Fitzgerald -Lorentz contraction

[mich]

Why would the Michelson-Morley experiment be the experiment which proves "length contractions" of measuring rods if everything within the experiment lie on the same frame of reference?
According to Relativity, no contraction should exist at all in this case.

If the mathematical derivative of this constant comes from the "calculated" results of this particular experiment, how can we be certain it isn't flawed?

Andre

[Doc Al]

Why would the Michelson-Morley experiment be the experiment which proves "length contractions" of measuring rods if everything within the experiment lie on the same frame of reference?
It didn't prove "length contraction", but it did provide evidence for the non-existence of an ether. (Lorentz did propose a contraction hypothesis to explain the null results, but that was replaced by special relativity.)
According to Relativity, no contraction should exist at all in this case.
Right. Nothing contracts.
If the mathematical derivative of this constant comes from the "calculated" results of this particular experiment, how can we be certain it isn't flawed?
What constant? :confused:

[mgb_phys]

I think the OP meant formula not constant.
If you assume the speed of light is constant then you can explain the results of the MM experiment by assuming a contraction in the length in the direction of motion.

Doesn't just a 'classical' picture of the experiment being contracted give you the lorentz contraction formula? I thought that's how lorentz came up with it.

[Doc Al]

I think the OP meant formula not constant.
I'll bet you're right.
If you assume the speed of light is constant then you can explain the results of the MM experiment by assuming a contraction in the length in the direction of motion.
If you assume the speed of light is constant for all observers, as did Einstein, the null results follow immediately. If you assume that light travels with respect to an ether, then you can explain the null results by assuming that motion through the ether produces a length contraction, as Lorentz proposed.

[heafnerj]

I suggest the OP read Arnold Arons' thorough exposition of this topic in his 1965 textbook Development of Concepts of Physics. It's a model of clarity!

[mich]

It didn't prove "length contraction", but it did provide evidence for the non-existence of an ether. (Lorentz did propose a contraction hypothesis to explain the null results, but that was replaced by special relativity.)

However, lenght contraction was the proposed solution to the nul result.


Right. Nothing contracts.

What constant? :confused:

If I'm not mistaken, the constant "c" of the light's velocity (a little pun on the side) was proposed by Dr.Einstein in order to rid of the ether theory, but also to explain the nul result of the M&M experiment.

The experiment was first proposed in order to detect the ether, which came from Maxwell's equations ; if I'm not mistaken, it seems that the qualities( permeability and permittivity) of free space were properties of the ether itself, which gave a constant velocity for light within the ether.It was because of Maxwell that Michelson believed light to have a constant velocity within the ether, in the first place; this, being needed, in order to afterwards calculate the velocity of the earth through space.

Now, since the theory Relativity implies there to be length contractions between moving frames; it must have been taken from the M&M experiment itself, for where else was this implied before?

But since within the Michelson and Morley experiment, nothing within the experiment moves, that is, observer relative to source, then no time dilations, nor length contractions are to be expected...which begs the question; why does the theory of Relativity imply there to be a contraction of measuring rods in the first place?

Andre

[mich]

I'll bet you're right.

If you assume the speed of light is constant for all observers, as did Einstein, the null results follow immediately. If you assume that light travels with respect to an ether, then you can explain the null results by assuming that motion through the ether produces a length contraction, as Lorentz proposed.


However, not because of any measuring rod contractions. The particle theory of light could predict the same nul result.

Andre

[granpa]

However, not because of any measuring rod contractions.

Andre

huh? whats that mean?

[Doc Al]

Now, since the theory Relativity implies there to be length contractions between moving frames; it must have been taken from the M&M experiment itself, for where else was this implied before?

But since within the Michelson and Morley experiment, nothing within the experiment moves, that is, observer relative to source, then no time dilations, nor length contractions are to be expected...which begs the question; why does the theory of Relativity imply there to be a contraction of measuring rods in the first place?

I am puzzled by your reasoning. Is "length contraction" between moving frames a consequence of special relativity? Sure, along with time dilation and the relativity of simultaneity (as described by Einstein in 1905). But that doesn't mean that "length contraction" has anything directly to do with the Michelson-Morley experiment.

For a nice discussion of the Michelson-Morley experiment, followed by lectures on the basic principles and consequences of special relativity, you might try this: The Michelson-Morley Experiment (http://galileoandeinstein.physics.virginia.edu/lectures/michelson.html)

[mich]

huh? whats that mean?

Simply that even by assuming the speed of light to be invariant, the Michelson and Morley experiment would not involve any measuring rod
contractions nor any time dilation factor. Yet, any book on Relativity will identify the Michelson and Morley experiment as being responsible for the development of theory of Relativity....the Lorentz transform was taken from this.

Now, we know the reason why Michelson suggested a length contraction....it was in order to explain the nul result....but why would Relativity need a length contraction in order to explain it's theory?

Andre

[granpa]

.but why would Relativity need a length contraction in order to explain it's theory?

Andre

this is simple relativity 101 stuff. dont they teach this stuff anymore? if you assume the speed of light is constant (which the Michelson and Morley experiment showed) then a few simple thought experiments are sufficient to prove that length contraction, time dilation, and loss of simultaneity must occur.

look up "light clock"

with a light clock one can measure distances by simply bouncing light off objects and measuring the travel time of the light pulse.

[mich]

this is simple relativity 101 stuff. dont they teach this stuff anymore? if you assume the speed of light is constant (which the Michelson and Morley experiment showed) then a few simple thought experiments are sufficient to preve that length contraction, time dilation, and loss of simultaneity must occur.

look up "light clock"

with a light clock one can measure distances by simply bouncing light off objects and measuring the travel time of the light pulse.

The measurements of time will lead me to believe there was a dilation of time experienced on the moving frame....now, tell me why I should believe there was any length contractions involved? What measurements made me assume this?

Andre

[granpa]

imagine a pulse of light bouncing between the front and back of the moving object.

[mich]

imagine a pulse of light bouncing between the front and back of the moving object.

Good; this is what I'm interested to know; I'm not interested in any thought experiments, but real experiments proving that length contraction exists.
In your though experiment, I can always think of the moving object as remaing the same length even if I assume the velocity of light is constant.

Andre

[mich]

I am puzzled by your reasoning. Is "length contraction" between moving frames a consequence of special relativity? Sure, along with time dilation and the relativity of simultaneity (as described by Einstein in 1905). But that doesn't mean that "length contraction" has anything directly to do with the Michelson-Morley experiment.

For a nice discussion of the Michelson-Morley experiment, followed by lectures on the basic principles and consequences of special relativity, you might try this: The Michelson-Morley Experiment (http://galileoandeinstein.physics.virginia.edu/lectures/michelson.html)

I will read the link that you gave me and get back to you; My question, I guess, would be, would the theory of Relativity have
developped even without the Michelson and Morley experiement?


Andre

[granpa]

Good; this is what I'm interested to know; I'm not interested in any thought experiments, but real experiments proving that length contraction exists.
In your though experiment, I can always think of the moving object as remaing the same length even if I assume the velocity of light is constant.

Andre

no, you cant. remember that to a person on-board the moving object the light pulse must move at c. we have already established the time dilation so the only way that light pulse can appear to move at c is for length contraction to take place. you need to do some math to see exactly how long the pulse will take to bounce back and forth from your persective. its not that hard really. its just algebra.

[mich]

no, you cant. remember that to a person on-board the moving object the light pulse must move at c. we have already established the time dilation so the only way that light pulse can appear to move at c is for length contraction to take place. you need to do some math to see exactly how long the pulse will take to bounce back and forth from your persective. its not that hard really. its just algebra.

It seems to me, granpa, that it's easily done. Suppose that a frame (train) is passing by me at "v", and as it passes by, it sends a light signal from the back of the train towards the front (at the time when the back of the train passes by me).Let the legth of the train be "x". The observers on the train measure the speed of light as being "c", while I also measure the same speed, instead of (c+v). Therefore, the distance the light will travel (in my frame of reference) would be x+ vt (t = the time for the light to travel from the back of the train to the front).

In this case t= [x+(c+v)] / c

on the moving frame, the distance "could still be x" while t' would have been dilated.

t'= x/c

No need for length contractions at all. However, if there are indeed length contractions, and certainly it's possible, then there ought to be some experiments which can prove this to be true.

Andre

[Doc Al]

It seems to me, granpa, that it's easily done. Suppose that a frame (train) is passing by me at "v", and as it passes by, it sends a light signal from the back of the train towards the front (at the time when the back of the train passes by me).Let the legth of the train be "x". The observers on the train measure the speed of light as being "c", while I also measure the same speed, instead of (c+v). Therefore, the distance the light will travel (in my frame of reference) would be x+ vt (t = the time for the light to travel from the back of the train to the front).

In this case t= [x+(c+v)] / c

on the moving frame, the distance "could still be x" while t' would have been dilated.

t'= x/c

No need for length contractions at all. However, if there are indeed length contractions, and certainly it's possible, then there ought to be some experiments which can prove this to be true.

