

Disclaimer: I watched only the anime. (And read
[Toarupedia](http://toarumajutsunoindex.wikia.com/wiki/))

In season 2 of Index we are introduced to Kihara Amata, one of the scientists
who worked on researching Accelerator's ability. He attacks him head on which
usually proves fatal for the attacker depending on their attack strength and
punches Accelerator. Then Accelerator creates a wind tornado and attacks
Amata. However, Amata dispels the attack into thin air.

Now the thing that bugs me is how he managed to punch Accelerator and the
anime's explanation. Amata narrates what he is deeply knowledgeable about the
inner workings of Accelerator's ability and points out that his redirection
shield switches the directions of objects which come in contact with it and
the trick to bypass it is to pull back your fist just at the moment of contact
and the shield will switch direction of the fist which would be backwards
right into Accelerator (or somehow the momentum of the punch and not the fist
it is not perfectly clear to be honest).

The problem in this explanation is that we are quite often told that
Accelerator's shield's speed is equal or faster than the speed of light
because his ability reflects UV and other radiation (at least UV travels in
speed close to speed of light) which means that to bypass the shield and trick
it into letting through an attack Amata would have to move at speeds which are
faster than the speed of UV rays or the speed of light, which is absurd even
given the fact that he apparently replaces his hand with artificial one.

To illustrate, imagine what you are standing before a big Vector field which
is slightly visible. It is of a size of a wall or something. You see Kihara
Amata on the other side running at the wall going for a punch. There are few
things what could happen here:

  1. Amata moves faster than the shield can react and punches you in the face.

  2. The shield's reaction time is much much higher than Kihara's time (and that includes calculations to determine whether the vector brings harm/no harm, as shown wth UV), Kihara moves his fist close to the edge of the field and then pulls back. Nothing happens, as the fist did not make contact.

  3. The shield's reaction time is still higher than Amata's punch,but this time Amata goes for the punch-pullback on the edge of the field, entering it slightly. As shield is too fast for him, Amata breaks his arm moment he even makes contact with the field, and there is no time for him (as per his relative slowness of movement) to even begin to pullback, as the shield already made calculations and reacted with at least speed higher than UV.

The only way the punch bypasses the shield is if it's movement speed is faster
than the field's reaction. This way, it is possible to just ignore the shield
as it does not even have time to make calculations if it is slower than the
vector's speed.

Even if for a moment we imagine that Amata invented a way for his hand to
harmlessly move at the speed of light or rather faster than the shield can
react as we are never told in the anime whether the shield has any speed
limits, his punch would just go through the shield, and if Amata pulls back,
shield still would not have time to react and nothing happens. However that
Amata can give a punch that travels at the speed of light is highly doubtful,
so we can assume that the trick is something else and he bluffs. But what is
it? Did the guy who wrote the script decide to include plot induced stupidity
or is there an _in-verse_ explanation which actually makes sense?

Edit: [@Ryans comment](https://anime.stackexchange.com/questions/47547/how-
does-kihara-amatas-device-to-bypass-accelerators-passive-redirection-
actua#comment61780_47550)'s about with theory of possible exploit of the the
shield got me thinking. If the field decides whether to reflect or not, and
let's some things in, perhaps it could be exploited by first just moving
slowly and entering the field. The field would mark the vector "non-harmful"
and allow the hand in. Then the hand would suddenly accelerate into opposite
direction of the shield's emittor (i.e. Accelerator) and the shield would mark
the vector as "harmful" and switch the directions of the hand. Though this
seems odd now that I think about it more, because the moment shield would
switch direction of the fist it would become "harmful" again, would not it
switch the direction of the vector again this time away from the field
emittor? Now, one other things I wonder about is why Kihara's hand when he
punches Accelerator does not fly towards him and instead the momentum of the
punch does? This is certainlynot the case with bullets, when the field
switches their directions, they fly away, not the momentum of theirs. But in
anime it is shown that when Kihara sends a punch to Accelerator, when he
executed his trick, it's not his fist what goes towards Accelerator after
being switched by the shield, but the momentum. Thois seems to leave much room
for speculation (for example, would the field even classify a hand, of any
velocity "non-harmful"? We never see in the anime and in other materials as
far as I can tell from Indexpedia anyone touch Accelerator in a non-harmful
way (aside from when Accelerator magines what a normal childhood would be like
in Railgun S, but this is obviously in his head).

So far, it seems there is not enough information but as far as I see, @Ryan's
theory seems to be the most plausible one.


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How does Kihara Amata's device to bypass Accelerator's passive redirection actually work, and should it work at all?


My theory is that Kihara Amata indeed bluffs and in reality he actually
implanted some subtle bug (perhaps activated by a particular soundwave as IIRC
Amata mentions something about soundwaves) in Accelerator's power while he
developed/researched it. We may ask why he did not just implant a bug which
allowed him to disable Accelerator's powers all together and possible answer
to that is that Amata could only create a bug subtle enough for Accelerator to
not notice while using his ability and the bug could be only extended as far
as to bypass Accelerator's redirection shield on contact and his "wind-based
powers". This is the only explanation I can come up with which may be in-verse
without just damping it on the authors despair to make a match for Accelerator
and thus include PIS in his story, though even then I think my theory might be
a bit far fetched considering that it's never mentioned again in the anime and
as far as I read Toaru wiki, in the source material as well in any form. I
admit this might be the only possible theory in-verse.


