Relativity of simultaneity objections

[mirrormirror]

Hi, I'm not a physicist, I'm 32 years old and I like physics. For the past years I've been troubled by an issue: the known thought experiment of Einstein with the train and the relativity of simultaneity notion.

Now, most probably I'm wrong or I'm missing something but I think that there is a mistake with the train thought experiment. The train experiment has two equivalent versions:

a) two lightning strikes hit the traincar both in the end-door and the front-door and the light goes towards the observer in the middle of the traincar. in this version, the event is simultaneous for the observer at the platform but not simultaneous for the observer in the traincar because the train is moving towards the light coming from the front of the wagon, thus it will reach the on-train observer faster.

b) the observer in the middle of the traincar emits light flashes going to both directions. in this version, the event ( of light reaching the doors ) is simultaneous for the train observer and not simultaneous forthe observer at the platform.To make things simple let's take version a) of the experiment.

I hereby claim ( i most probably am wrong somewhere ) that the measurement method of whether the events are simultaneous for the TRAINCAR observer is WRONG. Yes, the light beam coming from the front will reach him faster than the beam that comes from the back of the train, but he should NOT DEDUCE that the events are NOT simultaneous. The CORRECT way to measure the simultaneity would be to install TWO SYNCHRONISED clocks at both doors ( front and back ). That way when he checks the clocks he will notice that the lightnings hit both clocks at the same time.

This is also a way for someone inside an inertial frame of reference ( a cubic black room with no windows, moving at a constant speed ) to measure whether or not his inertial frame of reference is moving or not: Just install 6 SYNCHRONISED clocks ( it's doable with 2 as well ) at each wall of the room: left, right, back, forth, up, down and then emit with a device a laser beam simultaneously to all 6 ( or two ) directions. Since the big-black-room-with-no-windows is moving with a constant velocity, there will be differences at the times measured at the clocks on each wall, thus he can find out the velocity of the room ( frame of reference ) in each axis.

What do you say ?

Andreas T.

 

[Dale]

And how do you synchronize the clocks?

 

[mirrormirror]

And how do you synchronize the clocks?

I don't know, it's a technical issue I guess. Who cares? I think we already DO synchronise clocks in experiments, when we leave a clock on Earth and the other we put it on the spaceship. This is a thought experiment, I guess we shouldn't care about how we synchronise the clocks.

 

[Dale]

I don't know, it's a technical issue I guess.

Since your approach relies strongly on synchronized clocks this is an issue that you cannot gloss over.

Einstein provided a method for synchronizing clocks, known as the Einstein synchronization convention. Using that synchronization method, a pair of clocks which are synchronized in one reference frame will not be synchronized in another. So using your "CORRECT" way to measure simultaneity still results in the same situation that the train and the embankment frames disagree regarding the simultaneity of the strikes.

 

[mirrormirror]

Since your approach relies strongly on synchronized clocks this is an issue that you cannot gloss over.

Einstein provided a method for synchronizing clocks, known as the Einstein synchronization convention. Using that synchronization method, a pair of clocks which are synchronized in one reference frame will not be synchronized in another. So using your "CORRECT" way to measure simultaneity still results in the same situation that the train and the embankment frames disagree regarding the simultaneity of the strikes.

I don't know what was Einstein's method of synchronising clocks. Why would they not be synchronised outside that frame of reference?

But yes, let's say that the clocks were synchronised for the in-train passenger he would find too that the events were not simultaneous. The out of train observer would NOT use the two clocks for the measurement, he would use his one clock, because he doesn't need two clocks to measure in a CORRECT way the simultaneity. So the change of method of measurement concerns only the on-wagon observer. The current out-of-train observer had initially a CORRECT method of measuring. So it doesn't matter if the clocks are not synchronised in the external's observer frame, because he doesn't use them to measure the simultaneity

 

[Nugatory]

The CORRECT way to measure the simultaneity would be to install TWO SYNCHRONISED clocks at both doors ( front and back ). That way when he checks the clocks he will notice that the lightnings hit both clocks at the same time.

You have to be careful to specify how the clocks were synchronized. If you just say "two clocks that read the same at the same time, you're assuming what you're trying ton prove, namely that there is absolute simultaneity.

But with that said...

Yes, you are describing a correct way of establishing simultaneity. But... If the two clocks are synchronized and equidistant from the observer and both read the same time when the lightning strikes, then the flashes will both reach the observer at the same time. It has to be that way, because otherwise the observer, by doing a straightforward "speed equals distance divided by time" calculation, would find that the two flashes weren't both traveling at the speed of light - and we know from experiment that the speed of light is always c regardless of the motion of the source and destination.

Furthermore, the same has to be true for all observers: the one on the platform, or one in a plane flying at 400 kilometers/hr overhead, or an observer watching through a telescope from Mars which is moving at several miles a second relative to earth. (The Martian observer is important, because he can clearly see that the platform observer is no more "really at rest" than the train observer).

So now let's look at exactly how the two observers synchronized their clocks. Train observer, standing in the middle of the train, took two identically constructed clocks, set them both to read the same time (noon, let's say), and then instructed his two helpers to each carry a clock to each end of the train and leave it there.
Ground observer did the same thing with his two clocks.

