Speed of light for different observers

[PAllen]

Before this thread veers completely into new directions, I thought I would put some numbers and clarifications of the OP experiment. Refer to the picture in the OP.

We have a truck moving fast. At the moment its front passes a light, it flashes. The back of the truck receives the signal at some time and place. A ground observer, B, is standing where the back of the truck passes as it receives the signal. The question was how do they both measure the speed of light to be the same?

We need to add a lot of specificity. Assume some time in the past, we collected identical tape measures and clocks. Now we describe how B and A (a truck traveling scientist) set up to do their measurements.

B) B places a clock where the signal will be received as the back of the truck passes; rolls out tape measure to where the emitter is (carrying a clock) and places clock next to emitter. Assistant stands at this clock to record when the emitter flashes. Scientist walks back to reception point, to await the truck's arrival (where he will record the reception event on his clock).

A) Scientist enters truck with tape measure and two clocks, and waits in back of truck as it gets up to speed. Then leaves one clock at back of truck, carries other to front as he measures the truck. An assistant waits at the front to record emission time according this clock at front of truck. Scientist walks to back of truck to record reception time.

Ok, now for some numbers. All units are light seconds, speed of light is 1 (light second / second), and we imagine we have all the time in the world to walk back and forth many light seconds.

A measures his truck length as 100, the speed of the ground going by as .6 (c). For him, the light took 100 seconds to reach the back and lightspeed is 1.

Now it gets interesting. If everything has been properly set up so that events happen as described at the beginning, then B will measure the distance between emitter and reception point as 50. This is a combination of seeing the truck as length 80, and the fact that the truck will travel 30 between emission and reception. B will measure the time between emission and absorption as 50, getting c for the speed of light. The discrepancy between B's 50 seconds and A's 100 is a combination B seeing A's clock running only 80% as fast as his, but also seeing a large discrepancy between the front and back clocks on the truck. According to B, A's front clock is set 60 seconds ahead of the back clock. So the 100 seconds measured by A is 'really' only 40 of A's seconds. Then since A's clock is only running 80% the rate of B's, this 40 of A's seconds correspond to 50 of B's.

 

[sisoev]

Special Relativity (or any other theory) must take into account what actually happens, if by that you mean all measurements that any and all observers make.

But Special Relativity goes beyond what we can measure and observe and allows us to talk meaningfully about things which we cannot have knowledge of, specifically, where light is at any given moment in time.

That's what we do when we define a Frame of Reference which assigns coordinates to all of space and time. These coordinates are what we call "events", one value of time and three values of space (x, y, z). So events are always referenced from a stated FoR and only have meaning in that one FoR. We use the Lorentz Transform to see what any event in one FoR will be in a different FoR moving with respect to the first one.

The ladder paradox, as is true of virtually all SR paradoxes, is a result of combining the coordinates for events from two different FoR's. If you define part of a scenario in one FoR (oftentimes not explicitly stated) and another part in a different FoR, then you will get all kinds of "contradictions", just like you will if you use two different coordinate systems that are not relativistic. The resolution is to define everything according to one FoR and see what happens. Then you can use the Lorentz Transformation to see what happens according to a different FoR and there will never be any contradictions.

Let me present my point of view for Time.

It is an inflating balloon and all observers are placed on its outside surface.
Everything we observe is inside the balloon and it is past.
We observe and we act in the "present"(even acting is hard to be determined as present moment) but everything we observe is past.

Since all observers are placed on different places on the balloon they see the event differently, but the event is one and it is placed in one only point of time. It is the point where the balloon was when the event took place.

 

[sisoev]

Before this thread veers completely into new directions, I thought I would put some numbers and clarifications of the OP experiment. Refer to the picture in the OP.

We have a truck moving fast. At the moment its front passes a light, it flashes. The back of the truck receives the signal at some time and place. A ground observer, B, is standing where the back of the truck passes as it receives the signal. The question was how do they both measure the speed of light to be the same?

