## Left/Right

A forum for good logic/math puzzles.

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mudge
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### Left/Right

This isn't so much a puzzle as a thought experiment, but it seems puzzle-y enough. I can't remember where I heard this, and it's kind of hard to google.

I read this somewhere as an assertion, and I'm not convinced it's true (although, I haven't come up with a counter-example):

If we were able to contact an alien planet and communicate with them by only sending bits of data and they could translate everything but the concepts "left" and "right" it would be impossible to describe them.

So, the puzzle is, come up with a way to describe "left" and "right"
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phlip
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### Re: Left/Right

So, I take it we can define up/down and forward/backward OK, and we need to define the third axis (and thus some form of orientation for the universe)? And I take it that other chiral terms (like "clockwise") are out too, for fear of circular definitions?

In that case...
Spoiler:
How about electromagnetic stuff? If an electron is moving downwards through a magnetic field pointing away from you (both directions already defined) then the electron will start drifting left.

 Wait, I don't think that'll work...
Spoiler:
I can't see a way of defining the direction of a magnetic field or north/south without using left/right... Wikipedia says that electromagnetism is parity-symmetric (ie will work the same if left/right are swapped) so maybe I'm on the wrong track...

The universe as a whole isn't parity-symmetric though, so it's possible... just hope that the alien race has a similar quantum mechanical model to us, and have them recreate the cobalt-60 experiment mentioned in that article.

Code: Select all

`enum ಠ_ಠ {°□°╰=1, °Д°╰, ಠ益ಠ╰};void ┻━┻︵​╰(ಠ_ಠ ⚠) {exit((int)⚠);}`
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Owehn
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Location: Cambridge, UK

### Re: Left/Right

Yep, that's the first thing I thought of too. So long as time's pointing in the same direction for them as it is for us, you can manage it.
[This space intentionally left blank.]

mudge
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### Re: Left/Right

phlip wrote:So, I take it we can define up/down and forward/backward OK, and we need to define the third axis (and thus some form of orientation for the universe)? And I take it that other chiral terms (like "clockwise") are out too, for fear of circular definitions?

In that case...
Spoiler:
How about electromagnetic stuff? If an electron is moving downwards through a magnetic field pointing away from you (both directions already defined) then the electron will start drifting left.

 Wait, I don't think that'll work...
Spoiler:
I can't see a way of defining the direction of a magnetic field or north/south without using left/right... Wikipedia says that electromagnetism is parity-symmetric (ie will work the same if left/right are swapped) so maybe I'm on the wrong track...

The universe as a whole isn't parity-symmetric though, so it's possible... just hope that the alien race has a similar quantum mechanical model to us, and have them recreate the cobalt-60 experiment mentioned in that article.

Funny enough, after googling around, I found this:
Spoiler:
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NerfTW
Posts: 13
Joined: Fri Nov 23, 2007 3:55 pm UTC

### Re: Left/Right

Can't you just
Spoiler:
send a picture with easily discernable landmarks (AKA, a map of a planet), and the directions up/down/left/right on it?

Or, if a that isn't allowed:

Spoiler:
Send a message that says "Left and right are directions. The ones in the following piece of code are to the left of the zeroes, and the zeroes are to the right of the ones." then send the binary code 11110000

Stanford
Posts: 38
Joined: Fri Nov 02, 2007 6:52 pm UTC

### Re: Left/Right

NerfTW wrote:Or, if a that isn't allowed:

Spoiler:
Send a message that says "Left and right are directions. The ones in the following piece of code are to the left of the zeroes, and the zeroes are to the right of the ones." then send the binary code 11110000

Spoiler:
Do they use little endian or big endian (bitwise) representation of numbers?

Token
Posts: 1481
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Location: London

### Re: Left/Right

NerfTW wrote:Can't you just
Spoiler:
send a picture with easily discernable landmarks (AKA, a map of a planet), and the directions up/down/left/right on it?

Or, if a that isn't allowed:

Spoiler:
Send a message that says "Left and right are directions. The ones in the following piece of code are to the left of the zeroes, and the zeroes are to the right of the ones." then send the binary code 11110000

Neither of those work if the aliens don't write left-to-right.
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freddyfish
Posts: 69
Joined: Sat Oct 20, 2007 3:08 am UTC

### Re: Left/Right

Im assuming that we can communicate anything other than directly left and right.
so we can communicate cross products!
so say that down cross forward is right...
unless thats also not allowed...

Token
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Location: London

### Re: Left/Right

freddyfish wrote:Im assuming that we can communicate anything other than directly left and right.
so we can communicate cross products!
so say that down cross forward is right...
unless thats also not allowed...

It's not not allowed, it just doesn't work. We use right-handed cross products by convention, so you'd have to distinguish left and right to properly define it.
All posts are works in progress. If I posted something within the last hour, chances are I'm still editing it.

NerfTW
Posts: 13
Joined: Fri Nov 23, 2007 3:55 pm UTC

### Re: Left/Right

Token wrote:
NerfTW wrote:Can't you just
Spoiler:
send a picture with easily discernable landmarks (AKA, a map of a planet), and the directions up/down/left/right on it?

Or, if a that isn't allowed:

Spoiler:
Send a message that says "Left and right are directions. The ones in the following piece of code are to the left of the zeroes, and the zeroes are to the right of the ones." then send the binary code 11110000

Neither of those work if the aliens don't write left-to-right.

