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In the final part of Tenet, the Protagonist and Neil are trying to prevent one of the bad guys from dropping the Algorithm into the time capsule. Supposedly dropping it inside would be a way to deliver the Algorithm into the future and thus allowing humanity of the future to reverse entropy, destroying the past.

But why was it a big deal if its dropped in or not? Couldn't the Protagonist simply recover it afterwards, preventing it from staying in the time capsule? What exactly were they trying to prevent?

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  • There is no time capsule. It's only a hole in the ground. The idea was to drop the algorithm into that hole and trigger it by using a minor detonation. Sator's tracker causes the minor detonation, but the algorithm has been removed from that casing, so it doesn't get triggered. Thereby preventing Armageddon.
    – MovieMe
    Jan 6 at 2:51
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+100

I believe the first part of your question (your assumption regarding the algorithm being delivered into the future) is correct. The answer saying otherwise is unsourced; let's explore an alternative approach.

A Primer on Information (And Who Has It)

It is important to recognize throughout the film that most characters, The Protagonist (TP) included, do not posses all, most, or even a little bit of the information required to discern the nature of the plan enacted by Tenet beyond what is required for them to perform the actions or deliver the information for which they are personally responsible. This theme is highlighted in the following lines of dialogue:

FAY
There's a cold war. Cold as ice. To even know its true nature is to lose.


NEIL
The policy is to suppress.


IVES
Need to know. And you don't. Any other stupid questions?

This theme will guide us in the assumption that, unless otherwise noted, characters posses no information relevant to Tenet/the plan other than what has been specifically provided to them. This is made explicitly clear by Fay, at the close of their conversation on the deck of the ship:

FAY
This is knowledge divided – all I
have for you is a gesture –
(interlaces his fingers)
– in combination with a word –
‘tenet’. Use it carefully, it’ll
open the right doors, but some of
the wrong ones, too.

PROTAGONIST
That’s all they’ve told you?

FAY
The test you passed?
(beat)
Not everybody does.

These lines establish Fay as a pawn, given one role (transmit the two items of information) in accordance with his (lack of) ability--an implied unwillingness to commit suicide instead of divulging important information. This is further shown on a procedural/organizational level during TP's conversation with the scientist after his departure from the windmill ("no small talk").

We will assume that the ultimate source of the information disseminated by Tenet to its members is TP, in a plan enacted after* the events of the film. This is based on the conversation TP has with Neil prior to Neil and Ives departing from the hypocentre LZ. We will assume, based on the length and nature of their relationship (and narratively from Neil's role as a source of reliable exposition), that Neil has adequate information to accurately conclude that TP is indeed the founder. This is stated clearly at the end of the film, but the film is convoluted enough that nailing down the obvious is a good first step.

*From TP's perspective


Good on you if you've made it this far. So why is this extreme level of information compartmentalization necessary?


The Nature of the Conflict

The cold war referenced by Fay is an existential conflict between The Present and The Future, with The Future desiring to reverse the ecological destruction of the earth. From the conversation between TP and Sator by sat-phone in the hypocentre:

PROTAGONIST
How can they want to destroy us?

SATOR
(over phone)
Because their oceans rose and their
rivers ran dry. Don’t you see –?
Their world shrivelled because of
us. They have no choice but to turn
back, there’s no life ahead of
them. And we’re responsible.
Knowing this, do you still want me
to stop?

PROTAGONIST
Yes. Each generation looks out for
its own survival.

The two sides are locked in an annihilatory struggle (annihilate--"to move to nothing"--seems especially appropriate in this case). The Present is threatening the existence of the The Future through wanton ecological destruction. The Future, through implementing the algorithm, is threatening the existence of The Past.

The Nature of the Algorithm

PROTAGONIST
What’s the ‘algorithm’, Neil?

NEIL
The 241 is one section of it. One
out of nine. It’s a formula
rendered into physical form so it
can’t be copied or communicated. A
black box with one function.

...that function being...

NEIL
Inversion. But not objects or
people. The world around us.

... which is necessary because, in a conflict involving objects sent through time, the default direction of entropy favors The Present:

NEIL
Because the environment’s entropy
flows in our direction we dominate.
They’re always swimming upstream.

"Swimming upstream" is not rigorously defined in the logic of the movie ("don’t try flying a plane – it’d fall out of the sky"--a line that didn't make the final cut--suggests that the more complex the interaction the more resistance there is), but we'll assume that it's the dynamic responsible for TP's knife wound appearing as he travels back in time/the implied gradual appearance of the bullet holes in the glass in the Oslo vault (as opposed to them always being there).

