Trying to Understand Light in Motion: A Frustrating Puzzle

[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.

 

[solarflare]

each strike is said to hit the train

strike A is at t=0

strike B is at t=0

yet they occurred at different times ?

 

[solarflare]

the world lines show all events for both observers not just one

all of them

it shows the actual event and then where and when each observer sees that event

 

[Doc Al]

each strike is said to hit the train

strike A is at t=0

t = 0 in the platform frame.

strike B is at t=0

t = 0 in the platform frame.

yet they occurred at different times ?

They occur at the same time in the platform frame. (Obviously!)

They occur at different times in the train frame.

 

[harrylin]

the world lines show all events for both observers not just one

all of them

it shows the actual event and then where and when each observer sees that event

For this example world lines are counterproductive, you only need simple math (= the topic here).

 

[solarflare]

if you put two poles separated by the length of the train and the poles get hit by the lightning then it all works out as you say

but by making the bolts hit the train it does not

 

[Doc Al]

the world lines show all events for both observers not just one

all of them

it shows the actual event and then where and when each observer sees that event

Yes, so it's unfortunate that you do not know how to read space-time diagrams.

But harrylin is right. There is no need for space-time diagrams, just simple math and logic.

 

[solarflare]

worldlines are not from one frame thay show all frames it shows the event

it shows the light moving from that event to the platform guy - where they reach him simultaneously at a time AFTER the event

it also shows the light moving from the source to the other observer

it is NOT one frame

 

[solarflare]

A and B are not where the platform observer see the light

 

[solarflare]

Yes, tautologous but true. 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.

A and B are the actual event

C is where the platform observer sees the light simultaneously ( or what is the point of C )

he does NOT see them simultaneously at A and B

 

[Doc Al]

worldlines are not from one frame


thay show all frames it shows the event

it shows the light moving from that event to the platform guy - where they reach him simultaneously at a time AFTER the event

it also shows the light moving from the source to the other observer

it is NOT one frame

If you knew how to read the space-time diagram, then you could certainly confirm that the two observers disagree about whether the lightning strikes were simultaneous.

The unprimed axes represent the platform frame; the primed, the train frame.

 

[solarflare]

unless the observer sees the event before the light reaches him

 

[Doc Al]

Solarflare: Instead of continuing to waste everyone's time going around in circles, do this:

Go back to post #155 and discuss things in terms of the specific events that I outlined.

No more nonsense. Refer to specific events as seen by specific frames.

 

[solarflare]

if A and B are not the source for the primed axis then how can you draw lines from them to the primed axis

 

[Dale]

if the flashes occur simultaneously "on the train"

As has been explained to you many times already, this is a physically different scenario than the usual one we have been discussing.

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??

Yes. For this physically different scenario the result is that the platform observer will see first one strike and then the other and will conclude that they were not simultaneous. I already showed the math for this. It is unambiguous.

 

[solarflare]

at the time an observer sees the light - the event has already happened before - if the person is equal distance from the source when they see them then they must have happened simultaneously at the source

 

[Dale]

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

Again, this is a different scenario, but yes.

if the platform observer was equal distance from each strike when they occured 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, the light does not travel the same distance to the platform observer. Do you understand the difference between the two distances you mention here? Under what conditions are those two distances equal? Which of those two distances determines the travel time for the light?

 

[solarflare]

its for both because lines are drawn to both from there

A and B are the strikes ( in real life so to speak )

C is where the observer sees the light from that event

and where the lines cross on the primed axis is where the primed frame will see the light from that event

 

[Dale]

I'm lost. What are A and B and C? Are we referring to some specific post?

