To Infinity and Beyond
We take for granted today that the speed of light can be used to make highly accurate measurements of distance. But in the 17th century, when Sir Isaac Newton was formulating the laws of motion and attributing his success to the works of Galileo, Kepler, Copernicus, and others, scientists were only beginning to suspect that light had a finite speed at all. Prior to that time – going back to Aristotle – it was widely assumed that light’s motion was instantaneous. After all, no delay could be observed by the naked eye. It’s hard to notice a delay in something fast enough to circle the Earth more than seven times in a single second.
A Finite View
But who first suggested light wasn’t infinitely fast?
The earliest known hypothesis of a finite speed of light came from the ancient Greek philosopher Empedocles (5th century BCE), who argued that light must take time to travel. This idea didn’t gain much traction. It wasn’t until 1676 that compelling empirical evidence emerged — thanks to Danish astronomer Ole Rømer. By observing eclipses of Jupiter’s moon Io, Rømer noticed they appeared to occur later than expected when Earth was moving away from Jupiter, and earlier when Earth was moving closer. He reasoned that this discrepancy was due to the time it took light to travel varying distances — the first real measurement suggesting light had a finite speed.
Over time, the precision of these measurements improved. In the 19th century, scientists like Hippolyte Fizeau and Léon Foucault conducted clever laboratory experiments using rotating mirrors and toothed wheels to measure the speed of light more accurately.
A Modern Approach
In the mid-twentieth century, the approach changed from measuring the speed of light to defining it. Scientists defined the speed of light as 299,792,458 meters per second and constructed our unit of measurement around that definition. As such, the meter is defined as, “The length of the path traveled by light in vacuum during a time interval of 1⁄ 299,792,458 of a second.” They flipped the script.
Why hadn’t anyone done that sooner?
It wasn’t until the invention of the atomic clock that we could measure time with the precision required for this approach. These clocks became extraordinarily accurate — capable of measuring time down to billionths, even trillionths, of a second. With this level of precision, and knowing the exact speed of light (because it was now defined…or assigned), we can solve the equation: distance = speed × time. For example, if a laser pulse takes exactly 2 (round-trip) × 1⁄ 299,792,458 of a second to travel to a surface and return, you’ve just measured one meter. This isn’t just a neat trick — it highlights a profound principle of modern physics: the speed of light underpins our entire system of space, time, and measurement.
You might notice, that this approach is quite similar to lidar. So in a way, lidar doesn’t use the speed of light to calculate distance, lidar created the measurement of distance!
What Mysteries Remain?
Before signing off, I’ll present to you an interesting concept that is the basis of an excellent Veratasium YouTube video (embedded below).
The speed of light has never actually been measured! That sounds absurd, but technically, it’s true. We’ve only ever measured the round-trip speed of light — light leaving a source, hitting an object, and returning. No one has ever successfully measured the one-way speed of light.
Even Einstein acknowledged this. In his own words, the assumption that light’s speed is the same in both directions is “neither a supposition, nor a hypothesis, about the physical nature of light, but a stipulation… to arrive at a definition of simultaneity.” Basically, he had no evidence that light does travel the same speed in both directions but needed to treat it as such for practicality.
Fortunately, for those of us in the lidar world, this philosophical quandary doesn’t affect our work. Lidar relies solely on the round-trip time of flight — a measurement we can make with extraordinary precision. Our systems are built on this constant, and thanks to centuries of scientific advancement — from Rømer’s moons to Einstein’s relativity — we can map the world in exquisite detail at the speed of light.
For more foundational material about lidar, I recommend reading Understanding Pulse Interval and Pulse Duration in Lidar