The actual Cold War (ours, First vs. Second world, capitalism vs. communism) offers a potential explanatory metaphor: defection. If one side offers a higher quality of life individuals will naturally gravitate towards it, tangent to any ideological parameters. We can apply the metaphor to physical objects in a similar manner; they naturally gravitate towards the future, with objects travelling in the opposite direction inclined to "defect"/degrade/disappear. This metaphor is entirely my own attempt at an explanation, and can be disregarded at will.

It seems dicey to introduce the term "break causality" into such an already rickety structure, but we can speculate that reversing the flow of entropy would cause the present to pass out of existence much like TP's knife wound came into it; viewer speculation aside, we have the speculation of Neil in the film to go on, with the ultimate result of implementing the algorithm uncertain:

PROTAGONIST
I’ve been thinking... we’re their
ancestors. If they destroy us,
won’t that destroy them?

NEIL
Bringing us to the grandfather
paradox.

PROTAGONIST
The what?

NEIL
The classic thought experiment – if
you went back in time and killed
your own grandfather, how could you
have been born to commit the act?

PROTAGONIST
What’s the answer?

NEIL
There is no answer, it’s a paradox.
But in the future, those in power
clearly believe that you can kick
Grandpa down the stairs, gouge his
eyes and slit his throat without
consequence.

PROTAGONIST
Could they be right?

NEIL
It doesn’t matter. They believe it,
so they’re willing to destroy us.
Can I go back to sleep?

The explicit message of this conversation is that the effects of implementing the algorithm are A. uncertain but B. almost certain to result in the destruction of The Present. Whether that would result in the destruction of The Future is stated to not be clear, but it's indicated that the residents of The Future are desperate enough that they're willing to take the chance and "kill Grandpa". The scientist responsible for inventing the algorithm disagreed:

PRIYA
Think of our scientist as her
generation’s Oppenheimer – she
devises a method for inverting the
world, but becomes convinced that
by destroying us, they destroy
themselves.

...and so sends it into the past, as Oppenheimer might have taken steps to destroy the Manhattan project had he believed that detonating a nuclear weapon would ignite the atmosphere. (There's an interesting question there: if the scientist had been confident that The Future would survive the destruction of The Present would she consent to the activation of the weapon, like Oppenheimer did?)

The Functioning of the Algorithm

This brings us to the nuts and bolts of the functioning of the Algorithm, specifically as they relate to the events at the hypocentre. There are a few key points:

  1. It is not indicated that the detonation of the bomb at the top of the hypocentre shaft is responsible for the activation of the algorithm. From the conversation on the ship about the dead man's switch while TP, Neil, and Kat are spending a week inverting for the final operation:

    NEIL
    It’ll be linked to a switch.
    Probably a simple email burst,
    revealing the location of the dead
    drop, set to fire if his heart
    stops.
    
    PROTAGONIST
    In effect, his death activates the
    algorithm. He dies, the world ends
    – no one dares kill him.
    

    The operative term in this case being in effect. Sato's death/the sending of the dead-man's switch e-mail is not directly related to the explosion that happens at the top of the shaft, but is instead a fulfillment of his mandate from the future:

    PRIYA
    Sator’s lifelong mission, financed
    and guided by the future, has been
    to find and reassemble the
    algorithm.
    
    PROTAGONIST
    Why did they choose him?
    
    PRIYA
    The necessary combination of greed
    and ambition. But mostly, he was in
    the right place at the right time.
    

    ...to assemble the pieces of the algorithm and place them in a location where they can later be found, supported by the earlier establishment of the concept of the dead drop:

    PROTAGONIST
    Dead drops. He buries his time
    capsule, transmits the location,
    then digs it up to collect whatever
    inverted material they’ve sent.
    

    ...just in the case of the algorithm, instead of The Future placing inverted objects in the capsule to be dug up in the past, Sator is placing an un-inverted object to be found at a later date (a traditional time capsule).

  2. It is indicated that the algorithm will be activated in the future. From the sat-phone conversation with Sator in the hypocentre:

     PROTAGONIST
     (over phone)
     What’s more fanatical than trying
     to destroy the world?
    
     SATOR
     I’m not. I’m creating a new one.
     Somewhere, sometime, a man in a
     crystalline tower throws a switch
     and Armageddon is both triggered
     and avoided. Entropy inverts the
     same way the magnetic poles have
     switched 183 times over the
     millennia. Now time itself switches
     direction.
    

