Trying to Understand Light in Motion: A Frustrating Puzzle

[solarflare]

response to post 313

so she sees them simultaneously because they came from the train

so the light leaves the front of the train and the rear of the train simultaneously

each light pulse travels the same distance to the platform observer so he must see them simultaneously also

or do you say that the platform observer moves towards one flash and away from the other so he will see them seperately

 

[Doc Al]

response to post 313

so she sees them simultaneously because they came from the train

No, the light from both bulbs reaches her simultaneously because they flashed simultaneously.

so the light leaves the front of the train and the rear of the train simultaneously

Yes, in her frame.

each light pulse travels the same distance to the platform observer so he must see them simultaneously also

Not true at all.

or do you say that the platform observer moves towards one flash and away from the other so he will see them seperately

Of course.

 

[Doc Al]

each light pulse travels the same distance to the platform observer so he must see them simultaneously also

Let me expand on my previous answer:

Do the light pulses travel the same distance to the platform observer?
- According to the train observer, they do not.
- According to the platform observer, they do.

Do the light pulses reach the platform observer at the same time?
- No.

 

[solarflare]

B) The train passenger also knows that the platform observer is moving with respect to her and should be hit by the rear flash of the bulb before the front flash of the bulb.

this concept is wrong – the light moves out like an orb from the train – what you are saying is that the light is coming from the left and right side of the observer on the platform but it is not. The platform guy can only move towards the light if he moves along the path the light is coming at him from. Once they leave the moving train simultaneously they are not affected by any velocity of the train.

C) Both the train passenger and the platform observer must agree that he was hit by the rear flash before the front flash; they need not agree on why, but relativity demands that they agree on something physically meaningful, as the sequencing of when the flashes reach him is

how will the passenger know when the light reaches the platform observer – we follow it by use of orbs expanding to show the lights progress – but she can only see the platform guy after the light has reflected back after hitting him – not when it hits him – therefore any thing she sees cannot be used to say as a fact. She may believe it but it is not necessarily true. If you watch a live football match on TV – you will be seeing a delayed transmission. The cameraman will see what you see but he will see it before you. The cameraman and you will both agree what happened in the match even though you have different inertial frames of reference. (assuming you watched the match only from his camera) The only thing you will disagree on is what time you saw it. Simple maths will tell you that if you have two distances that are equal and something moves at the same speed they will cover that distance in the same time. My main issue is that the train strikes cannot be equidistant from the platform observer if he sees them simultaneously. One strike happens – the rear one (that is further away from the platform)– the light then begins its journey towards the platform then the closer strike happens . Then they both move together and reach the platform guy simultaneously.

 

[Muphrid]

B) The train passenger also knows that the platform observer is moving with respect to her and should be hit by the rear flash of the bulb before the front flash of the bulb.

this concept is wrong – the light moves out like an orb from the train – what you are saying is that the light is coming from the left and right side of the observer on the platform but it is not. The platform guy can only move towards the light if he moves along the path the light is coming at him from. Once they leave the moving train simultaneously they are not affected by any velocity of the train.

The light isn't affected by the motion, of course. Nevertheless, the platform observer is moving toward the rear of the train. He is equidistant from the sources at the time the orbs have yet to expand, but as they expand, he is moving toward the rear and must be struck by the rear expanding sphere of light first.

This is an easy calculation. Put two lightbulbs at $x = \pm 4$ units and give the platform observer velocity $\beta = 1/3$ toward the rear of the train. If both bulbs go off at $t= 0$, then after 3 units of time, the forward flash has expanded at most to $x = 1$, the rear flash has reached $x = -1$. But $x = -1$ is where the platform observer is at $t=3$ because of his velocity.

This does not contradict the notion that he was equidistant from the bulbs. He was equidistant at $t=0$, and that is what's important.

C) Both the train passenger and the platform observer must agree that he was hit by the rear flash before the front flash; they need not agree on why, but relativity demands that they agree on something physically meaningful, as the sequencing of when the flashes reach him is

how will the passenger know when the light reaches the platform observer [...]

Because at the time both flashes have reached her, the platform observer is further toward the rear of the train than her, and the forward flash from the bulb cannot possibly have reached the platform observer yet.

