# How long is a bit?

After watching this inspiring YouTube clip where Rear Admiral Hopper explains the length of a nanosecond, and discussing the fiber under Sydney Harbour with my friend Geoff Houston, I started to think about the length of a bit.

Geoff thought the new path was 2km shorter, but the article says it is only 400 meters shorter. Anyway, lets see what that means in reality.

Speed of light in fiber is slower than in air, which is slower than in vacuum. In this article it is calculated that in single mode fiber (which we talk about here), the speed is 186,282/1.467 = 124,188 miles per second or 199861213 m/s.

If we have a 10Gbps transmission (40G and 100G are multiplexes of 10G so far) we can transfer 10000000000 bits per second. And if we then do our math correctly, we see that **one bit is 1.99cm long**, and a byte 15.99cm, or just a bit more than half a foot.

Now, if we look at the 2 km shorter distance, we see that the party that uses the shorter distance can send a whopping 12.2kBytes over the shorter distance before the first bit arrives over the longer path. If the distance is only 400 m shorter, then we talk about 1.2kBytes.

Question is then how many transactions of something one can do within those first kBytes of data…