    The "man in the crystalline tower" is an individual in The Future ("sometime") activating the implementation of the algorithm, inverting entropy. This avoids the ecological Armageddon of future earth, and triggers the temporal Armageddon of The Present.

  3. The assumption that the method to implement the algorithm is to destroy the hardware on which it is implemented does not make a ton of sense. Additionally, the algorithm is specifically said to be sealed within the hypocentre during the conversation in which TP reveals the location of the dead drop to Ives:

     PROTAGONIST
     Ground zero for an underground
     nuclear test. Sir Michael Crosby
     told me about a detonation at
     Stalsk-12 on the 14th – the dead
     drop is at the bottom of the
     hypocentre. That explosion seals up
     the algorithm.
    
     IVES
     Then we’d better pull it out of
     that hole before the bomb goes off.
    

    Reinforced by the line from the script stating Volkov (final henchman/knuckle tattoo enthusiast/anti-hot sauce mastermind) is struggling for the winch release--if the purpose was to detonate a bomb next to the algorithm, what purpose is served by dropping it down a hole, away from the explosion?

     INT. TUNNEL, STALSK-12 – CONTINUOUS
     The Protagonist DIVES to where Ives is lying, GRABS his gun –
     Volkov TURNS, RUNNING FOR the winch-release –
    

    There is none.

Answering Your Question

At this point things get a little hazy. We've established that the movie took a pass on explaining the grandfather paradox; it's attempt at nailing down the nature of causality can be summed up in this exchange:

NEIL
What’s happened’s happened.
(smiles)
Which is an expression of faith in
the mechanics of the world, not an
excuse to do nothing.

PROTAGONIST
Fate?

NEIL
Call it what you want.

PROTAGONIST
What do you call it?

NEIL
Reality. Now let me go.

We assume that, given time and excavation equipment, TP/Tenet would have had no problem retrieving the algorithm from the hypocentre. But that assumption has a Present bias; based on the logic presented in the movie it would be equally valid to assume that, had the algorithm been successfully sealed in, The Future would have had no problem retrieving it and, from the perspective of The Present, immediately erasing the existence of the latter. This did not occur; what happened happened, and reality as the characters know it was a product of their actions.

That The Present continued to exist suggests that The Present/Tenet was "always" going to be successful in stopping the transfer of the algorithm to The Future. This requires viewing events in the future as functionally the same as events that have occurred: that they always were/always am, and that reality is constructed simultaneously instead of sequentially (tough to wrap your mind around when we obviously perceive it very differently).

You could imagine that someone struck and killed by a car could have been saved by standing three feet to the left, or you could imagine a car that swerves three feet to the right could strike and kill someone who would have otherwise been unharmed, but the individual is either struck by the car or they are not struck by the car. Consider the meta-physical car (Sarto assembling the algorithm and broadcasting its location) sent by The Future to kill The Present: whether it succeeds or fails, the fact remains that it can only be one possibility or the other. The turnstile technology, by turning time into a dimension that can be traversed like any other, and therefore removing the independent variable by which we usually define events as exclusive to each other, flattens our perception of what is and isn't possible--everything "just is", "always was", and "always will be", which would be the mechanics of the world that Neil expresses faith in*.

*Comments on nature of reality not substantiated, safety during time travel not guaranteed.

This interpretation is supported by the role of information in the film (coming full circle back to the importance of information security). During TP's first conversation with Priya:

PROTAGONIST
He can communicate with the future?

PRIYA
We all do. Credit cards, email,
text – anything that goes into the
record speaks directly to the
future. The question is – can the
future speak back? And, if so, what
are they saying?

The "always is, always was" nature of communication (where anyone can communicate into the future by making a record of it) means that nothing is ever "over", only lost and forgotten--The Future only has to "get lucky" once, while Tenet must be successful forever (but that The Present continues to exist implies that they are).

