Trying to Understand Light in Motion: A Frustrating Puzzle

[Doc Al]

Here is the transcript for the first minute of the video devoted exclusively to the platform frame:

Imagine two observers, one seated in the center of a speeding train car, and another standing on the platform as the train races by. As the center of the car passes the observer on the platform, he sees two bolts of lightning strike the car - one on the front, and one on the rear. The flashes of light from each strike reach him at the same time, so he concludes that the bolts were simultaneous, since he knows that the light from both strikes traveled the same distance at the same speed, the speed of light. He also predicts that his friend on the train will notice the front strike before the rear strike, because from her perspective on the platform the train is moving to meet the flash from the front, and moving away from the flash from the rear.​

I agree, sloppy use of the term 'see' in the second sentence. And sloppy illustration of the light flash reaching the platform observer while the train just sits there.

 

[ghwellsjr]

Now here's where they repeat the sequence again but this time it is an animation and it shows something different than what they showed earlier. And note the platform observer is not present so we can't tell exactly when he is supposed to see the flashes of light. This is further complicated by the fact that they are panning the image from left to right so the perspective is changing making it impossible to know where the platform observer is. The last image is where they "light up" the passenger to show that she sees the front flash first:

 

[ghwellsjr]

And now with further panning, they show the rear flash arriving at the train passenger in the last of these images but note the lightning strikes have disappeared:

 

[Doc Al]

Why don't we scrap the video and just discuss the scenario that the video was trying (badly) to illustrate, which is what we've actually been discussing for the most part. It's the standard Einstein train thought experiment, of course.

A train passes by a person on the platform. At the instant the center of the train passes the person, lightning strikes both ends of the train according to the platform frame. (These are the two events that I label #1 and #2 in post 155.)

Solarflare, given this setup, can you comment on my statements in post #155?

 

[solarflare]

how can you say scrap the video when my whole point is that the video is wrong ?

 

[Dale]

Your statements are also wrong, independently of any errors in the video. Your original mistakes were not even involving the same scenario as the one in the video.

 

[Muphrid]

how can you say scrap the video when my whole point is that the video is wrong ?

The situation the video describes is essentially correct; we want to do away with it anyway because the small flaws do not seriously jeopardize the larger argument, which is what you seem to have an issue with.

Nevertheless, it may help to start from a clean slate so we can pick out exactly what the issue is. So let's draw up a new scenario.

A man is rafting on a narrow stream. He moves at a constant velocity. There are two other boats, one downstream of the man and one upstream, both with mirrors. These boats (and the mirrors attached to them) move in some arbitrary, unspecified manner. They may decelerate and accelerate at will.

The man uses two lasers to shine beams off both mirrors. If these beams both return to him at the same moment, then he concludes that he must have been equidistant from both mirrors at the time the beams hit the mirrors. If he shines both beams at some time $t=0$ and the beams return to him at some time $t = 2 \delta$, then he concludes that the mirrors were each a distance $\delta$ from him at time $t = \delta$ according to his watch.

Now, let us presume that, at the man's time $t = \delta$, there is a child in another raft just beside him, except the child has some constant velocity downstream relative to the man.

Now, solarflare, some questions for you:
a) Would the child believe the man emitted both laser pulses at the same moment?
b) Would the child say that the pulses reflected off both mirrors at the same time according to his (the child's) watch?
c) Would the child receive both reflected pulses at his boat at the same moment?

 

[solarflare]

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.

take a spaceship with two lasers - one on each wing

observer 1 is in the centre moving directly away from the ship in another smaller ship
observer 2 is stationary also in the centre but at a greater distance.

the spaceship fires its lasers -

observer 1 sees the two lasers pass simultaneously before observer 2

observer 2 sees the two lasers pass him simultaneously also

 

[solarflare]

they both say the lasers pass them simultaneously but they disagree on the time that they pass

 

[Doc Al]

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.

Don't know what you mean by 'run the scenario for both observers'. There is just one scenario, described from two different frames of reference.

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

Your 'simple triangle' is wrong.

take a spaceship with two lasers - one on each wing

observer 1 is in the centre moving directly away from the ship in another smaller ship
observer 2 is stationary also in the centre but at a greater distance.

the spaceship fires its lasers -

observer 1 sees the two lasers pass simultaneously before observer 2

observer 2 sees the two lasers pass him simultaneously also

Your scenario is somewhat ambiguous:

Do you mean:

(A) At the moment that the ship fires its two lasers towards the middle, there is an observer sitting in the middle of the ship (observer 2) and a second observer (observer 1) in a small ship moving parallel to the big ship just passing the middle of the ship at that moment (according to the big ship frame).

In this case the light flashes from each laser reach the middle of the big ship at the same time, but they reach the small ship at different times. In any case, the observers in the small ship do not agree that the lasers were fired at the same time.

Or do you mean:

(B) The ship fires its two lasers towards the middle. There is an observer sitting in the middle of the ship (observer 2) and a second observer (observer 1) in a small ship moving parallel to the big ship who happens to pass by the middle point just as the light reaches the middle point.

