Today Special Relativity dies

[ArmoSkater87]

jcsd, I though pretty much everyone agreed on true, true, true for the first stage. So why are u saying only the last statement is true.

 

[jcsd]

There's ambiguities in the statements:

The first statement purpisely suggetss that the clocks will appear synchronized to all onsrevers this is not the case.

the second statement is true except for the bit in the brackets which is only true in one inertial frame.

 

[ArmoSkater87]

oh i thought we asume they are synchronized in every case and every frame

 

[jcsd]

oh i thought we asume they are synchronized in every case and every frame

But this is the crux, they can't be synchronised in every frame as they are separated by distance.

 

[ram1024]

all the clocks are synchronous in the stationary "picture-frame" as hurkyl would call it.

we're determining simultaneity/detection times for the observers which i believe to be immutable (meaning that no matter what frame you choose to look at there will never be a frame where events happening to one observer will change order)

in case 1 we're assuming the train is stationary (i know nothing can be truly stationary in SR, calm down let's not get bogged down)

in case 2 we're moving the train, but because it's a relativistic inertial frame we should get the same results as in case 1 right?

in case 3 we're moving the observer, we SHOULD get a different result than in step 2, right?

that was the main focus of this "gedankenexperiment". Hurkyl caught on a bit too quickly and foiled my plans, and now i can't get anyone to admit where they stand on any of the cases :D

 

[ArmoSkater87]

thanks for the clarification! :P

 

[russ_watters]

thanks for the clarification! :P

To simplify a little for your sake, picture two clocks that are 300,000km apart and synchronized from Hurkyl's "picture frame." Each clock can send a signal to the other saying what time it is: since it takes 1sec for the signal to reach the opposing clock, each clock compares its own time to the signal it receives and concludes the other clock is a second behind. The paradox can obviously be resolved easily enough knowing the speed of light and the distance between the clocks and calculating the transmission delay.

This is true in Galilean Relativity, ie, before you even start to consider Einstein's SR/GR. The only thing added by SR is that the speed of light is constant.

 

[ram1024]

if you're going to tell him it's constant at least tell him HOW[/color] it's constant.

 

[Alkatran]

actually it all suits my purposes. i don't care to retute SR as long as it sees things MY way ;D

I find that quote really amusin, RAM.

Anyways, I just though I'd come in and tell you why all of this time dilation and non-simultaneity is popping up. Consider this: You have two objects in a two-dimensional universe. One is standing still in the reference frame, while the other is moving to the right at a high speed. As soon as the second object is at the same position as the first light is emitted from where they are positioned.

Now, the un-moving object perceives the light as expanding uniformely around it (in a circle) and SO DOES THE MOVING ONE. But how is that possible?? Well, check the attacked picture for what Einstein was thinking for relativity.

Now, remember this is a 2d universe, the up/down part of the picture represents the passage of time. (Notice that the light is expanding as it goes up? That's light moving away. Now, notice how the moving object (the blue one) has a "reality" that is warped in relation to time? Notice how it's actual passage through time is warped (it moves slightly faster upwards). Finally, notice that in it's reality the light is in a circle around it.

Do you understand now? That really for us to have a concept of simultaneous we could define a certain speed as "0" and say only things in that frame are simultaneous? (Think about this, if the observers know of the effects or relativity, shouldn't they compensate for it when deciding wether or not something was simultaneous? )

 

[Janus]

all the clocks are synchronous in the stationary "picture-frame" as hurkyl would call it.

Earlier in this thread you said

the clocks in all cases emit photon simultaneously RELATIVE TO EACH OTHER.

these clocks are perfectly aligned and synchronized and in all cases they move within the same inertial frame so they can stay calibrated.

So which is it? Now, for cases 1 & 3 it doesn't matter, because the "picture frame" and the "clock frame" are one in the same.

But in case 2, the clock frame and the "picture frame move relative to each other, and thus the clocks can't emit their photons simultaneously in both frames. They can emit simultaneously in one or the other, but not both.

 

[David]

By the way, u just can't try to disprove the theory of one of the greatest geniuses of all time, SR has been proven correct in several experiments. (-_-)

No, that’s a myth. Lorentz theory has been proven correct in several experiments. Einstein stole many of his SR theory ideas from Lorentz and modified Lorentz’s ideas.

