Trying to Understand Light in Motion: A Frustrating Puzzle

[Muphrid]

the pilot is positioned where the I is - therefore the light from the targets will travel in the y direction to get to his eyes - in the same way light hitting a platform will travel to the platform observer

Then this is what I submit to you: because the pilot is moving in the x-direction, which is perpendicular to the y-direction, there is no difference in when he perceives the light from the targets being destroyed.

In the train example, either the platform observer or the train observer is moving in the x-direction, which is same direction as the light beams from the lightning strikes travel. That is the critical difference between the two scenarios--the direction of motion compared to the direction separating the two events.

Is that an idea that you can accept even if you're unsure how it can be justified?

 

[solarflare]

Then this is what I submit to you: because the pilot is moving in the x-direction, which is perpendicular to the y-direction, there is no difference in when he perceives the light from the targets being destroyed.

In the train example, either the platform observer or the train observer is moving in the x-direction, which is same direction as the light beams from the lightning strikes travel. That is the critical difference between the two scenarios--the direction of motion compared to the direction separating the two events.

Is that an idea that you can accept even if you're unsure how it can be justified?

the lightning comes down from the sky - how is that traveling in the same direction as the train?

 

[Muphrid]

The two points struck by lightning are separated by some vector.

The direction of that vector is the same direction as the velocity of the train.

 

[solarflare]

at the moment the lightning strikes the train - the direction of the light is not affected by the motion of the train - light moves out like an expanding sphere (as seen in the video)

if the lightning happens where the video says it happened then the platform observer will see them simultaneously -

because the lightning strikes are simultaneous when r1 = r2 in the platform frame

they must be simultaneous in the trains frame also because that is where the strikes take place.

and because they hit the train simultaneously in the trains frame the train passenger will aslo see them simultaneously

the point is that if it was anywhere other than r1 = r2 then the video would be accurate

 

[Nugatory]

the lightning comes down from the sky - how is that traveling in the same direction as the train?

It's the reflection from the flash that we're talking about it - that reflection is traveling from the point of impact to the observer, along the line of the tracks.

 

[Doc Al]

at the moment the lightning strikes the train - the direction of the light is not affected by the motion of the train - light moves out like an expanding sphere (as seen in the video)

if the lightning happens where the video says it happened then the platform observer will see them simultaneously -

because the lightning strikes are simultaneous when r1 = r2 in the platform frame

This is true.

they must be simultaneous in the trains frame also because that is where the strikes take place.

This is nonsense. Just because the lightning hit the train doesn't mean the strikes are simultaneous in the train frame.

and because they hit the train simultaneously in the trains frame the train passenger will aslo see them simultaneously

Which of course contradicts what we already know: That the light reaches the passenger at different times.

Sad that you are still making the same erroneous statements, over and over. No progress.

 

[Muphrid]

at the moment the lightning strikes the train - the direction of the light is not affected by the motion of the train - light moves out like an expanding sphere (as seen in the video)

if the lightning happens where the video says it happened then the platform observer will see them simultaneously -

because the lightning strikes are simultaneous when r1 = r2 in the platform frame

This is all true.

they must be simultaneous in the trains frame also because that is where the strikes take place.

No, this does not follow. This is what we've been trying to tell you. Do not assume that just because the strikes hit the train they "belong" in the train's frame. They don't. This is why I created the mirror boats on a river example. The lightning strikes need not even hit the train--two points on, say, a second train that is going by at some other speed will do and will not change the result.

 

[Muphrid]

Additionally, solarflare, tell me what you think would happen if we took your spaceship scenario and changed it like this:

The moving ship is halfway between the two targets and moving with velocity V toward the upper target at the moment the two targets are struck and obliterated.

 

[zbe]

Reading this thread just makes me nervous. And sad.

 

[universal_101]

at the moment the lightning strikes the train - the direction of the light is not affected by the motion of the train - light moves out like an expanding sphere (as seen in the video)

if the lightning happens where the video says it happened then the platform observer will see them simultaneously -

because the lightning strikes are simultaneous when r1 = r2 in the platform frame

they must be simultaneous in the trains frame also because that is where the strikes take place.

and because they hit the train simultaneously in the trains frame the train passenger will aslo see them simultaneously

the point is that if it was anywhere other than r1 = r2 then the video would be accurate

Let me ask you a very simple question, for you to understand why it happens.

There is a light source and a detector at some distance apart, does the time taken by the signal from source to reach the detector depends on their relative distance, of-course it does.

The same is applicable to the train scenario, since it does not matter if the lightning struck the train or some fixed poles on the ground which are at the same position where the lightning would have struck the train ends.

