Compression of random binary data

[Chris Angelico]

On Sun, Oct 29, 2017 at 2:08 PM, Gregory Ewing
<greg.ewing at canterbury.ac.nz> wrote:
> Stefan Ram wrote:
>>
>>   Well, then one can ask about the entropy of a data source
>>   that only is emitting this message.
>
>
> You can, but it's still the *source* that has the entropy,
> not the message.
>
> (And the answer in that case is that the entropy is zero.
> If there's only one possible message you can ever send, you
> don't need to send it at all!)

One bit. It might send the message, or it might NOT send the message.

And I have known situations in which that is exactly the system used.
Back before mobile phones were common, you could sometimes use a
payphone to cause someone's phone to ring, but you couldn't actually
speak on it. So you had one message you could send: "brrrring
brrrrring". A pre-arranged meaning for that message might be "I'm at
the railway station, please come and pick me up"... but there's still
*some* information in the mere fact of the call.

ChrisA