Speed of light for different observers

[sisoev]

He everybody :)

I guess I am not the only one who cannot comprehend the idea that the light has same speed for all observers, regardless of the velocity between the source and the observer.
In an attempt to get a picture of the idea, I constructed a thought experiment which I think could be performed.

[PLAIN]http://onegative.org/light1.jpg

(the image above) Let's imagine that a truck traveling with high speed has attached a light source in its front and a detector "A" on its back.
As the truck is moving with constant speed the light source emits a light pulse or a single photon if you prefer so.
Knowing the speed of the truck we can calculate after what traveled distance the light will reach detector "A" and on the same line we place detector "B".
Now both detectors will detect the light at the same time (simultaneously)

[PLAIN]http://onegative.org/light2.gif

The second image shows the position of the truck when the light hits both detectors.
We can see the difference in the distance traveled by the light to the two detectors for the same amount of time.

Initially I built this experiment with the truck moving in the right direction, but I found out that it is kind of deceiving to figure out the distance traveled for the light for both detectors.
I can post the other images if you think that it would be easier to explain the problem.

Respectfully looking forward for your explanation and help.

P.S. I'd like to ask Janus to make an animation for the above, which will make it easier for us to imagine the full path of the light as traved to both detectors.

 

[ghwellsjr]

It is not possible to determine or measure the one-way speed of a light pulse or a photon. We can only measure the round-trip time it takes for a light pulse to start from a source, traverse to a mirror, reflect off the mirror and traverse back to the source. This is experimental evidence for the universal constant value of the speed of light.

In Einstein's Special Relativity, a Frame of Reference is defined in which the two halves of the trajectory of the aforementioned experiment are assigned equal times. This is Einstein's second postulate. Read his 1905 paper.

 

[PeterDonis]

Your diagrams are misleading because they mix together distances defined using different rules. In particular, in your second diagram, the "distance covered by light for detector A" is defined using different rules than the "distance covered by light for detector B". The A distance assumes that the truck is motionless; the B distance assumes that the truck is moving. These are inconsistent assumptions, so the two distances are not consistently defined and you can't conclude anything by comparing them.

More precisely, you are assuming that, even though the distances in your second diagram are different, the *times* for A and B are the same. But since the distance definitions are not consistent, you can't assume that the time definitions are either. That is called the "relativity of simultaneity", and once it is taken into account, it becomes clear that, if you define the A distance as you have in your second diagram, so that it is longer than the B distance, then if you define A's time consistently with that definition of distance, you will find that A's time is longer as well, by just enough to make the speed of light, distance divided by time, the same for both.

To properly analyze this situation, you need to draw a spacetime diagram. I don't have the tools handy to do that right now, but doing so makes what I've said above clearer and easier to see since it is visually obvious from the diagram.

 

[Dale]

The second image shows the position of the truck when the light hits both detectors.
We can see the difference in the distance traveled by the light to the two detectors for the same amount of time.

Note that the second image incorrectly shows the distance covered by the light for detector A. The light travels a distance from the location of the source at the time of emission to the location of the detector at the time of detection. It does not travel from the location of the source at the time of detection to the location of the detector at the time of detection, which is what you have drawn. The distance traveled is the same for A and B.

That said, other than that small error I am not sure what your point is. You don't seem to be proposing any measurement that could even in principle distinguish between relativity and Newtonian mechanics, nor any other theory.

 

[sisoev]

It is not possible to determine or measure the one-way speed of a light pulse or a photon. We can only measure the round-trip time it takes for a light pulse to start from a source, traverse to a mirror, reflect off the mirror and traverse back to the source. This is experimental evidence for the universal constant value of the speed of light.

In Einstein's Special Relativity, a Frame of Reference is defined in which the two halves of the trajectory of the aforementioned experiment are assigned equal times. This is Einstein's second postulate. Read his 1905 paper.

I am well aware of that, ghwellsjr, but the experiment does not measure speed of light.
It measures difference in the speed.

 

[sisoev]

Your diagrams are misleading because they mix together distances defined using different rules. In particular, in your second diagram, the "distance covered by light for detector A" is defined using different rules than the "distance covered by light for detector B". The A distance assumes that the truck is motionless; the B distance assumes that the truck is moving. These are inconsistent assumptions, so the two distances are not consistently defined and you can't conclude anything by comparing them.

