The Relativity of Simultaneity

[Pete]

The reality is, light has to travel different distances to impact the observer on the train. That is due to the train observer's velocity. It just so happens in Einstein's example that the embankment has a zero velocity, because Einstein set it up that way, unintentionally for sure. The embankment could have had a velocity and the train a velocity, and things would have been different, but that isn't how the example is set up.

Sorry, MD, you're all wrong. Absolute rest has been out of date since Galileo.
Einstein's says that's reality "As judged from the embankment." The actual velocity of the embankment is arbitrary. It's certainly not supposed to be floating in space, unmoved by the Earth's orbit around the Sun, or the Sun's motion through the galaxy, or the galaxy's motion toward Andromeda. Where do you think Einstein says that the embankment is at absolute rest?

Look. Here's a 1972 experiment that measured the speed of light in a laboratory to within 1 m/s. They didn't adjust for the Earth's rotation or orbit. This measurement has been repeated and improved upon. No one has found it necessary to adjust for Earth's motion, yet they all get the same results.

What does that tell you?

 

[Motor Daddy]

Sorry, MD, you're all wrong. Absolute rest has been out of date since Galileo.
Einstein's says that's reality "As judged from the embankment." The actual velocity of the embankment is arbitrary. It's certainly not supposed to be floating in space, unmoved by the Earth's orbit around the Sun, or the Sun's motion through the galaxy, or the galaxy's motion toward Andromeda. Where do you think Einstein says that the embankment is at absolute rest?

Einstein says that just as the points line up the strikes occur. The embankment observer is midway between the points, and the lights reach the embankment observer simultaneously. That can only happen if the observer has a zero velocity, for if he had a velocity, he would have moved closer to one light and away from the other, resulting in one light taking less time than if he were at a true zero velocity, and the other light taking more time than if he were at a zero velocity.

 

[Janus58]

The points in the example coincide with each other as Einstein clearly states. Are you saying Einstein didn't mean to say the points coincide? They do, the diagram shows that fact, and that is what Einstein states. What you are saying with the front of the train's point lining up, and the rear not yet lined up means that the observer either isn't in the middle of the length of the train, or that he is in the middle of that length, and that his midpoint doesn't line up with the embankment observers midpoint, as is also clearly stated in the diagram and text. Read chapter 9 and look at that diagram. Einstein makes perfectly clear the diagram and the wording.

This is what Einstein says happens:

This first animation shows events from the embankment frame:

And this animation shows the same events from the train's frame:

Note that in both animations, the front of the train and the right red dot coincide when the lightning strikes the dot, and the rear of the train and the left dot coincide when that lightning strikes. Also note that in both frames the light from both strikes reach the embankment observer simultaneously. However, this doesn't mean that both lightning strikes occurred at the same time in the train frame.

 

[Pete]

Einstein says that just as the points line up the strikes occur. The embankment observer is midway between the points, and the lights reach the embankment observer simultaneously. That can only happen if the observer has a zero velocity, for if he had a velocity, he would have moved closer to one light and away from the other, resulting in one light taking less time than if he were at a true zero velocity, and the other light taking more time than if he were at a zero velocity.

Now you're just repeating yourself, MD.
Did you think about that experiment? How do you think those guys consistently measured the speed of light without taking the Earth's motion into account?

 

[AlphaNumeric]

This is your best post yet to validate the OP's argument.

Jack, why do you quote the entire post of mine and then quote again a specific paragraph you want to respond to? Either don't quote the entire post or just quote the relevant bit. You know, like everyone else does! I think even you can grasp the quote function, please try it.

So, each observer will see the strikes simultaneously as the OP contends and you just proved.

I like how you just jumped from what I said to the conclusion I said precisely the opposite. Obviously you 'proof skills' need improvement, as well as MD's. Maybe the two of you could take a class together.

Managed to put your physics where your mouth is and submitted to a journal yet Jack? Though not.

 

[Motor Daddy]

Note that in both animations, the front of the train and the right red dot coincide when the lightning strikes the dot, and the rear of the train and the left dot coincide when that lightning strikes. Also note that in both frames the light from both strikes reach the embankment observer simultaneously. However, this doesn't mean that both lightning strikes occurred at the same time in the train frame.

Albert Einstein (1879–1955). Relativity: The Special and General Theory. 1920.