Now try the thought experiment again, see what you see.

 

[WannabeNewton]

You must choose a consistent scheme of simultaneity and actually define what that simultaneity scheme is. This is what DaleSpam is saying.

 

[mirrormirror]

Yes, you are describing a correct way of establishing simultaneity. But... If the two clocks are synchronized and equidistant from the observer and both read the same time when the lightning strikes, then the flashes will both reach the observer at the same time. It has to be that way, because otherwise the observer, by doing a straightforward "speed equals distance divided by time" calculation, would find that the two flashes weren't both traveling at the speed of light - and we know from experiment that the speed of light is always c regardless of the motion of the source and destination.

The clocks will stop at the moment that the lightnings hit them. Let's say they both stop at 12:05:05. So the on-train observer goes to one end and checks one clock, then at the other end and checks the other other clock. Both clocks have stopped at the same time, thus it was simultaneous. It doesn't matter WHEN the flashes of the lightning reach the observer in the middle, YES they won't reach him simultaneously, but that DOESN'T mean that the speed of light is different! Neither does it mean that the events were not simultaneous. It only means that the train is MOVING to the direction of the light beam that reached him earlier.

about your second point and like i said before:

The nature of the experiment does not require the ground observer to use two clocks. He can just use his one clock with which he already found the events were simultaneous. Why he doesn't need to use two clocks? Because the light traveling from the doors towards the ground observer do not move in the direction of the train but vertical to it and he is not moving either.

 

[Nugatory]

I don't know, it's a technical issue I guess. Who cares? I think we already DO synchronise clocks in experiments, when we leave a clock on Earth and the other we put it on the spaceship. This is a thought experiment, I guess we shouldn't care about how we synchronise the clocks.

On the contrary, it's huge issue. If you synchronize the clocks in a way that assumes that there is no relativity of simultaneity, then of course you can conclude that there is no relativity of simultaneity - but your argument is circular so proves nothing.

Here you are assuming that if you synchronize two clocks (put them in the same place at rest relative to one another and set them to the same time) and then set them in motion relative to another they will remain synchronized. That's a very plausible and intuitive sort of assumption, one that everyone pretty much took for granted in the pre-relativity days. It's what behind your unspoken assumption that not only are the train clocks synchronized with each other (OK) and the ground clocks synchronized with each other (also OK) but that the train clocks and ground clocks are synchronized with each other (not OK).

But it turns out that if you make this assumption, as Einstein's train experiment showed, you can't also have the speed of light equal to c for all observers. And it is, so the assumption can't be valid.

 

[mirrormirror]

You must choose a consistent scheme of simultaneity and actually define what that simultaneity scheme is. This is what DaleSpam is saying.

Hm, why this scheme of simultaneity is not consistent? the on-train observer needs two clocks to measure it correctly and he cannot depend on the light beams because they travel along the axis of train, thus the speed of the train will interfere with the measuring. On the other hand the ground observer doesn't need two clocks and CAN use the light beams because he is not moving and the beams are moving vertically to the axis of the train movement.

 

[mirrormirror]

On the contrary, it's huge issue. If you synchronize the clocks in a way that assumes that there is no relativity of simultaneity, then of course you can conclude that there is no relativity of simultaneity - but your argument is circular so proves nothing.

Here you are assuming that if you synchronize two clocks (put them in the same place at rest relative to one another and set them to the same time) and then set them in motion relative to another they will remain synchronized. That's a very plausible and intuitive sort of assumption, one that everyone pretty much took for granted in the pre-relativity days. It's what behind your unspoken assumption that not only are the train clocks synchronized with each other (OK) and the ground clocks synchronized with each other (also OK) but that the train clocks and ground clocks are synchronized with each other (not OK).

But it turns out that if you make this assumption, as Einstein's train experiment showed, you can't also have the speed of light equal to c for all observers. And it is, so the assumption can't be valid.

i never said that the ground clock has to be synchronised with the train clocks. it doesn't need to be. The on-train observer needs two clocks to measure it correctly and he cannot depend on the light beams because they travel along the axis of train, thus the speed of the train will interfere with the measuring. On the other hand the ground observer doesn't need two clocks and CAN use the light beams because he is not moving and the beams are moving vertically to the axis of the train movement.

There is no requirement for the ground clock to be synchronised to anything. For the ground clock, the two beams will reach it simultaneously, thus the events were simultaneous.

 

[Chestermiller]

You wrote: I hereby claim ( i most probably am wrong somewhere ) that the measurement method of whether the events are simultaneous for the TRAINCAR observer is WRONG. Yes, the light beam coming from the front will reach him faster than the beam that comes from the back of the train, but he should NOT DEDUCE that the events are NOT simultaneous. The CORRECT way to measure the simultaneity would be to install TWO SYNCHRONISED clocks at both doors ( front and back ). That way when he checks the clocks he will notice that the lightnings hit both clocks at the same time.

If the clocks had been synchronized on the train, then when he checks the clocks, he will find that the lightning strikes did not hit both clocks at the same time. The strikes are observed to be simultaneous only by observers on the ground (using their own set of synchronized clocks).