Spoiler:
How so? They still understand left and right, they'd just be reversed. And the picture suggestion would work no matter what direction they write in, because it wasn't said that they don't understand up and down. To fix the second example, you would simply state that left and right are directions perpendicular to up and down, relative to the viewer's viewpoint.

I don't think it's all that hard. If they understand all other concepts and can translate, we can use a two dimensional graph to show it, directions (from your viewpoint looking towards Earth, the center of the Milky Way is to your left), or pictures. I don't think it's difficult at all.

phlip
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### Re: Left/Right

Hmm... maybe I should've made a solution thread instead of posting my answer in here... I couldn't tell if it warranted one or not... seems like it does... oh well.

Spoiler:
The picture idea won't work because there's no way to tell them how to interpret the picture... you could send them a picture of an arrow pointing left and say "this is left" but there'd be no way to describe how to build the picture without using left/right... they could easily follow every instruction but end up with a flipped picture, with an arrow pointing right. Sending a picture of landmarks won't work because we can't assume that the aliens would recognise any landmarks we know of... assume they're far enough away that we can't get a picture of their planet, and they can't send us one, because we wouldn't be able to reconstruct it without knowing their left and right.

Declaring left/right as perpendicular to up/down won't work either, because that still doesn't help you decide which is which...

Code: Select all

`enum ಠ_ಠ {°□°╰=1, °Д°╰, ಠ益ಠ╰};void ┻━┻︵​╰(ಠ_ಠ ⚠) {exit((int)⚠);}`
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btilly
Posts: 1877
Joined: Tue Nov 06, 2007 7:08 pm UTC

### Re: Left/Right

phlip wrote:
Spoiler:
I can't see a way of defining the direction of a magnetic field or north/south without using left/right... Wikipedia says that electromagnetism is parity-symmetric (ie will work the same if left/right are swapped) so maybe I'm on the wrong track...

The universe as a whole isn't parity-symmetric though, so it's possible... just hope that the alien race has a similar quantum mechanical model to us, and have them recreate the cobalt-60 experiment mentioned in that article.

Spoiler:
That works unless the aliens are made of anti-matter. (Theoretically anti-matter should have reversed parity from normal matter.) So as Feynman said, if you coordinate everything, meet them in the middle of space, and their left is your right, don't try to touch!!!
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phlip
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### Re: Left/Right

btilly wrote:
Spoiler:
That works unless the aliens are made of anti-matter. (Theoretically anti-matter should have reversed parity from normal matter.) So as Feynman said, if you coordinate everything, meet them in the middle of space, and their left is your right, don't try to touch!!!

Spoiler:
True, but look at mudge's link... it talks about CP too, near the bottom of the page (unfortunately, it looks like the good bit's been cut off). CPT might be a problem, but since we're sending messages and then receiving replies, we can assume they perceive time in the same direction as us... I suspect communicating with a time-reversed race would be a challenge... we'd probably know about it.

Also, an engineering solution (outside the bounds of the thought experiment, but fun nonetheless):[spoiler]How're we communicating with this alien race? Some sort of EM wave?

Well, then, set up a second transmitter at the same frequency and amplitude, and we'll get interference. twerk the frequency and phase shifts and such so that there is exactly one maximum on the planet (this'll take some time... be patient). Get them to mark it (it'll be a line along the planet's surface). Say that from our point of view, it's a horizontal line. twerk the phase shift a little to move it upwards (again, from our POV) and have them mark it again.

Rotate the whole contraption through 90 degrees, and do it again, to get two vertical lines. Then you can say something like "If you are at the intersection of the first and third lines, looking towards the intersection of the second and third lines, with "down" being towards the centre of the planet, then the fourth line is to your right." This defines left and right, with the added bonus of telling them where in their sky our planet is, and orienting the two planets together (we can tell them what direction our north pole points, for instance, relative to those 4 points we plotted on their planet's surface).

This particular suggestion assumes that the aliens are surface-dwellers on a solid planet, like us, but I'm it could be adjusted for other situations too. It also assumes that they're not time-reversed (since it uses "first"...) but as I said, I consider that very unlikely.

Code: Select all

`enum ಠ_ಠ {°□°╰=1, °Д°╰, ಠ益ಠ╰};void ┻━┻︵​╰(ಠ_ಠ ⚠) {exit((int)⚠);}`
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btilly
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Joined: Tue Nov 06, 2007 7:08 pm UTC

### Re: Left/Right

phlip wrote:
Spoiler:
btilly wrote:That works unless the aliens are made of anti-matter. (Theoretically anti-matter should have reversed parity from normal matter.) So as Feynman said, if you coordinate everything, meet them in the middle of space, and their left is your right, don't try to touch!!!

Spoiler:
True, but look at mudge's link... it talks about CP too, near the bottom of the page (unfortunately, it looks like the good bit's been cut off). CPT might be a problem, but since we're sending messages and then receiving replies, we can assume they perceive time in the same direction as us... I suspect communicating with a time-reversed race would be a challenge... we'd probably know about it.

Good point. You're right.

However verifying this is a lot of work. (Admittedly much less than meeting them in person.)

Which suggests another solution:

Spoiler:
Any aliens who we can establish 2-way communication with must be relatively close to us - certainly they are within our galaxy. Once you've established agreement on the locations of any 4 objects - say our galaxy and three others - you can distinguish left from right.
Some of us exist to find out what can and can't be done.

Others exist to hold the beer.