Trying to Understand Light in Motion: A Frustrating Puzzle

[Muphrid]

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.

There is a train frame, but the train itself is an extended object, and the only reasons there should be any difference in how the observers see the strikes' flashes are

a) there is a distance difference, and the observer is closer to a strike than the other
b) there is a time difference, meaning one of the strikes themselves happens before the other

Both of these explanations apply in the same frame as an observer's velocity--i.e. these are how an observer may describe the difference in what he himself sees. To describe what another observer sees (again, this is valid), there is a third explanation:

c) there is a velocity difference, and the other observer is moving toward or away from the light of each flash.

This is still a valid thing to say for a chosen frame.

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.

That may be literally true, but what happens in another frame can be used to gain some information about what must happen in the first frame for things to be consistent.

Simple analogy: you're facing north and there's a building 3 miles east. Now turn to face the west. That building must be 3 miles behind you. Different frames in spacetime are mathematically the same as choosing a different direction to face on a North-South-East-West plane than due north. You can always describe locations in terms of ahead or behind, left or right, no matter how you rotate the coordinate system, and information for one orientation is entirely valid to tell you how another orientation must look. Spacetime is the same way for frames.

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.

No, the same distance does not tell you that the strikes are simultaneous. If just tells you that if the train observer perceives both flashes at the same time, then she will conclude that both strikes are simultaneous. It is not a complete argument in and of itself that the strikes themselves are simultaneous. You need some argument to say that she will perceive both flashes at the same time in the first place. See what I mean?

 

[solarflare]

you say the platform observer must conclude that the flashes were simultaneous

but then you say he can use his knowledge of light and the fact the train is moving to predict that she will see them seperately. so why can't he use his knowledge - if what you say is true - to conlude that because he saw them simultaneously that they couldn't have been simultaneous. and then because he knows that they can't be simultaneous because he saw them simultaneously and he predicts that the passenger will also see them seperatley then they will both agree that they were not simultaneous

 

[Muphrid]

you say the platform observer must conclude that the flashes were simultaneous

but then you say he can use his knowledge of light and the fact the train is moving to predict that she will see them seperately. so why can't he use his knowledge - if what you say is true - to conlude that because he saw them simultaneously that they couldn't have been simultaneous. and then because he knows that they can't be simultaneous because he saw them simultaneously and he predicts that the passenger will also see them seperatley then they will both agree that they were not simultaneous

Let's be precise: it's a given that the platform observer sees the flashes at the same time. What he must conclude from there is that the lightning strikes themselves (which we all agree happened before those flashes reach him) both occurred at the same moment, again because he believes that both strikes were equidistant from him. Why he believes this (and what tools he would need to be certain of this) I can't say. Really, how he determines this distance is beyond the scope of the problem; let's just assume he can determine distances accurately (there are enough ways you can do this that I certainly don't see the need to worry about it).

Him saying the strikes are simultaneous in his frame is really inescapable with all the information given. That's not to say he concludes everyone must believe the strikes were simultaneous. Just from the train observer's velocity, he can conclude that (a) she will see the forward flash before the rear flash and that (b) based on her position on the train, she would conclude that the forward strike hit before the rear strike. He can figure all of this out just by geometry, really. It's no different than drawing a couple lines on a graph and seeing where they intersect, exactly as cepheid did.

He knows that saying these two events were simultaneous to him is as meaningful to someone else moving relative to him as me saying I'm standing between two buildings and they're exactly lined up with the way I'm pointing. It may be true for me, and anyone else looking at me can verify this is what I see, but anyone else pointed in any other direction will not say they're aligned with the buildings.

 

[Dale]

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.

This is exactly correct. Similarly, the platform observer notices the flashes from both sides at the same time. Again, because each light pulse travels the same distance at the same speed the platform observer must conclude that the strikes actually happened at the same time.

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.

Exactly. Simultaneity is relative. The platform observer is correct that the strikes happened simultaneously in the platform frame and the train observer is correct that the strikes happened at different times in the train 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.

Again, please see the math above. This is impossible. The strikes cannot be simultaneous in both frames.

