What makes you say that? Did you watch the video?stevmg said:Yes it does
At the same instant according to who?stevmg said:Doc Al -
Why aren't the lightning strikes to the train at the same instant?
Watch it one more time. The person on the platform deduces (correctly) that the light from the two flashes reaches the person in the center of the train at different times. This is a fact that everyone must agree on, including observers on the train. (This is what JesseM was explaining.)stevmg said:Doc Al -
That's where my hangup is...
In the video it is NOT explained why the lightning strikes on the train are at different times than from the platform. It is stated that they are different but not explained why. It is a tautology - a supposition is presumed true and then is proven true because it was "true" from before (the assumption.) Look at the video again - look for the explanation of why the strikes are different and there is none.
Yes, exactly. But then, why do you say you only have it "half way"? It sounds like you got it. I'll try spelling out the steps in the argument in more detail, maybe it'll help:stevmg said:Wait a minute - Jesse M - I am getting it half way.
Usin Einstein's example precisely as he states it, with reference to the ground, the light arrives at the center observer on the train at different times and WON'T explode the bomb. So, if we posited that the if the light flashes hit the observer at the same time in the frame of reference of the train, then we would have a contradiction (or "parallel universes" as you called it.) as the bomb would explode in that frame but not in the ground frame.
According to Einstein's example in section 9, it is agreed that acording to the ground frame, the flashes do not meet at the same time at the midpoint because of the motion of the train, so they likewise cannot do it in the train frame (here, that would mean that they left points A' and B' at different times in the train frmae.
danielatha4 said:I think Stevmg said it perfectly. The video makes it seem as though the observer observes the passenger to see the front strike first (which maybe he does) but because of his reference frame it is concluded that in her reference frame she sees the flashes not simultaneously??
What DocAl initially said makes sense too:
"The light would only reach the passenger in the middle of the train at the same time if the lightning struck the ends of the train at the same time according to the frame of the train. But it doesn't."
But why doesn't it? according to the train frame. the video lends no credibility as to why the train passenger wouldn't see them at the same time, other than: that's the way the observer on the platform sees it. Is this credible?
danielatha4 said:What DocAl initially said makes sense too:
"The light would only reach the passenger in the middle of the train at the same time if the lightning struck the ends of the train at the same time according to the frame of the train. But it doesn't."
But why doesn't it? according to the train frame. the video lends no credibility as to why the train passenger wouldn't see them at the same time, other than: that's the way the observer on the platform sees it. Is this credible?
TcheQ said:It's hard to 'think' relativistic without diagrams. The whole point is that a different timeline of events are observed depending on where you are due light having to travel further/shorter distances.. If we use "according to the frame of the train" we assume the train is at rest for that calculation, which would mean the observer according to us travels at speed. But it is the reverse of this in the 'actual' situation.
One thing they don't explicitly mention is that the first bolt the person in the train sees is blue, while the second is red. The observer in the train would, if they had a sensitive enough detector with them, be able to calculate their speed from the redshift+blueshift of the light spectrum.
Since she sees them non-simultaneously in spite of the fact that they happened at equal distances from her, that means that in her reference frame they happened non-simultaneously. Keep in mind that seeing events simultaneously is distinct from them happening simultaneously in your frame. For example, if in 2010 I see the light from an event 5 light-years away in my frame, and in 2015 I see the light from an event 10 light-years away in my frame, then in my frame both events happened simultaneously in 2005.danielatha4 said:I think Stevmg said it perfectly. The video makes it seem as though the observer observes the passenger to see the front strike first (which maybe he does) but because of his reference frame it is concluded that in her reference frame she sees the flashes not simultaneously??