Andre
You can't just pick and choose the parts of relativity that you like and ignore the others. Length contraction, time dilation, and the relativity of simultaneity all work together to give a consistent picture. Your calculations don't match those of special relativity, which has been amply confirmed by experiment.

If you call the rest length of the train (in its own frame) to be L', the time for the light to travel from one end to the other--according to train observers--will be t' = L'/c.

In the track frame, the light travels a distance L + vt, where L is the length of the train as measured by the track observers. Thus the time for the light to travel from one end to the other--according to track observers--must satisfy: ct = L + vt, thus t = L/(c-v). Of course, relativity tells us that the train is contracted, thus L = L'\sqrt{1 - v^2/c^2}.

If you are interested in experimental evidence for relativity, read the sticky at the top of this forum: FAQ: Experimental Basis of Special Relativity (http://75.126.60.30/showthread.php?t=229034)

[granpa]

yes that would work but you didnt look up light clock like I suggested. time dilation is the only way to explain the different travel times of the light in the light clock (perpendicular to the motion of the object). only length contraction is left to explain the difference in travel time parallel to the objects motion.

[mich]

I am puzzled by your reasoning. Is "length contraction" between moving frames a consequence of special relativity? Sure, along with time dilation and the relativity of simultaneity (as described by Einstein in 1905). But that doesn't mean that "length contraction" has anything directly to do with the Michelson-Morley experiment.

For a nice discussion of the Michelson-Morley experiment, followed by lectures on the basic principles and consequences of special relativity, you might try this: The Michelson-Morley Experiment (http://galileoandeinstein.physics.virginia.edu/lectures/michelson.html)

Ok; I've read through the document; I am not saying it was quite that easy.
I had some misunderstandings on some of Michelson's points of view. For the image of the river,he speaks of currents (being a resistance to flow) which is not found in the M&M experiment. The first swimmer remains beside the bank and swims to and fro with and against the current...ok this I can take. But instead of having the second swimmer aiming 90 degrees from the bank, he writes:

"It won't do simply to aim directly for the opposite bank-the flow will carry the swimmer downstream. To succeed in going directly across, the swimmer must actually aim upstream at the correct angle (of course, a real swimmer would do this automatically)."

But within the experiment, the sources of light are indeed separated by 90 degrees.

He goes on to write :

"Thus, the swimmer is going at 5 feet per second, at an angle, relative to the river, and being carried downstream at a rate of 3 feet per second".

However, if the swimmer points 90 degrees to the other bank (which is what the experiment denotes) then, he crosses the bank in
20 seconds or the distance (100 feet) / velocity (5 feet/sec).However, due to the influence ofthe second velocity, that of the river, he will arrive further away along the bank.

Andre

[mich]

You can't just pick and choose the parts of relativity that you like and ignore the others. Length contraction, time dilation, and the relativity of simultaneity all work together to give a consistent picture. Your calculations don't match those of special relativity, which has been amply confirmed by experiment.

I agree, Doc. But I am not picking and choosing; I am trying to understand for what reason Einstein thought about implementing a measuring rod contraction...What was measured in order for him to come up with such a notion?



If you call the rest length of the train (in its own frame) to be L', the time for the light to travel from one end to the other--according to train observers--will be t' = L'/c.

ok.


In the track frame, the light travels a distance L + vt, where L is the length of the train as measured by the track observers. Thus the time for the light to travel from one end to the other--according to track observers--must satisfy: ct = L + vt, thus t = L/(c-v).

ok.



Of course, relativity tells us that the train is contracted, thus L = L'\sqrt{1 - v^2/c^2}.

What argument is there for the train to be indeed contracted?What experiment was done to prove this assumption?

[/QUOTE]
If you are interested in experimental evidence for relativity, read the sticky at the top of this forum: FAQ: Experimental Basis of Special Relativity (http://75.126.60.30/showthread.php?t=229034)[/QUOTE]


Andre

[mich]

yes that would work but you didnt look up light clock like I suggested. time dilation is the only way to explain the different travel times of the light in the light clock (perpendicular to the motion of the object). only length contraction is left to explain the difference in travel time parallel to the objects motion.


I'm sorry granpa; was there a thread in particular you wanted me to look at for light clocks?Or just the internet?

Andre

[granpa]

a light clock is simply 2 mirrors with a pulse of light bouncing between them. each time it bounces off the mirror the clock ticks. the light path is perpendicular to the motion of the object (in the frame of the object). an observer at rest sees the light take a much longer path. time dilation is the ONLY way to explain it.

time dilation alone cant explain both the light path perpendicular and parallel to the motion of the object.

[Doc Al]

Ok; I've read through the document; I am not saying it was quite that easy.
I had some misunderstandings on some of Michelson's points of view. For the image of the river,he speaks of currents (being a resistance to flow) which is not found in the M&M experiment. The first swimmer remains beside the bank and swims to and fro with and against the current...ok this I can take. But instead of having the second swimmer aiming 90 degrees from the bank, he writes:

"It won't do simply to aim directly for the opposite bank-the flow will carry the swimmer downstream. To succeed in going directly across, the swimmer must actually aim upstream at the correct angle (of course, a real swimmer would do this automatically)."

But within the experiment, the sources of light are indeed separated by 90 degrees.

He goes on to write :

"Thus, the swimmer is going at 5 feet per second, at an angle, relative to the river, and being carried downstream at a rate of 3 feet per second".

However, if the swimmer points 90 degrees to the other bank (which is what the experiment denotes) then, he crosses the bank in
20 seconds or the distance (100 feet) / velocity (5 feet/sec).However, due to the influence ofthe second velocity, that of the river, he will arrive further away along the bank.

The path of the light (in the lab frame) in one arm is at 90 degrees to the other. That means that the light must be aimed slightly into the ether wind (if there were an ether wind), exactly like the analogy with the swimmer.
I agree, Doc. But I am not picking and choosing; I am trying to understand for what reason Einstein thought about implementing a measuring rod contraction...What was measured in order for him to come up with such a notion?
Length contraction is a consequence of the basic assumptions of special relativity, one of which is the fact that the speed of light is the same for all observers.
What argument is there for the train to be indeed contracted?What experiment was done to prove this assumption?
The argument for length contraction (and time dilation, etc.) is given in detail in the follow on lectures on the page I linked in post #9. Read it.

As far as I know, there is currently no direct experimental confirmation of length contraction (it's just too small to detect), but the experimental evidence for relativity as a whole (which requires length contraction) is overwhelming.
I'm sorry granpa; was there a thread in particular you wanted me to look at for light clocks?Or just the internet?

Once again, if you want to learn about time dilation and light clocks, follow that link in post #9.

[mich]

a light clock is simply 2 mirrors with a pulse of light bouncing between them. each time it bounces off the mirror the clock ticks. the light path is perpendicular to the motion of the object (in the frame of the object). an observer at rest sees the light take a much longer path. time dilation is the ONLY way to explain it.

time dilation alone cant explain both the light path perpendicular and parallel to the motion of the object.

Ok; so, since, according to the observer on the rest frame, measures the path of light horizontal to the moving frame, as experiencing a length contraction, while the path going perpendicular does not, then the observer will measure a difference in time between the arrival of the two light signals...which is simply the M&M experiment done in reverse (the observer being on a different frame from the experiment). This is what I'm asking...was this done? What experiment are we talking about in this case?

Andre

[granpa]

Ok; so, since, according to the observer on the rest frame, measures the path of light horizontal to the moving frame, as experiencing a length contraction, while the path going perpendicular does not, then the observer will measure a difference in time between the arrival of the two light signals...which is simply the M&M experiment done in reverse (the observer being on a different frame from the experiment). This is what I'm asking...was this done? What experiment are we talking about in this case?

Andre

length contraction is irrelevant. the 2 paths are not the same length yet each measures the speed of light to be the same. hence they measure different amounts of time.
the M&M experiment showed that the speed of light is independent of the observer. simple thought experiments suffice to show that time dilation, length contraction, and loss of simultaneity must follow

[mich]

The path of the light (in the lab frame) in one arm is at 90 degrees to the other. That means that the light must be aimed slightly into the ether wind (if there were an ether wind), exactly like the analogy with the swimmer.

But how is the source of light "aimed" ahead the ether wind if the experiment starts first with the light signal going through a half silvered mirror,splitting two rays at a 90 degree angle? From what I see, the light source is aimed at 90 degrees from the second light source and assumes the path of light to be longer instead.



Length contraction is a consequence of the basic assumptions of special relativity, one of which is the fact that the speed of light is the same for all observers.

Why does the constancy of light need to imply this?


The argument for length contraction (and time dilation, etc.) is given in detail in the follow on lectures on the page I linked in post #9. Read it.

ok; thanks; I will.


As far as I know, there is currently no direct experimental confirmation of length contraction (it's just too small to detect), but the experimental evidence for relativity as a whole (which requires length contraction) is overwhelming.

