Quick Answer
A consistent universe with a different speed of light would require changes to the underlying physical laws or constants. It would not simply make sunlight arrive later. The value of c appears in special relativity, electromagnetism and the relationship between mass and energy, so changing it while holding everything else fixed creates an internally inconsistent scenario. If a complete alternative theory changed c consistently, atomic structure, clocks, radiation and astrophysics could all differ.
The speed of light in vacuum, usually written c, is about 299,792,458 metres per second. In relativity it is the invariant speed that connects measurements of space and time. In electromagnetism, its value is related to the electric and magnetic constants of free space.
It would not just affect photons
The phrase 'speed of light' can make this sound like a property of light alone. In modern physics, c is much deeper: it is the causal speed limit in relativity. Changing it would affect how time and space relate for moving observers and would change the scale of relativistic effects.
The Sun would appear more delayed
If only the propagation speed changed while the rest of physics somehow remained usable, signals would take longer to cross space. Sunlight currently takes about eight minutes to travel from the Sun to Earth. A halved c would make that travel time about sixteen minutes. But that is only the easy part of the thought experiment.
Atomic physics gets complicated
The speed of light is related to the electromagnetic coupling and other dimensionless quantities through combinations of fundamental constants. Changing c alone while declaring all other constants unchanged can produce a theory whose physical meaning is unclear. A meaningful alternate universe would need a consistent set of laws, not one number edited in isolation.
Established scienceA useful distinction in fundamental-constant thought experiments is between changing a dimensional constant by definition and changing measurable dimensionless combinations. Observable physics depends on dimensionless quantities, so a serious model must specify which physical relationships are actually changing.
Relativity would become easier to notice
At everyday speeds, relativistic effects are tiny because ordinary velocities are much smaller than c. Halving c would make the same human and technological speeds a larger fraction of the causal speed limit, increasing relativistic corrections in situations that are currently almost classical.
The bottom line
A universe with half the speed of light cannot be modeled by simply changing 299,792,458 to half that number and leaving everything else untouched. The interesting consequence is that c sits inside the structure of modern physics itself. Change it consistently and the universe's clocks, atoms, radiation and spacetime relationships could all change.
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.