 

[mirrormirror]

You wrote: I hereby claim ( i most probably am wrong somewhere ) that the measurement method of whether the events are simultaneous for the TRAINCAR observer is WRONG. Yes, the light beam coming from the front will reach him faster than the beam that comes from the back of the train, but he should NOT DEDUCE that the events are NOT simultaneous. The CORRECT way to measure the simultaneity would be to install TWO SYNCHRONISED clocks at both doors ( front and back ). That way when he checks the clocks he will notice that the lightnings hit both clocks at the same time.

If the clocks had been synchronized on the train, then when he checks the clocks, he will find that the lightning strikes did not hit both clocks at the same time. The strikes are observed to be simultaneous only by observers on the ground (using their own set of synchronized clocks).

sorry, but why will he notice that the clocks did not stop at the same time? The clocks are AT the doors. One clock in the back door one in the front. Once they get hit by the strikes they stop.

 

[Nugatory]

The clocks will stop at the moment that the lightnings hit them. Let's say they both stop at 12:05:05. So the on-train observer goes to one end and checks one clock, then at the other end and checks the other other clock. Both clocks have stopped at the same time, thus it was simultaneous. It doesn't matter WHEN the flashes of the lightning reach the observer in the middle, YES they won't reach him simultaneously

The bolded bit is a misunderstandng. If the two train clocks were synchronized and placed as I describe, and if they are read the same time when they were stopped, then the two flashes will reach the train observer at the same time. That time won't be 12:05:05, it'll be just a bit later to allow for light travel time (12:05:05+D/c where D is one-half the length of the train) but it will be the same time.

It only means that the train is MOVING to the direction of the light beam that reached him earlier.

But the train observer says that the train isn't moving - as far he's concerned, he and the train are at rest while the platform is moving backwards. Now if we could do the experiment with the stopped clocks as you describe above and the light from the two flashes did not reach him simultaneously any time that the clocks were equidistant and read the same when they were stopped... Then we'd be able look for the one observer for whom the flashes were simultaneous, say that one is the one that is "really" at rest and the rest are "really" moving. But that's not how the world wors.

 

[Janus]

i never said that the ground clock has to be synchronised with the train clocks. it doesn't need to be. The on-train observer needs two clocks to measure it correctly and he cannot depend on the light beams because they travel along the axis of train, thus the speed of the train will interfere with the measuring.

But the whole point is that the "speed" of the train does not interfere with the measurement of the light for the train observer. You seem to be holding on to an idea that somehow you can say that it is the train that is "really" moving. This isn't the case.

For anyone in the Train, the speed of light is a constant, just like it is for someone on the embankment. What this means is that he will measure the speed of the light coming from the rear of the Train as traveling at the same speed as the light coming from the front as measured relative to himself. If he is at the midpoint of the train and the light from the two ends reach him at different times, he knows that the light left the ends at different times, since light takes equal times to cross equal distances. Just like the embankment observer knows that the flashes occurred at the same time because he sees the lights at the same time.

 

[mirrormirror]

The bolded bit is a misunderstandng. If the two train clocks were synchronized and placed as I describe, and if they are read the same time when they were stopped, then the two flashes will reach the train observer at the same time. That time won't be 12:05:05, it'll be just a bit later to allow for light travel time (12:05:05+D/c where D is one-half the length of the train) but it will be the same time.

The two train clocks ( one in the back, one in the front ) will stop BOTH at 12:05:05 when the lightning hits them. The light of the strike, moves from the clocks towards the observer in the middle, the beam from the front will reach t1 after the clocks stopped, the beam from the beam from the end will reach him t2 after the clocks have stopped. t1 != t2 because the train is moving*. Anyway this is of no importance to simultaneity because what matters for SIMULTANEITY is WHEN THE CLOCKS STOP. He can check the clocks whenever he wants and when he does he will see they both stopped at the same time.

But the train observer says that the train isn't moving - as far he's concerned, he and the train are at rest while the platform is moving backwards. Now if we could do the experiment with the stopped clocks as you describe above and the light from the two flashes did not reach him simultaneously any time that the clocks were equidistant and read the same when they were stopped... Then we'd be able look for the one observer for whom the flashes were simultaneous, say that one is the one that is "really" at rest and the rest are "really" moving. But that's not how the world wors.

* That's what I challenged too, the idea that an observer of an inertial frame of reference can't tell if his frame of reference is moving. I'm saying that he CAN.

So what I'm saying is:

Exactly because of the FACT that the speed of light is CONSTANT and does not depend on the speed of the source that produces it, it is possible to KNOW whether or not your inertial frame of reference is moving or not.

So the idea that the speed of light does not depend on the source that produces is mutually exclusive with the idea that inside an inertial frame of reference it's impossible to tell whether it's moving or not. I just described in my first post a way to measure whether an inertial frame of reference is moving. quoting it again:

This is also a way for someone inside an inertial frame of reference ( a cubic black room with no windows, moving at a constant speed ) to measure whether or not his inertial frame of reference is moving or not: Just install 6 SYNCHRONISED clocks ( it's doable with 2 as well ) at each wall of the room: left, right, back, forth, up, down and then emit with a device a laser beam simultaneously to all 6 ( or two ) directions. Since the big-black-room-with-no-windows is moving with a constant velocity, there will be differences at the times measured at the clocks on each wall, thus he can find out the velocity of the room ( frame of reference ) in each axis.