What about the M&M experiment produced by a sensor (observer) on a moving frame?



Once again, if you want to learn about time dilation and light clocks, follow that link in post #9.

ok; thanks.

Andre

[mich]

length contraction is irrelevant. the 2 paths are not the same length yet each measures the speed of light to be the same. hence they measure different amounts of time.

Actually, according to your "thought experiment" the observer stationnary, relative to the experiment will not see any light shifts ( difference of time the pulses arrive at the eye). This agrees with Relativity as well as with the Newtonian particle theory of light would have predicted. However, Relativity would predict the light signals must arrive out of sink when viewed from an observer on a moving frame, whereas the particle theory would not.

Was this experiment performed? If yes, where could I find such an experiment?


the M&M experiment showed that the speed of light is independent of the observer. simple thought experiments suffice to show that time dilation, length contraction, and loss of simultaneity must follow

I respectfully disagree with you on this one, granpa.Within the M&M experiment, the observer (eye, photographic plate) is always within the same frame as the source of light. Because of this, we cannot claim that the velocity of light is the same for all other inertial frames.

Andre

[granpa]

Actually, according to your "thought experiment" the observer stationnary, relative to the experiment will not see any light shifts ( difference of time the pulses arrive at the eye). This agrees with Relativity as well as with the Newtonian particle theory of light would have predicted. However, Relativity would predict the light signals must arrive out of sink when viewed from an observer on a moving frame, whereas the particle theory would not.

Was this experiment performed? If yes, where could I find such an experiment?



I respectfully disagree with you on this one, granpa.Within the M&M experiment, the observer (eye, photographic plate) is always within the same frame as the source of light. Because of this, we cannot claim that the velocity of light is the same for all other inertial frames.

Andre

wthayta?

the important thing about the M&M experiment is that the earth is moving around the sun and therefore changes speed constantly. if by some miracle the apparatus had been stationary at one point it certainly wasnt later on. also as I understand it the apparatus was rotated 90 degrees without producing any result. thats impossible unless the speed of light is constant for all observers.

what do you mean 'light shifts' and 'arrive at the eye'? there as only one pulse and there as no eye.

[mich]

wthayta?

What does that mean?:)



the important thing about the M&M experiment is that the earth is moving around the sun and therefore changes speed constantly. if by some miracle the apparatus had been stationary at one point it certainly wasnt later on. also as I understand it the apparatus was rotated 90 degrees without producing any result. thats impossible unless the speed of light is constant for all observers.

True; but the experiment was based on the wave theory of light.
Newton's particle theory of light would have predicted the same nul results as Relativity did...except in the case where "changes in speed"or accelerations are implied, something that even Relativity would not have predicted as well.



what do you mean 'light shifts' and 'arrive at the eye'? there as only one pulse and there as no eye.

If I understood you correctly, there was two paths of light; one horizontal to the moving frame, and the other perpendicular.

You wrote:

time dilation alone cant explain both the light path perpendicular and parallel to the motion of the object.

I was centrering on this particular view....sorry if I am not clear.
The idea was that the two light signals would arrive at the same time according to the observer within the frame of the experiment, because both light paths are equal, according to this observer. However, the observer on the moving frame would observe the light signals as arriving at different time since, in his frame of reference, one path is longer than the other.

Andre

[granpa]

If I understood you correctly, there was two paths of light; one horizontal to the moving frame, and the other perpendicular.


I was centrering on this particular view....sorry if I am not clear.
The idea was that the two light signals would arrive at the same time according to the observer within the frame of the experiment, because both light paths are equal, according to this observer. However, the observer on the moving frame would observe the light signals as arriving at different time since, in his frame of reference, one path is longer than the other.

Andre

thats actually even better than what I was talking about. now I finally see what the trouble is. 2 events that occur at the same place at the same time do so for all observers regardless of velocity. all observers see the light pulses arrive at the same time. thats the WHOLE POINT.

[mich]

thats actually even better than what I was talking about. now I finally see what the trouble is. 2 events that occur at the same place at the same time do so for all observers regardless of velocity. all observers see the light pulses arrive at the same time. thats the WHOLE POINT.


Actually, not really, granpa. Here, again, I apologize for my lack of skills in explaining things.
On the frame of reference of the experiment, the observers will measure both lengths of the light's paths (horizontal and vertical)
as being the same. If two light signals are sent at the same time from a source, one taking the horizontal path, the other taking the vertical path, the two light signals would return at the same time due to the invariant speed of light.
Now, for an observer on a moving frame, the horizontal path has been contracted, according to the theory of Relativity, while the vertical path is not contracted. Therefore, Relativity would predict the two light signals would not return at the same time; this could be detected by observing a shift in the light spectrum.
However, If the horizontal path in not affected by any contractions, then even the observer on the moving frame will agree that both light pulses arrived at the same time.

Andre

[granpa]

you've forgotten to take the objects motion into account.it is contracted but it is moving. you have to do a little algebra to determine the amount of time required for a round trap. it works out. trust me.

[mich]

you've forgotten to take the objects motion into account.it is contracted but it is moving. you have to do a little algebra to determine the amount of time required for a round trap. it works out. trust me.


From my point of view, if the light speed remains constant , the time will be t = distance (contracted) / c for the horizontal path, and
t= distance (non contracted) / c. So I cannot see it other than the observer as measuring a time difference between the two light signals arriving.

Andre

[granpa]

the light has to catch up with the front of the object which is moving in the same direction. the simplest way to calculate it as to pretend the distance remains the same and sue c-v for one leg of th etrip and c+v for the other.

[mich]

the light has to catch up with the front of the object which is moving in the same direction. the simplest way to calculate it as to pretend the distance remains the same and sue c-v for one leg of th etrip and c+v for the other.

ok....but why say "pretend"? ;)

But it works out close to being the same, I would guess.
Still the point being that by letting the distance as remaining the same, and "pretending" the light's velocity as being (c+v) (c-v), we still have a difference in time when the light pulse will return relative to the pulse travelling the vertical path. It's excactly, in my opinion,the same case as in the M&M experiment, except here, we have the observer on a moving frame (relative to the experiment).
Therefore, if that's the case, then, another type of Michelson's experiment can be performed, and here, according to Relativity, if I'm not mistaken, we ought to have a light spectral fringe shift; if the result is still nul, then it seems that the theory of length contraction would have been disproven.

Andre

[granpa]

I'm not going to do the math for you. if you dont want to do it yourself then look it up on the web. you assume length contraction has takes place but you use c-v and c+v. it works out exactly.

[RandallB]

Actually, not really, granpa. Here, again, I apologize for my lack of skills in explaining things.
On the frame of reference of the experiment, the observers will measure both lengths of the light's paths (horizontal and vertical) as being the same. If two light signals are sent at the same time from a source, one taking the horizontal path, the other taking the vertical path, the two light signals would return at the same time due to the invariant speed of light.
Now, for an observer on a moving frame, the horizontal path has been contracted, according to the theory of Relativity, while the vertical path is not contracted. Therefore, Relativity would predict the two light signals would not return at the same time; this could be detected by observing a shift in the light spectrum.
However, If the horizontal path in not affected by any contractions, then even the observer on the moving frame will agree that both light pulses arrived at the same time.

Andre No Andre you have miss stated the Michelson-Morley problem.
With two observers you do not have one source you have two, and if they are both together when they send their separate signals if they both have the same coordinate measures of distance for light to travel the same speed the signal for moving observer would have to travel ahead of the light sent from the stationary observer in order to reach the same distance away from its source in the moving frame as the light in the stationary frame.
Unless you point the beams “backwards” where the moving source would need to have its light slow down in that direction.
Combining the two effects in the Michelson-Morley experiments would show this difference as the vertical and horizontal light “could not arrive at the same time”!
That was the point of Michelson-Morley they could not detect the changes that had to be there for unchanging distances.

Lorentz offered the solution that would explain the observed results as objects physically change “shape” by becoming “shorter” in the direction of motion. Lorentz Contraction.

In order to get Lorentz Contraction to work with a the fixed value of “c” Michelson-Morley results were showing also requires Time Dilation be applied.
And that is how you get to Special Relativity.

[mich]

I'm not going to do the math for you. if you dont want to do it yourself then look it up on the web. you assume length contraction has takes place but you use c-v and c+v. it works out exactly.


I don't believe that I was arguing against your point, granpa. Would you agree, however, that the light travelling horizontally will not arrive at the same time as the light travelling vertically, as seen by the observer on the moving frame?

It's just a question of relative simultanety.

Andre

[granpa]

2 events that occur at the same place at the same time do so for all observers regardless of velocity. all observers see the light pulses arrive at the same time. thats the WHOLE POINT.

its 'pretend' because the light isnt moving at c+v its moving at c.

[RandallB]

Would you agree, however, that the light travelling horizontally will not arrive at the same time as the light travelling vertically, as seen by the observer on the moving frame?

