Trying to Understand Light in Motion: A Frustrating Puzzle

[Doc Al]

there are tree different events

the first is lightning hitting the train
the second is the light getting to the observer on the platform
the third is the the light getting to the passenger

the third has nothng to do with the second just the first

And because the lightning strikes hit the train ends at different times according to train frame clocks, the light from each gets to the passenger at different times.

 

[ghwellsjr]

there are tree different events

the first is lightning hitting the train
the second is the light getting to the observer on the platform
the third is the the light getting to the passenger

the third has nothng to do with the second just the first

the x-axis is the train frame because that is where the light originated from - there are two train frames - one when the lightning strikes the train and one when the light reaches the passenger

Solarflare, it is clear that you don't know what an event is nor what a frame is. All you are doing is taking one "sound bite" that you have heard from Special Relativity, "Light travels at the same speed for all observers" and misapplying to this scenario. Everyone is trying to use advanced concepts from SR to help you understand but they are all going over your head. I tried in post #74 to help you in a way that did not require any understanding of SR but you ignored it. Please read, try to understand and respond to my post which I repeat here:

Solarflare, it doesn't appear that we are making any headway with you on this issue. Perhaps it would help if we changed the scenario so that it basically happens in the reverse order.

Let's consider a totally different scenario:

We have a very long train, much longer than the length of the platform and so long we don't care where the front and rear are. But instead of just one observer, we have an observer at every window, all along the train.

Then on the ground, we have a platform of some arbitrary length. At the center of the platform, we have a flash bulb that will produce a bright flash of light sometime while the train is passing but we don't care when. Then at each end of the platform, we have a mirror placed at a 45 degree angle so that when the flash of light reaches it, it will reflect the light towards the train.

What we want to do is have whatever train observer is adjacent to the flash bulb when it flashes make note of the event. We'll call him Observer A. In a similar way, we want whichever two train observers see the light reflected off the two mirrors make note of those two events. We'll call the one toward the front of the train Observer F and the one toward the rear of the train Observer R.

Now isn't it obvious that by the time the light reaches the mirrors, the train has moved forward along with Observer A and so he will be closer to Observer F and farther from Observer R?

So now let's say that we take a video of this scenario and we play it backwards. We also pretend that Observer F and Observer R represent the ends of the train (except that now F and R are interchanged because the train will appear to be going backwards). Won't this be exactly the same scenario that we have been discussing in this thread, except that Observer A is not in the center of our now truncated train? And if Observer A sees the light from the two mirrors (which are analogous to the two lightning strikes) impenging on the flash bulb (analogous to the man in the original video) then Observer A cannot be the same as the woman on the train in the original video. In other words, if Observer A, who is not midway between Observer F and Observer R, sees the two flashes arriving simultaneously, then another observer who is midway between Observer F and Observer R cannot also see the two flashes arriving simultaneously.

Please note that this analysis does not require defining any frames of reference or synchronizing any clocks.

Does this make sense to you?

 

[solarflare]

im saying that the lightning strikes on the train - that happen before the platform observer sees them - must happen simultaneously in order for him to see them simultaneously in his frame of reference. how can he be equal distance from them but but they do not happen at the same time. forget the passenger on the train and go step by step

first the cause then the effect

the cause is lightning strikes
the effect is the observer on the platform seeing the strikes when the light reaches him

he cannot see them if they did not happen - if he was not equal distance he could see two strikes that occurred at different times but because he is equal distance they must happen at

the strikes happen simultaneously in the trains frame (at the time of the strikes)

the strikes happen simultaneously in the platform observers frame ( after the light from the strikes move to meet his eyes)

 

[cepheid]

Horizontal lines (parallel to the x-axis) are lines of constant t, with t being time
according to the observer in the track/platform frame. Since both strikes occur along the x-axis, which is the line corresponding to t = 0, both strikes are simultaneous *in the platform frame.*