The “time dilation” of atomic clocks was invented by Lorentz in the 1890s. So was “length contraction”, the “speed limit of c”, “mass increase due to motion”. See Lorentz's, “Versuch Einer Theorie Der Elektrischen Und Optischen Erscheinungen In Bewegten Körpern,” published in 1895 when Einstein was just 16 years old.

”To fill this gap, I introduced the principle of the constancy of the velocity of light, which I borrowed from H.A. Lorentz’s theory of the stationary luminiferous ether, and which, like the principle of relativity, contains a physical assumption that seemed to be justified only by the relevant experiments (experiments by Fizeau, Rowland, etc.).” A. Einstein, 1912

 

[David]

It isn't/doesn't have to be though, relativistic kinematics are suprisingly easy to understand.

Straight line non-accelerated relative motion can’t slow down the rate of any clock. As Lorentz pointed out, a “force” has to be placed on a clock timing mechanism, or removed from it, to change the clock’s rates. “Kinematics” without force changes no clock rates.

 

[ram1024]

So which is it? Now, for cases 1 & 3 it doesn't matter, because the "picture frame" and the "clock frame" are one in the same.

But in case 2, the clock frame and the "picture frame move relative to each other, and thus the clocks can't emit their photons simultaneously in both frames. They can emit simultaneously in one or the other, but not both.

no one's yet given me a reason to believe they'd emit photons non-simultaneously in EITHER frame, moving or not.

does the phrase "relative to each other" actually have a meaning in this case? i mean i just threw it out there without thinking of consequences because i cannot fathom the difference it would make.

so if you can, tell me what the difference would be if they emitted photons simultaneously in the "picture frame" and in the "relative to each other frame)

thanks in advance

 

[wespe]

no one's yet given me a reason to believe they'd emit photons non-simultaneously in EITHER frame, moving or not.

does the phrase "relative to each other" actually have a meaning in this case? i mean i just threw it out there without thinking of consequences because i cannot fathom the difference it would make.

so if you can, tell me what the difference would be if they emitted photons simultaneously in the "picture frame" and in the "relative to each other frame)

thanks in advance

Please read http://www.bartleby.com/173/9.html

 

[ArmoSkater87]

thanks to whoever helped me out :P

 

[ram1024]

Please read http://www.bartleby.com/173/9.html

Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A. Observers who take the railway train as their reference-body must therefore come to the conclusion that the lightning flash B took place earlier than the lightning flash A. We thus arrive at the important result

this case depicted is actually case 3 of my list, not case 2. note the sources are "lightning flashes" which are not in any way tied to the train.

the train moving towards B)flash is equivalent to my guy running towards B)emitter on top of the train in case 3.

case 2 is the crux of the argument. if "stationary" doesn't exist how can case 1 yield that light hits the observer simultaneously, yet in case 2 if you take that frame as being stationary (inertially relative frame) you DON'T get hit by light simultaneously?

i'm not sure i explained that well enough for you to understand :( sorry if not, i'll do better

 

[wespe]

Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A. Observers who take the railway train as their reference-body must therefore come to the conclusion that the lightning flash B took place earlier than the lightning flash A. We thus arrive at the important result

this case depicted is actually case 3 of my list, not case 2. note the sources are "lightning flashes" which are not in any way tied to the train.

the train moving towards B)flash is equivalent to my guy running towards B)emitter on top of the train in case 3.

case 2 is the crux of the argument. if "stationary" doesn't exist how can case 1 yield that light hits the observer simultaneously, yet in case 2 if you take that frame as being stationary (inertially relative frame) you DON'T get hit by light simultaneously?

i'm not sure i explained that well enough for you to understand :( sorry if not, i'll do better

Yes, it doesn't matter whether the sources are tied to the train or not, because speed of light is independent of its source.

So, what is the difference between your case #2 and #3?

The difference is: in case #2, the photons are emitted simultaneously relative to the emitters (which means also relative to the man). In case #3, the photons are emitted simultaneously relative to the emitters (as you said this is always the case), therefore they cannot be emitted simultaneously relative to the man. See, it matters in which frame they are emitted simultanously. So how do we know in which frame? It is the frame that the clocks (tied to the emitters) were synchronized in. And how were they synchronized? By sending two light signals to the emitters from the midpoint in that frame. Naturally it follows that: after this synchronization, simultaneous light signals will be received at the same time only at the midpoint in that frame (not some other frame where the midpoint moves [is somewhere else when the photons meet]).

