Imagine a person, let's call him John, floating in a space.

It's not a room, cause there are no walls. It's something bigger as you can't see any limits. And there is nothing you can see. Only white light.

John is floating, completely naked, has no objects as bracelets or earrings...

You are speaking to him with a microphone (not relevant) and he hears you, but as there are no speakers, hears you from everywhere. But you can't see him.

How can you tell him what is his right or his left?

Note: Someone told me this years ago. I couldn't find any puzzle like this, hope it is not a duplicate.

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    $\begingroup$ Is this similar? $\endgroup$ – Techidiot Dec 5 '16 at 9:28
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    $\begingroup$ @marius well... John is special. is a very specific situation as you see xD $\endgroup$ – lois6b Dec 5 '16 at 9:44
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    $\begingroup$ We used the teleporter / time travel thingy from "The Terminator" that cannot teleport clothes. Duh! $\endgroup$ – Marius Dec 5 '16 at 10:06
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    $\begingroup$ It's a bit disappointing that the accepted answer to "How to tell one's right/left when there is no reference?" is basically "I lied, there is a reference." And when a comment rightly points out that it's not a fully reliable reference, that comment is dismissed. $\endgroup$ – hvd Dec 5 '16 at 11:22
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    $\begingroup$ I would like to remark that this question is very literally about lateral thinking. $\endgroup$ – Gareth McCaughan Dec 5 '16 at 13:50

Tell him that left is the side on which his heart is beating, right is the other side

Can work with

any other organ in human body that is always on the same side and gives some indication of where it is

(not sure what they are but there most probably are some)

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    $\begingroup$ what if he has Situs Inversus ? $\endgroup$ – Marius Dec 5 '16 at 9:22
  • $\begingroup$ Then he would most probably know about it, and you should ask him if he has it :) $\endgroup$ – oleslaw Dec 5 '16 at 9:24
  • $\begingroup$ I know. I was trying to play the devils advocate. and spread some knowledge while at it. It's a good answer. :) $\endgroup$ – Marius Dec 5 '16 at 9:24
  • $\begingroup$ @oleslaw you are right :D too easy it seems... I'm ashamed $\endgroup$ – lois6b Dec 5 '16 at 9:25
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    $\begingroup$ Given that the heart is only very slightly laterally translated, is this even reliable? For the other organs, since they produce no (obvious) sensory output, does our hero have to cut himself open to find out? $\endgroup$ – James Webster Dec 5 '16 at 15:58

Assuming that John can see his hands, he can do this:

Hold up both of his hands in front of him. He has to see the back of his hands, then all he has to do is to lift his index and thumb.
His left hand will make an 'L', thus he will know his left and he can obviously deduce his right.

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    $\begingroup$ I'd be surprised if someone who can't tell left from right could still recognize the correct shape of a letter. $\endgroup$ – Lynn Dec 5 '16 at 11:23
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    $\begingroup$ @Lynn I'd be surprised if someone who couldn't tell left from right and didn't know the alphabet wound up in space, NASA must be hiring anyone these days. $\endgroup$ – DasBeasto Dec 5 '16 at 13:07
  • $\begingroup$ @Lynn You still can now your alphabet but not tell left from right : chron.com/life/article/… $\endgroup$ – IAmInPLS Dec 5 '16 at 19:08

Tell him to:

hold his arms out in front of him, hands pointing up and thumb as far away from the rest of the fingers as it can go.
The hand where the index and thumb make the letter 'L' is Left.
(This assumes this strange scenario allows for him to be able to read, although in that case, left is 'where the writing starts from'.)

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    $\begingroup$ Yo got ninja'd! $\endgroup$ – IAmInPLS Dec 5 '16 at 10:13

I think almost this exact question is explored in detail in the Feynman Lectures on Physics #52 - Symmetry in Physical Laws:

The solution is to have John perform an experiment using the weak force, which violates parity. Using that, he can determine left from right.


How about this?

Tell him to do complex logical thinking like math for a long time and that when the left side of his brain start to hurt.. He will know :P

  • $\begingroup$ If he has the common knowledge of right and left, then just tell him "the left is the left"... $\endgroup$ – oleslaw Dec 5 '16 at 9:26
  • $\begingroup$ hmmm, I think I got confused with east and west haha.... So.... he was born in this room and has no knowledge of anything whatsoever? $\endgroup$ – stack reader Dec 5 '16 at 9:27
  • $\begingroup$ well, he can speak and understand... i would say that he just doesnt know what left and right are. he knows they are directions but not which is it $\endgroup$ – lois6b Dec 5 '16 at 9:28
  • $\begingroup$ about your edit ..I loled but dont think thats reachable. $\endgroup$ – lois6b Dec 5 '16 at 9:35
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    $\begingroup$ The only pain I can identify in my head is the forehead part, also symmetrical $\endgroup$ – lois6b Dec 5 '16 at 9:38

Update: I'm editing this answer to say it won't work. See the comments below. But I'm just leaving it here incase someone with same misconception as mine stumbles across.

Ask him to spin. If he goes up, due to torque (the head side, if he cannot even recognize "up"), he is spinning counter-clock-wise and that is his left. If he goes down, that's his right.


There is an upward torque when a body spins counter-clock wise and downward torque when spinning the other way round.

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    $\begingroup$ The torque vector of a spinning body isn't actually a force that makes him move, it's just a definition that makes the vector match all of the equations, and the fact that it goes up for a counter-clockwise spinning body is arbitrary. We could just as easily say it was down and it would still match the equations. $\endgroup$ – Joel Keene Dec 5 '16 at 17:32
  • $\begingroup$ @JoelKeene oh, I thought it would, thanks :), I'll make a note of that.. $\endgroup$ – Aravind Voggu Dec 6 '16 at 6:41
  • $\begingroup$ It's logical to think that if we use a "right-hand rule" in determining a vector, that we could use that to tell left from right, however it turns out that every right hand rule is always cancelled out by another before we can actual observe the results. So we could also use a left hand rule and the result would be the same. This use covered in the lecture I mentioned in my answer. It's a good thought though. $\endgroup$ – Joel Keene Dec 6 '16 at 15:14

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