So, I was thinking about why doesn't Wizards use The Fidelius charm to hide important places, you know, like MoM or Hogwarts, during times of war.

After thinking about it for some time. I come to a theory (my head-cannon if you would) about The Fidelius charm.

Now, I know there could be simple and more logical reasons than my theory about this. For example :-

• The Fidelius charm is very difficult piece of Magic to use and it's very difficult to cast it on huge area.
• Most of important places are also public places, that lot of people use on daily basis. Casting Fidelius on those places would be kind on redundant.

My theory kind of ties in with the second point above, that those places are public and thus their secret is not one person's or family's to hide.

Take Hogwarts for example. Hogwarts isn't property of one single person of family. It is open for all young wizards and witches. Therefore one person can't hide it because not owned by one person or family, it's for all.

On the other end of the spectrum we have Grimmauld Place. It is property of Black family thus Sirius (last)Black can hide it from others.

So here it is, product of my sleep addled mind. What do you guys think?

 

It's always useful to go back to the source material in these cases:

I then follow this chain of thought:

1. It appears that a secret is not just a piece of abstract information; it has magical reality and a location, capable of being hidden within a person's soul.

2. The secret-keeper receives the secret into their soul. The act of receiving implies a corresponding act of giving, and raises the question: where was the secret located before the Fidelius?

3. The only place we know for sure that secrets can reside is in the soul - we might conclude, therefore, that before the Fidelius is cast, the secret in question lives within the soul of the secret-giver.

4. The mechanism of the Fidelius is therefore one of a transfer. The secret is transferred from its "natural" location, the soul of the person whose secret it is, into the soul of a third party, and thereby hidden.

5. This explains why one cannot be one's own secret keeper. There would be no transfer. The Potters (who may have shared ownership of the secret of Godric's Hollow, preventing them transferring to each other) had to transfer to Pettigrew; Sirius had to transfer the secret of Grimmauld Place to Dumbledore; Muriel had to transfer the secret of Shell Cottage to Bill (movie canon that Shell Cottage is Muriel's, but there's nothing in the books to contradict it, and it fits the system).

6. This also explains why you wouldn't be able to cast the Fidelius on a public space. The problem is that there is no person who can give the secret to a secret keeper, because there is no person whose secret it is.

 

@Taure, that's what I was somewhat trying to say above but thanks for expanding on it.

Also, can Fidelius hide something abstract like piece of information or something?

 

We've only ever seen it used on locations, so I would assume for the time being that the Fidelius can only be used to hide the secret of locations. However, if it could be used on ideas, I would assume the same dynamics would apply: you could only hide ideas which belong to a person, not public knowledge.

There are some people who propose that the Potters' secret was not the location of their cottage in Godric's Hollow, but rather the fact of their residence there, pointing towards Flitwick saying that Voldemort could have pressed his nose (lol) against the window and not realised they were there.

However, I don't think this is correct. If you look at OotP, the secret of Grimmauld Place is phrased in exactly the same way:

The secret is the information regarding who may be found there; the effect is nonetheless to hide the location itself.

What then of Voldemort pressing his nose to the window? I think this causes confusion simply because of people not distinguishing between the physical and visual effect of the Fidelius.

Visually, a person who doesn't know the secret cannot see the location which is hidden. When they are told the secret, the visual effect is the location appearing where before they could not see it. However, it does not follow that the location was not actually there, physically speaking. I don't think there's any evidence that the Fidelius removes a location from physical space - it just removes it from perception.

So a person who is not in on the secret can still interact physically with the hidden location. They just don't realise that they are doing so. So as Flitwick said, Voldemort could have interacted physically with the Potters' residence even before he knew the secret, but he would not have perceived it.

This is of course a major weakness of the Fidelius. If Voldemort had decided to flatten Godric's Hollow entirely, the Potters would presumably have died along with everyone else.

 

That would beg the question of whether the spell's mechanics could let him get non-specific information about someone's location, and if so where the limit is. Could he find out that they're hiding somewhere in Godric's Hollow without the aid of a secret keeper?

Or, to take a better example, Grimmauld Place. With Sirius on the run, you'd think one of the other Blacks (or ex-Blacks like Narcissa and Bellatrix) would check the place eventually. Sure, it was well-known that Sirius hated the house, but he has access to it and beggars can't be choosers. Not to mention they might have sentimental reasons to go check out the old household.

For that matter, Kreacher sought out Naricssa at one point. She knows he's the house elf of her old family home. While he couldn't give away the Secret, she knows he's bound to her old family home. Could she just tell Voldemort to blast all the houses on that block of Grimmauld Place? Does the spell make her forget where her old family house is? Would she then be conscious of not knowing the address?

 

But if the whole point is to keep magical areas secret from Muggles, every wizard and witch knowing the secret doesn't matter. The whole point would be to keep magic secret. That seems a reasonable thing to try to accomplish using the fidelius in such a manner.

 

It'd be a shame then if every young witch or wizard were to receive a letter telling them exactly where Diagon Alley is then, wouldn't it?