More precisely, you are assuming that, even though the distances in your second diagram are different, the *times* for A and B are the same. But since the distance definitions are not consistent, you can't assume that the time definitions are either. That is called the "relativity of simultaneity", and once it is taken into account, it becomes clear that, if you define the A distance as you have in your second diagram, so that it is longer than the B distance, then if you define A's time consistently with that definition of distance, you will find that A's time is longer as well, by just enough to make the speed of light, distance divided by time, the same for both.

To properly analyze this situation, you need to draw a spacetime diagram. I don't have the tools handy to do that right now, but doing so makes what I've said above clearer and easier to see since it is visually obvious from the diagram.

It is not correct to use in your explanation inconsistency with theory of relativity.
Time and simultaneity will differ only if the speed of light is the same for all observers.
This experiment puts that in doubt an it requires explanation which does not involve "relativity laws".
Otherwise there is no way to prove inconsistency in the theory in question.

 

[sisoev]

Note that the second image incorrectly shows the distance covered by the light for detector A. The light travels a distance from the location of the source at the time of emission to the location of the detector at the time of detection. It does not travel from the location of the source at the time of detection to the location of the detector at the time of detection, which is what you have drawn. The distance traveled is the same for A and B.

That said, other than that small error I am not sure what your point is. You don't seem to be proposing any measurement that could even in principle distinguish between relativity and Newtonian mechanics, nor any other theory.

The light source and detector A are in rest relative to each other.
The light has to travel the full length of the truck in order to reach detector A.

I agree that the place of emission for detector A does not match the position of the light source, but that is only when we compare them from outside of their own frame of reference.
We should not do that mistake in our comparison.
Remember that we are comparing the speed of light for two observers, which can be done only if we compare the distance traveled in their frames with the time for that travel.

We can easily predict that the light interference seen on both detectors will be different.

 

[ghwellsjr]

I am well aware of that, ghwellsjr, but the experiment does not measure speed of light.
It measures difference in the speed.

You started off your thread by stating that you "cannot comprehend the idea that the light has same speed for all observers, regardless of the velocity between the source and the observer."

Are you interested in comprehending this idea?

 

[sisoev]

You started off your thread by stating that you "cannot comprehend the idea that the light has same speed for all observers, regardless of the velocity between the source and the observer."

Are you interested in comprehending this idea?

Ha-ha :D
That's the point, ghwellsjr
If I agree with everything you say, I shouldn't have the problem of comprehending the idea.
Measuring speed and measuring difference in a speed are two different things to me.

We may still not know what the speed of the light is, but its difference for A and B can be determent by the above experiment.

 

[harrylin]

He everybody :)

I guess I am not the only one who cannot comprehend the idea that the light has same speed for all observers, regardless of the velocity between the source and the observer.
In an attempt to get a picture of the idea, I constructed a thought experiment which I think could be performed.
[..]

Hi sisoev,

Did you actually try to calculate (with simple numbers) what each will measure? Often such an exercise answers all questions.

 

[A.T.]

it requires explanation which does not involve "relativity laws".
Otherwise there is no way to prove inconsistency in the theory in question.

Nonsense. To show that a theory is inconsistent, you have to use only the laws of that theory, and show a contradiction between them.

 

[Dale]

The light source and detector A are in rest relative to each other.

Yes, clearly.

The light has to travel the full length of the truck in order to reach detector A.

No, this is not correct in any frame where the truck is moving. If the truck is moving in the opposite direction of the light then the distance will be shorter than the length of the truck and if the truck is moving in the same direction of the light then the distance will be longer than the length of the truck.

Note, this is not a distinction between Newtonian physics and relativity. Your drawing is incorrect regardless of which theory of physics you are using.

I agree that the place of emission for detector A does not match the position of the light source, but that is only when we compare them from outside of their own frame of reference.
We should not do that mistake in our comparison.
Remember that we are comparing the speed of light for two observers, which can be done only if we compare the distance traveled in their frames with the time for that travel.

Both drawings are done from the ground frame, and not the truck frame. If you wish to do a drawing in the truck's frame I would be glad to look at that also.

We can easily predict that the light interference seen on both detectors will be different.

What interference? You have to have two different light paths going to the same detector to get interference. Each detector has a single light path, there will not be any interference. Again, this is not a distinction between Newton and Einstein. Both theories would predict no interference for either.

In order to determine the interference expected you need to calculate the difference in the phase between the two different paths that reach the same detector from the same emitter.

 

[PAllen]

I am going to be generous and interpret your drawings the way I think you intended.

