How does Maxwell equation suggest that the speed of light is the same

  • #26
ghwellsjr
Science Advisor
Gold Member
5,122
146
Yes, it is a roundtrip measurement. Anyway ¿could you measure the speed of light from any source with this set-up?
Yes, as long as you understand it is a roundtrip measurement. You cannot measure the propagation speed of a one-way set-up.
 
  • #28
1
0
How does Maxwell equation suggest that the speed of light is same for all observer?
Under Galilean transformations ‘c’ is not the same in all frames of reference and M.E. are not invariant under Galilean transformation. Lorentz transformations can be arrived at by Einsteins postulates, the 2nd being that a light pulse must spread spherically in all frames. M.E. are invariant under Lorentz –- maybe a bit circular but nonetheless suggestive.
 
  • #29
46
0
I found this: http://arxiv.org/pdf/0911.3616v1.pdf

"In the October 2009 issue of the American Journal of Physics Greaves, Ro-
driguez and Ruiz-Camacho[1] report a measurement of the one-way speed of
light. ....
In the experiment they describe, a light beam is sent from a laser to a
photosensor, and then the signal from the photosensor is transmitted through
a coaxial cable back to the vicinity of the laser. The length of the cable ...“introduces a fixed
time delay of 79 ns”.
...
What the experiment ... actually
measures is the time for a round trip; the first leg of this round trip is the light
propagating from the laser to the photosensor
, and the second leg is the signal
going through the coaxial cable from the photosensor back
to the vicinity of the
laser.
It is the assumption that the second leg is accomplished with a known
speed
(in particular, the round-trip speed of light) that allows the speed of the
first leg to be determined."

The first leg is what I want to measure, is the speed of light.
The second leg is inside the coaxial cable. The signal that travels in this second leg is not the light whose speed I want to measure (yes, its light, its an EM wave, but its generated in my laboratory and travels inside my cable) and I believe the assumption
"the assumption that the second leg is accomplished with a known speed".
And you?
(and thank you for your time and patience)
 
  • #30
ghwellsjr
Science Advisor
Gold Member
5,122
146
I found this: http://arxiv.org/pdf/0911.3616v1.pdf

"In the October 2009 issue of the American Journal of Physics Greaves, Ro-
driguez and Ruiz-Camacho[1] report a measurement of the one-way speed of
light. ....
In the experiment they describe, a light beam is sent from a laser to a
photosensor, and then the signal from the photosensor is transmitted through
a coaxial cable back to the vicinity of the laser. The length of the cable ...“introduces a fixed
time delay of 79 ns”.
...
What the experiment ... actually
measures is the time for a round trip; the first leg of this round trip is the light
propagating from the laser to the photosensor
, and the second leg is the signal
going through the coaxial cable from the photosensor back
to the vicinity of the
laser.
It is the assumption that the second leg is accomplished with a known
speed
(in particular, the round-trip speed of light) that allows the speed of the
first leg to be determined."

The first leg is what I want to measure, is the speed of light.
The second leg is inside the coaxial cable. The signal that travels in this second leg is not the light whose speed I want to measure (yes, its light, its an EM wave, but its generated in my laboratory and travels inside my cable) and I believe the assumption
"the assumption that the second leg is accomplished with a known speed".
And you?
(and thank you for your time and patience)
You're very welcome.

Note that it is an assumption. It's not a measurement. It's the same as Einstein's second postulate that the light takes the same amount of time to travel to a mirror as it takes for the reflected light to get back. You could use other words like "stipulation" or "definition". But you can't say that it is an independent measurement.
 
Last edited:
  • #31
246
3
The fact that a theory is valid in a certain domain of applicability does not mean it is The Way The World Really Is, which I doubt we can ever Really Know.
It reminds me of the joke about the mathematician and the engineer. You just have to get close enough.
 

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