Trying to Understand Light in Motion: A Frustrating Puzzle

[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.

 

[solarflare]

response to post 313

so she sees them simultaneously because they came from the train

so the light leaves the front of the train and the rear of the train simultaneously

each light pulse travels the same distance to the platform observer so he must see them simultaneously also

or do you say that the platform observer moves towards one flash and away from the other so he will see them seperately

 

[Doc Al]

response to post 313

so she sees them simultaneously because they came from the train

No, the light from both bulbs reaches her simultaneously because they flashed simultaneously.

so the light leaves the front of the train and the rear of the train simultaneously

Yes, in her frame.

each light pulse travels the same distance to the platform observer so he must see them simultaneously also

Not true at all.

or do you say that the platform observer moves towards one flash and away from the other so he will see them seperately

Of course.

 

[Doc Al]

each light pulse travels the same distance to the platform observer so he must see them simultaneously also

Let me expand on my previous answer:

Do the light pulses travel the same distance to the platform observer?
- According to the train observer, they do not.
- According to the platform observer, they do.

Do the light pulses reach the platform observer at the same time?
- No.

 

[solarflare]

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.

this concept is wrong – the light moves out like an orb from the train – what you are saying is that the light is coming from the left and right side of the observer on the platform but it is not. The platform guy can only move towards the light if he moves along the path the light is coming at him from. Once they leave the moving train simultaneously they are not affected by any velocity of the train.

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

how will the passenger know when the light reaches the platform observer – we follow it by use of orbs expanding to show the lights progress – but she can only see the platform guy after the light has reflected back after hitting him – not when it hits him – therefore any thing she sees cannot be used to say as a fact. She may believe it but it is not necessarily true. If you watch a live football match on TV – you will be seeing a delayed transmission. The cameraman will see what you see but he will see it before you. The cameraman and you will both agree what happened in the match even though you have different inertial frames of reference. (assuming you watched the match only from his camera) The only thing you will disagree on is what time you saw it. Simple maths will tell you that if you have two distances that are equal and something moves at the same speed they will cover that distance in the same time. My main issue is that the train strikes cannot be equidistant from the platform observer if he sees them simultaneously. One strike happens – the rear one (that is further away from the platform)– the light then begins its journey towards the platform then the closer strike happens . Then they both move together and reach the platform guy simultaneously.

 

[Muphrid]

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.

this concept is wrong – the light moves out like an orb from the train – what you are saying is that the light is coming from the left and right side of the observer on the platform but it is not. The platform guy can only move towards the light if he moves along the path the light is coming at him from. Once they leave the moving train simultaneously they are not affected by any velocity of the train.

The light isn't affected by the motion, of course. Nevertheless, the platform observer is moving toward the rear of the train. He is equidistant from the sources at the time the orbs have yet to expand, but as they expand, he is moving toward the rear and must be struck by the rear expanding sphere of light first.

This is an easy calculation. Put two lightbulbs at x = \pm 4 units and give the platform observer velocity \beta = 1/3 toward the rear of the train. If both bulbs go off at t= 0, then after 3 units of time, the forward flash has expanded at most to x = 1, the rear flash has reached x = -1. But x = -1 is where the platform observer is at t=3 because of his velocity.

This does not contradict the notion that he was equidistant from the bulbs. He was equidistant at t=0, and that is what's important.

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

how will the passenger know when the light reaches the platform observer [...]

Because at the time both flashes have reached her, the platform observer is further toward the rear of the train than her, and the forward flash from the bulb cannot possibly have reached the platform observer yet.

 

[ghwellsjr]

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.

this concept is wrong – the light moves out like an orb from the train – what you are saying is that the light is coming from the left and right side of the observer on the platform but it is not. The platform guy can only move towards the light if he moves along the path the light is coming at him from. Once they leave the moving train simultaneously they are not affected by any velocity of the train.

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

how will the passenger know when the light reaches the platform observer – we follow it by use of orbs expanding to show the lights progress – but she can only see the platform guy after the light has reflected back after hitting him – not when it hits him – therefore any thing she sees cannot be used to say as a fact. She may believe it but it is not necessarily true. If you watch a live football match on TV – you will be seeing a delayed transmission. The cameraman will see what you see but he will see it before you. The cameraman and you will both agree what happened in the match even though you have different inertial frames of reference. (assuming you watched the match only from his camera) The only thing you will disagree on is what time you saw it. Simple maths will tell you that if you have two distances that are equal and something moves at the same speed they will cover that distance in the same time. My main issue is that the train strikes cannot be equidistant from the platform observer if he sees them simultaneously. One strike happens – the rear one (that is further away from the platform)– the light then begins its journey towards the platform then the closer strike happens . Then they both move together and reach the platform guy simultaneously.

The light isn't affected by the motion, of course. Nevertheless, the platform observer is moving toward the rear of the train. He is equidistant from the sources at the time the orbs have yet to expand, but as they expand, he is moving toward the rear and must be struck by the rear expanding sphere of light first.