Yes, it was part of the starting premise of the problem that the lightning strikes happened simultaneously in the platform frame, and you can use that to conclude that in the platform frame the light must hit the train-observer at different times. And as I said above, all frames must agree on local events, so the train frame must predict the light hit the train observer at different times too. Of course you'd be free to imagine a different case where the strikes happened simultaneously in the train frame and thus their light hit the train-observer simultaneously, but this would be a physically different situation that isn't compatible with the premises that were used to define the physical facts of this problem.danielatha4 said:What DocAl initially said makes sense too:
"The light would only reach the passenger in the middle of the train at the same time if the lightning struck the ends of the train at the same time according to the frame of the train. But it doesn't."
But why doesn't it? according to the train frame. the video lends no credibility as to why the train passenger wouldn't see them at the same time, other than: that's the way the observer on the platform sees it. Is this credible?
Do you understand that because of the relativity of simultaneity, clocks that are synchronized in one frame are unsynchronized in another? Specifically, if the clocks at rest at either end of the train are at synchronized in the train frame and a distance X apart in that frame, then in the ground frame where the train is moving at speed v, the two clocks are out-of-sync by vX/c^2 (the time on the trailing clock will be ahead of the time on the leading clock).stevmg said:Even though this satisfies the concept by JesseM as no parallel but different universes there appears to be a contradiction in the times of the train frame. If one had atomic clocks all along the train synchronized together and atomic clocks on the ground synchronized together, one would still assume that the time in the front of the train the same as the back of the train.
But because of the relativity of simultaneity, the notion that the strikes happened "when the two observers passed each other" has no frame-independent meaning. After all, "when the two observers passed each other" is equivalent to "the event of each strike happens simultaneously with the the event of the two observers passing each other", but because of the relativity of simultaneity, events at different locations which occur simultaneously in one frame occur non-simultaneously in other frames. And that's exactly what this thought-experiment is intended to show--that if we want to require that the speed of light be c in both frames, there's no way of avoiding the conclusion that the two frames disagree about whether the two strikes were simultaneous!stevemg said:If one were to synchronize the train such that the lightning strikes would occur when the two observers passed each other
But the point of the thought-experiment is to show conceptually how the relativity of simultaneity follows from the basic postulates of relativity without having to go through the trouble of deriving the full Lorentz transformation from the two postulates. Of course if you already have the Lorentz transformation, it's quite trivial to show that events which are simultaneous in one frame are non-simultaneous in another. For example, say that in the unprimed frame event #1 happens at coordinates (x=0, t=0) and event #2 happens at coordinates (x=X, t=0). Since both these events happen at t=0, they are simultaneous in this frame. But now use the Lorentz transformation to see what coordinates each of these events will have in the primed frame and see what happens...the Lorentz transformation equations are:stevemg said:Rather than using words, one might be best served by using Lorentz to explain this as words are just confusing.
It's even simpler than involving the Lorentz transforms. All we really need to know is that light travels at c to all observers regardless of the motion of the source. So let's say the platform has a width of d_p and that the light from the strikes is seen simultaneously by the platform observer. So the time the light takes to travel from each side of the platform according to that observer in the center is just t_p = (1/2 d_p) / c. Now let's look at what the train observer sees with a relative speed of v. From the perspective of the train, the platform has a width of d_t and the light from each side of the platform is still measured to be traveling at c, but the center of the platform is also moving at v while the light travels to the center of the platform as well. The time the light takes to travel from the lightning strike on the furthest side of the platform to the center while the center of the platform also moves away from the light over the same time at v is c t_f = 1/2 d_t + v t_f, t_f = (1/2 d_t) / (c - v). The time that the light takes to travel from the lightning strike on the closest side of the platform while the center of the platform also moves toward the light over the same time at v is c t_c = 1/2 d_t - v t_c, t_c = (1/2 d_t) / (c + v).stevmg said:Rather than using words, one might be best served by using Lorentz to explain this as words are just confusing.