And now since the distance between these fixed poles and the train passenger is different after some time which makes the signals out of sync.

That is, as it takes approx. 1.2 seconds for light to reach the moon from Earth, it would certainly takes less time if someone starts moving towards moon.

 

[Dale]

take the position of the train when it is in the centre of the platform -

and run the scenario for both observers - the result comes out the same.

if the strikes occur when r1 = r2 then a simple triangle shows that they must occur in both frames simultaneously but at different times.

I have proven this false multiple times already. If you disagree then please point out the error in my proof or provide your own formal proof of the contrary for my review.

 

[Dale]

they must be simultaneous in the trains frame also because that is where the strikes take place.

and because they hit the train simultaneously in the trains frame the train passenger will aslo see them simultaneously

Proven false.

 

[solarflare]

Additionally, solarflare, tell me what you think would happen if we took your spaceship scenario and changed it like this:

The moving ship is halfway between the two targets and moving with velocity V toward the upper target at the moment the two targets are struck and obliterated.

let me ask you what you think would happen - and then ill respond to that

 

[solarflare]

by the way the targets are attached to the ship - therefore the targets move with the ship

 

[solarflare]

but let's say that the ship was moving with velocity C and at the time of impact it was in the current position

 

[solarflare]

1) what is the frame of reference
2) what does each observer see

 

[Doc Al]

but let's say that the ship was moving with velocity C and at the time of impact it was in the current position

The ship will not be moving at the speed of light.

Rather that confuse things even more by changing scenarios, I suggest that you stick with the train example until you understand it.

 

[solarflare]

The ship will not be moving at the speed of light.

Rather that confuse things even more by changing scenarios, I suggest that you stick with the train example until you understand it.

ok say velocity v

 

[solarflare]

or just say it was moving at 500kph

 

[solarflare]

A--------------------B------------------------------------- C

---------------------0 -------------------------------------
)-------------------/---------------------------------------/
)-----------------I----------------------------------------D
)-------------------\---------------------------------------\
---------------------0 -------------------------------------

the spaceship at point B is moving with velocity V towards the letter B

the lasers hit the targets simultaneously at the point where the ship currently is

 

[Muphrid]

by the way the targets are attached to the ship - therefore the targets move with the ship

Not necessary, and in fact, I think this is what's confusing you.

                                    30 units
                          |==============================|
 Laser cannon
 |---                     0 Target
 |  \                      
 |  |                                                    
 |  |                                                    #
 |  |             Small -># +Y-velocity beta = 0.9       #<-Stationary other ship
 |  |              Ship     ]                            #
 |  |                       |-3 units
 |  /                       ]                            
 |---                     0 Target
 Laser cannon
     
     |====================|
          20 units

This diagram is in the frame of the stationary other ship. At some time t=0 in the stationary ship's frame, the stationary ship detects two flashes from the targets being obliterated by the laser cannons. The stationary ship knows, based on the sizes of these explosions, that it was equidistant from both explosions, so it concludes that the targets were obliterated at t = -30.15 units.

The small ship is traveling at \beta = 0.9 between the targets, which are confined to a line but otherwise have thrusters that allow them to move in an arbitrary manner along that line. The stationary ship monitors the small ship's trajectory and believes that the small ship was equidistant from the targets at t = -30.15, the time both targets were obliterated. If the stationary other ship is at y = 0, then it believes the light from the upper target will reach the small moving ship at t = -30.15 + 1.58, or 1.58 units of time after the explosion, and at y = 1.42. It also believes that the flash from the lower target will reach the small moving ship at t = -30.15 + 30, a full 30 units of time after the targets were obliterated, at y = 27.

Let's stop here before we go any further. Do you agree with the conclusions I have reached so far?

 

[solarflare]

Originally Posted by solarflare View Post

by the way the targets are attached to the ship - therefore the targets move with the ship
-----------------------------------------------------------------------------------
Not necessary, and in fact, I think this is what's confusing you.the train was a solid object - why can't my ship be?

 

[Doc Al]

A--------------------B------------------------------------- C

---------------------0 -------------------------------------
)-------------------/---------------------------------------/
)-----------------I----------------------------------------D
)-------------------\---------------------------------------\
---------------------0 -------------------------------------

the spaceship at point B is moving with velocity V towards the letter B

the lasers hit the targets simultaneously at the point where the ship currently is

Sounds like scenario B in my post #185. Can you verify?

Is that "towards the letter B" a typo?

 

[solarflare]

Sounds like scenario B in my post #185. Can you verify?

