Trying to Understand Light in Motion: A Frustrating Puzzle

[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.

 

[Doc Al]

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

If you understood RoS then you would not have made most of the statements you've made in this thread. You have not demonstrated any such understanding, at least as far as I could make out. Most of your statements are independent of the video.

 

[Doc Al]

How about this: Forget the video. Just describe Einstein's train experiment correctly. Show how the two frames disagree about the lightning strikes being simultaneous (and how that follows from the invariance of the speed of light).

I recommend that you start with the events that I have outlined in post #155.

 

[solarflare]

mathematics is like building a house - the scenario is like the foundation

you can build a perfect - strong house

but if you build it on the wrong foundation it will still collapse

 

[Doc Al]

mathematics is like building a house - the scenario is like the foundation

you can build a perfect - strong house

but if you build it on the wrong foundation it will still collapse

And your point is?

 

[solarflare]

look at my ship scenario

the middle ships targets have just been hit and send out light from the explosions

the ships pilot reports that the two targets were destroyed simultaneously because they occurred on his ship

but what does the stationary observer see

 

[solarflare]

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.

 

[Doc Al]

look at my ship scenario

the middle ships targets have just been hit and send out light from the explosions

the ships pilot reports that the two targets were destroyed simultaneously because they occurred on his ship

but what does the stationary observer see

Please describe your ship scenario clearly. I don't understand your diagram. (Where are the ships? Where are the targets? How are they moving?)

 

[solarflare]

the event occurs in the frame of the moving ship

the stationary observer see the flashes from that event

 

[solarflare]

the middle ship is moving in a direction from south to north as you look at it

the lasers are fired from west to east as you look at it

the targets are the 0 at the front of the ship and the 0 at the rear
if you look at the diagram it looks like this --------0----------- the target is the 0
-----------------------------------------------/
----------------------------------------------I ------- this is the position of the pilot

the ship is moving at 500 m/s from south to north

the two targets are hit simultaneously in the little ships frame

 

[solarflare]

the stationary ship is equidistant from the explosions when they happen

 

[Doc Al]

the event occurs in the frame of the moving ship

Events don't occur in a specific frame, they occur in all frames.

I don't understand your ship scenario, but if the events in question are separated along the x-axis and the moving ship moves along the y-axis, then there's no issue. Both frames will view them as simultaneous.

 

[Dale]

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.

I am going to call BS on this one. Nothing that you said above indicates that you were aware of any of the actual flaws in the video. Your mistakes appear to be entirely your own and not an identification nor a repetition of any flaws in the video.

 

[Dale]

mathematics is like building a house - the scenario is like the foundation

you can build a perfect - strong house

but if you build it on the wrong foundation it will still collapse

I am still waiting for you to provide a mathematical proof of any of your points or to identify a specific flaw in the math that I have presented or the assumptions I used. If the house is weak or the foundation wrong then you should be able to identify the specific flaw, particularly since you are so much more amazingly perceptive than all of the rest of us . If you cannot prove your own point, then you at least need to directly address the disproof.

 

[Dale]

the event occurs in the frame of the moving ship

Doc Al already mentioned that events occur in all frames. To expand on that, an event occurs in all frames, but each frame assigns that same event to different time and space coordinates. The equation which relates the coordinates assigned to any arbitrary event in one frame to the coordinates of the same event in another frame are given by the Lorentz transform:

http://en.wikipedia.org/wiki/Lorentz_transformation

 

[Dale]

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.

Oh, almost missed this one. It looks like you now once more agree with my point 6 here:
https://www.physicsforums.com/showpost.php?p=4032799&postcount=127

Can you confirm your agreement or explain why you don't agree even though your recent comments seem to indicate agreement?

 

[Muphrid]

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

The stationary other ship is seeing two events that happen to be simultaneous in the reference frame that he is stationary.

As yet, I haven't posted the complete conclusions. I just want to make sure we agree on the math. Read what I posted again (which I'm quoting so you don't need to flip back and forth). The stationary ship believes both targets were destroyed at the same time $t = -30.15$. It also believes that the flashes from those two events will reach the moving ship at different times. I have yet to calculate whether the moving ship will calculate that the two events took place at the same time in its frame because I want us to agree on these very basic principles first.

If you believe that connecting the two targets to the moving ship makes a difference, how does it make a difference to what has been calculated so far? Do you think that means the moving ship will receive the flashes at times other than I have calculated (times in the stationary other ship's frame)?

                                    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?

 

[Muphrid]

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.

No, no, absolutely not. The sources of the flashes do not "move with" the object the lightning/lasers hit. The sources are points in space and time. They do not move. They do not have worldlines. When the train observer sees the flashes, she is looking into the past, to the places and times the strikes hit, not to where the point of impact actually has moved to. That is why we keep saying it does not matter if the strikes hit the train or the platform.

In other words, it is only the platform observer who thinks the train observer has caught up to the forward flash and was closer to it. As long as the platform observer and the train observer were right beside each other and the events were equidistant from the platform observer, the events must be equidistant from the train observer (in her frame) whether or not the platform or the train was actually struck.