 

[ram1024]

Yes, it doesn't matter whether the sources are tied to the train or not, because speed of light is independent of its source

The difference is: in case #2, the photons are emitted simultaneously relative to the emitters (which means also relative to the man). In case #3, the photons are emitted simultaneously relative to the emitters (as you said this is always the case), therefore they cannot be emitted simultaneously relative to the man

i think you're confusing the emissions with the intercepts... if the sources do not matter in speed, remove them from the picture and view the motions of the observers in case 2 and case 3. they're identical. the photons from both frames were emitted at exactly the same time and place. the only difference is, one guy is running and one guy is riding the train.

are you telling me runners and train riders have different perspectives at the same speed?

<we're getting close to the point where you're going to have the same revelation that i had i think. good stuff>

 

[wespe]

i think you're confusing the emissions with the intercepts... if the sources do not matter in speed, remove them from the picture and view the motions of the observers in case 2 and case 3. they're identical. the photons from both frames were emitted at exactly the same time and place.

No I'm not confusing anything. Did you read carefully what I wrote? The sources can only emit light according to how they were synchronized (before the experiment started). How do you suppose they know when to emit the photons? It is this synchronization that makes the cases 2 and 3 different. Actually, change the sources with mirrors, and you can think of the emitted photons as reflected photons sent from the midpoint (they were sent to synchronize the clocks). In case 2, the man sent these photons to synchronize the clocks, so he receives them back at the same time. In case 3, the stationary midpoint sent them, so it receives them back at the same time, and the man does not. Yes it is this simple.

 

[ram1024]

i can see where you would get this confused.

go back to the link on the page. the lightning is synchronized simultaneous ACCORDING to the simultaneous intercepts of the embankment.

it is deriving simultaneity FROM that frame. and then comparing it to all others.

the clock/emitters in my experiment are NOT deriving synchronization or simultaneity FROM any frame. they are synchronized together (let's say zero-distance) and then methodically placed into position using exacting methods to ensure they are never subjected to anything that would "unsynch" them

in this case simultaneous MEANS true simultaneity not "according to how you look at the picture".

i'm going to take a wild stab in the dark that SR doesn't allow ANYTHING to be truly simultaneous unless it emanates from ONE location... right?

 

[wespe]

i can see where you would get this confused.
go back to the link on the page. the lightning is synchronized simultaneous ACCORDING to the simultaneous intercepts of the embankment.
it is deriving simultaneity FROM that frame. and then comparing it to all others.

Yes.

the clock/emitters in my experiment are NOT deriving synchronization or simultaneity FROM any frame. they are synchronized together (let's say zero-distance) and then methodically placed into position using exacting methods to ensure they are never subjected to anything that would "unsynch" them

No, after the acceleration they go out of synch, even if they both go under the same acceleration. They may be still in synch in their final rest frame, but in all other frames they will look out of synch. The end result is the same as if they were synchronized by a signal from the midpoint in their final rest frame.

in this case simultaneous MEANS true simultaneity not "according to how you look at the picture".

i'm going to take a wild stab in the dark that SR doesn't allow ANYTHING to be truly simultaneous unless it emanates from ONE location... right?

There is no such thing as true simultaneity or simultaneity in all frames [when events separated by distance]. That Enistein gedanken proves [edit:no, discusses] just that. Seems you didn't get the point from that page.

editing to add: The point is, if two distant clocks are synchronized in their frame, they can not be [look] synchronized in other frames.

 

[ram1024]

There is no such thing as true simultaneity or simultaneity in all frames [when events separated by distance]. That Enistein gedanken proves just that. Seems you didn't get the point from that page

that's because the page assumes simultaneity is relative to begin with.

i see I'm going to have to employ stricter methods to convince you :D

ONE EMITTER.

TWO OBSERVERS.

Case #5

[u](o)                    <-)|(->                    (o)[/u]

One emitter simultaneously shoots 2 photons towards two ovservers equal distance from the center (where the emitter is). the observers carry synchronized clocks to time their photon receptions

SR predicts that since this is a "inertial frame" light will hit both observers at the same time. (True / False) ?

