My apologies if you’ve already grasped the flaw, but I’d like to point out that you’ve included it clearly in your choice of diagrams. You’ve made two, they are mirror images, and there is no way to indicate which is which without the concept of left and right. You could talk about inside and outside, and above and below, but not left and right.
So which one are the aliens using? That matters, as in one case the magnet pole you want to tell the aliens about is pointing right, and in the other it is pointing left. You could use inside and outside, but the two mirror images you’ve created don’t consistently create the same pole inside the loop.
Sorry, Mike, you’re not getting it. The problem is that the aliens are distant, we can send each other signals at the speed of light, but can’t actually meet.
You tell them that the left hand looks like a letter L. Now describe to them what a letter L looks like: a vertical line, and a horizontal line that meet at the bottom left…
Or you could send them a picture encoded as a series of pixels. Then tell them that they should scan the picture starting at the top left …
Nitpick. Faraday cages do not exclude static magnetic fields. Somehow this has become a deeply rooted misconception that pervades everywhere. Excluding static magnetic fields (or really any magnetic field short of RF) is actually really difficult. However this doesn’t materially affect the rest of your experiment. (What you need is a room surrounded by mu-metal. Such things exist.)
If done on the Earth this is true, and all is fine. However, given the OP has posited another planet, it would be good if you were more explicit about the terminology here.What is it about humans calling it the “south pole” that is important here. Humans can only call it that on the Earth. They can call the direction it points on another planet the magnetic South Pole for that planet. But calling it generically “the south pole” is ill defined.
What I will again point out is that, even on the Earth, this experiment only works now, and would not give that answer 800,000 years ago, and won’t again sometime in the future. What your experiment does is provide way of creating an intermediate proxy that allows us to compare the direction of current in the wire versus the direction of current in the geo-dynamo. THis does allow you to tell the direction of the geo-dynamo in another planet - but only with reference to the
OK, it works on the Earth, and the magnet will point to what humans call the Earth’s magnetic south pole. What is the next step?
It seems to me that the solution is to point to some distant astronomical objects. If the aliens are on Mars, we could use the Southern Cross.
Tell them to take a 2D diagram, draw a line between Gacrux and Acrux, and another between Mimosa and Palida. Gacrux is “up”, Acrux is “down”. That should be enough to orient the diagram correctly. Then you can say that Mimosa is left and Palida is right.
If they are from a more distant star, we could do the same thing with distant galaxies.
If you can see *any *common object, then the solution becomes trivially easy. No need to use the southern cross and complex relative descriptions, you can just tell them Venus is to the left Aldebaran when they look up (assuming they are low in the sky).
I don’t think so. A line of stars might appear to run left to right in the North, vertically at the Equator, and right to left in the South. With a cross, you can state which is the top.
Not really. The angle of difference across even a large planet wouldn’t make a measurable difference to the relative positions of stars.
If Aldebaraan and Venus are both low in the sky, then you can only see them if you are facing that portion of the sky. Hence one always appears to the left of the other.
Try it for yourself. Go out at night when Venus rises and mark a bright star to the left of it. Ask any doper anywhere else on Earth to go out and have a look when Venus rises. They will also say the star is to the left.
As a result of your set up being flawed your description is wrong on a few points, and we’re not entirely sure which point indicates the faults in your understanding.
Let me try to play the alien and see if that works.
I create your set up, to the best of my understanding. Let’s say I create a current loop going counter clockwise and place my molten iron underneath the wire on the right hand side.
Of course we don’t have common language about ccw and right hand, so you can’t know that’s what I did or tell me to do so, but so everyone “observing” knows what’s going on, that’s what I did.
Know by Earth definitions the magnetic field lines go clockwise if you look in the direction of the current so my solidifying iron underneath the wire on the right hand side will form with a north pole to the left, pointing to the inside of the loop of wire.
We do not however have common language about the direction of magnetic field lines or north and south poles, since these are arbitrary directions. We could easily have picked the other direction and have had a left hand rule, which would be more convenient since I could look at that hand while writing and not have to switch back and forth between writing and checking the rule, but I digress.
So now I have a ccw current loop and a magnet on the right hand side with a north pole pointing inwards. But I could as easily have had a cw loop with a magnet on the left hand side and the north pole pointing outwards.
Without using outside references there is no way for you to indicate left/right, cw/ccw, or magnetic north/south.
But that’s exactly as arbitrary as the + and - of an electrical potential (which we got wrong).
Will we get a different field if we create an electromagnet turning the coils clockwise or anticlockwise? Because if we get the same field, then we can use that to define N and S and that in turn to define left and right.
Blake: “…left hand side would point towards what we humans call the south magnetic pole.”
Alien: “Er, we call the 2 poles of a magnet A and B. Which one is the one that you call the ‘south’ pole?”
The sign convention is arbitrary, but the quantity is not. You can send messages to the alien describing what electrons and protons are, and unambiguously communicate what we mean by a negative electrical charge. You can’t do the same for magnetic fields.
This isn’t even true on the same planet. An observer in London may see a star directly to the left of Venus; someone in Canberra will see that star below and slightly to the right of Venus.
Another (quite minor) hitch in the “magnetic field induced by a current argument” is ensuring that the alien is not in an anti-matter chunk of the Universe.
[They aren’t if you’re communicating with them in a reasonable way.]
The right hand side of the magnet will be a south magnetic pole; on a compass this would be a north seeking pole. For my own piece of mind, I just ran the experiment with a 9v battery, some wire, and a compass. With the anode at the top and the compass on top of the wire, the north seeking end of the compass deflected to face right.