Is that "towards the letter B" a typo?

it is moving from the bottom of the page towards the top of the page

 

[Muphrid]

the train was a solid object - why can't my ship be?

Because it makes no difference to the math how the targets are moving or whether they're rigidly attached to the ship. If you want, imagine that the targets are rigidly attached. I will work the problem as if we can't assume that--a general case.

Do you agree with my conclusions thus far?

 

[solarflare]

Because it makes no difference to the math how the targets are moving or whether they're rigidly attached to the ship. If you want, imagine that the targets are rigidly attached. I will work the problem as if we can't assume that--a general case.

Do you agree with my conclusions thus far?

well if it makes no difference to the math - why not do it my way instead

 

[solarflare]

i believe that it does make a difference to the outcome because there will be 3 objects instead of one

seems more compicated and i thought the idea was to keep it simple

 

[Muphrid]

well if it makes no difference to the math - why not do it my way instead

Because you've repeatedly tried to argue that because the objects are all connected then there must be simultaneity in that comoving frame. I will show you that whether the objects are connected or not the math is the same and there is no simultaneity in that frame.

Do you agree with my conclusions about the system thus far?

 

[Dale]

take the position of the train when it is in the centre of the platform -

and run the scenario for both observers - the result comes out the same.

if the strikes occur when r1 = r2 then a simple triangle shows that they must occur in both frames simultaneously but at different times.

I am still waiting for your attempted proof of this. You make assertions that not only cannot be proven, they have already been disproven.

 

[ghwellsjr]

All the images from the first minute of the video that I previously posted were described as being in the platform frame. Now I'm going to show the images that are described as being in the train frame after the first minute.

Note in the first image that the platform observer is standing in relative darkness and then the first lightning strike occurs and it illuminates the entire scene, even casting a shadow of the observer! Are we to believe that the observer is not at this point seeing the lightning? Now look at the third image. Here we see the beginning of the expanding sphere of light.

 

[ghwellsjr]

Now the scene goes dark again as the expanding sphere of light gets bigger until it reaches to the observer in the middle of the train but it quickly fades away before it gets to the platform observer. So apparently, we are supposed to believe that the platform observer saw the first lightning bolt earlier.

 

[ghwellsjr]

Just as quickly as the flash from the first lightning bolt fades away, the flash from the second lightning bolt illuminates the entire scene, casting another shadow just as the first bolt but in a different direction. Then the expanding sphere starts from the rear of the train car and by the time it reaches the platform observer the scene has gone dark again. So which one represents when the observer actually sees the flash?

 

[ghwellsjr]

Finally, the expanding sphere of light reaches the train observer.

 

[ghwellsjr]

If you look over all the posts of the images from the video, you see that none of them depicts anything resembling what either observer actually sees nor what either frame describes. In fact, I can see why solarflare would come to the conclusion, based on this video, that what the observers see is frame dependent and that the platform frame applies only to the platform observer and the train frame applies only to the train observer.

 

[ghwellsjr]

This video was discussed in detail a couple years ago in this thread:

https://www.physicsforums.com/showthread.php?p=2597760#post2597760

And at least part of it was deemed "misleading and sloppy" by Doc Al. I say that is true of the entire video. I would not recommend it to anyone except as an example of how someone with great video capabilities can produce a piece of junk.

Now if you look on the next page of the link I posted above, you will see another link in post #70 where yuiop (formerly kev) posted an animation that he made that depicts correctly how each frame determines what is happening in the scenario "described" in the video. It's not fancy like the video, but it's accurate. Here is a repeat of the link to the train animation:

http://i910.photobucket.com/albums/ac304/kev2001_photos/Etrain2e.gif

 

[solarflare]

Because it makes no difference to the math how the targets are moving or whether they're rigidly attached to the ship. If you want, imagine that the targets are rigidly attached. I will work the problem as if we can't assume that--a general case.

Do you agree with my conclusions thus far?

take a look at post 25 (my post) and then at post 28 (doc al's post )

by making the the targets separate you are trying to avoid this situation

and if george is right that this was discussed before and doc al admitted the video was not accurate then - then why when i said it this time why did he say it was accurate?

 

[solarflare]

This video was discussed in detail a couple years ago in this thread:

https://www.physicsforums.com/showthread.php?p=2597760#post2597760

And at least part of it was deemed "misleading and sloppy" by Doc Al. I say that is true of the entire video. I would not recommend it to anyone except as an example of how someone with great video capabilities can produce a piece of junk.