Well, the fact that clocks synchronized in their own rest frame are out-of-sync in other frames can be understood as just a consequence of the presupposition that every frame should measure the speed of light to be c. This implies that clocks in any frame must be synchronized in such a way that, if you set off a flash at the midpoint of the two clocks, they should both read the same time when the light reaches them. But now imagine a ship moving past you at high speed, with clocks on either end, and someone on board sets off a flash at the middle of the ship and sets the clocks to read the same time when the light hits them, so they're synchronized in the ship's frame. In your frame, the back clock was moving towards the point where the flash was set off, while the front clock was moving away from that point--so naturally if you assume the light travels at the same speed in both directions in your frame, the light should catch up to the back clock before it catches up to the front clock, meaning the clocks will necessarily be out-of-sync in your frame.stevmg said:To JesseM
Actually that does it (I'm almost there again) as you stated (I cannot as yet wrap my brain around it) that the lead clock is "behind" the trailing clock even though all these clocks on the train are in the same time frame. Accepting that I can understand what this is all about.
I cannot see (and you will not be able to explain to me as I am too stupid) any deeper than this but I DO appreciate the time you spent trying to enlighten me. You did get me to a higher level but I will never get any further than this.
Again, thanks for your help.
Your time equation is wrong there. The Galilei transform does not include any disagreements about simultaneity, the correct equations are:stevmg said:To JesseM
By using the Lorentz equations you HAVE demonstrated this effect. Under Galilean equations
x' = x - vt
t' = (t - vx/c^2)
But do you actually understand why the postulate that all frames measure c the same automatically implies disagreements about simultaneity? Or do you just understand that the Lorentz transformation implies disagreements about simultaneity, but not really understand how the Lorentz transformation follows from the two postulates of SR?stevemg said:Now, you may think this is strange but I understand that better than any of all the verbal explanations given so far by anyone.
Although Lorentz came up with the transformation equation, I believe Einstein was the first to propose that all laws of physics should be invariant under this transformation (not just the laws of electromagnetism), which is the physical content of special relativity. And so far all the most fundamental laws of physics discovered since have indeed obeyed Lorentz-invariant equations.stevemg said:Again the question - has anybody ever disproven Lorentz? He was not the Son of God and technically, this is a conjecture (although with more and likelighood of being a true theory.)
But then I take it you don't understand why the second postulate of SR (the one that says light moves at c in every inertial frame) implies the relativity of simultaneity, without the need to derive the Lorentz transformation? Did you read my post #25? Do you understand why the second postulate of SR implies that, in order for two clocks to be synchronized in their own rest frame, it must be true that if we set off a flash at the midpoint of the two clocks, they will read the same time when the light from the flash reaches them? If you understand that part, it wouldn't be hard to come up with a numerical example showing why, if a flash is set off at the middle of a ship and clocks at either end are set to the same time when the light reached them, then in a frame where the ship is moving, the light must reach the clocks at different times and therefore they must be out-of-sync in this frame. I can give you such a numerical example if you don't see how to construct it yourself...stevmg said:I still don't coonceive of this verbally, onlly mathematically.
One key point to understand is that in relativity all frames must agree about local events which occur at a single point in space and time. So, you can't have a situation where one frame predicts that two light rays hit an observer at the same moment in time and another doesn't, because this would involve a disagreement about local events (imagine that the observer has a bomb with light detectors on either side that will cause the bomb to explode if light hits both detectors within a very short time window--if different frames could disagree on whether the bomb exploded or not, that would essentially make different frames into parallel universes rather than just different ways of assigning space and time coordinates to events). That means that if the ground frame predicts the light hits the observer at the center of the train at different moments, then the train rest frame must say the same thing. But how can this be, given that both strikes happened at the same distance from the observer at the center of the train in the train rest frame, and the observer is at rest in this frame? The answer is that the strikes must have occurred at different times in this frame, so even though the light from each strike takes the same amount of time to reach him after the moment the strike occurred, since the strikes happened at different moments the light from each strike reaches him at different moments too.
