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?

 

[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, length contraction was the proposed solution to the nul result.

Right. Nothing contracts.

What constant?

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? what's 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: http://galileoandeinstein.physics.virginia.edu/lectures/michelson.html"

 

[mich]

huh? what's 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. don't 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. don't 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 remaining 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: 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 remaining 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: 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: 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: 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 can't 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 can't 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. that's 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. that's 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 can't 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. that's 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. that's 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 traveling 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 don't 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 don't 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 traveling horizontally will not arrive at the same time as the light traveling 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. that's the WHOLE POINT.

its 'pretend' because the light isn't moving at c+v its moving at c.

 

[RandallB]

Would you agree, however, that the light traveling horizontally will not arrive at the same time as the light traveling 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 traveling horizontally could not arrive at the same time as the light traveling 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 traveling horizontally will arrive at the same time as the light traveling 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 traveling horizontally will not arrive at the same time as the light traveling 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 isn't the problem, its the solution. without contraction the pulses of light won't arrive at the same time. I don't really understand how you can have this so messed up but its clear that you haven't done the math. the light appears to both observers to be moving at c. af you want to calculate it that way that's 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 isn't the problem, its the solution. without contraction the pulses of light won't arrive at the same time. I don't really understand how you can have this so messed up but its clear that you haven't done the math. the light appears to both observers to be moving at c. af you want to calculate it that way that's 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 don't 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 don't 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.