It's just a question of relative simultanety. Yes, That that was the point made by Michelson-Morley in the classical view where distances do not change in a reference frame. They first showed mathematically ”that the light travelling horizontally could not arrive at the same time as the light travelling vertically” to establish a prediction of what their Michelson-Morley experiment would show. Again based on a either, with frames using the same gauge for distances, both frames able to define relative simultaneity measures that are the same measured from both frames.

You are incorrectly assigning the MM expectations as a SR expectation – this is wrong.

You need to reread your source information on Michelson-Morley experiment – if you do not have that issue clear, you will not be able to understand Michelson-Morley.
You need to be clear on that BEFORE you can understand Lorentz Contraction.

Then you can add Time Dilation to understand Special Relativity. Where two frames will not agree on simultaneity measures!
And that is why by following SR rules “the light travelling horizontally will arrive at the same time as the light travelling vertically” just as Michelson-Morley unexpectedly found to be true in their experiments.

[JesseM]

I don't believe that I was arguing against your point, granpa. Would you agree, however, that the light travelling horizontally will not arrive at the same time as the light travelling vertically, as seen by the observer on the moving frame?

It's just a question of relative simultanety.
I hope you understand that "relativity of simultaneity" does not allow different frames to disagree in their predictions about localized events happening at a single point in space, only about the simultaneity of events with a spatial separation between them. So if one frame predicts that two beams of light arrive at a single point in space at the same moment, all frames must predict that. This is equally true in a Lorentz aether theory as it is in relativity.

[mich]

Sorry Randall; I wasn't ignoring you. Since I've gone back to work, my time for computer play is kinda limited. My reply is therefore directed not only to you but to granpa as well as others.

No Andre you have miss stated the Michelson-Morley problem.
With two observers you do not have one source you have two, and if they are both together when they send their separate signals if they both have the same coordinate measures of distance for light to travel the same speed the signal for moving observer would have to travel ahead of the light sent from the stationary observer in order to reach the same distance away from its source in the moving frame as the light in the stationary frame.
Unless you point the beams “backwards” where the moving source would need to have its light slow down in that direction.
Combining the two effects in the Michelson-Morley experiments would show this difference as the vertical and horizontal light “could not arrive at the same time”!
That was the point of Michelson-Morley they could not detect the changes that had to be there for unchanging distances.

Lorentz offered the solution that would explain the observed results as objects physically change “shape” by becoming “shorter” in the direction of motion. Lorentz Contraction.

In order to get Lorentz Contraction to work with a the fixed value of “c” Michelson-Morley results were showing also requires Time Dilation be applied.
And that is how you get to Special Relativity.

I wasn't speaking of two sources for simplicity sake. Just imagine the M&M experiment performed on one frame, where we all know the result will be null.
But now imagine a second observer on a moving frame who also sees the experiment. According to Relativity, what does he observe?
It seems to me obvious that the two light signals (horizontal path and vertical path) will not arrive at the same time since Relativity claims that one path will be forshortened while the other will not.

Granpa mentioned that the light will have a (c+v) (c-v) velocity relative to the frame where the experiment happens. I did tell him that I don't have any problems with this, since, if I assume the light remains c according to my measurements, then it must be going at (c+v) (c-v) relative to the moving frame. However, I personally disagree with his claim that we ought to pretend the distance remains
the same. This is what the whole thread is about, that is, length contractions of moving frames, so, according to Relativity, we ought to leave the moving frame contracted.

Therefore it seems that the moving observer will measure the path as being 2s*SQRT(1-v2/c2). The time t being
[2s*SQRT(1-v2/c2)] / (c+v) (c-v)

As for the vertical path we would have no contraction, so
t2 = 2s/c, however, because of a dilation of time, we would have
2s/c*SQRT(1-v^2/c^2).

So we would have a ratio of t2 / t1 = (c+v) (c-v), or (c^2 -v^2)

Andre

[granpa]

the shortening of the paths isnt the problem, its the solution. without contraction the pulses of light wont arrive at the same time. I dont really understand how you can have this so messed up but its clear that you havent done the math. the light appears to both observers to be moving at c. af you want to calculate it that way thats fine. its just easier to do it the way I told you. do the math.

[granpa]

show us some equations and we'll help you. otherwise, I'm done.

[mich]

the shortening of the paths isnt the problem, its the solution. without contraction the pulses of light wont arrive at the same time. I dont really understand how you can have this so messed up but its clear that you havent done the math. the light appears to both observers to be moving at c. af you want to calculate it that way thats fine. its just easier to do it the way I told you. do the math.


Well, you most probably are correct, for I'm not a scientist of any kind whatsoever....I'm mearly trying to understand something which seems confusing to me.

Now, I did try to explain in mathematical terms the problem I had even using the (c-v)(c+v) terms you asked me to use. So, I'm not quite certain as to what you want me to do.
I'll try something else;

For the horizontal path, the moving oberserver measures the light speed as being c, relative to himself, and therefore will measure the light speed as being (c+v) (c-v) relative to the experimental frame. Would you agree? Now what about the length of the path the light will travel? Will it be : s ( length measured at rest), or will it be s* gamma? I would suspect s* gamma, since Relativity predicts a shortening of measuring rods within moving frames.

I will leave you with this and we could go through it in small steps.

I am going to bed now, so I will write you back tomorrow, if you reply.

Andre

[granpa]

when I said that the distance remains the same I meant something completely different. if you use c+v and c-v then you use one distance. if you use just c then you have to use a different distance. its much easier to do the first.

[granpa]

if you are calculating the time as measured by an observer then you would use the distance as measured by that observer.

calculate one leg at a time.

the vertical path is more difficult than you are making. to the moving observer the light moves at an angle. you CAN get into sin and cos but you dont have to. a^2 + b^2 = c^2 is all you really need to solve it. its been a long time since I did it and I dont remember exactly what I did but I know that a lot of things cancel out.

[Doc Al]

I wasn't speaking of two sources for simplicity sake. Just imagine the M&M experiment performed on one frame, where we all know the result will be null.
But now imagine a second observer on a moving frame who also sees the experiment. According to Relativity, what does he observe?
It seems to me obvious that the two light signals (horizontal path and vertical path) will not arrive at the same time since Relativity claims that one path will be forshortened while the other will not.
Wrong again. Actually, if you understood the principle of relativity, you could immediately conclude that the two light signals must arrive at the same time.
For the horizontal path, the moving oberserver measures the light speed as being c, relative to himself, and therefore will measure the light speed as being (c+v) (c-v) relative to the experimental frame. Would you agree? Now what about the length of the path the light will travel? Will it be : s ( length measured at rest), or will it be s* gamma? I would suspect s* gamma, since Relativity predicts a shortening of measuring rods within moving frames.
If you insist on using the speed relative to the lab frame as measured by the moving observer (Rather convoluted, wouldn't you say? Why not just stick to measurements with respect to you?), then the speeds will be (c+v) and (c-v). The length of the path will be s/gamma (not s*gamma). Do the calculation and you'll find, as expected, that the time for the round trip path will be the same as the time measured in the lab frame multiplied by the time dilation factor gamma.

[RandallB]

I wasn't speaking of two sources for simplicity sake. Just imagine the M&M experiment performed on one frame, where we all know the result will be null.
But now imagine a second observer ……. NO that is not true.
Your problem is not in understanding SR. It is that you do not understand MM in the first place.
You need to get that straight first, and you won’t do that by applying SR when you clearly do not understand either M&M or SR.

M&M did not need to “a second observer” they used one observer making two observations (horizontal & vertical) and changed the direction of motion for that one observer (horizontal or vertical).
MOST important!! M&M did not agree that “we all know the result will be null” as you claim; you need to show why you think M&M expected a “null result”.

As you look at the details of exactly how M&M expected a NON-Null result you should see why the geometry you just provided for your relativistic length and time calculations is just plain wrong.
Don’t start with the relativistic solution – start by understanding the Math and Geometry used to show the M&M expectation of a Non-Null result.
Once your straight on two things
1) how they made the Non-Null Result predictions and
2) ALL their observations showed Null Results
you will then understand the paradox they and science had to deal with.

THEN apply the relativistic solution to the M&M math and geometry to solve the paradox.
(Note: according to Lorentz himself the SR relativistic solution was a more complete solution to the paradox than his Lorentz solution as the SR version introduced time dilation and the simultaneity issue. I.E. Lorentz does not include “simultaneity” only SR re-interpretations of Lorentz can apply “simultaneity”)

When you get your geometry and math correct to follow M&M you will see that only using relativistic SR can you predict a Null Result as observed; in contrast to the classical Non-Null results predicted by M&M that did not match the observations.

As this is the third time I’ve pointed you to this flaw in your approach – I trust you will take the time to research and ruminate on this one point. And then proceed with further application of what you learn about SR from that.
If you do that I’m confident you will need no additional help understanding the foundation of SR correctly. So I’ll unsubscribe from this thread and see you in another as you move on to more advanced topics.