Someone in the truck would measure the speed as c = L / t (all measurements made in the truck frame). Someone on the ground outside the truck would measure (same) c = L'/t' where L' < L, t' < t. Identically constructed clocks at A and B (moving relative to each other) would disagree on both rate and synchronization. That is, if A and B were synchronized with each other at emission time according to ground observer, they would not be synchronized according truck observer; and vice versa.

You will complain I have used relativity to explain what would be observed. Well, relativity is the theory that does explain what is observed here. You can say you don't like it, just like the person in ancient times who might believe that heavier objects must fall faster than light objects. However, reality is what it is. In a vaccuum, all object fall at the same speed; what I described in the prior paragraph is what would actually happen in your experiment.

 

[sisoev]

Nonsense. To show that a theory is inconsistent, you have to use only the laws of that theory, and show a contradiction between them.

You are absolutely right 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?)

 

[PAllen]

You are absolutely right 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?)

If the speed of light is assumed to behave like the speed of a material body, then Galilean relativity for your experiment makes the following predictions (among others):

1) The speed of light measured in the truck will be different from the speed measured on the ground.

2) Maxwell's equations will not hold inside the truck

Both of these are known to be false. The contradiction is with reality. Galilean relativity is mathematically self consistent.

 

[sisoev]

Yes, clearly.

No, this is not correct in any frame where the truck is moving. If the truck is moving in the opposite direction of the light then the distance will be shorter than the length of the truck and if the truck is moving in the same direction of the light then the distance will be longer than the length of the truck.

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.
I think we need animation here.

Note, this is not a distinction between Newtonian physics and relativity. Your drawing is incorrect regardless of which theory of physics you are using.

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.

Both drawings are done from the ground frame, and not the truck frame. If you wish to do a drawing in the truck's frame I would be glad to look at that also.

I don't understand your point.

What interference? You have to have two different light paths going to the same detector to get interference. Each detector has a single light path, there will not be any interference. Again, this is not a distinction between Newton and Einstein. Both theories would predict no interference for either.

In order to determine the interference expected you need to calculate the difference in the phase between the two different paths that reach the same detector from the same emitter.

Excuse my English, please.
I meant pattern, not "interference".
As I said above, a ball will release less energy for detector B compared to the energy released for A.
Same will be observed with photon or light pulse; the pattern for both detectors will be different.
Imagine the experiment in laboratory; one photon released, splits and hits two detectors at the same time and it will leave two different patterns on the two detectors.
Can we say that in one of the reference frames the photon was trapped in different time.

 

[sisoev]

I am going to be generous and interpret your drawings the way I think you intended.

Someone in the truck would measure the speed as c = L / t (all measurements made in the truck frame). Someone on the ground outside the truck would measure (same) c = L'/t' where L' < L, t' < t. Identically constructed clocks at A and B (moving relative to each other) would disagree on both rate and synchronization. That is, if A and B were synchronized with each other at emission time according to ground observer, they would not be synchronized according truck observer; and vice versa.

You will complain I have used relativity to explain what would be observed. Well, relativity is the theory that does explain what is observed here. You can say you don't like it, just like the person in ancient times who might believe that heavier objects must fall faster than light objects. However, reality is what it is. In a vaccuum, all object fall at the same speed; what I described in the prior paragraph is what would actually happen in your experiment.

Where did I say that t' < t ?
The whole point is that we set the experiment with t = t'

 

[Denius1704]

Let me ask a simple enough question. We have two identical trucks, one moving and the other one stationary and they both emmit a light from the one end and then a detector which is put on the other end of the truck, detects that light.

My question is, would both detectors detect that light for the same interval of time from the moment of emission? Meaning would it take both of them, let's say 1 second to detect the light (Yes that's a very long truck i know :) )?

 

[PAllen]

Where did I say that t' < t ?
The whole point is that we set the experiment with t = t'

Except it would not come out that way. Speed is measured by distance traveled in a time interval. The time interval between emission and reception in the truck, measured with clocks moving with the truck would come out greater than the time interval measured between emission and absorption for the ground observer using ground clocks [and using your particular set up, where the truck observer measures truck length, and the ground observer uses a shorter length to measure light speed]. Making the absorption events simultaneous between the two frames does not control for differences in measured interval. So you can claim t=t', just like you can claim heavy objects fall faster than light objects. Both claims are false. It's that simple.

 

[PeterDonis]

Let me ask a simple enough question. We have two identical trucks, one moving and the other one stationary and they both emmit a light from the one end and then a detector which is put on the other end of the truck, detects that light.