 

[ram1024]

i realize case 5 is a loaded question.

so I'm going to answer it myself so we can move on with no hard feelings :D

SR cannot make predictions as to the simultaneity of reception BECAUSE it does not believe in sychronous clocks at a distance.

fine. moving on to case #6

 

[ram1024]

Case #6

[u](o)                    <-)|(->                    (o)[/u]

Same thing as case #5 except this time the two clocks use laser light and the distance between them to synchronize. They synchronize in such a way that the light from the center hits them both at what appears to them to be the same "time"

the train is them sped up FASTER in the direction it was traveling (let's say to the right) and another light is pulsed.

SR predicts they receive light non-simultaneous now (because of clocks getting messed up) even though nothing happened that changed clock synch relative to the other clock. (True / False)

 

[wespe]

i see I'm going to have to employ stricter methods to convince you :D

You will either make me go insane, or I will give up on you. I'm not sure if you are reading what I'm saying. If you are thinking I don't qualify or something, just say so and I will leave you alone. Otherwise please try a bit harder and hopefully you will be able to answer your own questions.

Case #5

[u](o)                    <-)|(->                    (o)[/u]

One emitter simultaneously shoots 2 photons towards two ovservers equal distance from the center (where the emitter is). the observers carry synchronized clocks to time their photon receptions

SR predicts that since this is a "inertial frame" light will hit both observers at the same time. (True / False) ?

"At the same time" means "simultaneously". Simultaneity is meaningless unless you specify the frame it is measured in. So your question is meaningless. SR will predict that light will hit both observers at the same time in the frame of the emitter/observers/picture. There is no other frame seen here, but no doubt there will be in the next case. All this confusion is due to your omitting the frame bit.

Case #6

[u](o)                    <-)|(->                    (o)[/u]

Same thing as case #5 except this time the two clocks use laser light and the distance between them to synchronize. They synchronize in such a way that the light from the center hits them both at what appears to them to be the same "time"

the train is them sped up FASTER in the direction it was traveling (let's say to the right) and another light is pulsed.

SR predicts they receive light non-simultaneous now (because of clocks getting messed up) even though nothing happened that changed clock synch relative to the other clock. (True / False)

No, they will receive the light at the same time in the train frame. But will the clocks show the same time when they do? If the clocks remain synchronized according to the train frame*, yes. In any case, they will not look like receiving them at the same time in the picture frame. And even if the clocks remained synchronized in the train frame, they will not look synchronized in the picture frame.

*OK I can't talk so confidently when acceleration is involved. But I can say it depends on how the train is accelerated. Is it pulled from the front, or pushed from the back, or both "at the same time" (wrt what? If wrt themselves, then I -think- they will remain synchronized in the train frame)

 

[Alkatran]

Ram are you just ignoring what people have been trying to tell you?! It's been said time and time again that if something is "at the same time" in one frame, it doesn't mean it is in another! (most likely no others!)

This is where your flaw in thinking is: Just because your clock are undergoing the same speed changes doesn't mean they are in synch! They are only in synch in the "clock" frame ("train" frame)

If you are moving, your reality is distorted (see my first post, which you seem to have conveniently ignored) through time, so things that are NOW for me are LATER for you, or maybe BEFORE. This is how we can both be moving slow as seen by the other person!

 

[jcsd]

Straight line non-accelerated relative motion can’t slow down the rate of any clock. As Lorentz pointed out, a “force” has to be placed on a clock timing mechanism, or removed from it, to change the clock’s rates. “Kinematics” without force changes no clock rates.

The clocks don't 'slow down' there is no mystery force, they simply run at different rates.

 

[Janus]

no one's yet given me a reason to believe they'd emit photons non-simultaneously in EITHER frame, moving or not.

does the phrase "relative to each other" actually have a meaning in this case? i mean i just threw it out there without thinking of consequences because i cannot fathom the difference it would make.

so if you can, tell me what the difference would be if they emitted photons simultaneously in the "picture frame" and in the "relative to each other frame)

thanks in advance

Go back to the animations I gave earlier. In that example, if both observers do not agree that both flashes struck each other at the same time, we would have a paradox.

