Quick Answer
You cannot simply double the speed of light while leaving every other constant unchanged without changing the structure of modern physics. The consequences would depend on which fundamental relationships were altered. Light would arrive sooner over astronomical distances, but the deeper effects would involve relativity, electromagnetic interactions and the definition of causality.
The speed of light is not just a speed limit
In relativity, the constant c connects space and time and appears in the relationship between energy and mass. Changing it while keeping the rest of the equations untouched would produce a physically different universe.
Astronomical observations would change
Light from distant stars and galaxies carries information from the past. If c were larger, the travel time of that information would be shorter for newly emitted signals. Historical observations already in transit would also arrive according to the new propagation rules in the hypothetical scenario.
Causality is the deeper issue
Relativity's light cone defines which events can influence one another. Changing c changes the geometry of those cones. Faster propagation would expand the region of spacetime that can be causally connected over a given interval.
SpeculativeThere is no established physical model in which only c changes while every other fundamental constant and interaction remains identical. Any detailed prediction therefore requires specifying a new theory.
Technology would not simply become faster
Electronic circuits and optical communication are governed by many physical processes, not just the vacuum speed of light. Some systems would change, but the magnitude cannot be inferred by saying 'everything becomes twice as fast.'
The bottom line
Doubling c would mean rewriting fundamental physics rather than upgrading the universe's internet connection. The most important effects would appear in relativity, causality and the relationships among physical constants.
Changing the speed of light would affect much more than what we see in the sky. The value of c appears throughout electromagnetism, relativity and modern measurement, so changing it would require specifying which other physical constants remain fixed. If only the propagation speed changed while everything else stayed the same, familiar equations would no longer describe the same universe. That makes this scenario highly dependent on its assumptions.
What changes in practice?
A note on our approach: Every article on WhatIfLab separates what current science establishes from what remains genuinely speculative. Where we cite a figure or finding, it reflects published, mainstream research at the time of writing — not a prediction dressed up as fact.