My question is, would both detectors detect that light for the same interval of time from the moment of emission? Meaning would it take both of them, let's say 1 second to detect the light (Yes that's a very long truck i know :) )?

Your scenario differs from the OP because you stipulate that in each case, the light emitter and the light detector are both not moving relative to the truck. The only difference is in whether the whole assembly, truck, emitter, and detector together, is "moving". So as you pose the question, yes, both detectors would detect the light for the same interval of time, as measured by clocks traveling with the respective trucks.

 

[Denius1704]

I wasn't trying to make any point yet. My point is coming now :) If there is no difference between the measurement of the stationary and moving objects since their frame of reference is the same that would mean that in his example the detector A is actually detecting the full length of the truck and not shorter length as DaleSpam suggested. That is all i wanted to understand and clear up.
Now it comes down to the fact that according to Pallen (i believe), sisoev's conclusion of detector B detecting the light at that place at the same time as A would be wrong. So according to you, do we have to put B back as many times as A has moved forward so that they both see the light at the same time?

 

[PAllen]

I wasn't trying to make any point yet. My point is coming now :) If there is no difference between the measurement of the stationary and moving objects since their frame of reference is the same that would mean that in his example the detector A is actually detecting the full length of the truck and not shorter length as DaleSpam suggested. That is all i wanted to understand and clear up.
Now it comes down to the fact that according to Pallen (i believe), sisoev's conclusion of detector B detecting the light at that place at the same time as A would be wrong. So according to you, do we have to put B back as many times as A has moved forward so that they both see the light at the same time?

My understanding of the OP set up does achieve the following: Both A and B agree that the absorption event is simultaneous and colocated between them. What it fails to achieve is that their measurement of time interval between emission and absorption agree. OP seems to think that the first statement implies the second.

 

[ghwellsjr]

Let me ask a simple enough question. We have two identical trucks, one moving and the other one stationary and they both emmit a light from the one end and then a detector which is put on the other end of the truck, detects that light.

My question is, would both detectors detect that light for the same interval of time from the moment of emission? Meaning would it take both of them, let's say 1 second to detect the light (Yes that's a very long truck i know :) )?

As I responded to sisoev in post #2:

It is not possible to determine or measure the one-way speed of a light pulse or a photon. We can only measure the round-trip time it takes for a light pulse to start from a source, traverse to a mirror, reflect off the mirror and traverse back to the source. This is experimental evidence for the universal constant value of the speed of light.

In Einstein's Special Relativity, a Frame of Reference is defined in which the two halves of the trajectory of the aforementioned experiment are assigned equal times. This is Einstein's second postulate. Read his 1905 paper.

This is the essence of the problem. You cannot measure the time interval of the one-way speed of light.

If instead of detectors at the far end of both trucks, you placed mirrors there and measured how long it took for the light to make the trip to the rear of the trucks and back to the front of the trucks, you would get the same answer for both trucks. Let's say that answer is two seconds (yes, very long trucks).

Both truck drivers have no way to know if it took one second for the light to make the trip from the front of the truck to the rear. Einstein's brilliant idea was to say that either one could arbitrarily assign the time interval to be one second and you build a Reference Frame based on that assumption. But that means that it does not take one second for the other truck. You can use any Reference Frame you want but there is no Reference Frame in which it takes one second for both trucks.

 

[PAllen]

As I responded to sisoev in post #2:

This is the essence of the problem. You cannot measure the time interval of the one-way speed of light.

If instead of detectors at the far end of both trucks, you placed mirrors there and measured how long it took for the light to make the trip to the rear of the trucks and back to the front of the trucks, you would get the same answer for both trucks. Let's say that answer is two seconds (yes, very long trucks).

Both truck drivers have no way to know if it took one second for the light to make the trip from the front of the truck to the rear. Einstein's brilliant idea was to say that either one could arbitrarily assign the time interval to be one second and you build a Reference Frame based on that assumption. But that means that it does not take one second for the other truck. You can use any Reference Frame you want but there is no Reference Frame in which it takes one second for both trucks.

This is a long debated, interesting, philosophic debate, but it is not much related to the OP conclusions. We can assume, for example, that prior to the trucks arrival 'at the scene', still moving at constant speed relative to the ground, someone inside synchronized two identical clocks next to each other and walked one over the far end. Yes, you can turn around and argue about the hidden assumptions of slow clock transport, but this is really not related to the very basic misunderstanding involved in the OP.