[JesseM]

Maybe it would be helpful to do an actual calculation here? Suppose we have a Michelson-Morley type apparatus with two arms at right angles, each 20 light-seconds long in the rest frame of the apparatus. Now consider what will happen from the perspective of an observer who measures the apparatus to be moving at 0.6c to the right along the axis of the horizontal arm. In this frame, the vertical arm will still be 20 light-seconds long, but the horizontal arm is shrunk to 16 light-seconds due to Lorentz contraction, because 20*\sqrt{1 - 0.6c^2/c^2} = 16

Now suppose at t=0 some light is released at the point where the left end of the horizontal arm meets the bottom end of the vertical arm. Since the light is moving to the right at 1c while the horizontal arm is moving to the right at 0.6c, in this frame the distance between the light beam and the right end will be decreasing at a rate of 0.4c, and since the horizontal arm is 16 light-seconds long, the time for the light to reach the right end will be 16/0.4 = 40 seconds. Then the light is reflected from the right end back at the left end, and now the left end is approaching the light at 0.6c, so the distance between the light and the left end is decreasing at a rate of 1.6c. So, the time for the light to get back to the left end will be an additional 16/1.6 = 10 seconds. So, in this frame the total time for the light to go from the left end to the right end and back is 40 + 10 = 50 seconds.

Now consider the light going up and down the vertical arm. The top end of the vertical arm starts out 20 light-seconds directly above the point where the light is released, then after 25 seconds it is still 20 light-seconds above the release point on the vertical axis, while it has moved (25 seconds)*(0.6c) = 15 light-seconds away from the release point on the horizontal axis. So using the pythagorean theorem, the top end is now a distance of \sqrt{(20)^2 + (15)^2} = \sqrt{625} = 25 light-seconds away from the point where the light was released. Since the light travels 25 light-seconds in 25 seconds, this must be the time when the light catches up to the top end. Similar calculations show that 25 seconds after this, the bottom end of the vertical axis will be 25 light-seconds away from the position where the light hit the top end, so this must be the time the light returns to the bottom end. So, you can see that the total time for the light to go from the bottom to the top and back on the vertical arm is 25 + 25 = 50 seconds, exactly the same as the time for the light to go from the left to the right and back on the horizontal arm, so the two light waves will indeed meet at the same point in space and time. As I said before, it is a basic rule in relativity that if one frame says two events happen at the same position and time, this must be predicted in all frames.

[mheslep]

It didn't prove "length contraction", but it did provide evidence for the non-existence of an ether. (Lorentz did propose a contraction hypothesis to explain the null results, but that was replaced by special relativity.)...Hmm, I've just been glancing through Laughlin's "A Different Universe" where he says, IIRC, that it only proved the non-existence of a non-relativistic ether. The implication: a relativistic medium (whatever that might be) could exist.

Edit: yes, here, pg 121:
"The modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether. But we do not call it this because it is taboo."
http://books.google.com/books?id=4UnhVRvMELEC&printsec=frontcover&dq=Robert+laughlin&ei=NJrBSP2PKpKQzQTyyqGKDg&sig=ACfU3U0x-eArHjhmCeOGhaPzcejijI3q2A#PPA121,M1

[jtbell]

For better or worse, the word "ether" in physics is strongly tied to the classical pre-relativistic concept. Physicists probably avoid using it in connection with the modern quantum vacuum because there are so many crackpots out there who keep trying to revive the classical ether.

Calling a hippopotamus a horse doesn't actually make it a horse. If horses were extinct and everybody had forgotten about them, then it might be safe to rename "hippopotamus" to "horse." Otherwise we have a sure recipe for confusion.

We have enough trouble already with the word "mass" in relativity. :rolleyes:

[mich]

If you insist on using the speed relative to the lab frame as measured by the moving observer (Rather convoluted, wouldn't you say? Why not just stick to measurements with respect to you?), then the speeds will be (c+v) and (c-v). The length of the path will be s/gamma (not s*gamma).

Sorry; my mistake. :)

I used this frame because it was the prefered frame that granpa was using.

Andre

[mich]

Maybe it would be helpful to do an actual calculation here? Suppose we have a Michelson-Morley type apparatus with two arms at right angles, each 20 light-seconds long in the rest frame of the apparatus. Now consider what will happen from the perspective of an observer who measures the apparatus to be moving at 0.6c to the right along the axis of the horizontal arm. In this frame, the vertical arm will still be 20 light-seconds long, but the horizontal arm is shrunk to 16 light-seconds due to Lorentz contraction, because 20*\sqrt{1 - 0.6c^2/c^2} = 16

Now suppose at t=0 some light is released at the point where the left end of the horizontal arm meets the bottom end of the vertical arm. Since the light is moving to the right at 1c while the horizontal arm is moving to the right at 0.6c, in this frame the distance between the light beam and the right end will be decreasing at a rate of 0.4c, and since the horizontal arm is 16 light-seconds long, the time for the light to reach the right end will be 16/0.4 = 40 seconds. Then the light is reflected from the right end back at the left end, and now the left end is approaching the light at 0.6c, so the distance between the light and the left end is decreasing at a rate of 1.6c. So, the time for the light to get back to the left end will be an additional 16/1.6 = 10 seconds. So, in this frame the total time for the light to go from the left end to the right end and back is 40 + 10 = 50 seconds.

Now consider the light going up and down the vertical arm. The top end of the vertical arm starts out 20 light-seconds directly above the point where the light is released, then after 25 seconds it is still 20 light-seconds above the release point on the vertical axis, while it has moved (25 seconds)*(0.6c) = 15 light-seconds away from the release point on the horizontal axis. So using the pythagorean theorem, the top end is now a distance of \sqrt{(20)^2 + (15)^2} = \sqrt{625} = 25 light-seconds away from the point where the light was released. Since the light travels 25 light-seconds in 25 seconds, this must be the time when the light catches up to the top end. Similar calculations show that 25 seconds after this, the bottom end of the vertical axis will be 25 light-seconds away from the position where the light hit the top end, so this must be the time the light returns to the bottom end. So, you can see that the total time for the light to go from the bottom to the top and back on the vertical arm is 25 + 25 = 50 seconds, exactly the same as the time for the light to go from the left to the right and back on the horizontal arm, so the two light waves will indeed meet at the same point in space and time. As I said before, it is a basic rule in relativity that if one frame says two events happen at the same position and time, this must be predicted in all frames.

While I think I understand what you did, I'm not quite certain I understand why you did what you did.
Now, as granpa said, there are two ways to look at this. We can either view the problem with the light speed being c relative to us, or make the calculations accordingly with the light going at the speed relative to the moving frame. and as granpa said, we cannot switch over one frame onto another.
Now first, as you mentioned, we start with Michelson's experiment having two paths (legs), one horizontal, the other vertical, both having the same length.

The first step, I have no problems with;
At t=0, two light pulses are sent simultaneously; one on the horizontal path, the other on the vertical path. Now, you wrote:

Since the light is moving to the right at 1c while the horizontal arm is moving to the right at 0.6c, in this frame the distance between the light beam and the right end will be decreasing at a rate of 0.4c, and since the horizontal arm is 16 light-seconds long, the time for the light to reach the right end will be 16/0.4 = 40 seconds.

Since you wrote the speed of light will be calculated as being .4c, it must be relative to the moving frame, since it moves at the velocity of c relative to you.

So, t1 = L/ gamma * (c+v) (c - v)
This frame must be kept all along .

However, for the vertical path, you wrote:


...after 25 seconds it is still 20 light-seconds above the release point on the vertical axis, while it has moved (25 seconds)*(0.6c) = 15 light-seconds away from the release point on the horizontal axis.

This is where you lost me; notice that you are identifying the "diagonal" path. This path is the one that is observed by you, and is not relative to the moving frame.The light in this path is moving at "c" as you rightly identified as travelling 25 light seconds in 25 seconds.
However, notice your statement:


while it has moved (25 seconds)*(0.6c) = 15 light-seconds away from the release point on the horizontal axis.

On the moving frame, the light is certainly not moving away from the release point but remains in a straight vertical path.
In other words, the pythagorean triangle is reversed. The hypotheneuse is c, and the horizontal leg is v, leaving the vertical path as being the velocity of light relative to the moving frame....which is what we want. This leg or path alone is needed is this case....
Again, I'm not claiming to be right, but am only describing how I personally perceive the problem.
Now, since the path is not contracted, it must be L, while the light's velocity would be
c^2 = v^2 + C^2( C being the speed of light relative to the moving frame).

