Trying to Understand Light in Motion: A Frustrating Puzzle

[Doc Al]

so two separate lighteneing strikes happen in the trains frame

You can certainly view the lightning strikes from the train frame.

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

The first strike hits the front of the train, not the rear.

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

huh?

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

The two light flashes reach the platform observer at the same time--as seen in every frame. Realize that from the train viewpoint the platform observer is moving away from the front flash and towards the rear flash.

 

[solarflare]

ok

i admit i was wrong - its the not the last it is the paragraph before the last

But what does the passenger see? As her friend on the platform predicted, the passenger does notice the flash from the front before the flash from the rear. But her conclusion is very different. As Einstein showed, the speed of the flashes as measured in the reference frame of the train must also be the speed of light. So, because each light pulse travels the same distance from each end of the train to the passenger, and because both pulses must move at the same speed, he can only conclude one thing: if he sees the front strike first, it actually happened first.


what does the passenger see? - not what does the platform observer believes she will see

As Einstein showed, the speed of the flashes as measured in the reference frame of the train must also be the speed of light. - I am pretty sure this is talking about the trains reference frame

he can only conclude one thing: if he sees the front strike first, it actually happened first.

if i see a bolt of lightning tonight - and then see the stars tomorrow night. must i conclude that the lighteneing occurred before the light was emitted from the star?

well i can say for sure that using my knowledge i would conclude that the starlight was actually emitted millions of years ago before the lightening.

 

[Doc Al]

ok

i admit i was wrong - its the not the last it is the paragraph before the last

But what does the passenger see? As her friend on the platform predicted, the passenger does notice the flash from the front before the flash from the rear. But her conclusion is very different. As Einstein showed, the speed of the flashes as measured in the reference frame of the train must also be the speed of light. So, because each light pulse travels the same distance from each end of the train to the passenger, and because both pulses must move at the same speed, he can only conclude one thing: if he sees the front strike first, it actually happened first.

Sounds right to me.

what does the passenger see? - not what does the platform observer believes she will see

Why do you say that? The platform observer believes that the passenger will see the flashes arrive separately. At that's what she does see. Perfectly consistent!

As Einstein showed, the speed of the flashes as measured in the reference frame of the train must also be the speed of light. - I am pretty sure this is talking about the trains reference frame

The speed of light is the same as measured in any frame.

he can only conclude one thing: if he sees the front strike first, it actually happened first.

Right.

if i see a bolt of lightning tonight - and then see the stars tomorrow night. must i conclude that the lighteneing occurred before the light was emitted from the star?

well i can say for sure that using my knowledge i would conclude that the starlight was actually emitted millions of years ago before the lightening.

Your example has no relevance to the train situation.

If lightning bothers you, imagine there are two light bulbs at the ends of the train. Since the passenger in the middle of the train knows she is equidistant from the bulbs, when she sees the light from one of them arrive before the other she must conclude that they flashed at different times.

 

[solarflare]

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.
(post 28)

If lightning bothers you, imagine there are two light bulbs at the ends of the train. Since the passenger in the middle of the train knows she is equidistant from the bulbs, when she sees the light from one of them arrive before the other she must conclude that they flashed at different times.
(post 283)

both from Doc AL - you are contradicting yourself here as the lightning and the bulbs are both in her frame.

 

[Doc Al]

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.
(post 28)

If lightning bothers you, imagine there are two light bulbs at the ends of the train. Since the passenger in the middle of the train knows she is equidistant from the bulbs, when she sees the light from one of them arrive before the other she must conclude that they flashed at different times.
(post 283)

both from Doc AL - you are contradicting yourself here as the lightning and the bulbs are both in her frame.

As I've pointed out many times (most recently in post #274), there are two physically different scenarios here. You keep bouncing around from one to the other--no wonder you are confused!

I think that you think that changing frames means going from A to B (see post #274), but no. Each scenario can be viewed from each frame and leads to perfectly consistent results.

Stick to scenario A:

(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. (And will reach the train passenger at different times.)

 

[harrylin]

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 largely agree with that but not completely (and I slightly disagree with cepheid's comment too). A platform observer certainly is (or can be) in the frame of the platform, as all events necessarily occur in all frames - SR reference "frames" have infinite extension. It doesn't make sense to say that, for example, someone isn't in a frame in which a light flash hits him at a certain (x,t)! Commonly people use "in frame S" as a shorthand for "as measured with reference system S" (there is a subtle difference that often doesn't matter).