We need to add a lot of specificity. Assume some time in the past, we collected identical tape measures and clocks. Now we describe how B and A (a truck traveling scientist) set up to do their measurements.

B) B places a clock where the signal will be received as the back of the truck passes; rolls out tape measure to where the emitter is (carrying a clock) and places clock next to emitter. Assistant stands at this clock to record when the emitter flashes. Scientist walks back to reception point, to await the truck's arrival (where he will record the reception event on his clock).

A) Scientist enters truck with tape measure and two clocks, and waits in back of truck as it gets up to speed. Then leaves one clock at back of truck, carries other to front as he measures the truck. An assistant waits at the front to record emission time according this clock at front of truck. Scientist walks to back of truck to record reception time.

Ok, now for some numbers. All units are light seconds, speed of light is 1 (light second / second), and we imagine we have all the time in the world to walk back and forth many light seconds.

A measures his truck length as 100, the speed of the ground going by as .6 (c). For him, the light took 100 seconds to reach the back and lightspeed is 1.

Now it gets interesting. If everything has been properly set up so that events happen as described at the beginning, then B will measure the distance between emitter and reception point as 50. This is a combination of seeing the truck as length 80, and the fact that the truck will travel 30 between emission and reception. B will measure the time between emission and absorption as 50, getting c for the speed of light. The discrepancy between B's 50 seconds and A's 100 is a combination B seeing A's clock running only 80% as fast as his, but also seeing a large discrepancy between the front and back clocks on the truck. According to B, A's front clock is set 60 seconds ahead of the back clock. So the one 100 seconds measured by A is 'really' only 40 of A's seconds. Then since A's clock is only running 80% the rate of B's, this 40 of A's seconds correspond to 50 of B's.

He-he :D
Very thorough and entertaining explanation.
Thank You, PAllen.

Wouldn't it be easier to set a laboratory truck with emitter on it and two slits to split the emitted photon.
Then just examine the pattern left by the photon on the two detectors.
Same pattern, same speed.
Different pattern, different speed.
Would that be correct?

 

[Dale]

It will be shorter of longer for your frame of reference but in the inertial frame the path of the light will be the same (from the source to the detector) , no matter in which direction is moving the truck.

You are making some confusing statements here. Both the ground frame and the truck frame are inertial, so I don't know what you mean by "in the inertial frame". In the truck frame the light travels the distance equal to the length of the truck. In the ground frame the light travels a distance less than the length of the truck. That is true in both SR and Newtonian mechanics, the only difference being that in SR the length of the truck is shorter in the ground frame

I think we need animation here.

Do you know what a spacetime diagram is? I think that would be more clear than an animation.

Explain that please.
The experiment is the same as if it was done with a ball.
The ball would arrive to observer B with slower speed compared to observer A and would release less energy.

OK, the energy is completely different from interference. You are correct, the light detected by B will be less energetic than the light detected by A. This is called "Doppler shift". Relativity and Newtonian mechanics make different predictions for the amount of Doppler shift, with the relativistic predictions being slightly "redder" than the Newtonian predictions. The relativistic predictions have been experimentally validated to very high precision.

I don't understand your point.

In the ground frame B is stationary and A and the source are moving to the left. In the truck frame A and the source are stationary and B is moving to the right. Both of your drawings show A and the source moving to the left so both are drawn in the ground frame.

 

[Denius1704]

How has the time dilation been proven from the hundreds of experiments? Is it only with the differences that clocks seem to show?

 

[ghwellsjr]

Now that you mentioned Lorentz Ether Theory and the motionless ether popped in my mind, I'd like to ask about ether.
Today I read to friend of mine one of the many definitions for ether:
"An intervening substance through which something else is transmitted or carried on."
I asked him if he sees something missing in the definition.
Hi didn't.
When I said that the "space" word is missing, he said that it is obvious and shouldn't be mentioned. Of course it is space if something has to be transmitted or carried on through it.
No space, no transmission!
But if space is a main part of the medium why we were looking for a luminiferous medium?