So this is the chain of reasoning:

a) The train observer must believe herself to be equidistant from the sources of the flashes (yes, even if the sources were lightning strikes that hit the platform, not the train)
b) The platform observer sees that the train observer must detect the forward flash before the rear flash; this sequencing of events is something all observers must agree upon because it's the difference between two points in space and time
c) Because of (b), we know the train observer detects the forward flash before the rear flash. Because of (a), she can't attribute the difference in times to a difference in distance of the sources. Therefore, she attributes the difference in times to an actual difference of simultaneity, saying the forward strike happened before the rear strike.

 

[Chestermiller]

No, no, absolutely not. The sources of the flashes do not "move with" the object the lightning/lasers hit. The sources are points in space and time. They do not move. They do not have worldlines. When the train observer sees the flashes, she is looking into the past, to the places and times the strikes hit, not to where the point of impact actually has moved to. That is why we keep saying it does not matter if the strikes hit the train or the platform.

In other words, it is only the platform observer who thinks the train observer has caught up to the forward flash and was closer to it. As long as the platform observer and the train observer were right beside each other and the events were equidistant from the platform observer, the events must be equidistant from the train observer (in her frame) whether or not the platform or the train was actually struck.

So this is the chain of reasoning:

a) The train observer must believe herself to be equidistant from the sources of the flashes (yes, even if the sources were lightning strikes that hit the platform, not the train)
b) The platform observer sees that the train observer must detect the forward flash before the rear flash; this sequencing of events is something all observers must agree upon because it's the difference between two points in space and time
c) Because of (b), we know the train observer detects the forward flash before the rear flash. Because of (a), she can't attribute the difference in times to a difference in distance of the sources. Therefore, she attributes the difference in times to an actual difference of simultaneity, saying the forward strike happened before the rear strike.

This is to elaborate on Muphrid's answer for solarflare.

If there were additional train observers on board, not just at the center of the train, but also at the actual locations and times at which the lightning strikes occurred, and if these observers had clocks that were synchronized with the clock of the woman at the center of the train, then the time displayed on the clock of the forward observer when the forward strike occurred would be earlier than the time displayed on the clock of the rear observer when the rear strike occurred. So, as far as everyone on the train is concerned, the forward strike occurred first, and then the rear strike (according to all the synchronized clocks on the train). If the woman is half way between the forward and rear observers, she will receive the flash from the forward strike first, followed by the flash from the rear strike. This describes what is observed on the train whenever a similar set of 3 platform observers note using their synchronized clocks (i.e., synchronized within their frame of reference) that the two lightning strikes occurred simultaneously. This is a reality of special relativity, of the Lorentz Transformation, and an experimentally confirmed reality of our actual universe.

 

[bahamagreen]

Just some thoughts that might help (assuming I'm not in error)...

The train itself is not the same as the frame of the train, nor is an observer on the train necessarily in the frame of the train.
The platform itself is not the same as the frame of the platform, nor is a platform observer necessarily in the frame of the platform.

The train may be observed from the platform in either the frame of the platform or the frame of the train (if the platform observer is moving wrt the platform so to be at rest wrt to the train).
Likewise, the platform may be observed from the train in either the frame of the train or the frame of the platform (if the train observer is moving wrt the train so to be at rest wrt to the platform).
In fact, any observer might be in any arbitrary frame whatsoever. It all depends on precisely how the experiment is stated.

I think part of what is going on is that folks are saying that because the platform observer in the platform frame observes the two strikes at the same time in his frame, he will conclude that the train observer in relative motion wrt the platform MUST see the forward strike before the rear strike... and since she is in the center of the train, she must conclude that the strikes were separated in time.
This might be misunderstood as giving the platform observer the "correct" view of events, and the train observer observing an "illusion"; especially if the reasoning is that her mistake is a natural one because of her motion which intercepts the forward flash prior to the rear flash... as if the train strikes really were together in time, but the motion of the train forward causes the difference in arrival.

So a possible confusion might stem from assuming that the strikes at the same time in the platform frame entail that the train itself received strikes at the same time in the train frame. The reasoning for this would be that the light propagates at constant speed independent of the motion of the source... as if it did not matter that the train was moving wrt the platform observer or not. This is a confusion between the train and its frame... if the train was not in motion relative to the platform then the train frame and platform frame would be the same. The frames are separate as a result of the relative motion between them.

The effect of relative motion between frames shows in the relationship between the source location of a light flash and the center of its subsequent sphere of expansion... observed from within one's own frame, all observers will observe spheres of the same time radius to contain the flash source's original location of the flash at the center of the sphere. This is the same as saying that all observers agree on c... but it may help to see why observers in different frames disagree about other observations.

The platform observer will see the flash source locations hold still in his frame... the forward and rear ends of the train just happen to be at these locations as it moves by... and it keeps on moving past them. This makes him conclude that the train observer will see one flash before the other.
The train observer in her frame will see the flash source locations hold still in her frame, too... the same forward and rear ends of the same train itself, but for her these source locations ARE the ends of the train and continue to be so even as it continues to roll along because she is at rest wrt the train.