 

[mirrormirror]

But the whole point is that the "speed" of the train does not interfere with the measurement of the light for the train observer. You seem to be holding on to an idea that somehow you can say that it is the train that is "really" moving.

yes, exactly that. I'm saying that you CAN tell if the train is really moving. here's how:

This is also a way for someone inside an inertial frame of reference ( a cubic black room with no windows, moving at a constant speed ) to measure whether or not his inertial frame of reference is moving or not: Just install 6 SYNCHRONISED clocks ( it's doable with 2 as well ) at each wall of the room: left, right, back, forth, up, down and then emit with a device a laser beam simultaneously to all 6 ( or two ) directions. Since the big-black-room-with-no-windows is moving with a constant velocity, there will be differences at the times measured at the clocks on each wall, thus he can find out the velocity of the room ( frame of reference ) in each axis.

 

[Dale]

I don't know what was Einstein's method of synchronising clocks. Why would they not be synchronised outside that frame of reference?

Here are a couple of references to explain it:
http://www.fourmilab.ch/etexts/einstein/specrel/www/
http://en.wikipedia.org/wiki/Einstein_synchronisation

The out of train observer would NOT use the two clocks for the measurement, he would use his one clock, because he doesn't need two clocks to measure in a CORRECT way the simultaneity.

This isn't a legitimate way to avoid the issue To determine simultaneity of a pair of events always requires a clock of each event, and the clocks must be synchronized according to some convention.

 

[mirrormirror]

Here are a couple of references to explain it:
This isn't a legitimate way to avoid the issue To determine simultaneity of a pair of events always requires a clock of each event, and the clocks must be synchronized according to some convention.

i don't see why clocks at different frames of reference have to be synchronized in order to measure SIMULTANEITY. Yes the clocks on the trains need to be synchronized among them, the same goes for the clocks out of the train ( on the platform ). But why do the clocks on the platform need to be synchronized with the clocks on the train ? We are not measuring WHEN an event happened, we are measuring if a couple of events happened simultaneously and for that it's enough that ALL the clocks on the SAME reference frame be synchronized.Plus, what's your opinion about my example of the big black room with the six clocks in it? Can you or can you not measure whether the room is moving ?

 

[Doc Al]

Plus, what's your opinion about my example of the big black room with the six clocks in it? Can you or can you not measure whether the room is moving ?

Of course you cannot. Nothing you can do within the room can tell you whether the room is moving or not.

 

[mirrormirror]

Of course you cannot. Nothing you can do within the room can tell you whether the room is moving or not.

do you care to provide some evidence or counter-arguments to my example?

 

[Nugatory]

do you care to provide some evidence or counter-arguments to my example:

This is also a way for someone inside an inertial frame of reference ( a cubic black room with no windows, moving at a constant speed ) to measure whether or not his inertial frame of reference is moving or not: Just install 6 SYNCHRONISED clocks ( it's doable with 2 as well ) at each wall of the room: left, right, back, forth, up, down and then emit with a device a laser beam simultaneously to all 6 ( or two ) directions. Since the big-black-room-with-no-windows is moving with a constant velocity, there will be differences at the times measured at the clocks on each wall, thus he can find out the velocity of the room ( frame of reference ) in each axis.

The reason you can't measure the motion of the closed room using this technique is that no matter what the constant motion of the room is, the time it takes, as measured by synchronized clocks moving with the room, for a light beam to move from one side of the room to the other is the width of the room divided by c.

This has been experimentally confirmed by various forms of the Michelson-Morley experiment (and see also the FAQ at top of this forum on experimental support for relativity). This is the link to the most directly relevant part: http://www.edu-observatory.org/phys...iments.html#Tests_of_Einsteins_two_postulatesIt's also a somewhat intuitive result when you consider that over a period of six months the Earth changes from moving at some miles a second relative to the sun in one direction to moving at the same speed in the exact opposite direction - yet we do not expect the travel time for speed of light signals between various points on Earth to change with the seasons.

 

[PeterDonis]

This is also a way for someone inside an inertial frame of reference ( a cubic black room with no windows, moving at a constant speed ) to measure whether or not his inertial frame of reference is moving or not: Just install 6 SYNCHRONISED clocks ( it's doable with 2 as well ) at each wall of the room: left, right, back, forth, up, down and then emit with a device a laser beam simultaneously to all 6 ( or two ) directions. Since the big-black-room-with-no-windows is moving with a constant velocity, there will be differences at the times measured at the clocks on each wall, thus he can find out the velocity of the room ( frame of reference ) in each axis.

