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Science

What If Light Travelled Slower?

Short answer: your room would still feel instant for a long time. At a tenth of today’s speed, five metres takes 167 nanoseconds. At a hundredth, city distances turn into milliseconds and sunlight takes more than a day. Changing c for real would rewrite atoms, though.

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.

Written and checked by Aivora Digital · Last updated:

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Results update as you move the controls.

0.1×c

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

  1. Room

    1.67 × 10^−7 s

    Vision still feels instant at these fractions. The slider is not a slow-motion lounge.

  2. City

    333.6 microseconds

    Radio and optical links pick up a delay you might actually schedule around.

  3. GNSS-scale

    667.13 milliseconds

    Ranging systems become sluggish even before anyone mentions relativistic clocks.

  4. Sun

    83 minutes

    Old sunlight is older still. See also the Sun-disappeared experiment, which uses the real c.

Compare

Slow glass readinga delay line; vacuum c unchanged
New-c readinga 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

New speed of light = today’s speed (from SI / NIST) × the fraction you choose. Delays are distance ÷ new speed for 5 m, 10 km, a 20,000 km satellite and the Sun (about 499 seconds today, divided by the 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.

Assumptions and limitations

This is a simplified estimate. Actual results depend on factors the model does not include.

This is a simplified thought experiment. It uses transparent assumptions so you can see what changes — it is not a full physical simulation of the real world.

  • Distances are ordinary metres; only the speed in t = d/v changes.
  • A global change in c would break much of known physics; delays are still shown as a calculation.
  • Slow-glass (medium delay) is a different, physically allowed story.
  • No redesign of atoms, chemistry or GPS clocks beyond quoting light-time.
  • The Sun distance is 1 AU, exact.

Sources

All IfSketch sources · Methodology

Go further

Related experiments that pick up where this one leaves off.

  1. 01 · WorldWhat If the Sun Disappeared?
  2. 02 · ScienceWhat If You Could Stop Time?
  3. 03 · ScienceWhat If Humans Could See Infrared?
  4. 04 · ScienceWhat If Sound Travelled Twice as Fast?

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