 

[ghwellsjr]

Solarflare, please don't ignore my question:

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?

 

[solarflare]

Solarflare, please don't ignore my question:

this one requires more than one because it is talking about more than one

to ignore that is to pick and choose data that you want and that is not scientific

 

[Dale]

to ignore that is to pick and choose data that you want and that is not scientific

Well said. Let's make sure that we agree on the actual data:

1) A lightning flash strikes the front of the train, leaving a black mark on the front of the train and on the right end of the platform adjacent to the tracks.

2) A lightning flash strikes the rear of the train, leaving a black mark on the rear of the train and on the left end of the platform adjacent to the tracks.

3) A platform observer is located midway between the left and right platform marks, as measured by rods on the platform.

4) A train observer is located midway between the front and back train marks, as measured by rods on the train.

5) The platform observer receives the light from both flashes at the same time.

6) The train observer receives the light from the front flash before receiving the light from the rear flash.

7) The train is moving with velocity v to the right in the platform frame, and the platform is moving with velocity v to the back in the train frame.

Can you agree on all of those data points, even if you disagree on the interpretation?

 

[solarflare]

1) the lightning only hit the train - not the platform - the train is on tracks that are a distance from the platform r1

2) same as 1

3) the length of the platform is not an issue he could be 1/3rd from 1 end and 2/3rds from the other as long as he was midpoint along the train when he saw the flashes.

4) yes i agree

5) yes i agree

6) no i do not agree - you can not add velocity or subtract it from c if they hit the train simultaneously because the train observer considers herself stationay so the light must reach her at the same time. just as if the lightning struck the platform the platform guy would see them simultaneously because he is stationary in his frame.

7) yes i agree
the problem is simple - i say the strikes must happen simultaneously in both frames if r1 = r2 and others say that is not

 

[Muphrid]

1) the lightning only hit the train - not the platform - the train is on tracks that are a distance from the platform r1

No, there is no r1; the problem can be worked as if the man on the platform and the woman on the train could actually occupy the same space in 1 dimension.

6) no i do not agree - you can not add velocity or subtract it from c if they hit the train simultaneously because the train observer considers herself stationay so the light must reach her at the same time. just as if the lightning struck the platform the platform guy would see them simultaneously because he is stationary in his frame.

There is no reason for the flashes to reach her at the same time in her frame if the strikes themselves do not happen at the same time in her frame. Do you agree or disagree on this point?

7) yes i agree
the problem is simple - i say the strikes must happen simultaneously in both frames if r1 = r2 and others say that is not

What is r2?

 

[Dale]

1) the lightning only hit the train - not the platform - the train is on tracks that are a distance from the platform r1

2) same as 1

Sure, but for simplicity let's set r1 to some small value, say 1 cm. Then it is reasonable to neglect it and also to consider the lightning strikes to have marked both the train and the platform.

6) The train observer receives the light from the front flash before receiving the light from the rear flash.

6) no i do not agree -

You appear to be contradicting your previous statement where you explicitly and clearly said that the passenger does see the flash from the front before the flash from the rear. Please explain.

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

 

[Jimmy]

You appear to be contradicting your previous statement where you explicitly and clearly said that the passenger does see the flash from the front before the flash from the rear. Please explain.

That quote is directly from the video which Solarflare doesn't agree with.

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.​

He didn't mention that hence the confusion.

 

[Dale]

That quote is directly from the video which Solarflare doesn't agree with.

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.​

He didn't mention that hence the confusion.

Ah, that makes sense.

solarflare, can you confirm that Jimmy is correct? If so, then I would remind you of your own comment:

to ignore that is to pick and choose data that you want and that is not scientific

You cannot simply ignore data that you disagree with. If one observer sees the light from the two flashes reach him at the same time then the other cannot. You are making stuff up and getting wrong conclusions as a result.

 

[Dale]

Let two flashes occur, one at t0 and x0 and the other at t1 and x1. The light from those flashes satisfies the following equations:c^2(t-t_0)^2=(x-x_0)^2c^2(t-t_1)^2=(x-x_1)^2
Where x0<x1 and t0<t and t1<t.