Then I made my own definition for medium:
space with certain properties through which something else is transmitted or carried on

I think that we should not look at "space" as emptiness, filled with something, but rather as different spaces with different properties.
Then the motionless ether would start making sense to us and we will see the "empty" space in the universe is a medium through which light moves.

My question about medium:

If we accelerate space which contains water, we actually accelerate medium; why not to be able to accelerate space which contains few atoms of hydrogen.
If we can accelerate space filled with fiberglass through which travels light, why can't we accelerate space with few hydrogen atoms, through which (space) travels light?
And finally, why can't we accelerate space which contains vacuum through which travels light.

Now I'm sorry I mentioned LET. I thought you would reject it out of hand. I have no idea what you are talking about when you ask about accelerating space.

 

[ghwellsjr]

Let me present my point of view for Time.

It is an inflating balloon and all observers are placed on its outside surface.
Everything we observe is inside the balloon and it is past.
We observe and we act in the "present"(even acting is hard to be determined as present moment) but everything we observe is past.

Since all observers are placed on different places on the balloon they see the event differently, but the event is one and it is placed in one only point of time. It is the point where the balloon was when the event took place.

I thought you said you were not anti-relativist. These words sure sound like you are promoting an absolute concept of time.

Do you have any interest at all in learning Special Relativity?

 

[sisoev]

......
.......
In the ground frame the light travels a distance less than the length of the truck. That is true in both SR and Newtonian mechanics, the only difference being that in SR the length of the truck is shorter in the ground frame

I understand what are you saying, DaleSpam.
You are saying that I'll see purple door.

I'm trying to imagine how would that be seen with a single photon? (One successive wave)

OK, the energy is completely different from interference. You are correct, the light detected by B will be less energetic than the light detected by A. This is called "Doppler shift". Relativity and Newtonian mechanics make different predictions for the amount of Doppler shift, with the relativistic predictions being slightly "redder" than the Newtonian predictions. The relativistic predictions have been experimentally validated to very high precision.

I know what Doppler shift is, but the problem is that I cannot see it in relativistic way :D
When I think it your way, I see the light approaching me slower but passing by me with its real speed.
Kind of difficult to imagine :)
I know, you'll say that it does not approaches me slower but comes later because of the greater distance from the source.
Isn't that the same like ten cars on equal distance driving with 100 km/h and each of them is approaching me slower because I am accelerating from 1 to 99 km/h.
Wouldn't I measure different speed for every next car relative to my speed?
How is the speed of the car different in a vacuum than the speed of light?
It is a number no matter how great it is.

 

[sisoev]

Now I'm sorry I mentioned LET. I thought you would reject it out of hand. I have no idea what you are talking about when you ask about accelerating space.

Well, I cannot explain it in easier way than I already did.

 

[sisoev]

I thought you said you were not anti-relativist. These words sure sound like you are promoting an absolute concept of time.

Do you have any interest at all in learning Special Relativity?

I am non anti-relativist and I'm very much interested in learning Special Relativity.
I'm not any type of ANTI :)
I can love few styles of music, and if I don't like some music I am not ANTI ;)

What wrong did you see in my concept of time?
It explains time, relativity of simultaneity, time dilation...

 

[ghwellsjr]

I thought you said you were not anti-relativist. These words sure sound like you are promoting an absolute concept of time.

Do you have any interest at all in learning Special Relativity?

I am non anti-relativist and I'm very much interested in learning Special Relativity.
I'm not any type of ANTI :)
I can love few styles of music, and if I don't like some music I am not ANTI ;)

What wrong did you see in my concept of time?
It explains time, relativity of simultaneity, time dilation...

Well, let's take a look at your concept of time:

Let me present my point of view for Time.

It is an inflating balloon and all observers are placed on its outside surface.
Everything we observe is inside the balloon and it is past.
We observe and we act in the "present"(even acting is hard to be determined as present moment) but everything we observe is past.