 

[ghwellsjr]

B) The train passenger also knows that the platform observer is moving with respect to her and should be hit by the rear flash of the bulb before the front flash of the bulb.

this concept is wrong – the light moves out like an orb from the train – what you are saying is that the light is coming from the left and right side of the observer on the platform but it is not. The platform guy can only move towards the light if he moves along the path the light is coming at him from. Once they leave the moving train simultaneously they are not affected by any velocity of the train.

C) Both the train passenger and the platform observer must agree that he was hit by the rear flash before the front flash; they need not agree on why, but relativity demands that they agree on something physically meaningful, as the sequencing of when the flashes reach him is

how will the passenger know when the light reaches the platform observer – we follow it by use of orbs expanding to show the lights progress – but she can only see the platform guy after the light has reflected back after hitting him – not when it hits him – therefore any thing she sees cannot be used to say as a fact. She may believe it but it is not necessarily true. If you watch a live football match on TV – you will be seeing a delayed transmission. The cameraman will see what you see but he will see it before you. The cameraman and you will both agree what happened in the match even though you have different inertial frames of reference. (assuming you watched the match only from his camera) The only thing you will disagree on is what time you saw it. Simple maths will tell you that if you have two distances that are equal and something moves at the same speed they will cover that distance in the same time. My main issue is that the train strikes cannot be equidistant from the platform observer if he sees them simultaneously. One strike happens – the rear one (that is further away from the platform)– the light then begins its journey towards the platform then the closer strike happens . Then they both move together and reach the platform guy simultaneously.

The light isn't affected by the motion, of course. Nevertheless, the platform observer is moving toward the rear of the train. He is equidistant from the sources at the time the orbs have yet to expand, but as they expand, he is moving toward the rear and must be struck by the rear expanding sphere of light first.

This is an easy calculation. Put two lightbulbs at $x = \pm 4$ units and give the platform observer velocity $\beta = 1/3$ toward the rear of the train. If both bulbs go off at $t= 0$, then after 3 units of time, the forward flash has expanded at most to $x = 1$, the rear flash has reached $x = -1$. But $x = -1$ is where the platform observer is at $t=3$ because of his velocity.

This does not contradict the notion that he was equidistant from the bulbs. He was equidistant at $t=0$, and that is what's important.
Because at the time both flashes have reached her, the platform observer is further toward the rear of the train than her, and the forward flash from the bulb cannot possibly have reached the platform observer yet.

This is not the scenario under discussion in this thread. Can we please drop these other scenarios until we settle the issues with the train scenario described by Einstein and illustrated in the animation? Please?

 

[Muphrid]

My main issue is that the train strikes cannot be equidistant from the platform observer if he sees them simultaneously. One strike happens – the rear one (that is further away from the platform)– the light then begins its journey towards the platform then the closer strike happens . Then they both move together and reach the platform guy simultaneously.

So now we're in the lightning example again.

I really don't see why you think this is. That both events are equidistant from the platform observer is presented as a given piece of information, something no one should have to deduce. There are many ways he can determine the distance, from putting lines on the train to mark off distances to figuring out something about the brightness of the strikes or the sizes of the char marks the strikes leave behind. It is a given piece of information. Why do you think it cannot be?

Edit: if the train is moving left to right, do you not agree that the rear flash comes from the platform observer's left and the forward flash from his right?

 

[solarflare]

the only way the platform observer will move towards the light is if you say the light moves at C+V towards the platform observer. the platform observer cannot move towards the light unless he moves on a different axis.

once it happens it does not move with the train and therefore will be classed as stationary - but expanding outwards - towards both the train and the platform.

 

[ghwellsjr]

Solarflare: can you see in the animation that the platform observer sees both flashes hit him at 2 seconds according to his clock and that light from the front flash hits the train observer at 1 second according to her clock and the light from the rear flash hits her at 6 seconds according to her clock and that both frames display the same information?

 

[Muphrid]

the only way the platform observer will move towards the light is if you say the light moves at C+V towards the platform observer. the platform observer cannot move towards the light unless he moves on a different axis.

once it happens it does not move with the train and therefore will be classed as stationary - but expanding outwards - towards both the train and the platform.

On a different axis from what?