An equivalent experiment to this one has been done; it's called the Michelson-Morley experiment. As others have already pointed out, it did *not* give the result you predict in what I quoted above. Given the results of the M-M experiment, the result of the experiment you give above is that there will be *no* differences in the times measured in the clocks on each wall, regardless of the state of motion of the room: light emitted from the exact center of the room will reach clocks on each wall at exactly the same time. (The M-M experiment, in your setup, would involve putting a mirror on each wall instead of a clock, to reflect the light beams back to the center, and then measuring whether the reflected beams all arrive back at the center at the same time--and they do.)

So your whole mental model is based on an incorrect prediction about what experiments will show.

 

[mirrormirror]

An equivalent experiment to this one has been done; it's called the Michelson-Morley experiment. As others have already pointed out, it did *not* give the result you predict in what I quoted above. Given the results of the M-M experiment, the result of the experiment you give above is that there will be *no* differences in the times measured in the clocks on each wall, regardless of the state of motion of the room: light emitted from the exact center of the room will reach clocks on each wall at exactly the same time. (The M-M experiment, in your setup, would involve putting a mirror on each wall instead of a clock, to reflect the light beams back to the center, and then measuring whether the reflected beams all arrive back at the center at the same time--and they do.)

So your whole mental model is based on an incorrect prediction about what experiments will show.

The bold part is EXACTLY what's WRONG with the experiment of M-M if they did it this way with mirrors. Of course if you place mirrors, the light will take t1 seconds to go to the front wall, then t2 ( less than t1 ) to move back to the center. For the back wall it will take t2 seconds to go from center to back wall, then t1 seconds from the back wall to the center. So yes, t1+t2 = t2+t1 but the fundamental ERROR is that you have to put CLOCKS at each wall, NOT measure the roundtrip time, the roundtrip time will be equal...

 

[Nugatory]

The bold part is EXACTLY what's WRONG with the experiment of M-M if they did it this way with mirrors. Of course if you place mirrors, the light will take t1 seconds to go to the front wall, then t2 ( less than t1 ) to move back to the center. For the back wall it will take t2 seconds to go from center to back wall, then t1 seconds from the back wall to the center. So yes, t1+t2 = t2+t1 but the fundamental ERROR is that you have to put CLOCKS at each wall, NOT measure the roundtrip time, the roundtrip time will be equal...

One of the key aspects of the MM experiment is that they used two light beams sent at right angles to one another. Both light beams were reflected back by mirrors set at the same distance away from the source; and the measurement was whether if they left at the same time they both arrived back at the same time. Because the two light beams are traveling at right angles to one another, they cannot both be equally affected by the t2<t1 effect that you're expecting. (Actually they could, if the apparatus happens to be set up so that both light beams make a 45-degree angle with respect to the direction of travel - but it's easy to eliminate this possibility by rotating the entire apparatus to point in some different direction and then repeating the experiment).

The reason for doing the experiment this way is that we can measure differences in the arrival times of two light beams with far greater accuracy than we can measure the arrival time of a light beam at a clock.

(Edit: I should add that nonetheless the experiment HAS since been done with a single clock at the destination, using instruments not available to Michelson and Morley as advancing technology made it possible to build these instruments)You'll find some more detailed explanations at the link that I posted, and by googling around.

 

[mirrormirror]

One of the key aspects of the MM experiment is that they used two light beams sent at right angles to one another. Both light beams were reflected back by mirrors set at the same distance away from the source; and the measurement was whether if they left at the same time they both arrived back at the same time. Because the two light beams are traveling at right angles to one another, they cannot both be equally affected by the t2<t1 effect that you're expecting. (Actually they could, if the apparatus happens to be set up so that both light beams make a 45-degree angle with respect to the direction of travel - but it's easy to eliminate this possibility by rotating the entire apparatus to point in some different direction and then repeating the experiment).

The reason for doing the experiment this way is that we can measure differences in the arrival times of two light beams with far greater accuracy than we can measure the arrival time of a light beam at a clock.You'll find some more detailed explanations at the long that I posted, and by googling around.

hm so you are saying that the choice of mirrors does not affect the measurement and that t2 would NOT be different than t1? Because it seems obvious that it would, because in one direction (beam moving from center to wall) the wall is moving away from the beam and in the other direction (beam reflected from wall to the center) it's moving towards it?

 

[Janus]

do you care to provide some evidence or counter-arguments to my example?

As has been mentioned, an actual experiment has already been done that shows that your example would not work as you expect it to.

Consider this:

You have a light source at one end of your box, at the other you have a mirror. You time how long it takes for the light to travel from light to mirror and back.

By your reckoning if the box is motionless, the light will take equal times to travel both ways and you get an answer of 2*L/c, where L is the length of the box.

However, if the box is moving along the line joining the light an mirror, then according to you, the light will take longer going one way than it does the other. Now, at first you might guess that these two effects will cancel out, and that the round trip will still take 2*l/c. However, it wouldn't work that way.

Light going one way would take a time of L/(c+v) and light going the other would take a time of L(c-v), where v is the velocity of the box This makes the total trip time;

L/(c+v) + L/(c-v) = 2Lc/(c^2-v^2)

which is not equal to 2*L/c

This experiment has been effectively performed, and always gets a result of 2*L/c. In other words, real life does not give the results you assumed you would get. And in science we don't care about what we think should happen, but what actually happens.