These two equations have at most one solution which satisfies the inequalities, which is:x = (c (-t_0 + t_1) + x_0 + x_1)/2t=(c (t_0 + t_1) - x_0 + x_1)/2c

So if anyone observer receives the signals at the same time then any other observer who is not co-located with the first observer at the time of the signal reception must receive them at different times.

Note, this proof does not require the flashes to be simultaneous nor does it require the observer to be equidistant between the flashes. It is a general proof for flashes with arbitrary timing and location. It is simply not possible for both the platform observer and the train observer to receive the light from both flashes at the same time since they are not co-located at the time that they receive the light.

 

[Muphrid]

In other words, all observers must agree on the invariant interval between two events. The platform observer sees the worldlines of the train observer and the forward flash intersect separately from when the train observer and the rear flash intersect. Since both observers must agree on the length of the invariant interval, and in the train observer's frame this interval is entirely a time duration and not a distance, the conclusion for the train observer is that the strikes were not simultaneous.

 

[solarflare]

Sure, but for simplicity let's set r1 to some small value, say 1 cm. Then it is reasonable to neglect it and also to consider the lightning strikes to have marked both the train and the platform.

this is a clear example of of picking and choosing data.

 

[solarflare]

No, there is no r1; the problem can be worked as if the man on the platform and the woman on the train could actually occupy the same space in 1 dimension.



There is no reason for the flashes to reach her at the same time in her frame if the strikes themselves do not happen at the same time in her frame. Do you agree or disagree on this point?



What is r2?

so your saying that the strikes do happen simultaneously in both frames but because she is moving towards 1 and away from the other she will say that the front one struck first and the rear one struck second

 

[Nugatory]

so your saying that the strikes do happen simultaneously in both frames but because she is moving towards 1 and away from the other she will say that the front one struck first and the rear one struck second

He is saying that the strikes do NOT happen simultaneously in both frames.

Let's say that train guy has set up a clock at each end of the train. He's set them so that both clocks read the same time in his frame. Platform guy also sets up two clocks, also reading the same in his frame, at the two points where the two lightning bolts will strike.

Let's say that these clocks are designed so that they will stop when they're hit by lightning.

After the lightning hits, if train guy's two stopped clocks both show the same time (lightning hit both clocks and stopped them at the same time in train guy's frame) then platform guy's two stopped clocks will not both show the same time (lightning hit platform guy's two clocks at different times in his frame).

 

[solarflare]

here is how i see the relativity of simultaneity

if a two bolts of lightning hit the platform equidistant from the observer on the platform he will see the flashes simultaneously.

if the passenger on the train was equidistant from the flashes when they occurred in the platform frame - then by the time the flashes moved from the source to her eyes she will have moved towards one and away from the other. so she will see two flashes but not simultaneously.

the amount of time between each flash from the passengers frame will depend on the speed that she is travelling.

the problem is that the video says the flashes occurred in her frame then moved to his frame and then moved again from his frame back to her frame.

 

[ghwellsjr]

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

I took another close look at the video and you are correct, the video is wrong. In fact, it's pathetic. Even the audio is wrong. Whoever put that thing together doesn't know what they're talking about. I can see now why you are saying some of the things you are saying.

 

[solarflare]

i think the problem here is that when i said the video is wrong everyone thought i was saying the concept of relativity of simultanety was wrong.

its like a scoolboy pointing out the teachers error and the teacher instead of looking to see if he made a mistake just tells him he is wrong and continues to teach the wrong stuff

 

[Doc Al]

i think the problem here is that when i said the video is wrong everyone thought i was saying the concept of relativity of simultanety was wrong.

The video could have been improved a bit, but the general idea of it is correct. On the other hand, you have convincingly demonstrated through your posts in this thread that you do not understand the relativity of simultaneity. What has changed?

 

[Doc Al]

here is how i see the relativity of simultaneity

if a two bolts of lightning hit the platform equidistant from the observer on the platform he will see the flashes simultaneously.

