The Minimum Speed in the Universe: An Exploration of the Cosmic Limit

[ghwellsjr]

The idea it has to be measured by a defined method, is because it is invariant. otherwise there would be no simultaneity issues with moving clocks

definite is not the same word as defined. "always" & "definite velocity" is about invariance...not what ever the value maybe or how to measure it.

I don't know what you mean with " no experiment to show that light propagates at c". A method to measure the DEFINITE speed is defined. The fact that it's invariant, is still observed to be true.

Here is a link with a table of the history of calculations of the speed of light, first of which was in 1676 using predicted & observed orbits.

Here is another wording, speaking to the definition of a second and the implication..."This defines the speed of light in vacuum to be exactly 299,792,458 m/s." taken from here, which is different from the defined method to measure light traveling that speed.

OK, please tell me what is the defined method to measure the DEFINITE speed of light that you have mentioned so many times in this post.

 

[nitsuj]

OK, please tell me what is the defined method to measure the DEFINITE speed of light that you have mentioned so many times in this post.

Any measurement that ultimately "employs" the equation that "works around" the issue of RoS. It will definitely equal c

I can't do equations, but the one used to determine the "common time", that is defining simultaneity of the clocks assuming one way speed of light is 1/2 round trip.

tb-ta = t'a - tb ...best i can do...but it's in the paper you linked to. Just below the paragraph "defining" the "common time" required to ...

 

[Rob Benham]

I was just going to suggest introducing the CMB into the discussion. The use of the CMB to stabilize a NASA U2 aircraft to better measure the . . . CMB has fascinated me for years. It almost seemed to suggest some sort of spacetime datum, but of course, that's not possible. :-(

(George Smoot. Work done after the gondola series)

 

[ghwellsjr]

OK, please tell me what is the defined method to measure the DEFINITE speed of light that you have mentioned so many times in this post.

Any measurement that ultimately "employs" the equation that "works around" the issue of RoS. It will definitely equal c

I can't do equations,

Are you saying you don't understand what the above equation means?

but the one used to determine the "common time", that is defining simultaneity of the clocks assuming one way speed of light is 1/2 round trip.

What do you mean by "one way speed of light is 1/2 round trip"? It sounds like you are saying the one way speed of light is 1/2 of whatever you measure for the round trip. Is that what you mean?

tb-ta = t'a - tb ...best i can do...but it's in the paper you linked to. Just below the paragraph "defining" the "common time" required to ...

 

[nitsuj]

Are you saying you don't understand what the above equation means?

What do you mean by "one way speed of light is 1/2 round trip"? It sounds like you are saying the one way speed of light is 1/2 of whatever you measure for the round trip. Is that what you mean?

Yes I don't know what the above equation is. What is two times a times b? Or is it two times ab because its same as ba...by definition?

Yes that is what I mean...by Einstein's definition.

 

[ghwellsjr]

Yes I don't know what the above equation is. What is two times a times b? Or is it two times ab because its same as ba...by definition?

Einstein didn't give a good explanation of that equation so I will. AB is the distance between A and B measured with a rigid ruler, t_A is the time on the clock at A when a light pulse is emitted from A, which then propagates to a mirror at B which reflects it back to A and t'_A is the time on the same clock at A when the reflected light pulse gets back to A. To calculate the "average" round trip speed of the light pulse we take double the distance between A and B and divide it by the total time that the light was in transit which is the difference between the two clock readings. Apparently, Einstein's audience was familiar enough with the requirement to use a round trip when measuring the speed of light that he didn't go into any detail about it.

What do you mean by "one way speed of light is 1/2 round trip"? It sounds like you are saying the one way speed of light is 1/2 of whatever you measure for the round trip. Is that what you mean?

Yes that is what I mean...by Einstein's definition.

No, Einstein is saying that the time on the clock at B is set so that the light pulse takes 1/2 of the round trip time interval to make each leg of the trip. In other words it spends half the time getting to the mirror and half the time getting back and clock B is set accordingly. That makes the unmeasurable one-way speed of light equat to the measured round trip speed.

Can you see now why it is impossible to measure the one-way speed of light? (Look it up in wikipedia if you need more help.)