 

[darkhorror]

Ok so you sync they clocks on the train( in the trains frame of reference), then when the lightning flashes hit the clocks stop, you go and read the clocks, they read different times.

 

[Nugatory]

hm so you are saying that the choice of mirrors does not affect the measurement and that t2 would NOT be different than t1? Because it seems obvious that it would, because in one direction (beam moving from center to wall) the wall is moving away from the beam and in the other direction (beam reflected from wall to the center) it's moving towards it?

Yes, that is what I am saying. It seems obvious that it should make a difference, but with light in a vacuum it doesn't. This is Einstein's second postulate, and if you search some old threads in this forum you'll find a fair amount of discussion about why it actually makes sense if you think about it enough. (You'll find that the science advisers sometimes prefer to explain WHY it makes sense in different ways, but we'll all agree that it makes sense).

If I can arrange to put two synchronized clocks at two points in space separated by a given distance and at rest relative to me (so that distance doesn't change), and then send a light signal between those two points, in either direction, the travel time will always be distance divided by c. If I have a mirror and a clock at the destination so I can measure both legs as well as the total two-way round trip time, I'll find that the two-way time is twice the one way time, which is the same in both directions.

This is assuming that the clocks have been synchronized as I described above, of course.

 

[mirrormirror]

As has been mentioned, an actual experiment has already been done that shows that your example would not work as you expect it to.

Consider this:

You have a light source at one end of your box, at the other you have a mirror. You time how long it takes for the light to travel from light to mirror and back.

By your reckoning if the box is motionless, the light will take equal times to travel both ways and you get an answer of 2*L/c, where L is the length of the box.

However, if the box is moving along the line joining the light an mirror, then according to you, the light will take longer going one way than it does the other. Now, at first you might guess that these two effects will cancel out, and that the round trip will still take 2*l/c. However, it wouldn't work that way.

Light going one way would take a time of L/(c+v) and light going the other would take a time of L(c-v), where v is the velocity of the box This makes the total trip time;

L/(c+v) + L/(c-v) = 2Lc/(c^2-v^2)

which is not equal to 2*L/c

This experiment has been effectively performed, and always gets a result of 2*L/c. In other words, real life does not give the results you assumed you would get. And in science we don't care about what we think should happen, but what actually happens.

yes you are right the roundtrip wouldn't be the same.

But the most unexpected ( for me ) is that they measured it like you said and it's always 2*L/c even when the box is moving. that's weird! I don't understand why that happens. here is a possible explanation: c is not constant ( lol ).

 

[Dale]

But the most unexpected ( for me ) is that they measured it like you said and it's always 2*L/c even when the box is moving. that's weird! I don't understand why that happens.

First, you must accept that it in fact does happen. It is weird, but it is reality. Once you can accept that it does happen, then you can start forming a coherent picture of the universe that is consistent with the facts.

 

[darkhorror]

yes you are right the roundtrip wouldn't be the same.

But the most unexpected ( for me ) is that they measured it like you said and it's always 2*L/c even when the box is moving. that's weird! I don't understand why that happens.

Because the box isn't moving in it's own frame of reference.

 

[Dale]

i don't see why clocks at different frames of reference have to be synchronized in order to measure SIMULTANEITY.

You always have to have synchronization to measure simultaneity.

I have two clocks, struck by lightning, stopped when they were struck. Both clocks read 12:00:05, but by itself that doesn't tell you that the lightning strikes were simultaneous. Supplse that one was on Eastern time and the other was on Central time. Then the strikes occurred an hour apart. It is only if the clocks are synchronized that you can use their readings to determine simultaneity.

This goes back to the original problem. You must synchronize the clocks to determine simultaneity, so how do you synchronize the clocks? If you use Einstein's convention then clocks which are synchronized in one reference frame will not be synchronized in other reference frames. I.e. simultaneity is relative.

 

[Doc Al]

But the most unexpected ( for me ) is that they measured it like you said and it's always 2*L/c even when the box is moving. that's weird!

It's weirder than you think.

I don't understand why that happens. here is a possible explanation: c is not constant ( lol ).

Just the opposite. The speed of light is invariant. (Very weird!)

 

[mirrormirror]

First, you must accept that it in fact does happen. It is weird, but it is reality. Once you can accept that it does happen, then you can start forming a coherent picture of the universe that is consistent with the facts.

i didn't know that they measured it and it was 2*L/c both ways. ok i do accept that, and the first "rational" explanation that comes to mind is: c is not constant. in one way it's c+v in the other it's c-v.

i know of course that C is constant. just saying

 

[mirrormirror]

You always have to have synchronization to measure simultaneity.

I have two clocks, struck by lightning, stopped when they were struck. Both clocks read 12:00:05, but by itself that doesn't tell you that the lightning strikes were simultaneous. Supplse that one was on Eastern time and the other was on Central time. Then the strikes occurred an hour apart. It is only if the clocks are synchronized that you can use their readings to determine simultaneity.

