I'm trying to update a sobject (CollaborationGroupFeed) via apex, but I am unable to update a particular reference field (in this case: CreatedBy).

Here is my code:
<pre>CollaborationGroupFeed cgf = [SELECT Id, CreatedById
                                                 FROM CollaborationGroupFeed
                                                 WHERE Id =: feedElement.Id
                                                 LIMIT 1];

                    User u = [SELECT Id, Name FROM User WHERE FirstName = 'Steve' AND LastName = 'Jobs'];
                    cgf.CreatedById = u.Id;
                    update cgf;</pre>

Error message:
Field is not writeable: CollaborationGroupFeed.CreatedById