of the OP. But I fully agree that for a complete beginner it may be better to explain two-way speed of light measurements first (Fowler's lecture mixes them up however).Sorry, harrylin, but I believe you are making things even more confusing for a beginner by focusing (as you often do) on one-way speed of light issues.
Einstein focused on the one-way speed of light issue in his 1905 paper introducing Special Relativity. His second postulate focuses on the one-way speed of light.Sorry, harrylin, but I believe you are making things even more confusing for a beginner by focusing (as you often do) on one-way speed of light issues.
He's asking about measuring the one-way speed of light. We have to assume that he's asking about the speed of light in the direction that he is moving, not in the direction from where he is coming from, otherwise, he would have wondered if the light would be going slower than himself.Am a bit confused about the speed of light being constant, does this mean that whatever speed I am going at, up to and including the speed of light, I will always measure it as going 300000000 mts/sec faster than myself?
wait so the speed of the light going to the mirror is not the same as the speed of the light being reflected from the mirror? Only that the total distance over the total time is the same? That kinda makes sense since some energy is lost due to reflecting but I always thought that the amplitude was the lost energy not the velocityEinstein focused on the one-way speed of light issue in his 1905 paper introducing Special Relativity. His second postulate focuses on the one-way speed of light.
Beginners need to understand that when we are talking about measuring the speed of light being equal to c, we are always talking measuring the round-trip speed of light. When we are talking about the one-way speed of light, we are not talking about a measurement but rather an arbitrary assignment, an arbitrary definition, an arbitrary stipulation, an arbitrary assumption, an arbitrary postulate, an arbitrary axiom, according to Einstein.
Look at the OP's question:
He's asking about measuring the one-way speed of light. We have to assume that he's asking about the speed of light in the direction that he is moving, not in the direction from where he is coming from, otherwise, he would have wondered if the light would be going slower than himself.
If we point out to him that if he measures the round-trip speed of light by putting a mirror in front of him (like Einstein discussed in his 1905 paper), he will get the same answer that he will get if he does the same measurement in the opposite direction by putting the mirror behind him. This usually surprises beginners until they realize that it's the same measurement, the only difference being the two directions of light travel happen in the opposite order.
Then they have to realize that it is impossible to know if it takes the same time for the light to traverse the distance to the mirror as it takes for the reflection to get back to the observer and this is where Einstein's arbitrary assignment of those two times being equal comes in. This is where we get the unmeasurable one-way speed of light being equal to the same value as the measured two-way speed of light--it's by assignment.
This is the foundational basis of Einstein's argument for Special Relativity, both in his 1905 paper and in his 1920 book. I don't understand why we should hide this from beginners. It's how the theory began. I support harrylin's focus and if he hadn't been here prior to now, I would have been.
Nobody, certainly not Einstein, said the speeds were different. The point is that we can't know. We can't measure the difference. We can't tell if they are the same or if they are different. We don't know.[/QUOTE]wait so the speed of the light going to the mirror is not the same as the speed of the light being reflected from the mirror?
With regard to what we can measure, yes, it's only the total distance divided by the total time that always comes out the same.Only that the total distance over the total time is the same?
No, this has nothing to do with energy or amplitude, only velocity. Have you read Einstein's 1905 paper, especially the first couple of sections?That kinda makes sense since some energy is lost due to reflecting but I always thought that the amplitude was the lost energy not the velocity
Only the introduction and part 1, the explanation of how to set distant clocks are essential here. And yes, "inertial" is a term that people nowadays use for "Newtonian" or "Galilean" reference systems, which are systems in uniform rectilinear motion ("frames of reference for which the equations of mechanics hold good").Have tried to read the link but it seems long winded and difficult to follow. I assume that inertial means moving at a constant speed.
Not always "normally": light is not material, in certain ways it's more like sound or water waves than like bullets. The successful theory on which SR is based (Maxwell's electrodynamics), models light as a kind of wave in space. An essential feature of waves is that their speed c is independent of the motion of the source (to the precision that this has been verified).I am having problems with the fact that the speed of light is not dependant on the speed of the source, whereas normally the speed of something is dependant on its source i.e. if I fire a gun the speed of the bullet depends on the speed and direction of the gun although the speed is always the same relative to the gun.
As answered before: Yes, if you first synchronised your clocks at that velocity.[..] With light I get the impression that both would measure it as moving at c, is this the case?
Perception of speed relative to what?As you keep talking about synchronising clocks does time change somehow relative to the person doing the measurement so changing their perception of speed.
So does this mean if I am emitting the light it will always leave me at c whatever speed I am doing? If I am moving at say 1/2 c and measure the light to be leaving me at c how can someone who is not moving still measure that light at c?
i have trouble with "just like sound".Yes, just like sound: the speed of the emitter does not affect the speed of propagation.
First of all, the second postulate distinguishes SR from ballistic emission theories which just couldn't be made to work correctly.i have trouble with "just like sound".
it is the case that with both sound and light that the speed of the emitter does not affect the speed of propagation. [..] what must be kept in mind is that the speed of propagation of sound is relative to the medium that sound propagates in. [..]
With regard to what we can measure, yes, it's only the total distance divided by the total time that always comes out the same.Nobody, certainly not Einstein, said the speeds were different. The point is that we can't know. We can't measure the difference. We can't tell if they are the same or if they are different. We don't know.