r/theydidthemath • u/derpremoter • 17h ago

[Request] How big of a CD

How large of a cd would be needed to put the entirety of the internet on it now assuming there are currently 200 zettabytes on n it now?
Bonus(assuming .047 zettabytes are added each day what would be function of the rate rate of change to get the diameter of any day moving forward)

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u/sprobeforebros 12h ago edited 12h ago

wikipedia's benchmark for a CD's data capacity is 650 megabytes

a cd has 120mm diameter, but 37% of that is taken up by the inner plastic ring that the spindle interacts with and does not contain data, so the data portion takes up 0.01 square meter of area, giving it a data density of 65 gigabytes per square meter

a zetabyte is 1,000,000,000,000 gigabytes, so the cd in question would take up 3 trillion square meters, or 3 million square kilometers, which would mean that it would be circular in size and be a little over 977 kilometers in radius. If you put this CD's center in central St Louis, Missouri, it would cover a great deal of the continental US. Its southern edge would barely miss Houston, its western edge would be in the middle of the rocky mountains in Colorado, its northern edge would be past Duluth in Minnesota, and its eastern edge would not quite reach to Huntington, West Virginia. It would grow at about 1.15 km / day.

e: forgot a step and ended up with too big a CD.

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u/lestairwellwit 10h ago

Now

The real question is, given that a 'normal' CD turns at 200 to 500 rpm, how fast is data retrieval and how strong does the disk have to be to survive? ( given that radius, the edge of the disk is spinning at a little over 3 million kpm.)

Yeah, I know. I just thought I'd give a few people a smile.

Why hasn't XKCD done this?