Therefore, C = SQRT (c^2 - v^2); SQRT (c^2-v^2) being gamma. The time t2 would therefore be 2L/ SQRT (c^2 - v^2), or 2L/ gamma

t1 = 2L / gamma * (c+v) (c-v)

t2 = 2L/ gamma

Therefore : t1/t2 = (c+v) (c-v)

Andre

[JesseM]

While I think I understand what you did, I'm not quite certain I understand why you did what you did.
Now, as granpa said, there are two ways to look at this. We can either view the problem with the light speed being c relative to us, or make the calculations accordingly with the light going at the speed relative to the moving frame. and as granpa said, we cannot switch over one frame onto another.
What post of granpa's are you talking about? It's certainly not true that you can't switch from one frame to another, the whole point of relativity is that any physical situation can be analyzed in any inertial frame using the same laws of physics, and all frames will agree on physical predictions about local events.
Since you wrote the speed of light will be calculated as being .4c, it must be relative to the moving frame, since it moves at the velocity of c relative to you.
The speed of light is c in every inertial frame in relativity. What I was referring to was the rate that the distance between the end of apparatus and the light ray would be shrinking--this is sometimes called "closing speed" (the speed that one object is closing in on another one), it's different from the speed of a single object in a given frame. The speed of any single photon is always c in every inertial frame, but the closing speed between the photon and some other object can be less than c.
So, t1 = L/ gamma * (c+v) (c - v)
Not quite, t1 = (L/gamma*(c+v)) + (L/gamma*(c-v))
On the moving frame, the light is certainly not moving away from the release point but remains in a straight vertical path.
Where did you get that idea? As I've told you twice, all frames must agree in their predictions about local events. If their is a photodetector directly above the release point which is at rest in the frame of the apparatus, and in the frame of the apparatus the light goes straight up and hits the photodetector, you can't have other frames predicting that the light still goes straight up and therefore misses the photodetector which is moving in this frame! If it hits the photodector in the rest frame of the apparatus, it must do so in all frames. In general if you have some device which shoots a light beam straight up in its rest frame, it must shoot a light beam diagonally in other frames; in principle you should be able to prove this using classical electromagnetism, considering something like an oscillating charge at the bottom of a tube.
In other words, the pythagorean triangle is reversed. The hypotheneuse is c, and the horizontal leg is v, leaving the vertical path as being the velocity of light relative to the moving frame....which is what we want.
The velocity of light is always c along its path, in every inertial frame. If you don't understand this you've missed one of the most basic points about relativity.

On the other hand, in a Lorentz aether theory we can say that light only travels at c in the rest frame of the aether, while in any frame moving at v relative to the aether, it travels at c+v in one direction and c-v in another. Is this what you're trying to talk about? If so I can reanalyze the same problem from this perspective, under the assumption that objects shrink by a factor of 1/gamma when they are moving relative to the aether--it'll still work out that all frames agree that the two light rays return to the intersection of the horizontal and vertical arms at the same moment.

[granpa]

using c+v isnt a matter of looking at it from the frames point of view. its purely a mathematical convenience. it has nothing to do with relativity. we could be calculating the speed of cow manure hurled from one end of a moving truck to the other. or rather the time it takes to make the trip. we could use its actual speed and the actual distance it moves but why bother? just use the length of the truck and the speed relative to the truck.

we are obviously looking at it from the moving observers point of view since its all rather trivial from the stationary point of view.

[mich]

What post of granpa's are you talking about? It's certainly not true that you can't switch from one frame to another, the whole point of relativity is that any physical situation can be analyzed in any inertial frame using the same laws of physics, and all frames will agree on physical predictions about local events.

The two frames I meant was how the observer calculated the velocity of light; it's either c relative to him or it's a different speed relative to the moving frame, as seen by the observer (closing speed). One cannot switch from one perception to the other when calculation is done on both legs. One has to stick with either one.


The speed of light is c in every inertial frame in relativity. What I was referring to was the rate that the distance between the end of apparatus and the light ray would be shrinking--this is sometimes called "closing speed" (the speed that one object is closing in on another one), it's different from the speed of a single object in a given frame. The speed of any single photon is always c in every inertial frame, but the closing speed between the photon and some other object can be less than c.

I agree.



Not quite, t1 = (L/gamma*(c+v)) + (L/gamma*(c-v))

Sorry...forgot my brakets. t1 = L/ gamma * [(c+v) (c - v)]




Where did you get that idea? As I've told you twice, all frames must agree in their predictions about local events.

What about when two light signals are sent by a source located at the centre of the frame of refrence. The local events are the time when the light signals will hit the walls. An second observer, on a different frame, will view the events differently than the observer within the stationnary frame, relative to the source. Now, you might say, this is not one local event but two different events since the light pulse are separated. However for the M&M experiment, the source of light pointing towards the horizontal path could be located on one side of the frame while the other source, pointing towards the vertical path might be located on the opposite end of the frame, so I personally don't see any difference.



If their is a photodetector directly above the release point which is at rest in the frame of the apparatus, and in the frame of the apparatus the light goes straight up and hits the photodetector, you can't have other frames predicting that the light still goes straight up and therefore misses the photodetector which is moving in this frame! If it hits the photodector in the rest frame of the apparatus, it must do so in all frames. In general if you have some device which shoots a light beam straight up in its rest frame, it must shoot a light beam diagonally in other frames; in principle you should be able to prove this using classical electromagnetism, considering something like an oscillating charge at the bottom of a tube.

By release point, I know that you did not mean the source itself, but the coordinate where the source was located when it released the photon.
Therefore, at t =25 second, neither the detector nor the source is located there anymore....the frame at that particular coordinate no longer exists. Because of this, you cannot speak of "closing speed" anymore. You are now refering to the light speed, relative to the observer....which is c.


The velocity of light is always c along its path, in every inertial frame. If you don't understand this you've missed one of the most basic points about relativity.

When light is directly measured, Relativity promises us that it will always be c; I believe I know this, yes.


On the other hand, in a Lorentz aether theory we can say that light only travels at c in the rest frame of the aether, while in any frame moving at v relative to the aether, it travels at c+v in one direction and c-v in another. Is this what you're trying to talk about? If so I can reanalyze the same problem from this perspective, under the assumption that objects shrink by a factor of 1/gamma when they are moving relative to the aether.

No we're stricly speaking of Relativity, and not LET. I hope that you understand that I'm not claiming anything to be false or true, but am just trying to identify the problems I have.


Andre

[mich]

using c+v isnt a matter of looking at it from the frames point of view. its purely a mathematical convenience. it has nothing to do with relativity. we could be calculating the speed of cow manure hurled from one end of a moving truck to the other. or rather the time it takes to make the trip. we could use its actual speed and the actual distance it moves but why bother? just use the length of the truck and the speed relative to the truck.

we are obviously looking at it from the moving observers point of view since its all rather trivial from the stationary point of view.


I agree with everything you wrote granpa.

Andre

[JesseM]

The two frames I meant was how the observer calculated the velocity of light; it's either c relative to him or it's a different speed relative to the moving frame, as seen by the observer (closing speed).
If you're just talking about closing speed, saying a "different speed relative to the moving frame" is overly confusing--the closing speed of the photon and the end of the apparatus not the same as the speed of the photon in the rest frame of the apparatus, that speed is still c.
One cannot switch from one perception to the other when calculation is done on both legs. One has to stick with either one.
And do you think I did? On both legs I was calculating times and distances in the frame of the observer who sees the apparatus moving at 0.6c.
Not quite, t1 = (L/gamma*(c+v)) + (L/gamma*(c-v))
Sorry...forgot my brakets. t1 = L/ gamma * [(c+v) (c - v)]
Your expression is still wrong--if you're adding the fraction L/[gamma*(c+v)] to the fraction L/[gamma*(c-v)], the two fractions have different denominators, so you can fix that by multiplying both top and bottom of the first fraction by (c-v), and multiplying both top and bottom of the second fraction by (c+v), so the two fractions become [L*(c+v)]/[gamma*(c+v)*(c-v)] and [L*(c-v)]/[gamma*(c+v)*(c-v)]. Then you just add the numerators, which would give [L*(c+v) + L*(c-v)]/[gamma*(c+v)*(c-v)] = L*c / [gamma*(c+v)*(c-v)].