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  • So the answer is... the battle in Stalsk-12 was indeed unnecessary as the Protagonist had many years to successfully dig it out? Insanely good answer btw. Jan 4 at 6:07
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    Had things turned out differently, and the algorithm existed at the bottom of the hole with its location broadcast for even an instant, The Future would have been able to retrieve it "immediately" (from their perspective) and reverse entropy. The resulting destruction of The Present timeline would preclude any further recovery efforts.
    – clyf
    Jan 4 at 6:22
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    Yes, I see your point. "Had things turned out differently" => but they couldn't have turned out differently, as there's only one timeline in Tenet and therefore either the Past exists or it doesn't. But since the Protagonist cannot actually change his past or his future as free will doesn't exist, everything in the movie is basically immutable and doesn't have to make any sense. Jan 4 at 6:24
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    I gather you would appreciate a once-sentence summary: Were the algorithm dropped into the hypocentre and the location broadcast, the reality in which The Future can access the algorithm is the reality in which the characters live; The Future does so, erasing their existence.
    – clyf
    Jan 5 at 0:42
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    I think you're right to think that the moral problem of outright averting someone's existence would be trickier than, say, shooting them with a gun--averting someone's existence is common now (birth control, as an example). It's also worth noting that in the film the powers-that-be of The Future consider the erasure of the past to be a morally acceptable alternative to their own extinction (the characters in the present obviously don't agree), which is indicative if we are to assume that they are not entirely villainous/not villainous at all.
    – clyf
    Jan 5 at 1:06
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Your assumption that,

Supposedly dropping it inside would be a way to deliver the Algorithm into the future and thus allowing humanity of the future to reverse entropy, destroying the past.

is wrong.

Sator did not intend to keep the Algorithm in time capsule for the folks in the future to discover it.

He was going to detonate the Algorithm after enjoying his one last moment with Kat and Max on his yacht in Vietnam.

Also the detonation was to be done with a dead man's switch after he has had a cyanide pill. So that when he dies, the fitness tracker sends signal to detonate the Algorithm.

So they were trying to prevent the detonation of the Algorithm before Sator commits suicide.

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  • 1
    @JonathanReez: I don't understand what do you mean by destroyed. Destroyed by Posterity or detonated by Sator ?
    – Rahul
    Dec 21 '20 at 20:56
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    Sator's death does not destroys the Algorithm, it detonates. In other words it activates the Algorithm (like a bomb).
    – Rahul
    Dec 21 '20 at 21:00
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    Is that assumption wrong? My understanding was that Sator's death would trigger the end of the world by triggering the bomb, causing the Algorithm to be buried so the future could find it and use it to reverse time. When is it explained that "detonat[ing] the Algorithm" would destroy the world?
    – Mike S
    Jan 2 at 0:49
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    @galacticninja: Signal was sent and the bombs in tunnel did go off. But before the algorithm could activate(whose technicalities are unexplained) they were lifted out.
    – Rahul
    Jan 4 at 7:21
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    @MovieMe see the answer above which includes references to the script (and the movie is now streaming as well). Could you mention the script lines which support the theory of the bomb being needed to trigger the algorithm rather than to send it to the future? Jan 6 at 3:18
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The Protagonist and Neil speculate this:

It'll be linked to a switch, probably a simple e-mail burst that reveals the location of the dead drop, set to fire if his heart stops.He dies, the world ends. No one dares kill him.

The two of them don't know much about Sator, but Kat does. Kat corrects them saying:

No, you've missed the point. He's intending to end his life. He's dying. Inoperable pancreatic cancer. And he's taking the world with him. If he can't have her, no one can. He gets to choose the time and place for the end of the world.

Sator chooses the time of the world's end to be aligned to his own death.

If Sator was merely hiding the algorithm for the future to find:

  1. He didn't have to risk bringing them all together, he could have sent the 9 parts out in 9 different secret locations.
  2. He doesn't get to pick the time for the world to end because there is no saying when the future folks will choose to end the world once they get the device. It could be 10 years after Sator's death.
  3. The present could simply dig up the algorithm up now that they know the location. And Sator knows The Protagonist's team knows the location, so if not The Protagonist, someone else would dig it up.
  4. The future now needs to find another person like Sator to send the algorithm back in time to in order to detonate it. Remember, they want the bomb to detonate in the past, not in the future. The future people are seeking revenge, not looking to commit suicide.

Now to answer the question - Sator's fitness device connects to some sort of a trigger (a fuse) that will set the algorithm to trigger (just like a fuse that sets off a nuclear bomb). The bomb that we see in the drop zone is a timer bomb. This will detonate independent of Sator. It's purpose seems to be to seal the algorithm underground. Sator planned to kill himself on his boat soon after this. His death would trigger the algorithm destroying the world at that moment.

How can we say this? Because Kat says that Sator has picked the time for the world to end. That means by killing himself he picks the time to trigger Armageddon.

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  • Comments are not for extended discussion; this conversation has been moved to chat.
    – A J
    Jan 8 at 3:45

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