The observer should determine whether he is in some kind of game where the system will not allow him to die while at the same time hiding this fact from him. The system can use any unsuspicious means to save him, including doctors, weather events, actions by other people etc.

The system always prefers the more probable and unsuspicious ways to the less probable, magical and suspicious. For instance after getting a fatal wound the hero learns that some scientists recently discovered how to cure it.

The system in its acts cannot break any physical laws except the second law of thermodynamics.

  • $\begingroup$ @Starfish Prime I updated the question. The main point is, the survival each time looks the most natural way possible, within the laws of physics, deviating from them only statistically. For instance, you receive some fatal injuries and suddenly learn the health scientists just developed the cure against this kinds of wounds. $\endgroup$ – Anixx Sep 24 '19 at 11:44
  • $\begingroup$ Sorry, your edits do not seem to make the question any better. You state differences from the other question based on description you have not provided, then shift the immortality to be given by health care... I am lost $\endgroup$ – L.Dutch - Reinstate Monica Sep 24 '19 at 11:44
  • $\begingroup$ @L.Dutch the observer should determine whether he is in some kind of game where the system will not allow him die while at the same time hiding this fact from him. The system can use any means to save him, including doctors, wheather events, actions by other people etc. $\endgroup$ – Anixx Sep 24 '19 at 11:47
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    $\begingroup$ how did the protaganist come to the conclusion he's an immortal in a game? Fatal wounds don't occur very often for normal people, some never get them $\endgroup$ – Kilisi Sep 24 '19 at 11:50
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    $\begingroup$ I don't see how this would be a duplicate (beyond the question title). The proposed scenarios are quite different and none of the other question's answers would work for this one. $\endgroup$ – Ruther Rendommeleigh Sep 24 '19 at 14:28

No, not the way you set it up.


The way you set this up, "the universe" will find a way to save your immortal character in a manner which leave doubt about if it's luck or if he's really immortal. Unless he dies, he cannot be sure that he's immortal, since he's not immortal.

He's immortal until proven otherwise.

Knowing that he won't know if he's immortal (or not) unless he dies or do something so dangerous that it doesn't leave place to doubt - which we want to avoid -, this character can guess that he cannot be certain, only reasonably certain.

Enter statistics.

Assuming only the protagonist is "maybe immortal", and that he is not willing to endanger himself recklessly, he should live his normal life, and every time something dangerous happens, write it down. After enough time, he'll have something like a sample he may use to do statistics. Then he'll be able to theorize if he might just be immortal, and what % of certainty he can attach to this hypothesis.

The thing is: this is not an exciting answer. The initial situation doesn't leave a lot of room th breath. Yet, if he's really immortal... he have way enough time to start and try this - and if he's not, he won't suicide himself while verifying.

  • $\begingroup$ This is an answer but one I came to myself. I was looking for other answers. Particularly, he could just live normal life until he outlives everybody else by a lot (but the system can make the healthcare to advance a lot so to make other people also to live a lot so to hide his exceptionism). Alternatively he could try to statistically analyze past events so to find any statistically unlikely events that saved him. $\endgroup$ – Anixx Sep 24 '19 at 12:39
  • $\begingroup$ The thing is, there are a lot of people on this planet, and if you live for a very long time, who knows how many more there will be? With a potentially unlimited sample size, there's a decent chance of having more than one person survive "impossible" odds. $\endgroup$ – Ruther Rendommeleigh Sep 24 '19 at 12:43
  • $\begingroup$ @Anixx I almost discarded my answer before publishing it because I was afraid it wasn't helpful. In the end, I decided to send it anyway because, in a way, I think that the real answer is in inserting something else in the initial problem which will give some grasp to your protagonist on the problem. For an example, he may "notice" he's hard to kill after surviving an accident - which is fine in itself, but in this case, he may notice something special or uncommon related to his survival, which he could exploit to understand what is happening and how he can exploit this. $\endgroup$ – laancelot Sep 24 '19 at 12:43
  • $\begingroup$ @RutherRendommeleigh You are absolutely right, and this is why there is no 100% way to be sure - hence having a percentage of certainty instead of a certainty. Also, I kinda assume that only the protagonist is immortal, or else this would be waaaay easier. $\endgroup$ – laancelot Sep 24 '19 at 12:45
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    $\begingroup$ Survivorship bias is your enemy here. $\endgroup$ – NofP Sep 24 '19 at 18:07

By definition, you can't

If the only thing the universe (supposedly) actively protects you from is death, you cannot test this fact without risking death, simple as that. Any experiment you devise that stops short of killing yourself in some way or other will yield no usable data because the universe needn't interfere in the first place.