 

[Denius1704]

You guys are so confusing :)

ghwellsjr are you saying that it's impossible to measure the truck's length by just sending an impulse from the one side to the other side? (and when i say "length" i mean it is measured in time needed for the pulse to come from the emission's point to the absorption point)

Pallen are you saying that at the absorption event the moving truck's detector would read let's say 1 sec and the stationary detector would read 0.8 sec?

 

[sisoev]

Except it would not come out that way. Speed is measured by distance traveled in a time interval. The time interval between emission and reception in the truck, measured with clocks moving with the truck would come out greater than the time interval measured between emission and absorption for the ground observer using ground clocks [and using your particular set up, where the truck observer measures truck length, and the ground observer uses a shorter length to measure light speed]. Making the absorption events simultaneous between the two frames does not control for differences in measured interval. So you can claim t=t', just like you can claim heavy objects fall faster than light objects. Both claims are false. It's that simple.

I need help on this one, please :D
If the time interval differs for A and B, that would mean that the simultaneity set in the experiment will be lost for them. Is that correct?

 

[PAllen]

You guys are so confusing :)

ghwellsjr are you saying that it's impossible to measure the truck's length by just sending an impulse from the one side to the other side? (and when i say "length" i mean it is measured in time needed for the pulse to come from the emission's point to the absorption point)

Pallen are you saying that at the absorption event the moving truck's detector would read let's say 1 sec and the stationary detector would read 0.8 sec?

If you're trying to measure the speed of light, you certainly don't want to define distance in terms of light. Use rulers.

I'm saying if the an observe in the truck measured its length ahead of time using rulers, then, after it was moving at desired speed, but before reaching the experiment setup, they walked a clock from one end to the other; then, recorded the time the front clock passed the emission event, and then the time for the signal to reach the back clock, they would get a different time then someone on the ground measuring the time between emission and absorption.

 

[PAllen]

I need help on this one, please :D
If the time interval differs for A and B, that would mean that the simultaneity set in the experiment will be lost for them. Is that correct?

In my very first post on this, I emphasized the following:

- ground and truck clocks will disagree on rate.
- if the ground observer synchronized clocks across some distance, the truck observer will see them out of synch; thus events at different places deemed simultaneous by the ground observer will not be simultaneous observed from the truck
- In the above statement, replace ground->truck, truck->ground, and the same statement is true.

 

[sisoev]

.......
......... will not be simultaneous observed from the truck

I like the "observed" used in the context of "simultaneity", but since I know that you are referring to the relativity of simultaneity I'd like to ask; if the simultaneity is universal and only observed differently, would that equalize the time for both observers? I mean, then we could really measure time and distance according to the simultaneity we set in the experiment?

 

[PAllen]

I like the "observed" used in the context of "simultaneity", but since I know that you are referring to the relativity of simultaneity I'd like to ask; if the simultaneity is universal and only observed differently, would that equalize the time for both observers? I mean, then we could really measure time and distance according to the simultaneity we set in the experiment?

You could choose to do all measurements in one frame. However, you have to do all of them that way, not mix and match. Then it is trivial that you get the same speed for light for A and B, because for (e.g. the ground frame) the light has traveled the same distance in both cases and the same time interval. But if you want to use length of truck as measured in the truck, then you need clocks synchronized in the truck to measure the time interval. If you mix and match, you just get nonsense.

 

[ghwellsjr]

ghwellsjr are you saying that it's impossible to measure the truck's length by just sending an impulse from the one side to the other side? (and when i say "length" i mean it is measured in time needed for the pulse to come from the emission's point to the absorption point

Yes, it's impossible.

What you do, according to Einstein, as I keep repeating, is set the time on the clock that is at the rear end of the truck so that the two parts of the round-trip take an equal amount of time. This is the process of synchronizing the two clocks.

After you do all that, you can confirm that the length of the truck is equal to the previously measured speed of light (using a physical ruler) multiplied by the time interval as defined by the two previously synchronized clocks.

It might be worth pointing out that modern laser measuring devices work on a round-trip speed of light and you could measure the length of the truck without previously synchronizing any clocks by measuring the round-trip time it takes for the light to traverse the distance and back and multiplying by the time interval and dividing by two.

 

[sisoev]

You could choose to do all measurements in one frame. However, you have to do all of them that way, not mix and match. Then it is trivial that you get the same speed for light for A and B, because for (e.g. the ground frame) the light has traveled the same distance in both cases and the same time interval. But if you want to use length of truck as measured in the truck, then you need clocks synchronized in the truck to measure the time interval. If you mix and match, you just get nonsense.