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Incidentally, there's a wikia page for this technique. The fans call it the
[Kihara Counter](https://toarumajutsunoindex.wikia.com/wiki/Kihara_Counter).

As far as I can tell, the explanation is meant to be taken at face value.
Later on in the light novels, other people have attempted to replicate the
technique (or at the very least, demonstrated the potential to replicate the
technique) with varying levels of success.

For example, in volume 19, chapter 3, part 8, Sugitani manages to land a hit
on Accelerator using this technique, although he injures his hand in the
process because his execution of the technique isn't perfect.

Wandering more into speculation territory...

This technique does seem a bit farfetched. However, I don't think the
technique involves any light speed shenanigans. Both Kihara and Sugitani are
humans without any special augmentations, as far as I can tell (although
Sugitani does happen to be a ninja), so the technique ought to be achievable
using human-level reflexes and speed (or thereabouts).

Keep in mind that although Accelerator's passive reflection may seem
straightforward (just reflect all the things, right?) there is lots of
complexity to it that leaves it vulnerable to exploitation. Let's review the
details of [his passive reflection
ability](https://toarumajutsunoindex.wikia.com/wiki/Accelerator_\(ability\)#Redirection):

  1. His ability can reverse any oncoming vector that comes towards the field.
  2. His ability acts passively and automatically. In other words, his ability can reflect high velocity projectiles such as bullets that he obviously could not consciously react to.
  3. He subconsciously establishes a filter that analyzes everything as harmful or not. This filter reflects any harmful things.
  4. His filter allows certain non-harmful things to pass through out of convenience. This includes gravity, air, pressure, light, heat, and sound.

The main vulnerability here is his harmful/not harmful filter. If his filter
doesn't recognize the projectile as harmful, then it will get through. For
example, Teitoku's Dark Matter wings manage to bypass Accelerator's
redirection during their fight in volume 15 because Accelerator's filter
doesn't recognize the exotic matter.

If you think about it, all of the harmful vectors that his ability reflects
are incoming vectors. Bullets that go right toward him. UV rays that go right
towards him. Shockwaves, explosion fronts, and so on that all go directly
towards him. As a result, Accelerator's harmful/not harmful filter is probably
very good at recognizing these sorts of threats.

However, a punch that goes right towards him and then _stops_ right on the
edge of his AIM field and then _reverses_ is a very unusual edge case. I doubt
his filter has much experience with vectors that follow this pattern. As a
result, it's not implausible his subconscious filter would misclassify the
fist as "not harmful" and then perhaps "harmful" again when the hand suddenly
begins to accelerate in a reverse direction, and then perhaps get confused
with the outgoing direction and cause it to reflect inwards.

If you're familiar with neural networks, there are lots of models these days
that approach superhuman performance in various classification tasks, such as
the [ImageNet Large Scale Visual Recognition
Challenge](http://karpathy.github.io/2014/09/02/what-i-learned-from-competing-
against-a-convnet-on-imagenet/). However, it easy to construct adversarial
data inputs that fool the classifier (for example, see [High Confidence
Predictions for Unrecognizable Images](https://arxiv.org/pdf/1412.1897.pdf)
and [One Pixel Attack Defeats Neural
Networks](https://www.youtube.com/watch?v=SA4YEAWVpbk)).

In a similar vein, with Kihara knowing what he does about Accelerator's
powers, I think it's quite plausible that with enough computing power, Kihara
could figure out an adversarial vector sequence that could fool Accelerator's
harmful/not harmful filter. Or indeed, as [Moe Epo's
answer](https://anime.stackexchange.com/a/47549) suggests, it's also a
possibility that Kihara implanted the bug/exploit intentionally.

However, it should be noted that once Accelerator figures out the exploit, he
can easily adjust his calculations to take that into account. He has indeed
done so with Dark Matter during his battle against Teitoku in volume 15.


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Amata is pulling off a [Supertask](https://en.wikipedia.org/wiki/Supertask)
here. (Yes, that is as ridiculous as it sounds).

At every point in time _before_ his fist actually reaches Accelerator, it is
still moving forward, though decelerating. At the _exact instant_ his fist
actually touches Accelerator, it stops moving - nothing to reflect.

He then pulls his fist back.

Because his fist is touching Accelerator, it gets reflected - but because it's
accelerating _away_ from Accelerator, reflecting it causes it to accelerate
_into_ Accelerator.

It's not really a punch - more of a hard shove. This is especially obvious in
a later scene where Accelerator's collar runs out of charge, and Amata starts
'kicking' Accelerator while he is down. The novel notes that if he was kicking
him normally, he probably would have killed him.

(Remember, Accelerator is actually really fragile with his powers off - Touma
_jabs_ at him, not punches, because he is terrified Accelerator will grab his
forearm and make him explode.)


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