Since all observers are placed on different places on the balloon they see the event differently, but the event is one and it is placed in one only point of time. It is the point where the balloon was when the event took place.

The surface of a balloon is two-dimensional but we live in a three-dimensional spatial world. How do you reconcile this?

What does the center of your balloon correspond to--the beginning of time--the big bang?

How does it explain time dilation, especially reciprocal time dilation? The truck driver in the speeding truck sees the clock in the stationary truck as running slower than his own and the truck driver in the parked truck sees the clock in the speeding truck as going slower than his own. How does your balloon explain that?

 

[Dale]

I understand what are you saying, DaleSpam.
You are saying that I'll see purple door.

That's not even close to what I said. Please do not put words in my mouth.

Isn't that the same like ten cars on equal distance driving with 100 km/h and each of them is approaching me slower because I am accelerating from 1 to 99 km/h.
Wouldn't I measure different speed for every next car relative to my speed?

Yes.

How is the speed of the car different in a vacuum than the speed of light?

The speed of the car is frame variant and the speed of light is frame invariant. That is how they are different.

There are only two possibilities which are compatible with the principle of relativity, one is the Galilean transform where the invariant speed is infinite and the other is the Lorentz transform where the invariant speed is finite. In both cases there is only one invariant speed so it is qualitatively different from other speeds.

 

[harrylin]

[...] So, relativity does not take in account what actually happened.
For relativity the observation equals the actual event, and this is what's bugging me around.
[...]

SR only makes claims about what will be really observed; on purpose it does not make claims about invisible (unmeasurable) reality. Because of that Lorentz and Einstein promoted the theory together, despite the fact that they had - at least the start - quite different interpretations of it.

[...] I think that we should not look at "space" as emptiness, filled with something, but rather as different spaces with different properties.
Then the motionless ether would start making sense to us and we will see the "empty" space in the universe is a medium through which light moves.

My question about medium:

If we accelerate space which contains water, we actually accelerate medium; why not to be able to accelerate space which contains few atoms of hydrogen.
If we can accelerate space filled with fiberglass through which travels light, why can't we accelerate space with few hydrogen atoms, through which (space) travels light?
And finally, why can't we accelerate space which contains vacuum through which travels light.

Concerning the ether concept, SR is only compatible with the concept of a motionless ether: a physical space that cannot be put in motion (a kind of Lorentz ether, as Einstein later admitted). And the same is the case for QM, for even matter has wave properties. Waves are propagating vibrations, they do not themselves propagate the medium. But of course, according to SR we can never measure our velocity with respect to such a medium: that's the basic issue behind your question.

I hope this helped.

Harald

 

[A.T.]

Now try to show the inconsistency in Newton's theory by explaining the above experiment with Newton's theory and show where it has flaws. (Where the theory contradicts itself in the proposed experiment?)

Classical mechanics (Galilean transformation) doesn't contradict itself, but it contradicts experimental results. Lorentz transformation agrees with experimental results.

 

[harrylin]

[..]
Isn't that the same like ten cars on equal distance driving with 100 km/h and each of them is approaching me slower because I am accelerating from 1 to 99 km/h.
Wouldn't I measure different speed for every next car relative to my speed?
How is the speed of the car different in a vacuum than the speed of light?
It is a number no matter how great it is.

You would similarly measure light approaching you slower if you keep the same reference system, so that you would measure yourself to be accelerating. That is just how the GPS system functions.

The main difference between the speed of a car and that of light is that light propagates at the limit speed; it's a special (extreme) case. At low relative speeds and small to medium distances the effects are negligibly small, so that one can use classical physics instead. What texts did you read about relativity?

 

[ghwellsjr]

You would similarly measure light approaching you...

How, exactly, do you propose measuring the speed of light approachining you--without using a previously defined speed for that light?

 

[sisoev]

Well, let's take a look at your concept of time:

The surface of a balloon is two-dimensional but we live in a three-dimensional spatial world. How do you reconcile this?