IX. The Relativity of Simultaneity


UP to now our considerations have been referred to a particular body of reference, which we have styled a “railway embankment.” We suppose a very long train travelling along the rails with the constant velocity v and in the direction indicated in Fig. 1. People travelling in this train will with advantage use the train as a rigid reference-body (co-ordinate system); they regard all events in reference to the train. Then every event which takes place along the line also takes place at a particular point of the train. Also the definition of simultaneity can be given relative to the train in exactly the same way as with respect to the embankment. As a natural consequence, however, the following question arises: 1
Are two events (e.g. the two strokes of lightning A and B) which are simultaneous with reference to the railway embankment also simultaneous relatively to the train? We shall show directly that the answer must be in the negative.

FIG. 1.

2
When we say that the lightning strokes A and B are simultaneous with respect to the embankment, we mean: the rays of light emitted at the places A and B, where the lightning occurs, meet each other at the mid-point M of the length A —> B of the embankment. But the events A and B also correspond to positions A and B on the train. Let M' be the mid-point of the distance A —> B on the travelling train. Just when the flashes 1 of lightning occur, this point M' naturally coincides with the point M, but it moves towards the right in the diagram with the velocity v of the train. If an observer sitting in the position M’ in the train did not possess this velocity, then he would remain permanently at M, and the light rays emitted by the flashes of lightning A and B would reach him simultaneously, i.e. they would meet just where he is situated. Now in reality (considered with reference to the railway embankment) he is hastening towards the beam of light coming from B, whilst he is riding on ahead of the beam of light coming from A. Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A. Observers who take the railway train as their reference-body must therefore come to the conclusion that the lightning flash B took place earlier than the lightning flash A. We thus arrive at the important result: 3
Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (relativity of simultaneity). Every reference-body (co-ordinate system) has its own particular time; unless we are told the reference-body to which the statement of time refers, there is no meaning in a statement of the time of an event. 4
Now before the advent of the theory of relativity it had always tacitly been assumed in physics that the statement of time had an absolute significance, i.e. that it is independent of the state of motion of the body of reference. But we have just seen that this assumption is incompatible with the most natural definition of simultaneity; if we discard this assumption, then the conflict between the law of the propagation of light in vacuo and the principle of relativity (developed in Section VII) disappears. 5
We were led to that conflict by the considerations of Section VI, which are now no longer tenable. In that section we concluded that the man in the carriage, who traverses the distance w per second relative to the carriage, traverses the same distance also with respect to the embankment in each second of time. But, according to the foregoing considerations, the time required by a particular occurrence with respect to the carriage must not be considered equal to the duration of the same occurrence as judged from the embankment (as reference-body). Hence it cannot be contended that the man in walking travels the distance w relative to the railway line in a time which is equal to one second as judged from the embankment. 6
Moreover, the considerations of Section VI are based on yet a second assumption, which, in the light of a strict consideration, appears to be arbitrary, although it was always tacitly made even before the introduction of the theory of relativity. 7


Note 1. As judged from the embankment.

In your animations the reference points don't line up when the strikes occur. As I said before, this example could be redone with the train observer at the midpoint of the train having a switch that activates two lights, one on each end of the train. When the observer activates the switch, the lights simultaneously emit light, and the lights each hit the observer at different times. The observer will refuse to believe the lights were activated simultaneously, even though there was only one switch and he operated it. That phenomena is due to his velocity and would not occur if he didn't posses that velocity.

Also, what do you think the directional arrow represents, and what is the direction in reference to?

 

[Motor Daddy]

Now you're just repeating yourself, MD.
Did you think about that experiment? How do you think those guys consistently measured the speed of light without taking the Earth's motion into account?

The speed of light has nothing to do with another object's motion. The speed of light is simply the distance the light travels and the duration of travel, as compared to the standard second.

If you measured the light coming from the sun in a spherical way, how big would the light sphere be after 1 second? ~186,000 mile radius? Correct. But what if during that one second the sun traveled? Does that change the diameter of the 1 second light sphere? NO! It only means the sun is no longer in the center of that 1 second light sphere. The sun's motion has nothing to do with the speed of light.

 

[Janus58]

"Then every event which takes place along the line also takes place at a particular point of the train."

If you look carefully at the animations, you will note that when a flash reaches any given point of the embankment, the same point of the train is adjacent to it in both animations. For example, the left flash reaches the rightmost red dot when the train observer reaches that dot in both animations. My animation fulfills this requirement

Also the definition of simultaneity can be given relative to the train in exactly the same way as with respect to the embankment."

Earlier is the same book he gives the definition of simultaneity that he refers to here. It is this: If you are standing exactly at the midpoint between two events and you see them at the same time, then for you those events are simultaneous.
It is this definition that also applies relative to the train. If the train observer is halfway between the ends of the train and see events at the end of the train at the same time ,hen those events are simultaneous for the train observer.

