Light is incredibly fast. In fact, according to Einstein, nothing surpasses its speed. It’s so fast that we often perceive it as instantaneous, though that isn’t entirely accurate. Even flipping a light switch creates a tiny, unnoticeable delay before the room illuminates.
This delay is imperceptible because Newton’s laws of motion function perfectly for everyday speeds. Objects like us, vehicles, airplanes, and even bullets move too slowly for the speed of light to significantly impact them. However, communication exists in a different realm. We use light to transmit information through fiber optic cables, and its counterpart, radio waves, also belong to the electromagnetic spectrum and are bound by the same speed constraints. Surprisingly, this speed limit, as determined by Einstein, can pose real challenges.
Imagine having a conversation with someone in Tokyo while you’re in New York City, a distance of 6,760 miles. Light travels at approximately 186,000 miles per second, which translates to 186 miles per millisecond. Therefore, the fastest a signal could travel between these cities is around 36 milliseconds. Since fiber optic cables, which utilize laser light, carry most of the call, and light travels slower in glass than in a vacuum or air (about 65% slower, or 120,900 miles per second), the delay between your speech and the other person hearing it is at least 56 milliseconds.
This 56-millisecond transmission delay is known as latency. In reality, phone calls experience a latency three times longer because fiber optic cables don’t run in a straight line between cities. The path is longer, going around landmasses and under oceans. Additionally, electronic regenerators along the way contribute to the delay, pushing the initial 56 milliseconds to at least 84 to 168 milliseconds. For a call halfway across the globe (approximately 12,500 miles), the speed of light alone causes a delay of 67 milliseconds, resulting in a total latency of 101 to 202 milliseconds. Once the delay reaches 250 milliseconds, it becomes noticeable.
But when does 250 milliseconds truly make a difference? Imagine playing a real-time action game hosted on a server on the opposite side of the world. Every mouse movement or keyboard entry creates a round trip delay of 500 milliseconds, or half a second, for the screen to update. This half-second lag is definitely noticeable.
Another example is hosting a concert with musicians from different countries. When connecting various studios through digital fiber optic cables to combine vocals and instruments on a mixing board, a singer located halfway around the globe will experience a quarter-second delay between their performance and hearing the mixed audio. Similarly, their voice will reach the mixing board a quarter of a second after they sing. This half-second discrepancy significantly impacts the performance, as no two musicians will hear the same mix, causing timing issues.
At least making a phone call is still possible, right? Yes, as long as you avoid routing it through a satellite in geosynchronous orbit. These satellites orbit 22,300 miles above Earth. Since the signal travels almost entirely through a vacuum at the speed of light (186,000 miles per second), it takes 120 milliseconds for the satellite to receive your voice and another 120 milliseconds to transmit it to the recipient, totaling 240 milliseconds at best. If each data packet required acknowledgment before acceptance, the delay would double to 480 milliseconds minimum. Therefore, for voice calls, it’s more efficient to transmit packets without waiting for confirmation and hope they arrive intact. However, additional latencies from landline transport and routers, which add about 10 milliseconds each, extend the delay, making satellite VoIP calls more akin to walkie-talkie conversations than full-duplex phone calls. Ping times, measuring round-trip latency, have been reported to exceed one second for two-way geosynchronous satellites, averaging around 850 milliseconds.
Want to call someone on Mars? Considering its distance of 35 million miles, expect a 3-minute delay each way. Forget hold music; you’ll need background music while awaiting responses. Thankfully, the moon is much closer. Once we establish a base there, calls to friends 239,000 miles away will only have a delay of about one and a quarter seconds, resulting in a total round-trip time of two and a half seconds at the speed of light.
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