harry potter - Do the diary Riddle and Voldemort share memory?

In Harry Potter and the Chamber of Secrets, Tom Riddle is interested to know how Harry survived Voldemort's spell. This implies that Tom Riddle from the diary could not deduce/guess what happened that night.

The strange thing is that in Harry Potter and the Goblet of Fire, Voldemort seems to know exactly how the spell backfired (while this process has only taken a few seconds)*. Since Tom Riddle could not guess the cause, it's very unlikely Voldemort could work out this magical challenge himself.

Since the diary was destroyed after Harry's encounter with Tom Riddle, there is no way Voldemort could have learned what Harry told Tom in the chamber. Does this mean both "identities" share a memory and that Voldemort simply has access to the knowledge Tom (from the diary) accumulated at Hogwarts? If not, how did Voldemort work out why the spell backfired?

Answer

Does diary Riddle share memories with the “real” Voldemort?

I don’t think so. Voldemort is unaware that his horcruxes are being destroyed (implying he has no psychic connection to them).

Also, as I explain in my answer to How did the diary horcrux know about recent events?, the diary Riddle seems to have a third-hand and incomplete account of the events of Godric’s Hollow. That’s only possible if it’s not sharing memories with the real Voldemort.

How did adult Voldemort know that Harry survived if diary Riddle did not?

There are several factors which give adult Voldemort an edge:

• 
  

  Adult Voldemort has a spy close to Dumbledore.

  
  

  With Crouch Jr impersonating Moody, an old and trusted friend of Dumbledore, it’s quite possible he’s learnt Dumbledore’s theories about Harry’s survival, and passed them to Voldemort. By contrast, diary Riddle only has Ginny’s accounts, and she doesn’t know anything useful about how Harry survived.

  
  


• 
  
  

  Voldemort witnessed his defeat; Riddle only heard the aftermath.

  
  

  Again referring to the quotes in another of my answers, Riddle’s account of Godric’s Hollow is based only on Ginny’s retelling. She doesn’t know that Lily cast herself between Voldemort and Harry, or the exact sequence of events. Without that information, it may be impossible for Riddle to work out what happened.

  
  


• 
  

  Voldemort has about forty extra years of accrued knowledge and wisdom.

  
  

  It’s very possible that he only learnt about the magic of sacrificial protection after he left school. It’s an extremely rare and poorly understood branch of magic (and not one the teenage Riddle would have been actively seeking out). Since the two of them don’t share memories, it may be that teenage Riddle just doesn’t know enough to put the pieces together.

  
  

  (Forty: the Chamber of Secrets was last opened fifty years before Harry’s second year. Voldemort was defeated twelve years prior to that, so about forty years between horcrux creation and destruction.)

  
  


• 
  

  Voldemort knows what happened in Philosopher’s Stone.

  
  
  

  When Quirrell tries to touch Harry, he gets a nasty surprise:

  
  

  
  

  Harry’s scar was almost blinding him with pain, yet he could see Quirrell howling in agony.

  
  

  “Master, I cannot hold him – my hands – my hands!”

  
  

  And Quirrell, though pinning Harry to the ground with his knees, let go of his neck and stared, bewildered, at his own palms – Harry could see they looked burnt, raw, red and shiny.

  
  

  — Philosopher’s Stone, chapter 17 (The Man with Two Faces)

  
  

  
  

  This is another detail which could be useful in working out why Harry is protected, to which Ginny wouldn’t be privy. She knows he defeated Voldemort, but not how.