Now, the important thing to realize is that what the train observer experiences as time (t') runs in a *different direction* through spacetime than what the track observer experiences as time. In particular, for the train observer, the lines of constant t' are the *slanted* lines that are parallel to the x'-axis. So, in order for two events to be simultaneous in the train frame, they both have to have the same t' coordinate, which means they both must lie along the same one of these *slanted* lines. You can see that the right and left flashes don't satisfy this condition. Taking the right flash, and running from it to the t'-axis along a slanted line parallel to the x'-axis, you see that this line intercepts the t'-axis at a point four tick marks *below* the x'-axis, which is the line of t' = 0. So the first flash occurs at t' = -4 (in whatever units correspond to the arbitrary grid spacing I drew). In contrast, if you project the left flash event onto the t'-axis in a similar manner, you find that the line of constant t' that it lies on is 4 grid spacings *above* the t' = 0 line (which is the x'-axis). Therefore the left flash occurs at t' = +4 in my arbitrary units. So the diagram clearly shows that the two flash events do not occur simultaneously in the train observer's frame. It also shows that, as a result, they don't *arrive* simultaneously at the train either. They arrive at the two different points on the t'-axis that I've labelled 'not simultaneous' with two arrows.

It is the Lorentz transformation that has the effect of rotating and skewing the coordinate axes of the train observer in the manner depicted, so that he experiences space and time differently from the platform observer.

We're a page on from my diagram and still going on, which I didn't expect. I'm sorry if it caused any additional confusion. My intention was the opposite: to provide clarity.

 

[solarflare]

you keep talking about the train observer seeing the light

im talking about when the train get hits by the lightening. the observers cannot see light that has not yet been emitted

 

[solarflare]

your saying that the lightning in reality does not strike the train simultaneously

yet the fact that he is equal distance means that they must have

 

[Muphrid]

the strikes happen simultaneously in the trains frame (at the time of the strikes)

the strikes happen simultaneously in the platform observers frame ( after the light from the strikes move to meet his eyes)

No, as has been repeatedly shown, these statements are contradictory. They cannot both be true. Look at the diagram. The strikes are at the same time according to the platform observer. The train observer is moving toward the forward strike, so that strike's light reaches him sooner.

That the train observer believes instead that that that forward strike is sooner (and he's not moving relative to the light). Relativitiy of simultaneity is exactly what resolves the issue, so that the train observer still perceives the foward strike sooner and still measures the speed of light to be the same relative to him.

you keep talking about the train observer seeing the light

im talking about when the train get hits by the lightening. the observers cannot see light that has not yet been emitted

We can still talk about the location and time that these events happened according to one observer or the other's coordinate system.

your saying that the lightning in reality does not strike the train simultaneously

yet the fact that he is equal distance means that they must have

To one observer, yes. You keep missing the point that just because one observer believes two events to be simultaneous means any other observer might not.

 

[solarflare]

No, as has been repeatedly shown, these statements are contradictory. They cannot both be true. Look at the diagram. The strikes are at the same time according to the platform observer. The train observer is moving toward the forward strike, so that strike's light reaches him sooner.

That the train observer believes instead that that that forward strike is sooner (and he's not moving relative to the light). Relativitiy of simultaneity is exactly what resolves the issue, so that the train observer still perceives the foward strike sooner and still measures the speed of light to be the same relative to him.



We can still talk about the location and time that these events happened according to one observer or the other's coordinate system.



To one observer, yes. You keep missing the point that just because one observer believes two events to be simultaneous means any other observer might not.

you keep talking about the passenger on the train so it is you that is missing the point.

the strike on the train must occur before any observer can see them - in order for the platform guy to see the light at the same time the strikes must have happened at the same time. forget about the passenger on the train she does not matter

 

[solarflare]

if the platform observer was anywhere but in the centre what you are saying would be correct but if he is central then it cannot be

 

[solarflare]

if he was in line with the rear of the train and saw them strike simultaneously he would conclude that because he saw the strikes happen at the same time and he is not equal distance from each strike that the person on the train would not see them at the same time because she is in the centre of the train.

 

[Muphrid]

you keep talking about the passenger on the train so it is you that is missing the point.

the strike on the train must occur before any observer can see them - in order for the platform guy to see the light at the same time the strikes must have happened at the same time. forget about the passenger on the train she does not matter

To say that the strikes happened at the same time, you must choose a frame of reference. There are frames of reference where the man on the platform sees both strikes at the same time yet the strikes themselves are not simultaneous.