This is an easy calculation. Put two lightbulbs at x = \pm 4 units and give the platform observer velocity \beta = 1/3 toward the rear of the train. If both bulbs go off at t= 0, then after 3 units of time, the forward flash has expanded at most to x = 1, the rear flash has reached x = -1. But x = -1 is where the platform observer is at t=3 because of his velocity.

This does not contradict the notion that he was equidistant from the bulbs. He was equidistant at t=0, and that is what's important.
Because at the time both flashes have reached her, the platform observer is further toward the rear of the train than her, and the forward flash from the bulb cannot possibly have reached the platform observer yet.

This is not the scenario under discussion in this thread. Can we please drop these other scenarios until we settle the issues with the train scenario described by Einstein and illustrated in the animation? Please?

 

[Muphrid]

My main issue is that the train strikes cannot be equidistant from the platform observer if he sees them simultaneously. One strike happens – the rear one (that is further away from the platform)– the light then begins its journey towards the platform then the closer strike happens . Then they both move together and reach the platform guy simultaneously.

So now we're in the lightning example again.

I really don't see why you think this is. That both events are equidistant from the platform observer is presented as a given piece of information, something no one should have to deduce. There are many ways he can determine the distance, from putting lines on the train to mark off distances to figuring out something about the brightness of the strikes or the sizes of the char marks the strikes leave behind. It is a given piece of information. Why do you think it cannot be?

Edit: if the train is moving left to right, do you not agree that the rear flash comes from the platform observer's left and the forward flash from his right?

 

[solarflare]

the only way the platform observer will move towards the light is if you say the light moves at C+V towards the platform observer. the platform observer cannot move towards the light unless he moves on a different axis.

once it happens it does not move with the train and therefore will be classed as stationary - but expanding outwards - towards both the train and the platform.

 

[ghwellsjr]

Solarflare: can you see in the animation that the platform observer sees both flashes hit him at 2 seconds according to his clock and that light from the front flash hits the train observer at 1 second according to her clock and the light from the rear flash hits her at 6 seconds according to her clock and that both frames display the same information?

 

[Muphrid]

the only way the platform observer will move towards the light is if you say the light moves at C+V towards the platform observer. the platform observer cannot move towards the light unless he moves on a different axis.

once it happens it does not move with the train and therefore will be classed as stationary - but expanding outwards - towards both the train and the platform.

On a different axis from what?

Never said anything of the kind about light moving faster than c. The flashes from both events move at c. Nevertheless, most observers tend to have velocities and can move toward an expanding "sphere of light". The only constraint that relativity imposes is that in an observer's own frame, the observer measures the speed of light to be c.

The whole point of the thought experiment is that someone is moving toward an expanding light wavefront and moving away from another wavefront in the same way. We impose that that person still measure c to be the same in their frame, though, and this is what leads us to conclude the different arrival times are caused by relative simultaneity instead of relative velocity with respect to the light.

 

[solarflare]

the point is that you continue to look at the light with the motion of the train after the strikes - you cannot because the light will be classed as stationary in both frames. therefore the platform observer will not have any relative motion to the source.

 

[Muphrid]

the point is that you continue to look at the light with the motion of the train after the strikes - you cannot because the light will be classed as stationary in both frames. therefore the platform observer will not have any relative motion to the source.

Of course he has no relative motion to the source. The sources are points that may as well wink in and out of existence as soon as they've put out their pulses of light.

You don't mean the light is stationary. We already talked about expanding spheres and stuff.

You're taking the idea that all observers measure the same speed of light too far. Any given observer can perceive other objects as moving toward or away from an expanding sphere of light such that those objects' "relative velocity" to the expanding sphere can be greater or less than c. This is fine (well, properly defined it's fine). Forcing that all observers perceive the speed of light to be the same in their respective frames does not change that statement. What it does change are their respective notions of simultaneity and distance, which resolves the apparent contradiction.

Have you looked at ghwellsjr's animation? I too think this neatly resolves what's going on in this lightning scenario.

 

[solarflare]

if you want to say that there is relative motion then the source would move with the train carrying the whole orb of light with it meaning that the light moving towards the front of train is traveling at C+V and the light moving at C-V away from the rear of the train

 

[Doc Al]

if you want to say that there is relative motion then the source would move with the train carrying the whole orb of light with it meaning that the light moving towards the front of train is traveling at C+V and the light moving at C-V away from the rear of the train

No, the light just moves at speed C. If the light moves to the left at speed C and the passenger moves to the right at speed V, then they will get closer at a rate of C + V as seen by platform observers. But the speed of light hasn't changed. (And certainly nothing is moving at a speed of C + V.)

 

[Muphrid]

if you want to say that there is relative then the source would move with the train carrying the whole orb of light with it meaning that the light moving towards the front of train is traveling at C+V and the light moving at C-V away from the rear of the train

No, emphatically no. The sources are gone as soon as they put out their pulses of light. They do not persist. They do not move or go anywhere in any frame. They are gone as soon as the pulses begin to expand.

Lightning example:
The platform observer perceives two pulses of light moving at speed c from opposite directions that hit him at the same moment. He believes the train passenger has velocity c+v with respect to the forward flash and c-v with respect to the rear flash.

Nevertheless, the train passenger will believe her relative velocity with respect to both flashes is c. There is no contradiction here.