However, while a platform observer may be moving relative to the platform, by common definition the "platform frame" is in rest wrt the platform. Such expressions as "platform frame" and "train frame" are unambiguous.
In such examples the platform observer supposedly uses the platform frame, and the train observer uses the train frame. But in fact, the people can conveniently be left out completely (clearer but less colourful!).

BTW, where did the maths go?? It appears that the topic has been abandoned...

[..] The platform observer will see the flash source locations hold still in his frame... [..] The train observer in her frame will see the flash source locations hold still in her frame, too... [..]

Lightning typically takes place in air that is (nearly) in rest wrt the platform. However, the speed of light is independent of the motion of the source and the flash duration is neglected; the motion of the flash source is therefore completely irrelevant for the time delay.

 

[Chestermiller]

Solarflare,

Suppose you did an actual experiment in which the setup was exactly the same as in the train-and-platform scenario you have been discussing. The platform guy was equidistant from the two locations on the ground that the two strikes hit, and the train gal was equidistant from the two locations on the train that the two strikes hit. Now the guy on the platform reports back that the two flashes arrived at his location simultaneously, while the gal on the train reports that the flash from the front arrived at her location before the flash from the rear. These are the experimental facts. What would you conclude from these facts? Do you think that this is possible, or do you think that something is wrong somewhere?

 

[solarflare]

Solarflare,

Suppose you did an actual experiment in which the setup was exactly the same as in the train-and-platform scenario you have been discussing. The platform guy was equidistant from the two locations on the ground that the two strikes hit, and the train gal was equidistant from the two locations on the train that the two strikes hit. Now the guy on the platform reports back that the two flashes arrived at his location simultaneously, while the gal on the train reports that the flash from the front arrived at her location before the flash from the rear. These are the experimental facts. What would you conclude from these facts? Do you think that this is possible, or do you think that something is wrong somewhere?

if lightning srikes the platform equidistant from the platform guy - would he see them seperately because acording to the train observer the platform was moving?

 

[solarflare]

Stick to scenario A:

(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. (And will reach the train passenger at different times.)

yes this is obvious

but the platform observer would not be equidistant from the flashes when they ACTUALLY occurred - when they hit the train - and as the train is in the trains frame the flashes ACTUALLY occur at different times

 

[Dale]

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.

Btw, I know that this terminology is common and in context it is well-defined, but I absolutely hate it. Unfortunately, the phrase "in the platform frame" conveys the mistaken idea that a frame is some sort of a container which it is possible to be in or out of and it is possible to enter or leave the container. Everything is in every frame, and it is not possible to enter or leave a frame in the sense of a container. It is possible to be moving wrt a frame or stationary wrt a frame, but not in or out of a frame.

Nothing you said is wrong, and I know that this is my own personal preference, but I always cringe when I read the phrase "in a frame".

 

[cepheid]

if lightning srikes the platform equidistant from the platform guy - would he see them seperately because acording to the train observer the platform was moving?

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.

 

[solarflare]

look at the situation in reverse

the observer sees two flashes of light

they move back towards the train (that is in motion)

how can the light originate in the same place as where the observer sees it

if you say this then you say light travels instantly

 

[solarflare]

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

 

[solarflare]

i have watched the video A LOT of times and i at first did not think that there was anything wrong.

but like George after going over it step by step it becomes clear - i suggest everyone does it and I am sure you will see what I am saying

 

[solarflare]

when doc al said that if two bulbs were turned on in the train simultaneously the woman would see the front one first - (train observer in train frame)

it is the same as saying that if two bulbs were turned on on the platform that the platform guy would see one first. (platform observer in platform frame)

 

[ghwellsjr]

i have watched the video A LOT of times and i at first did not think that there was anything wrong.

but like George after going over it step by step it becomes clear - i suggest everyone does it and I am sure you will see what I am saying

Now that we all agree that the video is complete nonsense, we need to abandon it. I pointed you to an animation that is correct:

http://i910.photobucket.com/albums/ac304/kev2001_photos/Etrain2e.gif

Please use this animation to understand what happens according to the two different frames of reference.

Then read Einstein's explanation of the scenario here:

http://www.bartleby.com/173/9.html

Nothing could be clearer. There is no need to belabor this anymore.

 

[Muphrid]

Stick to scenario A:

(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. (And will reach the train passenger at different times.)

yes this is obvious

but the platform observer would not be equidistant from the flashes when they ACTUALLY occurred - when they hit the train - and as the train is in the trains frame the flashes ACTUALLY occur at different times

solarflare, it is a given that the platform observer was equidistant from the strikes in his frame. Why do you think he wouldn't be? Because the bolts hit the train and the train is moving? This has nothing to do with it. This is why we keep saying what the bolts hit doesn't matter. All that matters are the coordinates (in time and space) of the points the bolts hit.