What does the center of your balloon correspond to--the beginning of time--the big bang?

How does it explain time dilation, especially reciprocal time dilation? The truck driver in the speeding truck sees the clock in the stationary truck as running slower than his own and the truck driver in the parked truck sees the clock in the speeding truck as going slower than his own. How does your balloon explain that?

My “Time balloon” can be related to the events and to the space.
(and yes, the center of the balloon is the Big Bang)
I already explained how the events are viewed with the balloon.
If you want to look at the surrounding space using the time balloon, the three-dimensional world wouldn’t be a problem.
Since we are always on the end of the Universe in a time sense, our three-dimensional drawing will look like a mirror image of what we observe, placed inside the balloon.
The nearest to us object will be nearest to us in the balloon.

Why putting the surrounding world in the balloon?
Since the light needs time to reflect from the objects in our eyes, and then time for the brain to draw the image, we can say that this light is from a millisecond, from a second, from a minute ago.
So what we see is past.

One would say that if we don’t observe events, we could not refer to our observation as to past.
That would be wrong.
The fact that we don’t see events doesn’t mean that they did not happen a millisecond after our observation. We will probably see it in the next second.
Further more, the world is not motionless and we cannot say that there is no motion (events) in our observation.

OK, but what if we start traveling toward the object for which we say that is in the past?
It will still be in the past if in the moment of our arrival it exist in the state we observed it, but that never happens :)
We will arrive to a different object. It will not be the object which we observed few hours ago. It changed.

But aren’t we walking toward the past.
No.
We are walking in space.
If we want to connect time and space, we should imagine ourselves as still staying on the outside of the time balloon which expands and in the same time brings to us past-observed space, which we placed as three-dimensional drawing in the balloon.

I’m afraid that my “enthusiasm” in explaining my views may be taken in a wrong way, so I’ll stop here :)
Yes Time balloon explains time dilation, simultaneity and more, but I’ll need drawings and I don’t think that this topic is the place to discuss it.

 

[Dale]

Sounds like standard FLRW cosmology for positive curvature. None of that can be used to avoid relativity, in particular time dilation.

 

[harrylin]

How, exactly, do you propose measuring the speed of light approachining you--without using a previously defined speed for that light?

We use a standard inertial reference system; in this case you are accelerating wrt your reference system. That's also how GPS calculates (although the ECI frame is only approximately inertial). The one-way speed is previously defined as isotropic wrt that system. Consequently it is c-v relative to you, as you are moving at v relative to your coordinate system - in modern jargon, the closing speed is c-v.

 

[harrylin]

[..]
I’m afraid that my “enthusiasm” in explaining my views may be taken in a wrong way, so I’ll stop here :)
Yes Time balloon explains time dilation, simultaneity and more, but I’ll need drawings and I don’t think that this topic is the place to discuss it.

Indeed this isn't the place for personal theories :

"It is against our Posting Guidelines to discuss, in the PF forums or in blogs, new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional mainstream scientific discussion. Non-mainstream or personal theories will be deleted."
- https://www.physicsforums.com/showthread.php?t=414380

 

[sisoev]

Indeed this isn't the place for personal theories :

"It is against our Posting Guidelines to discuss, in the PF forums or in blogs, new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional mainstream scientific discussion. Non-mainstream or personal theories will be deleted."
- https://www.physicsforums.com/showthread.php?t=414380

I know, I know
It would be helpful though if you have a section where to discuss personal views in physics.
You'll be surprised how faster people learn in discussion, especially with opponents like you guys.
And who knows, you may meet a new Einstein one day

 

[sisoev]

I don't want to annoy you anymore with my presence
I have two more questions and I'll be gone.

1.)
I asked in a post:

Wouldn't it be easier to set a laboratory truck with emitter on its front and two slits to split the emitted photon.
.......