Never said anything of the kind about light moving faster than $c$. The flashes from both events move at $c$. Nevertheless, most observers tend to have velocities and can move toward an expanding "sphere of light". The only constraint that relativity imposes is that in an observer's own frame, the observer measures the speed of light to be $c$.

The whole point of the thought experiment is that someone is moving toward an expanding light wavefront and moving away from another wavefront in the same way. We impose that that person still measure $c$ to be the same in their frame, though, and this is what leads us to conclude the different arrival times are caused by relative simultaneity instead of relative velocity with respect to the light.

 

[solarflare]

the point is that you continue to look at the light with the motion of the train after the strikes - you cannot because the light will be classed as stationary in both frames. therefore the platform observer will not have any relative motion to the source.

 

[Muphrid]

the point is that you continue to look at the light with the motion of the train after the strikes - you cannot because the light will be classed as stationary in both frames. therefore the platform observer will not have any relative motion to the source.

Of course he has no relative motion to the source. The sources are points that may as well wink in and out of existence as soon as they've put out their pulses of light.

You don't mean the light is stationary. We already talked about expanding spheres and stuff.

You're taking the idea that all observers measure the same speed of light too far. Any given observer can perceive other objects as moving toward or away from an expanding sphere of light such that those objects' "relative velocity" to the expanding sphere can be greater or less than $c$. This is fine (well, properly defined it's fine). Forcing that all observers perceive the speed of light to be the same in their respective frames does not change that statement. What it does change are their respective notions of simultaneity and distance, which resolves the apparent contradiction.

Have you looked at ghwellsjr's animation? I too think this neatly resolves what's going on in this lightning scenario.

 

[solarflare]

if you want to say that there is relative motion then the source would move with the train carrying the whole orb of light with it meaning that the light moving towards the front of train is traveling at C+V and the light moving at C-V away from the rear of the train

 

[Doc Al]

if you want to say that there is relative motion then the source would move with the train carrying the whole orb of light with it meaning that the light moving towards the front of train is traveling at C+V and the light moving at C-V away from the rear of the train

No, the light just moves at speed C. If the light moves to the left at speed C and the passenger moves to the right at speed V, then they will get closer at a rate of C + V as seen by platform observers. But the speed of light hasn't changed. (And certainly nothing is moving at a speed of C + V.)

 

[Muphrid]

if you want to say that there is relative then the source would move with the train carrying the whole orb of light with it meaning that the light moving towards the front of train is traveling at C+V and the light moving at C-V away from the rear of the train

No, emphatically no. The sources are gone as soon as they put out their pulses of light. They do not persist. They do not move or go anywhere in any frame. They are gone as soon as the pulses begin to expand.

Lightning example:
The platform observer perceives two pulses of light moving at speed $c$ from opposite directions that hit him at the same moment. He believes the train passenger has velocity $c+v$ with respect to the forward flash and $c-v$ with respect to the rear flash.

Nevertheless, the train passenger will believe her relative velocity with respect to both flashes is $c$. There is no contradiction here.

 

[solarflare]

it seems to me that the platform observer sees the second flash before it even hits the train on that animation

 

[Muphrid]

it seems to me that the platform observer sees the second flash before it even hits the train

Edit: responded before "in that animation" was added.

What do you mean "before it even hits the train"? The sphere of light expands only after the lightning bolt hits the rear of the train.

 

[Dale]

All I need to say is that the points that were hit have specific coordinates, and all light rays from these points will go along well-defined trajectories. Do you agree that this is what happened?

In keeping with this, let's say that the coordinates of the point hit was $(x_0,y_0,z_0)$ and that it hits at time $t_0$. Then, given that, the "well-defined trajectory" for the light rays is given by:
$c^2(t-t_0)^2=(x-x_0)^2+(y-y_0)^2+(z-z_0)^2$

Which is the equation of a sphere of radius $c(t-t_0)$ centered on $(x_0,y_0,z_0)$. Do you agree with that?

 

[Muphrid]

In keeping with this, let's say that the coordinates of the point hit was $(x_0,y_0,z_0)$ and that it hits at time $t_0$. Then, given that, the "well-defined trajectory" for the light rays is given by:
$c^2(t-t_0)^2=(x-x_0)^2+(y-y_0)^2+(z-z_0)^2$

Which is the equation of a sphere of radius $c(t-t_0)$ centered on $(x_0,y_0,z_0)$. Do you agree with that?

solarflare was quoting me when he said that (about light rays having well-defined trajectories), and I'm not sure he ever agreed with it.