In other words, this is what happened: in the platform observer's coordinate system (where he is always at x=0 at all times t), there is a train that is 8 units long, extending from x=-4 to x=+4 at time t=0. At that time t=0, two lightning strikes hit, one at x=-4 and another at x=+4.

That's it. I haven't specified what object was hit because I don't need to. 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?

 

[Doc Al]

Stick to scenario A:

(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. (And will reach the train passenger at different times.)

yes this is obvious

If it were that obvious, this thread would not be as long as it is.

but the platform observer would not be equidistant from the flashes when they ACTUALLY occurred - when they hit the train - and as the train is in the trains frame the flashes ACTUALLY occur at different times

You still seem to think that the lightning strikes 'actually occurred' simultaneously in the train frame. You seem to think, since they hit the train, that they are somehow 'in the train frame'. How many times do we have to correct you?

In scenario A, which is what we are discussing, the lightning strikes are simultaneous in the platform frame.

As far as the platform observer is concerned: When the strikes ACTUALLY occur he is right in the middle of the train.

As far as the train observer is concerned: When the first strike ACTUALLY occurs the platform observer has not yet passed the middle of the train.

 

[Doc Al]

when doc al said that if two bulbs were turned on in the train simultaneously the woman would see the front one first - (train observer in train frame)

If the bulbs were turned on simultaneously according to the platform frame, then the woman passenger would claim the front one flashed first.

it is the same as saying that if two bulbs were turned on on the platform that the platform guy would see one first. (platform observer in platform frame)

No it isn't.

 

[solarflare]

If lightning bothers you, imagine there are two light bulbs at the ends of the train. Since the passenger in the middle of the train knows she is equidistant from the bulbs, when she sees the light from one of them arrive before the other she must conclude that they flashed at different times.

 

[Muphrid]

solarflare, quote button please. Who are you quoting there? You don't use capital letters like that.

What point are you trying to make by quoting that statement?

 

[solarflare]

actually there is no point in going on because i came on here to say the video is wrong and now everyone agrees the video is wrong

job done

we will agree to disagree on the rest

 

[solarflare]

if they come on at the same time in her frame then she will see them simultaneously wether the train is moving or not

 

[Muphrid]

if they come on at the same time in her frame then she will see them simultaneously wether the train is moving or not

Yes, that statement follows, key word being if. The point is that they cannot come on simultaneously in both the train frame and the platform frame.

 

[solarflare]

if she was in the centre of the platform when they come on then the platform observer will also see them simultaneously because he was equal distance from them too

 

[solarflare]

the two lightbulbs come on simultaneously IN HER FRAME what does she see?

she was in the middle of the platform when they came on.

what does the platform observer see IN HIS FRAME?

 

[solarflare]

That's it. I haven't specified what object was hit because I don't need to. 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?

you havnt specified it because it does matter

 

[solarflare]

ahhh i think i know where you are going wrong

the first bolt hits the train - then the train moves a bit - then the second strike hits

you use the same co ordinates for the second strike for where the train was when the first strike happened

 

[Muphrid]

if she was in the centre of the platform when they come on then the platform observer will also see them simultaneously because he was equal distance from then too

Okay, at last we've reached something that really gets at the crux of the matter. What you're thinking is not unreasonable, yet strangely enough, it's not correct. Both observers are equidistant from these events, yes. But here is the picture of what happens as described in both frames:

A) In the train passenger's frame, the light from both bulbs reaches her simultaneously, she knows the distances to the two are the same, and so she concludes that both lit up at the same moment.

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.

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

D) Nevertheless, the platform observer believes he was equidistant from both light bulbs, yet because he must agree that he was hit by the flash from the rear bulb before the front bulb, he infers that the rear bulb was lit before the front bulb because the speed of light is the same in both directions.

E) This apparent contradiction with the train observer's view (that both bulbs lit up simultaneously) is the point of the argument: that two observers moving relative to one another need not agree that two events happened at the same moment. You can see it follows from the speed of light being constant for all observers (mentioned in part D) and that all observers agree on the spacetime interval between two events (point C).

Now, I expect you will not agree with all of points ABCDE. Let me go in more detail over some of the things you have said that I expect will cause issues.

First, how can we know the platform observer was equidistant from both bulbs according to his frame? It seems reasonable to suggest that he may have perceived the rear flash before the forward flash of the bulbs because he was actually closer to the rear flash. I admit, it's difficult for me to prove this without falling back on the Lorentz transformation. That's why it's easier to argue the lightning strike example because then the train observer knows she was halfway between the source of the strikes by using the rest of the train as a ruler. Nevertheless, please do tell me if this is something you'd like to see argued, as it seems central to your issue with the video (which, though it may have some flaws, is not nearly as flawed as I think you're making it out to be).

