Science
What If Light Travelled Slower?
If c were a fraction of itself, what delays show up in a room, a city, a sky?
c is 299 792 458 metres per second, exact, by definition of the metre. In a room it is already overkill: five metres of air is 17 nanoseconds. Drop c to a tenth and that room is 167 nanoseconds, still not a blink. Drop it to a hundredth and a city block becomes milliseconds, GPS-style ranges become sluggish, and the Sun is more than a day away by light. There are two different games. One is slow glass: light delayed in a medium while vacuum c stays put. The other is changing c itself, which would rearrange atoms, chemistry and E = mc². This page computes delays as if you could scale c, then says plainly that a global change is not a parlour trick. It is a different universe.
Change this
Results update as you move the controls.
0.1 is 10 percent of 299 792 458 m/s.
Your result
1.67 × 10^−7 s
light-time across a 5 m room
3.00 × 10^7 m/s
- c in this run
- 3.00 × 10^7 m/s
- 5 m room
- 1.67 × 10^−7 s
- 10 km city link
- 333.6 microseconds
- 20 000 km satellite
- 667.13 milliseconds
- 1 AU (Sun)
- 83 minutes
1 hours 23 minutes
What does that mean?
At 10% of c, light crawls at 3.00 × 10^7 m/s. A 5 m room is 1.67 × 10^−7 s (today 1.67 × 10^−8 s) — still not a human-scale lag. Ten kilometres is 333.6 microseconds. A 20 000 km satellite link is 667.13 milliseconds. Sunlight takes 83 minutes instead of 8 minutes 19 seconds. If this is 'slow glass,' those are delay-line numbers in a medium. If this is a new vacuum c, atoms and relativity would not be the ones you know; this page is not that universe, only the delays.
Timeline
Room
1.67 × 10^−7 s
Vision still feels instant at these fractions. The slider is not a slow-motion lounge.
City
333.6 microseconds
Radio and optical links pick up a delay you might actually schedule around.
GNSS-scale
667.13 milliseconds
Ranging systems become sluggish even before anyone mentions relativistic clocks.
Sun
83 minutes
Old sunlight is older still. See also the Sun-disappeared experiment, which uses the real c.
Compare
| Slow glass reading | a delay line; vacuum c unchanged |
|---|---|
| New-c reading | a different universe; not computed beyond t = d/v |
Compare scenarios
Rooms stay prompt for a long time
Human perception of lag starts around tens of milliseconds. To make a 5 m glance feel late you would need c down by factors of millions, which this slider does not allow — on purpose. The interesting delays at 0.01c to 0.5c are civic and astronomical: a 10 km radio link, a GNSS satellite, the 8-minute sunlight becoming hours. Everyday vision would still look instantaneous. That is worth saying so the demo does not fake a slow-motion living room.
Relativity is not a costume you take off
Special relativity weaves c into simultaneity, mass–energy, and the way magnets work. Turn the number down globally and you have not made a movie effect; you have proposed new physics. GPS already corrects for relativity at the real c. At 0.1c the raw light-time to a 20 000 km satellite is two-thirds of a second, which would make a 'ranging' system a different machine even before the relativistic terms. This page quotes the light-time. It does not rewrite Maxwell.
Slow glass versus a new c
Media can delay light a great deal: optical fibre, cold atomic gases, the science-fiction idea of slow glass. Vacuum c is the same; the extra time is interaction with matter. That is allowed physics. Changing c in vacuum is not something you can do with a filter. If the control feels like a medium, read the delays as a delay line. If it feels like a new constant of nature, read the disclaimer twice.
How we calculated this
c_run = c × fraction, with c from SI / NIST. Delays are distance / c_run for 5 m, 10 km, a 20 000 km satellite, and 1 AU (physics.LIGHT_TIME_SUN_S / fraction). No medium dispersion, no rewrite of atomic energy levels. The text distinguishes a delay-line ('slow glass') reading from a constants-of-nature reading.
Go further
A curated rabbit hole from this question. Each link is a real experiment, not a random suggestion.
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