Lie Detector
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From fretje:
When you die, you go to this place where there are two doors: one goes to heaven, the other one goes to hell. There’s a guard in front of each door. You don’t know which door is which. The only thing you know is that one of the guards is always telling the truth, while the other one is always lying. You can ask one question to one of the guards.
How do you know which door to take to get to heaven?
Get Puzzles Delivered:
You ask either of the guards “Do both doors go to heaven?”.
If the guard answers “yes”, he is standing infront of the door for hell. If he answers “no”, he guards the door for heaven.
@Paul Richards: we don’t know if the lying gard is in front of the hell door,
we can ask any of the guards:
“if i ask the other guard ‘which door is the door of hell?’ which door will he show me?”
and i can be sur that they will show me the door of heaven
Ask one guard what the other guard would say. Really? cribbing from Smullyan?
The guard in front the door of hell is facing the door.
i’d second gicque… it should work
Like any /real/ coders haven’t seen Labyrinth. What’s next, putting poison in two goblets?
“Guard #1: Would guard #2 tell me that your door is the door to heaven?”
If Guard #1 says yes and he’s the liar, he’d be indicating that the truthful person would be saying no, and thus his door would be the gate to hell.
If Guard #1 says yes and he’s the truthful person, he’d be indicating that the liar would tell him it’s the gate to heaven, thus the gate is the gate to hell.
If Guard #1 says no and he’s the liar, he’d be indicating that the truthful person would say that it’s the gate to heaven.
If guard #1 says no and he’s telling the truth, he’d be indicating that the liar would say no, and thus the gate is to heaven.
Either liar or truth teller, adding parity allows you to determine the state of the door. Yes = hell, no = heaven.
If you I asked you if this is the door to heaven and you said “yes”, would you be telling the truth?
Ask one of the guards:
Can the guard, who is standing in front of Heaven’s door, speak?
If answer is yes – you know your door to heaven.
@Nathan Mahon:
Are are assuming that the liar guard is standing before hell’s door?
If that’s the case then a simple question might also do:
Ask one of the guard whether he can speak?
lol
Ask a guard if he knows his shoe is untied. While he looks at his shoe, bum rush him and go into the door. You got a 50% shot of getting into heaven.
Say to the guard “did you know that they’ve relaxed the rules for getting into heaven just for today?”.
Either the guard leads you to heaven, or the guard leads you to hell, but has to live with the suspicion that he missed a chance to get into heaven.
The solution works regardless of *how* the guard lies.
(shamelessly adapted from a readers response from one of Martin Gardner’s books)
After elaborate analysis, one finds that, on asking the question ” is this heaven?”,
if both the guards reply “yes” then the guard infront of the heaven always tells the truth and the one infront of hell tells lie whereas if both of them tell “no”, then the one infront of heaven tells false whereas the one infront of hell tells the truth.
More formally, one guard gives a true answer (let’s call it +1) and the other false (-1). You don’t know what you get, but if you “multiply” the answers by asking one guard about the other’s answer (1 * -1) you’re sure to get a -1. That’s the classic answer. Another approach is to multiply an answer by itself (reference to another answer by an identical guard), which guarantees a +1:
Ask either guard, “If I asked you yesterday which door leads to heaven, what would you have said?” – then go there. Works even if there’s only one guard present
Are you a guard?
The guard in front of heaven will say Yes, the guard in front of hell will say No.
Jan V. – You assume the guard telling the truth is standing in front of the door leading to heaven.
I have the same answer that Ido Gendal gave.
Ask one guard what the other guard would answer to the question ‘Which door leads to heaven’. Then take the other door.
This is because if you ask the truth speaking guard the question he will tell you the exact answer the lying guard told him. (so take the other door)
Unless you want to go to hell of course, then you just walk into the door they tell you heaven is behind.
If you ask the lying guard the question he will tell you the opposite of what the truth speaking guard told you. (so take the other door)
Assuming lying gets you to Hell, you could ask either guard “Where do you live?”
The one that always lies will point to Heaven, and the one that always tells the truth will also point to Heaven.
Ask any simple question whose answer you know
then if guards answers matches yours its Heven else Hell
For example:
Heaven is a beautiful place?