What about when two light signals are sent by a source located at the centre of the frame of refrence.
What do you mean by "center"? A frame of reference in SR is an infinite coordinate system assigning coordinates to every point in space and time--do you just mean the origin of the coordinate system, or the physical center of a room?
The local events are the time when the light signals will hit the walls.
But times are not physical events unless you are talking about readings on a particular set of physical clocks. If there are clocks mounted on the walls which are synchronized in the frame of the walls, and a flash is set off at the midpoint between the two clocks, then all frames will make the same prediction about the reading on each clock at the moment the light from the flash first hits them--they'll all predict that the two clocks read the same time at the moment the light hits them. But this doesn't mean the two events actually happen at the same coordinate time in all frames, some other frame will say that the light actually reached one clock before the other, but in this frame the clocks were out-of-sync by just the right amount so they showed the same readings at the two different times the light hit them.
Now, you might say, this is not one local event but two different events since the light pulse are separated. However for the M&M experiment, the source of light pointing towards the horizontal path could be located on one side of the frame while the other source, pointing towards the vertical path might be located on the opposite end of the frame, so I personally don't see any difference.
Like I said, a frame is an infinite coordinate system, so talking about "one side of the frame" doesn't really make sense, I'd guess you are probably just using "frame" to mean the physical MM apparatus here. Anyway, if you have one light source located on the right end of the horizontal arm and pointed towards the left end (where the horizontal arm intersects with the bottom end of the vertical arm), and another light source located on the top end of the vertical arm and pointed at the bottom end, then if the two sources are turned on in such a way that one frame predicts the light rays reach the intersection point at the same time, then it must be true that all frames predict this, since the meeting of the two rays is a localized event.
By release point, I know that you did not mean the source itself, but the coordinate where the source was located when it released the photon.
Therefore, at t =25 second, neither the detector nor the source is located there anymore....the frame at that particular coordinate no longer exists.
Again, are you using "frame" to refer to the physical apparatus? I agree the apparatus is no longer at that coordinate, but the observer still determines the distance the light ray traveled by looking at the distance interval between the position coordinate in his frame that the ray was released and the position coordinate where it was reflected, and likewise he determines the time by looking at the time interval between the time coordinate the ray was released and the time coordinate it was reflected, and he defines speed in his frame in terms of (distance interval)/(time interval).

By the way, note that the coordinates of a particular observer's frame are imagined to be grounded in the readings on a grid of rulers and synchronized clocks which are at rest with respect to the observer. So the observer could look at what marking on his ruler the bottom of the vertical arm was next to at the moment the light was released, and what the clock affixed to that marker read at that moment, and then compare with the marking on his ruler that the top of the vertical arm was next to at the moment the light was reflected, and what the clock affixed to that marker read at the moment of reflection. So he's assigning coordinates based purely on local readings that were right next to each event as they occurred.
Because of this, you cannot speak of "closing speed" anymore. You are now refering to the light speed, relative to the observer....which is c.
Sure. In the observer's frame, the distance between the position the light was released and the position of the top arm when it was reflected was 25 light-seconds, and the event reflection happened 25 seconds after the event of release, so the speed of light was indeed c in his frame. With the horizontal arm I did make use of the notion of closing speed to calculate the time for the light to get from the left end to the right end and back, but this wasn't really necessary, I could have just as well said that the light was released at time t=0 and position x=0, and at that moment the right end was at position x=16 light-seconds (because the horizontal arm is 16 light-seconds long in this frame), and that 40 seconds later the light would naturally be at position x=40 l.s. because it's moving at c, and since the right end was moving at 0.6c for 40 seconds it would have covered a distance of 24 light seconds, and since it started at x=16 it would now be at x=16+24=40 too, so 40 seconds must be the time for the light to catch up with the right end. This is exactly the same answer as the one you get if you take the length of the arm in this frame, 16 light seconds, and divide by the closing speed of 0.4c. Closing speed is just a convenient shortcut for figuring out when one moving object will catch up with another moving object, nothing more.

[mich]

If you're just talking about closing speed, saying a "different speed relative to the moving frame" is overly confusing--the closing speed of the photon and the end of the apparatus not the same as the speed of the photon in the rest frame of the apparatus, that speed is still c.

I agree


And do you think I did?

At first, I thought you did, but I was wrong.



On both legs I was calculating times and distances in the frame of the observer who sees the apparatus moving at 0.6c.

I agree


Your expression is still wrong--if you're adding the fraction L/[gamma*(c+v)] to the fraction L/[gamma*(c-v)], the two fractions have different denominators, so you can fix that by multiplying both top and bottom of the first fraction by (c-v), and multiplying both top and bottom of the second fraction by (c+v), so the two fractions become [L*(c+v)]/[gamma*(c+v)*(c-v)] and [L*(c-v)]/[gamma*(c+v)*(c-v)]. Then you just add the numerators, which would give [L*(c+v) + L*(c-v)]/[gamma*(c+v)*(c-v)] = L*c / [gamma*(c+v)*(c-v)].

I was not adding those two fractions at all; I was identifying a length contraction for L, making it L/gamma. This over the velocities (c+v) (c-v) gave me t1.


What do you mean by "center"? A frame of reference in SR is an infinite coordinate system assigning coordinates to every point in space and time--do you just mean the origin of the coordinate system, or the physical center of a room?

Physical center


But times are not physical events unless you are talking about readings on a particular set of physical clocks. If there are clocks mounted on the walls which are synchronized in the frame of the walls, and a flash is set off at the midpoint between the two clocks, then all frames will make the same prediction about the reading on each clock at the moment the light from the flash first hits them--they'll all predict that the two clocks read the same time at the moment the light hits them. But this doesn't mean the two events actually happen at the same coordinate time in all frames, some other frame will say that the light actually reached one clock before the other, but in this frame the clocks were out-of-sync by just the right amount so they showed the same readings at the two different times the light hit them.

If an observer on a moving frame can observe an event, coming from what we can claim to be a stationnary frame, as being non simultaneous while the other claims it to be simultaneous, then this is all I'm saying about the M&M experiment observed from a moving frame.


Like I said, a frame is an infinite coordinate system, so talking about "one side of the frame" doesn't really make sense, I'd guess you are probably just using "frame" to mean the physical MM apparatus here.

Yes, and no; along with the moving apparatus, or train, if you like, I see with it, a plane of coordinates having a xyzt axis. This is the reason why I prefer to call it a frame of reference.



Anyway, if you have one light source located on the right end of the horizontal arm and pointed towards the left end (where the horizontal arm intersects with the bottom end of the vertical arm), and another light source located on the top end of the vertical arm and pointed at the bottom end, then if the two sources are turned on in such a way that one frame predicts the light rays reach the intersection point at the same time, then it must be true that all frames predict this, since the meeting of the two rays is a localized event....


After reading over your post, it struck me that you were right. Of coarse it doesn't matter how the observer will measure the light's velocity c in his frame, or xc relative to another moving frame. His
time measurement would indeed be the same for the event in question no matter which frame of reference the observer would use (his or the moving frame's).
Yet, I still could not understand how the the ratio between t1/t2 could be 1 since we are clearly dealing with a length contraction for the horizontal distance and a non contracted distance for the vertical distance.I looked at what I wrote down and could not see anything that could help me see where my error was.
And so I went through your figures and found something questionable. However, I've been out of school for quite a while and so I certainly maybe doing a miscalculation....but this miscalculation seem to work my way.

It concerns your measured length contraction for the moving platform. You wrote down 16.
I got 20* SQRT (1-.6)
= 20 * sqrt (.4)
=20 * .632
= 12.64

So, with this contraction, we continue:

L/(c-v) = 12.64 / .4 = 31.6
L/ (c+v) = 12.64/ 1.6 = 7.9

31.6 + 7.9 = 39.5
Therefore, t1 = 39.5

t2/t1 = 25/39.5
= .6329

The ratio I had calculated was
(c+v) (c-v)
= 1.6 * .4
= .64


Andre

[JesseM]

I was not adding those two fractions at all; I was identifying a length contraction for L, making it L/gamma. This over the velocities (c+v) (c-v) gave me t1.
But where did you get that equation? The two fractions I gave are correct for the time for the light to go from the left end to the right end of the horizontal arm, and the time for the light to go from right end to left end. After all, the length of the arm is L/gamma, and the closing speed from left to right is (c - v) while the closing speed from right to left is (c + v), and the time for a faster-moving object to get from one end to the other of a slower-moving object is always going to be (length of slower object)/(closing speed between faster object and the end of the slower object it's moving towards).
If an observer on a moving frame can observe an event, coming from what we can claim to be a stationnary frame, as being non simultaneous while the other claims it to be simultaneous, then this is all I'm saying about the M&M experiment observed from a moving frame.
Disagreements about simultaneity can only happen for events at different locations--as I keep saying, if two events are predicted in one frame to coincide at the same point in space and time, then all other frames must predict this as well. Imagine that at the intersection of the two arms of the MM apparatus we place a small photosensitive bomb that will go off only if the photons from each arm hit it at the same moment--if different frames made different predictions about whether the bomb goes off, that would either mean we could find a preferred frame by seeing whether or not it actually does go off, or it would mean different frames were parallel universes with distinct histories!
Like I said, a frame is an infinite coordinate system, so talking about "one side of the frame" doesn't really make sense, I'd guess you are probably just using "frame" to mean the physical MM apparatus here.
Yes, and no; along with the moving apparatus, or train, if you like, I see with it, a plane of coordinates having a xyzt axis. This is the reason why I prefer to call it a frame of reference.
When you say "yes, and no" does that mean you are referring to the apparatus as a "frame", and the plane of coordinates as a "frame of reference"? Normally "frame" is just used as shorthand for "frame of reference", and only refers to the coordinate system, not to any physical apparatus (unless we're talking about the imaginary grid of rulers and clocks which are used to define the coordinate system physically). Regardless of how you've used these terms up until now, is it OK with you if from now on we just use "frame" in the standard way, to refer to the type of coordinate system you describe above?
Yet, I still could not understand how the the ratio between t1/t2 could be 1 since we are clearly dealing with a length contraction for the horizontal distance and a non contracted distance for the vertical distance.
Yes, but the light is not just traveling the length of each arm, since the arms themselves are moving in this frame--to calculate the time you need to take into account that in this frame, the light has to travel on diagonals to go to the top end of the vertical apparatus and back, and the light going on the horizontal arm has to catch up with the right end which is moving away from where the light originates, and then has a much shorter distance to travel when it is reflected back from the right end and the left end is rushing forward towards it.
And so I went through your figures and found something questionable. However, I've been out of school for quite a while and so I certainly maybe doing a miscalculation....but this miscalculation seem to work my way.