For instance: Suppose that there is an explosive device at the point where they pass each other that has to be triggered by both. Each observer will only trigger the device if he sees the flashes arrive simultaneously. It is obvious that in the frame of the embankment observer that the flashes arrive at the same instant as the observer on the railway car. Thus both should trigger the device and set off the explosive. If the Railway car observer did not see the flash arrive at the same instant, he would not trigger the device from his side and the device would not go off. This would set up a paradox where for one observer the track is destroyed,and for the other it isn't.

Now we include the postulates of Relativity.

1. The laws of physics remain the the same for all observers in all frames regardlesss of their relative motion.

I don't think we have any argument there because if this weren't true then we'd really have a case of different realities in different frames.


2. The speed of light(in a vacuum) is invarient for all observers regardless of their relative moton.

IOW, the speed of light is a constant when measured relative to an observer by that observer.

Now this is a sticking point for some. In fact, earlier you said that is was just an assumption. But it is much more than that. It in itself is a consequence of the first postulate.

The speed of light is determined by two universal constants : permeability and permitivity. These two constants also are what determine the strengths of the magnetic and electrostatic fields. Thus in order for the speed of light to have different values relative to an observer as measured by that observer, these constants would have to have different values for this observer also(the strength of magnets would change, etc.). The laws of physics would have to change for every observer depending on his relative motion to others.

It is actually worse than that. For if two beams of light were directed at the observer from two sources each moving at different velocities from the observer, in order for that observer to see the light from each be dependant on the velocity of each source (Galileian addition of velocities), these two constants would each have to have two different values each at the same time.

In fact, it is possible to show that if the speed of light wasn't invarient for every observer, light produced in one frame could not even be detected by a frame that was moving relative to that frame.

Add to this the fact that the invarience of the speed of light has been confirmed by multiple experiments.

The animations I provided show what happens according to the frames of both observers in order to uphold the principles above. As a result, simultanity is found to fail at distance between frames.

But this is a small price to pay, since at least this doesn't lead to any real paradoxes.

 

[russ_watters]

Straight line non-accelerated relative motion can’t slow down the rate of any clock. As Lorentz pointed out, a “force” has to be placed on a clock timing mechanism, or removed from it, to change the clock’s rates. “Kinematics” without force changes no clock rates.[emphasis added]

Its sloppy grammar to say clocks slow down in SR, though I admit I do it too. In actuality (and of course, you already know this, as you've been part of this same discussion many, many times), time itself passes at different rates for clocks in different frames, while the clocks themselves are, to a local observer, unaffected.

 

[russ_watters]

Ram, just to hammer this in a little more, please please remember that most of the simultenaity issues we're dealing with here are the same for Galilean Relativity as they are for Einstein's. You seem to be trying to argue against SR, but in actuality, you're arguing against the original basis of physics.

One more little thought experiment:

-Suppose you have three identical clocks next to each other and synchronized.
-Move two of the clocks 300,000km apart and keep the third (ignore SR effects).
-At a pre-determined time (call it T0), both clocks emit a signal.
-If you are 200,000km from one clock and 100,000 from the other, at what time according to your clock do you receive each signal (T0+X, T0+Y)?
-Are the clocks synchronized in your frame?

 

[ram1024]

of course they are. I know the speed of light and i know the distance from the emitters. they won't hit me simultaneously, but i can easily deduce the time of emission from distance and arrival times.

 

[ram1024]

http://home.teleport.com/~parvey/train1.gif

http://home.teleport.com/~parvey/train2.gif

i'll have you look at your own animations.

look at the first animation and then look at the second one.

what looks fishy about the second animation?

that's right, the speed of the bubble expansion from the left emitter is MUCH faster than the speed of the bubble on the right. in order to satisfy "constant relative to all viewers". SOMEONE made up length contraction. but wait. if we contract the distance to the right of the train it just leads to the light getting to the train even FASTER. we need to contract the distance BEHIND the train so that light arriving from that direction will arrive simultaneously. OH NO! protect our precious light speed!

 

[ram1024]

Ram are you just ignoring what people have been trying to tell you?! It's been said time and time again that if something is "at the same time" in one frame, it doesn't mean it is in another! (most likely no others!)