Sure, if they struck at the same time in the platform frame. (As is the case here.)

if the passenger on the train was equidistant from the flashes when they occurred in the platform frame - then by the time the flashes moved from the source to her eyes she will have moved towards one and away from the other. so she will see two flashes but not simultaneously.

the amount of time between each flash from the passengers frame will depend on the speed that she is travelling.

OK. But the key point is that one can use these facts to deduce that the lightning strikes were not simultaneous in her frame.

the problem is that the video says the flashes occurred in her frame then moved to his frame and then moved again from his frame back to her frame.

Huh?

The lightning strikes are events that happen. They occur in all frames!

 

[solarflare]

The video could have been improved a bit, but the general idea of it is correct. On the other hand, you have convincingly demonstrated through your posts in this thread that you do not understand the relativity of simultaneity. What has changed?

at least now you are not saying the video is 100% accurate - like i was saying all along

the video is wrong because according to the video they will both see the strikes simultaneously -

you say that they can't see them smultaneously using mathematical data - therefore agreeing with what I am saying

so wheres the problem - all you have to do is study the video and see where the person who made it went wrong

 

[Doc Al]

at least now you are not saying the video is 100% accurate - like i was saying all along

I would have changed one of the illustrations, but the main point of the video is correct.

the video is wrong because according to the video they will both see the strikes simultaneously -

Where does it say that? (It's not that wrong!) The video clearly states otherwise.

you say that they can't see them smultaneously using mathematical data - therefore agreeing with what I am saying

But you have been arguing in this thread that they were simultaneous in all frames!

so wheres the problem - all you have to do is study the video and see where the person who made it went wrong

Please state exactly what you think is wrong with the video.

 

[solarflare]

the trains frame is the origin of the strikes - the person uses that frame and says r1 = r2 and because the light travels the same distance from each strike he sees them simultaneously.

if the strikes happen seperatley in the trains frame like you say - then he can not be equidistant from each strike. he must be further away from one of them if they occur to him as simultaneous.

 

[Doc Al]

the trains frame is the origin of the strikes

No, the strikes occur in all frames!

- the person uses that frame and says r1 = r2 and because the light travels the same distance from each strike he sees them simultaneously.

This is your erroneous conclusion--nothing to do with what the video said. You seem to think that just because the distance is the same that the light must reach the center of the train simultaneously. Why?

if the strikes happen seperatley in the trains frame like you say - then he can not be equidistant from each strike. he must be further away from one of them if they occur to him as simultaneous.

When you say 'he' I'm not sure who you refer to. But no matter. From the platform frame, the platform observer is exactly in the middle of the strikes when they occur. And the train passenger is certainly in the middle of the train.

So?

 

[solarflare]

the strikes happen in one frame first - the trains frame
they then occur in the platform guys frame only after the light has traveled from the trains frame to the platform frame.

 

[Dale]

this is a clear example of of picking and choosing data.

What is wrong with setting r1 to 1 cm, or 1 mm, or 1 Angstrom? We are choosing a scenario (ie setting boundary conditions), so we are free to do that any way we want. We are picking boundary conditions, not data.

What we are not free to do is to pick and choose what data arises from the boundary conditions like you want to do. I have proven mathematically two different ways that your claim that both observers receive the light from both strikes at the same time cannot be true. You cannot pick any set of boundary conditions which lead to the data you are claiming.

Can you derive any logical inconsistency with my setting r1 to an arbitrarily small value? Do you understand your logical inconsistency I have shown two different ways?

 

[Dale]

its like a scoolboy pointing out the teachers error and the teacher instead of looking to see if he made a mistake just tells him he is wrong and continues to teach the wrong stuff

No, this is the usual situation where a student insists they know better than the teacher but the student is simply wrong. You are hardly the first student to have trouble understanding the relativity of simultaneity.

Regardless of the video, YOUR statements are wrong, as I have proven multiple times.

 

[Doc Al]

the strikes happen in one frame first - the trains frame
they then occur in the platform guys frame only after the light has traveled from the trains frame to the platform frame.

No. The lightning strikes happen--in all frames. When viewed from the platform frame, those strikes happen at the same time; when viewed from the train frame, at different times. But they are the same strikes.