 

[Dale]

the strikes happen simultaneously in the trains frame (at the time of the strikes)

the strikes happen simultaneously in the platform observers frame ( after the light from the strikes move to meet his eyes)

These two ststements cannot both be true. I posted the math earlier. It clearly shows you to be wrong.

 

[solarflare]

i choose the trains frame of reference at the point the lightning strikes ( there is no passenger on the train - the train is completely passengerless - there is no reason to mention when the light reaches the centre of the train because that is not the issue)

if the lightning hits the train simultaneously and the platform observer is equal distance from the two bolts will he see the strikes simultaneously

 

[solarflare]

These two ststements cannot both be true. I posted the math earlier. It clearly shows you to be wrong.

so your saying it takes light longer to travel from one end of the train to the platform observer than the other? even though light travels at the same speed over the same distance.

and bear in mind that I am not talking about the passenger seeing the light - I am talking about when the two strikes happen

 

[solarflare]

and even in the video it says what I am saying - it freezes the train and shows the light leaving both ends and reaching the platform observer at the same time.

if you say I am wrong then you say the video is wrong - and as i am saying the video is wrong then you are agreeing with me

 

[Muphrid]

i choose the trains frame of reference at the point the lightning strikes ( there is no passenger on the train - the train is completely passengerless - there is no reason to mention when the light reaches the centre of the train because that is not the issue)

if the lightning hits the train simultaneously and the platform observer is equal distance from the two bolts will he see the strikes simultaneously

No, in the train's frame of reference, the person on the platform is moving backwards, and the light from the strike at the rear will reach him first because he's moving toward it.

Edit: what you're describing is emphatically not the situation presented in the video, which is from the platform's frame of reference.

Edit edit: here is a transcript of the video.

Imagine two observers, one seated in the center of a speeding traincar, 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 to his eyes at the same speed of light. He also predicts that his friend on the train will notice the front strike before the rear strike because from his perspective, the train is moving to meet the flash from the front and moving away from the flash at the rear.

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 traveles the same distance from each end of the train to the passenger, she can only conclude one thing: if she sees the front strike first, it actually happened first.

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.

There is nothing wrong with this video. The person on the platform draws a reasonable conclusion: the flashes reach him at the same time, he was an equal distance from both ends, so the strikes must've occurred at the same time. The person on the train also draws a reasonable conclusion: the forward flash reaches her first, and the distances between both ends are the same, so the forward strike must've been first. There is no contradiction here with the theory. They necessarily disagree on whether the strikes were simultaneous, and the theory of special relativity explains why.

 

[cepheid]

you keep talking about the train observer seeing the light

im talking about when the train get hits by the lightening. the observers cannot see light that has not yet been emitted

I don't know if you are responding to the preceding post (which was my #94) here, but if you are, then it means that you failed to read anything that I said in it, which is frustrating and unfortunate.

I was NOT talking just about when the flashes arrived at the train observer. In fact, I spent the vast majority of my post talking about when they occurred (i.e. when they hit the rails), according to the train observer, and explaining that the diagram clearly showed that they did not occur at the same time in the train's reference frame. The forward flash hit the ground first, and the rear flash didn't hit until later. They may have both occurred at points along the track that were equidistant from the position of both observers at t = t' = 0, but according to the train observer, one occurred first, and then the second one occurred later.

I've updated my diagram (with the stuff in red) to show what I was trying to explain verbally in post #94:

The red dashed lines project the flash events onto the t' axis, so that we can see what their time coordinates are in the primed (') coordinate system. You can also think of these lines as the lines of constant t' that pass through each of the flash events. CLEARLY THE FLASH EVENTS DO NOT LIE ALONG THE SAME LINES OF CONSTANT t', WHICH MEANS THAT THEY OCCURRED AT DIFFERENT VALUES OF t'.

This will be my last attempt to explain this.