Second, at times you've given the impression that you think one observer can't reason anything about what another observer would see. This is intimately tied to a distinction between what is "real" or "concrete" or "physical" (which all observers must agree on) and what is relative (which no two observers need agree on). All we're saying, really, is that the spacetime intervals between events are something all observers agree on (in magnitude). So when the light flashes reach an observer, what we're really doing is talking about when the worldline of the light crosses the wordline of an observer. This is how the train passenger (in this example) can talk about when the flashes from the bulbs reach the platform observer. All she's doing is extrapolating out how the worldlines of the flashes must cross the platform observer's worldline, and it should be apparent that they can't both cross his worldline at the same event in space and time.

If there's anything else you need explained (or you have some other issue with points ABCDE), please do quote them.

ahhh i think i know where you are going wrong

the first bolt hits the train - then the train moves a bit - then the second strike hits

you use the same co ordinates for the second strike for where the train was when the first strike happened

So now we're back to the lightning bolt example.

The train is only moving according to the platform observer. In the lightning strike example, he believed both strikes hit the train at the same moment in his frame, so the train would not have moved at all.

And the train is not moving according to the reference frame of the train passenger, so when each bolt hits the front and rear of the train, there is no motion to account for in the time between the strikes.

 

[bahamagreen]

You still seem to think that the lightning strikes 'actually occurred' simultaneously in the train frame. You seem to think, since they hit the train, that they are somehow 'in the train frame'.

THIS opens the heart of the problem right here - confusion between "strikes at the same time on the train in the platform frame" and "strikes on the train" as the train itself, leading to the mistake that the strikes on the train in the train frame must also be at the same time.

This is why I mentioned that the "train itself" is not the same as the "frame of the train". In fact the "train itself" cannot be observed except from some frame... nothing can be inferred about what is observed in the train frame by thinking about what happened to the "train itself" based on observations from any other frame.

In a strong sense, there is no "train itself", as if thinking that the "train itself" is independent from being observed from a frame, in the same way there is no meaning in speaking of a current time and present location without respect to a frame.

 

[ghwellsjr]

actually there is no point in going on because i came on here to say the video is wrong and now everyone agrees the video is wrong

job done

Yes, well done, thanks.

we will agree to disagree on the rest

No, we can't agree to disagree. You need to learn the truth about Special Relativity. We need to focus on the one scenario where the platform observer sees the two flashes arriving at his location at the same time and the train observer sees the two flashes arriving at her location at different times. These are different locations. Have you watched the animation to see this happening in both frames of reference? Have you read Einstein's explanation of what is happening?

Now that we all agree that the video is complete nonsense, we need to abandon it. I pointed you to an animation that is correct:

http://i910.photobucket.com/albums/ac304/kev2001_photos/Etrain2e.gif

Please use this animation to understand what happens according to the two different frames of reference.

Then read Einstein's explanation of the scenario here:

http://www.bartleby.com/173/9.html

Nothing could be clearer. There is no need to belabor this anymore.

 

[Chestermiller]

if lightning srikes the platform equidistant from the platform guy - would he see them seperately because acording to the train observer the platform was moving?

No. He would see them at the same time. The experimental description says that the platform guy sees them at the same time, and train gal sees them at different times. Do you believe that such a counterintuitive experiment can actually be performed, and that the results would be as described?

Here is something that will really blow your mind. It is possible in this geometric arrangement (if the lightning strikes are timed properly) for the platform guy to observe the two lightning flashes such that the rear flash is seen first, followed by the front flash, while the train gal sees them in the reverse order, such that the front flash is seen first followed by the rear flash.

Solarflare, you are asking the wrong questions. You should be asking, "If experimental observations such as these can actually occur within our real universe, what must be the fundamental geometric and kinematic structure of the universe that could give rise to such unexpected results?"

 

[Doc Al]

the two lightbulbs come on simultaneously IN HER FRAME what does she see?

she was in the middle of the platform when they came on.

Now we are discussing what I have called Scenario B. Since she's in the middle of the train and the lights flash simultaneously, she of course receives the light from each at the same time.

what does the platform observer see IN HIS FRAME?

The platform observer agrees that the light from each bulb reaches the passenger at the same time. But he says that the light at the rear of the train flashed first.

 

[Dale]

actually there is no point in going on because i came on here to say the video is wrong

The video is wrong, but not for ANY of the reasons you have given.

 

[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.

This is correct.