 

[solarflare]

Since the events are placed along the axis of train movement, their time coordinates become projected to different time coordinates in the moving train's inertial frame. Events which occurred at space coordinates in the direction of train movement (in the stationary frame), happen earlier than events at coordinates opposite to the direction of train movement. In the moving train's inertial frame, this means that lightning will strike the front of the traincar before two observers align (face each other).

http://en.wikipedia.org/wiki/Relativity_of_simultaneity#The_train-and-platform_thought_experiment

 

[solarflare]

I'm lost. What are A and B and C? Are we referring to some specific post?

post 351

 

[solarflare]

before they align not when they are aligned

 

[Doc Al]

A and B are the actual event

C is where the platform observer sees the light simultaneously ( or what is the point of C )

he does NOT see them simultaneously at A and B

Realize that Mentz114's diagram in post 351 is referring to a different scenario in which the lightning strikes are simultaneous in the train frame.

C is where the train passenger sees the light simultaneously.

 

[Dale]

post 351

Thanks, And by the way, thanks for using the quote feature!

 

[solarflare]

Realize that Mentz114's diagram in post 351 is referring to a different scenario in which the lightning strikes are simultaneous in the train frame.

C is where the train passenger sees the light simultaneously.

that is true

but A and B are still the source for what both frames see

and C is the position of the observer when they see it - they do not see it at A and B

 

[solarflare]

to make it the complete reverse the strikes would have to be said to hit the platfom

and the graph still would show that the strikes hit the platform simultaneously yet the platform observer sees them seperately

 

[Doc Al]

that is true

but A and B are still the source for what both frames see

and C is the position of the observer when they see it - they do not see it at A and B

C is at the position of the train observer. C represents the event of the light from each end reaching the train observer.

So what?

 

[solarflare]

C is at the position of the train observer. C represents the event of the light from each end reaching the train observer.

So what?

so that means that A and B are the source of the light

 

[solarflare]

and as the sources are both at t = 0

then they must be simultaneous

 

[Doc Al]

and as the sources are both at t = 0

then they must be simultaneous

Simultaneous according to whom?

 

[solarflare]

I made this spacetime diagram a while ago, and I *think* it corresponds to the original scenario that is being discussed here. Maybe it will help. The "worldline" of the train (which is its path through spacetime) is clearly indicated, and it of course coincides with the t' axis:

The worldline of the photon that is coming in from the front (i.e. from the positive side) clearly intercepts the worldline of the train before (i.e. at a smaller value of t') the worldline of photon that is coming in from the rear (negative side) does.

The coordinate grid I've drawn is for the train observer, in the primed (t',x') coordinate system.

this was posted to show the videos worldline

the strikes are along the X axis both at t = 0

as they occur on the train - they must be simultaneous

the light then travels from the sources to the platform observer at t = 4

the lines also cross the t prime line but the x prime line is not used to show where they cross on the t prime line

if the x-axis used to show the strikes for both frames then t = 0 is where they originated from for the t prime line

this means that they struck the train simultaneously in the trains frame also but the train moves forward to meet the front strike and away from the rear

 

[Doc Al]

this was posted to show the videos worldline

Once again, you have switched scenarios. This diagram is for the standard scenario in which the lightning strikes are simultaneous in the platform frame.

the strikes are along the X axis both at t = 0

Yes, in the platform frame.

as they occur on the train - they must be simultaneous

The fact that they occur 'on the train' is irrelevant. They are simultaneous in the platform frame, not the train frame.

the light then travels from the sources to the platform observer at t = 4

Sure, it takes time for the light to travel to the platform observer.

the lines also cross the t prime axis but the x prime line is not used to show where they cross on the t prime line

You should be able to draw in the lines to see where and when the events (the lightning strikes) happen in the train frame.

 

[solarflare]

it is not irrelevant when the graph says they occur on the train simultaneously

and again the lines cross t prime from the source on the x-axis

if you want to say that t prime is relevant then the source of the lines that cross t prime must also be relevant

 

[Doc Al]

This thread is obviously going nowhere (and very slowly at that). So it's time to shut it down.

Solarflare: All your questions have been answered several times over. Repeating the same thing isn't going to help. If you're serious, I suggest getting an intro book on relativity. (Or you can read Einstein's own words--see George's link.) Then you can go back and read through this thread again.

Thanks to everyone who contributed their time and effort to this thread.