From: veritasium
The speed of light is conventionally defined as exactly 299,792,458 meters per second [00:00:11]. Since 1983, this value has been used to define the length of a meter [00:00:20]. A meter is the distance light travels in a vacuum in 1/299,792,458ths of a second [00:00:24]. This definition ensures the speed of light is precisely this number, with no decimals [00:00:31].
Challenges in Measuring the Oneway Speed of Light
Despite this precise definition, no one has actually measured the one-way speed of light [00:00:46]. Measuring the speed of light directly, like measuring the speed of a baseball (distance divided by time) [00:01:13], presents unique challenges [00:00:50].
The Problem of Clock Synchronization
To measure the one-way speed of light over a distance (e.g., one kilometer), one would ideally start a timer at the moment a laser beam is fired and stop it when the beam reaches the end [00:01:53]. However, if the clock is at the starting point, there’s no way to know precisely when the light reaches the end [00:02:00].
This leads to the need for two synchronized clocks: one at the laser’s origin and one at the destination [00:02:07]. But synchronizing these clocks without already knowing the one-way speed of light proves impossible:
- Synchronizing with a wire: Sending a pulse through a wire to synchronize clocks introduces a time delay equal to the time it takes for light to travel that distance [00:02:21]. Subtracting this delay requires knowing the speed of the pulse, which is the very thing being measured [00:02:29].
- Synchronizing and moving: If clocks are synchronized at the same location and then one is moved to the destination, special relativity dictates that moving clocks tick slower relative to stationary observers [00:02:53]. Thus, they would no longer be in sync upon reaching the destination [00:03:01]. Even moving them very slowly would not resolve this if the speed of light depends on direction [00:11:07].
- Central synchronizing device: Using a device equidistant from two clocks to send out simultaneous pulses would perfectly synchronize them if the speed of light is the same in both directions [00:09:48]. However, if the speed of light differs, one clock would be ahead, yet any measurement would still yield the conventional speed ‘c’ [00:10:00]. This is why GPS synchronized clocks won’t work for this purpose; the entire GPS system assumes the speed of light is uniform in all directions [00:10:16].
Essentially, to measure the one-way speed of light, synchronized clocks are needed, but to synchronize clocks, the one-way speed of light must already be known [00:11:25].
Measuring the Roundtrip Speed of Light
The only practical solution is to measure the roundtrip speed of light [00:03:08]. This involves using a single clock at the starting point and reflecting the light back with a mirror at the end [00:03:13]. The clock times the full two-kilometer round trip [00:03:18].
Historical Measurement
The first experimental measurement of the speed of light was performed by Hippolyte Fizeau in 1849 [00:03:35]. He shone a beam of light through the teeth of a rapidly spinning gear to a mirror eight kilometers away [00:03:39]. By increasing the gear’s speed, he reached a point where the reflected light passed through the next gap, allowing it to be observed [00:03:46]. Fizeau measured the speed of light to be 313,000 kilometers per second [00:03:55], which is within 5% of the currently accepted value [00:03:59].
However, Fizeau’s experiment, like all subsequent successful measurements, determined the two-way speed of light, not the one-way speed [00:04:08]. Even modern techniques like high-speed cameras or fiber optic cables inherently measure the two-way speed due to the light bouncing back or undergoing multiple internal reflections [00:09:05], [00:09:43].
Einstein Synchronization Convention and its Implications
The idea that the speed of light is the same in all directions is a convention, not an experimentally verified fact [00:06:34]. Einstein himself pointed this out in his famous 1905 paper, “On the Electrodynamics of Moving Bodies” [00:06:42]. He addressed the problem of synchronizing clocks at different locations and stated that a meaningful comparison of their times requires a definition that the time light takes to travel from A to B equals the time it takes to travel from B to A [00:06:55]. This is known as the Einstein synchronization convention [00:07:20].
Einstein later described this as a “stipulation” made “of my own free will” to arrive at a definition of simultaneity, rather than a hypothesis about the physical nature of light [00:07:28]. This means the speed of light ‘c’ is defined for the round trip [00:08:08]. This is also why light clocks are always depicted with light bouncing back and forth [00:08:13]. The two-way speed of light is the only constant value across all inertial observers [00:08:22].
The Unknowable One-Way Speed
It is theoretically possible that the speed of light is not the same in all directions [00:04:16]. For example, it could be half of ‘c’ (c/2) in one direction and instantaneous on the return journey [00:04:46]. In such a scenario, communication delays would still appear symmetrical to observers, making it impossible to detect the asymmetry [00:05:03]. This is because any method used to measure the one-way speed would inherently rely on the assumption of symmetric light travel time, leading to a measurement of ‘c’ regardless of the actual one-way speeds [00:10:05].
Physicists have developed internally consistent theories where the speed of light is different forwards and in reverse, as long as the round trip averages out to ‘c’ [00:05:45], [00:15:22]. This could imply a preferred direction through spacetime, similar to asymmetries observed in the universe (e.g., matter/anti-matter imbalance) [00:05:31].
Impact on Simultaneity
If the speed of light is not uniform in all directions, it profoundly impacts the concept of simultaneity across distances [00:11:37]. For instance, if a message from Earth takes 20 minutes to reach Mars (due to c/2 speed) and returns instantaneously, but observers on both planets assume the Einstein convention (10 minutes each way), their clocks would be out of sync by 10 minutes, with no way for them to detect or correct this error [00:12:27]. What one considers “right now” on Mars would be unknowable [00:13:08], [00:16:30]. This flexibility in defining simultaneity is represented in spacetime diagrams [00:13:50].
Even if the one-way speed of light were instantaneously fast in one direction, Earth could see Mars in real-time, and stars hundreds of light-years away as they are right now, rather than centuries ago [00:14:24]. Since one only knows about light when it reaches them, an instantaneous interpretation of that light is just as valid as one where it takes ‘c’ to reach us [00:14:51].
While many physicists adhere to Occam’s razor, preferring the simpler convention that light travels at the same speed in all directions [00:15:51], it remains a convention rather than an empirically verified fact [00:16:04]. The inability to measure the one-way speed of light might be a crucial clue for future advancements in General Relativity, Quantum Mechanics, space, and time [00:16:45].