There seems to be a number of ways to block an Avada Kedavra (Physical barriers conjured by magic, hitting the spell with Expelliarmus, etc) so why do people consider it "unblockable"?
You just need to have quick reflexes, right?
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To the best of my knowledge, it's not truly unblockable. What has been said is that there is "no defense against it, no counter-curse" (By the fake Moody in book 4, during his lesson on Unforgiveable Curses). FakeMoody also says it can't be blocked, but please keep in mind that FakeMoody was a lying liar who tells lies, and things he says cannot necessarily be taken as completely honest.
FakeMoody then makes the point that no one has been struck by it and survived, save Harry.
FakeMoody lied when he said it was unblockable. Indeed, the opposite is extremely true. Dumbledore does clever things with statues in the Ministry, blocking killing curses. During every major battle scene from book 5 onwards, the Death Eaters make extensive use of Avada Kedavra. It, like any spell, can be blocked by physical objects (though it doesn't seem to be stopped by magical shields), it can be dodged, and (as we see in the climax of book 4) it can be struck by other spells. Book 4's climax was, admittedly, a special case, but it does demonstrate that the Killing Curse can be affected by other magic (at least Harry's magic).
Avada Kedavra isn't an Unforgivable Curse because it's unblockable, it's Unforgivable because the ONLY purpose of the spell is to kill. It can't wound, it can't incapacitate, it can only deliver death.
Avada Kedavra is said to be unblockable in Goblet of Fire
Moody raised his wand, and Harry felt a sudden thrill of foreboding.
‘Avada Kedavra!’ Moody roared.
There was a flash of blinding green light and a rushing sound, as though a vast, invisible something was soaring through the air – instantaneously the spider rolled over onto its back, unmarked, but unmistakably dead. Several of the girls stifled cries; Ron had thrown himself backwards and almost toppled off his seat as the spider skidded towards him. Moody swept the dead spider off the desk onto the floor.
‘Not nice,’ he said calmly. ‘Not pleasant. And there’s no counter-curse. There’s no blocking it. Only one known person has ever survived it, and he’s sitting right in front of me.’
Harry felt his face redden as Moody’s eyes (both of them) looked into his own.
Goblet of Fire - page 191 - Bloomsbury - chapter 14, The Unforgivable Curses
I have a feeling someone will argue with me that this doesn't count because it was Barty Crouch Jr. as Mad-Eye Moody who said it, but it's totally consistent with canon. It counts.
Avada Kedavra is not a curse that can be blocked. I mean, someone could cast Avada Kedavra at someone else, yet miss (see Dumbledore and Voldemort's duel in the atrium at the Ministry of Magic in Order of the Phoenix). However, if the curse hits a person, that person is killed.
Avada Kedavara is unblockable in the sense, it can't be intercepted by another magic spell. Exceptions:
Situation of Priori Incantatem, when the caster and his opponent's wands and spells are locked together.
Sacrificial Protection which uses magic of love.
And yes, physical barriers can block the spell. Fast reflexes can save you from this spell,
but its like escaping from gun fire. Certainly, there's no Spider-Man in the Harry Potter universe.
A Shield (protego or non-verbal) charm will not stop it is probably what Moody meant by 'block', and no known counter-curse will reverse it .
Defenses seem to be:
Interrupt the caster before completing the spell--hit the caster with a quicker spell or folding chair
Disapparate -- Dumbledore did this in the Death Chamber, didn't he?
Priori incantatum -- this just happens, not really a tactic
Use a physical barrier, conjured, "animated" or existing (hide behind a pole), to stop the "jet"
Another living being gets between the caster and the target-- conceptually the same as a physical barrier. Fawkes reacted quickly enough to "take the jet" to save the intended target, right? He "swallowed" the entire jet. This implies you could possibly pull someone into the way if you react quickly enough, if they block the entire jet. Regarding "sacrificial protection" -- my interpretation of the book was 3 separate AV spells were cast-- Voldemort cast AV, and killed James downstairs. Then, he went upstairs, and told Lily to step aside. She didn't, so he cast AV again which killed Lily, Then, he cast another AV against Harry, which rebounded because of the sacrifice. Movie showed it differently. Maybe I misinterpreted (or misremember) the book. I think it is unclear whether AV can kill more than one victim, if both were somehow struck by the jet. The movie shows it billowing out like flames from a flame thrower-- the book seems to indicate more of a focused beam, right?
Caster "misses". Didn't Voldemort miss Dumbledore in the Death Chamber? Since a "miss" is possible and it's 6 syllable curse--you have a half second or better, implying you could do one of the following
Outlier reason-- wand allegiance. This one is beyond me, and maybe the movie is clouding my memory. AV from the elder wand "killed" Harry in the Forbidden Forest, but not at Hogwarts-- Harry's expelliarmus stopped it cold. Anybody? I guess in the Forest, Voldy killed the part of him that was in Harry, not Harry himself. That's pretty corny.