 

[Doc Al]

im saying that the lightning strikes on the train - that happen before the platform observer sees them - must happen simultaneously in order for him to see them simultaneously in his frame of reference. how can he be equal distance from them but but they do not happen at the same time. forget the passenger on the train and go step by step

first the cause then the effect

the cause is lightning strikes
the effect is the observer on the platform seeing the strikes when the light reaches him

he cannot see them if they did not happen - if he was not equal distance he could see two strikes that occurred at different times but because he is equal distance they must happen at

So far, all is correct.

the strikes happen simultaneously in the trains frame (at the time of the strikes)

You just ASSUME this is true, but it's not.

the strikes happen simultaneously in the platform observers frame ( after the light from the strikes move to meet his eyes)

The strikes happen simultaneously in the platform frame before the light meets the platform observers eyes.

 

[Doc Al]

if the lightning hits the train simultaneously and the platform observer is equal distance from the two bolts will he see the strikes simultaneously

Once again, you insist on mixing up scenarios, rendering this whole exercise rather pointless.

You haven't grasped the point--the entire point of Einstein's train example--that if the lightning strikes are simultaneous in one frame they cannot be simultaneous in the other.

Once again, I urge you to stick to one scenario: The one in the video, in which the lightning strikes are simultaneous in the platform frame. Follow the simple logic that leads to the inevitable conclusion that those strikes must occur at different times in the train frame.

 

[solarflare]

No, in the train's frame of reference, the person on the platform is moving backwards, and the light from the strike at the rear will reach him first because he's moving toward it.

Edit: what you're describing is emphatically not the situation presented in the video, which is from the platform's frame of reference.

Edit edit: here is a transcript of the video.


There is nothing wrong with this video. The person on the platform draws a reasonable conclusion: the flashes reach him at the same time, he was an equal distance from both ends, so the strikes must've occurred at the same time. The person on the train also draws a reasonable conclusion: the forward flash reaches her first, and the distances between both ends are the same, so the forward strike must've been first. There is no contradiction here with the theory. They necessarily disagree on whether the strikes were simultaneous, and the theory of special relativity explains why.

The person on the platform draws a reasonable conclusion: the flashes reach him at the same time, he was an equal distance from both ends, so the strikes must've occurred at the same time.

yes they occurred simultaneously in the frame of the train when the bolts hit - and they occur simultaneously when the platform observer sees them in his frame

 

[solarflare]

what any observer thinks about what happened in another frame does not matter.

what einstein meant is that they will not agree on the time that the event occurred becaue the platform observer will see the light after the event.

so in the trains frames at the time of the bolts hitting will be t=0
and the platform frame will be if he was 1 light second away t=1

that is the disagreement - not wether they hit simultaneously

 

[solarflare]

so why can't the observer on the train see the light and think that because she saw light hit at different times and she is moving towards one that the flashes could have happened simultaneously - just like the observer on the platform predicts that the passenger will see them seperately

 

[Doc Al]

so why can't the observer on the train see the light and think that because she saw light hit at different times and she is moving towards one that the flashes could have happened simultaneously - just like the observer on the platform predicts that the passenger will see them seperately

She knows she's exactly in the middle of the train, equidistant from the lightning strikes. So when she sees the light arrive at different times, she must conclude that the strikes occurred at different times. Simple as that.

 

[Doc Al]

The person on the platform draws a reasonable conclusion: the flashes reach him at the same time, he was an equal distance from both ends, so the strikes must've occurred at the same time.

That's true. The situation was arranged so that the lightning strikes occurred at the same time according to the platform observer.

yes they occurred simultaneously in the frame of the train when the bolts hit - and they occur simultaneously when the platform observer sees them in his frame

No, you cannot simply assume that the lightning strikes occurred simultaneously in the frame of the train. In fact, you can prove that they did not.

 

[Muphrid]

yes they occurred simultaneously in the frame of the train when the bolts hit - and they occur simultaneously when the platform observer sees them in his frame

Saying this repeatedly does not make it true.

what any observer thinks about what happened in another frame does not matter.

what einstein meant is that they will not agree on the time that the event occurred becaue the platform observer will see the light after the event.

so in the trains frames at the time of the bolts hitting will be t=0
and the platform frame will be if he was 1 light second away t=1

that is the disagreement - not wether they hit simultaneously

Both observers use the time when the light reaches them to infer something about when the event that generated that light actually occurred.