 

[ash64449]

Measuring the speed of light in a round trip is possible since the emission and receiving events can be measured readily by a single clock.

By understanding the total distance traveled, and dividing this number by duration of time gives you the measured speed of light.

But measuring the speed of light in a single half-way of round trip is impossible since you need for that two clocks.

Well,it would have been possible if we synchronize the clocks beforehand.But to synchronize the clocks by Einstein's definition in his published paper requires you to assume that light travels at the speed c and then by setting that clock B, clocks get synchronized.

It means clocks are synchronized in such a manner that light speed has a value of c when measured in a half-way trip.

Is this what you meant ghwellsjr? is that the reason why measuring halfway round trip speed of light impossible?

 

[Rob Benham]

Given that measuring the speed of light over short distances requires a very precise clock, is there any significant time taken for the photons to be 'processed'* by the mirror to send them on their return journey?

*I've just read a little on what takes place but it seemed even more complex than I remember. Nowhere did I find a typical time taken.

 

[nitsuj]

Einstein didn't give a good explanation of that equation so I will. AB is the distance between A and B measured with a rigid ruler, t_A is the time on the clock at A when a light pulse is emitted from A, which then propagates to a mirror at B which reflects it back to A and t'_A is the time on the same clock at A when the reflected light pulse gets back to A. To calculate the "average" round trip speed of the light pulse we take double the distance between A and B and divide it by the total time that the light was in transit which is the difference between the two clock readings.

I found Einstein explained it well and far more structured.

No, Einstein is saying that the time on the clock at B is set so that the light pulse takes 1/2 of the round trip time interval to make each leg of the trip. In other words it spends half the time getting to the mirror and half the time getting back and clock B is set accordingly.

That makes the unmeasurable one-way speed of light equate to the measured round trip speed.

There is no "this makes". That IS the defined part. That the round trip of light is equal to the one way trip.Yes I do know why we cannot measure one-way, a main reason is because the speed c is invariant, clocks that move relative to each other are not synchronized. One cannot carry a clock from the start point to the finish and have the clocks (start clock / finish clock) share a "common time" (be synchronized).

While A-B and B-A are lengths, Einstein effectively used light to equate that as "time", the exact time required to get from a-b-a. Same idea as the light second unit.

Again from the paper you are referring to "We have not defined a common “time” for A and B, for the latter cannot be defined at all unless we establish by definition that the “time” required by light to travel from A to B equals the “time” it requires to travel from B to A."

If AB BA were truly lengths, he would have explicitly stated AB + BA since both would be known exactly. This was about the time for LIGHT to travel those distances. In turn only AB is used and it's defined as 2 times AB for the total time.

So since someone can determine the time for an a-b-a trip by definition one can deduce (rather simply, though not provable) that is the same value as the 2AB.

It took 4 seconds for a full trip of 2 light seconds each way. 2*2ls / 4 = 1 Ha and all I needed were definitions.

 

[ghwellsjr]

Measuring the speed of light in a round trip is possible since the emission and receiving events can be measured readily by a single clock.

By understanding the total distance traveled, and dividing this number by duration of time gives you the measured speed of light.

But measuring the speed of light in a single half-way of round trip is impossible since you need for that two clocks.

Well,it would have been possible if we synchronize the clocks beforehand.But to synchronize the clocks by Einstein's definition in his published paper requires you to assume that light travels at the speed c and then by setting that clock B, clocks get synchronized.

It means clocks are synchronized in such a manner that light speed has a value of c when measured in a half-way trip.

Is this what you meant ghwellsjr? is that the reason why measuring halfway round trip speed of light impossible?

Yes.

 

[ghwellsjr]

Given that measuring the speed of light over short distances requires a very precise clock, is there any significant time taken for the photons to be 'processed'* by the mirror to send them on their return journey?

*I've just read a little on what takes place but it seemed even more complex than I remember. Nowhere did I find a typical time taken.

Nobody is going to try to measure the speed of individual photons. How would you know when the photon was emitted at the source? If you want to pursue this subject, I suggest you open a new thread in the quantum forum.

 

[Dale]

OK, the OP is banned and his/her question was fully and completely answered in post 2. Is there any point in continuing?