Trying to Understand Light in Motion: A Frustrating Puzzle

[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.

 

[Mentz114]

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

Yes, tautologous but true.

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

No he won't. Do you know how to interpret a space-time diagram ? If you can I'll post one that shows the scenario.

A,B are the lightning strikes, seen simultaneously by the train observer ( green worldline). The platform observer (light blue) does not see them simultaneously, despite being colocated with the train observer at the time of the strike.

 

[ghwellsjr]

I also know that they both traveled at speed c relative to me. Yet, I saw the forward flash

 

[ghwellsjr]

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.

 

[ghwellsjr]

No, I'm stationary, and you are moving backwards past me. Therefore you were receding away from the source of the forward flash,

 

[ghwellsjr]

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."

 

[solarflare]

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."

i see - so in order for one to see them simultaneously - and the other to see them seperately -------

they cannot be equal distances

 

[solarflare]

the world lines suggest that at t = 0

there are two simultaneous strikes "on the train"

the platform guy because he was equal distacnce from the strikes sees them simultaneously.

and the train observer sees them seperately because she is moving towards one and away from one

 

[solarflare]

if you say the strikes are on the train then A and B (where the strikes come from)on that grid means they actually srike the train simultaneously

 

[harrylin]

i see - so in order for one to see them simultaneously - and the other to see them seperately -------
they cannot be equal distances

In Einstein's thought experiment of the train, the flashes occur at equal distances. You can also see that in the animation of ghwellsjr. What about the math or the animation don't you understand? This example boils down to simply applying Δt=Δx/c.

Note: an easier to understand variant (as it's more straightforward) is presented here:
http://en.wikipedia.org/wiki/Relativity_of_simultaneity#The_train-and-platform_thought_experiment

The clocks at the ends are simply set at the same time when the light reaches them; that's how they are "synchronized".

 

[Doc Al]

if you say the strikes are on the train then A and B (where the strikes come from)on that grid means they actually srike the train simultaneously

The strikes do hit the train simultaneously--according to the platform frame, not the train frame.

Just because the strikes hit the train does make them simultaneous in the train frame, if that's what you are thinking.

Imagine that there are firecrackers inside the train at the ends. Do the firecrackers have to explode at the same time, just because they are on the train? Don't be silly.