Fitzgerald -Lorentz contraction

[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.

 

[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=4U...=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.

 

[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 preferred 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 traveling 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 isn't 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 referring 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 isn't 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 referring 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.