# Why must the speed light be constant?

Discussion in 'Physics & Math' started by Saint, May 30, 2021.

1. ### SsssssssRegistered Senior Member

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3. ### Q-reeusBannedValued Senior Member

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5. ### Sherlock HolmesRegistered Member

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Not quite sure I can fully picture what your saying.

The speed of light measured in a vacuum is the same for all observers providing they are in inertial reference frames.

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7. ### SsssssssRegistered Senior Member

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From the thread I linked it appears TonyYuan wants you to state the 1d relativistic velocity addition formula which he will misapply to a poorly specified 2d Newtonian (or partially Newtonian anyway) model of a relativistic scenario and then claim that the resulting mess is a problem with relativity.

8. ### phytiRegistered Senior Member

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Tony;

Relative to the glass rod problem:

The speed of light in glass c1 = c/1.5 =.67c. Rod length d = 1 m.

In the lab, t = 2d/c1 = 3.00 sec.

The lab is in the earth ref. frame, moving at v = 30 km/sec.

Round trip time t = d/c1[1/(1-v/c1)+(1+v/c1)]

Neglecting v/c1, t=3 sec., 300 if d = 100.

Substituting glass for air would not change the outcome in the MMX, where td and lc work together to nullify the interference.

The propagation speed of light is relative to space, and independent of objects.
The reason c±v enters into equations reflects two objects in relative motion, the rod and photons in this case. It is commonly referred to as 'closing speed'. One object moving at c, one object moving at v, nothing moving at c±v. Both are necessary to solve the equations.

In the lab there is no interference, yet the lab is moving. For the brief time of the experiment, the lab may be considered an inertial frame. By definition there is no motion within an inertial frame, i.e. all elements have the same velocity or (are at rest).
Thus the processes function independently of the ref. frame velocity.
An observer moving with the rod would not detect any change, in agreement with SR postulate 1, laws of physics are the same in all inertial frames.

Last edited: Jul 9, 2021