The Speed of Light Sucks

We're used to treating the speed of light as instantaneous in our everyday lives. Light is, indeed, very fast -- approximately 300 million meters (186,000 miles) per second fast in a vacuum.

But, move halfway around the world and the speed of light can become a very real concern. Tokyo is, as the crow flies, about 10,000km (6,800 miles) away from New York. If I want to send some data to New York, it moves at the speed of light from my PC to the data center in Manhattan.

If light was moving through a vacuum, it would cover this distance in about 33 milliseconds (.033 seconds). 33 milliseconds is not very long. In fact, according to some theories of the mind, it's just above the theshold of what we can consciously perceive.

But, just getting to New York from Tokyo doesn't accomplish that much. If we send a request there, we'd probably like to get something back. So double it, so we can get a reply: 66 millseconds.

Err, but wait. When we send a signal from Tokyo to New York, it doesn't move thorugh a vacuum, but through tiny strands of glass. Our signal will spend most of its time in these fiber optic cables, winding its way across the Pacific, then across the North American continent, until it finally ends up in a data center in central Manhattan.

Unfortunately for us, though, light moves considerably slower in glass than in a vacuum. A trip that takes us 33ms in a vacuum will take us 1/3rd longer in fiber. Now we are looking at almost 100 milliseconds to get to New York and back again. If you inject 100ms into a one-on-one conversation, very observant folks will notice that something is slightly "off".

Well, 100ms isn't too bad, right? Even if we are using VoIP, most people we call won't notice the delay. Oh, but Tokyo isn't actually 10,000km from New York, because the fiber optic cables that cross the Pacific and the American plain don't run in a perfect great circle route between the two cities. We can increase the point-to-point distance by 50% and probably still be pretty conservative. That brings our total roundtrip time to 150ms.

Throw in a DSL modem, routers, media converters and everything else that sits in the 18 hops between Tokyo and New York, and you've probably got another 50 milliseconds of roundtrip latency on your hands. In fact, it takes 8ms (16ms roundtrip) just to reach the other side of my DSL line. We're at 200ms now, only 100ms away from the point where delays in a conversation become extremely annoying.

In fact, the measured latency between my office and a server in a New York data center is between 210-220ms. The majority of this delay is a direct result of the physics of the universe we live in. No matter how high-tech our routers become, the speed of light is immutable, so this delay is simply unavoidable.