It will travel through the truck and hit detector A
It will travel from the point of emission and hit detector B

If it traveled same distance for both detectors, on both of them we should observe same pattern.
If not, the patterns would differ.
Same would be true if it traveled same distance with different speed.

What is your prediction for the patterns?

2.)
If the https://www.physicsforums.com/showthread.php?p=3500067#post3500067" proves that simultaneity happens but is wrong observed, how can we say that A and B will not be hit simultaneously by the light?
Is it because the Purple Door is not approved by authorities or there is another explanation?

 

[Dale]

1.)
I asked in a post:

It will travel through the truck and hit detector A
It will travel from the point of emission and hit detector B

If it traveled same distance for both detectors, on both of them we should observe same pattern.
If not, the patterns would differ.
Same would be true if it traveled same distance with different speed.

What is your prediction for the patterns?

You need to pay attention to previous discussions and use proper terminology. By "pattern" do you mean "interference pattern" (there is none), or "energy" (Doppler shift)?

2.)
If the https://www.physicsforums.com/showthread.php?p=3500067#post3500067" proves that simultaneity happens but is wrong observed, how can we say that A and B will not be hit simultaneously by the light?
Is it because the Purple Door is not approved by authorities or there is another explanation?

I have no clue what you think is proven by the purple door thought experiment. The color of the doors is completely irrelevant to the ladder paradox.

 

[sisoev]

You need to pay attention to previous discussions and use proper terminology. By "pattern" do you mean "interference pattern" (there is none), or "energy" (Doppler shift)?

Sorry for the wrong terminology, but you could easily answer both.
Why wouldn't there be interference pattern?

I have no clue what you think is proven by the purple door thought experiment. The color of the doors is completely irrelevant to the ladder paradox.

Are you serious?
Doesn't it show that we cannot talk about simultaneity between red and blue, if we observe red and purple?

 

[Dale]

Sorry for the wrong terminology, but you could easily answer both.
Why wouldn't there be interference pattern?

You need to have two paths with a phase difference arriving at the same detector in order to get interference. There is only one path for each detector, so no interference.

Doesn't it show that we cannot talk about simultaneity between red and blue, if we observe red and purple?

No, why would the color have anything to do with talking about simultaneity? The doors can be opened simultaneously regardless of their colors. I think that this whole line of thought is based on the misconception that relativity is about appearances.

 

[sisoev]

You need to have two paths with a phase difference arriving at the same detector in order to get interference. There is only one path for each detector, so no interference.

OK, I know that, but there are two slits for each detector.
If we repeat numerous times the experiment will we see the same interference on both detectors?

About the red light shift:
I don't understand very well the frequency of a photon and I'd appreciate some help here.
If the photon travels same distance for A and B in my experiment and it does it for the same time, how that would change its frequency for detector B (you already agreed in a previous post that the photon will hit detector B with less energy)
Would you explain, please?

No, why would the color have anything to do with talking about simultaneity? The doors can be opened simultaneously regardless of their colors. I think that this whole line of thought is based on the misconception that relativity is about appearances.

I think you did not understand the "purple door" experiment.
If we set an experiment to show simultaneity we set that simultaneity between the opening of a blue and red doors.
If you say that you saw the purple door to open later that would only prove that your observation of the event was wrong because there is no purple door in the experiment setting.
If we don't take in consideration the properties of the objects in one experiment we cannot get the right results from that experiment.

Note that in the first question from above I agreed with you that the photon traveled same distance for A and B. I agreed with the "purple door" for the sake of the conversation, but the experiment is set with the full length of the truck and just because we see it differently doesn't mean that the light didn't travel the full truck length

 

[Dale]

OK, I know that, but there are two slits for each detector.
If we repeat numerous times the experiment will we see the same interference on both detectors?

OK, if there is a two-slit apparatus between the source and the detectors then that gives you the two paths required for interference. The fringes will be different because the frequency is different for each detector due to Doppler shift.