Now if you look on the next page of the link I posted above, you will see another link in post #70 where yuiop (formerly kev) posted an animation that he made that depicts correctly how each frame determines what is happening in the scenario "described" in the video. It's not fancy like the video, but it's accurate. Here is a repeat of the link to the train animation:

http://i910.photobucket.com/albums/ac304/kev2001_photos/Etrain2e.gif

actually now it all makes sense to me - you do know that it is wrong but instead of just admitting it you worked out a way of avoiding the situation my making the flash points independent so that you could argue that they do not have to adhere to the part of rellativity that proves it wrong.

________________________________

C.L.GOLDING ( just copying george :-) )

 

[solarflare]

Not necessary, and in fact, I think this is what's confusing you.

                                    30 units
                          |==============================|
 Laser cannon
 |---                     0 Target
 |  \                      
 |  |                                                    
 |  |                                                    #
 |  |             Small -># +Y-velocity beta = 0.9       #<-Stationary other ship
 |  |              Ship     ]                            #
 |  |                       |-3 units
 |  /                       ]                            
 |---                     0 Target
 Laser cannon
     
     |====================|
          20 units

This diagram is in the frame of the stationary other ship. At some time t=0 in the stationary ship's frame, the stationary ship detects two flashes from the targets being obliterated by the laser cannons. The stationary ship knows, based on the sizes of these explosions, that it was equidistant from both explosions, so it concludes that the targets were obliterated at t = -30.15 units.

The small ship is traveling at \beta = 0.9 between the targets, which are confined to a line but otherwise have thrusters that allow them to move in an arbitrary manner along that line. The stationary ship monitors the small ship's trajectory and believes that the small ship was equidistant from the targets at t = -30.15, the time both targets were obliterated. If the stationary other ship is at y = 0, then it believes the light from the upper target will reach the small moving ship at t = -30.15 + 1.58, or 1.58 units of time after the explosion, and at y = 1.42. It also believes that the flash from the lower target will reach the small moving ship at t = -30.15 + 30, a full 30 units of time after the targets were obliterated, at y = 27.

Let's stop here before we go any further. Do you agree with the conclusions I have reached so far?

the first paragraph i agree with -

the second paragraph i do not agree with - the train was a single object - (see posts 25 and 28 )

 

[Muphrid]

take a look at post 25 (my post) and then at post 28 (doc al's post )

by making the the targets separate you are trying to avoid this situation

and if george is right that this was discussed before and doc al admitted the video was not accurate then - then why when i said it this time why did he say it was accurate?

Because you made it sound like you were saying "let's assume the strikes happen at the same time in the train's frame of reference". The response was consistent with this statement, not realizing that you were implicitly contradicting that the strikes were meant to be simultaneous in the frame of the platform observer.

The point you're missing is that when we say the "train's" frame of reference, we mean "a reference frame going with the same velocity of the train observer". Because the train is a rigid object, we don't often distinguish between the frame of the train and the frame of the train observer, but when asking about simultaneity, it's much clearer to go back to the observer, not the train as an extended object.

Again, it is not important that the strikes actually hit the train (instead of, say, two adjacent points on the ground or two points on another passing train). All that matter are the position and time that the light from these lightning strikes originates from--the sources of these light rays are single points in spacetime.

So yes, I am trying to avoid the situation because you keep attributing false meaning to both strikes hitting the train. So, to further probe this point, perhaps you can clarify for me what the difference between the following three scenarios would be in your opinion:

a) The two strikes hit the train simultaneous in the platform observer's frame
b) The two strikes hit points on the platform that are also equidistant from the platform observer and simultaneous in the platform observer's frame
c) The two strikes hit points on a second passing train (with different velocity from the firs train) that are also equidistant from the platform observer and simultaneous in the platform observer's frame

Do any of these scenarios imply something about simultaneity in the reference frame of the train observer that the others do not? As I've made abundantly clear by now, I see no difference between these three scenarios.

the second paragraph i do not agree with - the train was a single object - (see posts 25 and 28 )

What about that do you disagree with? Nothing in that paragraph changes if the moving ship is tethered to the targets. That paragraph only consists of statements about what the stationary other ship thinks the moving ship will see.

 

[solarflare]

the point is that the stationary observer is seeing an event where the ship ACTUALLY was hit simultaneously in that frame.

if your saying it doesn't matter then you are sayin reality has nothing to do with it - and if you say that then you also say relativity has nothing to do with reality

 

[solarflare]

if they hit the platform and not the train then by the time the flashes move to where the train observer will see them - she will no longer be equidistant from the flashes- therefore she will see them separately.

 

[solarflare]

look at my ship scenario

and instead of having the targets on middle ship have them on the stationary one

the big ship fires its lasers when the middle ship is in the centre - but by the time the flashes hit the stationary ship and move to meet the small ship - the small ship will no longer be equidistant from each flash.