What do you mean? By definition, the inertial rest frame of an object moving inertially is the frame where it's always at rest, that's just what "rest frame" means, in both Newtonian physics and relativity. In the train's rest frame, each part of the train remains at a fixed position coordinate while the ground moves at constant velocity past it.ValenceE said:I’m having difficulty following this, especially with the bolded segments. Imo, the only time we can use/say that the train’s frame is at rest is for the very short moment both strikes hit the train
"Single point in space and time" means a single time coordinate and a single position coordinate--the two lightning strikes happen at different position coordinates, in both frames.ValenceE said:(later shown to be simultaneous, as confirmed by the stationary observer situated precisely at an equidistant location from the train’s back and front when the strikes hit), and that would be what you refer to as the single point in space and time.
At rest relative to what? Rest and motion have no absolute meaning in relativity, you can only define them relative to particular objects or particular coordinate system. If you are at rest relative to the ground, you are moving relative to the train (and thus moving relative to the train's rest frame), while if you are at rest relative to the train, then you are moving relative to the ground (and thus moving relative to the ground's rest frame).ValenceE said:At that point, if you take an instantaneous snapshot of the scene when the strikes hit, both passenger and ground observer, in their own respective frames, are located exactly at the same distance from each end of the train, while being perfectly aligned orthogonally. Here, if we could remain at rest
But the whole point of the thought-experiment is to show that different frames define "simultaneity" differently--if the flashes are simultaneous in the ground frame, they cannot also be simultaneous in the train frame without violating the postulate that every frame should measure the speed of all light rays to be c.ValenceE said:and let the flashes follow their course, everyone would be happy to see that all happens simultaneously
I don't understand, why does the passenger need two clocks? Are they at different positions on the train? And why is he stopping the clocks when he receives the reflected light from the mirrors (and where are the mirrors positioned on the ground?) as opposed to stopping them when he receives the light from the flashes themselves?ValenceE said:give mirrors to the ground observer and synchronised detector/clocks to the passenger and he will observe that both reflected light flashes stop the clocks simultaneously.
Why do you say "reflected" light waves? It sounds like you're just talking about the ordinary light waves that proceed directly from the flashes to each observer here, no?ValenceE said:Let it roll on and the reflected light waves from both strike impact locations will travel, at c, eventually being perceived simultaneously by the ground observer, who is stationary with respect to the strikes, while the passenger will see a slight difference because, while the light waves travel at c, he is moving away from the back / towards the front strike locations, making it appear that they were not simultaneous when indeed they were.
Again, "in motion" means nothing in itself in relativity. The train is in motion relative to the ground's rest frame, and the ground is in motion relative to the train's rest frame. Also, it's not clear you understand that the phrase "train's rest frame" specifically refers to the inertial coordinate system where the train is at rest, i.e. its position coordinate remains unchanged as the time coordinate varies in this coordinate system.ValenceE said:The ground observer’s predictions are not for the train’s rest frame, they are for the train’s frame as it is in motion.
But in the ground frame, the strikes occurred simultaneously at equal distances from the ground observer, no? Therefore, if we assume the light moves at c in the ground frame, we must predict in the ground frame that the light from each strike reaches the ground observers simultaneously. And this means that the events of both light rays reaching the ground observer happen at the same position and time in the ground frame, so this is a fact about local events coinciding which different frames must agree on. Thus it must also be true in the train frame that the light from each strike reached the ground observer simultaneously.ValenceE said:So, I think that the agreement about preserving local events when viewed from the passenger’s perspective should be that, although they did, she sees that the strike flashes have not reached the stationary ground observer simultaneously, keeping in line with observations made in her own frame.
Still don't understand what you mean by "reflected light"--reflected from where? The thought-experiment as Einstein stated it was only meant to deal with the light traveling directly from the strikes to each observer, not with any light reflected off mirrors.ValenceE said:This will always appear to be true as the passenger is in motion with respect to the reflected light from the ground observer, exactly as it was inside the train with respect to the original strikes.
stevmg said:Sports Fans -
I am a doctor (MD) so give me a break here.
Blood doesn't move that fast and we cannot prove Fizeau's experiment with it...