2
If an observer sitting in the position M’ in the train did not possess this velocity, then he would remain permanently at M, and the light rays emitted by the flashes of lightning A and B would reach him simultaneously, i.e. they would meet just where he is situated. Now in reality (considered with reference to the railway embankment)[\color]

Which is repeated with this note at the bottom of the passage:

Note 1. As judged from the embankment.

Look, I've read this passage. I have the book. You are misinterpreting the meaning of his statements.

In your animations the reference points don't line up when the strikes occur. As I said before, this example could be redone with the train observer at the midpoint of the train having a switch that activates two lights, one on each end of the train. When the observer activates the switch, the lights simultaneously emit light, and the lights each hit the observer at different times. The observer will refuse to believe the lights were activated simultaneously, even though there was only one switch and he operated it. That phenomena is due to his velocity and would not occur if he didn't posses that velocity.

This your example, the light would come on simultaneously in the train frame and the train observer would see them simultaneously. However they would not come on simultaneously for the observer on the embankment, nor would he see the lights simultaneously. All this version does is change which frame the lights are simultaneous in.

 

[Pete]

The speed of light has nothing to do with another object's motion. The speed of light is simply the distance the light travels and the duration of travel, as compared to the standard second.

But don't you see? In the lab, the light receiver moves with the Earth between between sending the light and receiving it. The experimenters did not allow for that motion, and yet they still got a consistent measurement.

You can't argue with reality, MD.

 

[Motor Daddy]

Earlier is the same book he gives the definition of simultaneity that he refers to here. It is this: If you are standing exactly at the midpoint between two events and you see them at the same time, then for you those events are simultaneous.
It is this definition that also applies relative to the train. If the train observer is halfway between the ends of the train and see events at the end of the train at the same time ,hen those events are simultaneous for the train observer.

Just because two snowballs hit you simultaneously doesn't mean the snowballs traveled the same distance or speed. Two lights hitting you at the same time says nothing about the simultaneity of the emission of those lights.

This your example, the light would come on simultaneously in the train frame and the train observer would see them simultaneously.

Absolutely not! What you are saying is that the lights will always impact the midpoint observer simultaneously, regardless of the motion of the train. That is an absurd statement!

 

[Motor Daddy]

But don't you see? In the lab, the light receiver moves with the Earth between between sending the light and receiving it. The experimenters did not allow for that motion, and yet they still got a consistent measurement.

You can't argue with reality, MD.

I'll ask again:

If you measured the light coming from the sun in a spherical way, how big would the light sphere be after 1 second? ~186,000 mile radius? Correct. But what if during that one second the sun traveled? Does that change the diameter of the 1 second light sphere? NO! It only means the sun is no longer in the center of that 1 second light sphere. The sun's motion has nothing to do with the speed of light.

 

[Pete]

I'll ask again:

If you measured the light coming from the sun in a spherical way, how big would the light sphere be after 1 second? ~186,000 mile radius? Correct. But what if during that one second the sun traveled? Does that change the diameter of the 1 second light sphere? NO! It only means the sun is no longer in the center of that 1 second light sphere. The sun's motion has nothing to do with the speed of light.

You're not really trying to get it, are you?

Look, imagine the experimenters are on the train.

They know the length of the train, they have synchronized clocks at each end.
A light flashes at one end, it is received at the other.
The experimenters measure the time elapsed between light emitted and light received using their synchronized clocks, and they measure the distance travelled from the length of the train.
They divide distance by time to get the speed. Note that they don't concern themselves over whether the train (or the Earth) is moving or not.

Now this isn't exactly how the experiment was done, of course. In practice there are many devilish details built up from previous experiments. But the principle is close enough.

 

[Motor Daddy]

You're not really trying to get it, are you?

Look, imagine the experimenters are on the train.

They know the length of the train, they have synchronized clocks at each end.
A light flashes at one end, it is received at the other.
The experimenters measure the time elapsed between light emitted and light received using their synchronized clocks, and they measure the distance travelled from the length of the train.
They divide distance by time to get the speed. Note that they don't concern themselves over whether the train (or the Earth) is moving or not.

Now this isn't exactly how the experiment was done, of course. In practice there are many devilish details built up from previous experiments. But the principle is close enough.

So no matter what, the distance the light travels in the train experiment is always the same, regardless of the train's motion?

I'll ask another way, if you do the test when the train is going 60 MPH, will the distance the light travels be the same as when you do the experiment when the train is going 120 MPH?

 

[Pete]

So no matter what, the distance the light travels in the train experiment is always the same, regardless of the train's motion?

In the train's reference frame, yes.