You keep talking about the platform being some distance away. I think you're getting too hung up on the metaphor, thinking that there must be some real separation between the person on the platform and the train and that that comes into play. As far as our math cares, the person "on the platform" could be inside the train, flying backwards with respect to the train and right next to the person sitting inside the train.

Saying that what observers think happened in another frame doesn't matter is missing the point. Saying what happens according to another observer is not guessing. As long as you know their velocity, you know how they will measure things. It is entirely correct for the platform observer to say that the train observer will detect the flash from the foward strike first because he knows something about the train observer's velocity.

so why can't the observer on the train see the light and think that because she saw light hit at different times and she is moving towards one that the flashes could have happened simultaneously - just like the observer on the platform predicts that the passenger will see them seperately

The reason she doesn't think she is moving toward the strike is because this is a light pulse, and its invariant speed doesn't tell her anything about whether she's moving toward it or away from it. That is the point of saying all observers measure the same speed of light. As far as the train observer is concerned, she's still and the world is moving. It makes little sense to say this on earth, but put the situation into space, and you see why it makes no sense to say one person is moving and the other isn't. The same situation with two spacecraft makes it clear.

 

[Dale]

so your saying it takes light longer to travel from one end of the train to the platform observer than the other? even though light travels at the same speed over the same distance.

No, I am saying that IF the strikes are simultaneous in one frame then they cannot be simultaneous in the other. See the math above, it is unambiguous on this point.

 

[Chestermiller]

so why can't the observer on the train see the light and think that because she saw light hit at different times and she is moving towards one that the flashes could have happened simultaneously - just like the observer on the platform predicts that the passenger will see them seperately

Because there is no measurement she can make that can convince her that she is moving toward one flash and away from the other flash. As far she is concerned, she is stationary, and it is the platform and the landscape that are moving (backwards).

 

[ghwellsjr]

Solarflare, I have been studying your posts and have come to the conclusion that your idea of frames includes the following:

1) There are multiple frames in the scenario.

2) There are separate and multiple frames for the train and for the platform.

3) A frame for the train does not include the platform and a frame for the platform does not include the train.

4) The platform frame at the time when the lightning bolts strike is a different frame for the time when the flashes reach the platform observer.

5) The train frame at the time when the lightning bolts strike is a different frame for the time when the flashes reach the train observer.

6) Although the platform frame and the train frame at the time when the lightning bolts strike are different frames, if the lightning bolts are simultaneous in one, then they are simultaneous in the other one.

7) What the platform observer sees in the platform frame has no bearing on what the train observer sees in the train frame and vice versa.

Have I pretty much summarized your ideas of frames? Please respond with which ones you agree with and which ones you disagree with.

 

[solarflare]

1)

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 traveles the same distance from each end of the train to the passenger, she can only conclude one thing: if she sees the front strike first, it actually happened first.

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.

this part of the video is in the trains frame of reference - therefore the video is talking about two different frames of reference not just the one of the observer on the platform.

the video is saying that what the passenger sees is dependent on what the observer on the platform sees

what the observer on the platform sees is dependent on the fact that the strikes happen simultaneously in the trains frame.

there has to be at least two frames of reference or there would be no need to make any transformations.

2) the train has its own frame - of which the passenger is a part of - the platform observer has his own frame.

3) no both frames depend on the train - the trains frame with the passenger does not depend on the observer in the platform frame. she would still see 2 strikes even if the man on the platform was not there - and the man on the platform would also see two strikes even if there was no passenger on the train.

4) unless the light hits him at exactly the same time as the strikes happen yes - there must be a time delay between the strikes happening in the trains frame and the strikes happening in the platform observers frame. if you do not think that then you are saying light moves instantly between two places. the distance given in the video is r1 = r2 - this means that the time between the two will be how long it takes light to travel the distance r1.

5) no - what the woman in the trains frame sees is dependent on what happened in the trains frame not what happens in the platform observers frame.

6) r1 = r2 tells you that the strikes must happen simultaneously in both frames. the motion of the train does not affect the motion of the light that heads to the observeron the platform.

7) yes

 

[ghwellsjr]

Thanks for your answers. I just have one more question:

In your mind, is it always a requirement when defining, describing and analyzing scenarios like this to use multiple frames or can it all be done with just one frame?