What if light traveled as slowly as a bicycle?

When you turn on a light, the room lights up at once. It seems like no time at all!

But light does take a tiny bit of time to travel. What if it were as slow as a bike?

Flip a light switch and the room seems to light up instantly. Look at a friend and you seem to see them right now. But light takes time to travel, and seeing anything means waiting for its light to reach your eyes.

Light is so fast that we never notice the wait. So what if it moved only as fast as a racing bicycle? This thought experiment starts from real physics and a real laboratory experiment, then imagines the rest.

Light's finite speed means every observation is delayed: we see the Sun as it was about 8.3 minutes ago and the Andromeda galaxy as it was 2.5 million years ago (NASA). Everyday delays are far too small to notice.

The thought experiment: if light in our surroundings moved at about 17 m/s, a speed physicists have actually reached in a special laboratory medium, which everyday experiences would change? The physics of light-travel time and the Doppler effect are facts; the slow-light world is an idea.

Light is super, super fast. It gets from the Moon to us in about one second!

That's why seeing feels instant.

In empty space, light travels at 299,792,458 meters per second, about 300,000 kilometers every second. Since 1983, scientists have used that exact number to define the meter.

At that speed, light reaches the Moon in 1.3 seconds and crosses from the Sun to Earth in about 8.3 minutes. So when you look at the Sun (never directly!), you see it as it was 8 minutes ago. Across a room, the delay is billionths of a second, far too short to notice.

NIST's CODATA value for the speed of light in vacuum is 299,792,458 m/s, exact by definition. Light crosses the 384,400 km to the Moon in 1.3 s and the 150 million km from the Sun in about 8.3 min (NASA).

All electromagnetic waves share this speed in empty space, and it is the fastest possible speed (OpenStax). In materials it is lower: Harvard Magazine notes that a pane of glass slows light by about 30 percent as it passes through. At human scales, light-travel delays are nanoseconds.

Now imagine light moves only as fast as a racing bike.

Your friend waves at you across the playground. You see the wave one second later! Everything you see would be a little bit old.

Now imagine light moved at 17 meters per second, about as fast as a top cyclist sprints. Your friend across a 17-meter playground waves, and you see the wave a whole second later. A car driving toward you might look different from one driving away.

This isn't only imaginary. In 1999, physicist Lene Hau and her team slowed light down to 17 meters per second, in a tiny cloud of extremely cold atoms. Outside that cloud, light went right back to full speed.

Take 17 m/s, the speed Hau's group measured in 1999, as our imagined everyday light speed. Then distance in meters divided by 17 gives the delay in seconds: 1 s across 17 m, about 6 s across a 100 m field. Simple arithmetic puts the Moon about 8.6 months of light-travel away and the Sun about 280 years.

In the real experiment, the slow-down happened only inside a 0.1 mm Bose–Einstein condensate of sodium atoms cooled to billionths of a degree above absolute zero. A world-wide slow light is an idea, not something physics predicts.

We see things when light from them reaches our eyes. So we always see things a tiny bit late.

If light were slow, 'a tiny bit late' would become 'very late'!

You see an object when light leaving it reaches your eyes. The farther away it is, the longer the light takes, so you see it as it was a moment ago. NASA puts it this way for space: the farther away an object is, the farther in the past we see it.

Light's speed also controls colors from moving things. Light from something moving toward you is squeezed to shorter, bluer waves, and from something moving away, stretched to redder waves. In our world the change is tiny at everyday speeds. If light were slow, it might become easy to see.

Two consequences of a finite light speed scale with how fast things move compared with it. Light-travel time means we see objects as they were, not as they are. The Doppler effect shifts light from approaching sources toward blue and from receding ones toward red (OpenStax); at everyday speeds the shift is minute because v ≪ c.

If c were comparable to everyday speeds, Live Science reports, changes in color, brightness, time and length predicted by special relativity would become noticeable. A game built at the MIT Game Lab by Gerd Kortemeyer, Philip Tan and colleagues shows such effects (Live Science); how a real slow-light world would behave is an idea.

Scientists really slowed light down! They used a tiny cloud of super-cold atoms.

Inside it, light went as slow as a racing bike. Later, they even stopped light for a moment!

At Harvard and the Rowland Institute, Lene Hau's team cooled sodium atoms to a few billionths of a degree above absolute zero, making a strange state of matter called a Bose–Einstein condensate. Shining laser light through it, they slowed light pulses to 17 meters per second, about 38 miles per hour: speeds a Tour de France cyclist could beat in a sprint.

Two years later, her team stopped a light pulse completely, stored its information in the atoms, and sent it back on its way.

Hau's 1999 Nature paper reported light slowed by a factor of about 20 million, to 17 m/s, through a laser-dressed Bose–Einstein condensate of sodium atoms only about 10 by 100 microns in size (Harvard Magazine). In 2001 her group stopped light: photons imprinted their information on the atoms and, milliseconds later, the pulse was reconstituted (Physics Today).

Possible uses mentioned for such work include optical switches, delay lines and quantum technologies. Slow light in a lab medium does not change c in vacuum.