The reason it is described as unblockable is that the use of the shield charm is ineffective against a killing curse, as explained by fake Alastor Moody in Goblet of Fire. You can, however, protect yourself by summoning physical shields like Dumbledore did in the Battle of the Ministry of Magic. I must also point out that powerful wizards can cast non-verbal spells, including spells like expelliarmus so this negates the fact that spells take time to cast.
Think of Avada Kedavra as a death ray beam gun with shield piercing capabilities. If it hits, you die; armor can't protect you and shields (short of a sacrificial spell) can't deflect it. The only way to survive it is to not get hit by it in the first place, but because it's line-of-sight and very fast that means that much like firearms you have to dodge the caster, not the spell itself.
The big issue with countercursing is the same thing -- if it hits, it's instantly fatal. No middle ground, no injury, just death... and the incantation is a mere six syllables long. As Aditya points out in their answer, it takes roughly five seconds or less to cast, which puts a rather narrow maximum cap to what you can do about it.
(Research in countercharms is further hampered by the fact that there's only one way to test whether a possible counter charm would work and you don't get to try again if the answer turns out to be "Nope, doesn't work". Hope you left good notes for your successors...)
Technically, "Expelliarmus" could be considered an effective counter (can't cast the curse without a wand in your hand) but even if you assume instantaneous effect, if you aren't aren't already pointing your wand and casting by the time you hear the "va" syllable, you'll be dead before you finish casting.
In anything short of a straight-up face-to-face duel, that means you have roughly one short spoken syllable's worth of time to realize someone is about to murder you and that you need to get your wand up and out now or it won't matter anymore. That's on the order of half a second or so (try it yourself) and that's before we take into account the time it takes for the sound to reach your ears and your nerve impulses to draw and cast reach your limbs, lungs and vocal chords...
... I suppose taken in a purely tactical context both "reducto" and "Crucio" would be even more devastatingly effective because they require half as many syllables to cast and someone with body parts (possibly vital ones) missing or writing on the ground in incapacitating pain isn't going to be able to defend themselves against your follow-up any time soon, but both of those are survivable if help arrives in time (well, unless reducto took your head off).
Avada Kedavra is not. That's why it's considered so damn scary, why it's one of the Unforgivable Curses, and why Harry's survival is regarded as a Big thing.
(Side note: "Reducto" would be a better counter to AK than Expelliarmus because it takes two less syllables to cast so you have a wider window, but you'd have to make sure to hit either their wand arm or their head to prevent them from finishing the curse, so...)
As stated it has never been specifically said as unblockable.However it has been infered there is a specific limit of spell protection in the potterverse as demonstrated in the weasley's profitable hats,cloaks and gloves which have been comfirmed that they only work well against minor offensive spells unlike major jinxes and curses.
From this it can be infered that 'the killing curse' with holds much penetrating power which is capable of destroying shield charms however not much can be said if another offensive spell intercepts the curse head on such as the expelliarmus
There is no counter curse to the Avada Kedavra (Expelliarmus only worked for Harry because his wand was twined with Voldemort's and could not be forced to do battle with each other).
Yes physical objects can be moved into the path, but this is not a counter curse and it takes an extremely skillful wizard to do so quicker than the curse - particularly in a life and death situation.
The only direct magical defences are Horcruxes; which if you have one (or more), will keep your soul earthbound even after your body dies. Or "Self-Sacrifice"; if someone willingly takes an Avada Kedavra curse, without trying to defend themselves, to protect someone else (as Harry's mother did for him and as Harry himself did for all his friends at the end of The Deathly Hallows) then the person who was protected cannot be touched by the wizard who cast the Avada Kedavra. Any curse they attempt to cast on them will be unbinding and another Avada Kedavra will be rebounded.
This is what happened the night Voldemort tried to kill Harry in Godrics Hollow, Voldemort only survived because of his Horcuxes.
What Moody ment it is that it is not reversable. If you are hit by avada kedavra there is no counter curse to reverse it effect. Also it feels like he mean that it obviously penetrate any protective charm or something like that. What is my problem its that sources says that it cant be cast withou incantation. That means u cant cast it without saying loud avada kedavra yet in movies it looks like nothing - lriory to make fights dynamic but i just do not like. But I think if you hit avada kedavra spell with your spell its like magic energy versus magic energy and it somehow disrupts ewch other or rebounds. Avada can destroy surface when it miss target or rebound from surface.
But there is protection from avada. Horcrux