This goes back to the original problem. You must synchronize the clocks to determine simultaneity, so how do you synchronize the clocks? If you use Einstein's convention then clocks which are synchronized in one reference frame will not be synchronized in other reference frames. I.e. simultaneity is relative.

yes but we had already established that the two clocks on the train reference (one front door one back door) were already synchronised. we are not talking about two clocks one in the train one in the platforms

 

[Nugatory]

But the most unexpected ( for me ) is that they measured it like you said and it's always 2*L/c even when the box is moving. that's weird! I don't understand why that happens.

The very flippant response would be that the universe is under no obligation to act the way you think it ought to.

A somewhat less flippant response is to point out that the speed of light is so high that none of us have any natural experience observing things moving at relativistic velocities, so we have to be a bit careful about trusting our intuition here. It's worth noting that the equations of special relativity reduce to those of classical mechanics if you assume that all the speeds involved are small compared to the speed of light.

An even less flippant answer is that light waves in a vacuum are fundamentally different than (for example) water waves in water (which do behave as you're expecting). The difference is that the water waves are moving at a constant speed relative to the water, and when I'm measuring my speed relative to them I can look down at the water, see if I'm moving relative to the water or are "really" at rest. You can't do that in a vacuum - there's just you and the light wave.

My favorite argument (other than the experimental results, which pretty much trump all the arguing of course) is that the speed of light in a vacuum can be calculated from the laws of electricity and magnetism, which do not care how fast you're moving. So if there's a light wave in my vicinity, I expect that I'll measure its velocity to be c - but someone moving past me had better get the same result too, because he's supposed to be subject to the same laws of electricity and magnetism.

The history here is interesting. Maxwell discovered these laws in the 1860s, and for the next half-century the single greatest unsolved problem in physics was how to reconcile these laws with our intuition based on the way that water waves work with water and sound waves work with air, and so forth. Special relativity was that resolution, and MM-style experiments confirmed that it's a good one.

In historical context it's not at all surprising that the title of Einstein's classic 1905 paper on special relativity was "On the electrodynamics of moving bodies". You can find copies on line; it's not exactly a gentle tutorial introduction but the math is surprisingly undemanding and it's a good read.

 

[Dale]

yes but we had already established that the two clocks on the train reference (one front door one back door) were already synchronised.

Again. How are they synchronized? You never specified that. You simply tried to wave your hand and gloss over it.

This thread has not progrressed anywhere since post 2.

 

[Dale]

in one way it's c+v in the other it's c-v.

That doesn't give you 2L/c. That gives you 2L/(c-v²/c).

Here are a list of experiments showing that the speed of light does not depend on the motion of the source:
http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#moving-source_tests

 

[mirrormirror]

The very flippant response would be that the universe is under no obligation to act the way you think it ought to.

A somewhat less flippant response is to point out that the speed of light is so high that none of us have any natural experience observing things moving at relativistic velocities, so we have to be a bit careful about trusting our intuition here. It's worth noting that the equations of special relativity reduce to those of classical mechanics if you assume that all the speeds involved are small compared to the speed of light.

An even less flippant answer is that light waves in a vacuum are fundamentally different than (for example) water waves in water (which do behave as you're expecting). The difference is that the water waves are moving at a constant speed relative to the water, and when I'm measuring my speed relative to them I can look down at the water, see if I'm moving relative to the water or are "really" at rest. You can't do that in a vacuum - there's just you and the light wave.

My favorite argument (other than the experimental results, which pretty much trump all the arguing of course) is that the speed of light in a vacuum can be calculated from the laws of electricity and magnetism, which do not care how fast you're moving. So if there's a light wave in my vicinity, I expect that I'll measure its velocity to be c - but someone moving past me had better get the same result too, because he's supposed to be subject to the same laws of electricity and magnetism.

The history here is interesting. Maxwell discovered these laws in the 1860s, and for the next half-century the single greatest unsolved problem in physics was how to reconcile these laws with our intuition based on the way that water waves work with water and sound waves work with air, and so forth. Special relativity was that resolution, and MM-style experiments confirmed that it's a good one.

In historical context it's not at all surprising that the title of Einstein's classic 1905 paper on special relativity was "On the electrodynamics of moving bodies". You can find copies on line; it's not exactly a gentle tutorial introduction but the math is surprisingly undemanding and it's a good read.

I can understand why C is constant and I accept that. What I don't understand is why the time is always 2*L/c. for the front door, since the door is moving away from the light (despite the fact that we don't know it yet), it makes sense that the light beam would need more time to reach it ( from the center ) than the other light beam going from the center to the back door ( clock ) which is moving towards the light.

 

[Doc Al]

I can understand why C is constant and I accept that.

It's not clear that you do.

What I don't understand is why the time is always 2*L/c. for the front door, since the door is moving away from the light (despite the fact that we don't know it yet), it makes sense that the light beam would need more time to reach it ( from the center ) than the other light beam going from the center to the back door ( clock ) which is moving towards the light.

From the viewpoint of observers moving within the box, the doors are stationary.

Viewed from some other frame, one in which the box (and its doors) are moving, you could say that the doors are moving away from or towards the beam of light. But not from the frame of the box itself.

