What if c is variable rather than constant

[Azhar]

TL;DR

If c is variable rather than constant then travel more than light velocity is not impossible

What if c is not constant,example:
c_2=c+1000 km/s
v_1=c_2
v_2=c

(v_1+v_2)÷(1+v_1×v_2÷c_2²)
The result is c_2 so to calculate particle that the velocity is more than light the limit should be more than light too,maybe particle that travel faster than light is not impossible but doesn't exist in tangible nature,what if 0 invariant mass mean the particle travel in the limit so if the limit is c then photon invariant mass is 0 but if the limit is c_2 photon invariant mass is non zero,what if the formula is accurate because the formula measure particle that travel in light speed or less than light speed,what if tachyon is result of more than light velocity but calculate with light velocity limit

 

[Baluncore]

TL;DR: If c is not constant then light speed is not limit

What if c is not constant, ...

In physics today, light is defined to have a constant velocity.
c = 299,792,458 m/s.

 

[Azhar]

In physics today, light is defined to have a constant velocity.
c = 299,792,458 m/s.

The light velocity is constant but what if the c is not,like c is variable in formula that act as limit rather than constant

 

[Ibix]

It depends what else you vary. Because the fine structure constant is $\alpha=e^2(4\pi\varepsilon_0\hbar c)^{-1}$ you can't vary $c$ without also varying at least one of $e$ (the electronic charge), $\varepsilon_0$, $\hbar$ or $\alpha$. It turns out that if you also vary $\alpha$ then the change has physical consequences. Otherwise, the other thing(s) you changed make balancing changes and it turns out that the whole process is nothing more than a unit redefinition.

We have looked for variation in the fine structure constant and have not seen any evidence.

 

[Roberto Pavani]

Undergraduate answer:

There are two different "c"s. One is just a unit conversion between space and time, its numerical value depends on your choice of units, so "changing it" can be nothing more than a unit redefinition.

The other is the physical principle that there's a single invariant top speed for information/causality; that's the real content, and it's never been contradicted.
Your example doesn't beat the limit, putting c₂ into the formula just gives c₂ back, so you've only renamed the limit, not exceeded it.

 

[Azhar]

Undergraduate answer:

There are two different "c"s. One is just a unit conversion between space and time, its numerical value depends on your choice of units, so "changing it" can be nothing more than a unit redefinition.

The other is the physical principle that there's a single invariant top speed for information/causality; that's the real content, and it's never been contradicted.
Your example doesn't beat the limit, putting c₂ into the formula just gives c₂ back, so you've only renamed the limit, not exceeded it.

But the result is c_2 and c_2>c so the c_2 result break c