In this case both observers see the light simultaneously (since they are at the middle when the light arrives). But the observers in the small ship do not agree that the lasers were fired at the same time.

Note that in either scenario the lasers are only fired simultaneously in the frame of the big ship. The small ship will think that they were fired at different times.

 

[Doc Al]

they both say the lasers pass them simultaneously but they disagree on the time that they pass

If you say they reach the observers simultaneously, then you are talking about the second version (B) of the scenario (per my last post).

Again, you are hung up on the times at which the light flashes reach the observer. But the real interesting deal is what they conclude about whether the lasers fired at the same or different times. The different frames disagree about that!

 

[Muphrid]

Solar, the lasers have to point in opposite directions to to see the difference we're talking about.

You understand that the point if the train example is that the light from the strikes approach the observers from two different directions, right?


You also keep talking about triangles. There are no triangles necessary. Put the platform and train observers right next to each other.

 

[solarflare]

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

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

the big ship fires two lasers at the two tragets from position A
the smaller ship that flying away from the big ship with velocity V sees the targets get hit by the lasers at position B
the targets are the 0
the pilot of the small ship reports that the lasers hit the targets simultaneously
the observer at position C sees the two targets get hit simultaneously also

 

[Doc Al]

Please choose one of the two laser scenarios that I offered in post #185, A or B. (They are both similar to the train example.)

If you insist on creating yet another scenario, please describe it in complete detail.

 

[Muphrid]

What you describe is correct; it's also nothing like the train situation because there is no light ray moving opposite the the small ship. Why do you think this illustrates the same thing as the train?

 

[solarflare]

my ship scenario shows that two observers in drifferent inertial frames of reference can agree that an event was was simultaneous

 

[solarflare]

but the event itself is relative to the time that they see it

 

[Doc Al]

my ship scenario shows that two observers in drifferent inertial frames of reference can agree that an event was was simultaneous

That's easy to arrange. One way: As long as two events take place at the same time and place, they will be simultaneous in all frames. But that's not particularly interesting.

Events 3 & 4 in the train example, for instance. (Post #155)

 

[Muphrid]

my ship scenario shows that two observers in drifferent inertial frames of reference can agree that an event was was simultaneous

While observers in two different frames of reference can agree that two events are simultaneous, the point of the train example is to show that there are circumstances where they don't agree.

In your example, the observers agree on simultaneity because the targets are separated in a direction perpendicular to the velocities of the observers.

In the train example, the observers disagree on simultaneity because the places the lightning bolts strike are separated in a direction parallel to the velocities of the observers.

 

[Nugatory]

they must occur in both frames simultaneously but at different times.

OK, I think I've found the problem here.
----------------------------------------

(Expanding a bit: Dude, "simultaneously" means "at the same time". Thus, the text above says "they must occur in both frames at the same time but at different times", which is of course nonsense. Is it possible that you're thinking that there's a single "real" time out there in which events can "really" be simultaneous, although that may disagree with what the observers observe?)

 

[Doc Al]

In your example, the observers agree on simultaneity because the targets are separated in a direction perpendicular to the velocities of the observers.

Ah... I'm glad you were able to decipher what that example was all about.

 

[solarflare]

What you describe is correct; it's also nothing like the train situation because there is no light ray moving opposite the the small ship. Why do you think this illustrates the same thing as the train?

because the train and the observers in the video are all lined up like my ship scenario -

by saying r1 = r2 the person who made the video is describing the same thing as my ship scenario.

 

[Muphrid]

because the train and the observers in the video are all lined up like my ship scenario -

by saying r1 = r2 the person who made the video is describing the same thing as my ship scenario.

No, the scenarios are nothing alike. Let's look at it again:

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

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

the big ship fires two lasers at the two tragets from position A
the smaller ship that flying away from the big ship with velocity V sees the targets get hit by the lasers at position B
the targets are the 0
the pilot of the small ship reports that the lasers hit the targets simultaneously
the observer at position C sees the two targets get hit simultaneously also

If points A, B, and C all lie along the x-direction, are the targets (0 and 0) separated in the y-direction? Is the velocity of the small ship in the x-direction?

If you answered yes to both, then this situation is nothing like the train, where the "targets" (the points the lightning bolts strike) are separated in the same direction as the velocity.

Contrast with the train diagram:

                   Train observer
                          T
 Rear of train R==========|==========F  Front of train
                          P
                  Platform observer

T & P need not be separated by any actual distance in the y-direction. The train moves left to right. F and R are separated left to right.

 

[solarflare]

No, the scenarios are nothing alike. Let's look at it again:



If points A, B, and C all lie along the x-direction, are the targets (0 and 0) separated in the y-direction? Is the velocity of the small ship in the x-direction?

If you answered yes to both, then this situation is nothing like the train, where the "targets" (the points the lightning bolts strike) are separated in the same direction as the velocity.

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

 

[Doc Al]

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

That's a red herring in the train scenario. Imagine the platform observer one inch from the tracks, but one mile from the ends of the train (at the moment of interest). You can safely ignore that one inch. The only distance of interest is along the direction of travel.

 

[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.