About the red light shift:
I don't understand very well the frequency of a photon and I'd appreciate some help here.
If the photon travels same distance for A and B in my experiment and it does it for the same time, how that would change its frequency for detector B (you already agreed in a previous post that the photon will hit detector B with less energy)
Would you explain, please?

Do you understand the Doppler shift? If so, that is all this frequency shift is. The distance traveled doesn't matter, only the relative speed between the source and the detector.

http://en.wikipedia.org/wiki/Doppler_effect
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/reldop2.html
http://www.fourmilab.ch/cship/doppler.html

I think you did not understand the "purple door" experiment.
If we set an experiment to show simultaneity we set that simultaneity between the opening of a blue and red doors.
If you say that you saw the purple door to open later that would only prove that your observation of the event was wrong because there is no purple door in the experiment setting.

So what? We know that the speed of light is finite. We account for that in determining simultaneity.

Note that in the first question from above I agreed with you that the photon traveled same distance for A and B. I agreed with the "purple door" for the sake of the conversation, but the experiment is set with the full length of the truck and just because we see it differently doesn't mean that the light didn't travel the full truck length

You are simply factually wrong on this point. Not only are you wrong according to SR, but you are also wrong according to Newtonian mechanics.

Let a light pulse be emitted from the source at x=0, t=0. Then the equation of motion for the light pulse is $x=ct$. If the truck is of length L then the equation of motion for the detector is $x=L-vt$. That is two equations in two unknowns so we can easily solve to obtain $x=L/(1+v/c)$ and $t=L/(c+v)$. Clearly x is smaller than L for any v>0.

Please stop repeating this error.

 

[sisoev]

OK, if there is a two-slit apparatus between the source and the detectors then that gives you the two paths required for interference. The fringes will be different because the frequency is different for each detector due to Doppler shift.

Thank You.

Do you understand the Doppler shift? If so, that is all this frequency shift is. The distance traveled doesn't matter, only the relative speed between the source and the detector.

http://en.wikipedia.org/wiki/Doppler_effect
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/reldop2.html
http://www.fourmilab.ch/cship/doppler.html

I understand very well the Doppler shift.
It is because every next successive wave is approaching us later because of the increased distance (red shift)
I cannot connect it with the photon frequency though, since the photon is one wave.
The moment it hits the detector it stop existing for it as wave = one successive wave.
That's why I said that I don't understand very well the photon frequency, when compared to light frequency, especially in connection with Doppler shift.

So what? We know that the speed of light is finite. We account for that in determining simultaneity.

And does that turns the door from purple to blue?

You are simply factually wrong on this point. Not only are you wrong according to SR, but you are also wrong according to Newtonian mechanics.

Let a light pulse be emitted from the source at x=0, t=0. Then the equation of motion for the light pulse is $x=ct$. If the truck is of length L then the equation of motion for the detector is $x=L-vt$. That is two equations in two unknowns so we can easily solve to obtain $x=L/(1+v/c)$ and $t=L/(c+v)$. Clearly x is smaller than L for any v>0.

Please stop repeating this error.

I'm trying very hard, DaleSpam (to stop repeating the errors)
My thousands apologies for me being that annoying.

I don't understand why we have to make calculations if all parameters are given in the experiment.
It is irrelevant what we observe, from which coordinate system and from which point in it we observe it.
The light will travel the full length of the truck.

By the way, why no one attempted to use length contraction in explaining this experiment?

 

[PAllen]

By the way, why no one attempted to use length contraction in explaining this experiment?

What??! See my long post on a numeric example of the OP. It used length contraction as part of the comparison of different frame's measurements.

 

[sisoev]

What??! See my long post on a numeric example of the OP. It used length contraction as part of the comparison of different frame's measurements.

Sorry, Pallen.
I missed it.
I'll read it tomorrow.
It is already 00:14 here.
Time for bed.

Thank You guys.
Good day and good night :)

 

[Dale]

the photon is one wave.

Where did you get that idea? It is not generally correct.