 

[ghwellsjr]

Have you looked at ghwellsjr's animation? I too think this neatly resolves what's going on in this lightning scenario.

It's not my animation, it was made by yuiop as I said in post #235.

it seems to me that the platform observer sees the second flash before it even hits the train on that animation

I think you must be looking at both frames at the same time. Don't do that. First look at just one and then later on you can look at the other one.

The platform frame is the one on top. The train frame is the one on the bottom. If you look at either one all by itself, you will see that the flash hits the rear of the train before the platform observer sees it.

 

[Dale]

solarflare was quoting me when he said that (about light rays having well-defined trajectories), and I'm not sure he ever agreed with it.

&$@?#!

solarflare, USE THE QUOTE FEATURE!

If you aren't smart enough to figure that out, how do you think you can possibly convince anyone that you are smarter than Einstein, Feynman, Hawking, etc. Hint, it's the button labeled "quote" in the bottom right hand corner of every single post on this forum.

 

[cepheid]

Huh?

Chestermiller is specifying that the result of this experiment is that the platform observer sees the flashes occurring at the same time. Since they both struck at the same distance away, and both travel at c relative to him, the platform observer concludes that they must have occurred simultaneously.

so if in the trains frame the two strikes are simultaneous then the same must be true as
she considers herself stationary and the platform is moving

you seem to think that in both scenarios its only the train that is in motion

Of course I'm now three pages behind (EDIT: make that four) , but since you made this assertion, which, as with many of your assertions in this thread, is false, I feel compelled to address it.

First of all, they are not two different "scenarios." They are the same scenario, from two different points of view. This is a point that many people (Doc Al, DaleSpam, Muphrid, myself) have tried to make to you numerous times. Events in spacetime happen in all frames, only their coordinates (i.e. where and when they happened) differ from frame to frame. Secondly, NO, I don't think that the train is in motion in both frames. I don't think that at all, and I have no idea why you would think that I think that.

Here is what each of the observers would describe as having occurred:

Platform observer

"I know that when the forward strike occurred, it was the same distance ahead of me as the rear strike was behind me when it occurred. I also know that they both traveled at speed c relative to me. I saw them both at the same time. Therefore, since both had the same distance to travel at the same speed, I conclude that they occurred simultaneously. [Addressing the train observer:] I'm stationary, and you are moving forwards past me. Therefore you were approaching the source of the forward flash, and receding away from the source of the rear flash. That is why you saw the forward flash before you saw the rear one. The forward flash had a shorter distance to travel before reaching you than the rear one did."

Train observer

"I know that when the forward strike occurred, it was the same distance ahead of me as the rear strike was behind me when it occurred. I also know that they both traveled at speed c relative to me. Yet, I saw the forward flash before I saw the rear flash. This is in spite of the fact that both had to travel the same distance to get to me, and at the same speed. Therefore, I conclude that the flashes did NOT occur simultaneously: the forward one occurred first, and then the rear one occurred later. [Addressing the platform observer:] No, I'm stationary, and you are moving backwards past me. Therefore you were receding away from the source of the forward flash, and approaching toward the source of the rear flash. That is why you saw the two flashes arrive at you at the same time. Even though the forward flash struck the ground first, it had a longer distance to travel to reach you than the rear flash did. So, the forward flash, starting earlier, and traveling for a longer distance, reached you at the same time as the rear flash, which started later and traveled a shorter distance to get to you."

****************************************************************************

THIS is the scenario, solarflare. Whether you like it or not, this is what happens.

 

[Dale]

solarflare, although I was mis quoting you, I would still appreciate an answer to 333. Do you agree that is the correct form for a flash of light emanating from a given time and place? Do you think that equation is modified in any way if there is a moving object at the given time and place?

Btw, cepheid just posted an excellent summary which you should examine in detail.