I'll ask another way, if you do the test when the train is going 60 MPH, will the distance the light travels be the same as when you do the experiment when the train is going 120 MPH?

How far do you drive to work?
Does it depend on which way the Earth is moving?

 

[Motor Daddy]

In the train's reference frame, yes.

The distance the light travels is not related to a frame. You didn't comment on the light sphere. Do you agree the light sphere will have a ~186,000 mile radius 1 second after emission? Does the motion of that source change the distance traveled?

How far do you drive to work?
Does it depend on which way the Earth is moving?

Relate that to the train. As far as the train observer is concerned, the light always travels from one end of the train to the other. If that distance is 100 feet then certainly the train observer can say the light traveled from point a to point b, which is 100 feet in his frame. BUT, the train observer must understand that the distance light travels is not measured against his own frame. The distance light travels is not compared to a frame.

Going back to the light sphere, if light is emitted from a source in space, the sphere will have a radius of ~186,000 miles after 1 sec of travel, regardless of the motion of the source.

 

[Pete]

The distance the light travels is not related to a frame.

That's what relativity is all about, MD. All measurement of distance and time are relative to some arbitrary rest reference.

Relate that to the train. As far as the train observer is concerned, the light always travels from one end of the train to the other. If that distance is 100 feet then certainly the train observer can say the light traveled from point a to point b, which is 100 feet in his frame. BUT, the train observer must understand that the distance light travels is not measured against his own frame.

What reference frame do you think it's measured against, MD? The Earth? The Sun? The galaxy?

It's Relativity. Relative to a reference frame. Any reference frame. Get used to it.

Going back to the light sphere, if light is emitted from a source in space, the sphere will have a radius of ~186,000 miles after 1 sec of travel, regardless of the motion of the source.

That's right, rabbit. But we're talking about moving observers, not moving sources.

You didn't answer the question about driving to work.
If you live 1 mile from your office, that's how far you drive, right?

 

[Motor Daddy]

That's right, rabbit. But we're talking about moving observers, not moving sources.

We are talking about the speed of light, which is measured in the distance and time the light travels, which is not dependent on an observer.

You didn't answer the question about driving to work.
If you live 1 mile from your office, that's how far you drive, right?

I answered the question, related to the train. Yes, if you measured 1 mile on the Earth's surface, then it is one mile on the Earth's surface. That doesn't mean light always takes the same amount of time to go from the starting point to the ending point, regardless of the Earth's velocity.

You can be going 60 MPH on the highway in your car, and strangely enough, if the car 20 feet in front of you also travels 60 MPH, you will not increase or decrease the distance between the cars. Both have a speed of 60 MPH, and yet, the distance between remains the same. HMMMMMMM, let's see if light does the same. I turn on my headlights, and during the time it takes for the light to travel from my car to the car 20 feet in front of me, the car in front of me moves forward, away from the light, so the light has to travel more than 20 feet to reach the car. Does the light travel 20 feet just because the distance between the cars remained 20 feet? No.

 

[Pete]

That doesn't mean light always takes the same amount of time to go from the starting point to the ending point, regardless of the Earth's velocity.

That is exactly what Einstein is going on about, MD and exactly what we find in practice.

You can be going 60 MPH on the highway in your car, and strangely enough, if the car 20 feet in front of you also travels 60 MPH, you will not increase or decrease the distance between the cars. Both have a speed of 60 MPH, and yet, the distance between remains the same. HMMMMMMM, let's see if light does the same. I turn on my headlights, and during the time it takes for the light to travel from my car to the car 20 feet in front of me, the car in front of me moves forward, away from the light, so the light has to travel more than 20 feet to reach the car. Does the light travel 20 feet just because the distance between the cars remained 20 feet? No.

How far does it go, MD? How fast it the Earth, the Sun, and the Galaxy moving?
Using the cars as as refernce, the light travels just 20 feet. That's relativity.

 

[Motor Daddy]

How far does it go, MD? How fast it the Earth, the Sun, and the Galaxy moving?
Using the cars as as refernce, the light travels just 20 feet. That's relativity.

If light has to "chase" an object in absolute space, it takes more time to "catch it" than if the object is moving towards the light, obviously.

Light always travels at c. How much time it takes for the light to reach another object is only dependent on the absolute motion of that other object, and the distance between the source and other object at emission.

 

[Pete]

If light has to "chase" an object in absolute space, it takes more time to "catch it" than if the object is moving towards the light, obviously.

Light always travels at c. How much time it takes for the light to reach another object is only dependent on the absolute motion of that other object, and the distance between the source and other object at emission.

Again with the absolute motion.
Go read up on the Michelson Morley experiment, and stop arguing with reality.