 

[solarflare]

Again, it is not important that the strikes actually hit the train (instead of, say, two adjacent points on the ground or two points on another passing train). All that matter are the position and time that the light from these lightning strikes originates from--the sources of these light rays are single points in spacetime.

So yes, I am trying to avoid the situation because you keep attributing false meaning to both strikes hitting the train. So, to further probe this point, perhaps you can clarify for me what the difference between the following three scenarios would be in your opinion:

------------------------------------------------------------------------------------------------------------------the things you say are not important - are

this is what i was saying about picking and choosing data - you can not decide what parts are and what are not important

 

[solarflare]

Not true. (Unless those strikes are simultaneous in the platform frame.)


The strikes occur simultaneously in the platform frame.

Don't keep flipping back and forth between the two physically different scenarios:
(1) The lightning strikes are simultaneous in the platform frame.
(2) The lightning strikes are simultaneous in the train frame.

They cannot both be true. Stick to scenario 1 (as in the video) until you understand it.

my ship scenario shows just one physical scenario - but seen by two different observers

 

[Doc Al]

my ship scenario shows just one physical scenario - but seen by two different observers

Even though I think it's a waste of time, I will ask you to describe your ship scenario in words. Your diagram is not self-explanatory.

Why do I think it's a waste of time? Because the train scenario is very easy to analyze yet you resist it. And it illustrates the relativity of simultaneity nicely.

And even after progress is made (if it ever is) with your ship scenario, you'll still have to go back and understand the Einstein train scenario.

 

[solarflare]

You should take that quote to heart. You've been repeatedly proven wrong in this thread.

1) it was you that said the video was accurate - then you admitted it wasnt
2) george now says he believes the video is wrong
3) when the video was shown to have flaws you wanted to move the discussion away from
the video

i would say it was the other way around

 

[Doc Al]

if they hit the platform and not the train then by the time the flashes move to where the train observer will see them - she will no longer be equidistant from the flashes- therefore she will see them separately.

Again you miss the point.

From the viewpoint of the platform frame, the light flashes reach her at different times because she is moving. This has nothing to do with the lightning hitting the train instead of the platform.

From her train frame viewpoint she is always equidistant from the flashes. The speed of light with respect to her is constant. Yet she agrees that the light flashes reach her at different times.

 

[Doc Al]

1) it was you that said the video was accurate - then you admitted it wasnt
2) george now says he believes the video is wrong
3) when the video was shown to have flaws you wanted to move the discussion away from
the video

i would say it was the other way around

How about we deal the the actual Einstein train experiment, which has been described very clearly--and repeatedly--in this thread. (Forget the poor attempts in the video.)

(So far, most of the problems with the video were pointed out by George, not you. Most of your objections seem based on your misunderstandings of relativity. Unfortunately, the video's sloppiness doesn't help.)

 

[Doc Al]

3) when the video was shown to have flaws you wanted to move the discussion away from
the video

You're right about that! I wanted to discuss Einstein's thought experiment, not someone's mistaken description of it. And I thought that's what you were interested in also. Most of the statements you made were independent of the video.

Alas, it seems that you were attempting to learn relativity from that (admittedly sloppy) video, which is unfortunate. (Perhaps that explains the misconceptions that you've posted.)

If you want to discuss the flaws of the video, first demonstrate that you understand the real Einstein thought experiment and the relativity of simultaneity. (Your statements in this thread show that you do not.) Then you are free to pick apart the video, like George did.

Several folks have gone to great lengths in this thread to describe the (real) Einstein train scenario and point out your errors. I strongly suggest you take advantage of that. It's easy! (But subtle.)

 

[solarflare]

How about we deal the the actual Einstein train experiment, which has been described very clearly--and repeatedly--in this thread. (Forget the poor attempts in the video.)

(So far, most of the problems with the video were pointed out by George, not you. Most of your objections seem based on your misunderstandings of relativity. Unfortunately, the video's sloppiness doesn't help.)

so let me get this right - i say the video is wrong

george says I am wrong - along with many others

i explain why i think the video is wrong - you say I am wrong the video is accurate -

george then quotes me - and then says - now i understand what you mean -

george then goes on to explain what is wrong with the video
( admittedly beter then me )

you then agree the video in not accurate ( agreeing with what i said at the beggining )

you then want to move away from the wrong video - and make a different scenario

--------------------------------------------------------------------------------------

i could see the flaws in the video - and you could not -

if i did not understand RoS then i would not have seen the flaws.