 

[Nugatory]

I can understand why C is constant and I accept that. What I don't understand is why the time is always 2*L/c. for the front door, since the door is moving away from the light (despite the fact that we don't know it yet), it makes sense that the light beam would need more time to reach it ( from the center ) than the other light beam going from the center to the back door ( clock ) which is moving towards the light.

Go back to the train again. You, standing in the middle of the train, send a flash of light out in both directions.

I, standing on the platform say the train is moving forwards at speed S, the front of the train is moving away from the light, and the back of the train is moving towards the light.

You on the train say that you're at rest, while I, the platform, and the ground are moving backwards at speed S. You also say that the distance between the you and the front of the train is not changing and the front of the train is not moving away from the light; and likewise the back of the train is at rest and not moving towards the light.

We both have to be able to describe this situation consistently, such that the distance (as we see it) traveled by the light, divided by the travel time (as measured with our synchronized clocks at the various points along the light's path) comes out to be c. It can be done, but only if we accept that clocks moving relative to one another cannot stay in sync so will eventually disagree about which events happen at the same time. And that's where relativity of simultaneity (which started this thread) comes from.

 

[darkhorror]

Sounds to me like it isn't just simultaneity messing you up but the very basics of relativity.

Lets say there are two people one standing on earth, and another in a spaceship they are moving away from each other at .5c. How fast are they moving and with respect to what? How fast will each see light moving away from it?

 

[mirrormirror]

It's not clear that you do.


From the viewpoint of observers moving within the box, the doors are stationary.

Viewed from some other frame, one in which the box (and its doors) are moving, you could say that the doors are moving away from or towards the beam of light. But not from the frame of the box itself.

yes, the observer within the box cannot fathom whether or not the box is moving (but it's still moving even if he doesn't know it). That's why I thought that by installing two clocks one at the front and one at the back, it would be possible to actually have evidence on whether it is moving or not. Because if indeed the box is moving, light will reach later the front door than the back door thus he can determine if it's moving. But Janus said that they made the experiment and there was no time difference. THAT is that part which i don't understand HOW it happens.

 

[WannabeNewton]

Because if indeed the box is moving, light will reach later the front door than the back door thus he can determine if it's moving.

Nono. In his frame the box is at rest with respect to him. As far as he's concerned the two light rays reach the respective ends of the box at the same time.

 

[mirrormirror]

Sounds to me like it isn't just simultaneity messing you up but the very basics of relativity.

Lets say there are two people one standing on earth, and another in a spaceship they are moving away from each other at .5c. How fast are they moving and with respect to what? How fast will each see light moving away from it?

hm, if one is moving at 0.5c from point M and the other at -0.5c from point M ( minus stands for opposite direction ) then they each will "see" the other moving at -c from them.

 

[mirrormirror]

Nono. In his frame the box is at rest with respect to him. As far as he's concerned the two light rays reach the respective ends of the box at the same time.

I guess I'm getting confused because most probably i have in my mind a notion of "absolute frame of reference" where no-matter what each observer "thinks" (whether he is stationary or moving ), there is some absolute movement.

 

[mirrormirror]

Go back to the train again. You, standing in the middle of the train, send a flash of light out in both directions.

I, standing on the platform say the train is moving forwards at speed S, the front of the train is moving away from the light, and the back of the train is moving towards the light.

You on the train say that you're at rest, while I, the platform, and the ground are moving backwards at speed S. You also say that the distance between the you and the front of the train is not changing and the front of the train is not moving away from the light; and likewise the back of the train is at rest and not moving towards the light.

We both have to be able to describe this situation consistently, such that the distance (as we see it) traveled by the light, divided by the travel time (as measured with our synchronized clocks at the various points along the light's path) comes out to be c. It can be done, but only if we accept that clocks moving relative to one another cannot stay in sync so will eventually disagree about which events happen at the same time. And that's where relativity of simultaneity (which started this thread) comes from.

hm, but the fact that I ( who am on the train ) think that I'm at rest, doesn't really mean that I'm at rest! The fact that it's difficult for me to tell who's moving, doesn't mean that I'm not moving and that the other guy is. There "must" be some - even theoretical - frame of reference.

 

[WannabeNewton]

I guess I'm getting confused because most probably i have in my mind a notion of "absolute frame of reference" where no-matter what each observer "thinks" (whether he is stationary or moving ), there is some absolute movement.

Ah ok. Now we're getting somewhere. If we are talking about uniform motion (constant velocities) then there exists no such absolute frame of reference (this isn't special to special relativity by the way this is also true in Newtonian mechanics as described ages past by Galileo; in fact that aspect of Newtonian mechanics is called Galilean relativity). Uniform motion is relative.

 

[Doc Al]

Because if indeed the box is moving, light will reach later the front door than the back door thus he can determine if it's moving.

Why do you think that? Within the box the doors are stationary, yet light moves at speed c. So it takes the same amount of time for light to reach any door, as long as the distance is the same.

Do you understand why, given the fact that the speed of light is invariant, that someone moving with the box cannot determine that the box is moving?

Of course, observers in a different frame, who see the box and its doors as moving, will see the light reach the doors at different times. But not the observers moving with the box.