This is where your flaw in thinking is: Just because your clock are undergoing the same speed changes doesn't mean they are in synch! They are only in synch in the "clock" frame ("train" frame)

that's why i made the point in case #6 that we're determining "simultaneous synchro" by the setup in case #5.

give me your analysis for case #6 if you will be so kind :D

 

[ArmoSkater87]

no comment, for the most part...WOOSHHHHHHHHHHHH :P

 

[Doc Al]

case #5

Why I'm jumping in I have no idea. Masochism?

ONE EMITTER.

TWO OBSERVERS.

Case #5

[u](o)                    <-)|(->                    (o)[/u]

One emitter simultaneously shoots 2 photons towards two ovservers equal distance from the center (where the emitter is). the observers carry synchronized clocks to time their photon receptions

SR predicts that since this is a "inertial frame" light will hit both observers at the same time. (True / False) ?

I presume that the observer's clocks are synchronized in their own frame. If so, then the answer is: When the two observers detect the light, their clocks will read the same time. (Of course a third observer in relative motion to these two guys will disagree that the clocks were ever synchronized according to his frame.) So what?

 

[jdavel]

I have a suggestion.

Restate this problem using spacetime diagrams. ram's snapshot diagrams may seem unambiguous to him, but it's obvious that they don't seem that way to everyone else.

There's a reason that physicists discuss and explain relativity in terms of spacetime: it works!

And nothing has to be conceded on either side of the argument; spacetime is just as valid a concept in Galilean relativity as it is in SR.

 

[ram1024]

I presume that the observer's clocks are synchronized in their own frame. If so, then the answer is: When the two observers detect the light, their clocks will read the same time. (Of course a third observer in relative motion to these two guys will disagree that the clocks were ever synchronized according to his frame.) So what?

don't jump in and do one case and expect a revelation :D the "paradox" only comes after you realize the issue from multiple vantage points.

 

[russ_watters]

of course they are. I know the speed of light and i know the distance from the emitters. they won't hit me simultaneously, but i can easily deduce the time of emission from distance and arrival times.

Well herein lies the basic problem you are having with these thought experiments: you don't know what we're talking about when we say "simultaneous." You essentially answered 'yes, they are simultaneous in my frame' and then provided an explanation that says 'no, they are not simultaneous in my frame, but you can calculate that they are simultaneous to an outside observer.'

Frame, frame, frame, frame, frame, frame, frame!

In your frame, the events are not simultaneous - and that's the question I asked. That's what we mean when we say two events are not simultaneous according to a specific observer. You need to get on board with that concept. Its the root of the misunderstanding here.

 

[russ_watters]

in order to satisfy "constant relative to all viewers". SOMEONE made up length contraction. but wait. if we contract the distance to the right of the train it just leads to the light getting to the train even FASTER. we need to contract the distance BEHIND the train so that light arriving from that direction will arrive simultaneously. OH NO! protect our precious light speed!

While it may be easy enough to ignore length contraction for a train moving at 100km/hr, it is not easy to ignore for a train moving at a significant fraction of the speed of light as shown in the animations.

I mentioned before that all these thought experiments may be counterproductive: since thought experiments exist only in your head, you may start thinking the data exists only in your head too. It doesn't. The data has been collected from real experiments and demonstrates that C is constant. Maybe we should start looking at real experiments instead of thought experiments.

 

[ram1024]

see that's the thing, if simultaneity can be real at a single point, then simultaneity MUST be able to be real at a distance. not "according to an observer" but according to "reality".

to say it doesn't happen is like saying "no two things in the universe EVER happen at the same time"

whether or not they happen "at the same time to you" is merely a matter of perception and is NOT reality

 

[ram1024]

if you have any real experimental data handy you can share it, but i unfortunately do not have access to any

 

[wespe]

what looks fishy about the second animation?

that's right, the speed of the bubble expansion from the left emitter is MUCH faster than the speed of the bubble on the right. in order to

No, it doesn't look much faster to me, it looks very close. In fact, they should be the same, if the animation is accurate. Maybe you are fixing your eyes on the red dots? Remember this is the train's perspective. The speeds must be the same wrt the train.