So, the two frames disagree about the flash source locations and those source positions with respect to the subsequent flash sphere centers.

 

[cepheid]

Just some thoughts that might help (assuming I'm not in error)...

The train itself is not the same as the frame of the train, nor is an observer on the train necessarily in the frame of the train.
The platform itself is not the same as the frame of the platform, nor is a platform observer necessarily in the frame of the platform.

What we're defining as the "train reference frame" is the *rest frame* of the train. In other words, it's the inertial reference frame in which the train is stationary.Therefore, any observer who is NOT moving relative to the train is in the "train frame" (i.e. that observer's rest frame is the same as the train's). This obviously includes the observer on the train in our scenario.

EDIT: Likewise for the platform. When we say that an observer is "in the platform frame", we mean that that observer is at rest relative to the platform. So both the platform and the "platform observer" are "in the platform frame" by definition.

 

[Muphrid]

And this is a key point about reference frames. Objects can be stationary or moving with respect to a frame, but events (like the lightning strikes we've been talking about) are not objects and they do not move, and the distances measured to events are simply the distance light travels to reach an observer (a distance which depends on the particular frame chosen, of course).

 

[solarflare]

No, no, absolutely not. The sources of the flashes do not "move with" the object the lightning/lasers hit.
The sources are points in space and time. They do not move. They do not have worldlines. When the train observer
sees the flashes, she is looking into the past, to the places and times the strikes hit, not to where the point of
impact actually has moved to.That is why we keep saying it does not matter if the strikes hit the train or the
platform. (taken from post 267)Originally Posted by solarflare View Post

let me ask the same question but in definate terms.

the two bolts of lightning do actually strike at the same time in the trains frame of reference

the observer on the platform is exactly the same distance from each strike

does the woman see the two strikes at the same time?

Sure. The lightning struck the ends at the same time in her frame and since they traveled the same distance they
reach her at the same time. No mystery there. (taken from post 28) (doc al)maybe doc al can explain to murprid why she will see them simultaneously if they strike the train simultaneously?

 

[solarflare]

now on the other hand if the flashes hit the tracks and not the train - then yes she would see them seperately because the train and the tracks are moving at different speeds in her frame.

 

[Doc Al]

Originally Posted by solarflare View Post

let me ask the same question but in definate terms.

the two bolts of lightning do actually strike at the same time in the trains frame of reference

the observer on the platform is exactly the same distance from each strike

does the woman see the two strikes at the same time?

Sure. The lightning struck the ends at the same time in her frame and since they traveled the same distance they
reach her at the same time. No mystery there. (taken from post 28) (doc al)maybe doc al can explain to murprid why she will see them simultaneously if they strike the train simultaneously?

It is you, solarflare, who are confused not Muphrid.

(A) If the lightning strikes hit the train ends simultaneously in the platform frame, the light flashes from each will reach the platform observer at the same.

(B) If the lightning strikes hit the train ends simultaneously in the train frame, the light flashes from each will reach the train observer at the same.

These are two physically distinct scenarios. They cannot both apply to the same situation--it's one or the other, not both.

The one we've been discussing is scenario A.

 

[Doc Al]

now on the other hand if the flashes hit the tracks and not the train - then yes she would see them seperately because the train and the tracks are moving at different speeds in her frame.

You are still stuck with the idea that if the lightning strikes hit the train that they become simultaneous in the train frame but if they hit the tracks they would be simultaneous in the track frame. This is completely wrong, as has been explained in great detail throughout this (comically long) thread.

 

[solarflare]

you seem to forget that the video says that the platform observer was equal distance from each strike

the only way the platform observer can see two simultaneous strikes is if the strikes happen seperately at DIFFERENT distances

 

[Doc Al]

you seem to forget that the video says that the platform observer was equal distance fron each strike

Right.

the only way the platform observer can see two simultaneous strikes is if the strikes happen seperately at DIFFERENT distances

You seem to forget that the strikes were simultaneous in the platform frame. So of course he sees the flashes reach him at the same time.

 

[solarflare]

the video is saying that an event happens

and then describes what BOTH observers see from that event

the video is using both frames.

look at the last paragraph of the transcript

 

[solarflare]

so two separate lighteneing strikes happen in the trains frame

the first strike hits the rear - the light begins to move towards the platform

then - the front strike happens when the light from the rear strike reaches the front of the train

now - both light beams are traveling towards the platform together and so the platform observer sees the strikes simultaneously

that is what the video should have shown

 

[Doc Al]

the video is saying that an event happens

and then describes what BOTH observers see from that event

the video is using both frames.

look at the last paragraph of the transcript

Yes, the events happen. Yes, they can be described from both frames.

So what?

The last paragraph of the transcript is:

Whose interpretation is correct - the observer on the platform, who claims that the strikes happened simultaneously, or the observer on the train, who claims that the front strike happened before the rear strike? Einstein tells us that both are correct, within their own frame of reference. This is a fundamental result of special relativity: From different reference frames, there can never be agreement on the simultaneity of events.​

Sounds good to me. What's the problem?