Trying to Understand Light in Motion: A Frustrating Puzzle

[Dale]

this states that in some reference frames the two accidents may happen at the same time. yet you claim that it is not possible for two frames to agree

It is not possible for these two frames to agree in this scenario because the relative motion is in the x direction and the lightning strikes are separated in the x direction. See the formula I posted earlier which clearly shows the impossibility of agreement here.

The wiki article appears to refer to the possibility of two frames moving in the x direction to agree on events separated only in y or z. That does not apply here.

 

[Doc Al]

this states that in some reference frames the two accidents may happen at the same time. yet you claim that it is not possible for two frames to agree

In one frame the two (non causally connected) accidents may happen at the same time; but in a frame moving with respect to that one, they will not happen at the same time. You won't find them happening at the same time in two frames moving with respect to each other (along the line between the two accidents).

 

[Muphrid]

I think the problem is that you're trying to assume certain things are the same in both frames that aren't. Let's turn the problem around a little bit.

Let's say two lightning strikes hit opposite ends of a stationary platform again. Person P, stands in the middle of the platform; person T is riding a train. The strikes both hit at the same time according to person P, and at that moment, person T is in the middle of the platform (or as close as he can be while riding a train passing it by) as well.

Now, let's equip both person P and person T with two simple light sensors that tell when they have seen the light from each lightning strikes.

Person P sees both light flashes at the same time, and both his sensors light up at the same time. Now, according to person P, person T can't possibly see both flashes at the same time because T is moving forward and will catch up to the forward light pulse while running away from the backward light pulse. T will see the forward strike first, and then the backward strike.

Now, T doesn't agree on why he sees the two strikes at different times. He thinks the strikes must've happened at different times, with the forward strike first and hte backward strike second. But, he thinks person P is moving backward and that the time delay between the strikes is enough so P will still see both strikes at the same time.

In other words, P and T both believe P will see the strikes at the same time and that T will see them at different times. That is the physically meaningful statement they can make. That is what both frames agree on. Everything else, well, is relative.

 

[ghwellsjr]

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]

I think the problem is that you're trying to assume certain things are the same in both frames that aren't. Let's turn the problem around a little bit.

Let's say two lightning strikes hit opposite ends of a stationary platform again. Person P, stands in the middle of the platform; person T is riding a train. The strikes both hit at the same time according to person P, and at that moment, person T is in the middle of the platform (or as close as he can be while riding a train passing it by) as well.

Now, let's equip both person P and person T with two simple light sensors that tell when they have seen the light from each lightning strikes.

Person P sees both light flashes at the same time, and both his sensors light up at the same time. Now, according to person P, person T can't possibly see both flashes at the same time because T is moving forward and will catch up to the forward light pulse while running away from the backward light pulse. T will see the forward strike first, and then the backward strike.

Now, T doesn't agree on why he sees the two strikes at different times. He thinks the strikes must've happened at different times, with the forward strike first and hte backward strike second. But, he thinks person P is moving backward and that the time delay between the strikes is enough so P will still see both strikes at the same time.

In other words, P and T both believe P will see the strikes at the same time and that T will see them at different times. That is the physically meaningful statement they can make. That is what both frames agree on. Everything else, well, is relative.

the problem is that your using the results from the platform frame to explain what the person in the train sees - they are different frames and so what the person on the platform believes the person on the train will see is not actually what they will see - for all the person on the platform knows the person on the train might be blind and won't see anything.

 

[cepheid]

I made this spacetime diagram a while ago, and I *think* it corresponds to the original scenario that is being discussed here. Maybe it will help. The "worldline" of the train (which is its path through spacetime) is clearly indicated, and it of course coincides with the t' axis:

The worldline of the photon that is coming in from the front (i.e. from the positive side) clearly intercepts the worldline of the train before (i.e. at a smaller value of t') the worldline of photon that is coming in from the rear (negative side) does.

The coordinate grid I've drawn is for the train observer, in the primed (t',x') coordinate system.

