Quick Answer
A universe with a lower speed of light would not simply have slower internet. The speed of light appears throughout relativity and electromagnetic theory, so changing it while holding everything else fixed would require changing the relationships among fundamental constants. The exact consequences depend on which constants are allowed to change with it.
The speed of light in vacuum is about 299,792 kilometres per second. In modern physics it is not just a speed limit for spacecraft; it is built into the structure of spacetime and electromagnetic theory.
Relativity would change
The Lorentz factor depends on the ratio between an object’s speed and the speed of light. Halving the light-speed constant would make relativistic effects appear at lower everyday speeds than they do now.
Communication would have a harder limit
Signals across Earth would still travel extremely quickly compared with human reaction times, but delays across the Solar System would double. A message to Mars, for example, would take roughly twice as long to cross the same distance.
Mass and energy
Einstein’s E=mc² contains the square of the speed of light. If c changed while mass remained defined in the same way, the numerical relationship between rest mass and energy would change dramatically.
Atomic physics becomes complicated
The speed of light is linked to electromagnetic constants and the fine-structure constant. Changing it alone would create an internally inconsistent version of physics unless other constants changed too.
SpeculativeThere is no experimentally supported scenario in which the speed of light changes independently while every other physical constant remains untouched. The thought experiment is useful precisely because the constants of physics are interconnected.
The bottom line
Halving c would rewrite the rules behind relativity and electromagnetism. It cannot be treated as a simple change to the maximum speed of a spacecraft.
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.