 

[wespe]

whether or not they happen "at the same time to you" is merely a matter of perception and is NOT reality

Why can't that perception be your reality? Nothing can interact faster than light. And aren't you one of those observers? Will you prefer someone else's perception as your reality?

 

[Doc Al]

I mentioned before that all these thought experiments may be counterproductive: since thought experiments exist only in your head, you may start thinking the data exists only in your head too. It doesn't. The data has been collected from real experiments and demonstrates that C is constant. Maybe we should start looking at real experiments instead of thought experiments.

Russ makes an excellent point. While these thought experiments may serve to illustrate what relativity says in various situations and to show that it's perfectly self-consistent, they cannot prove that relativity is in fact how the world really works. Only experiment can do that.

 

[Hurkyl]

what looks fishy about the second animation?

that's right, the speed of the bubble expansion from the left emitter is MUCH faster than the speed of the bubble on the right.

I had a look at them again; they look the same speed to me.

The motion of the track kind of plays an optical illusion, though; if you still think the left bubble expands faster than the one on the right, crop the image so you can't see the track (maybe by shrinking your browser window and scrolling the image partially off the window) and see if you still think one expands faster than the other.

 

[Alkatran]

http://home.teleport.com/~parvey/train1.gif

http://home.teleport.com/~parvey/train2.gif

i'll have you look at your own animations.

look at the first animation and then look at the second one.

what looks fishy about the second animation?

that's right, the speed of the bubble expansion from the left emitter is MUCH faster than the speed of the bubble on the right. in order to satisfy "constant relative to all viewers". SOMEONE made up length contraction. but wait. if we contract the distance to the right of the train it just leads to the light getting to the train even FASTER. we need to contract the distance BEHIND the train so that light arriving from that direction will arrive simultaneously. OH NO! protect our precious light speed!

They're expanding at the same speed. The train track gives the illusion that one is moving more quickly than the other (remember that this is lgiht speed relative to the train and NOT the track!)

Watch the bubbles (the left sides of them) at each frame they move approximately one "track" (little brown line). They are moving at the same speed.

 

[Hurkyl]

no one's yet given me a reason to believe they'd emit photons non-simultaneously in EITHER frame, moving or not.

Take case two. Suppose each clock is reset to zero when the photons are emitted simultaneously in the picture frame.

Now, note the times at which each clock receives the others photon; they will be different. (as can easily be shown in the picture frame)


If the photons were emitted simultaneously in the clock frame, then the clocks would be synchronized in the clock frame. Furthermore, it takes the same time for the photon to get from A to B as it does from B to A. (Remember that the clocks are stationary in their rest frame!) Thus, the clocks must read the same time when they receive the other's photon.

Since the clocks, in fact, do not read the same time when they receive the other's photon, we conclude that the photons were not emitted simultaneously in the clock frame.

 

[jcsd]

Or just imagine a device to sychronize them:

at the half way point we have a device that emits a photon to each clock, when that photon arives at the clocks they tick. This synchronises the clocks in the staionary frame, but in a moving frame one photon will have further to travel than the other so they CANNOT tick at the same time in the moving frame. As our photon device sychronises the clocks perfectly in the rest frame this must hold true for all clocks that are synchronised in their rest frame whether we use this device or not.

 

[ram1024]

They're expanding at the same speed. The train track gives the illusion that one is moving more quickly than the other (remember that this is lgiht speed relative to the train and NOT the track!)

Watch the bubbles (the left sides of them) at each frame they move approximately one "track" (little brown line). They are moving at the same speed.

wondering if you guys have eyes...

look at example 2 yet again:

http://home.teleport.com/~parvey/train2.gif

now halfway through the animation, the light from emitter(L) starts. it covers a FULL distance in that time while light from emitter(R) travels half the remaining distance.

add that to the fact that the picture is skewed (look at the bubbles in the last frame they're not even centered to the sources anymore)

the light on the left is traveling about 4x faster than the light on the right.

 

[Hurkyl]

now halfway through the animation, the light from emitter(L) starts. it covers a FULL distance in that time while light from emitter(R) travels half the remaining distance.

So? The left emitter was much closer to the meeting point when it fires than the right emitter was.

add that to the fact that the picture is skewed (look at the bubbles in the last frame they're not even centered to the sources anymore)

The sources moved. What else would you expect?