 

[solarflare]

post 337

if the flashes occur simultaneously "on the train" the relative motion of the train will not affect what the passenger "on the train" will see. what you are describing is two strikes occurring on the tracks - which are stationary to the platform observer

now take the position of the train observer who sees two simultaneous flashes on the platform and she saw them simultaneously when she was at the centre of the platform equal distance from each strike.
occording to her the platform is moving - therefore she will say that the platform observer is moving towards one and so the platform observer will see one strike first and then the other. does the platform observer see them seperately??

the problem with equations is that they are useless if you are inputting the wrong data into them.

 

[ghwellsjr]

Yes, cepheid has posted an excellent summary and to help you examine it in detail, I'm going to post snapshots from the animation.

First of all, they are not two different "scenarios." They are the same scenario, from two different points of view. This is a point that many people (Doc Al, DaleSpam, Muphrid, myself) have tried to make to you numerous times. Events in spacetime happen in all frames, only their coordinates (i.e. where and when they happened) differ from frame to frame. Secondly, NO, I don't think that the train is in motion in both frames. I don't think that at all, and I have no idea why you would think that I think that.

In order to help you focus on one frame of reference at a time, I'm going to first show only the top half of the animation labeled "Point of view of the observer on track embankment" which is what we have been calling the platform frame and then I'm going to show only the bottom half of the animation labeled "Point of view of observer on the train" or the train frame.

Just in case you haven't already figured it out, the red bar is the platform observer, the blue bar is the train observer, the green bar is the location on the track where the lightning strikes the front of the train and the brown bar is the location on the track where the lightning strikes the rear of the train.

Here is what each of the observers would describe as having occurred:

Platform observer

"I know that when the forward strike occurred, it was the same distance ahead of me as the rear strike was behind me when it occurred.

 

[ghwellsjr]

I also know that they both traveled at speed c relative to me.

 

[ghwellsjr]

I saw them both at the same time.

Therefore, since both had the same distance to travel at the same speed, I conclude that they occurred simultaneously.

 

[ghwellsjr]

[Addressing the train observer:] I'm stationary, and you are moving forwards past me. Therefore you were approaching the source of the forward flash,

 

[ghwellsjr]

and receding away from the source of the rear flash. That is why you saw the forward flash before you saw the rear one. The forward flash had a shorter distance to travel before reaching you than the rear one did."

 

[solarflare]

if the strikes occur simultaneously "on the train" the train passenger will say she saw them simultaneously

if the platform observer was equal distance from each strike when they occurred simultaneously "on the train" that was in the centre of the platform when they occured. what will he see?

the light travels the same distance from each strike so he will see them simultaneously also

 

[solarflare]

if the train passenger was 1 light second away from the lightning strike on the train

would she see the flash after 1 second or 1 second minus the time she traveled forward before the light reached her?

 

[Doc Al]

if the strikes occur simultaneously "on the train" the train passenger will say she saw them simultaneously

I assume you mean "in the frame of the train". That's a different scenario than the one we are discussing, but yes, if the strikes are simultaneous in the train frame, the light will reach the passenger simultaneously.

if the platform observer was equal distance from each strike when they occurred simultaneously "on the train" that was in the centre of the platform when they occured. what will he see?

the light travels the same distance from each strike so he will see them simultaneously also

Since you keep repeating this silly argument over and over, I'm beginning to think you're just trolling.

For the nth time, the light obviously doesn't travel the same distance to the platform observer. He's moving! Unless you believe that the light travels infinitely fast, by the time the light reaches him he is no longer in the middle; he's moved closer to one flash and further from the other. So of course they reach him at different times. This is just basic kinematics. If you cannot grasp this, there is really no point in you worrying about relativity.

 

[solarflare]

or the platform guy could say - from my perspective you are in motion so to me it looks like you should catch up with the front and move away from the rear - but in reality you see the flashes at the same time also.

 

[Doc Al]

or the platform guy could say - from my perspective you are in motion so to me it looks like you should catch up with the front and move away from the rear - but in reality you see the flashes at the same time also.

The platform guy would say "From my perspective, you are in motion and thus are moving towards the front and away from the rear. The strikes did not occur simultaneously, but the light from each does reach you at the same time since the rear strike--the light from which has further to travel--happens first."

 

[ghwellsjr]

Train observer

"I know that when the forward strike occurred, it was the same distance ahead of me

as the rear strike was behind me when it occurred.