 

[Muphrid]

the problem is that your using the results from the platform frame to explain what the person in the train sees - they are different frames and so what the person on the platform believes the person on the train will see is not actually what they will see - for all the person on the platform knows the person on the train might be blind and won't see anything.

If that's what you think, you're sorely mistaken. Both observers agree on spacetime intervals. This is fundamental to the theory. Cepheid has pretty clearly shown how the points of reception are not the same. Both observers must agree on the proper time between the two reception events for the train observer--or that they're different at all.

 

[solarflare]

I made this spacetime diagram a while ago, and I *think* it corresponds to the original scenario that is being discussed here. Maybe it will help. The "worldline" of the train (which is its path through spacetime) is clearly indicated, and it of course coincides with the t' axis:

The worldline of the photon that is coming in from the front (i.e. from the positive side) clearly intercepts the worldline of the train before (i.e. at a smaller value of t') the worldline of photon that is coming in from the rear (negative side) does.

The coordinate grid I've drawn is for the train observer, in the primed (t',x') coordinate system.

this diagram says that if the stikes happen simultaneously - on the tracks - the the observer will see them simultaneously also. just at a different time.

 

[solarflare]

now look at post 49 and post 50.

 

[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.

the diagram shows that the lightning struck in the trains frame of reference simultaneously and also in the platform observers frame.

so according to Doc Al's statement above the train observer will see the bolts a the same time also.

 

[Doc Al]

this diagram says that if the stikes happen simultaneously - on the tracks - the the observer will see them simultaneously also. just at a different time.

The observer on the tracks will see them simultaneously, not the observer on the train.

the diagram shows that the lightning struck in the trains frame of reference simultaneously and also in the platform observers frame.

No it doesn't. The diagram shows that the flashes are not simultaneous in the train frame.

so according to Doc Al's statement above the train observer will see the bolts a the same time also.

And no again.

 

[MikeLizzi]

If anyone is interested, here is an analysis of a train/lightening problem using straightforward math, no diagrams. It's not the same as the OP original but I believe it addresses the same issues.

http://www.relativitysimulation.com/Documents/Train and Lightning.htm

 

[solarflare]

two simultaneous strikes on the train - cause - the observer on the platform to see the two strikes simultaneously because he is the same distance from each strike.

you seem to be ignoring the cause of the strikes which occur simultaneously - if they did not the man on the platform could not see them simultaneously

 

[Doc Al]

two simultaneous strikes on the train - cause - the observer on the platform to see the two strikes simultaneously because he is the same distance from each strike.

Not true. (Unless those strikes are simultaneous in the platform frame.)

you seem to be ignoring the cause of the strikes which occur simultaneously - if they did not the man on the platform could not see them simultaneously

The strikes occur simultaneously in the platform frame.

Don't keep flipping back and forth between the two physically different scenarios:
(1) The lightning strikes are simultaneous in the platform frame.
(2) The lightning strikes are simultaneous in the train frame.

They cannot both be true. Stick to scenario 1 (as in the video) until you understand it.

 

[solarflare]

the lines being drawn from the x-axis are at -4 and +4 but both are at t=0

 

[Doc Al]

the lines being drawn from the x-axis are at -4 and +4 but both are at t=0

Yes. The lightning strikes are simultaneous in the platform frame.

 

[solarflare]

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

 

[solarflare]

Yes. The lightning strikes are simultaneous in the platform frame.

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

 

[Muphrid]

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

The x-axis measures distance according to someone on a stationary platform (that is, someone standing talong the track).

The diagram is very clear. The two photons' worldlines cross the platform/track observer's worldline at the same time (as expected) but do not cross the train observer's worldline at the same time. Hence, they do not reach the train observer at the same time. They cannot be perceived simultaneously in this situation.

 

[Doc Al]

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

No, the x-axis is the platform frame. The lightning strikes do not originate in a frame, they happen in all frames. The x' frame is the train frame.

Don't confuse events (such as: lightning striking the front of the train, lightning striking the rear of the train, light from the front of the train reaching the passenger) with frames. In this analysis there are two frames of interest: the platform frame and the train frame.

 

[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