I don't understand why we have to make calculations if all parameters are given in the experiment.

It is often possible to make qualitative predictions without making any calculations, but quantitative predictions always require calculations. The quantitative predictions can also be used to confirm or correct the qualitative predictions.

The light will travel the full length of the truck.

No it won't, as I have both explained and derived. Again, please stop repeating this same error.

By the way, why no one attempted to use length contraction in explaining this experiment?

I mentioned it earlier. But length contraction is a second order effect, and you are not even correctly analyzing the first order (Newtonian) effects. Once you learn Newton then we can start working on Einstein.

 

[harrylin]

[..] I don't understand why we have to make calculations if all parameters are given in the experiment.
It is irrelevant what we observe, from which coordinate system and from which point in it we observe it. [..]

In my experience - and probably most others who studied physics - without also doing calculations (and/or graphical constructions), I could not get a full and thorough understanding of such complex matters. Consequently, I'm afraid that you cannot either.

 

[sisoev]

Where did you get that idea? It is not generally correct.

The above is answer to my understanding that the photon represents one wave.
Perhaps my terminology is wrong again
As I understand it, according to complementarity principle a photon can behave as either particle or wave, but not both at the same time.
Why would we say that the photon is not one successive wave, since it "dies" as wave the moment it exhibits its particle behavior by hitting the detector.

It is often possible to make qualitative predictions without making any calculations, but quantitative predictions always require calculations. The quantitative predictions can also be used to confirm or correct the qualitative predictions.

Fully agree with you.
In our case we don't have to predict the path of the light for observer A. It is given; from one side of the truck to the other.
I account the fact that for a third observer the light travels from the place of emission to the detector, but in the same time I account the other fact, that for observer A the point of emission moved with the truck.
For A we are moving and for us A is moving.
In this case, should we not compare both light paths as in stationary frames?

No it won't, as I have both explained and derived. Again, please stop repeating this same error.

OK, it is equally annoying for me as it is for you, to go in a loop.
I consider the possibility that I'm a little slow in understanding, but do you think that your explanation is good enough for my question.
I understand where the problem is, and I'll try later to put the question in a better way.

I mentioned it earlier. But length contraction is a second order effect, and you are not even correctly analyzing the first order (Newtonian) effects. Once you learn Newton then we can start working on Einstein.

Kind of agree on this

 

[sisoev]

In my experience - and probably most others who studied physics - without also doing calculations (and/or graphical constructions), I could not get a full and thorough understanding of such complex matters. Consequently, I'm afraid that you cannot either.

Ha-ha
Obviously I have a problem, otherwise I wouldn't be here, asking "stupid" questions ;)

 

[Dale]

The above is answer to my understanding that the photon represents one wave.
Perhaps my terminology is wrong again
As I understand it, according to complementarity principle a photon can behave as either particle or wave, but not both at the same time.
Why would we say that the photon is not one successive wave, since it "dies" as wave the moment it exhibits its particle behavior by hitting the detector.

If you have a detector which detects the frequency of the photon then you will measure a Doppler shift. Personally, I think you need to focus on classical physics and not even worry about quantum effects for now. Once your grasp on classical physics is firm then learning QM is feasible.

In our case we don't have to predict the path of the light for observer A. It is given; from one side of the truck to the other.

It is given that it goes from the source at the time of emission to the detector at the time of detection. That distance is not equal to the length of the truck in any frame where the truck is moving, as explained and derived.

do you think that your explanation is good enough for my question.

Yes, I explained it multiple times, pointed out the error in your reasoning, and derived the correct result. That seems to be a complete explanation.

Before you pursue this line of discussion any further, spend a little effort of your own. Sit down and take a close look at each step of my brief derivation. Either the derivation is right or it is wrong. If it is right then you have no choice but to acknowledge that the distance traveled by the light is not equal to the length of the truck in any frame where the truck is moving. If it is wrong, then point out the specific error, and post the correct derivation which supports your claim quantitatively.