Now we are getting somewhere :)
Now I'm starting to get the essence of relativity.
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.

I don't intend to shift the discussion, but would also like to make my point.
I'll quote my experiment with the purple door, which experiment I presented in the https://www.physicsforums.com/showthread.php?p=3494901#post3494901" topic.

We take red and blue transparent garage doors (http://en.wikipedia.org/wiki/Ladder_paradox" ) and put them behind each other on the two ends of the garage.
Then we set a timer to open the doors simultaneously after some time.
So, we set simultaneity between one red and one blue doors, and we go on some distance to watch what will happen.
From our position we see one red and one purple door behind it.
(the blue door is seen purple behind the red door)
suddenly the red door opens and we see purple door behind it, which opens a moment after the red one.
But why PURPLE?
The simultaneity was set between red and blue doors?
Now we know that the time of appearance of the purple door is the time for the light from the opened blue door to travel trough the garage.
The door wasn't actually there, because if it was, we would see it blue.
And now we also know that the two events were simultaneous but we observed the simultaneity differently.
Our conclusion should be: simultaneity is universal, but observed differently.

If relativity of simultaneity and length contraction are in question for me, how do you expect me to understand the rest.
It is not that I am ANTI-relativist. I am not.
Like many of you I also use and I appreciate GPS navigation :)

 

[ghwellsjr]

Now we are getting somewhere :)
Now I'm starting to get the essence of relativity.
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.

I don't intend to shift the discussion, but would also like to make my point.
I'll quote my experiment with the purple door, which experiment I presented in the https://www.physicsforums.com/showthread.php?p=3494901#post3494901" topic.

We take red and blue transparent garage doors (http://en.wikipedia.org/wiki/Ladder_paradox" ) and put them behind each other on the two ends of the garage.
Then we set a timer to open the doors simultaneously after some time.
So, we set simultaneity between one red and one blue doors, and we go on some distance to watch what will happen.
From our position we see one red and one purple door behind it.
(the blue door is seen purple behind the red door)
suddenly the red door opens and we see purple door behind it, which opens a moment after the red one.
But why PURPLE?
The simultaneity was set between red and blue doors?
Now we know that the time of appearance of the purple door is the time for the light from the opened blue door to travel trough the garage.
The door wasn't actually there, because if it was, we would see it blue.
And now we also know that the two events were simultaneous but we observed the simultaneity differently.
Our conclusion should be: simultaneity is universal, but observed differently.

If relativity of simultaneity and length contraction are in question for me, how do you expect me to understand the rest.
It is not that I am ANTI-relativist. I am not.
Like many of you I also use and I appreciate GPS navigation :)

If you want simultaneity to be universal, then you need to promote the Lorentz Ether Theory in which there is a preferred reference frame, although no one knows where it is.

Since I don't believe you really want that and you say that you are not anti-relativist, then I assume that you want to understand Special Relativity. That means you have to give up on the notion of universal simultaneity and on the notion of a universal time (they are the same thing). It is not just an observational issue although that enters into it. It's both an observational issue caused by the finite speed of light and it involves time dilation.

The issue of symultaneity has to do with the way we define the times on remote clocks as part of our definition of what a Frame of Reference is (remember my first post #2?). You have two choices: you can claim that there exists an absolute ether rest state in which the speed of light is exclusively constant in all directions and for which times, distances, and simultaneities are absolute, or you can claim that all rest states are equally valid and define times, distances and simultaneities relatively according to the definition of anyone of those rest states.

 

[ghwellsjr]

Now we are getting somewhere :)
Now I'm starting to get the essence of relativity.
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.

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.

 

[sisoev]

If you want simultaneity to be universal, then you need to promote the Lorentz Ether Theory in which there is a preferred reference frame, although no one knows where it is.

Since I don't believe you really want that and you say that you are not anti-relativist, then I assume that you want to understand Special Relativity. That means you have to give up on the notion of universal simultaneity and on the notion of a universal time (they are the same thing). It is not just an observational issue although that enters into it. It's both an observational issue caused by the finite speed of light and it involves time dilation.

The issue of symultaneity has to do with the way we define the times on remote clocks as part of our definition of what a Frame of Reference is (remember my first post #2?). You have two choices: you can claim that there exists an absolute ether rest state in which the speed of light is exclusively constant in all directions and for which times, distance, and simultaneities are absolute, or you can claim that all rest states are equally valid and define times, distances and simultaneities relatively according to the definition of anyone of those rest states.

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.

 

[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?