Any experiment that might result in your death will yield data points that are exactly as reliable as the likelyhood of you dying, i.e. if there's a 50% chance to survive the experiment, and you do survive, you're only 50% sure that the universe protects you. Thus, the risk you take determines the precision of your answer. And unless you accept a lot of risk, you can never be (close to) sure.

Playing with the parameters

I can think of only one way to kind of cheat this, and that'll only work if you're patient: Find something that is very likely (i.e. nigh unavoidable) to kill you in the future (ideally, longer than your normal life expectancy) but has observable effects in the present. You won't know for sure until it actually happens (or, hopefully, doesn't), but you might be able to observe something that hints at the outcome. This way, you gain some information in the present while deferring the risk (of death) to a point in the future where, if you were wrong, it's not likely to matter anymore because you already died of old age (or something else entirely).

Caveat: I can't think of anything in the real world that's that lethal and that slow acting and not potentially curable with 50 years of research, that doesn't also make your life a living hell. But perhaps your universe contains such a thing?

  • $\begingroup$ "if there's a 50% chance to survive the experiment, and you do survive, you're only 50% sure that the universe protects you" - but if you survived an experiment that has 90% chance to kill you, then you can be much more sure the universe protects you (not 90%, but close, lazy to calculate). $\endgroup$ – Anixx Sep 24 '19 at 12:43
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    $\begingroup$ @Anixx yes, but you also took a much higher risk. That's what I'm saying, the more sure you want to be, the bigger the risks you must take. $\endgroup$ – Ruther Rendommeleigh Sep 24 '19 at 12:45

Elitzur-Vaidman Bomb Tester

SMBC comics Source of the comic above

Quantum mechanics are weird.

Remember Schrödinger's cat? The cat is an analogy for a subatomic particle - you can't have a whole cat in superposition (at least, not with nowadays technology). However, the cat experiment evolved into a series of other experiments. My favorite one is the Elitzur-Vaidman bomb tester, which extrapolated to the macroscopic world would be like detecting whether a bomb is real or a dud without triggering it.

Basic setup goes like this: the bomb is triggered by an extremely sensitive light sensor, so it blows up when it detects even a single photon. You get a photon in superposition as per Vaidman's experiment:

Experiment setup

This image is from the wiki linked at the top of my answer.

The result is that if the bomb is real, you can always assert it, and it only has a 50% chance to actually detonate! The full scientific elaboration is in the wiki.

The practical thing for your hero to do is to assemble dozens of bombs that would be able to kill him, triggered by the same sensors used in the experiment I've described. Now, instead of testing it on himself, he takes the bombs to a lab and runs Elitzur-Vaidman on all of them. If none of the bombs detonate, then he knows that something in the universe might be protecting him. Otherwise, well... Fortunately he knows how to science things out from a safe distance.

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    $\begingroup$ Even assuming that an interaction-free measurement is possible, and that the experiment works exactly as described, how does this protect the protagonist from the to-be-expected explosions? A 50% chance of the bomb going off is still a 50% chance of death, isn't it? And if it isn't, how does he gain any usable data points? $\endgroup$ – Ruther Rendommeleigh Sep 24 '19 at 14:10

He could go to war, as a mercenary or soldier, volunteer for the front line and any dangerous missions. If he survives where everyone else doesn't then he might as well go into the war business for himself and carve out an Empire.

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    $\begingroup$ But this would involve a lot of risk. $\endgroup$ – Anixx Sep 24 '19 at 12:01

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