Trying to Understand Light in Motion: A Frustrating Puzzle

[Muphrid]

And this is a key point about reference frames. Objects can be stationary or moving with respect to a frame, but events (like the lightning strikes we've been talking about) are not objects and they do not move, and the distances measured to events are simply the distance light travels to reach an observer (a distance which depends on the particular frame chosen, of course).

 

[solarflare]

No, no, absolutely not. The sources of the flashes do not "move with" the object the lightning/lasers hit.
The sources are points in space and time. They do not move. They do not have worldlines. When the train observer
sees the flashes, she is looking into the past, to the places and times the strikes hit, not to where the point of
impact actually has moved to.That is why we keep saying it does not matter if the strikes hit the train or the
platform. (taken from post 267)Originally Posted by solarflare View Post

let me ask the same question but in definate terms.

the two bolts of lightning do actually strike at the same time in the trains frame of reference

the observer on the platform is exactly the same distance from each strike

does the woman see the two strikes at the same time?

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. (taken from post 28) (doc al)maybe doc al can explain to murprid why she will see them simultaneously if they strike the train simultaneously?

 

[solarflare]

now on the other hand if the flashes hit the tracks and not the train - then yes she would see them seperately because the train and the tracks are moving at different speeds in her frame.

 

[Doc Al]

Originally Posted by solarflare View Post

let me ask the same question but in definate terms.

the two bolts of lightning do actually strike at the same time in the trains frame of reference

the observer on the platform is exactly the same distance from each strike

does the woman see the two strikes at the same time?

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. (taken from post 28) (doc al)maybe doc al can explain to murprid why she will see them simultaneously if they strike the train simultaneously?

It is you, solarflare, who are confused not Muphrid.

(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.

(B) If the lightning strikes hit the train ends simultaneously in the train frame, the light flashes from each will reach the train observer at the same.

These are two physically distinct scenarios. They cannot both apply to the same situation--it's one or the other, not both.

The one we've been discussing is scenario A.

 

[Doc Al]

now on the other hand if the flashes hit the tracks and not the train - then yes she would see them seperately because the train and the tracks are moving at different speeds in her frame.

You are still stuck with the idea that if the lightning strikes hit the train that they become simultaneous in the train frame but if they hit the tracks they would be simultaneous in the track frame. This is completely wrong, as has been explained in great detail throughout this (comically long) thread.

 

[solarflare]

you seem to forget that the video says that the platform observer was equal distance from each strike

the only way the platform observer can see two simultaneous strikes is if the strikes happen seperately at DIFFERENT distances

 

[Doc Al]

you seem to forget that the video says that the platform observer was equal distance fron each strike

Right.

the only way the platform observer can see two simultaneous strikes is if the strikes happen seperately at DIFFERENT distances

You seem to forget that the strikes were simultaneous in the platform frame. So of course he sees the flashes reach him at the same time.

 

[solarflare]

the video is saying that an event happens

and then describes what BOTH observers see from that event

the video is using both frames.

look at the last paragraph of the transcript

 

[solarflare]

so two separate lighteneing strikes happen in the trains frame

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

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

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

that is what the video should have shown

 

[Doc Al]

the video is saying that an event happens

and then describes what BOTH observers see from that event

the video is using both frames.

look at the last paragraph of the transcript

Yes, the events happen. Yes, they can be described from both frames.

So what?

The last paragraph of the transcript is:

Whose interpretation is correct - the observer on the platform, who claims that the strikes happened simultaneously, or the observer on the train, who claims that the front strike happened before the rear strike? Einstein tells us that both are correct, within their own frame of reference. This is a fundamental result of special relativity: From different reference frames, there can never be agreement on the simultaneity of events.​

Sounds good to me. What's the problem?

 

[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.