It concerns your measured length contraction for the moving platform. You wrote down 16.
I got 20* SQRT (1-.6)
= 20 * sqrt (.4)
=20 * .632
= 12.64
1/gamma is \sqrt{1 - v^2/c^2}, not \sqrt{1 - v/c}. So you have to take 0.6c/c and square it, giving 0.36, then take the square root of (1 - 0.36), which is the same as the square root of 0.64, and the square root of 0.64 is 0.8.

[mich]

1/gamma is \sqrt{1 - v^2/c^2}, not \sqrt{1 - v/c}. So you have to take 0.6c/c and square it, giving 0.36, then take the square root of (1 - 0.36), which is the same as the square root of 0.64, and the square root of 0.64 is 0.8.

Ha! You got me. I munched on all the meat and 'tatos you wrote and I'd like to thank you for your patience because it clarified many things.

As I was trying to visualize the experiement, it dawned on me that
leaving the length contraction out of the equation for the horizontal leg would leave me with the galilean form only....and this cannot be for the constancy of light's speed. So I do agree that the transform is needed for the horizontal path.
However, my problem concerns strickly length contractions,not time dilation. Therefore I'd like to visualise the experiment a little differently. By the way, I appologize for all the mathematical mistakes; I've really out of practice. :)

Also, I've come to understand most of your other explanations as well; In the experiment, having the local event out of synch for one observer and in synch for the other would indeed violate the first postualte.So, now I also agree that Relativity cannot predict this to happen.

So in looking at the scenario again, but in keeping with only time dilations and leaving length contraction aside, I would say first:

O1 = observer stationnary to the experiment
O2 = observer on a moving frame

For the horizontal path.

For O1; In both cases (horizontal and vertical path) T1(t1)= 2L/c

For O2; The horizontal path would be
T2 (t1) =[(L + (vt)) + (L - (vt)] / c
= L[ (1+(vt) + (1 - (vt)]/c
= L /c [ 1-(vt)^2]


T2/(T1) = c/2L * L/c [ 1-(vt)^2] = [ 1-(vt)^2] /2

For O2; vertical path:

T2 = 2[SQRT L^2 +(vt)^2] / c
2[ SQRT (ct)^2 + (vt)^2]/c
2[ SQRT t^2 (c^2+v^2)]/c
2t/c [SQRT (1 + (v^2/c^2)]

....ok; I think I get now....L is measured by both observers as being
ct. However, due to the time dilation inlvolved, L cannot be the same for both frames.

Case closed... thanks again.

Andre

[JesseM]

For O2; The horizontal path would be
T2 (t1) =[(L + (vt)) + (L - (vt)] / c
I don't get it, what is "t" supposed to represent here? It's true that the distance the light travels going left to right is L + v*t1, where t1 is the time to get from the left end to the right end, and then the distance the light travels going right to left is L - v*t2, where t2 is the time to get from the right end to the left end, but t1 and t2 will be unequal.

Also, you seem to use L for the length of the uncontracted arms, why are you also using it for the length of the contracted horizontal arm here? You should really use a different symbol like L' = L*sqrt(1 - v^2/c^2)

So, if the point where the light is first sent out is defined as x=0, then the position of the right end as a function of time is x(t) = L' + vt, while the position of the light ray is x(t) = ct, so this tells you the light reaches the right end when L' + vt = ct, and solving for t gives L'/(c - v). Then if we redefine x=0 to be the position of the left end at the moment the light is reflected from the right end, then the position of the left end as a function of time is x(t) = vt, and the position of the light as a function of time is x(t) = L' - ct, so the light will meet the left end when vt = L' - ct, and solving for t gives L'/(c + v). So the time for the light to go from left to right and back must be L'/(c - v) + L'/(c + v), or L'*(c + v)/[(c + v)*(c - v)] + L'*(c - v)/[(c + v)*(c - v)], which is equal to 2*L'*c/[(c + v)*(c - v)] = 2*L'*c/[c^2 - v^2] = 2*L'*c/[c^2*(1 - v^2/c^2)] = 2*L'/[c*(1 - v^2/c^2)]. And then if you substitute in L' = L * sqrt(1 - v^2/c^2), you get 2*L/[c * SQRT(1 - v^2/c^2)] for the total time for the light to go from bottom to top and back.
For O2; vertical path:

T2 = 2[SQRT L^2 +(vt)^2] / c
2[ SQRT (ct)^2 + (vt)^2]/c
It looks like you substituded L = ct here, but why? In this case L is supposed to be the length of the vertical arm, and t is supposed to be the time to get from the bottom to the top of the vertical arm, right? But because light is traveling along a diagonal path in this frame, it travels a greater distance than L in time t--specifically it travels a distance of SQRT[L^2 + (vt)^2]. And since in time t light always travels a distance ct, you should really have ct = SQRT[L^2 + (vt)^2], or (ct)^2 = L^2 + (vt)^2, which you can solve for t to give t^2 = L^2 /(c^2 - v^2) = L^2 / [c^2 * (1 - v^2/c^2)], which gives you t = L/[c*SQRT(1 - v^2/c^2)]. Of course that's just the time to go from the bottom to the top of the vertical arm, the time to get back to the bottom is twice that, or 2*L/[c*SQRT(1 - v^2/c^2)], which is exactly the same as the time we previously got for the light to go from left to right and back on the horizontal arm.

[mich]

I don't get it, what is "t" supposed to represent here? It's true that the distance the light travels going left to right is L + v*t1, where t1 is the time to get from the left end to the right end, and then the distance the light travels going right to left is L - v*t2, where t2 is the time to get from the right end to the left end, but t1 and t2 will be unequal.

Also, you seem to use L for the length of the uncontracted arms, why are you also using it for the length of the contracted horizontal arm here? You should really use a different symbol like L' = L*sqrt(1 - v^2/c^2)

So, if the point where the light is first sent out is defined as x=0, then the position of the right end as a function of time is x(t) = L' + vt, while the position of the light ray is x(t) = ct, so this tells you the light reaches the right end when L' + vt = ct, and solving for t gives L'/(c - v). Then if we redefine x=0 to be the position of the left end at the moment the light is reflected from the right end, then the position of the left end as a function of time is x(t) = vt, and the position of the light as a function of time is x(t) = L' - ct, so the light will meet the left end when vt = L' - ct, and solving for t gives L'/(c + v). So the time for the light to go from left to right and back must be L'/(c - v) + L'/(c + v), or L'*(c + v)/[(c + v)*(c - v)] + L'*(c - v)/[(c + v)*(c - v)], which is equal to 2*L'*c/[(c + v)*(c - v)] = 2*L'*c/[c^2 - v^2] = 2*L'*c/[c^2*(1 - v^2/c^2)] = 2*L'/[c*(1 - v^2/c^2)]. And then if you substitute in L' = L * sqrt(1 - v^2/c^2), you get 2*L/[c * SQRT(1 - v^2/c^2)] for the total time for the light to go from bottom to top and back.

It looks like you substituded L = ct here, but why? In this case L is supposed to be the length of the vertical arm, and t is supposed to be the time to get from the bottom to the top of the vertical arm, right? But because light is traveling along a diagonal path in this frame, it travels a greater distance than L in time t--specifically it travels a distance of SQRT[L^2 + (vt)^2]. And since in time t light always travels a distance ct, you should really have ct = SQRT[L^2 + (vt)^2], or (ct)^2 = L^2 + (vt)^2, which you can solve for t to give t^2 = L^2 /(c^2 - v^2) = L^2 / [c^2 * (1 - v^2/c^2)], which gives you t = L/[c*SQRT(1 - v^2/c^2)]. Of course that's just the time to go from the bottom to the top of the vertical arm, the time to get back to the bottom is twice that, or 2*L/[c*SQRT(1 - v^2/c^2)], which is exactly the same as the time we previously got for the light to go from left to right and back on the horizontal arm.


Jesse, when I wrote down "case closed", I had just realized how right you were, and everything was becoming clear. I was starting to understand my errors now....Case closed meaning you win, I loose.

The L=ct was not part of the equation I was trying out. It just dawned on me that both observers measure distances with ct. Therefore it made me see that clearly length contractions are needed in Relativity.

Thanks again for your time

Andre